LIVE-LINE WORKING AND EVALUATION OF RISK ON 400kV TRANSMISSION LINE

LIVE-LINE WORKING AND

EVALUATION OF RISK ON 400kV

TRANSMISSION LINE

A THESIS SUBMITTED TO THE UNIVERSITY OF MANCHESTER

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

FACULTY OF SCIENCES AND ENGINEERING

2017

Pietro Martini

School of Electrical and Electronic Engineering

2

Table of Contents

ABSTRACT…………………………………………………………………………....19

DECLERATION ……………………………………………………………………...20

COPY RIGHT STATEMENT …………………………………………………….....21

AKNOWLEDGMENT ……………………………………………………………….22

TERMS, DEFINATION…………………………………………………………… 23

CHAPTER 1............................................................................................................... 27

1.1. Introduction ....................................................................................................... 27

1.2. Introduction to Live-line Working ..................................................................... 28

1.3. Live-line Working Tools and Methods .............................................................. 28

1.3.1. Hot stick ..................................................................................................... 28

1.3.2. Bare Hand (Potential Method) .................................................................... 29

1.3.3. Helicopter Techniques ................................................................................ 30

1.3.4. Ground-Based Robots ................................................................................. 31

1.4. Live-line Working Risk and Challenges ............................................................ 32

1.5. Minimum Approach Distance (MAD) ............................................................... 34

1.6. Objectives and Conclusion ................................................................................ 35

CHAPTER 2............................................................................................................... 37

2.1. IEC 61472, Live-line Working Safety Standards ............................................... 37

2.2. IEEE 516-1995 Standard ................................................................................... 39

2.3. IEC 61472 Description of Calculation Procedure............................................... 41

3

2.3.1. Correction Factors ...................................................................................... 44

2.4. Impact of Correction Factors on MAD .............................................................. 52

2.4.1. At Tower .................................................................................................... 53

2.4.2. At Mid-Span ............................................................................................... 54

2.5. Discussion of Standards .................................................................................... 56

CHAPTER 3............................................................................................................... 59

3.1. Introduction ....................................................................................................... 59

3.2. Travelling Waves .............................................................................................. 60

3.2.1. Wave Velocity on Overhead Lines.............................................................. 61

3.2.2. Wave Velocity on Cables............................................................................ 65

3.2.3. Wave Reflection and Line Characteristics Impedance ............................... 67

3.3. Transient Classification ..................................................................................... 70

3.4. Lightning Overvoltage....................................................................................... 71

3.5. Review of Main Sources of Switching Overvoltages ......................................... 71

3.5.1. Line Energisation, re-energisation and Disconnection: ................................ 72

3.6. Switching Impulse Strength ............................................................................... 82

3.6.1. Effect of Wave shape .................................................................................. 82

3.6.2. The “U-Curve” ........................................................................................... 84

3.6.3. Wave Polarity ............................................................................................. 87

3.6.4. Effect of Atmospheric Conditions ............................................................... 89

3.7. Discussion and Conclusion ................................................................................ 90

CHAPTER 4............................................................................................................... 92

4

4.1. Introduction ....................................................................................................... 92

4.2. Simulation Methodology ................................................................................... 93

4.2.1. PSCAD Goodness-of-Fit Testing for Weibull Distribution.......................... 96

4.3. Parameters Influencing the Overvoltage on Transmission Line ....................... 98

4.3.1. Transmission Line Effect ............................................................................ 99

4.3.2. Type and Length of cable Section ............................................................. 100

4.3.3. Cable Section Position on transmission Line ............................................. 102

4.3.4. Capacitor bank.......................................................................................... 107

4.4. Network for Overvoltage Studies..................................................................... 112

4.5. Overvoltage Simulation Results....................................................................... 115

4.6. Calculation of Minimum Approach Distance ................................................... 120

4.7. Influence of Atmospheric Conditions .............................................................. 123

4.8. Influence of Floating object on Minimum approach distance ........................... 126

4.9. Discussion ....................................................................................................... 129

CHAPTER 5............................................................................................................. 131

5.1. Introduction ..................................................................................................... 131

5.2. Live-line Working Risk Evaluation ................................................................. 132

5.3. Risk Assessment ............................................................................................. 133

5.4. Methodology for Risk Assessment (Standard Switching Transient) ................. 134

5.4.1. Stress on the gap ....................................................................................... 135

5.4.2. Strength of the gap.................................................................................... 136

5.4.3. Intersection area ....................................................................................... 138

5

5.5. Methodology for Risk Assessment (Non-standard Switching Transient) .......... 140

5.6. Evaluation of Risk Based on Simulation Results.............................................. 142

5.7. Discussion: ...................................................................................................... 147

CHAPTER 6............................................................................................................. 149

6.1. Conclusion ...................................................................................................... 149

6.2. Impact of different Parameters on Minimum Approach Distance ..................... 151

6.3. Future Work .................................................................................................... 153

References......................................................................................................................156

7. Appendices........................................................................................................ 166

Word Count: 43474

6

Table of Figures

Figure 1-1: Live-Line Work Using Hot sticks, A: Fibre Glass Ladder, B: Hot Stick, C:

Bare Hand [1.12] ....................................................................................... 29

Figure 1-2: Live-Line Work Bare Hand or Potential Method, Where the Linesmen Are at

Same Potential as the Live Part and Isolated From the Earth [1.13] ............ 30

Figure 1-3: Live Men on 400kV Using the Live-lines Helicopter Method (Pictures

Provided by National Grid) ........................................................................ 30

Figure 1-4: Single Pick Robotic Arm which captures the Energized Conductor above the

H-Frame Structure [1.14-1.15]. .................................................................. 31

Figure 1-5: Typical Live-Line Working Task [IEC 624/13].......................................... 35

Figure 2-1: Flow Chart Illustrating the Calculation Procedure for the Minimum

Approach Distance .................................................................................... 40

Figure 2-2: Flow Chart Illustrating the Calculation Procedure for the Minimum

Approach Distance .................................................................................... 42

Figure 2-3: Electrical Distance for 0-1000m altitude at L6 tower, With and Without

Floating Object .......................................................................................... 53

Figure 2-4: Electrical Distance for 0-1000m Altitude at Mid-span L6 Tower, With and

Without Floating Object ............................................................................ 54

Figure 3-1: Pi-section Presentation of Overhead Line and Cable .................................. 61

Figure 3-2: Small Section of Transmission Line ........................................................... 61

Figure 3-3: Simple PSCAD Power System Model ........................................................ 62

Figure 3-4: Surge travelling time: Top: E_sending; The Voltage at the Sending and

Bottom: E_receiving; The Voltage at the Receiving End of the Line .......... 65

7

Figure 3-5: National Grid direct buried cable diagram .................................................. 65

Figure 3-6: Impulse Generator Used in PSCAD ........................................................... 66

Figure 3-7: Voltage at Sending Point (Blue Curve) Due to Current Impulse where Ea and

Eb are the sending and receiving voltages respectively .............................. 67

Figure 3-8: PSCAD Simulation Travelling Wave; Top: Voltage at Beginning, Bottom:

Voltage at the End of Transmission Line ................................................... 68

Figure 3-9: Behaviour of Voltage Travelling Wave at Transition Point ........................ 68

Figure 3-10: Sum of reflected voltage and current and sending waves .......................... 73

Figure 3-11: Voltage at The Sending and Receiving End Due to Energisation of 60km

Line on 400kV System............................................................................... 75

Figure 3-12: PSCAD Simulation Model of Trapped Charge ......................................... 77

Figure 3-13: Energising of a Line, Top; Without Trapped Charge, Bottom; With Trapped

Charge ....................................................................................................... 77

Figure 3-14: Voltage Due to Top; Energisation, Middle; Re-energisation, Bottom;

Disconnection ............................................................................................ 79

Figure 3-15: Oscillatory Transient Due to Interruption of Fault Current on PSCAD

Model- ES: Voltage Sending Point, EL: Voltage along the Line, Earc:

Circuit Breaker Arc Voltage. ..................................................................... 81

Figure 3-16: Standards Switching Impulse Where V50 is a half the time to crest of Crest

of a Transient Wave [4.1] .......................................................................... 82

Figure 3-17: U-Curves Obtained with Impulse Voltages of Various Time-to-Crests (Tcr

µs) Applied to Rod-Plane Gaps. Atmospheric Humidity in These

Experiments Was Varied [3.22, 3.23]......................................................... 85

Figure 3-18: A; Switching Impulse Flashover Voltage of Rod-Plane Gap, B; Estimation

of CRIEPI’s Equation ................................................................................ 86

8

Figure 3-19: Rod-Plane Gap; 1- Minute Critical Withstand AC and DC Voltages; 50%

Percent Spark Over Voltage with Standard and Long Front Impulses [3.26].

.................................................................................................................. 88

Figure 4-1: Model of Event Occurrence in Simulation ................................................. 94

Figure 4-2: Switching Overvoltage Distribution (pu).................................................... 94

Figure 4-3: Overvoltage Weibull Distribution Plot ....................................................... 97

Figure 4-4: Sample PSCAD Model of Transmission Line ............................................ 98

Figure 4-5: Overhead Model ...................................................................................... 100

Figure 4-6: PSCAD Model of Line-Cable Combination ............................................. 100

Figure 4-7: Change of Overvoltage at Beginning and End of Cable Section Due to

Changing the Length ................................................................................ 101

Figure 4-8: Overvoltage at Beginning and End of Cable Section vs. Cable Type ........ 102

Figure 4-9: Schematic Model of Transmission Line ................................................... 103

Figure 4-10: Time Required for Wave to Travel along the Cable................................ 103

Figure 4-11: Overvoltage, Sending (Blue Curve) And Receiving (Green Curve) With

Cable Section at Beginning of the Line .................................................... 105

Figure 4-12: Schematic Model of Transmission Line with Cable Section Place in the

Middle of the Line ................................................................................... 106

Figure 4-13: Overvoltage, Sending (Blue Curve) and Receiving (Green Curve) With

Cable Section at Middle of the Line ......................................................... 106

Figure 4-14: Maximum Overvoltage, Sending (Blue Curve) and Receiving (Green

Curve) with Cable-Line at End of Transmission Line ............................... 107

Figure 4-15: Series Capacitor Bank Modelling with a 41.91µF series Capacitor ......... 109

Figure 4-16: Overvoltage with 20% Series Compensation .......................................... 110



9

Figure 4-19: A; PSCAD Model of Transmission Line, B; Schematic diagram of the

network ................................................................................................... 113

Figure 4-20: Top; P-E, Bottom; P-P. Influence of Length of Transmission Line on the

Minimum Approach Distance .................................................................. 125

Figure 4-21: Top; P-E, Bottom; P-P - Minimum Approach Distance Influenced by

Altitude and Fault Levels ......................................................................... 126

Figure 5-1: Risk and Hazard Explanation [5.1]........................................................... 131

Figure 5-2: Risk Management Process ....................................................................... 132

Figure 5-3: Live-Line Working Risk Evaluation Process ............................................ 134

Figure 5-4: Switching Overvoltage Distribution ......................................................... 135

Figure 5-5: Flowchart Illustrating the Steps Undertaken for Calculation of Gap Strength

................................................................................................................ 136

Figure 5-6: Air Gap Voltage Breakdown Probability .................................................. 138

Figure 5-7: Combination of Air Gap Voltage Breakdown Probability and Switching

Overvoltage Distribution.......................................................................... 139

Figure 5-8: Risk as the Function of Time to Crest ...................................................... 142

Figure 5-9: Risk of Failure as a Function of Time to Crest on Different Towers for Top:

P-E and Bottom: P-P Voltage................................................................... 145

Figure 5-10: Risk of Failure for P-E Voltage as the Function of Changing the Gap Size,

Bottom: The Zoom in Graph of the Top Graph ........................................ 146

Figure 7-1: Conductor Coordinates of Overhead Line- Refer to Table 59 ................... 169

Figure 7-2: PSCAD Fault Type and Time Selection Modules ..................................... 171

Figure 7-3: PSCAD Overhead Line Model ................................................................. 171

Figure 7-4: P-E Calculation Design ............................................................................ 172

Figure 7-5: P-P Calculation Modules ......................................................................... 172

10

Figure 7-6: Rod to Plane Sparkover versus Gap Length D, CRIEPI_ Figure 5-2 [2.23]

................................................................................................................ 193

Figure 7-7: Switching Impulse Flashover Voltage of Rod-Plane Gap, Estimation of

CRIEPI’s Equation .................................................................................. 194

11

List of Tables

Table 2-1: Minimum Approach Distances DA for Several Countries [2.2] .................... 38

Table 2-2: Comparisons of Minimum Approach Distance, IEC Correction Factors: Gap

Factor=1.2, Altitude Factor=0.94, Insulation Factor=0.95 and Floating

Factor =0.85. ............................................................................................. 39

Table 2-3: Gap factors for some actual phase to earth configurations [2.6]. The gap

factor (kg) in Table 2-3 is presented by "k". ................................................ 45

Table 2-4: Values of Exponents, ‘m’ of Air Density Correction and ‘w’ For Humidity

Correction as the Function of Parameter ‘g’ (IEC 60060) - [2.7] ................ 48

Table 2-5: Average ka Value [2.8] ................................................................................ 49

Table 2-6: Set of P-E and P-P Overvoltages ................................................................. 50

Table 2-7: Effect of Humidity of the Minimum Approach Distances at a temperature of

20oC and a pressure of 101.3kPA ............................................................... 50

Table 2-8: Comparison of the Calculation Results for The Minimum Clearances Based

on IEEE 516-1995 and IEC 61472 Method [2.21] ...................................... 56

Table 3-1: Surge Impedance and Propagation Constant for Normal and Lossless Line

[3.3] ........................................................................................................... 62

Table 3-2: Generator Parameters .................................................................................. 63

Table 3-3: Overhead Line and Circuit Breakers’ Parameters ........................................ 63

Table 3-4: Sample Cable Data for 400kV Single Core Cable, 1200mm2 ABB XLPE

Cable [3.6] ................................................................................................. 66

Table 3-5 CIGRE Classification of Overvoltage Based on Frequency [2.6] .................. 70

Table 3-6 IEC Classification of Overvoltage Based on Time Duration [3.8] ................. 70

Table 3-7: Shapes and Classes of Overvoltages Standards Voltage [3.29] .................... 83

Table 3-8: U50 of Rod-Plane for Fast and Slow Wave Shape [3.21] .............................. 83

12

Table 3-9: U50 of Rod-Plane as the Function of Wave Shape, Non-Standard Switching

Wave Form [3.21]...................................................................................... 84

Table 3-10: Effect of Polarity on Rod-Plane Gap [3.15], [3.19] .................................... 88

Table 4-1: U2 Value Comparison Achieved by PSCAD and Excel ............................... 95

Table 4-2: Simulation Result of MATLAB Output File ................................................ 97

Table 4-3: Magnitude of Switching Overvoltage Due to Various Length of Transmission

Line ........................................................................................................... 99

Table 4-4: Three Types of Cable Specification Used by National Grid ....................... 101

Table 4-5: Cable and Overhead Line Specification ..................................................... 103

Table 4-6: Series Capacitor Size ................................................................................ 110

Table 4-7: Overvoltage Results for Line Energisation ................................................ 116

Table 4-8: Overvoltage Results for Line Re-Energisation ........................................... 116

Table 4-9: Overvoltage Results for Line Dis-Connection ........................................... 116

Table 4-10: Overvoltage Results for Fault & Clearance.............................................. 117

Table 4-11: Overvoltage Results for Fault & Clearance Due to Simulation Setting..... 117

Table 4-12: Overvoltage Results for Fault & Clearance Due to 80% LG Faults, 17% LL

Faults,2% LLG Faults and 1% LLL Faults ............................................... 118

Table 4-13: Overvoltage Results Due to Fault & Clearances with Inductive

Compensation .......................................................................................... 119

Table 4-14: Overvoltage Results Due to Fault & Clearances with Capacitive

Compensation .......................................................................................... 119

Table 4-15: Overvoltage Results Due to 80% LG Faults, 17% LL Faults, 2% LLG Faults

And 1% LLL Faults with Inductive Compensation................................... 119

Table 4-16: Overvoltage Results Due to 80% LG Faults, 17% LL Faults, 2% LLG Faults

and 1% LLL Faults with Capacitive Compensation .................................. 119

Table 4-17: Example Selection Table for ka ............................................................... 120

13

Table 4-18: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios (With

No Ergonomic Distance DA) .................................................................... 122

Table 4-19: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios with

Inductive Compensation (With No Ergonomic Distance DA) ................... 122


Capacitive Compensation (With No Ergonomic Distance DA).................. 122


80% LG Faults, 17% LL Faults, 2% LLG Faults And 1% LLL Fault

Probability (With No Ergonomic Distance DA) ........................................ 122


Inductive Compensation With 80% LG Faults, 17% LL Faults, 2% LLG

Faults And 1% LLL Fault Probability (With No Ergonomic Distance DA) 123


Capacitive Compensation With 80% LG Faults, 17% LL Faults, 2% LLG

Faults And 1% LLL Fault Probability (With No Ergonomic Distance DA) 123

Table 4-24: Influence of Altitude on Electrical Distances (Du) Due to Fault and

Clearances (Without Compensation) - With No Ergonomic Distance DA . 124


Clearances (Inductive Compensation) - With No Ergonomic Distance DA 124


Clearances (Capacitive Compensation) - With No Ergonomic Distance DA

................................................................................................................ 124

Table 4-27: Electrical Distances for Fault & Clearance Simulation Scenarios at 500m

Altitude With Floating Object With 2m Length in Direction of Phases (With

No Ergonomic Distance DA) .................................................................... 127

14

Table 4-28: Electrical Distances for Fault & Clearance Simulation Scenarios with

Inductive Compensation at 500m Altitude with Floating Object with 2m

Length in Direction of Phases (With No Ergonomic Distance DA) ........... 128

Table 4-29: Electrical Distances for Fault & Clearance Simulation Scenarios with

Capacitive Compensation at 500m Altitude with Floating Object With 2m

Length in Direction of Phases (With No Ergonomic Distance DA) ........... 128

Table 5-1: Calculation Extracted from Simulation Results in Figure 5-4 .................... 136

Table 5-2: Minimum Approach Distance’s Risk of Failure Obtained from Probability of

Air Gap Breakdown and Switching Overvoltage Distribution .................. 139

Table 5-3: Estimation of Risk Based on Transient Time-to-Crest ............................... 141

Table 5-4: Calculated Risk for Fault & Clearance Simulation Scenarios..................... 143

Table 5-5: Calculated Risk for Fault & Clearance Simulation Scenarios with Inductive

Compensation .......................................................................................... 143

Table 5-6: Calculated Risk for Fault & Clearance Simulation Scenarios with Capacitive

Compensation .......................................................................................... 143

Table 5-7: Calculated Risk for Fault & Clearance Simulation Scenarios with 80% LG

Faults, 17% LL Faults, 2% LLG Faults And 1% LLL Fault Probability ... 144

Table 5-8: Calculated Risk for Fault & Clearance Simulation Scenarios with Inductive

Compensation with 80% LG Faults, 17% LL Faults, 2% LLG Faults and 1%

LLL Fault Probability .............................................................................. 144

Table 5-9: Calculated Risk for Fault & Clearance Simulation Scenarios with Capacitive

Compensation with 80% LG Faults, 17% LL Faults, 2% LLG Faults and 1%

LLL Fault Probability .............................................................................. 144

Table 5-10. Rate of Change of the Risk Due to Change of Wave Time to Crest.......... 148

Table 7-1: Atmospheric Factor ka for Different Reference Altitudes and Values of U90_

(IEC 61472) ............................................................................................. 167

15

Table 7-2: Average ka Values IEC 61472 ................................................................... 167

Table 7-3: Floating Conductive Object Factor kf ........................................................ 168

Table 7-4: Conductor Coordinates (Including Sag) for Overhead Line Designs [2.1].. 169

Table 7-5: PSCAD Configuration of L2 Tower .......................................................... 170




Table 7-9: PSCAD Configuration of L12 Tower ........................................................ 171

Table 7-10: Overvoltage Simulation Results for Fault and Clearance ......................... 173

Table 7-11: Overvoltage Simulation Results for Fault and Clearance, Inductive

Compensation .......................................................................................... 173

Table 7-12: Overvoltage Simulation Results for Fault and Clearance, Capacitive

Compensation .......................................................................................... 174

Table 7-13: Minimum Approach Distance for Fault and Clearance at Sea Level ........ 174

Table 7-14: Minimum Approach Distance for Fault and Clearance, Inductive

Compensation at Sea Level ...................................................................... 174

Table 7-15: Minimum Approach Distance for Fault and Clearance, Capacitive

Compensation at Sea Level ...................................................................... 175

Table 7-16: Minimum Approach Distance for Fault and Clearance at 500m Altitude . 175


Compensation at 500m Altitude ............................................................... 175


Compensation at 500m Altitude ............................................................... 176

Table 7-19: Minimum Approach Distance for Fault and Clearance at 1000m Altitude 176


compensation at 1000m altitude ............................................................... 176

16


Compensation at 1000m Altitude ............................................................. 177

Table 7-22: Overvoltage Simulation Results for Fault and Clearance & Weighted Fault

Type ........................................................................................................ 177

Table 7-23: Overvoltage Simulation Results for Fault and Clearance, Inductive

Compensation & Weighted Fault Type .................................................... 178

Table 7-24: Overvoltage Simulation Results for Fault and Clearance, Capacitive

Compensation & Weighted Fault Type .................................................... 178

Table 7-25: Minimum Approach Distance for Fault and Clearance at Sea Level &

Weighted Fault Type .................................................................................. 178


Compensation at Sea Level & Weighted Fault Type ................................ 179


Compensation at Sea Level & Weighted Fault Type ................................ 179

Table 7-28: Minimum Approach Distance for Fault and Clearance at 500m Altitude &

Weighted Fault Type ............................................................................... 179


Compensation at 500m Altitude & Weighted Fault Type ......................... 180


Compensation at 500m Altitude & Weighted Fault Type ......................... 180

Table 7-31: Minimum Approach Distance for Fault and Clearance at 1000m Altitude &

Weighted Fault Type ............................................................................... 180


compensation at 1000m altitude & Weighted Fault Type ......................... 181


Compensation at 1000m Altitude & Weighted Fault Type ....................... 181

17

Table 7-34: Minimum Approach Distance for Fault and Clearance at Sea Level with

Floating Object of 2m .............................................................................. 182


Compensation at Sea Level with Floating Object of 2m ........................... 182


Compensation at Sea Level with Floating Object of 2m ........................... 183

Table 7-37: Minimum Approach Distance for Fault and Clearance at 500m Altitude with

Floating Object of 2m .............................................................................. 183


Compensation at 500m Altitude with Floating Object of 2m .................... 183


Compensation at 500m Altitude with floating object of 2m ...................... 184

Table 7-40: Minimum Approach Distance for Fault and Clearance at 1000m Altitude

with Floating Object of 2m ...................................................................... 184


Compensation at 1000m Altitude with Floating Object of 2m .................. 184


Compensation at 1000m Altitude with Floating Object of 2m .................. 185

Table 7-43: Minimum Approach Distance for Fault and Clearance at Sea Level with

Floating Object of 2m (Weighted Fault Type) .......................................... 185


Compensation at Sea Level With Floating Object Of 2m (Weighted Fault

Type) ....................................................................................................... 186


Compensation at Sea Level with Floating Object of 2m (Weighted Fault

Type) ....................................................................................................... 186

18

Table 7-46: Minimum Approach Distance for Fault and Clearance at 500m Altitude with

Floating Object of 2m (Weighted Fault Type) .......................................... 186


Compensation at 500m Altitude with Floating Object of 2m (Weighted Fault

Type) ....................................................................................................... 187


Compensation at 500m Altitude with Floating Object of 2m (Weighted Fault

Type) ....................................................................................................... 187


with Floating Object of 2m (Weighted Fault Type) .................................. 187


Compensation at 1000m Altitude with Floating Object of 2m (Weighted

Fault Type) .............................................................................................. 188


Compensation at 1000m Altitude with Floating Object of 2m (Weighted

Fault Type) .............................................................................................. 188

Table 7-52: Rod to Plane Gap Experimental Sparkover Data, Positive polarity

(Continue), CRIEPI_ Table 5-1 [2.23] ..................................................... 192

Table 7-53: 50% Flashover Voltage (kV) as the Function of Gap Size and Time to Crest

Based on Table 7-7 .................................................................................. 195

Table 7-54: Estimated formulae for Calculation of U50 Voltage as the Function of Gap

Size for Each Transient Time to Crest ...................................................... 196

19

ABSTRACT

Power industries in transmission and distribution level are obligated to maintain andreplace their electrical equipment. Maintaining the quality and continuity of supply istheir priority to avoid customers' complaints and financial penalisation. Live-lineworking as one of the most important methods of maintenance has been used since the1900s where the new methods in 1960s made the live-line workers enabled to work onthe higher voltage levels up to 800kV. Various industries adopt different techniques tocalculate the minimum approach distance (MAD) during the live-line work. A suitablemethod reduces the risk to live-line workers and provides adequate safety distancesbetween the live parts and linesmen. Therefore, setting an appropriate safety distancebetween the linesmen and live parts ensures the safety of the workers and minimise therisk of flashover.In this thesis, different methods of calculation of the minimum approach distance aredescribed, and results from overvoltage simulations are used as an input to themethodology outlined in IEC 61472. Also, this thesis highlights and investigates theimpact of a range of factors within 400kV transmission line on the minimum approachdistance (MAD). Factors examined include the time to crest of the overvoltage (waveshape), the fault type, the probability of occurrence of each type of fault, fault level andthe type of overhead line and towers.Furthermore, the minimum approach distances and also associated risk due to eachfactor and scenario have been calculated. The calculated risk in this thesis presents therisk of failure of a gap against the switching overvoltages due to the simulation ofsources of overvoltage. A new set of estimated equations is developed to consider theinfluence of wave shape in the calculation of the minimum approach distance (MAD).This thesis does not propose a method to replace the international standards, but it couldbe used in many situations including where utility companies wish to develop acomplete understanding of the risk associated with live-line working.Calculation of the minimum approach distance (MAD) within the National Grid UK isbased on the methodology described in the IEC 61472, whereas EDF Energy uses theIEEE method to calculate the minimum approach distance. The choice of a smaller /larger minimum approach distance (MAD) using different methods will have an impacton the risk associated with live-line working. Previous works intend to investigate themagnitude of switching overvoltages on one part of a network and calculate theappropriate minimum approach distance for the work in that section.This work is based on the examination of the switching overvoltages under the worstcase scenarios. As a result, the simulated overvoltages in this work are higher thanexpected overvoltages in National Grid network. Also as in practice, the magnitude ofswitching overvoltages in National Grid network is controlled by different protectionsequipment therefore, the simulated results and the calculated minimum approachdistances in this work are very conservative.

20

DECLARATION

No portion of the work referred to in this thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other

institute of learning.

21

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22

ACKNOWLEDGMENTS

First and foremost, I would like to express my sincere gratitude to my supervisor, Prof.

Ian Cotton, for his continuous guidance and support throughout these years with his very

positive attitude and openness. I appreciate his helpful comments and discussions which

have contributed a lot to this achievement.

A special acknowledgment goes to the Engineering and Physical Sciences Research

Council (EPSRC) and National Grid UK who have sponsored this project. I would

particularly like to thank all the members in National Grid and Mr. Chris Land, live

Working Operation Engineer. I would also like to thank all my friends and colleagues

in the Electrical Energy and Power Systems (EEPS) research department at the

University of Manchester. Their good companionship and the excellent opportunities

they provided to me to develop ideas and exchange knowledge have made this journey

very enjoyable.

I warmly thank and appreciate my parents’ (Nasser & Sima) continues help and support,

to whom I wish to dedicate this Thesis to. Words cannot express how grateful I am to

them for their love, patience, kindness and support. I owe them my life, career, future

and who I am and I will be. Special thanks to my mom’s Dampayees which pushed me

to where I am standing now.

Finally, I would like to thank my wife Mobina for being always next to me and

unconditionally supporting me throughout all these years.

Pietro Martini

February 2017

Manchester

23

Terms, definitions and symbols

For the purpose of this thesis, the following terms, definitions and symbols apply.

Damaged insulator

Insulator having any type of manufacturing defect or in-service deterioration which

affects its insulating performance.

Electrical distance (DU)

Distance in air required to prevent a disruptive discharge between energized parts or

between energised parts and earthed parts during live working [IEC 60050-651: 651-21-

12].

Ergonomic component of distance (DE)

Distance in air added to the electrical distance, to take into account inadvertent

movement and errors in judgement of distances while performing work [IEC 60050-651:

651-21-13].

Fifty per cent disruptive discharge voltage (U50)

Peak value of an impulse test voltage having a fifty per cent probability of initiating a

disruptive discharge each time the dielectric testing is performed [IEC 60050-604:1987,

604-03-43].

Highest voltage of a system (Us)

Highest value of operating voltage which occurs under normal operating conditions at

any time and any point in the system (phase to phase voltage) [IEC 60050-601:1985,

601-01-23].

24

Minimum working distance (DA)

Minimum distance in air to be maintained between any part of the body of a worker,

including any object (except tools appropriate for live working) being handled directly,

and any part(s) at different electric potential(s).

Minimum approach distance (MAD)

The minimum approach distance is the sum of the electrical distance appropriate for the

maximum nominal voltage and of the selected ergonomic distance [IEC 60050-651:

651-21-11].

Ninety per cent statistical impulse withstand voltage (U90)

Peak value of an impulse test voltage at which insulation exhibits, under specified

conditions, a ninety per cent probability of withstand [IEC 60050-604:1987, 604-03-42].

Part

Any element present in the work location, other than workers, live working tools and

system insulation.

Gap

The gap refers to the space between the phases of overhead line or phase to ground/

tower’s body where the live-line working takes place.

Per unit value (pu)

Expression of the per unit value of the amplitude of an overvoltage (or of a voltage).

25

Transient overvoltage

Short duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory,

usually highly damped [IEC 60050-604:1987, 604-03-13].

Two per cent statistical overvoltage (U2)

Peak value of a transient overvoltage having a 2 per cent statistical probability of being

exceeded.

Work location

Any site, place or area where a work activity is to be, is being, or has been carried out

[IEC 60050-651: 651-26-03].

Ad: Length of damaged insulator or number of damaged units in an insulator of length

Ao, not shunted by long arcing horn or grading ring

Ao: Length of undamaged insulator or number of undamaged insulator units not shunted

by long arcing horn or grading ring

β: Ratio of the total length in the direction of the gap axis of the floating conductive

objects (s) to the original air gap length

D: Length of the remaining air gap phase to earth

DA: Minimum Approach Distance

DE: Ergonomic distance

DU: Electrical distance necessary to obtain U90

DLins: Minimum residual insulation length

(d1, d2, d3, d4): Distances between the worker(s) and parts of the installation at different

electric potentials

F: Sum of all lengths, in the direction of the gap axis, of all floating conductive objects

in the air gap (in metres)

Ks: Statistical safety factor

26

Kt: Factor combining different considerations influencing the strength of the gap

ka: Atmospheric factor

kd: Coefficient characterizing the average state of the damaged insulators

kf: Floating conductive object factor

kg: gap factor

ki: Damaged insulator factor

kic: Damaged composite insulator factor

kis: Damaged insulator strings factor

ks: Standard statistical deviation factor

Lf: Original air gap length

P: Length of the remaining gap phase to phase

r: Distance of a conductive object from the axis of the gap

se: Normalized value of the standard deviation of U50 expressed in per cent

Ue2: Two per cent statistical overvoltage between phase and earth

Ue90: Ninety per cent statistical impulse withstand voltage phase to earth

Up2: Two per cent statistical overvoltage between two phases

Up90: Ninety per cent statistical impulse withstand between two phases

ue2: Per unit value of the two per cent statistical overvoltage phase to earth

up2: Per unit value of the two per cent statistical overvoltage between two phases

Us: Highest voltage of a system between two phases

PE: Phase to earth voltage

PP: Phase to phase voltage

a.c.: Alternative current

Dc: Direct current

CHAPTER 1. Introduction to Live Line Working 27

CHAPTER 1

Introduction to Live Line Working

1.1. IntroductionTransmission and distribution companies spend millions of pounds to maintain and

replace the power transmission lines, cables and equipment to ensure the continuity and

reliability of power supply to their customers.

Where the continuity and quality of power supply are necessary, disconnecting the

power consumers from the power supply can be very costly. Quality and continuity of

supplying electricity have always been a primary consideration for all transmission

companies and this fact becomes indispensable where supplying the public and industry

with power is concerned. To guarantee the quality and continuity of the service,

continual and efficient plant maintenance without taking the plant out of service can be

necessary.

On the another hand, working on energised equipment reduces the need for a spare line

and increases the utilisation and operational continuity of existing lines and also it has

more financial and environmental advantages [1.1].

In the past, the maintenance on the transmission lines and substations required

disconnection of some parts of the network. Therefore, live-line working began

operation in 1975 with the purpose of the maintenance of electrical component and


transmission lines operating at medium or high voltage, while the whole system is in

service.

1.2. Introduction to Live-line WorkingIn the early years of the 20th century, live-line working techniques were developed to

prevent the power shortage and blackout at the time of plant maintenance. In the 1960s,

some methods were tested in the laboratory to establish a safe process of working in

much closer contact with high voltage lines. Nowadays, these methods are still in use by

transmission companies where live-line working takes place. However, forward

movement of technology provides the staff and their companies with new tools,

techniques and equipment but, in general there are four primary methods of performing

the live-line work as explained in the next sections [1.2].

1.3. Live-line Working Tools and MethodsThe fibreglass ladder, hot sticks, conducting suit and insulating rubber gloves are some

of the common equipment in live-line working whereas some methods, such as bare

hand, aerial service or ground-based robots, are used to ensure that the live-line working

is as safe as possible.

1.3.1. Hot stick

Hot stick (live-line tool) was invented towards the end of the 20th century. Linesmen

could do some limited jobs such as screwing, moving poles and switching by standing at

a safe distance from live equipment in the voltage range of 0.05kV- 800kV [1.12]. The

invention of fibreglass has improved usage of the hot stick. Unlike the wooden stick, the


fibreglass stick did not get any damp or moist, and this is one of the most important

advantages of the fibreglass stick.

Figure 1-1 shows the application of the hot stick during live-line working away from live

parts. Unlike the hot stick, the linesmen are energised by working on a live line while

they are standing on a fibreglass ladder.

Figure 1-1: Live-Line Work Using Hot sticks, A: Fibre Glass Ladder, B: Hot Stick, C:

Bare Hand [1.12]

At lower voltages up 36kV, linesmen wear insulating rubber gloves to be able to work in

direct mechanical contact with live parts.

1.3.2. Bare Hand (Potential Method)

The first procedures for bare hand working were initiated in 1960. In the bare hand

(potential method) shown in Figure 1-2, the linesman body’s potential needs to be raised

to the same electrical potential (voltage) as the live equipment.

Wearing the insulating rubber gloves enables the linesmen to work in direct contact with

live parts. In this method, the linesman and the live part or line are at the same electrical

potential, and they are isolated from the surrounding. However, there is only a small

flow of current through the linesman’s body [1.3]. Before establishing a contact

between a linesman and a live part, the linesman’s body needs to reach the same

A B C


electrical potential as the live part. This process is initiated by using a conducting tool

which gets hooked on the live part – refer to Figure 1-2.

Figure 1-2: Live-Line Work Bare Hand or Potential Method, Where the Linesmen Are atSame Potential as the Live Part and Isolated From the Earth [1.13]

After finishing the live working task, the process needs to be reversed to disengage the

linesman from the live part. The advantage of this method compared to the hot sticks

method is that the linesman can do more varieties of tasks such as line splicing, vibration

damper or conductors’ spacers replacement, etc., – refer to Figure 1-2.

1.3.3. Helicopter Techniques

Linesmen can also work on high voltage live-lines while they are isolated from the

ground potential by standing in a basket which is attached via special insulated rope(s) to

a helicopter or a crane.

Figure 1-3: Live Men on 400kV Using the Live-lines Helicopter Method (PicturesProvided by National Grid)

In this method, linesmen are wearing Faraday suits, overalls made from conducting

fibres, conducting gloves and socks. The linesmen can work of a platform fixed to the


side of the helicopter where, the body of the helicopter, linesmen and hanging basket are

at the same potential as the transmission line while they are isolated from the earth -

refer to Figure 1-3.

Linesmen can be lowered by a line attached to the helicopter or crane while standing

inside the basket. Similar to the bare hand method, as the linesman approaches the wire,

an arc will form between the live part and linesman’s body. The worker must

immediately bond to the line to prevent further arcing. The linesman may use a

conducting band during the approach to make the connection.

1.3.4. Ground-Based Robots

In this method, a ground-based insulated long robotic arms can capture, remove or lift

the heavy live conductors and parts. This method is used when there is difficulty in

excitation of a project by use of other methods. These robots are remotely controlled

from the ground by use of a radio controller device. This method is used up to 500kV

[1.14] and [1.15], during some live-line projects, such as replacement of rotten poles, re-

conductoring of existing transmission lines, substation repairs including nuclear plants,

and replacement and re-insulating existing structures.

Figure 1-4: Single Pick Robotic Arm which captures the Energized Conductor above theH-Frame Structure [1.14] and [1.15].


1.4. Live-line Working Risk and Challenges

In the UK, many electricity companies such as National Grid, Scottish Power, EDF

Energy, etc., have applied live-line working as part of their maintenance scheme.

Different methods and tools have been used to mitigate the considerable hazards

involved with working on the live lines. Being in contact with a live-line, exposes the

linesmen to high electromagnetic fields (EMFs). To prevent receiving the exposures

above the relevant limits specified by Table 2 of ICNIRP guidelines [1.11], the linesmen

usually wear the conducting suits which screen them against the electric field.

Apart from live-line working tools and techniques explained in the previous sections,

different organisation such as IEC, IEEE, OSHA, etc., developed a guideline for the

calculation of the minimum approach distance (MAD). Each guideline introduces a

method for the calculation of the minimum approach distance for phase to ground (P-E)

or phase to phase (P-P) cause which must be maintained by the linesmen when they are

exposed to energised parts. These guidelines are set to mitigate the risk involved in live-

line working.

Determination of maximum electrical stress due to transient overvoltages is the first step

of calculation of the minimum approach distances. By knowing the magnitude of

electrical stress at the work site during the live-line working, the minimum approach

distance can be calculated.

The magnitude of stress and also the strength of the gap are influenced by many factors

such as system parameters, gap geometry, atmospheric conditions and altitude, the

presence of the insulation in the air gap, surge wave shape and presence of tools or

floating object in the air gap.

Therefore, working standards are developed to minimise the risk of flashover in the air

gap where live-line working takes place. EC/TC78 was initially created to standardise


the tools and equipment used in the live-line working in North American and European

countries [1.4]. Later, IEC 61472 as one of the TC78 projects provided the minimum

approach distance required for live-line working.

Recently National Grid as the UK’s transmission company uses this method for the

calculation of the minimum approach distances for the purpose of live-line working.

This project initiated by the National Grid as part of their requirement to ensure the

safety of their live-line staffs.

Currently, there are not many pieces of research or investigations concerning the

minimum approach distances, influencing parameters or risk involved with live-line

working.

Although these methods and equipment intend to minimise the risk facing the linesmen,

yet working on live system involves risks which can have fatal results. To address the

importance of the safety, some world- known associations such as IEEE, EPRI, CIER,

CIGRE, LWA, etc., actively work on live working safety, but still according to

UNIPEDE survey, there were 171 accidents and five fatalities due to live-line working

[1.5].

Some academic studies highlighted the importance of influencing factors such as

altitude, humidity or broken insulator on the flashover voltage of the air gap [1.6]- [1.8].

At the same time, some studies suggested a new insulation coordination approach to

address the effect of wave shape on the voltage breakdown of the gap [1.9], [1.10].

It has been noticed by the author of this thesis that there is a missing link between the

influencing parameters affecting the air gap flashover and risk involved with the

minimum approach distance. Also, as it is shown further in Chapter 2 of this thesis, the

inconsistency between the calculated minimum approach distances using different

available methods features the very first objective of this project.


1.5. Minimum Approach Distance (MAD)

As this project considers the Barehand and helicopter methods explained in section 1.4

of this thesis, it is necessary to examine the safety factors concerning the live-line

workers.

When a linesman climbs on a tower or uses a ladder to hang from any part of a tower,

the minimum phase-earth (PE) or phase-phase (PP) safety clearance have been

interrupted. This is because the linesman’s body will provide a conductive shortcut

between the phases or phase to ground. As a result, his/her body can conduct any

possible flashover from closer phase to another phase or tower’s body/ earth.

Therefore, to prevent any flashover due to the presence of the linesman, a safe distance

needs to be defined which can be referred to the minimum approach distance (MAD).

According to IEC 61472, Ed 3.0, the minimum approach distance (MAD) is “the

minimum distance in air to be maintained between any part of the body of a worker,

including any object (except tools appropriate for live working) being handled directly,

and any part(s) at different electric potential(s)” [2.8].

Therefore, to process the live-line work, a required withstand voltage and a minimum

approach distance (MAD) need to be calculated. Based on IEC 61472, in the calculation

of the minimum approach distance (MAD) between phase-phase or phase-ground, the

presence of the hanging basket or the linesman’s body on the transmission line can be

considered to reduce any possible risk of flashover that can cause severe or fatal injuries.

Figure 1-5 presents various live working tasks and MAD configurations in which can

occur. Each individual scenario in Figure 1-5 shows the minimum approach distances

involved with live line working, i.e. d1 in “A”, d1 and d2 in “B”, d1+d3 and d3 in “C” and

d1, d2, d3 and d4 in “D” are the minimum approach distances in each case. These tasks

can also be done with the presence of a hanging basket from a helicopter where the


minimum approach distance (MAD) is interrupted due to the presence of a floating

conductive object between the phases or phase to earth.

Figure 1-5: Typical Live-Line Working Task [IEC 624/13]

In Figure 1-5, the minimum approach distance (MAD) varies based on the linesmen’s

position on the tower. In towers A and B, the minimum approach distance has to be

bigger than d1. In Tower C, the minimum approach distance has to be bigger than

d1+d3 and d2+d3 whereas, in Tower D, the minimum approach distance has to be larger

than d1, d2, d3 or d4. 'd' is the minimum approach distance between the linesmen and

live or tower structure.

1.6. Objectives and Conclusion

The aims of the research described in this thesis were to investigate different available

methodologies that can be used to determine the safety of live-line workers, all of which

carry a range of assumptions for calculation of the minimum approach distance.

This thesis considered 400kV transmission lines and different type of towers used in the

UK’s HV transmission network to investigate the factors influencing the minimum

approach distances during the live-line working. Due to inconsistency in the calculated

minimum approach distances using different methods and also due to the lack of


understanding the risk involved with live-line working, the aims of these project are as

follow;

· Review of existing insulation co-ordination methodology and the method used by

National Grid,

· Review of the historical background behind the definition of existing safety

clearances and the guidance of standardisation bodies such as IEC and IEEE in

this area,

· To carry out a literature review to highlight the existing work or method,

· Investigate the parameters influencing the minimum approach distance and

magnitude of switching overvoltages, such as tower and overhead lines type, the

length of transmission lines, cable, fault level, the effect of fault type, time-to-

crest, etc.

· Propose a fundamental model of power network and investigate the effect of

different component of transmission line on the minimum approach distances,

· Propose a new set of minimum approach distances,

· Evaluate the risk involved with live-line working due to standard switching

transients.

· Investigate the risk involved with live line working due to non-standard

switching transients and propose a method for investigation of effect of wave

shape on the risk.

CHAPTER 2. Analysis of International Standards 37

CHAPTER 2

Analysis of International Standards

2.1. IEC 61472, Live-line Working SafetyStandards

In 1975, live working standards was developed to address standardisation needs of North

American and European countries in the field of live-line working [1.4]. TC78 standard

committee was concerned with tools and equipment used for live working. IEC (The

International Electro Technical Commission) is an organisation that sets, prepares and

publishes international standards for all electrical, electronic and related technologies.

After many years and drafts, IEC 61472 standard [2.8] was published, and method of

calculation of the minimum approach distance was set. Since then, the method

introduced by IEC 61472 has been used by transmission companies. IEC 61472 defines

the calculation method of the minimum approach distance for live working for a voltage

range between 1kV up to 800kV.

National Grid applies the IEC TC78 method as a fundamental approach for calculation

of safety clearances. Based on National Grid technical guidance note (TGN (T) 54)

[2.1], it is assumed that the deployed method used by National Grid may provide slightly

larger minimum approach distances (phase-to-earth) compared to the method used by

EDF Energy. Based on their assumption, the IEC results are 10% greater than those

produced using the ANSI/IEEE Standard 516, 1987 [2.1].


In some countries, like Canada and within some other power companies, different

minimum approach distances values are in use [2.2]. These approach distances are

based on various experiences, empirical and analytical methods. The phase- to - ground

minimum approach distances in live-line working, implemented in various countries are

shown in Table 2-1. Unfortunately the actual test conditions and the method used within

each individual country in Table 2-1 are not available. However, as shown further in

this thesis, the weather conditions can have a large impact on the minimum approach

distances.

Table 2-1: Minimum Approach Distances DA for Several Countries [2.2]

CountryVoltage of Design Us (kV)

220 362DA(m) DA(m)

US 1.42 2.59France 1.65 2.31Sweden 1.82 3.15

China (dry air) 2.20 3.00Finland (low humidity) 2.80 3.70

Voltage of Design Us (kV)245 420

DA(m) DA(m)Mexico 1.58 2.8

As an example, Table 2-2 shows some calculation results to illustrate the difference

between the IEC 61472 method and other standards accepted by the United States’

Department of Labour (OSHA). Two different switching transient magnitudes of 2.3pu

and 3.5pu are used for these calculations. The altitude used by OSHA method is

considered to be any altitude less than 900m, whereas 500m altitude is set for IEC 61742

method. The results in both Table 2-1 and Table 2-2 indicate a large difference in

results calculated by different standards and countries. Unfortunately, the test condition

and deployed methods are not available for further investigations.


Table 2-2: Comparisons of Minimum Approach Distance, IEC Correction Factors: GapFactor=1.2, Altitude Factor=0.94, Insulation Factor=0.95 and Floating Factor =0.85.

SystemVoltage

(kV)

MaximumTransient pu

IEC MinimumApproach Distance

(m)

Accepted OSHAMinimum Approach

Distance (m)Phase-

GroundPhase-Phase

Phase-Ground

Phase-Phase

Phase-Ground

Phase-Phase

400 2.3 3.5 2.33 3.75 2.07 6.35275 2.3 3.5 1.54 2.36 1.43 3.69

Floating object included400 2.3 3.5 2.89 4.81 2.23 7.28275 2.3 3.5 1.87 2.93 1.54 4.05

Table 2-2 shows that the minimum approach distances deployed by IEC 61472 and

OSHA [2.3] are different. As a result, there is a need for further investigation and

reviewing of the calculated minimum approach distances by standards. Apart from IEC

standard, IEEE Std 516-1995 [2.4] also provides a calculation method for the minimum

approach distance which is also based on experimental results of U50 (fifty percent

disruptive discharge voltage) on a particular length of the air gap. In HV and EHV

systems, the voltages that cause the highest risk of flashover are those associated with

lightning and switching operations. These voltages determine the external insulation

design under their large magnitudes. In the next sections, two different standards

available for calculation of the minimum approach distance have been examined in more

details.

2.2. IEEE 516-1995 StandardIn 1987, after the publication of several papers regarding the safety aspects of the live-

line maintenance, the IEEE Transmission and Distribution Committee published a full

used ANSI/IEEE standard for the purpose of the live-line working. In 1990, the

ESMOL Subcommittee (Engineering in Safety, Maintenance, and Operation of Lines)

revised the standards to update the guidelines to conformance with other international


standards. The flow charts below show the full methodology used to calculate the

minimum approach distance based on the IEEE 516-1995 method.

Figure 2-1: Flow Chart Illustrating the Calculation Procedure for the MinimumApproach Distance

In this method, calculation of withstand voltage for selection of air gap was done based

on 13 laboratories experiments over 30 years. According to the IEEE 516-1995 method,

the minimum approach distance (d) depends on two factors;

1. rms phase to ground voltage of the system

2. The maximum per-unit switching overvoltage factor (T)

The minimum approach distance (d) is calculated by use of Equation (2.1) which is

designed to fit the experimental curve obtained from withstand voltage of different size

air gaps [2.4].

d= (C1.C2+a).T. kVLG (m) (2.1)

Where:

d: Insulation distance, (m);

C1: 1% phase-ground system voltage (kV)

Step1

• Measuring system voltage based on simulation or experimental test

Step2

• Calculation of rms phase-ground voltage of the system, kVLG

Step3

• Calculation of saturation factor

Step4

• Calculation of MAD• d= (C1.C2+a).T. kVLG

Step5

• Calculation of truncation value

Step6

• Estimation of U2 voltage

Step7

• Multiplication of MAD by altitude factor


C2: 1% for presence of no tools in air gap

a: Saturation factor of the crest √2. . kV of voltage 630kV and above, this factor can

be approximated to within 2% by use of Equation (2.2);

kVLG: rms system Phase-ground voltage- actual.

a= (0.0075 . kVLG – 4.75) . 10-3 (2.2)

T: maximum per-unit switching overvoltage factor, i.e. truncation value of distribution

of overvoltage which no other overvoltage occurs after that point.

2.3. IEC 61472 Description of CalculationProcedure

IEC 61472 describes a method for calculation of the minimum approach distances at

maximum voltages between 72.5 kV and 800 kV for the purpose of live-line working.

The required withstand voltage and also the minimum approach distances described in

the IEC standard are evaluated taking into consideration the followings:

· Workers are trained for, and skilled in working in the live working zone;

· The anticipated overvoltages do not exceed the value selected for the determination

of the required minimum approach distance;

· Transient overvoltages are the determining overvoltages;

· Tool insulation has no continuous film of moisture or measurable contamination

present on the surface;

· No lightning is seen or heard within 10 km of the work site;

· Allowance is made for the effect of conducting components of tools;


· The effect of altitude, insulators in the gap, etc., on the electric strength, is taken

into consideration.

The flow chart of Figure 2-2 shows the full methodology used to calculate the minimum

approach distance based on IEC 61472.

Figure 2-2: Flow Chart Illustrating the Calculation Procedure for the MinimumApproach Distance

The statistical analysis assumes that switching overvoltages are distributed according to

a given probability law, i.e. a normal distribution. In the IEC 61472 method, the U2 (2%

Statistical Switching Overvoltage) voltage is used for calculation of the minimum

approach distance [2.5]. This value can be obtained by Monte Carlo procedure and

usually performed by use of a digital computer.

However, 2% statistical switching overvoltages in this project are obtained from

simulation results which are explained in Chapter 4. The required withstand voltage for

live-line working is taken to be equal to U90 which refers to a ninety percent probability

of withstand voltage [2.6].

Step 1• 2% Statistical Overvoltage obtained from Network or Simulation (Ue2 &

Up2)

Step 2• Calculation of U90: Multiplying the Ue2& Up2 by Statistical Correction

Factor

Step 3• Consideration of correction factors (Kt)

Step 4• Calculation of Electrical Clearance

Step 5• Calculation of the Minimum Approach Distance (MAD)


The Ue2 and UP2 voltages (U2 voltage for Phase-Earth and Phase-Phase respectively,

expressed in kV) are extracted from simulation results and multiplied by KS which is a

statistical safety factor.

The electrical stress at the work place during the live-line working is described as the

statistical overvoltage that may be presented at the work location. In a three-phase a.c.

power system, the statistical overvoltage Ue2 (phase to earth_ kV), Up2 (phase to phase_

kV), (phase to earth_ pu) and (phase to phase_ pu) are calculated by Equations

(2.3) – (2.5) extracted from IEC 61472. The Us is the system nominal voltages.

= × × (kV) (2.3)

= × × (kV) (2.4)

Ue90 = KS Ue2 & UP90 = KS Up2 (kV) (2.5)

As the minimum approach distance (MAD) consists of electrical and ergonomic

distance, when no ergonomic distance is used, the value of 1.1 is recommended for KS to

reduce the overall risk of breakdown of the insulation to a level that correlates with other

electrical work operations.

If the per unit phase to phase data are not available, an approximate value can be derived

from ue2 by the Equation (2.6).

= 1.35 + 0.45 (2.6)

In order to examine the validity of the modelling used in this work, Equation (2.6) was

applied to a set of simulations. The results from calculations show a small difference

less than 5% in all cases.

As a result, the U90 (ninety percent probability of withstand voltage) and the minimum

electrical distance (DU) can be calculated by Equations (2.7) and (2.8).


U90=KS x U2 (kV) (2.7)

D = 2.17 e ( )⁄ − 1 + F (m) (2.8)

As it will be explained later, the strength of the gap is influenced by different factors

which can be combined in a correction factor (Kt) to produce Equation (2.9). Equation

(2.9) presents the Kt which is combination of various factors influencing the strength of

the air gap.

Kt=kS.ka.ki.kg.kf (2.9)

Also, the factor F in Equation (2.8) is the sum of all lengths of any floating conductive

objects (in meters) in the direction of the air gap axis. Moreover, the minimum approach

distance (DA) is calculated by adding the electrical distance ‘DU’ and the ergonomic

distance ‘DE’. These distances are further defined in IEC 61472 as:

DU “Distance in air required to prevent a disruptive discharge between energised parts or

between energised parts and earthed parts during live-line working.”

DE “Distance in air to take into account inadvertent movement and errors in judgement

of distances while performing work.”

Therefore, the minimum approach distance is given by:

DA = DU+DE (m) (2.10)

2.3.1. Correction Factors

ØStandard Statistical Deviation Factor kS

‘kS’ factor is the statistical nature of the breakdown voltage, and its value is calculated

by IEC 61472 based on the relationship between the statistical withstands voltage, U90

and the 50% disruptive discharge voltage, U50 as below;

U90 = U50-0.0128. se .U50 (kV) (2.11)


Where 'Se' is the normalised value of the standard deviation of U50 expressed in percent.

Therefore, the kS can be defined by Equation (2.12).

kS=1- 0.0128se (2.12)

Unless the value of Se is known from the tests representative of the gap configuration

and distance concerned, a value of Se = 5 % should be assumed. Equation (2.13) then

becomes:

kS = 0.936 (2.13)

ØGap Factor kg

The gap factor (kg) accounts for the varying electric field distribution in the gaps of

varying shapes. The gap factor (kg) depends on the gap configuration. The gap factor is

used to adjust the strength of a gap of a specific geometry to the reference rod-plane

case. The fundamental gap factor equal to one is calculated for the rod-plan gap,

whereas, typical gap factor values for standard configurations of gap factor and other

parameters are shown in Table 2-3 which is reproduced from CIGRÉ 72 and also

presented by IEC 60071-2. The gap factor kg in Table 2-3 is presented as "k" and it is

permitting the calculation for different gap configurations.

Table 2-3: Gap factors for some actual phase to earth configurations [2.6]. The gapfactor (kg) in Table 2-3 is presented by "k".

Configuration Formula Typicalvalue

k= 1.45


k=1.25

k= 1.15 forconductor–plane to

1.5 ormore

k=1.45

k1=1.3

k2= 1+0.6H

Equation (2.14) extracted from Table 2-3, and it is used to calculate kg for National

Grid’s towers.

18

2

1 1

1.45 0.015 6 0.35 0.2 0.135 1.5S

dg

dHk ed d

-æ ö æ öæ ö= + - + - + -ç ÷ ç ÷ç ÷è øè ø è ø

(2.14)

Where;

H = Height of overhead line conductor from the ground (m)

d1 = Distance from the conductor up to the point where it is connected to the cross-arm

(m)

d2 = Horizontal distance between the conductor and the tower structure (m)

S = Thickness of the tower along the distance d2 (m)


It should be noted that this equation is valid only for the conditions where:

· 2m ≤ d1 ≤ 10m

· 1 ≤ d2/ d1 ≤ 2

· 0.1 ≤ S/d1 ≤ 1

· 2 ≤ H/d1 ≤ 1

This project uses the National Grid towers and transmissions lines specifications for the

purpose of overvoltage studies. For different type of towers used by National Grid, the

lowest values of kg were chosen to be used in the calculations of the minimum approach

distances as these values will give the most conservative results. Therefore, to comply

with National Grid calculations, the kg values used in this thesis are taken to be equal to

1.2 and 1.45 for the phase to earth and phase to phase respectively as stated in TGN (T)

54 Technical Guidance Note [2.1].

ØAtmospheric Factor ka

In the calculation of 50% voltage breakdownof the gap, the atmospheric factor takes into

account the effect of air density influenced by temperature, humidity and altitude. The

effect of temperature and humidity is negligible in comparison with altitude. For an

instant, U50 decreases at a location higher than the reference altitude [2.23] and, as a

result the required distance increases and this can be determined by multiplying the

electrical distance by an altitude correction factor. The atmospheric factor can be

calculated as below:

ka=K1K2 (2.15)


ØAir Density Correction Factor, K1

The air density correction factor ‘k1’ depends on the relative air density ‘δ’ where

temperatures ‘t’ and ‘t0’ are expressed in degrees Celsius, and the atmospheric pressure

‘P’ and ‘P0’ are expressed in the same units.

K1= δm (2.16)

δ =p

p ×273 + t273 + t (2.17)

Ø Calculation of Exponents m and w:

To calculate the exponents ‘m’ and ‘w’, the following formula is being used and values

of ‘m’ and ‘w’ are extracted from Table 2-4 as below:

g =U

500Lδk(2.18)

Where L is the minimum discharge path in meter, δ is the relative air density, and k is

the dimension less parameter defined by Equation (2.19).

Table 2-4: Values of Exponents, ‘m’ of Air Density Correction and ‘w’ For HumidityCorrection as the Function of Parameter ‘g’ (IEC 60060) - [2.7]

ǥ m w

<0.2 0 00.2 to 1.0 ǥ (ǥ – 0.2) /0.8 ǥ (ǥ – 0.2) /0.81.0 to 1.2 1.0 1.01.2 to 2.0 1.0 (2.2 – ǥ ) (2.0 – ǥ)/ 0.8

≥ 2.0 1.0 0

ØHumidity Correction Factor, K2:

The effect of humidity on the voltage breakdownis more complex compared with

previous cases. It is usually accounted for using a factor defined as ‘k’ in IEC 60060-1,

the value of which is empirically related to humidity, and an exponent ‘w’, which


depends on the gap length and its configuration and the wave shape. Thus, the humidity

correction factor ‘K2’ is:

K2=kw (2.19)

The value of ‘w’ factor, as well as the exponent ‘m’ for the relative air density can be

determined by the methods given in IEC 60060-1 and explained in more details in next

section.

In Equation (2.19), ‘k’ is a parameter that depends on the type of test voltage and it may

be obtained as a function of the ratio of absolute humidity (h) to the relative air density

(δ) and can be calculated using Equation (2.20) [2.7].

(2.20)

The appropriate value (average) of ka can be selected from the Table 2-5 for the average

value of ka or can be calculated for specific altitudes according to the calculation method

explained.

Table 2-5: Average ka Value [2.8]

Altitude(m)

ka

average

0 1.000100 0.995300 0.983500 0.972

1000 0.9411500 0.9092000 0.8752500 0.8413000 0.805

Table 2-7 presents the effect of humidity on a set of P-E and P-P overvoltages

respectively which are shown in Table 2-6. The calculation of the minimum approach


distances is done using the IEC 61472 method – refer to section 2.3. During this

examination, the pressure and temperature remain at 20oC and 101.3kPA respectively.

Increasing the absolute humidity decreases the minimum approach distances due to the

increase of the voltage breakdown of the gap. The examination results shown in Table

2-7 comply with IEC 60060-1 standard section 4.3.2 [2.7].

Table 2-6: Set of P-E and P-P Overvoltages

Overvoltage Samples (kV)P-E P-E

Min Overvoltage (kV) 345.7 598.8Max Overvoltage (kV) 720.2 959.2Mean Overvoltage (kV) 392.6 615.3

U2 Overvoltage (kV) 473.8 728.7

Table 2-7: Effect of Humidity of the Minimum Approach Distances at a temperature of20oC and a pressure of 101.3kPA

Minimum Approach Distance (m)P-E P-P

Relative Humidity 5% 1.42 2.53Relative Humidity 10% 1.42 2.53Relative Humidity 15% 1.42 2.53Relative Humidity 20% 1.42 2.53Relative Humidity 40% 1.53 2.76Relative Humidity 60% 1.44 2.57Relative Humidity 80% 1.35 2.40Relative Humidity 90% 1.31 2.32

Relative Humidity 100% 1.28 2.25

ØFloating Object Factor kf

The ‘kf’ takes into account the presence of floating objects within the gap. In the

absence of a floating object in the air gap, the value of ‘kf’ will be equal to 1; otherwise,

‘kf’ must be calculated. Based on IEC 61472, for long or flat shaped conductive objects

situated perpendicular to the air gap or where no specific experimental data is available,

a conservative value of kf equal to 0.75 may be assumed. The value of ‘kf’ can be

selected from Tables provided in Appendix 1. However, in this project, the value of ‘kf’


was taken to be equal to 0.85 to match the value used by National Grid’s assumption in

[2.1].

ØDamaged Insulator Factor ki

A damage insulation can have a significant impact on the withstand voltage of an air gap

at a live-line work location. As a result, the number and location of the damaged units

and also the degree of their damage can have a significant effect on the strength of the

gap and as a result, on the minimum approach distance during the live-line working.

The strength of an air gap can be reduced significantly in the case of glass insulators as

the glass insulators are made of pre-stressed toughened glass, and they always shatter

completely in the event of any incident. Regarding composite insulators, the strength

reduction is significantly larger with conductive or semi-conductive defects. The

strength of composite insulators becomes null when a conductive damage involves the

whole insulation length.

‘ki’ is the insulation string factor concerning the insulators’ damage and allowing for the

system or tool in the gap. ‘kis’ for cap or pin insulators can be calculated based on IEC

61472 using Equation (2.21).

kis=1-0.8 kd (Ad/Ao) (2.201)Where;

Ad is the number of damaged insulator units in the string;

Ao is the number of insulator units in the string;

kd is assumed 1.0 for glass and 0.75 for porcelain;

kis is the damaged insulation string factor.

‘ki’ also can be calculated for composite insulators. The insulation factor for composite

insulator will be used for consideration of damaged insulators and allowing for the

system or tool insulation in the gap. ‘kic’ for composite insulators can be calculated

based on IEC 61472 using Equation (2.22).


kic=1- (ld/lo) (2.22)Where;

ld is the damaged length along insulator axial direction;

lo is the insulating length of the insulator;

kic is the damaged composite insulator factor.

There have been many suggestions to detect the faulty composite insulators on HV

power lines i.e. the electric field measurement [2.9], [2.10] which can be beneficial for

calculation of the gap strength for the purpose of live-line working and method used for

the insulator replacement task.

In this thesis, it has been assumed that ki is equal to 0.95, which is a recommended value

in IEC 61472.

In summary, throughout this thesis correction factors with values of 1.2, 1.45, 0.95 and

0.85 have been used for P-E gap factor, P-P gap factor, insulation factor and floating

factor respectively.

2.4. Impact of Correction Factors on MAD

Unlike the IEEE 516-1995 method, the method introduced by IEC 61472 is very simple

to apply in different circumstances by changing some correction factors. Below, the

effects of atmospheric conditions on the minimum approach distances are presented for

both at tower and at mid-span of an overhead line.

Two overvoltages with a magnitude equal to 1.46 and 3.64 were assumed for this study

where the altitude ranging from the reference altitude (sea level) to 1000m above the sea

level was considered. The gap factor (kg) was set to 1.346 for the tower and 1.36 for

mid-span.


2.4.1. At Tower

The results from calculations based on Equation (2.8) show that increasing altitude

increases the minimum approach distances. The calculation results show 10%

differences in minimum electrical distances between sea level and 1000m for the

overvoltage levels about 500kV (1.46pu), whereas, by increasing the magnitude of

overvoltage, this difference reduced to 6% for the voltage levels around 1248 (3.64pu).

Therefore, increasing the altitude has a smaller effect on a system with higher voltage.

This statement also can be used for a gap with a floating object. The results of

calculations can be found in Figure 2-3.

Figure 2-3: Electrical Distance for 0-1000m altitude at L6 tower, With and WithoutFloating Object

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

0 200 400 600 800 1000

Min

imum

Elec

tric

alDi

stan

ce(m

)

Altitude (m)

Without Floating Object

1.46 pu

3.64 pu

1.5

2

2.5

3

3.5

4

4.5

0 200 400 600 800 1000

Min

imum

Elec

tric

alDi

stan

ce(m

)

Altitude (m)

With a Floating Object

1.46 pu

3.64 pu


2.4.2. At Mid-Span

The results from calculations based on Equation (2.8) show that increasing altitude

increases the minimum approach distances. The calculation results show 9% differences

in minimum electrical distances between sea level and 1000m for the overvoltage levels

about 500kV (1.46pu), whereas, by increasing the magnitude of overvoltage, this

difference reduced to 6% for the voltage levels around 1248 (3.64pu). Therefore,

increasing the altitude has a smaller effect on a system with higher voltage. This

statement also can be used for a gap with a floating object. The results of calculations

can be found in Figure 2-4.

Figure 2-4: Electrical Distance for 0-1000m Altitude at Mid-span L6 Tower, With andWithout Floating Object

11.21.4

1.61.8

22.2

2.42.62.8

3

0 200 400 600 800 1000

Min

imum

Elec

tric

alDi

stan

ce(m

)

Altitude (m)

Without Floating Object

1.46 pu

3.64 pu

1.5

2

2.5

3

3.5

4

4.5

0 200 400 600 800 1000

Min

imum

Elec

tric

alDi

stan

ce(m

)

Altitude (m)

With a Floating Object

1.46 pu

3.64 pu


The dielectric strength of a gap is also proportional with the air density in gaps less than

2m. However, in a gap with larger distance, the air breakdown is less proportional with

air density [2.6].

This means that air density has a small effect on the strength of a gap, and as a result,

there is a negligible effect on the minimum approach distance at a live-line working

location where the air gap is limited to the gap sizes larger than 2m. However, pressure

is the main influencing factor of atmospheric condition on the flashover voltage of a gap

during the live-line working [2.2]. Decreasing the pressure due to increasing altitude

reduces the voltage breakdown of a gap, and as a result, a smaller magnitude of

switching overvoltage is required to cause a flashover within the gap. Therefore, the

minimum approach distance will increase as a result of increasing the altitude or

decreasing the pressure.

As shown in Figure 2-4, the magnitude of switching overvoltages has a greater influence

than atmospheric factors on the minimum approach distance of the towers with a line

spacing larger than 2m.

In the calculation of minimum approach distance, it is important to consider that most of

the UK’s lands with low plains and downs with the major hill regions situated in the

north (mostly Scotland and Wales), and some places in the west and south-east of the

country. The elevations of these lands do not rise above 305 metres (1,000 feet) at any

point.


2.5. Discussion of Standards

Table 2-6 presents some example calculations of the electrical distances based on both

IEC and IEEE methods. These values are obtained based on the assumption of existence

of no broken insulators and no floating object when Ks=1.1, ks=0.936, kg=1.2, ki=1.0,

kf=1 and F=0. The Kt value calculated based on Equation (2.8), and it is equal to 1.12.

In Table 2-6, the altitude assumed to be 900m to adjust the results from IEC method

with equivalent IEEE method.

Table 2-8: Comparison of the Calculation Results for the Minimum Clearances Based onIEEE 516-1995 and IEC 61472 Method [2.21]

US (kV) 121 242 362 362 550 550 800 800kVLG 70 10 209 09 318 318 462 462

T 3.0 3.0 2.0 3.0 1.5 2.4 1.5 2.0a - - 0 0.0018 0.0003 0.0033 0.0025 0.0050u2 2.6 2.6 1.8 2.6 1.4 2.12 1.4 1.8

U2 (kV) 257 514 532 768 629 952 914 1176U90 (kV) 283 565 585 845 692 1047 1006 1293

IEC DU(m) 0.57 1.29 1.35 2.19 1.67 2.98 2.80 4.13IEEE D(m) 0.64 1.28 1.28 2.27 1.51 3.11 2.64 4.22

The correction factor for altitude in IEEE method does not take into account the altitude

below 900m whereas, in the IEC method, the effects of different parameters such as

altitude, weather conditions (temperature, humidity and pressure) and also the effects of

a broken insulator and floating objects have been considered. Based on Table 2-6, the

IEEE provides a method that recommends a smaller electrical distance in comparison

with IEC method, but both approaches agree on the higher values of U2 overvoltage.

In the IEEE method, the factor ‘T’ is interpreted as the maximum anticipated

overvoltage (truncation value of the overvoltage which no other overvoltage occurs after

that point) is different from U2 values used by IEC method.

Neither of the two methods contains the exact nature of the tested gaps and test

conditions, but considering the safety matter, using the IEC method provides a larger


distance as the IEC standard takes into account different correction factors. However,

the IEEE method could be adequate. In the calculation method deployed by IEEE,

details of the exact nature of the tested gaps and test conditions have been lost, and more

work needs to be done on altitude correction factors. Also, the effect of floating objects

within the phases has been ignored, whereas live-line working can be carried out by use

of a hot-stick or a basket hanging from a helicopter.

Both methods have considered the system maximum operational voltage whereas in

reality, this might not be possible. The electrical system normally operates at a voltage

that the system components are designed. This voltage usually is 5 to 10 percent below

the maximum system voltage [2.22]. Neither of the two methods, consider the live-line

working duration and the probability of occurrence of the maximum overvoltage at live-

line working location as the location of the live-line working site might not coincide

with the maximum overvoltage due to the switching. At the same time, half of

switching overvoltages are not severe as they might have negative polarity.

Previous experimental results proved that the sparkover strength of an air gap and, as a

result, the minimum safety clearances are varied according to the wavefront (time to

crest) of transient switching overvoltage [2.11]-[2.13] and transient wave shape [2.14] -

[2.19]. As shown later in Chapters 4 and 5, different line length/ source inductance

influence the time to crest of the transient wave and, hence the probability of flashover

can be affected. Therefore, there is a missing link between the calculated minimum

approach distance using the IEC and IEEE methods and the wave shape.

Although the existing minimum approach distances set by the IEC standard are more

conservative than the IEEE method, due to the importance of human safety factor, there

is a need for further investigation on the competency of these clearances according to the

network specifications.


Unlike IEEE, in the calculation of the minimum approach distance developed by IEC,

the effects of altitude, floating objects, weather conditions and broken insulators are

taken into account. Therefore, as the IEEE method does not directly account for some

details, and at the same time the IEC method is more general and flexible and provides

larger and more conservative safety distances, IEC method is more applicable in a

calculation of the minimum safety approach. Therefore, this research used the IEC

method as it is also confirmed by the British Standards Institution (BSI) for calculation

of the minimum approach distances. This project intended to use the following standards

in its calculations;

· IEC 61472:2004, Live working — Minimum approach distances for A.C systems

in the voltage range 72.5 kV to 800 kV — method of calculation.

· IEC Standard 60060-1:2010, High-voltage test techniques, Part 1: General

definitions and test requirements.

· IEC Standard 60071-1:2006, Insulation co-ordination — Part 1: Definitions,

principles and rules.

· IEC/TC78 “Live Working”: Background, Structure, Program of Work, and

Market, Relevance.

· PD IEC/TR 60071-4:2004, Insulation coordination. Computational guide to

insulation coordination and modelling of electrical networks.

CHAPTER 3. Transient and Air Breakdown in Power System 59

CHAPTER 3

Transients and Air Breakdown in PowerSystems

3.1. IntroductionWhen the voltage in whole or part of the system exceeds the nominal or design voltage

limit, this phenomenon called overvoltage. In HV and EHV systems, the voltages that

cause the most risk of flashover within air gaps are those associated with lightning and

switching operations.

Circuit breaker opening/closing due to the fault and clearances, maintenance or network

requirement, changing in load demand and power generation, etc., can affect the power

system steady state, which needs to be settled down and reverted to the initial steady

state situation. Thus, exchanging electromagnetic and electromechanical energy

between the system components takes some time to push the power system back to the

initial steady state which causes a short burst of energy in a very short time which is

defined as transient [2.22].

On the other hand, the most important transient overvoltages are switching surges [3.1].

Whether, these overvoltages caused by energisation, disconnection, re-closing of the

circuit breakers or by nature, i.e. lightning, fault due to unpredicted accident; the design,

structure and performance of the network will be set according to the system’s nominal

voltage, magnitude of fault level and overvoltages.


In Chapter 2, the strength of the gap and the methods used for calculation of the

minimum approach distance have been investigated. However, as calculation of the

minimum safety distance is based on both stress (switching overvoltage) and strength of

the gap, in this Chapter, different sources of overvoltages (stress) have been

investigated.

These overvoltages have been studied further by use of PSCAD (Power System

Computer Aided Design) simulation tool to illustrate the switching transient’s behaviour

along the transmission line. In the first part of this Chapter, different types of switching

transients have been studied. In the second part of this Chapter, factors influencing the

magnitude of switching transients have been reviewed. These factors are directly

accounted for modification of the minimum safety distance for live-line working.

Throughout this project, PSCAD [3.28] is used as an Electromagnetic Transient

Simulation Program (EMTDC). Before its release in 1992, PSCAD has been

extensively tested in North America, Japan, Australia and Europe. PSCAD is a

graphical user interface program which represents and solves differential equations in

the time domain. Users are enabled to run a simulation, analyse the results and manage

the data in a graphical environment.

3.2. Travelling Waves

Overhead lines and cables are presented by a pi-section to demonstrate a switching

transient's characteristics- refer to Figure 3-1. In the pi-section models, electric and

magnetic field properties are shown by the capacitance (C) and inductance (L). In

Figure 3-1, by closing the switch, the current flows through the first inductor (L1) and

charges the first capacitor C1. A gradual gathering of charge on the first capacitor (C1)

creates a voltage that causes a current to flow through the second inductor (L2). Once


again, this current charges the second capacitor (C2) and accumulation of the charges on

the second capacitor causes a current flow to the third inductor (L3) and so on.

Figure 3-1: Pi-section Presentation of Overhead Line and Cable

This travelling wave propagates along the overhead lines and cables near to the speed of

light due to disturbance of the steady state in a power system. They reflect back when

reaching the open end of the line or where the impedance of the system is changing due

to different component’s connection. They could cause very high overvoltages which

can cause insulation failure in the power system components. Also, they can cause a

flashover between air insulated conductors. The high-speed travelling waves are known

as Transverse Waves that are oscillating perpendicular to the direction of propagation.

Although, these waves are explained by Maxwell’s equations, however in a power

system, analysing the overvoltage caused by travelling wave is done by travelling wave

equations.

3.2.1. Wave Velocity on Overhead Lines

Electric and magnetic Transverse waves that exist on the transmission lines appear on

two or more separate conductors [3.2]. Therefore, by dividing the line into smaller

sections as shown in Figure 3-2, the wave equation can be presented by the use of Table

3-1 [3.3].

Figure 3-2: Small Section of Transmission Line


Table 3-1: Surge Impedance and Propagation Constant for Normal and Lossless Line[3.3]

Propagation Constant γ(ω) = Y(ω). Z(ω)Shunt Admittance Y(ω) = G + jωC

Series Impedance of the line Z(ω) = R + jωLSurge Impedance Zc = Z(ω)/Y(ω)

Propagation Constant Lossless line (R=G=0) jω√LCSurge Impedance Lossless line (R=G=0) L/C

Calculation of the line capacitance (C) and the line inductance (L) per unit length (m) of

the overhead line are shown in (3.1) and (3.2) for a single phase line where the

inductance and capacitance depend on conductor radius (r) and conductors spacing (Dab).

L = 4 ∗ 10 ∗ ln(D

r )(H/m) (3.1)

C =πε

ln Dr(F/m) (3.2)

To illustrate transmission line characteristics, Figure 3-3 presents a single-phase

transmission line where the line fed from an ideal generator via a circuit breaker (BRK1)

at the beginning of the line. The transmission line is 100km of L6 tower used within

National Grid network. The line and tower specifications are shown in Tables 3-2 and

3-3. The line contains a bundle of two Zebra overhead lines with 0.5m spacing. To

simplify the model and alsoto better understanding the wave propigation, in this model

the circuit breaker closes at the start point of the simulation.

Figure 3-3: Simple PSCAD Power System Model


The circuit breaker at the beginning of the ‘L6_1’ line is set to be closed at the peak

voltage while the BRK2 is always open. The model contains an ideal generator to

illustrate the line inductance and capacitances. All tower configurations used in this

thesis are presented in Appendix 2.

Table 3-2: Generator Parameters

Source Voltage (Line Voltage) 400 kV

Frequency 50 HzPhase Angle 0Inductance (Series) 0 [H]Resistance (Series) 0 [ohm]Resistance (Parallel) 0 [ohm]

Table 3-3: Overhead Line and Circuit Breakers’ Parameters

For All Overhead line Cable Short Line Case

Tower Type L6 Single CircuitSteady State Frequency [Hz] 50Number of Conductor 1Total Overhead Line Length [km] 100Shunt Conductance [mho/m] 1.0e-011Conductor Radius [m] 0.01431DC Resistance [Ω /m] 0.03206 e-3Height from ground [m] 30Ground resistivity [Ω *m] 100Number of Sub-conductor 2Sub-conductor space: Dab [m] 0.5Breakers Open Resistance [Ω] 1.0e6Breakers Closed Resistance [Ω] 0.1

Based on Equations (3.1) and (3.2), the capacitance and inductance of each phase will be

calculated as follows;

= 4 ∗ 10 ∗ ( ) = 1.42143μ

= 1.4214 /

=π ∗ 8.85 ∗ 10

ln( )= 7.8238

pFm = 7.8238nF/km


Therefore, the surge impedance (Z0) and the time required for the wave to travel from

the beginning to the end of the line were calculated using Equation (3.3) where ‘R’ and

‘G’ losses have been ignored in the analysis of the surge phenomena.

=++ [Ω] (3.3)

Therefore, the surge impedance becomes as follow;

= [Ω] = 1.4214 ∗ 107.8238 ∗ 10 = 426.23Ω

As shown above, the surge impedance was found to be equal to 426.23Ω which is in the

range of typical overhead line surge impedance of 200 to 500Ω [3.4]. By using the line

inductance and capacitance calculated previously, the wave velocity is calculated as

below;

(Travellingwavevelocity) =1

√=

1√7.8238 ∗ 10 ∗ 1.4214 ∗ 10

= 299869968.8

This calculation gives a wave velocity equal to 299.86 x 106 m/s, which is very close to

the speed of light (299.79 x 106 m/s). Therefore, the time required for the wave to travel

along 100km of the mentioned line is about 333µs which is very close to PSCAD

simulation plot in Figure 3-4 with a value equal to 310µs. It needs to be highlighted that

either both the calculation and the simulation may consist of many sources of error, such

as plotting error, rounding up the values and also curve examination error due to human

vision error. Therefore, these errors cause the calculation value to be slightly higher

than the speed of light or the reading to be differing by 23µs.


Figure 3-4: Surge travelling time: Top: E_sending; The Voltage at the Sending andBottom: E_receiving; The Voltage at the Receiving End of the Line

3.2.2. Wave Velocity on Cables

In order to illustrate the wave velocity on cables, the cable used at this part of the thesis

contains a single circuit three-phase 400kV cable used by National Grid, which is buried

in a trench in the ground [3.5]. This cable is equivalent to 400kV, 1200mm2 XLPE,

ABB cable specification [3.6] presented by Table 3-4. The picture in Figure 3-5 shows a

typical position of cables in the ground used by National Grid.

Figure 3-5: National Grid direct buried cable diagram


The cables are spaced horizontally with 400mm between each phase and buried

approximately 0.9m-1.1m deep, depending on the location. The specification of the

cable core used in this model is shown in Table 3-4. All binder, semi-conducting screen,

insulation and conductor screen are merged into the insulator layer in PSCAD model.

Table 3-4: Sample Cable Data for 400kV Single Core Cable, 1200mm2 ABB XLPECable [3.6]

Cross

section of

conductor

Diameter

of

conductor

Insulation

thickness

Diameter

Over

insulation

Cross

section

of

screen

Outer

diameter

of cable

CapacitanceCharging

current

Inductance

Surge

Impedance

mm2 mm mm mm mm2 mm µf/km A/km mH/km mH/km Ω1200 42.8 27 101.8 185 120.4 0.18 13.3 0.40 0.53 31.9

In order to measure the propagation speed and the surge impedance of the travelling

wave, the model in Figure 3-6 is used. The impulse generator injects 10kA, 1.2/50µs

current into 1 km of the cable section at the sending point and the time for the wave to

travel along the cable and the surge impedance are calculated by monitoring the open

end of the cable before any reflection.

Figure 3-6: Impulse Generator Used in PSCAD

In Figure 3-7, the PSCAD voltmeter at the sending end (Ea) was recording 318kV that is

indicating a surge impedance of 31.80Ω which is very close to real data on Table 3-4.

The propagation speed was calculated as v=102382km/s.


Figure 3-7: Voltage at Sending Point (Blue Curve) Due to Current Impulse where, Eaand Eb are the sending and receiving voltages respectively

3.2.3. Wave Reflection and Line Characteristics Impedance

When a travelling wave reaches either an end of a line with a higher or a lower

impedance of the current path, some portion or even the whole wave reflects back

toward the original propagation source. The polarity and magnitude of reflected waves

depend on the transmission line’s impedance and reflection coefficient of the

transmission line’s discontinuity [3.7].

This can be explained with the help of Figure 3-8 and PSCAD model of transmission

line shown in Figure 3-3. After the circuit breaker closure (BRK1) at the beginning of

the line (point A on Figure 3-8), the voltage wave (Blue Curve) travels along the line

and reaches the open end of the line (point B on Figure 3-8) at a time equal to . After

the reflection, it takes “2 ” from the start time, for the travelling wave to reach the

beginning of the line and causes a peak voltage (point C on Figure 3-8). The magnitude

of this peak depends on the line inductance and capacitance. The period of the travelling

surge will be equal to “4 ” and, in this case, it is equal to 1.048ms. Therefore, the

following formulae confirm the results from the PSCAD.


= (Hz) (3.4)( ) = (linelength(km))/(Wavevelocity(km/s)) (3.5)

Figure 3-8: PSCAD Simulation Travelling Wave; Top: Voltage at Beginning, Bottom:Voltage at the End of Transmission Line

This phenomenon can also be explained by the travelling wave theory by simplifying the

PSCAD model to a schematic diagram shown in Figure 3-9. By considering the source

impedance (ZS) and transmission line surge impedance, (ZL), the following equations can

be produced to demonstrate the impacts of the source on reflected travelling waves.

Figure 3-9: Behaviour of Voltage Travelling Wave at Transition Point

In Figure 3-9, point ‘A’ is a transition point where there is a change of circuit constant

due to a junction between the transmission line and the generator. The impinging

travelling wave/ incident wave (e) from the generator faces the reflected wave (er) from

the transition point of the line (Point A). At this instance, the rising wave due to the


conflict of the incident wave and reflected wave at the transition point would be

reflected back into the transmission line which is known as transmitted wave (et).

Incident Wave: Z = [Ω] (3.6)Reflected Wave: Z = [Ω] (3.7)

Transmitted wave: e + er = et .... Z = (3.8)

Therefore, Equations (3.9) and (3.10) can be derived from Equations (3.6)-(3.8) as

follows;

e =Z − ZZ + Z ∗ e (3.9)

e =2Z

Z + Z ∗ e (3.9)

Where (er) and (Ir) are reflected voltage and current waves respectively, and (et) and (It)

are transmitted voltage and current waves respectively. Therefore, by considering

Equations (3.9) and (3.10), based on the simulation results shown in Figure 3-8, after

262µs, the travelling wave reaches the open end of the line (point B), it takes almost

524µs (point C) for the wave to reach the transition junction again (between the

transmission line and generator). By considering the direction of the surge from the end

of the line toward the transition junction, Equations (3.11) and (3.12) can be derived

from Equations (3.9) and (3.10) to calculate the magnitude of reflected wave toward the

open end of overhead line.

= ∗ (V) (3.10)

= ∗ (V) (3.11)

Therefore, the characteristic impedance of the transmission line and also load impedance

can influence the polarity and magnitude of reflected waves. When a travelling wave is

not facing a higher or lower impedance, there would be no reflection, however, when the

wave meets the line-cable junction with higher impedance or open end of a transmission


line, the reflected wave can be double in magnitude and reflects back toward to the

source [3.7].

3.3. Transient Classification

Transients are defined and classified based on their origin into atmospheric or switching,

or based on their transient generation mode into electromagnetic or electromechanical

transients. In general, transients can be classified based on frequency and rate of voltage

rise. According to CIGRE Classification of Overvoltage Based on Frequency [2.6]and

IEC classification [3.8], transients are categorised into five groups based on their

frequency ranges (Table 3-5), whereas, according to IEC 60071, the magnitude and

duration of overvoltages classify the transient overvoltages as shown in Table 3-6.

Table 3-5 CIGRE Classification of Overvoltage Based on Frequency [2.6]

Classification Abbrev The Origin FrequencyRange Magnitude

TemporaryOvervoltages TOV Earth fault & Load

Rejection Seconds Up to1.5pu

Low-frequencyOscillation ---- Load rejection & Fault

clearing 0.1Hz-3kHz Up to4puSlow-front surges SFO Line switching 50 Hz-20 kHz

Fast-front surges FFO Reignition & prestrike/lightning 10kHZ-3MHz Up to

7puVery-fast-frontsurges VFFO Disconnection

switching in GIS100kHz-50MHz

Table 3-6 IEC Classification of Overvoltage Based on Time Duration [3.8]

Nature of the Transient Phenomena Time Duration

Lightning 0.1 μs –1.0 ms

Switching 10 μs to less than asecond

Sub-synchronous resonance 0.1 ms–5 sTransient stability 1 ms–10 s

Dynamic stability, long-term dynamics 0.5–1000 sTie line regulation 10–1000 s

Daily load management, operatoractions Up to 24 h


Therefore, transients can be classified based on their causes or nature. In other words,

transients in the power system can be due to external sources such as lightning or

internal sources as a result of switching or temporary overvoltages.

3.4. Lightning Overvoltage

Fast Front Overvoltages (FFO) are mostly caused by a lightning strike with a magnitude

up to 7pu of nominal system voltage [2.6]. The first step of a lightning discharge is the

formation of leader stroke due to the potential difference between the positively charged

ionosphere and the negatively charged earth. At the earthing point, a large impulse

current equal to tens of kilo amperes occurs which causes damage to the power system,

and as a result, it causes a large magnitude of a transient wave at the point of strike. The

transient waves caused by the lightning strike move along the transmission line and

tower body and can cause an overvoltage up to 7pu [CIGRE Classification of

Overvoltage Based on Frequency] in some parts of the network. The stroke damage

with a speed very close to half the speed of light and a temperature up to 20,000o C, time

to crest of few seconds and decay time of 10-100 microseconds is devastating.

However, as live-line working takes place in good weather condition, the lightning

overvoltage is not considered in this project.

3.5. Review of Main Sources of SwitchingOvervoltages

In theory, the switching action on electrical circuit occurs by a single break action

(opening) and a single making action (closing), and as a result, the magnitude of thr

switching overvoltage can exceed even more than twice the system voltage. However,

in reality, due to the interaction of the system and switching factors, the switching


operation differs from the ideal. These factors can be one or combination of many

factors such as; line and source impedance, transformer excitation characteristics, the

existence of system compensation, Ferranti effect, circuit breaker characteristics, etc.

The switching surge is a voltage transient/spike with a high amplitude and a different

waveform from the system nominal voltage at any point or part of the system. The

switching transient can take any shape depending on system configuration and transient

source. The switching surge wavefront duration and its rate of rise determine the

magnitude of the switching transient (stress), and, as a result, it has a significant impact

on the voltage breakdown of a gap. Therefore, as switching transients (stress) are the

most common sources of overvoltages, this project intends to investigate the switching

transients to illustrate their influences on the minimum approach distances. The

switching transient can be initiated by various events such as switching on/off the

transmission or distribution network or a circuit with inductance and capacitance. The

sources of switching transients are classified as below;

· Line energisation,

· Line re-energisation,

· Line disconnection,

· Fault initiation and fault clearance,

· Switching off small capacitive or inductive currents.

3.5.1. Line Energisation, re-energisation and Disconnection:

The predominant switching overvoltage in the majority of HV and EHV systems are

those caused by energisation or re-energisation of the unloaded line. Due to closing

between the poles of circuit breakers and also electrical coupling between the phases, the

maximum switching overvoltage on transmission lines can be severe.


Furthermore, the magnitude of this overvoltage can be higher and even more severe if

the circuit breaker re-striking or circuit breaker re-closure happened on a transmission

line with a trapped charge.

By closing the circuit breaker at the sending point of an open-end transmission line,

voltage and current travelling waves rush into the transmission lines. The electrical

circuit configuration influences the magnitude and waveform of switching surge

(travelling wave) on both sides of the circuit breaker.

In practice, before a circuit breaker mechanical closure, the electrical contact can be

made due to the formation of circuit breaker prestrike. The time for prestrike

occurrence (arc flash) depends on the voltage at the terminal of the circuit breaker and

the withstand voltage across the breaker's terminals.

In this part of Chapter 3, the source of switching transients such as energisation, re-

energisation, disconnection and fault and clearance will be analysed by use of travelling

wave theory and PSCAD simulation tool.

I. Energisation:

In Figure 3-10, energisation of an open-end transmission line by closing the circuit

breaker produces a transient wave (e), which reflects back (e’) after reaching the open

end of the line. In theory, switching surges due to energisation of a line with no trapped

charge can create a value of overvoltage not exceeding twice of the system voltage.

Figure 3-10: Sum of reflected voltage and current and sending waves


By applying an open end resistance (Zk) and a line resistance (Z) in the travelling wave

theory, the following calculations can be produced:

e= iZ I”= i-i”e’= i’Z e”= e+e’

e”= i” Z

Total voltage due to reflection: e”= 2Zk / (Z+Zk)

Therefore, the total reflected voltage can be calculated by use of Equation (3.12).

e”= 2Zk / (Z+Zk) (3.12)

In theory, switching surges due to energisation of a line with no trapped charge can

reach a value up to twice of the system voltage, however, in reality switching surge can

rise to 3pu-3.5pu [3.9]. Some of the system parameters influencing the magnitude of

switching transients are line length and impedance, effect of series compensation or

shunt reactors, source X/R ratio, transformer excitation, the behaviour of circuit breakers

at the time of opening/closing and effect of surge arresters [3.12].

Figure 3-11 presents a transient simulation due to energisation of the 400kV

transmission line with 120km of L6 overhead line shown in Figure 3-3. Point ‘A’

presents the time when the circuit breaker closed at the beginning of the line, whereas

point ‘B’ shows the time when the travelling wave reaches the open end of the line.


Figure 3-11: Voltage at the Sending and Receiving End Due to Energisation of 60kmLine on 400kV System

The simulation step time (∆t) was set to 100µs. The time required for the wave to travel

120km at a speed of light is about 400µs which is very close to results from the

simulation which is about 390µs. The voltage at the sending point (ESending) is about

327kV, and this voltage ramps up to the maximum value of 641kV (EReceiving) after

reflecting back and forward along the line. The oscillating section (C) in Figure 3-11

shows the effect of travelling wave reflection on the transmission line. These

oscillations wade away, depending on the characteristic impedance of the line, and the

system returns to steady state after some period of time.

II. Re-energisation:

In ≥245kV systems, applying voltage to a no-load line without any trapped charge or

open-end circuit can create a travelling wave as big as 2pu once it reflects back from the

end of the line. At the same time, re-closing the circuit breaker at the beginning of a line

with a trapped charge of -1.0pu can cause a total overvoltage up to 3pu in case of an

ideal circuit [3.10].


The magnitude of a switching surge depends on the size of the trapped charge and the

point of voltage wave at which circuit breaker closure happens. The magnitude of

switching transient for the case of re-energisation of a line for a single phase

transmission line is different from 3-phase lines. This is due to interphase coupling and

sequential pole closure of the circuit breaker on each line. The trapped charge stays for

10-100s on transmission lines if no wound voltage transformers (VTs), power

transformer and reactors are connected to the transmission line. The only losses will be

due to corona and leakages and, therefore, the losses and decay depend on the weather

conditions [3.12].

The magnitude of the overvoltage at the end of the transmission line will rise to the

highest value if the circuit breakers close at the opposite polarity voltage to the residual

voltage on the line.

Based on travelling wave theory, the maximum overvoltage occurs when the supply

voltage is at its peak and the residual voltage is at its peak of opposite polarity. Under

this condition, the voltage at the sending point has a magnitude of up to 2pu and when it

reaches the open end of the line, it would rise to a value up to 3pu (phase to earth).

The PSCAD simulation model of a transmission line with a trapped charge is shown in

Figure 3-12. In order to simulate the re-energisation of the transmission line, the circuit

breaker (BRK1) in the model stayed closed at the beginning of the simulation while the

circuit breaker (BRK2) at the end of the line stayed open throughout the simulation.

Then, 0.085 seconds after starting the simulation, the circuit breaker (BRK1) opened and

re-closed at a random time again. The simulation repeated and the circuit breaker

reclosing occurred 500 times within the voltage full cycle to achieve the highest

overvoltage produced by circuit breaker closure.


Figure 3-12: PSCAD Simulation Model of Trapped Charge

The highest overvoltage was observed when there was a trapped charge of ~(-1)pu on

the line. In Figure 3-13, the circuit breaker was opened 170ms after starting the

simulation and after further 8ms the re-closure occurred. The maximum transient

overvoltage at the end of transmission line reached 2.1pu whereas, in the case of

energisation, the maximum overvoltage that appeared on the line was only 1.5pu.

Figure 3-13: Energising of a Line, Top; Without Trapped Charge, Bottom; With TrappedCharge


III. Disconnection

Switching overvoltage due to disconnection events are generated when a system in a

steady state is disconnected by a circuit breaker. Disconnection overvoltages could

happen due to disconnection of an open-end line or a capacitor, open circuit transformer

or disconnection of the line due to clearing a fault in the system.

Before disconnection of a circuit even at current zero, the overhead lines, cables and

even transformers contain some magnetic energy. The sudden interruption of current or

the steady state of the system destabilising the changes to the system. Therefore,

disconnection of a line can produce an additional transient that superimposes the

instantaneous condition of the system.

Figure 3-14 compares the simulation results due to energisation, re-energisation and

disconnection of a simple 60km open-end of transmission line.


Figure 3-14: Voltage Due to Top; Energisation, Middle; Re-energisation, Bottom;Disconnection

The green curve in Figure 3-14 presents the voltage at the receiving end of the line. As

shown in all switching configurations, the voltage at the end of the line is higher than the

voltage at the sending point of the line. The computed maximum overvoltages due to

energisation, re-energisation and disconnection are equal to 561kV, 775kV and 379kV

respectively where the nominal voltage set to 400kV.


IV. Fault and Clearance

At the time of a fault, circuit breakers interrupt the current at the zero crossing. The

fault could appear either after the breaker terminal (bolted terminal fault) or somewhere

further on the transmission line. At the time of a fault, the line will be left with a charge

at the instant of current interruption. This charge is at its maximum value on the breaker

side, and it is equal to zero at the fault side. Therefore, the network tries to balance

itself, and as this balancing of the voltage potential cannot take place instantaneously, an

overshoot of voltage occurs and produces a travelling wave on the transmission line part.

At the same time, the charge on the breaker contacts changes from zero to the

instantaneous value of power frequency and creates a Transient Recovery Voltage

(TRV) at the circuit breaker terminals which can also generate a travelling wave along

the line.

This oscillatory transient is due to the sudden change of the voltage or current in the

steady state condition with polarity influenced by the polarity of the system nominal

voltage wave. The rate of change of voltages and its magnitude depend on the length of

the line, characteristics of the line and distance from the fault location with a frequency

determined by inductance and capacitance of the line. However, the magnitude of the

TRV on a circuit breaker depends on the rms value and the interrupted current (load

current, fault current, etc.) [3-12].

As in practice, power systems are inductive under a fault condition, the power factor of

the circuit from the circuit breaker’s side is zero and lagging, and the power frequency of

TRV is at its peak value at the instant of current zero when the interruption occurs.


Figure 3-15: Oscillatory Transient Due to Interruption of Fault Current on PSCADModel- ES: Voltage Sending Point, EL: Voltage along the Line, Earc: Circuit Breaker

Arc Voltage.

Figure 3-15 shows oscillatory transients due to the circuit breaker opening after 50ms

from the fault time. At the point ‘A’, when the system voltage (phase 1: green curve) is

at its maximum value, the circuit breaker terminals will be disconnected at current zero,

and the green wave would bounce back and forward between the fault location and open

terminal of the transmission line. The time to crest of each tooth shape travelling wave

depends on transmission line’s surge impedance. The red curve shows the TRV

imposed on the circuit breakers' open terminals where its frequency is determined by the

inductance and capacitance seen from the breakers looking upstream into the network.

This TRV could be worse if a fault happens a few hundred meters up to a couple of

kilometres away from the circuit breaker. This phenomenon is because the travelling

wave on the line side has very high frequency and superposition of TRV and travelling

wave creates a very high overvoltage on circuit breaker terminal and features a transient

wave on the line side.


3.6. Switching Impulse Strength

In HV and EHV systems, the voltages that cause the highest risk of flashover are those

associated with lightning and switching operations. These overvoltages determine the

external insulation design due to their high magnitudes.

There are many factors which are influencing the breakdown voltages of uniform and

non-uniform air gap. Therefore, the strength of the switching surge is very dependent on

the maximum overvoltage and condition of the surrounding where the live-line working

takes place. These factors have an impact on the strength under switching surge and the

minimum voltage breakdown of the gap, and they are used in the determination of

electrical distance and insulation coordination within a power system.

As the gap flashover and its strength depend on a number of parameters, in the next

section, the effect of these parameters and their influence on the strength under

switching impulses and also their impact on the minimum approach distance are briefly

explained.

3.6.1. Effect of Wave shape

The switching wave shape is described by its time to crest and time to half value on their

tail- refer to Figure 3-17.

Figure 3-16: Standards Switching Impulse Where V50 is a half the time to crest of aTransient Wave [4.1]


Based on IEC60071-1, the shapes and classes of overvoltages with a standard voltage

shape are shown in Table 3-7.

Table 3-7: Shapes and Classes of Overvoltages Standards Voltage [3.29]

The time to crest (tcr) is a primary factor that influences the formation of flashover due to

transient overvoltages. The wave that produces the lowest value of U50 (the voltage that

has fifty percent probability of flashover) is called a critical wave of the gap where the

air breakdown happens at or near the peak of the transient wave.

At the same time, if the critical wave is shorter than time-to-crest, the voltage

breakdownoccurs after the peak of transient, and it has a higher value of U50 [3.19],

[3.20]. This is shown in Tables 3-8 and 3-9.

Table 3-8: U50 of Rod-Plane for Fast and Slow Wave Shape [3.21]

D(m)U50(kV)

Fast TOV (1.2/50 µs) Slow TOV0.4 281 216 (tcr= 52µs)1 625 380 (tcr=112 µs)2 1195 820 (tcr=375 µs)


As shown in Table 3-7, the slow front transients are those with time to peak between

20µs and 5000µs and time to half value equal or less than 20ms. Switching overvoltages

are slow front transients whereas lightning is a fast front transient. As shown in Table 3-

8, a larger gap has a higher value of U50 voltage. Also, as the critical wave of a fast front

transient is shorter than its time-to-crest, the voltage breakdown occurs after the peak of

the transient wave and it causes a higher value of U50 in comparison to the slow front

transient. As shown in Table 3-9, some wave shape are classified based on their time to

crest (tcr), whereas some of the wavshape are presented by their time to crest (tcr) x the

time to half value of the tail after the peak of the wave.

Table 3-9: U50 of Rod-Plane as the Function of Wave Shape, Non-Standard SwitchingWave Form [3.21]

D(m) U50(kV) and Wave shape

1 42160x2500

412(tcr= 72µs)

400(tcr= 70µs)

41680x1000

440220x2100

510350x3200

2 732(tcr= 100µs)

700(tcr= 1052µs)

752120x4000

756220x2100

875350x3200

887420x4000

Further in Chapter 4, the influence of the line length on time to crest if transient wave

has been investigated.

3.6.2. The “U-Curve”

As explained earlier, the shape of the impulse has an impact on the strength of a gap. As

a result, the insulation strength of a gap is influenced by the wave shape as the function

of time to crest and time to the half value.

By plotting U50 values against the time to crest of transient overvoltages for a gap size, a

U-shaped curve will be formed. This curve shows the voltage breakdown of the gap as a

function of the time to crest of the transient overvoltage. Also, it indicates the minimum


value that is corresponding to the critical wave and lowest U50. The voltage data points

along the curve are the voltages at which the strength of the gap is a minimum and the

stress due to transient overvoltages causes the flashover within the gap.

The U50 (50% possibility of voltage breakdown) spark overvoltages as a function of time

to crest with different spacing under different atmospheric conditions are shown in

Figures 3-17 and 3-18. The curve is called U-curve which is widely used in the

calculation of insulation coordination [3.22, 3.23]

Figure 3-17: U-Curves Obtained with Impulse Voltages of Various Time-to-Crests (Tcr

µs) Applied to Rod-Plane Gaps. Atmospheric Humidity in These Experiments WasVaried [3.22, 3.23]

As shown in both Figures 3-17 and 3-18, small changes of time to crest of switching

transients do not have a significant effect on U50 of the gap. That means, breakdown

voltages of the small gaps are not massively influenced by transient’s time to crest.

However, the bottom values of the U-curves (red arrow in Figure 3-17) show the

minimum voltage required to form a flashover within the gaps. In other words, these

points are where a gap has its minimum strength against the different transient times to

crest.


Therefore, based on experimental results shown in Figures 3-17 and 3-18, the minimum

required voltage to form a flashover within a gap is very close to the bottom of the U-

curve. At the same time, each point along the U-curve presents the voltage

breakdowndue to the different times to crest of each air gap.

Figure 3-18: A; Switching Impulse Flashover Voltage of Rod-Plane Gap, the picture onright corner of Figure A, indicates the rod-plan gap, B; Estimation of CRIEPI’s Equation

The results from experiments led to the assumption of Equation (3.13), where according

to IEC 60060, the standard switching overvoltages is assumed to have the time to crest

of 250µs. Equation (3.13) used by CRIPEI [3.21] calculates the U50 of an air gap, and it

A

B


is more complicated than the previous equations introduced by Paris, Gallet or Cortina

and Herbec formulae [3.15], [3.19] and [3.21].

U50RP = 1080 ln(0.46d + 1) (kV) (3.133)

Equation (3.13) has been achieved by plotting an estimated curve connecting the critical

points of experimental results from other researchers, and it has the advantage of being

adjusted for the larger air gaps. It is also closer to experimental results when smaller

gaps are in used. The formula has been adopted by IEC standards and used and

developed by many utility companies as the fundamental formula in the calculation of

the minimum safety distance.

In live-line working, Equation (3.13) is used to calculate the 50% sparkover of a rod-

plane gap with a length of d (meter) which is estimated from the lowest part of the ‘U-

curve’ of different gaps where the voltage breakdown is at its lowest value.

The lowest values of voltage breakdown of the gaps give the highest possibility of

flashover over where the smallest stress due to transient overvoltage overcome the

strength of the air gap. By considering the minimum value of U50 at the bottom of a U-

curve, the risk of flashover will be at the minimum value.

3.6.3. Wave Polarity

The switching surge flashover and also the strength of the gap depend on the polarity of

the surge. As the gap between the electrodes is non-uniform [3.24], the positive polarity

switching surge strength is lower than that under negative polarity [2.6]. A negative

discharge applied to a field has less ramification and shorter length in comparison to

positive surge [2.6]. Table 3-10 shows the effect of polarity on rod-plane gap for

standard switching transients [3.15], [3.19].


Table 3-10: Effect of Polarity on Rod-Plane Gap [3.15], [3.19]

D(m) U50 (kV) Wave shape (µs)Positive Negative0.5 420 580 1.5x501 800 1050 1.5x501 400 700 120x40002 710 1300 120x4000

As shown in Table 3-10 and report in [3.25], the positive polarity voltage breakdowns

are lower than corresponding negative polarities. Therefore, in the case of negative

switching transients, higher voltage breakdowns will be required to form a flashover in a

gap. As a result, for the purpose of live-line working, the positive polarity flashovers are

considered for calculation of the strength of a gap. In very rare cases, due to different

atmospheric conditions, gaps under negative polarity surges have a lower voltage

breakdown compares with positive polarity switching surges [3.26].

Figure 3-19 shows experimental results of the rod-plane gap spark over voltages for both

positive and negative polarities of DC and AC voltages. As shown in Figure 3-19, the

positive flashover voltages have a lower magnitude of voltage breakdown than negative

flashovers.

Figure 3-19: Rod-Plane Gap; 1- Minute Critical Withstand AC and DC Voltages; 50%Percent Spark Over Voltage with Standard and Long Front Impulses [3.26].


3.6.4. Effect of Atmospheric Conditions

The voltage breakdown of an air gap depends on atmospheric conditions at which the air

breakdown or flashover occurs and it is influenced by three factors; pressure, humidity

and temperature. In order to adjust the test results in any weather condition according to

standard weather condition with a temperature equal to t0=20, a pressure of P0=101.3

kPa and humidity of 11g/m3, the correction factor (ka) is used.

The voltage breakdown of the gap increases with the air density and humidity whereas,

rain and its combination with a large variety of agents such as; coal and cement dust, fly

ashes, salt spray, etc., can reduce the voltage breakdown of the gap and porcelain

insulations [3.27].

Reducing the voltage breakdown of the gap influences the gap strength, and as a result,

the minimum approach distances need to be increased for the purpose of live-line

working.


3.7. Discussion and Conclusion· PSCAD is world known transient simulation tool with slightly slower

computation speed in comparison with other simulation software such as ATP-

EMTP. However, due to having more type of controllers, power generators,

recording components and also easier graphical interface, this project used

PSCAD for the purpose of transient studies.

· In the calculation of the minimum approach distance, the influence of lightning

overvoltage has been ignored, and only switching overvoltage needs to be

considered.

· Switching transient due to energisation of an open-end line can create a reflected

wave with the same polarity and a magnitude as twice as original wave.

· Switching transient due to re-energisation of an open-end line can create a

reflected wave with the same polarity and a magnitude bigger than original wave.

· Large reflected wave with the same polarity means at the open-end of the circuit;

the voltage can be varied while the current is zero.

· In the case of short circuit line, the reflected wave has an opposite polarity in

comparison with the original voltage wave.

· Increasing the temperature or decreasing the air pressure will decrease the air

voltage breakdown while increasing the humidity can increase the air voltage

breakdown.

· Although IEC 61472 provides a guideline for calculation of the minimum

approach distances, the influence of some factors such as wave shape, time to

crest, the worst case atmospheric conditions, polarity, etc., has not been clearly

investigated. These factors clearly introduce a considerable safety margin into

the calculation of minimum safety distances.


· The waveshape can have a massive impact on voltage voltage breakdown of a

gap whereas in IEC 61472, calculation of minimum approach distance has been

done only by introducing the standard wave shape transient.

Simulation scenarios in the next Chapters show that wave shape can be influenced

by many factors within the power system and the risk of gap breakdown due to non-

standard switching transient is affected by transient wave shape.

CHAPTER 4. Network Studies and Calculation of MAD 92

CHAPTER 4

Network Studies, Overvoltage Levels andResulting MAD

4.1. IntroductionCalculation of the minimum approach distance (MAD) for live-line working can be done

using the methodology explained by IEC 61472 in Chapter 2. This project uses U2

voltage (2% statistical overvoltage) from simulations as an input into the method

described in IEC 61472. This methodology delivers a minimum approach distance

based on the simulation results of the modelled network refer to Figure 4-19.

This chapter as the main body of this thesis intends to investigate the parameters

influencing the magnitude of switching transients on a transmission line. A simple

PSCAD model (Figure 4-19) presents each switching transient scenario to demonstrate

the relationship between overvoltage level and source of transient i.e. energisation, re-

energisation, fault /clearance, etc.

Moreover, a fundamental transmission line model is produced to establish a new

suggested set of minimum safety approach distances for the 400kV transmission line. In

the simulation of switching overvoltage in this section, PSCAD software is used as a

transient simulation tool.


4.2. Simulation Methodology

Throughout this project, each simulation has been repeated up to a maximum value of

2400 runs to achieve the highest accuracy of the results. Multiple run function in

PSCAD has been used to simulate each scenario within a full voltage cycle of 20ms

(50Hz). The incident time selections have been achieved in two ways:

1. Random

2. Sequential

For example, Figure 4-1 presents the 27 points along the voltage wave when the

switching or fault could occur (it assume that the time distances between points are

equal). During the random selection, each simulation event such as switching or fault

takes place at any random time within 20ms window. These event times do not

necessary need to be at each exact incident point (green point) along the wave –refer to

Figure 4-1.

In the sequential method, 20ms time window is divided by the number of simulation

runs with equal interval time, i.e. 27 simulation events- refer to Figure 4-1 where a full

cycle is given as an example of 20ms time window. The simulation starts with 1st

simulation run at first green point, and it continues to occur at each event point (green

points) - refer to Figure 4-1. PSCAD tool produces a U2 voltage at the end of each set of

simulation.

The simulation step times (∆t) throughout this study were set to 100µs. Smaller steps

could have also been used, however due to memory restriction and time consumption of

smaller steps <100µs, the 100µs found to be adequate.


Figure 4-1: Model of Event Occurrence in Simulation

In order to calculate the minimum approach distance for live-line working, the U2

voltage (2% statistical overvoltage) is used. This is because the low-voltage tail of

overvoltage distribution does not cause a flashover or it has very small or almost

negligible probability of flashover.

The U2 overvoltage can be obtained by use of PSCAD simulation tool. It also can be

obtained by use of PERCENTILE formula in Excel. In order to demonstrate the method

for calculation of U2 voltages, the distribution of switching overvoltages in one set of

simulation has been plotted against the frequency of occurrence of each overvoltage

magnitude- refer to Figure 4-2. The highest probability of switching transient in Figure

4-2, features the first peak of the transient distribution graph.

Figure 4-2: Switching Overvoltage Distribution (pu)


To include the impact of the smallest magnitude of overvoltages on the minimum

approach distance, the switching overvoltage distribution presented in Figure 4-2 is set

based on 0.01pu scaling value (1pu = 343kV). Table 4-1 compares various values of U2

voltage achieved by 2400 simulation runs produced by PSCAD and PERCENTILE

formula from Excel.

Table 4-1: U2 Value Comparison Achieved by PSCAD and Excel

Method Switching Overvoltage (kV)

Case 1 Case 2 Case 3 Case 4

Data from2400runsin PSCAD

Minimum (kV) 380.9535 567.1365 381.9098 567.1389Maximum (kV) 418.2729 647.6249 438.6966 642.6575

Mean (kV) 392.084 590.6614 404.8095 596.0857Std Deviation 13.96909 33.51238 19.28206 30.91897

PSCAD 98% Level 363.395 521.8354 365.209 532.58592% Level (U2) 420.773 659.4874 444.41 659.5855

PERCENTILEEXCEL

98% Level 381.4424 567.1365 382.0037 567.13892% Level (U2) 417.7662 647.6234 436.9012 641.6552

As shown in Table 4-1, the magnitudes of U2 overvoltages from PSCAD multiple runs

analyser are lower than Excel calculation. The Excel calculation was simply obtained by

use of the PERCENTILE formula for 0.98 of switching overvoltages. By plotting the

simulation values into a histogram probability plot, such as in Figure 4-2, the U2 value

will be slightly higher than PSCAD value and very close to Excel calculation.

Therefore, in this project, analysing the data and achieving the U2 values are performed

by using the PERCENTILE formula in excel as higher values of overvoltage achieved

by PERCENTILE formula in excel can feature more conservative results in the

calculation of the minimum approach distance.


4.2.1. PSCAD Goodness-of-Fit Testing for Weibull

Distribution

As shown in the Figure 4-2, there are some infrequent gaps along the distribution of

switching overvoltages especially within the range of 1.15pu to 1.19pu and 1.21pu to

1.22pu. As Weibull distribution is a continuous distribution, the probability of dropping

the variables to zero is almost zero. Considering of an existence of many falls to zero

value along the switching overvoltage distribution could lead to the suggestion of using

other methods such as moment estimators [4.2] which are not affected by zeros.

However, there are lots of ways [4.3] to deal with zero data and estimation of the

Weibull parameters which is beyond this project focus.

As the distribution of overvoltages is not always a normal Gaussian distribution, IEC

60071-2 [4.1] suggests to use a Weibull distribution which is calculated based on U50

and U16 of Gaussian distribution, and it is truncated at three standard deviations (3σ)

from U50. U50 and U16 are values of 50% and 16% discharge voltage of self-restoring

insulations.

In PSCAD, the Weibull function is used to present the transient simulation results and

the two percent statistical overvoltage. Figure 4-3 shows the Weibull overvoltage

distribution plot on MATLAB, from simulation results of the model in the previous

section.


Figure 4-3: Overvoltage Weibull Distribution Plot

The two percent overvoltage (U2) is found to be 421.2kV which is the very close to

PSCAD U2 value. The result of MATLAB output file is shown in Table 4-2.

Table 4-2: Simulation Result of MATLAB Output File

Cases Overvoltage (kV)Maximum 417.27Minimum 380.95

Mean 392.0898% level 371.052% level 421.2

Standard Deviation 13.96

Table 4-2 illustrates the results from MATLAB which are very close to the result in

Table 4-1. However, in this project, to obtain the U2 voltage the PERCENTILE formula

in Excel is used, as this approach provides more conservative estimation of U2 voltage.

The value of U2 voltage obtained from this method produces a more conservative value

for the minimum approach distances.

416407389 398

Overvoltages (kV)


4.3. Parameters Influencing the Overvoltage

on Transmission Line

The magnitude of transients on transmission line depends on the circuit parameters,

performance characteristics of circuit breakers, system voltage profile and the time when

the switching or fault occurs on the voltage wave cycle [4.4].

At the same time, many parameters within the power system are influencing the

magnitude of switching transient on a transmission line. These parameters are length

and type of transmission line, tower types, the presence of cable section, the probability

of occurrence of different fault type and compensations.

In this section of the thesis, a very simple model is produced to illustrate the impacts of

different parameters on the magnitude of switching overvoltage on a transmission line.

As a result, later in this Chapter, the minimum approach distance will only be considered

for the worst case scenario for each study.

In order to investigate the effect of different parameters on the magnitude of switching

overvoltages, the model of transmission line system in Figure 4-4 is presented.

Figure 4-4: Sample PSCAD Model of Transmission Line

The complete list of towers and transmission lines data are provided in Appendix 2. The

model in Figure 4-4 comprises a single circuit three-phase overhead lines which consist

of four towers with an equal span distance. The 400kV model also consists of two

circuit breakers at the both ends of the transmission line where they are connected to


generators. The type, length of transmission line and towers are changed based on

nature of each study.

4.3.1. Transmission Line Effect

The magnitude of switching overvoltages can be influenced by changing the length of

transmission line. This phenomenon is explained by setting a different length of a

transmission line on the PSCAD simulation model in Figure 4-4. The model consists of

L6 towers used by National Grid network. For this study, the same values of X/R ratio

were used in each set of simulation to investigate the impact of transmission line length

on travelling wave transient. The generators in this section were set to have resistances

and inductances to observe the effect of travelling wave on the impinging wave from

generators.

For the following study, the length of the transmission line was set to be 10km, 40km,

80km and 90km where the fault level was set to be 10kA. Furthermore, different

switching scenarios have been investigated using a various lengths of transmission line.

For each length of transmission line, the test was repeated 200 times sequentially to

obtain the maximum overvoltage along the transmission line.

As shown in Table 4-3, the maximum U2 switching overvoltages due to energisation

were observed on the longest line due to higher line inductance.

Table 4-3: Magnitude of Switching Overvoltage Due to Various Length of TransmissionLine

Study Case

Length of Transmission Line (km)

10 40 80 90

P-E P-P P-E P-P P-E P-P P-E P-P

Energisation 2.18 3.19 2.24 3.20 2.29 3.26 2.35 3.30Re-energisation 1.86 2.71 1.90 2.71 1.95 2.75 1.99 2.77Disconnection 2.23 3.24 2.27 3.24 2.33 3.29 2.38 3.31

Fault & Clearances 2.27 3.27 2.31 3.27 2.36 3.31 2.40 3.33


As explained in Chapter 2, by increasing the length of a line, the time required for the

wave to travel along the line will be increased. For clarification, the line can be

considered as the model shown in Figure 4-5, where both line capacitances and

inductances are responsible for the rise of voltage at the end of a transmission line. If

the time required for the wave to bounce back from the open end of the line and reach

the sending point is smaller than the voltage rise time at the sending point of the line, the

peak of overvoltage will have a maximum positive value. Figure 4-5 was explained by

use of Figure 3-1 in section 3.3 of Chapter 3.

Figure 4-5: Overhead Model

4.3.2. Type and Length of cable Section

In order to investigate the effect of type and length of cable section, the following model

in Figure 4-6 has been used. Length and type of the cable used in the cable section were

individually set in PSCAD model according to Table 4-4. The PSCAD model consists

of a 30km overhead line connected to a cable section- refer to Figure 4-6. The

simulation was repeated 200 times for each case, and the circuit breaker (Breaker one)

closed at a random time.

Figure 4-6: PSCAD Model of Line-Cable Combination


By increasing the length of the cable section, the magnitude of overvoltage observed at

both ends of the cable section was increased, whereas the overvoltage observed at the

beginning of transmission line (Sending point) was decreased. The rate of change of

overvoltage due to the different length of cable section is shown in Figure 4-7.

Figure 4-7: Change of Overvoltage at Beginning and End of Cable Section Due toChanging the Length

In order to illustrate the effect of cable type used by power networks on transient

overvoltage, the data from Table 4-4 are used, and cable specifications in PSCAD model

were set. Cable data in Table 4-4 are extracted from National Grid databases for the

cable used on their network around the UK.

Table 4-4: Three Types of Cable Specification Used by National Grid

CorrugatedAluminium sheath

800mm2 XLPE 1600mm2 XLPE 2500mm2 XLPE

Diameter (mm) Diameter (mm) Diameter (mm)

Conductor 33.7 52 66Binder 33.7 53.6 67.6

Conductor screen 35.7 55.6 69.6Insulation 93 112.4 126.4

Insulation screen 95 114.4 128.4Bedding 96.5 115.9 129.9

Copper wire Screen 96.5 115.9 129.9Equalizing tape 96.5 115.9 129.9Screen binder 96.5 115.9 129.9

Clearance 110.8 130.1 144.1Sheath 125.8 146.9 162.7

Over sheath 133 154. 169.9


Dc resistance 29.04 ohm/km 16 ohm/km 11.37 ohm/kmDielectric losses 2.43 W/m 3.31 W/m 3.90 W/m

Conductor temperature 90 0C 90 0C 90 0C

As shown in Figure 4-8, increasing the size of a cable used within a transmission line

increases the magnitude of switching overvoltage at both ends of the cable section. This

phenomenon is due to the decreasing of the cable surge impedance. In larger cables, the

inductance is lower whereas the capacitance due to thicker insulation layer is higher and

As a result, the surge impedance Z0 will be smaller in a cable such as the 2500mm2

XLPE cable.

Figure 4-8: Overvoltage at Beginning and End of Cable Section vs. Cable Type

4.3.3. Cable Section Position on transmission Line

Including the cable line section can increase the overvoltage that appears at the open end

of the line. This is due to having different surge impedances on the overhead line and

the cable section which increases the reflection coefficient that leads to increasing the

voltage magnitude at cable-line junction. In order to investigate the overvoltage that

appears on a transmission line, a cable section has been placed in 3 different positions

along the line as shown in Figure 4-9 where the line and cable specification are shown in

Table 4-5.


Figure 4-9: Schematic Model of Transmission Line

Table 4-5: Cable and Overhead Line Specification

For All Overhead line Cable Cable Overhead line sections

Steady state Frequency 100 x 103 Hz 100 x 103 HzNumber of Conductor 3 3

Segment Length 15.5[km] 72 [km]Total Length 15.5[km] 284.5 [km]

Conductor Radius 0.028 [m] 14.31 x 10-3 [m]Total Impedance (Ω) 2.8 ohm 13.7 ohm

1 Section Inductance (µH) 1.29 3.53 x 101

1 Section Capacitance (µF) 3.23 x 10-03 2.47 x 10-4

Surge Impedance (Ω) 20.00 377.91

The time required (τ) for the wave to travel along the cable due to energisation is 98.7µs

as shown in Figure 4-10. Therefore, the propagation speed of the wave on a cable found

to be 1.56 x 108 m/s which is almost half the value of wave propagation speed on an

overhead line.

Figure 4-10: Time Required for Wave to Travel along the Cable


By placing the cable section at the beginning of a transmission line, and due to the lower

surge impedance of the cable, the reflected wave toward the generators will be smaller

than other cases. At the same time, compared to other two cases that will be explained

later, it takes shorter time for the voltage wave to reach its peak value (5.6ms and 8.1ms

if the cable section placed at the middle and end of the line respectively).

When the travelling wave on the cable reaches the cable-line junction (cable-overhead

line joining point), and due to the high transmission coefficient (β), a significant portion

of the travelling wave goes through the overhead line. At the same time, only a very

small portion of the travelling waves reflected back into the cable section, and that is due

to smaller reflection coefficient (ϒ). The transmission and reflection coefficients can be

calculated by (4.1) and (4.2) where Zoh and Zcable are overhead line and cable surge

characteristic impedances.

β=2Zoh/ (Zoh+Zcable) (4.1)ϒ= (Zoh-Zcable) / (Zoh+Zcable) (4.2)

In Figure 4-11, the blue curve is the voltage at the beginning of the cable section, and as

it was explained earlier, due to a small reflection at the cable-line junction, the rate of

rise of the voltage was small.

When the wave reaches the open end of the line, it bounces back toward the cable

section. Due to a small transmitted coefficient (δ) of the line-cable section, only a very

small portion of the wave would travel into the cable section, whereas due to high

reflection coefficient (Φ) of the line-cable section, most of the travelling wave returns to

the open end of the transmission line. The transmission and reflection coefficient can be

calculated as follow;

δ = 2Zcable / (Zcable+Zcb) (4.3)

Φ = (Zcable-Zoh) / (Zcable+Zcb) (4.4)


As shown in Figure 4-11, it takes 2ms for the surge to reach the maximum value of

484kV (at sending end). Due to smaller reflection coefficient at the cable-line junction

in comparison to the cable section, it takes 98.7 µs for the reflected surge to reach the

open end of the line. As the voltage reaches its highest peak value, the maximum

overvoltage at the end of the line occurred 2.9ms after circuit breaker closing time with

the highest value of 978kV.

Figure 4-11: Overvoltage, Sending (Blue Curve) And Receiving (Green Curve) WithCable Section at Beginning of the Line

By placing the cable section at the middle of the transmission line, the maximum

overvoltage and the time to reach this value differ from the previous case. In this case

shown in Figure 4-12, the first transmitted travelling wave sent from the circuit breaker

travels through the overhead line, and it reaches the cable-line junction.

After reaching the cable junction, due to lower transmission coefficient of the cable in

comparison with an overhead line, a small portion of travelling wave goes through the

cable section and a large part of the travelling wave reflects back toward the generator

and conflicts with the voltage wave supplied by the generator.


In this process, the beginning end of the transmission line experiences its maximum

voltage peak around half cycle after the circuit breaker closing time (point D in Figure 4-

13). Due to the higher overhead line impedance compared to that of the cable section,

the voltage peak is higher than the previous case. In this case, the overvoltage at the

open end of the line reaches the maximum value of -757kV after 8.5ms from the circuit

breaker closing time which is longer than the previous case.

Figure 4-12: Schematic Model of Transmission Line with Cable Section Place in theMiddle of the Line

Figure 4-13: Overvoltage, Sending (Blue Curve) and Receiving (Green Curve) WithCable Section at Middle of the Line

By placing the cable section just before the open end of the line, the maximum

overvoltage of -879.75kV will be observed just 11ms after the circuit breaker closing

time. It takes longer for the wave to reach the maximum value as the most of the wave

facing the cable section (line-cable junction) reflects back toward the sending point.


However, as the wave bounces back from the cable section, a reflection with a negative

magnitude reaches the overhead line junction. Due to the higher impedance of the line,

its magnitude increases and a higher magnitude of reflected wave will be penetrating

back through the cable section and reaches the open end of the line- refer to Figure 4-14.

Figure 4-14: Maximum Overvoltage, Sending (Blue Curve) and Receiving (GreenCurve) with Cable-Line at End of Transmission Line

4.3.4. Capacitor bank

Capacitor banks are used to increase the power system's efficiency, maintain and control

the transmission voltage profile within the limit, power factor correction and mainly

regulation of reactive power. Therefore, insertion of reactive power elements into

transmission line can provide the following:

• Reducing line voltage drops

• Limiting load-dependent voltage drops

• Influencing load flow in parallel transmission lines

• Increasing sign transfer capability

• Reducing transmission angle

• Increasing system stability


Energisation of a capacitor bank can give rise to transient in a network [4.7]. In general,

if the system and overhead line resistance are ignored, the following equations express

the inrush current into the capacitors.

( ) = sin(4.5)

Where:

=(4.6)

=1√

(4.7)

Increasing the size of a capacitor increases the inrush current and decreases its

frequency. Therefore, in the case of a larger capacitor, a higher magnitude of switching

transient will be observed. Therefore, the magnitude of switching surge and, as a result,

the minimum approach distance for a transmission line connected to a capacitor bank is

also influenced by the size of the capacitor bank.

The series capacitor banks are generally used in a long transmission line or large

generating power plant. Their main aim is to increase the efficiency of a transmission

line. “The series capacitor bank shall be capable of withstanding the rated continuous

current, system swing currents, emergency loading, and power system faults and, in

some applications, harmonic currents; these quantities normally are specified by the

purchaser” [4.5].

The series compensation on the transmission line is selected based on system power

flow, system stability, short circuit and synchronous resonance and cost of the

equipment. The main issue with series compensated line is that DC component of

current associated with most faults would not decay, and instead, it generates an AC

transient component of current on fault inception with a frequency equal to Equation

(4.8).


ƒ = ∗ (4.8)

Also, the degree of series compensation is calculated by the ratio of capacitive reactance

of series compensation over inductive reactance of the line and shown in Equation (4.9);

= × 100% (4.9)

The percentage selected for series compensation can be in the range of 20% to 80% of

transmission line’s impedance. If the degree to be set at 100%, a large current will flow

into the system in the presence of small fault or disturbance and also it can cause the

series resonant at the fundamental frequency. To investigate the effects during the line

energisation, a single series capacitor bank was inserted in the middle of 300km L6

overhead line –refer to Figure 4-15.

Figure 4-15: Series Capacitor Bank Modelling with a 41.91µF series Capacitor

The properties of the line under investigation were set to be the same as in previous

sections. The compensation value was changed in a range of 20 to 80 percent of the

total line inductive reactance – refer to Table 4-6.

Zoom


Table 4-6: Series Capacitor Size

Inductive compensation of the line20% 50% 80%

Total per phase line inductance [mH] 483.42 483.42 483.42Inductive reactance of 300km line [Ω] 151.87 151.87 151.87

Capacitive reactance [Ω] 30.37 75.935 121.496Size of capacitors [µF] 104.7e-6 41.91e-6 26.19e-6

Adding a series compensation increases the transmission capacity. However, the voltage

characteristics of a transmission line will also be changed. By increasing the degree of

series compensation on the transmission line, the maximum overvoltages due to

energisation of a line will be reduced. This phenomenon is due to the reduction of

inductive reactance on a transmission line. According to Figures 4-16 to 4-18, the

overvoltage due to energisation of the open end or lightly loaded line with 20%, 50%

and 80% of the line inductive reactance are 1.92pu (660kV), 1.87pu(644kV) and

1.08pu(358kV) respectively for a 400kV network where the base value assumed to be

343kV. Therefore, increasing the compensation level can decrease the maximum

overvoltage on the line.

Figure 4-16: Overvoltage with 20% Series Compensation




In all above cases, a capacitor bank was inserted in the middle of the line as there is no

set of standards concerning the location and number of series capacitors installed on a

transmission line. Typically series capacitor banks are installed as a set of two, i.e. one

at each end of the line. Series capacitors increase the voltage within the power system

whereas, the inductive reactance of a transmission line can cause a voltage drop. The

current through the capacitive reactance of the bank terminals cancel out the voltage

drop and maintain the acceptable voltage profile. Therefore, by splitting the series

capacitors and inserting them at two different locations, excessive voltage rise will be

divided into two parts of the line. This case and also compensation at both ends of the

line has been investigated in section 4.4.


4.4. Network for Overvoltage Studies

In order to illustrate the results from different switching events and calculate the

minimum approach distances, a fundamental model of a power system transmission line

has been created. The power network model in Figure 4-19 represents a section of the

UK transmission system. A double circuit overhead line with a length varying between

10 and 120km (a range of tower models representing the different versions found in the

UK 400kV system) is connected to a 400kV substation at both ends.

Live-line work takes place on one of the two circuits on this line. The two substations

(substations A & B) have two further 120km double circuit connections to remote

substations. A basic generator model consisting of a voltage source and an impedance

appropriate to represent the fault level of X/R ratio is connected at these remote

substations – each substation having the same generator type. Overvoltages on the

system are monitored at five locations; each end of the double circuit line and at 25%,

50% and 75% distances along the line. Both phase to earth (PE) and phase to phase

voltages (PP) on the live circuit (the one on which workers are active) and the coupled

circuit are monitored. The PSCAD model and also schematic diagram of the network is

presented by Figure 4-19.


Figure 4-19: A; PSCAD Model of Transmission Line, B; Schematic diagram of thenetwork

The maximum P-E and P-P voltages are recorded in a range of overvoltage scenarios as

below. The complete results from simulation will be presented in Appendix 3, whereas

only the highest values of switching overvoltages will be considered for calculation of

the minimum approach distances. The scenarios are as below:

A

B


· Energisation of the coupled circuit (with and without trapped charge) while live-

line work takes place on the live circuit. Energisation takes place at a random

time on the AC cycle with no significant scatter assumed between the closing

times of the three phases.

· Disconnection of the coupled circuit while live-line work takes place on the live

circuit. Disconnection takes place at a random time on the AC cycle with no

significant scatter assumed between the closure times of the three phases.

· Faults and resulting clearance through operation of both lines end circuit

breakers on both the live and coupled circuits (a range of fault types and

locations being simulated).

The fault is applied at a random time on the AC cycle with the circuit breaker being

commanded to operate after a fixed time delay representing the operating time of the

projection. Faults are cleared at current zero on each phase with no current chopping

being assumed. These overvoltage studies have been carried out in one of two ways

below;

1. 9600 runs of the overvoltage simulations have been carried out with each type of

fault being applied equally.

2. Another set of simulations has separated the runs for line-ground (LG), line-line

(LL), line-line-ground (LLG) and line-line-line (LLL) faults. In this case, it is

possible to generate an overvoltage profile based on an uneven mix of fault

types. The use of an 80% LG, 17% LL, 2% LLG and 1% LLL ratio is an

example of a more realistic distribution of fault type.

The distribution of the fault type is obtained from test results in [4.8], and it could differ

from one network to another.


Each generator at a feeding substation provides one-quarter of the fault level

contribution to set the fault level at the main substation around 10kA to 40kA. The

overhead lines were modelled using the Frequency Dependent (Phase) model. This is

the most accurate overhead line model available in PSCAD.

The model takes the geometric arrangement of overhead lines into the simulation along

with the size of conductors used. From this information, PSCAD automatically

computes the line parameters that are used in the model. L2, L6, L8, L9 and L12 towers

were modelled in this work, these representing a small and large tower size used on the

400kV network. Information about the tower geometry and conductor types were taken

from TGN (E) 166 and entered into PSCAD model. The geometric data was entered

into PSCAD. Phase sequencing was rotated on the two circuits (abc and cba).

Conductors’ co-ordinations and data are shown in Appendix 2.

4.5. Overvoltage Simulation Results

The following tables present the U2 value of switching overvoltages (which exceeded in

only 2% of all cases) obtained from the various simulation scenarios. In the majority of

simulations, the U2 voltage was extremely close to the maximum overvoltage observed

and, in some simulations, was higher (particularly when a high standard deviation

existed in the overvoltage levels). The U2 overvoltages shown in the following tables

are given in per-unit on a base of 343kV. The peak phase to earth voltage on a 400kV

system is taken to be operating above nominal at a voltage of 420kV.

In all cases, the highest value of overvoltage achieved on a longer length of the

transmission line when the fault level was set to 40kA during the fault and clearance

scenario. The simulations in which the circuit breaker energised the coupled line (with

the other circuit breaker remains open) yielded the following results.


Energisation of the coupled circuit (without -1pu trapped charge) occurred while live-

line work takes place on the live circuit. Tables 4-7 and 4-8 present the results of

energisation and re-energisation of 120km of different tower types.

Table 4-7: Overvoltage Results for Line Energisation

Tower Type

L2 L6 L8 L9 L12

P-E P-P P-E P-P P-E P-P P-E P-P P-E P-POvervoltage (pu) 2.42 3.31 2.43 3.36 2.40 3.35 2.42 3.36 2.39 3.33

Table 4-8: Overvoltage Results for Line Re-Energisation

Tower Type

L2 L6 L8 L9 L12


In the event of line disconnection, the live circuit remains live at all times. The parallel

circuit on the double circuit tower is then energised at one end by closing the circuit

breaker at the substation. Disconnection of the coupled circuit occurred while live-line

work took place on the live circuit. Disconnection took place at a random time on the

AC cycle with no significant scatter assumed between the closing times of the three

phases.

Table 4-9: Overvoltage Results for Line Dis-Connection

Tower Type

L2 L6 L8 L9 L12


As mentioned earlier, the overvoltage studies due to the fault and clearance scenario in

this thesis have been carried out in one of two ways;


1. 9600 runs of the overvoltage simulations have been carried out with each type of

fault being applied equally.

2. Another set of simulations has separated the runs for line-ground (LG), line-line

(LL), line-line-ground (LLG) and line-line-line (LLL) faults. In this case, it is

possible to generate an overvoltage profile based on an uneven mix of fault

types. The use of an 80% LG, 17% LL, 2% LLG and 1% LLL ratio is an

example of a more realistic distribution of fault type.

In the first method, an equal number of fault types at a random time were set. The

circuit breakers at each end of the overhead line were instructed to open 50ms after the

application of the fault with clearance taking place at the current zero following the open

instruction - refer to Table 4-10.

Table 4-10: Overvoltage Results for Fault & Clearance

Tower Type

L2 L6 L8 L9 L12


In the second method, simulation of different fault types were separated, and Table 4-11

produced to compare the influence of each type of fault with respect to the likelihood of

that fault type occurring within the simulation for both P-E and P-P voltages. The

results are based on an analysis of 120km L6 towers with a fault level at 40kA.

Table 4-11: Overvoltage Results for Fault & Clearance Due to Simulation Setting

FaultProbability:

Fault

Type

Total no

of runs

U2 (pu)

P-E P-P

Individualfault type

LG 2400 2.35 3.32LL 2400 2.42 3.41

LLG 2400 2.45 3.44LLL 2400 2.60 3.48

25% of each fault type(LG, LL, LLG AND LLL 9600 2.48 3.40


80% LG faults, 17% LL faults,2% LLG faults and 1% LLL faults 9600 2.37 3.21

As shown in Table 4-11, the U2 voltages for each fault type are different from the values

of the U2 voltages in the other two scenarios with a combination of entire overvoltage

distributions. P-E and P-P of U2 voltages are equal to 2.48pu and 3.40pu when an equal

number of fault type occurs in one complete set of simulations. These values are

reduced even further to 2.37pu and 3.21pu when the fault types are weighted according

to their likelihood of occurrence. This fact compares with values of 2.60pu and 3.48pu

if the U2 voltage is simulated for each individual fault type and the worst case is

selected.

Table 4-12: Overvoltage Results for Fault & Clearance Due to 80% LG Faults, 17% LLFaults,2% LLG Faults and 1% LLL Faults

Tower Type

L2 L6 L8 L9 L12


In the simulation of a line with compensation, reactive compensation was included

through the addition of either an inductance or capacitance on the busbar of the main

substation (substation A). For a 420kV line, the capacitive charging current of the line is

approximately 1A per km (0.25 MVAR per phase), and ,for instance, a 200km line

would typically require 40MVAr of shunt reactive compensation per phase depending

on system's operational requirement [3.10].

The reactive compensation had a value of 225MVAR (lagging or leading) which has

been taken as a maximum representative value from the National Grid Seven Year

Statement [3.30]. 75MVAR per phase at a phase voltage of 231kV gives a load

impedance of 711Ω which is equivalent to an inductance of 2.26H or a capacitance of

4.48µF at the compensation bank – refer to Tables 4-13 and 4-14.


Table 4-13: Overvoltage Results Due to Fault & Clearances with InductiveCompensation

Tower Type

L2 L6 L8 L9 L12


Table 4-14: Overvoltage Results Due to Fault & Clearances with CapacitiveCompensation

Tower Type

L2 L6 L8 L9 L12


The results from fault and clearance simulation of a transmission line with compensation

can also be modified for consideration of the uneven probability of fault type occurrence

as shown in Tables 4-15 and 4-16.

Table 4-15: Overvoltage Results Due to 80% LG Faults, 17% LL Faults, 2% LLG FaultsAnd 1% LLL Faults with Inductive Compensation

Tower Type

L2 L6 L8 L9 L12


Table 4-16: Overvoltage Results Due to 80% LG Faults, 17% LL Faults, 2% LLG Faultsand 1% LLL Faults with Capacitive Compensation

Tower Type

L2 L6 L8 L9 L12



4.6. Calculation of Minimum Approach

Distance

As an example, the transient overvoltage results listed in Table 4-10, section 4.5 (Fault

& Clearance) for the L6, 120km and 40kA fault current, are used to illustrate the

calculation procedure. It must be noted that, in the calculations presented here, the

ergonomic distance ‘DE’, the presence of a floating object and, thus, the floating object

distance ‘F’ are excluded. This implies that the factor kf =1.0.

a. Determination of U90

Ue2 = 3.40 puUsing Equation (2.6):Ue90 = 1.1 x 3.40 pu = 3.74 pu (= 1282.82 kV)Known values: kf = 1, ki = 1, ks = 0.936 and kg = 1.346

b. Determination of ka

The gap factor for L6 Tower was calculated, and it is equal to kg = 1.346. Using

Equations (2.14) to (2.19), Table 4-17 was constructed which presents the values of ka at

different altitudes and different voltage levels.

Table 4-17: Example Selection Table for ka

Altitude(m)

Range of U90 (kV)

<199 200-399 400-599 600-799 800-999 1000-1199 >1200

199 399 599 799 999 1199 12000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

100 0.988 0.990 0.992 0.994 0.996 0.997 0.999300 0.965 0.971 0.977 0.982 0.987 0.991 0.995500 0.942 0.952 0.962 0.970 0.978 0.985 0.991

1000 0.888 0.906 0.922 0.938 0.952 0.964 0.9761500 0.835 0.860 0.883 0.904 0.923 0.940 0.9572000 0.785 0.815 0.843 0.868 0.892 0.913 0.9332500 0.738 0.772 0.803 0.832 0.859 0.884 0.9063000 0.693 0.729 0.764 0.795 0.825 0.852 0.877


For the voltage Ue90 = 1282.82 kV, this falls in the range of >1200 kV. Given the

selection of a sea level altitude, this gives a value of ka = 0.991.

c. Determination of DA

After obtaining all the factors (kf = 1, ka = 0.991, ki = 0.95, kg = 1.346, ks = 0.936),

Equation (4.10) was used to calculate Kt:

Kt = kf ka ki kg ks (4.10)

Therefore, the correction factor is calculated and found to be equal to 1.186. Finally,

Knowing that Ue90 = 3.74 p.u (=1282.82 kV) and by letting the floating object distance

‘F’ and the ergonomic distance DE to be zero, Equation (4.11) explained in Chapter 2,

was used to calculate the DA:

D = 2.17 e ( )⁄ − 1 + F (4.11)

D = 2.17 e.

( . ) − 1 = 3.67m

Based on the above method, the minimum approach distances can be calculated to

correspond with the switching overvoltages in section 4.5. Tables 4-18 to 4-20 present

the electrical distances (Du) calculated based on the fault and clearance scenarios. The

clearances in Tables 4-18 to 4-20 are calculated based the selection of a 500m altitude, a

conservative value for the most locations in the UK, where kf = 1, ki = 0.95, kg = 1.346

and ks = 0.936. The values in the below tables are only concerned with the fault and

clearances scenarios as their overvoltages yield the highest values. The full calculated

electrical distances (Du) for all scenarios can be found in Appendix 3. The minimum

approach distances can be calculated by adding 0.4m (ergonomic distance) to the

calculated electrical distances (Du).


Table 4-18: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios (WithNo Ergonomic Distance DA)

Tower Type

L2 L6 L8 L9 L12P-E P-P P-E P-P P-E P-P P-E P-P P-E P-P

Minimum ApproachDistances (m) 2.36 3.65 2.38 3.73 2.34 3.71 2.37 3.73 2.31 3.67

Table 4-19: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios withInductive Compensation (With No Ergonomic Distance DA)

Tower Type



Table 4-20: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios withCapacitive Compensation (With No Ergonomic Distance DA)

Tower Type



As mentioned in section 4.5, U2 overvoltages can be affected by different fault type

configurations and as a result, the required minimum approach distance will be varied.

Tables 4-21 – 4-23 present the electrical distances (Du) due to an uneven mix of fault

types by use of 80% LG, 17% LL, 2% LLG and 1% LLL fault.

Table 4-21: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios with80% LG Faults, 17% LL Faults, 2% LLG Faults And 1% LLL Fault Probability (With

No Ergonomic Distance DA)

Tower Type




Table 4-22: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios withInductive Compensation With 80% LG Faults, 17% LL Faults, 2% LLG Faults And 1%

LLL Fault Probability (With No Ergonomic Distance DA)

Tower Type



Table 4-23: Electrical Distances (Du) for Fault & Clearance Simulation Scenarios withCapacitive Compensation With 80% LG Faults, 17% LL Faults, 2% LLG Faults And

1% LLL Fault Probability (With No Ergonomic Distance DA)

Tower Type



4.7. Influence of Atmospheric Conditions

As explained in Chapter 2, the atmospheric conditions such as pressure, temperature and

air density have an impact on the calculation of the minimum approach distance. The

tables in Appendix 3 present the influence of weather condition on the electrical

distances calculated in this investigation.

As explained earlier, changing air density and pressure has a larger effect on the

minimum approach distance than temperature. The results shown in Tables 4-24 – 4-26

are produced to present the effect of altitude on the electrical distances. However, for

the purpose of the investigation, the tables only present the electrical distances due to the

simulation of fault and clearance as it yields the higher magnitude of switching

overvoltages in comparison to the energisation, disconnection, etc.


Table 4-24: Influence of Altitude on Electrical Distances (Du) Due to Fault andClearances (Without Compensation) - With No Ergonomic Distance DA

Altitude (m)

Tower Type

Minimum Approach Distances (m)


0 2.28 3.60 2.30 3.68 2.26 3.66 2.29 3.68 2.24 3.62100 2.30 3.61 2.32 3.68 2.28 3.67 2.31 3.68 2.26 3.63300 2.33 3.63 2.35 3.71 2.31 3.69 2.34 3.71 2.29 3.65500 2.36 3.65 2.38 3.73 2.34 3.71 2.37 3.73 2.31 3.67

1000 2.45 3.74 2.47 3.82 2.43 3.80 2.46 3.82 2.40 3.761500 2.56 3.86 2.58 3.95 2.54 3.92 2.57 3.95 2.51 3.882000 2.69 4.02 2.71 4.11 2.66 4.09 2.70 4.11 2.64 4.042500 2.84 4.22 2.87 4.31 2.82 4.29 2.85 4.31 2.79 4.243000 3.02 4.45 3.05 4.55 2.99 4.53 3.03 4.55 2.96 4.47

Table 4-25: Influence of Altitude on Electrical Distances (Du) Due to Fault andClearances (Inductive Compensation) - With No Ergonomic Distance DA

Altitude (m)

Tower Type



0 2.24 3.54 2.27 3.63 2.21 3.60 2.25 3.63 2.20 3.56100 2.25 3.54 2.28 3.64 2.23 3.61 2.26 3.64 2.21 3.57300 2.28 3.56 2.31 3.66 2.26 3.63 2.29 3.66 2.24 3.59500 2.31 3.59 2.34 3.68 2.28 3.65 2.32 3.68 2.27 3.61

1000 2.40 3.67 2.43 3.77 2.37 3.74 2.41 3.77 2.36 3.701500 2.51 3.79 2.54 3.89 2.48 3.86 2.52 3.89 2.47 3.822000 2.64 3.95 2.67 4.06 2.60 4.02 2.65 4.05 2.59 3.982500 2.79 4.14 2.82 4.25 2.75 4.22 2.80 4.25 2.74 4.173000 2.96 4.36 3.00 4.49 2.92 4.45 2.97 4.49 2.90 4.40

Table 4-26: Influence of Altitude on Electrical Distances (Du) Due to Fault andClearances (Capacitive Compensation) - With No Ergonomic Distance DA

Altitude (m)

Tower Type



0 2.32 3.65 2.33 3.75 2.31 3.70 2.32 3.72 2.29 3.68100 2.33 3.65 2.34 3.75 2.32 3.70 2.33 3.72 2.30 3.69300 2.36 3.68 2.37 3.78 2.35 3.73 2.36 3.75 2.33 3.71500 2.39 3.70 2.41 3.80 2.38 3.75 2.40 3.77 2.36 3.74


1500 2.60 3.91 2.61 4.02 2.59 3.97 2.60 3.99 2.57 3.952000 2.73 4.07 2.75 4.19 2.72 4.13 2.73 4.16 2.70 4.122500 2.89 4.27 2.90 4.39 2.87 4.34 2.89 4.36 2.85 4.323000 3.07 4.51 3.08 4.64 3.05 4.58 3.07 4.60 3.03 4.56

Figures 4-20 presents the influence of transmission line length and fault level for L6

tower at 500m altitude on the minimum approach distance.

Figure 4-20: Top; P-E, Bottom; P-P. Influence of Length of Transmission Line on theMinimum Approach Distance

The drop at the highlighted area in Figure 4-20, is due to the tooth effect of bouncing

traveling wave on a short circuit between the circuit breaker and fault location. As this

bouncing current has a high voltage potential and a very short distance to travel, the

magnitude of the switching transient at this type of fault rapidly increases which causes a

need to larger minimum approach distances.


It is clear that increasing the fault level and the transmission line length increases the

magnitude of switching overvoltage. Figure 4-21 presents the influence of altitude on

the minimum approach distances when the 120km line is under consideration.

Figure 4-21: Top; P-E, Bottom; P-P - Minimum Approach Distance Influenced byAltitude and Fault Levels

4.8. Influence of Floating object on Minimumapproach distance

The minimum approach distance can be reduced by an introduction of tools, hanging

basket or equipment required for live-line working, into the air gap. Nevertheless, the

presence of the floating conductive object(s) reduces the net electrical length of the air

gap [IEC 61472]. According to IEC 61472, the minimum strength of the gap in the

presence of a floating conductive object can be estimated by use of Equation (4.12). In

Equations (4.12) –(4.14), the floating object correction factor is kf where Lf and F are the


overall length of an air gap and the maximum dimension of floating object along the gap

axis respectively.

U = 1080. ln(0.46(L − F) + 1) kV (4.52)

Therefore, the ninety percent withstand voltage (U90) of the gap, and minimum electrical

distance (DU) can be calculated as follow;

U = 1080. k . ln(0.46(L − F) + 1)(kV) (4.13)

D = 2.17 e . − 1 + F(kV) (4.14)If the length of floating in the axis of an air gap between the overhead lines to be

assumed as 2m, based on Table 1 of IEC 61472, the floating object correction factor will

be equal to 0.85. Therefore, the switching overvoltages obtained from simulation of

fault and clearance can be used to calculate the minimum approach distances for

different towers with a presence of floating object. These clearances are shown in

Tables 4-27 to 4-29 when the switching overvoltage for fault and clearances yields the

highest values. The electrical distances are calculated based on the assumption that the

floating object has a width of 2 meters between the air gaps. These calculations also

have been done base on 500m altitude.

Table 4-27: Electrical Distances for Fault & Clearance Simulation Scenarios at 500mAltitude With Floating Object With 2m Length in Direction of Phases (With No

Ergonomic Distance DA)

Tower Type

L2 L6 L8 L9 L12

P-E P-P P-E P-P P-E P-P P-E P-P P-E P-PMinimum Approach

Distances (m) 4.98 6.76 5.01 6.84 4.96 6.84 5.00 6.87 4.93 6.79


Table 4-28: Electrical Distances for Fault & Clearance Simulation Scenarios withInductive Compensation at 500m Altitude with Floating Object with 2m Length in

Direction of Phases (With No Ergonomic Distance DA)

Tower Type

L2 L6 L8 L9 L12


Distances (m) 4.92 6.67 4.96 6.80 4.89 6.76 4.94 6.80 4.87 6.71

Table 4-29: Electrical Distances for Fault & Clearance Simulation Scenarios withCapacitive Compensation at 500m Altitude with Floating Object With 2m Length in

Direction of Phases (With No Ergonomic Distance DA)

Tower Type

L2 L6 L8 L9 L12


Distances (m) 5.03 6.83 5.05 6.97 5.02 6.90 5.04 6.93 4.99 6.88

As shown in the Tables 4-27 – 4-29, the highest value of P-E electrical distance for L2,

L6, L8, L9 and L12 towers with a presence of floating object are 5.03m, 5.05m, 5.02m,

5.04m and 4.99m. These values for the P-P clearances are 6.83m, 6.79m, 6.90m, 6.93m

and 6.88m for each tower respectively.

Based on Equation (4.12), these values can be reduced or increased based on the

geometry of floating object i.e. 5.03m when the floating object is 2 meters wide or

4.03m and 6.03 when the floating object has a width of 1 and 3 meters respectively.


4.9. Discussion

The results from simulations in this section illustrate the effect of different parameters

on U2 overvoltage and as a result, on the minimum approach distances. By examining

the simulation results obtained from PSCAD, it is clear that some factors, such as

trapped charge on a transmission line, fault level and source configuration, transmission

line length and compensation, have a significant impact on the electrical clearances

while other factors such as altitude have less impact.

These results help the live-line operators to estimate the expected changes on the

minimum approach distances due to different parameters.

Also, it can assist the power system operator to estimate the magnitude of switching

transients due to various events. This fact can help them to process the required

preparation before a live-line working task. Also, it can be used to investigate the

feasibility of live-line operation at different sections of a transmission line. As shown in

the next section, the risk can also be estimated from the minimum approach distances of

an air gap, and this calculation can provide the operator with an extra safety assurance.

These studies illustrated that a longer length of transmission line causes a higher

magnitude of switching overvoltage, however, if the length of a line is very long, the

magnitude of switching transient can be decreased, and this phenomenon is due to the

high impedance of extra-long transmission line.

Simulation results indicate that there is little difference between the overvoltages seen on

L6 and L9 towers, whereas the magnitude of switching overvoltages for these two

towers are higher than those for the case of L2, L8 and L12 towers. A higher magnitude

of switching overvoltages in L6 and L9 could be due to higher line inductance as these

towers consist of a bundle of four overhead conductors.


Based on the simulation conditions in this work, fault and clearance scenarios yield the

highest value of switching overvoltages and, as a result, calculation of the minimum

approach distance was only base on transient simulation of fault and clearances. By

considering the results from all simulations, fault and clearance scenarios with a

presence of a capacitor bank(s) yield the highest magnitude of switching overvoltage.

At the same time, increasing the number or size of capacitor bank on transmission line

increases the maximum inrush current and reduces the overvoltage at the capacitor bank

station.

In practice, the system protection, load losses, transmission capacitance and many other

factors can reduce the overvoltages. However, these overvoltages can be controlled by

using the pre-insertion resistors and inductors, surge arresters or synchronous closing

and at the same time any of these methods can fail due to human error, under-sizing the

equipment, equipment contaminations, etc. [4.6].

While system parameters have a significant influence on the magnitude of transients

and, consequently, on the electrical clearances, atmospheric conditions have an impact

on the strength of the air gap. For instance, increasing the pressure due to higher altitude

decreases the voltage breakdown of the air gap and, as a result, larger minimum

approach distances will be required. As shown in Chapter 4, increasing the pressure due

to increasing altitude has more influence on the minimum approach distance than other

atmospheric factors.

And finally, this work is based on the examination of the switching overvoltages under

the worst case scenarios. As a result, the simulated overvoltages in this work are higher

than expected overvoltages in National Grid network. Also as in practice, the magnitude

of switching overvoltages in National Grid network is controlled by different protections

equipment therefore, the simulated results and the calculated minimum approach

distances in this work are very conservative.

CHAPTER 5. Live-line Working Risk Evaluation 131

CHAPTER 5

Live-line Working Risk Evaluation

5.1. IntroductionIn general, a workplace hazard is any possible potential damage or harm whether the

cause is the work materials, work method, the condition of work or equipment that can

affect someone’s health. They can be originated from different sources such as; knife,

benzene, electricity, wet floor, etc., whereas, a risk is a probability or chance, high or

low, that any source of hazards harms someone –refer to Figure 5-1.

Figure 5-1: Risk and Hazard Explanation [5.1]

Risk consists of two factors of severity and probability. Throughout this section, the

probability of risk of failure of an air gap's safety distance will be assessed. This

assessment is beneficial for the live-line operator to provide the lowest possible

likelihood of risk of failure. Also, the calculation in this section provides a better

understanding of the risk concern to the live-line workers. If the hazard and the risk are


measurable, therefore, by using a correct risk management process, they can be

controlled. Figure 5-2 illustrates the risk management cycle.

Figure 5-2: Risk Management Process

5.2. Live-line Working Risk Evaluation

As mentioned before, the risk in live-line working consists of two factors: severity and

probability. The probability factor takes into account the probability or likelihood of

occurrence of flashover in the gap where the live-line work takes place. This likelihood

or probability is compared against the estimated probability of failure of the minimum

approach distance’s calculation method.

As mentioned in previous Chapters, weakening the air gap strength by a higher

magnitude of stress due to transient overvoltage or certain atmospheric conditions can

cause a flashover. Therefore, the air gap insulation failure can be the result of either

higher switching overvoltage or lower air gap insulation strength.

Therefore, a wrong estimation of switching overvoltage or an air gap insulation strength

can cause failure of an air gap and, as a result, it can cause an accident during the live-

line working.

Furthermore, calculation of risk of failure of a transmission line is necessary as any

accident due to live-line working can result in a severe or fatal injuries. Although some

IDENTIFY

REVIEW

CONTROL

EVALUATE


world-known association such as IEC, CIER, CIGRE, OSHA, LWA, etc., actively work

on live-line working safety, but still according to UNIPEDE survey [5.2], there were 171

accidents and five fatalities due to live-line working.

Unfortunately, the statistics of the scope and type of accident in individual countries are

not unified, but live-line working accident and its fatal outcomes suggest the need for an

expert benchmarking research to examine the existing minimum approach distance and

associated risk with the method of calculation of these clearances.

The IEC 61472 method only produces a value of MAD, and it contains no way of

assessing the risk to a live-line worker. However, based on the IEC standards 61472,

following statements had a minimising effect on the overall risk of gap breakdown.

· The actual system voltage is not always at a maximum value;

· The location of the work is not likely to correspond to the place where a transient

overvoltage is at maximum value;

· The stress of the actual transient overvoltage wavefront is less than the critical

front;

· Approximately, half of the transient overvoltages will be of negative polarity,

and are less severe;

· The frequency and amplitude of transient overvoltages are reduced by restricting

re-closing of circuit breakers.

5.3. Risk AssessmentTo investigate and evaluate the risk of failure at the time of live-line working, the

following assumptions are set;

· Transient waves are divided into two categories: standards and non-standards

transient waves.


· The standard switching transients under investigation are those with time to crest

of 250µs and time to half value of 2500µs as stated in Table 3-7 of Chapter 3 of

this thesis.

· Any transient wave with a shape outside the above time classification (250/2500

µs) assumed to be non-standard waves.

· As this project used the IEC 61472 method for calculation of the minimum

approach distance, all the assumptions set by IEC 61472 and explained in the

previous section should be complied with.

5.4. Methodology for Risk Assessment(Standard Switching Transient)

In this method, simulation results from Chapter 4 are used and the assumption is made

that all switching transients are standard switching transients where the minimum

voltage breakdown of the gap has a time to crest equal/close to 250µs with a time to half

value of wave equal to 2500µs. Figure 5-3 presents the risk calculation methodology

applied in this project.

Figure 5-3: Live-Line Working Risk Evaluation Process

The IEC 61472 only produces a value of MAD, and it contains no way of assessing the

risk to a live-line worker. The probabilistic method used for calculation of the risk to a

Calculationof risk

involvedwith

particularlive-line

work

Evaluation

Time to crest,live-line

working time,lineman

position, faulttype, tower

type and etc.

Control

Calculation ofvoltage

breakdown ofeach particular

gap

Risk Review

Switchingovervoltage

measurement

RiskIdentification


live-line worker in this Chapter is based on stress–strength analysis. The probability of a

specific value of switching overvoltage is combined with the probability of gap failure

when that overvoltage is applied. This method is illustrated in Equation (5.1) where R is

the risk per event, Pb is the probability of a specific value of overvoltage and P0 is the

probability of the breakdown of a gap of a particular size for that particular voltage.

= ( ). ( ) (5.1)

5.4.1. Stress on the gapSwitching overvoltage values from each set of the simulation are used to obtain the

probability of switching overvoltage distribution. Microsoft Excel was used to process

the PSCAD results. An example of overvoltage distribution produced by the analysis of

the network model shown in Figure 4-19, is presented by the use of Figure 5-4. Figure

5-4 presents the overvoltage distribution caused by faults and clearance where the faults

took place on the coupled circuit of the overhead line. The overvoltage magnitude

ranges found to be from 2.11pu to 3.11pu (1pu being 343kV, the peak phase voltage of

the 400kV system).

Figure 5-4: Switching Overvoltage Distribution

0.00%0.20%0.40%0.60%0.80%1.00%1.20%1.40%1.60%1.80%2.00%

726.

073

774

875

977

078

179

280

381

482

583

684

785

886

988

089

190

291

392

493

594

695

796

897

999

010

0110

1210

2310

3410

4510

5610

67

Prob

abili

ty

Switching Overvoltage (kV)

S W I T C H I N G O V E R V O L T A G E D I S T R I B U T I O N


This overvoltage distribution is used to calculate the voltage that has a 2% probability of

being exceeded (U2). Then, the U2 value is converted to the required U90 that can be

applied to a gap with a ninety percent probability of withstand voltage. This voltage is

then used to calculate the MAD. The results from the distribution of switching

overvoltage and also associated minimum approach distance based on IEC 61472 are

presented in Table 5-1.

Table 5-1: Calculation Extracted from Simulation Results in Figure 5-4

Voltage Type Minimum (kV) Maximum(kV)

U2(kV) Std Deviation Du

(m)P-P 725.70 1071.16 1070.00 108.90 2.98

5.4.2. Strength of the gapIn order to calculate the risk, this project uses the IEC method for calculation of the

minimum approach distances. The flow chart in Figure 5-5 presents the steps and

influencing parameters in the calculation of gap strength.

Figure 5-5: Flowchart Illustrating the Steps Undertaken for Calculation of Gap Strength


As shown in Figure 5-5, the atmospheric condition affects the strength characteristics of

the gap (U50) and, as a result, U90 of the gap will be affected. As explained in Chapter 2

and in [5.3], the standard deviation (σ) decreases when the absolute humidity (h) and

relative air density (δ) increase and, as a result, the nature of standard deviation can be

expressed by Equation [5.2].

σCFO = [9.7 + 0.7(h − 11)]/(1 + δ) (5.2)

At this stage, the minimum approach distance value calculated in Table 5.1 will be used

to calculate the strength of the gap. The minimum approach distance of 2.98m is used to

obtain the strength of the gap shown in Figure 5-6. In Figure 5-6, the U50 voltage is

equal to 1175kV where the probability of flashover is 50%. This value is calculated

based on IEC equation as shown below:

U50= 1080 ln (0.46d +1) (5.3)

Equation (5.3), used by IEC, is based on the CRIPEI’s formula [5.5], which is more

complex than other equations used previously. This Equation is extracted from the

relationships between the gap length and U50 voltage (possibility of 50% sparkover

voltage) during various experimental tests. Some of these results are shown in Appendix

4.

Compared to other existing formulae, the CRIPEI’s formula is being adjusted for larger

air gaps, and it is closer to experimental results when smaller gaps are used. The

formula has been adopted by IEC standards and used and developed by many utility

companies as the fundamental formula for calculation of the minimum safety distance.

According to the IEC 60060-2 and [5.4], only the upper tail of switching overvoltage

distribution and lower tail of strength distribution up to a maximum of three standard

deviations are required for calculation of the risk. Therefore, the calculated U50


influenced by correction factors will be considered only within the range of ±3σ. Figure

5-6 presents the probability of voltage breakdown of a particular gap.

Figure 5-6: Air Gap Voltage Breakdown Probability

5.4.3. Intersection area

The probability of each specific set of switching overvoltages obtained in section 5.4.1 is

then combined with the probability of a gap failure for a particular gap size in section

5.4.2 when that overvoltage is applied.

Figure 5-7 illustrates the application of Equation (5.1) and the orange curve presents the

strength of the gap and the blue columns show the distribution of switching overvoltage

along the x-axis.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1000 1050 1100 1150 1200 1250 1300 1350

Prob

abili

ty

Air Gap Voltage Breakdown (kV)

U50


Figure 5-7: Combination of Air Gap Voltage Breakdown Probability and SwitchingOvervoltage Distribution

The probability of failure of the gap during a live-line work can be obtained from a

product of the probability of switching overvoltages at and beyond the U2 voltage and

the probability of the gap failure at those corresponding voltages. Table 5-2 below

presents the probability of switching overvoltage and gap failure for each corresponding

voltage during the above simulation.

Table 5-2: Minimum Approach Distance’s Risk of Failure Obtained from Probability ofAir Gap Breakdown and Switching Overvoltage Distribution

SwitchingOvervoltage

(kV)

No ofOccurrence

Probability ofSwitching

Overvoltage

Probability ofGap Strength

Failure

Risk ofFailure ofLLW Gap

726.0 1 0.0025 0 0727 0 0 0 0728 0 0 0 0729 1 0.0025 0 0730 1 0.0025 0 0731 1 0.0025 0 0

.

.

.

.

.

.

.

.

.

.

.

.0

1066 0 0 0.000943733 01067 1 0.0025 0.001038356 01068 1 0.0025 0.001141611 01069 3 0.0075 0.001254195 01070 5 0.0125 0.001376852 01071 5 0.0125 0.001510376 1.88797E-05


As shown in Table 5-2, the risk as a product of two probabilities; gap breakdown and

switching overvoltage, has a value equal to 1.88 × 10-05 at 1071kV. This value is

obtained as a result of multiplying the switching overvoltage probability and air gap

failure probability (0.0125 × 0.001510376). Although, at some voltages below 1071kV,

the probabilities for both switching and gap failure might have a value > 0 but, the

product of these probabilities causing no risk to the gap where the live-line takes place.

This is because the minimum approach distance for live-line working is calculated based

on U2 voltage and, as a result, the risk of failure is only considered for the switching

overvoltages that are ≥ U2 voltage.

In Table 5-2, the value of 1.88 × 10-05 correspondence to the risk of failure of the gap,

needs to be divided by two as this value contains the risk for both positive and negative

switching overvoltages. As explained in Chapter 1, in the case of live-line working,

only positive switching transients are considered because a lower positive polarity needs

to cause a flashover within a gap in comparison to the negative switching transient. As a

result, the actual value of risk for the air gap found to be equal to 9.40 × 10-06.

5.5. Methodology for Risk Assessment (Non-standard Switching Transient)

The steps undertaken to calculate the risk of non-standard switching transient are similar

to the procedure explained in the previous section. However, the probability of the

breakdown of a gap needs to be estimated based on the transient wave shape.

Based on the calculated MAD and assuming the worker is operating at this minimum

approach distance, the risk associated with the full overvoltage distribution can be

estimated. This estimation is carried out using equations that relate the gap sizes and


voltage breakdowns of the gaps as a function of time to crest. These equations are based

on data for rod-plane gap sparkover with positive polarity extracted from Table 5-1 of

[5.5]. Table 7-54 in Appendix 5 presents these Equations for different gap size as the

function of time-to-crest. As an example, Equation (5.4) is applied for calculation of U50

of any gap size within the range of 1m-10m with a time to crest of 50µs.

U = −1.9641 × d + 17.854 × d + 243.08 × d + 189.47 (5.4)

The U90 of the gap is then obtained by multiplying the U50 by a correction factor Kt to

form the ninety percent statistical withstand voltage of the gap as explained in Chapter 2.

The U90 and U50 are then used to obtain the probability of voltage breakdownof the gap.

Table 5-3 shows the risk involved with live-line working at the calculated MAD of

2.98m shown in Table 5-1 for both standard and non-standard switching overvoltages.

These values are for a worker within a phase to earth gap.

Table 5-3: Estimation of Risk Based on Transient Time-to-Crest

Case Risk of flashover per overvoltageevent

50% positive voltages / standard time-to-crest 1.88 × 10-6

50% positive voltages / 50 µs rise time crest 3.29 × 10-9


50% positive voltages / 200 µs rise time crest 8.76 ×10-7

50% positive voltages / 250µs rise time crest 9.13 × 10-6


Figure 5-8 presents, the calculated risk as the function of time to crest for the same gap

size under switching overvoltage distribution shown in Figure 5-4. As shown in the

Figure 5-8, the surge with a time to crest equal to 250 µs has the highest probability of

failure with a value equal to 9.13 × 10-6. As explained previously, this is because the


gap has the lowest voltage breakdown at the bottom of U-curve where the surges’ time

to crest is around 200-250µs, at the bottom of U-curve.

Figure 5-8: Risk as the Function of Time to Crest

The risk calculated based on time to crest can also be influenced based on length of

transmission line, tower type and also other system influencing factors. In the next

section, risks are calculated for each simulation result presented in Chapter 4.

5.6. Evaluation of Risk Based on Simulation

Results

Anticipated risks involved with live-line working in this section are based on simulation

results shown in Tables 4-18 to 4-23 of Chapter 4 where the maximum switching

overvoltages yield the highest value due to fault and clearance of a model with and

without compensation. The risk is purely estimated based on simulation results for each

particular network where positive overvoltages with different time to crest have

participated in risk calculation.

0.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

9.00E-06

1.00E-05

50 75 100 125 150 175 200 225 250 275 300 350 400 450 500 550

Risk

ofFa

ilure

Time-to-crest (µs)


As shown in Tables 5-4 – 5-6, the risks are calculated for the worst case scenarios when

the fault and clearances happen on the 120km overhead line with 40kA fault level.

These risks can be even further reduced if the probability of each fault type is considered

in simulation method. The results suggest that the risk is higher when the worker is

performing a task in proximity to the shield wire (and hence vulnerable to phase to earth

overvoltages). The risk remains very close to 1 in 100,000 per overvoltage event.

However, if the ergonomic distance to be added to the calculated electrical distance in

previous sections, the total risk value would be lower than 1 in 100,000 per overvoltage

result.

Table 5-4: Calculated Risk for Fault & Clearance Simulation Scenarios

Tower TypeL2 L6 L8 L9 L12

P-E P-P P-E P-P P-E P-P P-E P-P P-E P-P

RISK6.57x10-6

1.16x10-6

4.84x10-6

9.89x10-6

4.96x10-6

6.51x10-6

4.14x10-6

4.35x10-6

6.75x10-6

9.63x10-6

Table 5-5: Calculated Risk for Fault & Clearance Simulation Scenarios with InductiveCompensation



RISK1.55x10-6

1.00x10-6

1.43x10-6

1.57x10-6

3.60x10-6

2.97x10-6

2.18x10-6

1.43x10-6

3.43x10-6

4.56x10-6

Table 5-6: Calculated Risk for Fault & Clearance Simulation Scenarios with CapacitiveCompensation



RISK 1.78x10-6

1.37x10-6

9.08x10-6

1.21x10-6

2.24x10-6

1.48x10-6

3.99x10-6

1.13x10-6

7.09x10-6

1.21x10-6

Tables 5-7 to 5-10 present the impact of changing the simulation method and

distribution of switching overvoltage on the calculated risk. The Tables 5-7 – 5-9


present the risk involved with live-line working when the weighted type of fault are

considered.

Table 5-7: Calculated Risk for Fault & Clearance Simulation Scenarios with 80% LGFaults, 17% LL Faults, 2% LLG Faults And 1% LLL Fault Probability



RISK 6.05x10-6

1.13x10-6

4.36x10-6

9.60x10-6

4.47x10-6

5.93x10-6

4.10x10-6

4.18x10-6

6.42x10-6

8.86x10-6

Table 5-8: Calculated Risk for Fault & Clearance Simulation Scenarios with InductiveCompensation with 80% LG Faults, 17% LL Faults, 2% LLG Faults and 1% LLL Fault

ProbabilityTower Type


RISK1.41x10-6

9.35x10-6

1.32x10-6

1.52x10-6

3.35x10-6

2.76Ex10-6

2.12x10-6

1.33x10-6

3.09x10-6

4.11x10-6

Table 5-9: Calculated Risk for Fault & Clearance Simulation Scenarios with CapacitiveCompensation with 80% LG Faults, 17% LL Faults, 2% LLG Faults and 1% LLL Fault

Probability



RISK1.77x10-6

1.36x10-6

8.36x10-6

1.15x10-6

2.09x10-6

1.47x10-6

3.60x10-6

1.09x10-6

6.88x10-6

1.13x10-6

Figure 5-9 compares the risk of failure for the calculated electrical distance presented in

Table 4-17. The figure compares the risk for both P-E and P-P voltages where the time

to crest of the surges are varied. As it shown, the highest value of the risk for all type of

towers is when the surge has a time to crest value around 200-250µs.


Figure 5-9: Risk of Failure as a Function of Time to Crest on Different Towers for Top:P-E and Bottom: P-P Voltage

The risk of failure caused by changing the length of the gap for one set of switching

overvoltage distribution is shown in Figure 5-10. The calculated length of the gap based

on IEC method found to be ~1.63m for the set of the simulation model.

0.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

50 75 100 125 150 175 200 225 250 275 300 350 400 450 500 550

Risk

ofFa

ilure

Time-to-crest (us)

L2L6L8L9L12

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

50 75 100 125 150 175 200 225 250 275 300 350 400 450 500 550

Risk

ofFa

ilure

Time-to-crest (us)

L2

L6

L8

L9

L12


Figure 5-10: Risk of Failure for P-E Voltage as the Function of Changing the Gap Size,Bottom: The Zoom in Graph of the Top Graph

-1.00E-04

2.40E-03

4.90E-03

7.40E-03

9.90E-03

1.24E-02

1.49E-02

1.74E-02

1.99E-02

1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2

Risk

ofFa

ilure

Air Gap (m)

P-E Risk of Failure

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

1.00E-04

1.55 1.57 1.59 1.61 1.63 1.65 1.67 1.69

Risk

ofFa

ilure

Air Gap (m)

Risk of Failure: 2.63E-05 at 1.6346m gap


5.7. Discussion:

The risk to the live line workers in this project, was evaluated based on the presumption

of occurrence of the highest magnitude of overvoltage at the same location as line

workers. This assumption provides pessimistic results in the calculation of the risk

involved with live-line working.

This risk can be reduced even further, as the position of the linesmen and a fault at each

part/section of a network might be different. The calculated risks show a lower value of

risk involved with live-line working when there is a consideration of fault type

probability in the simulation method. Consideration of uneven fault distribution is a

more realistic assessment of the risk in live-line working and as it shown, the risk clearly

influenced by the probability distribution of fault types.

As the risk of a fault and clearance event is higher than the risk during an energisation,

re-energisation or disconnection events, the values shown in all tables within this chapter

are based on the fault and clearances scenario. These results yield a conservative value

of risk for the purpose of live-line working. The results in Table 5-3 and Figure 5-9

present the influence time to crest on the calculated risk. The highest risk remains in all

towers when the time to crest is at the critical value while lower risks exist for

waveforms with time to crests outside of this value. Table 5-10 presents the rate of

change of the risk for different wave shapes for both P-E and P-P clearances at L6

Tower. For example, in the case of P-E risk, by changing the time to crest of the

transient 50µs to 100µs, 100µs to 200µs and 200µs to 250µs, the risk values increase

and equal to 6.88 x 10-9, 5.45 x 10-07 and 4.29 x 10-6 respectively.

Also for the same P-E risk values, if time to crest of the transient changes from 250µs to

300µs or 300µs to 450µs, the values of risk will be reduced to -4.27 x 10-6 and -5.70 x

10-7. Table 5-10 clearly illustrates the influence of the change of wave shape on the


magnitude of the risk involved with live-line working. Throughout the study, the risk

has been predicted to change dramatically, due to various transient times to crest.

RiskTime to Crest (µs) 50 to100 100 to 200 250 to 250 250 to 300 300 to 450

P-E 6.88 x 10-9 5.45 x 10-7 4.29 x 10-6 -4.27 x 10-6 -5.70 x 10-7

P-P 1.40 x 10-8 1.11 x 10-6 8.77 x 10-6 -8.72 x 10-6 -1.16 x 10-6

Table 5-10. Rate of Change of the Risk Due to Change of Wave Time to Crest

REFERENCES 149

CHAPTER 6

Conclusion and Further Works

6.1. ConclusionThis thesis assessed the impact of the various variables within a particular section of a

transmission line on the magnitude of switching overvoltages and relative minimum

approach distances. These assessments were based on the simulation of switching

overvoltages, using 400kV transmission line model in PSCAD. These simulations only

considered switching overvoltages, as live-line working only takes place in good

weather conditions and, as a result, lightning overvoltages are irrelevant.

This thesis also takes into account the method used by other standards, and it has been

found that the IEC method provides a more comprehensive approach compared with the

IEEE method. The IEEE method does not take into account the altitude below 900m

whereas, in the IEC method, the effects of different parameters such as altitude, weather

conditions (temperature, humidity and pressure) and also the effects of a broken

insulator, floating objects have been considered.

According to the conducted calculations in this thesis and also based on the IEC

approach, it has been assumed that the highest magnitude of overvoltage occurs at the

same location as the line workers. This value provides pessimistic results in the

calculation of the risk involved with live-line working. The magnitude of switching

transient can be reduced due to travelling surge along the transmission line. Therefore,

REFERENCES 150

the position of linesmen and also an average number of occurrence of a fault within one

part or section of a network can change the risk value.

Consideration of existing surge arresters along the transmission lines, also the existence

of protection devices, circuit breakers and switches within transmission network can

reduce/ control the magnitude of switching overvoltages, and, as a result, they lessen the

value of the risk involved with live-line working. However, due to a very high

importance of safety, the primary consideration of this research was to ignore all the

possible limiting conditions and consider the worst case scenario that an incident could

happen. Any incident could happen due to, human error, equipment failure, atmospheric

contamination (i.e. surge arrester failure), equipment ageing and deploying the wrong

equipment (under sizing), etc., which can alter or even abandon the performance of

protection devices.

This work is based on the examination of the switching overvoltages under the worst

case scenarios. As a result, the simulated overvoltages in this work are higher than

expected overvoltages in National Grid network. As in practice, the magnitude of

switching overvoltages in National Grid network is controlled by different protections



This work is based on the examination of the switching overvoltages under the worst

case scenarios. As a result, the simulated overvoltages in this work are higher than

expected overvoltages in National Grid network. Also as in practice, the magnitude of

switching overvoltages in National Grid network is controlled by different protections



Therefore, the minimum approach distances calculated by IEC 61472 method and used

by National Grid are adequate and can be applied. The minimum approach distances

REFERENCES 151

used by National Grid have a very small risk below one out of 100,000 events which is

even less than accepted risk of flashover within an airgap clarified by the IEC standards.

6.2. Impact of different Parameters onMinimum Approach Distance

During simulation of different sources of overvoltage, the fault and clearance scenario

was found to have the highest magnitude of switching overvoltages compared to

energisation, re-energisation and disconnection events. It also has been found that the

magnitude of switching overvoltage is higher when the transmission line is connected to

a capacitive compensation bank(s). Therefore, the results of simulations shown in all

tables within the main body of the thesis are based on simulation of fault and clearance

scenarios on transmission lines.

Results from the simulation of events such as energisation, re-energisation and

disconnection could be very pessimistic as in this project, the circuit breaker opening

and closing occur at a random/ sequential time within a time window of 20ms.

However, in reality, circuit breakers operation are manually controlled, and they are

operated (opening/ closing) at a particular time along the system voltage wave. This

method of operation can reduce the magnitude of switching overvoltage due to circuit

breaker closuring time.

The results show that overvoltages are more likely to yield higher values in the case of

L6 and L9 towers. The L6 and L9 towers’ conductors have bundles of four ACSR Zebra

whereas L2, L8 and L12 towers have bundles of two conductors – refer to Appendix 2.

Therefore, the line inductance and capacitance of L6 and L9 transmission lines are

higher than other towers under investigation in this thesis. Therefore, the magnitude of

REFERENCES 152

switching overvoltages was found to be higher and, as a result, the minimum approach

distance required for these types of overhead lines were larger.

Increasing the length of transmission line increases the magnitude of switching

overvoltage along the line and, as a result, the required minimum approach distance

would be larger. It has also been demonstrated that increasing both fault level and

transmission line length increases the magnitude of switching overvoltages in all

simulation cases and, as a result, higher fault level and longer transmission line yield the

highest switching overvoltage.

External influencing parameters found to have a direct impact on the strength of the gap

and, as a result, they have a bearing on the minimum approach distance. For an

instance, the humidity will increase the voltage breakdown of the air gap, whereas

increasing the altitude decreases the strength of the gap and its voltage breakdown.

It has been found in this thesis work and other literature reviews that altitude has more

influence on the minimum approach distance than other atmospheric conditions and

more likely dictates the voltage breakdown of a gap where live-line working takes place

at various altitudes. This is due to the changing of the pressure at different altitudes.

Therefore, decreasing the pressure due to increasing the altitude decreases the voltage

breakdown of the gap and, as a result, a smaller magnitude of switching overvoltage is

required to form a flashover within the gap.

Therefore, the minimum approach distance will be increased as a result of increasing the

altitude or decreasing the pressure. The results from calculations performed in this work

shows 12% difference in the minimum electrical distances when the altitude changes

from the sea level to 1000m for voltages lower than 500kV. At the same time, by

increasing the voltage, this difference reduced to 4% for voltages above 900kV.

A broken/ contaminated insulator can reduce the strength of the gap and the result; a

smaller voltage breakdown will be required to form a flashover within a gap. Therefore,

REFERENCES 153

a larger minimum approach distance will be required for such scenarios. This fact can

be true when the live-line working takes place at the tower.

The simulation of transients on transmission line illustrates that when the time to crest of

transient overvoltages are around 200µs to 300µs, the voltage breakdown of the gap is at

the lowest value. The IEC method for calculation of U50 of the gap calculates the

electrical distance where the gap is at the venerable time. In other words, the method

used by IEC calculates the worst case scenario for all gap sizes. The suggested method

used for calculation of the risk of failure in this thesis confirms that the IEC method is

adequate for all switching transient wave shape and illustrates the validity of the method

for all the gap sizes.

This thesis has illustrated a framework that could be used to assess the risk to a live-line

worker at the time of a switching event. It is not proposed that this method replaces

other international standards, but it could be of use in many situations including where

utility companies wish to develop a complete understanding of the risk associated with

live-line working.

However, the new proposed method in Chapter 5 can be applied to calculate the risk

based on different wave shapes and peak voltages.

Finally, based on the simulation of a particular section of the network and a suggested

method for calculation of the risk, the minimum approach distances using the IEC

approach founded to be very conservative and adequate with a low-risk value.

6.3. Future Work· Although the work presented within this thesis has fulfilled all of the research aims,

nevertheless, due to the vast application of live-line working, there are some areas

where this research could be extended.

REFERENCES 154

· This thesis has explained the method structure and also the calculation of the risk

involved with the minimum approach distance obtained from the IEC method.

However, both calculation methods can be applied for voltage range between the

72.5kV and 800kV. Therefore, the same method of calculation can be implemented

for 275kV, 500kV and 750kV (transmission level).

· Same calculation method can be applied to investigate the minimum approach

distances at substations. However, the gap factor (kg) values are different for each

scenario.

· The impacts of climate change can be considered and need to be studied. It would

be valuable to extend the research to examine the effects of climate change on the

current thermal capacity of cables and overhead lines, performances of transformers,

surge arrester, insulators, circuit breakers, etc. As the magnitudes of switching

overvoltages and also the probability of insulator failure can be directly affected by

the climate change/ atmospheric conditions, further investigation may be possible.

· The work can also be extended to renewable sections as some renewable sources

such as Batteries can have various export capacities. For instance, the output of a

battery farm could change from a maximum export to a maximum import with

massive voltage step change in just a few millisecond and, as a result of changing

power flow direction, the minimum approach distances can be affected.

· Throughout this project, the position of the linesmen assumed to correspond to the

location of the maximum switching overvoltage/ U2 voltage whereas, in practice, the

maximum switching overvoltage could appear hundreds of kilometres away.

Therefore, the overvoltage to be seen at the linesmen's location might be lower than

the actual maximum overvoltage and, as a result, a smaller minimum approach

distance would be required. Therefore, the risk and the minimum approach distance

might need to be calculated by considering the location of linesmen and the fault

within the system.

REFERENCES 155

· Distribution of fault type used by this project is based on [4.8] whereas, further

investigation can be conducted to estimate the fault type probabilities at different

transmission levels. This information needs to be obtained from power system

operator for each part of the network.

· Furthermore, the value of the risk can vary if the statistics of the fault events, the

number of faults occurrence and key parameters of the network under study is

available.

REFERENCES 156

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1.9 Y. Li, J. He, J. Yuan, C. Li, J. Hu and R. Zeng, "Failure Risk of UHV AC

Transmission Line Considering the Statistical Characteristics of Switching

Overvoltage Waveshape," in IEEE Transactions on Power Delivery, vol. 28, no.

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on Power Delivery, vol. 24, no. 4, pp. 2434-2440, Oct. 2009.

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Switching Overvoltage Waveshape—Part II: Application and Results," IEEE

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1.13 Live-Line Work Using Hot sticks s, Available at: http://ramelex.com/

1.14 Live-Line Work Bare Hand or Potential Method, Available at:

http://www.cablejoints.co.uk/

1.15 Single pick robotic arm, Available at: http://quanta-technology.com/

1.16 Single pick robotic arm, Available at:

http://www.cce.umn.edu/documents/CPE-Conferences/MIPSYCON-

PowerPoints/2015/DSI, Robots for Energised Transmission LineWork.pdf

2.1 “Live Line Working, Minimum Safe Working Distances on 275 and 400kV

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Appendix 166

7. Appendices

Appendix 1:

The standard proposes a range of voltages to construct a Table in which there is a range

of values for ka which varies with altitude 0 m -3000 m. With the use of the Equations

in Chapter 2 and shown below, Table 56 can be constructed in which for each value of

altitude and range of voltages a value of ka exists:

1500 1

3

OO

g

Ug

kd

=-é ù

+ê úë û

UO = Voltage breakdownunder standard conditions (kV, at a temperature of 20oC, at a

pressure of 101.3kPA and at a humidity of 11g/m3)

gO = Undefined factor used in the calculation of ka

1. Calculate:

( )1.61 0.81.4

1 0.2O

gO

gT k

g-

=-

T = Undefined factor used in the calculation of ka

2. Calculate:

( )8150H

O

p epd-æ ö= =ç ÷

è ø

δ = Relative air density

p = Actual pressure (kPa)

H = Height above sea level (m)

This makes an assumption that δ is affected by mean pressure only and not by

temperature.

Appendix 1673. Finally, calculate ka by:

( )( ) ( )( )

0.8 1 1 0.2/ 0.2

1 0.2O

a OO

gk U U

gd d+ - -é ùë û= = +

-

Altitude(m)

U90 Voltage (kV)

<199 200-399 400-599 600-799 800-9991000-1199 >1200

0 1 1 1 1 1 1 1100 0.988 0.99 0.992 0.994 0.996 0.997 0.999300 0.965 0.971 0.977 0.982 0.987 0.991 0.995500 0.942 0.952 0.962 0.97 0.978 0.985 0.991

1000 0.888 0.906 0.922 0.938 0.952 0.964 0.9761500 0.835 0.86 0.883 0.904 0.923 0.94 0.9572000 0.785 0.815 0.843 0.868 0.892 0.913 0.9332500 0.738 0.772 0.803 0.832 0.859 0.884 0.9063000 0.693 0.729 0.764 0.795 0.825 0.852 0.877

Table 7-1: Atmospheric Factor ka for Different Reference Altitudes and Values of U90_

(IEC 61472)

The average of ka can be assumed to be according to Table 7-2 as below:

Altitude(m)

ka

average

0 1.000100 0.995300 0.983500 0.972

1000 0.9411500 0.9092000 0.8752500 0.8413000 0.805

Table 7-2: Average ka Values IEC 61472

Table 7-3 presents a simplified criterion for the kf determination independence of β and

Lf. The kf values are derived from the interpolation of the data shown in Annex F of IEC

61472. Table 7-3 contains the values of β in function of the original gap length Lf rather

than in function of the remaining air gap length D because the original gap length L f is

one of the important quantities that characterise the constructed a.c. system.

Appendix 168

Table 7-3: Floating Conductive Object Factor kf

Appendix 169

Appendix 2:

Below Table presents the conductor coordinates for L2, L6, L8, L9 and L12 used by

National Grid UK. These coordinates are used for simulation and construction of

models throughout this thesis.

Table 7-4: Conductor Coordinates (Including Sag) for Overhead Line Designs [2.1]

Figure 7-1: Conductor Coordinates of Overhead Line- Refer to Table 59

Appendix 170

Table 7-5: PSCAD Configuration of L2 Tower




Cond. #Phasing # Phasing #

44.040.0 [m]Eliminated1123

19.47-6.09 11.63-5.71-5.48 27.241

23

Connection

4 5.48 27.2456

564

5.716.09

19.4711.63

X (from X (fromtower centre)

GW. # Connection

Tower: L2 TowerConductors: Quad Zebra Ground_Wires: Zebra

Tower Centre 0.0 [m]

Ytower centre) (at tower)

Y(at tower)

0.3 [m]

Circuit #



21.79-6.93 [m] 32.26

-10.16-8.33 12.951

23

Connection

4 8.33 12.9556

564

10.166.93

21.7932.26


GW. # Connection

Tower: L6TowerConductors: Quad Zebra Ground_Wires: Zebra



Y(at tower)

0.3 [m]

Circuit #



20.57-6.7 12.57-8.53-5.94 30.011

23

Connection

4 5.94 30.0156

564

8.536.7

20.5712.57


GW. # Connection




Y(at tower)

0.3 [m]

Circuit #



8.19-6.86 7.28-14.17-4.72 17.341

23

Connection

4 7.72 17.3456

564

14.176.86

8.197.28


GW. # Connection

Tower: L9 -TowerConductors: Quad Zebra Ground_Wires: Zebra



Y(at tower)

0.3 [m]

Circuit #

Appendix 171


Figure 7-2: PSCAD Fault Type and Time Selection Modules

Figure 7-3: PSCAD Overhead Line Model



21.50-7.12 12.80-9.12-6.30 30.801

23

Connection

4 6.30 30.8056

564

9.127.12

21.5012.80


GW. # Connection




Y(at tower)

0.3 [m]

Circuit #

MultipleRun

Ch. 1

Ch. 2

Ch. 3

V1

V2

V3

Ch. 4

Meas-Enab

.

.

.

1

FLTTime

F4 F5F3F2F1

Fault

TimedFaultLogicFault

1 2 3 4 5 6

Select Data

6 Channel Decoder

F6

1 2 3 4 5 6

Select Data

6 Channel Decoder

FType

FT4 FT5FT3FT2FT1 FT6

FType

BTIME

FLTLoc

0.05

D+

F +

L2PE2

L2PE1

L2PP1

L2PP2

L21A

L12_1

1

L12_1

1

L12_2

1

L12_2

1

L12_3

1

L12_3

1

L12_4

1

L12_4

1L21B

L21CL21D

L21E

L21F

L22A

L22B

L22C

L22D

L22E

L22F

L23A

L23B

L23CL23D

L23E

L23F

L24A

L24B

L24CL24D

L24E

L24F

L25A

L25B

L25C

L25D

L25E

L25F

L12_1L12_2 L12_3 L12_4

Appendix 172

Figure 7-4: P-E Calculation Design

Figure 7-5: P-P Calculation Modules

Max

BCD

EF

G

L21C

L21B

L21A

L22BL22A

L22C

MaxB

C D

EF

G

L23C

L23BL23A L2

4BL2

4AL2

4C

Max

AB

C

EF

G

L25BL25C

L25A

L2PE

1

Max

BCD

EF

G

L21E

L21D

L21F

L22DL22F

L22E

MaxB

C D

EF

G

L23D

L23EL23F L2

4EL2

4FL2

4D

Max

AB

C

EF

G

L25BL25C

L25A

L2PE

2

D +

F

-

L21A

L21B

D +

F

-

L21B

L21C

D +

F

-

L21C

L21A

Max

C

D

E F

L21PP1

D +

F

-

L22A

L22B

D +

F

-

L22B

L22C

D +

F

-L22C

L22A

Max

C

D

E F

L22PP1

D +

F

-

L23A

L23B

D +

F

-

L23B

L23C

D +

F

-

L23C

L23A

Max

C

D

E F

L23PP1

D +

F

-

L24A

L24B

D +

F

-

L24B

L24C

D +

F

-

L24C

L24A

Max

C

D

E F

L24PP1

L25AL25B

L25C

D +

F

-

L25B

D +

F

-

L25C

D +

F

-

L25A

Max

C

D

E F

L25PP1

Max

BC

D

E F

L2PP

1

Max

BC

D

E F

L2PP

2

L25PP1

L24PP1L23PP1

L22PP1

L21PP1

L25PP2

L24PP2L23PP2

L22PP2

L21PP2

Appendix 173

Appendix 3:

The study result of the fundamental model of a transmission line network in Chapter 4 is

shown by following Tables. These Tables are presenting the maximum U2 Overvoltages

due to the simulation of fault and clearance at sea level, 500m and 1000m altitudes. The

following Tables also present the result of simulation with and without floating object

with a length of 2m.

Case 3A: Without Floating Object

Tower Type FaultCurrent (kA)

Voltage (kV)P-E P-P P-E P-P P-E P-P P-E P-P P-E P-P

L210 2.27 3.22 2.31 3.22 2.35 3.25 2.38 3.27 2.42 3.2930 2.31 3.24 2.32 3.25 2.37 3.29 2.39 3.31 2.43 3.3340 2.35 3.26 2.36 3.27 2.39 3.31 2.42 3.33 2.46 3.35

L610 2.27 3.27 2.31 3.27 2.36 3.31 2.40 3.33 2.44 3.3630 2.35 3.34 2.33 3.33 2.39 3.34 2.42 3.36 2.46 3.3940 2.37 3.35 2.38 3.34 2.41 3.37 2.44 3.37 2.48 3.40

L810 2.35 3.27 2.35 3.27 2.37 3.29 2.40 3.31 2.42 3.3330 2.35 3.31 2.37 3.31 2.39 3.33 2.42 3.35 2.43 3.3740 2.37 3.31 2.40 3.32 2.42 3.35 2.44 3.38 2.45 3.39

L910 2.31 3.28 2.34 3.29 2.40 3.33 2.42 3.35 2.45 3.3630 2.33 3.31 0.00 3.31 2.42 3.36 2.45 3.37 2.46 3.3940 2.38 3.32 2.39 3.33 2.45 3.37 2.46 3.39 2.47 3.40

L1210 2.30 3.25 2.31 3.27 2.35 3.31 2.37 3.33 2.40 3.3430 2.31 3.27 2.32 3.28 2.37 3.31 2.40 3.34 2.42 3.3640 2.34 3.28 2.35 3.29 2.40 3.33 2.42 3.34 2.43 3.36

Table 7-10: Overvoltage Simulation Results for Fault and Clearance



L210 2.21 3.15 2.24 3.16 2.31 3.21 2.34 3.24 2.39 3.2630 2.24 3.18 2.26 3.20 2.32 3.24 2.34 3.27 2.40 3.2940 2.28 3.19 2.30 3.20 2.33 3.25 2.38 3.30 2.43 3.31

L610 2.21 3.21 2.25 3.21 2.31 3.26 2.37 3.29 2.42 3.3430 2.27 3.27 2.27 3.28 2.35 3.30 2.38 3.32 2.43 3.3640 2.30 3.29 2.32 3.29 2.36 3.32 2.39 3.34 2.45 3.37

L810 2.28 3.20 2.29 3.22 2.32 3.24 2.36 3.28 2.40 3.3130 2.28 3.23 2.31 3.25 2.33 3.29 2.37 3.31 2.41 3.3440 2.30 3.23 2.34 3.26 2.37 3.29 2.40 3.35 2.41 3.35

L910 2.24 3.22 2.28 3.23 2.35 3.27 2.38 3.31 2.43 3.3330 2.26 3.24 -- 3.25 2.37 3.30 2.41 3.33 2.43 3.3540 2.30 3.25 2.33 3.27 2.39 3.31 2.42 3.34 2.44 3.37

L1210 2.23 3.18 2.24 3.21 2.29 3.27 2.33 3.29 2.37 3.3230 2.24 3.20 2.27 3.22 2.32 3.26 2.36 3.30 2.39 3.3340 2.27 3.22 2.30 3.23 2.35 3.28 2.38 3.31 2.40 3.33

Table 7-11: Overvoltage Simulation Results for Fault and Clearance, InductiveCompensation

Appendix 174Tower Type Fault

Current (kA)Voltage (kV)


L210 2.35 3.29 2.37 3.27 2.40 3.31 2.41 3.31 2.45 3.3330 2.39 3.32 2.39 3.31 2.42 3.35 2.43 3.35 2.46 3.3640 2.42 3.34 2.43 3.32 2.44 3.36 2.47 3.37 2.49 3.38

L610 2.34 3.34 2.36 3.32 2.41 3.36 2.45 3.38 2.48 3.3930 2.42 3.41 2.39 3.39 2.45 3.39 2.46 3.41 2.50 3.4140 2.44 3.42 2.44 3.40 2.47 3.42 2.48 3.41 2.50 3.44

L810 2.42 3.34 2.42 3.33 2.41 3.35 2.43 3.35 2.46 3.3530 2.42 3.38 2.42 3.37 2.44 3.37 2.45 3.39 2.46 3.3940 2.45 3.38 2.47 3.38 2.48 3.39 2.48 3.42 2.48 3.41

L910 2.38 3.35 2.40 3.36 2.45 3.38 2.46 3.39 2.48 3.4030 2.39 3.39 0.06 3.37 2.46 3.41 2.49 3.42 2.49 3.4240 2.45 3.39 2.44 3.39 2.51 3.42 2.50 3.42 2.49 3.42

L1210 2.38 3.31 2.36 3.33 2.41 3.37 2.42 3.37 2.42 3.3730 2.38 3.33 2.39 3.35 2.41 3.36 2.45 3.38 2.46 3.3940 2.41 3.35 2.41 3.35 2.45 3.38 2.46 3.38 2.47 3.40

Table 7-12: Overvoltage Simulation Results for Fault and Clearance, CapacitiveCompensation


Minimum Approach Distance (m)P-E P-P P-E P-P P-E P-P P-E P-P P-E P-P

L210 2.04 3.38 2.08 3.39 2.14 3.44 2.17 3.47 2.22 3.5030 2.09 3.42 2.10 3.44 2.16 3.50 2.18 3.53 2.24 3.5640 2.13 3.45 2.15 3.46 2.19 3.53 2.23 3.57 2.28 3.60

L610 2.04 3.47 2.08 3.46 2.15 3.54 2.20 3.57 2.25 3.6230 2.13 3.58 2.11 3.57 2.19 3.59 2.23 3.62 2.28 3.6640 2.16 3.60 2.17 3.59 2.21 3.63 2.25 3.64 2.30 3.68

L810 2.13 3.47 2.14 3.47 2.16 3.50 2.20 3.54 2.23 3.5730 2.14 3.53 2.16 3.54 2.18 3.57 2.22 3.60 2.24 3.6340 2.16 3.53 2.20 3.55 2.23 3.60 2.25 3.65 2.26 3.66

L910 2.09 3.48 2.12 3.50 2.20 3.56 2.23 3.60 2.26 3.6230 2.11 3.54 0.00 3.53 2.22 3.61 2.26 3.64 2.28 3.6640 2.17 3.55 2.18 3.56 2.26 3.63 2.28 3.66 2.29 3.68

L1210 2.07 3.43 2.08 3.46 2.13 3.54 2.16 3.56 2.20 3.5930 2.09 3.46 2.10 3.49 2.16 3.54 2.20 3.58 2.23 3.6140 2.12 3.49 2.14 3.50 2.20 3.56 2.23 3.59 2.24 3.62

Table 7-13: Minimum Approach Distance for Fault and Clearance at Sea Level



L210 1.97 3.27 2.01 3.30 2.09 3.37 2.13 3.41 2.19 3.4430 2.00 3.32 2.03 3.36 2.10 3.41 2.13 3.46 2.20 3.5040 2.05 3.33 2.07 3.35 2.12 3.44 2.18 3.51 2.24 3.54

L610 1.97 3.36 2.02 3.37 2.09 3.44 2.16 3.49 2.23 3.5830 2.04 3.47 2.04 3.47 2.14 3.51 2.18 3.55 2.24 3.6240 2.07 3.49 2.10 3.49 2.15 3.54 2.19 3.59 2.27 3.63

L810 2.05 3.34 2.06 3.38 2.10 3.41 2.15 3.49 2.21 3.5330 2.05 3.40 2.09 3.43 2.12 3.50 2.17 3.54 2.21 3.5840 2.08 3.41 2.13 3.45 2.16 3.50 2.20 3.59 2.21 3.60

L910 2.01 3.38 2.05 3.39 2.14 3.47 2.17 3.52 2.24 3.5730 2.02 3.41 -0.04 3.43 2.16 3.51 2.21 3.56 2.23 3.5940 2.08 3.43 2.11 3.47 2.20 3.54 2.23 3.59 2.25 3.63

L1210 1.98 3.31 2.00 3.38 2.06 3.47 2.12 3.51 2.16 3.5530 2.00 3.35 2.04 3.38 2.10 3.44 2.15 3.52 2.19 3.5740 2.04 3.38 2.08 3.41 2.13 3.48 2.17 3.54 2.20 3.56

Table 7-14: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at Sea Level


Current (kA)Minimum Approach Distance (m)


L210 2.14 3.50 2.16 3.47 2.20 3.53 2.21 3.54 2.26 3.5630 2.19 3.55 2.18 3.53 2.23 3.59 2.24 3.60 2.28 3.6240 2.22 3.57 2.24 3.55 2.26 3.62 2.29 3.64 2.32 3.65

L610 2.12 3.58 2.15 3.54 2.22 3.61 2.27 3.65 2.30 3.6730 2.23 3.70 2.19 3.66 2.26 3.66 2.28 3.70 2.33 3.7140 2.26 3.72 2.25 3.68 2.29 3.72 2.31 3.71 2.33 3.75

L810 2.23 3.58 2.23 3.57 2.22 3.59 2.25 3.59 2.28 3.6130 2.22 3.65 2.23 3.64 2.26 3.64 2.27 3.66 2.27 3.6740 2.26 3.65 2.29 3.65 2.31 3.67 2.31 3.73 2.31 3.70

L910 2.18 3.60 2.20 3.61 2.27 3.66 2.27 3.67 2.30 3.6830 2.19 3.67 0.04 3.63 2.28 3.70 2.32 3.71 2.32 3.7340 2.26 3.66 2.25 3.67 2.34 3.73 2.33 3.72 2.32 3.72

L1210 2.17 3.54 2.15 3.57 2.21 3.63 2.23 3.64 2.23 3.6330 2.17 3.56 2.19 3.59 2.22 3.62 2.26 3.65 2.28 3.6740 2.22 3.60 2.22 3.61 2.27 3.64 2.28 3.65 2.29 3.68

Table 7-15: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at Sea Level



L210 2.11 3.43 2.15 3.44 2.21 3.49 2.24 3.52 2.29 3.5530 2.16 3.47 2.17 3.49 2.23 3.55 2.25 3.58 2.31 3.6140 2.20 3.50 2.22 3.51 2.26 3.58 2.30 3.62 2.36 3.65

L610 2.11 3.52 2.15 3.51 2.22 3.59 2.27 3.62 2.33 3.6730 2.20 3.63 2.18 3.62 2.26 3.64 2.30 3.67 2.36 3.7140 2.23 3.65 2.24 3.64 2.28 3.68 2.33 3.69 2.38 3.73

L810 2.20 3.52 2.21 3.52 2.23 3.55 2.27 3.59 2.30 3.6230 2.21 3.58 2.23 3.59 2.25 3.62 2.29 3.65 2.31 3.6840 2.23 3.58 2.27 3.60 2.30 3.65 2.33 3.70 2.34 3.71

L910 2.16 3.53 2.19 3.55 2.27 3.61 2.30 3.65 2.34 3.6730 2.18 3.59 0.00 3.58 2.29 3.66 2.34 3.69 2.36 3.7140 2.24 3.60 2.25 3.61 2.34 3.68 2.36 3.71 2.37 3.73

L1210 2.14 3.48 2.15 3.51 2.20 3.59 2.23 3.61 2.27 3.6430 2.16 3.51 2.17 3.54 2.23 3.59 2.27 3.63 2.30 3.6640 2.19 3.54 2.21 3.55 2.27 3.61 2.30 3.64 2.31 3.67




L210 2.03 3.35 2.07 3.37 2.16 3.42 2.20 3.46 2.26 3.4930 2.07 3.39 2.10 3.40 2.16 3.46 2.20 3.51 2.27 3.5540 2.12 3.37 2.14 3.40 2.19 3.48 2.25 3.56 2.31 3.59

L610 2.03 3.41 2.08 3.42 2.16 3.49 2.23 3.54 2.30 3.6330 2.10 3.52 2.10 3.52 2.20 3.56 2.25 3.60 2.32 3.6840 2.14 3.54 2.17 3.54 2.22 3.59 2.26 3.64 2.34 3.68

L810 2.11 3.39 2.12 3.43 2.17 3.46 2.22 3.54 2.28 3.5830 2.11 3.45 2.15 3.48 2.18 3.55 2.24 3.59 2.28 3.6340 2.15 3.45 2.19 3.50 2.23 3.55 2.27 3.64 2.28 3.65

L910 2.07 3.43 2.12 3.44 2.20 3.52 2.24 3.57 2.31 3.6230 2.08 3.46 -0.04 3.48 2.23 3.56 2.28 3.61 2.31 3.6540 2.14 3.48 2.18 3.52 2.26 3.59 2.30 3.64 2.32 3.68

L1210 2.05 3.39 2.07 3.42 2.12 3.52 2.18 3.56 2.23 3.6030 2.06 3.39 2.10 3.43 2.17 3.49 2.22 3.57 2.26 3.6240 2.11 3.43 2.14 3.45 2.20 3.53 2.24 3.59 2.27 3.61

Table 7-17: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 500m Altitude




L210 2.20 3.55 2.23 3.52 2.27 3.58 2.28 3.59 2.34 3.6230 2.26 3.60 2.25 3.58 2.30 3.64 2.31 3.65 2.35 3.6840 2.29 3.63 2.31 3.60 2.33 3.67 2.36 3.69 2.39 3.70

L610 2.19 3.63 2.22 3.59 2.29 3.66 2.34 3.70 2.38 3.7230 2.30 3.76 2.26 3.71 2.33 3.71 2.36 3.76 2.40 3.7640 2.33 3.77 2.32 3.73 2.36 3.77 2.38 3.76 2.41 3.80

L810 2.30 3.63 2.30 3.62 2.29 3.64 2.32 3.64 2.35 3.6630 2.29 3.70 2.30 3.69 2.33 3.69 2.34 3.71 2.35 3.7340 2.33 3.70 2.36 3.70 2.38 3.73 2.38 3.78 2.38 3.75

L910 2.25 3.65 2.27 3.66 2.34 3.71 2.35 3.72 2.37 3.7330 2.26 3.72 0.04 3.68 2.35 3.75 2.39 3.77 2.39 3.7840 2.33 3.72 2.32 3.72 2.41 3.78 2.40 3.77 2.40 3.77

L1210 2.24 3.59 2.22 3.62 2.28 3.68 2.30 3.69 2.30 3.6830 2.24 3.61 2.26 3.64 2.29 3.67 2.34 3.70 2.35 3.7240 2.29 3.65 2.29 3.66 2.34 3.69 2.35 3.70 2.36 3.74

Table 7-18: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 500m Altitude



L210 2.19 3.51 2.23 3.52 2.30 3.57 2.33 3.60 2.38 3.6430 2.24 3.55 2.25 3.57 2.32 3.64 2.34 3.67 2.40 3.7040 2.29 3.58 2.31 3.59 2.35 3.67 2.39 3.71 2.45 3.74

L610 2.19 3.60 2.23 3.59 2.31 3.68 2.36 3.71 2.42 3.7630 2.29 3.72 2.26 3.71 2.35 3.73 2.39 3.76 2.45 3.8040 2.32 3.74 2.33 3.73 2.37 3.77 2.42 3.78 2.47 3.82

L810 2.29 3.60 2.30 3.60 2.32 3.64 2.36 3.68 2.39 3.7130 2.30 3.67 2.32 3.68 2.34 3.71 2.38 3.74 2.40 3.7740 2.32 3.67 2.36 3.69 2.39 3.74 2.42 3.79 2.43 3.80

L910 2.24 3.61 2.27 3.64 2.36 3.70 2.39 3.74 2.43 3.7630 2.26 3.68 0.00 3.67 2.38 3.75 2.43 3.78 2.45 3.8040 2.33 3.69 2.34 3.70 2.43 3.77 2.45 3.80 2.46 3.82

L1210 2.22 3.56 2.23 3.59 2.29 3.68 2.32 3.70 2.36 3.7330 2.24 3.59 2.25 3.62 2.32 3.68 2.36 3.72 2.39 3.7540 2.27 3.62 2.30 3.64 2.36 3.70 2.39 3.73 2.40 3.76




L210 2.10 3.46 2.15 3.49 2.24 3.50 2.28 3.54 2.35 3.5730 2.14 3.51 2.17 3.48 2.25 3.54 2.28 3.60 2.35 3.6440 2.20 3.46 2.22 3.48 2.27 3.57 2.33 3.65 2.40 3.67

L610 2.10 3.49 2.16 3.50 2.24 3.57 2.32 3.63 2.38 3.7230 2.18 3.60 2.18 3.61 2.29 3.65 2.34 3.69 2.41 3.7740 2.22 3.63 2.25 3.63 2.31 3.68 2.35 3.73 2.43 3.77

L810 2.19 3.47 2.21 3.51 2.25 3.54 2.30 3.62 2.36 3.6630 2.19 3.54 2.24 3.57 2.27 3.63 2.32 3.67 2.37 3.7240 2.23 3.54 2.28 3.59 2.31 3.64 2.36 3.73 2.37 3.74

L910 2.15 3.51 2.20 3.52 2.29 3.61 2.33 3.66 2.40 3.7130 2.16 3.54 -0.04 3.57 2.31 3.65 2.37 3.70 2.39 3.7440 2.22 3.56 2.26 3.60 2.35 3.68 2.39 3.73 2.41 3.77

L1210 2.12 3.51 2.14 3.50 2.21 3.61 2.27 3.64 2.31 3.6930 2.14 3.47 2.18 3.51 2.25 3.57 2.30 3.66 2.35 3.7140 2.19 3.51 2.23 3.54 2.28 3.62 2.33 3.67 2.36 3.70

Table 7-20: Minimum Approach Distance for Fault and Clearance, Inductivecompensation at 1000m altitude




L210 2.29 3.63 2.32 3.61 2.36 3.67 2.37 3.68 2.43 3.7030 2.35 3.68 2.34 3.66 2.39 3.73 2.40 3.74 2.44 3.7740 2.38 3.71 2.40 3.69 2.42 3.76 2.46 3.78 2.48 3.79

L610 2.27 3.72 2.30 3.68 2.37 3.75 2.43 3.79 2.47 3.8130 2.39 3.85 2.35 3.80 2.42 3.81 2.45 3.85 2.49 3.8540 2.42 3.86 2.41 3.82 2.45 3.87 2.47 3.85 2.50 3.89

L810 2.39 3.72 2.38 3.71 2.38 3.73 2.41 3.73 2.44 3.7530 2.38 3.79 2.39 3.78 2.42 3.78 2.43 3.80 2.44 3.8240 2.42 3.80 2.45 3.79 2.48 3.82 2.47 3.87 2.48 3.84

L910 2.34 3.74 2.35 3.75 2.43 3.80 2.44 3.81 2.47 3.8230 2.35 3.81 0.04 3.77 2.44 3.84 2.48 3.86 2.49 3.8740 2.42 3.81 2.41 3.81 2.51 3.87 2.50 3.86 2.49 3.86

L1210 2.33 3.68 2.31 3.71 2.37 3.77 2.39 3.78 2.39 3.7730 2.33 3.70 2.34 3.73 2.38 3.76 2.43 3.79 2.45 3.8140 2.37 3.74 2.37 3.75 2.43 3.78 2.44 3.79 2.46 3.83

Table 7-21: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 1000m Altitude

Case 3B: Without Floating Object & Weighted Distribution of Fault Type



L210 2.08 2.93 2.14 2.97 2.20 3.01 2.24 3.07 2.30 3.1130 2.12 2.95 2.15 3.01 2.20 3.05 2.26 3.11 2.34 3.1340 2.13 2.96 2.17 2.99 2.23 3.06 2.29 3.12 2.36 3.17

L610 2.07 2.98 2.14 2.99 2.21 3.09 2.28 3.12 2.34 3.1930 2.12 3.06 2.14 3.07 2.25 3.12 2.30 3.15 2.35 3.2040 2.17 3.07 2.21 3.10 2.27 3.15 2.32 3.17 2.37 3.21

L810 2.15 2.99 2.17 3.02 2.22 3.07 2.27 3.11 2.31 3.1530 2.13 3.01 2.19 3.06 2.23 3.11 2.30 3.14 2.34 3.1940 2.17 3.03 2.22 3.07 2.29 3.13 2.30 3.17 2.34 3.21

L910 2.11 3.01 2.15 3.03 2.25 3.09 2.29 3.13 2.34 3.1930 2.12 3.03 -0.19 3.04 2.27 3.12 2.33 3.16 2.35 3.1940 2.18 3.04 2.19 3.07 2.28 3.13 2.34 3.18 2.37 3.22

L1210 2.10 2.98 2.13 3.00 2.19 3.08 2.24 3.11 2.31 3.1530 2.10 2.98 2.15 3.01 2.22 3.07 2.27 3.14 2.30 3.1940 2.14 3.01 2.18 3.04 2.23 3.09 2.29 3.14 2.34 3.19

Table 7-22: Overvoltage Simulation Results for Fault and Clearance & Weighted FaultType


Current (kA)Voltage (kV)


L210 2.03 2.86 2.10 2.90 2.16 2.94 2.21 3.02 2.27 3.0730 2.07 2.87 2.10 2.94 2.16 2.99 2.23 3.06 2.31 3.0940 2.08 2.89 2.12 2.93 2.18 3.00 2.25 3.06 2.33 3.13

L610 2.02 2.91 2.10 2.93 2.17 3.03 2.25 3.07 2.31 3.1430 2.07 2.99 2.09 3.00 2.21 3.06 2.26 3.10 2.32 3.1640 2.12 3.00 2.17 3.03 2.23 3.10 2.29 3.12 2.35 3.17

L810 2.10 2.92 2.12 2.95 2.18 3.02 2.23 3.06 2.28 3.1030 2.07 2.94 2.15 2.99 2.19 3.05 2.27 3.09 2.32 3.1540 2.12 2.96 2.17 3.01 2.25 3.08 2.27 3.12 2.31 3.17

L910 2.05 2.94 2.11 2.96 2.21 3.04 2.25 3.08 2.31 3.1530 2.07 2.96 -0.24 2.98 2.24 3.06 2.30 3.10 2.32 3.1540 2.13 2.97 2.15 3.01 2.24 3.07 2.31 3.13 2.35 3.18

L1210 2.05 2.91 2.09 2.94 2.15 3.02 2.21 3.06 2.29 3.1030 2.04 2.91 2.10 2.94 2.18 3.01 2.24 3.09 2.27 3.1540 2.09 2.94 2.13 2.98 2.19 3.04 2.25 3.09 2.32 3.15

Table 7-23: Overvoltage Simulation Results for Fault and Clearance, InductiveCompensation & Weighted Fault Type



L210 2.18 3.07 2.22 3.10 2.28 3.13 2.31 3.17 2.36 3.2030 2.22 3.09 2.23 3.13 2.29 3.17 2.32 3.21 2.38 3.2340 2.24 3.11 2.27 3.13 2.31 3.18 2.36 3.22 2.41 3.26

L610 2.17 3.13 2.22 3.13 2.29 3.20 2.34 3.23 2.39 3.2730 2.23 3.20 2.23 3.20 2.32 3.23 2.36 3.26 2.41 3.2940 2.27 3.21 2.29 3.22 2.34 3.26 2.38 3.27 2.43 3.31

L810 2.25 3.13 2.26 3.14 2.29 3.18 2.33 3.21 2.37 3.2430 2.24 3.16 2.28 3.19 2.31 3.22 2.36 3.25 2.39 3.2840 2.27 3.17 2.31 3.20 2.36 3.24 2.37 3.28 2.39 3.30

L910 2.21 3.14 2.25 3.16 2.33 3.21 2.36 3.24 2.39 3.2830 2.23 3.17 -0.10 3.18 2.34 3.24 2.39 3.27 2.41 3.2940 2.28 3.18 2.29 3.20 2.36 3.25 2.40 3.28 2.42 3.31

L1210 2.20 3.11 2.22 3.13 2.27 3.20 2.31 3.22 2.36 3.2530 2.21 3.12 2.23 3.15 2.29 3.19 2.34 3.24 2.36 3.2740 2.24 3.15 2.27 3.17 2.32 3.21 2.36 3.24 2.39 3.28

Table 7-24: Overvoltage Simulation Results for Fault and Clearance, CapacitiveCompensation & Weighted Fault Type



L210 1.81 2.93 1.88 2.99 1.96 3.05 2.00 3.15 2.07 3.2230 1.86 2.96 1.89 3.05 1.96 3.12 2.02 3.21 2.12 3.2540 1.87 2.98 1.92 3.03 1.99 3.13 2.06 3.22 2.15 3.31

L610 1.81 3.02 1.88 3.03 1.97 3.18 2.05 3.23 2.12 3.3330 1.86 3.13 1.88 3.14 2.01 3.22 2.07 3.28 2.14 3.3540 1.92 3.15 1.97 3.19 2.03 3.27 2.10 3.31 2.17 3.37

L810 1.89 3.03 1.92 3.06 1.98 3.15 2.03 3.22 2.09 3.2730 1.87 3.06 1.95 3.13 1.99 3.21 2.07 3.26 2.13 3.3440 1.92 3.08 1.98 3.15 2.06 3.25 2.08 3.31 2.12 3.37

L910 1.84 3.05 1.90 3.08 2.01 3.18 2.06 3.25 2.12 3.3430 1.86 3.09 -0.12 3.11 2.04 3.22 2.11 3.28 2.14 3.3440 1.93 3.10 1.95 3.15 2.05 3.24 2.13 3.32 2.17 3.39

L1210 1.83 3.00 1.87 3.04 1.94 3.17 2.00 3.21 2.09 3.2830 1.83 3.01 1.89 3.06 1.98 3.15 2.04 3.25 2.08 3.3340 1.88 3.06 1.93 3.10 1.99 3.18 2.06 3.26 2.13 3.34

Table 7-25: Minimum Approach Distance for Fault and Clearance at Sea Level &Weighted Fault Type

Appendix 179



L210 1.76 2.83 1.83 2.89 1.91 2.95 1.96 3.07 2.04 3.1530 1.81 2.85 1.84 2.95 1.91 3.02 1.99 3.13 2.09 3.1740 1.81 2.87 1.86 2.93 1.93 3.04 2.02 3.13 2.12 3.24

L610 1.75 2.91 1.83 2.93 1.92 3.09 2.02 3.15 2.09 3.2630 1.80 3.02 1.82 3.04 1.97 3.14 2.03 3.20 2.11 3.2840 1.86 3.04 1.92 3.09 1.99 3.19 2.06 3.23 2.14 3.30

L810 1.83 2.92 1.86 2.97 1.93 3.07 1.99 3.14 2.06 3.2030 1.81 2.95 1.89 3.03 1.94 3.12 2.04 3.18 2.10 3.2740 1.86 2.98 1.92 3.05 2.02 3.16 2.04 3.22 2.09 3.30

L910 1.78 2.95 1.85 2.98 1.97 3.09 2.02 3.16 2.09 3.2830 1.80 2.99 -0.15 3.01 2.00 3.12 2.07 3.20 2.11 3.2640 1.87 2.99 1.89 3.05 2.00 3.14 2.09 3.23 2.14 3.32

L1210 1.78 2.90 1.82 2.94 1.89 3.08 1.97 3.13 2.07 3.2030 1.77 2.90 1.84 2.96 1.93 3.06 2.00 3.17 2.05 3.2740 1.82 2.96 1.88 3.01 1.94 3.09 2.02 3.18 2.10 3.28

Table 7-26: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at Sea Level & Weighted Fault Type



L210 1.93 3.15 1.98 3.19 2.05 3.24 2.09 3.31 2.15 3.3630 1.98 3.18 2.00 3.24 2.06 3.30 2.10 3.37 2.18 3.4040 2.00 3.21 2.03 3.24 2.09 3.33 2.15 3.39 2.22 3.45

L610 1.92 3.24 1.98 3.24 2.06 3.36 2.13 3.40 2.19 3.4730 2.00 3.35 2.00 3.35 2.10 3.40 2.15 3.45 2.21 3.5040 2.04 3.37 2.07 3.38 2.12 3.45 2.18 3.47 2.24 3.52

L810 2.01 3.24 2.03 3.26 2.07 3.32 2.12 3.38 2.16 3.4230 2.01 3.29 2.05 3.33 2.08 3.38 2.15 3.43 2.19 3.4840 2.04 3.30 2.09 3.35 2.15 3.42 2.17 3.48 2.19 3.51

L910 1.97 3.26 2.01 3.29 2.11 3.37 2.15 3.42 2.19 3.4830 1.99 3.31 -0.06 3.31 2.13 3.41 2.19 3.46 2.21 3.5040 2.05 3.32 2.06 3.35 2.16 3.43 2.21 3.48 2.23 3.53

L1210 1.95 3.21 1.98 3.25 2.03 3.35 2.08 3.38 2.15 3.4330 1.96 3.23 2.00 3.27 2.07 3.34 2.12 3.41 2.16 3.4740 2.00 3.27 2.03 3.30 2.10 3.37 2.15 3.42 2.19 3.48

Table 7-27: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at Sea Level & Weighted Fault Type



L210 1.87 2.97 1.94 3.03 2.02 3.09 2.07 3.19 2.14 3.2630 1.92 3.00 1.95 3.09 2.02 3.16 2.09 3.25 2.19 3.2940 1.93 3.03 1.98 3.07 2.05 3.17 2.13 3.26 2.22 3.36

L610 1.86 3.06 1.94 3.07 2.03 3.22 2.12 3.28 2.19 3.3830 1.92 3.17 1.94 3.19 2.08 3.27 2.14 3.32 2.21 3.4040 1.98 3.19 2.03 3.23 2.10 3.32 2.17 3.36 2.24 3.42

L810 1.95 3.07 1.98 3.11 2.04 3.20 2.10 3.26 2.16 3.3130 1.93 3.10 2.01 3.17 2.05 3.25 2.14 3.30 2.20 3.3940 1.98 3.12 2.04 3.19 2.13 3.29 2.15 3.35 2.19 3.41

L910 1.90 3.09 1.96 3.12 2.08 3.23 2.13 3.29 2.19 3.3930 1.92 3.13 -0.13 3.15 2.11 3.26 2.18 3.33 2.21 3.3940 1.99 3.14 2.01 3.19 2.12 3.28 2.20 3.36 2.24 3.43

L1210 1.89 3.04 1.93 3.08 2.00 3.21 2.07 3.25 2.16 3.3230 1.89 3.05 1.95 3.10 2.04 3.20 2.11 3.30 2.15 3.3840 1.94 3.10 1.99 3.15 2.06 3.23 2.13 3.31 2.20 3.39

Table 7-28: Minimum Approach Distance for Fault and Clearance at 500m Altitude &Weighted Fault Type




L210 1.83 2.89 1.91 2.96 1.97 3.02 2.02 3.14 2.10 3.2230 1.88 2.91 1.92 3.02 1.96 3.09 2.05 3.20 2.16 3.2540 1.88 2.94 1.92 2.99 1.99 3.11 2.08 3.21 2.18 3.32

L610 1.82 2.97 1.91 2.99 1.98 3.16 2.08 3.22 2.15 3.3430 1.87 3.09 1.90 3.11 2.03 3.21 2.10 3.27 2.17 3.3640 1.92 3.11 1.97 3.16 2.05 3.26 2.13 3.31 2.20 3.38

L810 1.91 2.99 1.92 3.03 1.99 3.14 2.05 3.21 2.12 3.2730 1.88 3.01 1.95 3.10 2.00 3.19 2.10 3.25 2.17 3.3540 1.92 3.04 1.98 3.13 2.08 3.24 2.10 3.30 2.15 3.37

L910 1.86 3.01 1.92 3.05 2.03 3.17 2.08 3.24 2.15 3.3530 1.88 3.05 -0.16 3.08 2.06 3.20 2.14 3.27 2.17 3.3440 1.93 3.06 1.95 3.12 2.06 3.22 2.16 3.31 2.21 3.39

L1210 1.85 2.97 1.90 3.01 1.95 3.15 2.03 3.20 2.13 3.2730 1.84 2.96 1.92 3.02 1.99 3.13 2.07 3.25 2.11 3.3440 1.90 3.02 1.93 3.08 2.00 3.17 2.08 3.26 2.17 3.35

Table 7-29: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 500m Altitude & Weighted Fault Type



L210 1.99 3.22 2.04 3.26 2.11 3.32 2.15 3.39 2.21 3.4030 2.04 3.26 2.06 3.32 2.12 3.38 2.17 3.41 2.25 3.4540 2.06 3.29 2.10 3.32 2.15 3.37 2.21 3.44 2.29 3.50

L610 1.98 3.31 2.04 3.32 2.12 3.40 2.19 3.45 2.26 3.5230 2.06 3.40 2.06 3.40 2.17 3.45 2.22 3.49 2.28 3.5540 2.10 3.42 2.13 3.43 2.19 3.50 2.25 3.52 2.31 3.57

L810 2.07 3.32 2.09 3.34 2.13 3.37 2.18 3.42 2.23 3.4730 2.07 3.37 2.12 3.38 2.15 3.43 2.21 3.48 2.26 3.5340 2.10 3.38 2.15 3.39 2.21 3.47 2.23 3.52 2.26 3.56

L910 2.03 3.34 2.07 3.36 2.17 3.42 2.21 3.47 2.26 3.5330 2.05 3.39 -0.06 3.39 2.20 3.46 2.26 3.51 2.28 3.5540 2.11 3.37 2.13 3.40 2.22 3.48 2.28 3.53 2.30 3.58

L1210 2.01 3.29 2.04 3.32 2.10 3.40 2.15 3.43 2.22 3.4830 2.02 3.30 2.06 3.35 2.13 3.39 2.19 3.46 2.22 3.5240 2.06 3.35 2.10 3.38 2.16 3.42 2.21 3.47 2.26 3.53

Table 7-30: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 500m Altitude & Weighted Fault Type



L210 1.94 3.04 2.01 3.10 2.09 3.16 2.14 3.27 2.22 3.3430 1.99 3.07 2.02 3.16 2.09 3.23 2.17 3.33 2.27 3.3740 2.00 3.10 2.05 3.14 2.12 3.25 2.21 3.34 2.30 3.44

L610 1.93 3.13 2.01 3.14 2.10 3.30 2.20 3.35 2.27 3.4630 1.99 3.25 2.01 3.26 2.16 3.35 2.22 3.40 2.29 3.4840 2.05 3.27 2.10 3.31 2.18 3.40 2.25 3.44 2.32 3.50

L810 2.02 3.14 2.05 3.18 2.11 3.27 2.18 3.34 2.24 3.3930 2.00 3.17 2.08 3.25 2.12 3.33 2.22 3.38 2.28 3.4740 2.05 3.20 2.11 3.27 2.21 3.37 2.23 3.43 2.27 3.50

L910 1.97 3.16 2.03 3.19 2.15 3.30 2.21 3.37 2.27 3.4730 1.99 3.21 -0.13 3.22 2.19 3.34 2.26 3.41 2.29 3.4740 2.06 3.22 2.08 3.27 2.20 3.36 2.28 3.44 2.32 3.52

L1210 1.96 3.11 2.00 3.16 2.07 3.29 2.15 3.33 2.24 3.4030 1.96 3.12 2.02 3.17 2.11 3.27 2.19 3.38 2.23 3.4640 2.01 3.17 2.06 3.22 2.13 3.30 2.21 3.39 2.28 3.47

Table 7-31: Minimum Approach Distance for Fault and Clearance at 1000m Altitude &Weighted Fault Type




L210 1.92 2.98 2.00 3.06 2.04 3.12 2.10 3.25 2.18 3.3330 1.97 3.01 2.00 3.12 2.04 3.20 2.12 3.31 2.24 3.3540 1.97 3.04 1.99 3.09 2.07 3.21 2.16 3.31 2.27 3.43

L610 1.90 3.07 2.00 3.09 2.05 3.27 2.16 3.33 2.24 3.4530 1.96 3.19 1.99 3.22 2.11 3.32 2.18 3.38 2.25 3.4740 1.99 3.21 2.05 3.27 2.13 3.37 2.21 3.42 2.29 3.49

L810 2.00 3.09 1.99 3.13 2.06 3.25 2.13 3.32 2.20 3.3830 1.97 3.11 2.03 3.20 2.07 3.30 2.18 3.36 2.25 3.4640 1.99 3.14 2.05 3.23 2.16 3.34 2.18 3.41 2.23 3.49

L910 1.94 3.11 2.01 3.15 2.10 3.27 2.16 3.34 2.23 3.4730 1.96 3.15 -0.16 3.18 2.14 3.30 2.22 3.38 2.25 3.4540 2.00 3.16 2.02 3.22 2.14 3.32 2.24 3.42 2.29 3.51

L1210 1.93 3.06 1.98 3.11 2.02 3.25 2.10 3.30 2.21 3.3830 1.93 3.06 2.00 3.12 2.06 3.24 2.14 3.36 2.19 3.4640 1.99 3.12 2.00 3.18 2.08 3.27 2.16 3.37 2.25 3.47

Table 7-32: Minimum Approach Distance for Fault and Clearance, Inductivecompensation at 1000m altitude & Weighted Fault Type



L210 2.06 3.33 2.12 3.37 2.19 3.43 2.24 3.50 2.30 3.4830 2.11 3.37 2.13 3.43 2.20 3.50 2.25 3.49 2.34 3.5340 2.14 3.40 2.18 3.43 2.24 3.45 2.30 3.52 2.37 3.59

L610 2.06 3.43 2.12 3.43 2.20 3.48 2.28 3.53 2.34 3.6130 2.13 3.48 2.13 3.48 2.25 3.54 2.31 3.58 2.37 3.6440 2.18 3.50 2.21 3.51 2.27 3.58 2.33 3.61 2.40 3.66

L810 2.15 3.43 2.17 3.45 2.21 3.45 2.27 3.50 2.32 3.5530 2.15 3.48 2.20 3.46 2.23 3.51 2.30 3.56 2.34 3.6240 2.18 3.49 2.24 3.47 2.30 3.55 2.32 3.61 2.35 3.65

L910 2.10 3.45 2.15 3.48 2.26 3.50 2.30 3.55 2.35 3.6130 2.12 3.50 -0.07 3.51 2.28 3.54 2.34 3.59 2.37 3.6340 2.19 3.45 2.21 3.48 2.31 3.56 2.36 3.62 2.39 3.67

L12 10 2.09 3.40 2.11 3.44 2.18 3.48 2.23 3.51 2.30 3.5630 2.10 3.42 2.13 3.46 2.21 3.47 2.27 3.55 2.31 3.6040 2.14 3.46 2.18 3.49 2.25 3.50 2.30 3.56 2.34 3.61

Table 7-33: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 1000m Altitude & Weighted Fault Type

Appendix 182Case 3C: With Floating Object



L210 4.57 6.38 4.62 6.39 4.70 6.46 4.74 6.51 4.81 6.5530 4.63 6.44 4.65 6.46 4.73 6.55 4.75 6.59 4.83 6.6340 4.69 6.48 4.71 6.49 4.77 6.59 4.82 6.64 4.89 6.69

L610 4.57 6.51 4.62 6.49 4.71 6.60 4.78 6.64 4.85 6.7130 4.69 6.66 4.66 6.64 4.77 6.67 4.82 6.71 4.89 6.7740 4.73 6.69 4.74 6.67 4.79 6.73 4.85 6.74 4.91 6.80

L810 4.69 6.51 4.70 6.51 4.73 6.55 4.78 6.60 4.82 6.6430 4.70 6.59 4.73 6.60 4.75 6.64 4.81 6.69 4.83 6.7340 4.73 6.59 4.78 6.62 4.82 6.69 4.85 6.76 4.86 6.77

L910 4.63 6.52 4.67 6.55 4.78 6.63 4.82 6.69 4.86 6.7130 4.66 6.60 2.00 6.59 4.81 6.70 4.86 6.74 4.89 6.7740 4.74 6.62 4.75 6.63 4.86 6.73 4.89 6.77 4.90 6.80

L1210 4.61 6.45 4.62 6.49 4.69 6.60 4.73 6.63 4.78 6.6730 4.63 6.49 4.65 6.53 4.73 6.60 4.78 6.66 4.82 6.7040 4.67 6.53 4.70 6.55 4.78 6.63 4.82 6.67 4.83 6.71

Table 7-34: Minimum Approach Distance for Fault and Clearance at Sea Level withFloating Object of 2m



L210 4.47 6.23 4.52 6.26 4.63 6.37 4.68 6.42 4.76 6.4730 4.52 6.29 4.55 6.35 4.64 6.43 4.68 6.50 4.77 6.5540 4.58 6.31 4.61 6.34 4.67 6.46 4.75 6.56 4.83 6.60

L610 4.46 6.35 4.54 6.36 4.63 6.47 4.73 6.54 4.81 6.6630 4.56 6.51 4.56 6.51 4.69 6.56 4.75 6.62 4.83 6.7240 4.61 6.54 4.65 6.54 4.71 6.61 4.77 6.67 4.87 6.73

L810 4.57 6.33 4.59 6.39 4.64 6.43 4.71 6.53 4.78 6.5930 4.57 6.41 4.63 6.46 4.66 6.54 4.73 6.60 4.79 6.6640 4.62 6.42 4.68 6.48 4.72 6.55 4.78 6.68 4.79 6.69

L910 4.52 6.38 4.58 6.40 4.69 6.51 4.74 6.58 4.82 6.6530 4.54 6.42 1.96 6.46 4.72 6.56 4.79 6.64 4.82 6.6840 4.61 6.45 4.66 6.50 4.77 6.60 4.81 6.67 4.84 6.73

L1210 4.49 6.29 4.52 6.37 4.59 6.51 4.66 6.56 4.72 6.6230 4.51 6.33 4.56 6.38 4.65 6.47 4.71 6.57 4.76 6.6440 4.57 6.38 4.61 6.42 4.69 6.52 4.74 6.60 4.78 6.63

Table 7-35: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at Sea Level with Floating Object of 2m




L210 4.69 6.54 4.73 6.51 4.78 6.59 4.79 6.60 4.86 6.6430 4.76 6.61 4.75 6.59 4.81 6.68 4.83 6.69 4.88 6.7240 4.80 6.65 4.82 6.62 4.85 6.72 4.90 6.74 4.93 6.75

L610 4.67 6.66 4.71 6.61 4.80 6.70 4.86 6.76 4.91 6.7830 4.82 6.83 4.77 6.77 4.86 6.77 4.89 6.83 4.94 6.8440 4.85 6.85 4.84 6.80 4.89 6.85 4.92 6.84 4.95 6.89

L810 4.82 6.66 4.81 6.64 4.80 6.67 4.84 6.68 4.88 6.7030 4.81 6.75 4.81 6.74 4.85 6.74 4.87 6.77 4.87 6.7940 4.86 6.76 4.89 6.75 4.92 6.79 4.92 6.86 4.92 6.82

L910 4.75 6.68 4.77 6.70 4.87 6.77 4.87 6.78 4.91 6.7930 4.76 6.78 2.04 6.73 4.88 6.82 4.93 6.84 4.93 6.8640 4.86 6.78 4.84 6.78 4.96 6.86 4.95 6.85 4.94 6.85

L1210 4.74 6.60 4.71 6.64 4.79 6.73 4.81 6.74 4.82 6.7230 4.74 6.63 4.76 6.67 4.80 6.71 4.86 6.76 4.88 6.7840 4.80 6.69 4.79 6.70 4.86 6.75 4.88 6.75 4.90 6.80

Table 7-36: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at Sea Level with Floating Object of 2m



L210 4.65 6.45 4.71 6.46 4.79 6.53 4.83 6.57 4.90 6.6230 4.72 6.50 4.73 6.53 4.82 6.62 4.84 6.66 4.93 6.7040 4.78 6.55 4.80 6.56 4.86 6.66 4.91 6.72 4.98 6.76

L610 4.65 6.57 4.71 6.56 4.80 6.67 4.87 6.72 4.94 6.7930 4.78 6.73 4.75 6.72 4.86 6.74 4.91 6.79 4.98 6.8440 4.82 6.76 4.83 6.74 4.89 6.80 4.94 6.81 5.01 6.87

L810 4.78 6.57 4.79 6.57 4.82 6.62 4.87 6.67 4.91 6.7230 4.79 6.66 4.82 6.67 4.84 6.72 4.90 6.76 4.93 6.8040 4.82 6.66 4.87 6.69 4.91 6.76 4.94 6.83 4.96 6.84

L910 4.72 6.59 4.76 6.62 4.87 6.70 4.91 6.76 4.96 6.7930 4.75 6.67 2.00 6.66 4.90 6.77 4.96 6.81 4.98 6.8440 4.83 6.69 4.84 6.70 4.96 6.80 4.98 6.84 5.00 6.87

L1210 4.69 6.52 4.71 6.56 4.78 6.67 4.82 6.70 4.87 6.7430 4.72 6.56 4.73 6.60 4.82 6.67 4.87 6.73 4.91 6.7740 4.76 6.60 4.79 6.62 4.87 6.70 4.91 6.74 4.93 6.79

Table 7-37: Minimum Approach Distance for Fault and Clearance at 500m Altitude withFloating Object of 2m



L210 4.54 6.34 4.60 6.37 4.72 6.44 4.77 6.49 4.86 6.5330 4.60 6.40 4.64 6.41 4.73 6.49 4.77 6.56 4.86 6.6240 4.67 6.37 4.69 6.41 4.76 6.53 4.84 6.63 4.92 6.67

L610 4.54 6.42 4.62 6.43 4.72 6.53 4.82 6.61 4.90 6.7330 4.64 6.57 4.65 6.58 4.78 6.63 4.84 6.69 4.93 6.7940 4.69 6.61 4.74 6.61 4.80 6.68 4.86 6.74 4.96 6.80

L810 4.65 6.40 4.67 6.45 4.73 6.49 4.80 6.60 4.87 6.6630 4.66 6.48 4.71 6.52 4.75 6.61 4.83 6.67 4.88 6.7440 4.70 6.48 4.76 6.55 4.81 6.62 4.87 6.75 4.89 6.76

L910 4.60 6.45 4.67 6.47 4.78 6.58 4.83 6.65 4.92 6.7230 4.62 6.49 1.96 6.52 4.81 6.63 4.88 6.71 4.91 6.7540 4.70 6.52 4.74 6.57 4.86 6.67 4.91 6.74 4.94 6.80

L1210 4.57 6.40 4.60 6.44 4.67 6.58 4.75 6.63 4.81 6.6930 4.59 6.40 4.64 6.45 4.74 6.53 4.80 6.64 4.85 6.7140 4.65 6.45 4.70 6.48 4.77 6.59 4.83 6.67 4.87 6.71

Table 7-38: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 500m Altitude with Floating Object of 2m




L210 4.78 6.61 4.82 6.58 4.87 6.66 4.89 6.67 4.96 6.7130 4.85 6.68 4.84 6.66 4.90 6.75 4.92 6.76 4.97 6.7940 4.90 6.72 4.92 6.69 4.95 6.79 4.99 6.81 5.03 6.83

L610 4.76 6.73 4.80 6.68 4.89 6.78 4.96 6.83 5.01 6.8530 4.91 6.91 4.86 6.85 4.95 6.85 4.98 6.91 5.04 6.9140 4.95 6.93 4.94 6.87 4.99 6.93 5.01 6.91 5.05 6.97

L810 4.91 6.73 4.90 6.72 4.90 6.75 4.93 6.75 4.98 6.7730 4.90 6.82 4.91 6.82 4.94 6.82 4.96 6.84 4.97 6.8640 4.95 6.83 4.99 6.83 5.02 6.86 5.02 6.94 5.02 6.90

L910 4.84 6.75 4.86 6.78 4.96 6.84 4.97 6.86 5.01 6.8730 4.85 6.86 2.04 6.80 4.98 6.90 5.03 6.92 5.03 6.9440 4.95 6.85 4.94 6.86 5.06 6.94 5.05 6.93 5.04 6.93

L1210 4.83 6.67 4.80 6.71 4.88 6.80 4.90 6.81 4.91 6.8030 4.83 6.71 4.85 6.75 4.89 6.78 4.96 6.83 4.98 6.8640 4.89 6.76 4.89 6.77 4.96 6.82 4.97 6.83 4.99 6.88

Table 7-39: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 500m Altitude with floating object of 2m



L210 4.76 6.56 4.82 6.58 4.90 6.65 4.95 6.69 5.02 6.7430 4.83 6.62 4.84 6.65 4.93 6.74 4.96 6.78 5.05 6.8240 4.89 6.66 4.92 6.68 4.98 6.78 5.03 6.84 5.11 6.88

L610 4.76 6.69 4.82 6.68 4.92 6.79 4.99 6.84 5.06 6.9130 4.89 6.85 4.86 6.84 4.98 6.87 5.03 6.91 5.11 6.9740 4.93 6.88 4.95 6.87 5.00 6.93 5.06 6.94 5.14 7.00

L810 4.89 6.69 4.90 6.69 4.93 6.74 4.99 6.79 5.03 6.8430 4.90 6.78 4.93 6.79 4.96 6.84 5.02 6.88 5.05 6.9340 4.93 6.78 4.99 6.81 5.03 6.88 5.06 6.96 5.08 6.97

L910 4.83 6.71 4.87 6.74 4.99 6.82 5.03 6.88 5.08 6.9130 4.86 6.79 2.00 6.78 5.02 6.90 5.08 6.94 5.11 6.9740 4.95 6.81 4.96 6.82 5.08 6.93 5.11 6.97 5.12 7.00

L1210 4.80 6.63 4.82 6.68 4.89 6.79 4.93 6.82 4.99 6.8730 4.83 6.68 4.84 6.72 4.93 6.79 4.99 6.85 5.03 6.9040 4.87 6.72 4.90 6.74 4.99 6.82 5.03 6.87 5.05 6.91

Table 7-40: Minimum Approach Distance for Fault and Clearance at 1000m Altitudewith Floating Object of 2m



L210 4.65 6.49 4.71 6.53 4.83 6.55 4.88 6.61 4.97 6.6530 4.70 6.56 4.74 6.52 4.84 6.61 4.88 6.68 4.98 6.7440 4.77 6.49 4.80 6.52 4.87 6.64 4.95 6.75 5.04 6.79

L610 4.64 6.53 4.72 6.54 4.83 6.65 4.93 6.73 5.02 6.8630 4.75 6.69 4.75 6.70 4.89 6.75 4.96 6.81 5.05 6.9240 4.80 6.73 4.85 6.73 4.92 6.80 4.97 6.86 5.09 6.93

L810 4.76 6.51 4.78 6.57 4.84 6.61 4.91 6.72 4.99 6.7830 4.77 6.60 4.82 6.64 4.86 6.73 4.94 6.79 5.00 6.8640 4.81 6.60 4.88 6.67 4.93 6.74 4.99 6.87 5.01 6.88

L910 4.71 6.56 4.77 6.58 4.89 6.70 4.95 6.77 5.04 6.8530 4.73 6.61 1.95 6.64 4.93 6.75 5.00 6.83 5.03 6.8840 4.80 6.63 4.85 6.69 4.98 6.80 5.03 6.87 5.06 6.93

L1210 4.67 6.56 4.70 6.55 4.78 6.70 4.86 6.75 4.93 6.8130 4.69 6.51 4.75 6.56 4.85 6.65 4.91 6.77 4.97 6.8440 4.76 6.56 4.81 6.60 4.89 6.71 4.95 6.79 4.99 6.83

Table 7-41: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 1000m Altitude with Floating Object of 2m

Appendix 185

Tower Type Fault Current (kA) Minimum Approach Distance (m)P-E P-P P-E P-P P-E P-P P-E P-P P-E P-P

L210 4.89 6.73 4.93 6.70 4.99 6.78 5.00 6.79 5.08 6.8330 4.97 6.80 4.96 6.78 5.02 6.87 5.04 6.89 5.10 6.9240 5.02 6.85 5.04 6.81 5.07 6.91 5.12 6.94 5.16 6.95

L610 4.87 6.85 4.91 6.80 5.01 6.90 5.08 6.96 5.14 6.9830 5.03 7.04 4.98 6.97 5.07 6.98 5.11 7.04 5.17 7.0440 5.07 7.06 5.06 7.00 5.11 7.06 5.14 7.04 5.18 7.10

L810 5.03 6.86 5.02 6.84 5.02 6.87 5.05 6.87 5.10 6.8930 5.02 6.95 5.03 6.94 5.07 6.94 5.09 6.97 5.09 6.9940 5.08 6.96 5.11 6.95 5.14 6.99 5.14 7.07 5.14 7.03

L910 4.96 6.88 4.98 6.90 5.08 6.97 5.09 6.99 5.13 7.0030 4.97 6.99 2.05 6.93 5.10 7.03 5.15 7.05 5.16 7.0740 5.07 6.98 5.06 6.98 5.19 7.07 5.17 7.06 5.16 7.06

L1210 4.94 6.80 4.92 6.84 5.00 6.93 5.02 6.94 5.03 6.9230 4.94 6.83 4.97 6.87 5.01 6.91 5.08 6.96 5.10 6.9940 5.01 6.89 5.01 6.89 5.08 6.95 5.10 6.95 5.12 7.01

Table 7-42: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 1000m Altitude with Floating Object of 2m

With Floating Object and Weighted Fault Type



L210 4.27 5.76 4.35 5.84 4.45 5.92 4.52 6.06 4.61 6.1530 4.33 5.80 4.36 5.92 4.45 6.02 4.54 6.14 4.67 6.2040 4.34 5.84 4.40 5.90 4.49 6.04 4.59 6.16 4.71 6.28

L610 4.25 5.88 4.35 5.90 4.47 6.10 4.58 6.17 4.67 6.3130 4.33 6.03 4.35 6.05 4.53 6.17 4.60 6.24 4.70 6.3440 4.40 6.06 4.46 6.11 4.55 6.23 4.64 6.28 4.73 6.37

L810 4.36 5.89 4.40 5.94 4.48 6.07 4.55 6.15 4.63 6.2330 4.34 5.94 4.44 6.03 4.49 6.14 4.61 6.21 4.68 6.3240 4.40 5.97 4.48 6.06 4.59 6.20 4.62 6.28 4.67 6.36

L910 4.30 5.92 4.38 5.96 4.53 6.11 4.59 6.20 4.67 6.3330 4.33 5.98 1.86 6.00 4.57 6.16 4.66 6.25 4.70 6.3340 4.41 5.99 4.44 6.06 4.58 6.18 4.68 6.29 4.73 6.39

L1210 4.29 5.86 4.34 5.91 4.43 6.08 4.52 6.14 4.63 6.2430 4.29 5.86 4.36 5.94 4.48 6.07 4.57 6.20 4.62 6.3140 4.35 5.94 4.41 6.00 4.50 6.11 4.59 6.22 4.68 6.33

Table 7-43: Minimum Approach Distance for Fault and Clearance at Sea Level withFloating Object of 2m (Weighted Fault Type)


Current (Ka)Minimum Approach Distance (M)


L210 4.19 5.62 4.29 5.71 4.39 5.79 4.46 5.96 4.56 6.0630 4.25 5.65 4.30 5.79 4.39 5.89 4.49 6.03 4.63 6.0940 4.26 5.68 4.33 5.76 4.42 5.91 4.54 6.04 4.67 6.19

L610 4.18 5.73 4.29 5.76 4.40 5.98 4.53 6.06 4.63 6.2130 4.24 5.88 4.28 5.91 4.47 6.04 4.55 6.13 4.65 6.2440 4.32 5.91 4.40 5.98 4.50 6.12 4.59 6.17 4.69 6.27

L810 4.29 5.75 4.33 5.81 4.42 5.95 4.50 6.05 4.59 6.1330 4.25 5.78 4.37 5.90 4.43 6.02 4.56 6.10 4.65 6.2340 4.32 5.82 4.40 5.93 4.54 6.08 4.56 6.16 4.62 6.26

L910 4.22 5.78 4.31 5.83 4.47 5.99 4.54 6.08 4.62 6.2330 4.25 5.84 1.83 5.86 4.51 6.03 4.61 6.13 4.65 6.2240 4.34 5.85 4.36 5.92 4.51 6.05 4.63 6.18 4.69 6.29

L1210 4.21 5.72 4.27 5.78 4.36 5.96 4.46 6.03 4.60 6.1330 4.21 5.72 4.30 5.79 4.42 5.94 4.52 6.10 4.57 6.2240 4.28 5.79 4.35 5.87 4.44 5.99 4.54 6.11 4.65 6.23

Table 7-44: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at Sea Level With Floating Object Of 2m (Weighted Fault Type)



L210 4.41 6.06 4.48 6.11 4.57 6.19 4.63 6.28 4.70 6.3530 4.48 6.11 4.50 6.19 4.59 6.28 4.65 6.36 4.75 6.4140 4.51 6.15 4.56 6.19 4.63 6.31 4.70 6.40 4.80 6.48

L610 4.41 6.18 4.48 6.19 4.59 6.35 4.68 6.41 4.76 6.5130 4.50 6.34 4.50 6.34 4.65 6.41 4.71 6.47 4.79 6.5540 4.56 6.36 4.60 6.39 4.67 6.48 4.74 6.51 4.82 6.58

L810 4.52 6.19 4.55 6.22 4.60 6.30 4.67 6.37 4.72 6.4330 4.52 6.25 4.58 6.31 4.62 6.39 4.70 6.45 4.76 6.5240 4.56 6.27 4.63 6.33 4.70 6.44 4.73 6.51 4.76 6.56

L910 4.47 6.21 4.52 6.25 4.65 6.36 4.70 6.44 4.76 6.5230 4.49 6.28 1.93 6.29 4.68 6.42 4.76 6.49 4.79 6.5440 4.57 6.30 4.59 6.34 4.72 6.45 4.78 6.53 4.82 6.59

L1210 4.45 6.15 4.48 6.20 4.56 6.34 4.62 6.38 4.71 6.4530 4.46 6.17 4.50 6.23 4.60 6.33 4.67 6.43 4.72 6.5040 4.51 6.23 4.56 6.27 4.64 6.36 4.70 6.44 4.76 6.52

Table 7-45: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at Sea Level with Floating Object of 2m (Weighted Fault Type)



L210 4.34 5.82 4.43 5.90 4.53 5.98 4.60 6.12 4.69 6.2130 4.40 5.85 4.44 5.98 4.53 6.07 4.62 6.20 4.76 6.2640 4.41 5.89 4.48 5.95 4.57 6.10 4.68 6.22 4.80 6.35

L610 4.32 5.93 4.43 5.95 4.54 6.16 4.66 6.24 4.76 6.3730 4.40 6.09 4.43 6.11 4.61 6.23 4.69 6.30 4.78 6.4140 4.48 6.12 4.54 6.17 4.64 6.30 4.73 6.35 4.83 6.44

L810 4.44 5.95 4.48 6.00 4.56 6.13 4.64 6.21 4.72 6.2930 4.41 5.99 4.52 6.09 4.57 6.20 4.69 6.28 4.77 6.3940 4.48 6.03 4.56 6.12 4.68 6.26 4.70 6.34 4.76 6.43

L910 4.38 5.98 4.45 6.02 4.61 6.17 4.68 6.26 4.76 6.3930 4.40 6.04 1.85 6.06 4.65 6.22 4.74 6.31 4.78 6.3940 4.49 6.05 4.52 6.12 4.66 6.24 4.77 6.36 4.83 6.46

L1210 4.36 5.92 4.41 5.97 4.51 6.14 4.60 6.21 4.72 6.3030 4.36 5.92 4.44 5.99 4.56 6.13 4.65 6.27 4.70 6.3840 4.43 5.99 4.49 6.06 4.58 6.17 4.68 6.28 4.77 6.39

Table 7-46: Minimum Approach Distance for Fault and Clearance at 500m Altitude withFloating Object of 2m (Weighted Fault Type)




L210 4.29 5.71 4.39 5.80 4.47 5.89 4.54 6.05 4.64 6.1630 4.35 5.74 4.40 5.89 4.46 5.98 4.57 6.13 4.72 6.1940 4.36 5.77 4.40 5.85 4.50 6.00 4.62 6.14 4.75 6.29

L610 4.27 5.82 4.39 5.85 4.48 6.08 4.61 6.16 4.71 6.3230 4.34 5.98 4.38 6.01 4.55 6.15 4.64 6.23 4.74 6.3540 4.40 6.01 4.48 6.08 4.58 6.22 4.68 6.28 4.78 6.38

L810 4.39 5.84 4.40 5.90 4.50 6.05 4.58 6.15 4.67 6.2330 4.35 5.87 4.45 5.99 4.50 6.12 4.64 6.20 4.73 6.3340 4.40 5.92 4.48 6.03 4.62 6.18 4.65 6.27 4.71 6.37

L910 4.32 5.87 4.41 5.92 4.55 6.08 4.62 6.18 4.71 6.3430 4.35 5.93 1.82 5.96 4.59 6.13 4.69 6.23 4.74 6.3340 4.41 5.94 4.44 6.02 4.59 6.15 4.72 6.28 4.78 6.40

L1210 4.31 5.81 4.37 5.87 4.44 6.06 4.54 6.13 4.68 6.2330 4.30 5.81 4.40 5.89 4.50 6.04 4.60 6.20 4.65 6.3340 4.37 5.89 4.42 5.96 4.51 6.08 4.62 6.21 4.73 6.34

Table 7-47: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 500m Altitude with Floating Object of 2m (Weighted Fault Type)



L210 4.49 6.17 4.57 6.21 4.66 6.29 4.71 6.39 4.79 6.4130 4.56 6.21 4.59 6.29 4.67 6.38 4.73 6.43 4.84 6.4840 4.59 6.25 4.64 6.29 4.71 6.37 4.79 6.46 4.89 6.55

L610 4.49 6.29 4.57 6.29 4.67 6.41 4.77 6.47 4.85 6.5830 4.59 6.40 4.59 6.41 4.73 6.48 4.80 6.54 4.88 6.6240 4.65 6.43 4.69 6.45 4.76 6.54 4.84 6.58 4.92 6.65

L810 4.61 6.30 4.63 6.32 4.69 6.37 4.75 6.44 4.81 6.5030 4.60 6.36 4.67 6.38 4.71 6.45 4.79 6.51 4.85 6.5940 4.65 6.38 4.71 6.40 4.79 6.50 4.82 6.58 4.86 6.63

L910 4.55 6.32 4.61 6.36 4.74 6.43 4.79 6.50 4.86 6.5930 4.57 6.39 1.93 6.40 4.77 6.49 4.85 6.56 4.88 6.6140 4.66 6.36 4.68 6.40 4.81 6.52 4.88 6.60 4.91 6.66

L1210 4.53 6.25 4.56 6.30 4.64 6.40 4.71 6.45 4.80 6.5230 4.54 6.27 4.59 6.33 4.69 6.39 4.76 6.49 4.81 6.5740 4.59 6.33 4.64 6.37 4.73 6.43 4.79 6.51 4.85 6.59

Table 7-48: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 500m Altitude with Floating Object of 2m (Weighted Fault Type)



L210 4.43 5.91 4.52 5.99 4.63 6.08 4.70 6.23 4.80 6.3230 4.50 5.95 4.54 6.08 4.63 6.18 4.73 6.31 4.87 6.3740 4.51 5.99 4.58 6.05 4.67 6.20 4.78 6.33 4.91 6.46

L610 4.42 6.03 4.52 6.05 4.65 6.27 4.77 6.34 4.87 6.4930 4.50 6.19 4.52 6.22 4.71 6.33 4.80 6.41 4.90 6.5240 4.58 6.22 4.65 6.28 4.74 6.41 4.84 6.46 4.94 6.55

L810 4.54 6.05 4.58 6.10 4.66 6.23 4.74 6.32 4.83 6.4030 4.51 6.09 4.62 6.19 4.67 6.31 4.80 6.39 4.88 6.5040 4.58 6.13 4.66 6.23 4.78 6.37 4.81 6.45 4.87 6.54

L910 4.47 6.08 4.55 6.12 4.71 6.28 4.78 6.37 4.87 6.5130 4.50 6.14 1.84 6.16 4.76 6.33 4.85 6.42 4.90 6.5040 4.59 6.15 4.62 6.22 4.77 6.35 4.88 6.47 4.94 6.57

L1210 4.46 6.01 4.51 6.07 4.60 6.25 4.70 6.31 4.83 6.4130 4.46 6.02 4.54 6.09 4.66 6.23 4.76 6.38 4.81 6.4940 4.52 6.09 4.59 6.16 4.69 6.28 4.78 6.39 4.88 6.51

Table 7-49: Minimum Approach Distance for Fault and Clearance at 1000m Altitudewith Floating Object of 2m (Weighted Fault Type)




L210 4.40 5.83 4.50 5.93 4.57 6.02 4.64 6.20 4.75 6.3130 4.47 5.87 4.52 6.02 4.56 6.13 4.67 6.28 4.83 6.3540 4.47 5.91 4.50 5.98 4.60 6.14 4.72 6.29 4.86 6.45

L610 4.38 5.95 4.50 5.98 4.58 6.23 4.72 6.31 4.82 6.4830 4.45 6.12 4.49 6.15 4.65 6.29 4.74 6.38 4.85 6.5140 4.49 6.15 4.57 6.23 4.68 6.37 4.79 6.43 4.89 6.54

L810 4.50 5.98 4.50 6.04 4.59 6.19 4.68 6.29 4.78 6.3830 4.47 6.01 4.54 6.14 4.60 6.27 4.75 6.35 4.85 6.4940 4.49 6.05 4.58 6.17 4.72 6.33 4.75 6.42 4.82 6.53

L910 4.43 6.01 4.53 6.06 4.65 6.23 4.72 6.33 4.82 6.5030 4.46 6.07 1.81 6.10 4.69 6.28 4.80 6.39 4.85 6.4840 4.51 6.08 4.54 6.16 4.69 6.30 4.83 6.44 4.90 6.56

L1210 4.42 5.94 4.49 6.01 4.54 6.20 4.65 6.28 4.79 6.3930 4.42 5.94 4.52 6.02 4.59 6.18 4.70 6.35 4.76 6.4940 4.49 6.02 4.52 6.10 4.61 6.23 4.72 6.36 4.85 6.50

Table 7-50: Minimum Approach Distance for Fault and Clearance, InductiveCompensation at 1000m Altitude with Floating Object of 2m (Weighted Fault Type)



L210 4.59 6.32 4.67 6.37 4.77 6.45 4.82 6.55 4.91 6.5330 4.66 6.36 4.69 6.45 4.78 6.54 4.84 6.54 4.96 6.5940 4.69 6.41 4.74 6.45 4.82 6.48 4.91 6.58 5.01 6.67

L610 4.58 6.44 4.67 6.45 4.78 6.53 4.88 6.59 4.97 6.7030 4.69 6.52 4.69 6.52 4.84 6.60 4.91 6.66 5.00 6.7440 4.75 6.54 4.79 6.57 4.87 6.66 4.95 6.70 5.04 6.77

L810 4.71 6.45 4.74 6.48 4.79 6.48 4.86 6.55 4.93 6.6230 4.70 6.52 4.77 6.49 4.81 6.57 4.91 6.63 4.97 6.7140 4.75 6.54 4.82 6.51 4.91 6.62 4.94 6.70 4.97 6.75

L910 4.65 6.48 4.71 6.51 4.85 6.54 4.91 6.62 4.97 6.7130 4.67 6.55 1.92 6.56 4.89 6.61 4.96 6.68 5.00 6.7340 4.76 6.47 4.79 6.52 4.92 6.63 4.99 6.71 5.03 6.78

L1210 4.63 6.40 4.66 6.46 4.74 6.52 4.82 6.56 4.91 6.6330 4.64 6.43 4.69 6.49 4.79 6.51 4.87 6.61 4.92 6.6940 4.70 6.49 4.74 6.53 4.84 6.54 4.91 6.62 4.97 6.71

Table 7-51: Minimum Approach Distance for Fault and Clearance, CapacitiveCompensation at 1000m Altitude with Floating Object of 2m (Weighted Fault Type)

Appendix 4

Table 7-53 is extracted from Table 5-1 and Figure 5-2 of CRIEPI [2.23] and it contains

U50 values for Rod to Plane gap with respect to their time-to-crest. Most of the Table

data are switching type impulses (time to crest greater than 100µsec). The Equation for

calculation of U50 suggested by CRIPEI is extracted from Table 5-1 of [2.23], and it has

been presented in Table 7-53.

Appendix 189

Appendix 190

Appendix 191

Appendix 192

Table 7-52: Rod to Plane Gap Experimental Sparkover Data, Positive polarity(Continue), CRIEPI_ Table 5-1 [2.23]

Y: Critical Wave“-“: Not a critical WaveT: Data from TableTF: Data from Figure

Figure 7-6 presents the data from Table 7-53 where the U50 voltages have been plotted

against the gap sizes.

Appendix 193

Figure 7-6: Rod to Plane Sparkover versus Gap Length D, CRIEPI_ Figure 5-2 [2.23]

Appendix 5

The extracted data to produce the suggested Equations in Chapter 5 used the data shown

in Figure 7-7. The U50 of the gap has been extracted from the gap size and time to crest

of the transient wave. As a result, Table 7-54 has been produced where the presented

U50s are the product of the various time to crest and gap sizes.

Appendix 194

Figure 7-7: Switching Impulse Flashover Voltage of Rod-Plane Gap, Estimation of CRIEPI’s Equation

Appendix 195

Time to Crest (us)

50% Flashover Voltage (kV)

Gap Size (m):

1 2 3 4 5 6 7 8 9 10 11 1250 448.4399 731.3332 1026.365 1321.752 1605.708 1866.448 2092.19 2271.147 2391.535 2441.57 2409.467 2283.441

100 426.7733 668.6424 933.5611 1206.783 1473.563 1719.153 1928.808 2087.782 2181.328 2194.7 2113.152 1921.938150 442.2654 696.0952 947.8898 1191.49 1420.735 1629.466 1811.524 1960.749 2070.981 2136.06 2149.827 2106.123200 458.5475 704.513 944.0514 1172.517 1385.265 1577.65 1745.026 1882.748 1986.171 2050.65 2071.539 2044.193250 486.6331 722.9593 948.8488 1161.432 1357.839 1535.2 1690.645 1821.305 1924.31 1996.79 2035.875 2038.696300 521.6518 753.4886 972.0526 1175.306 1361.211 1527.73 1672.826 1794.459 1890.593 1959.19 1998.212 2005.621350 525.6677 778.8998 1005.155 1205.541 1381.166 1533.14 1662.569 1770.564 1858.231 1926.68 1977.019 2010.355400 517.8626 814.3008 1059.18 1259.266 1421.325 1552.122 1658.422 1746.991 1824.595 1898 1973.971 2059.273450 514.2359 828.2412 1082.687 1286.016 1446.668 1573.084 1673.708 1756.979 1831.339 1905.23 1987.093 2085.369500 670.498 893.092 1094.782 1275.568 1435.45 1574.428 1692.502 1789.672 1865.938 1921.3 1955.758 1969.312550 669.033 896.992 1103.037 1287.168 1449.385 1589.688 1708.077 1804.552 1879.113 1931.76 1962.493 1971.312600 695.69 918.89 1121.33 1303.01 1463.93 1604.09 1723.49 1822.13 1900.01 1957.13 1993.49 2009.09650 667.142 908.528 1125.658 1318.532 1487.15 1631.512 1751.618 1847.468 1919.062 1966.4 1989.482 1988.308700 747.4843 959.5072 1152.259 1325.739 1479.948 1614.885 1730.551 1826.945 1904.068 1961.92 2000.5 2019.809750 745.0034 957.2036 1150.551 1325.044 1480.685 1617.472 1735.407 1834.488 1914.715 1976.09 2018.611 2042.28800 556.313 1025.502 1540.857 2102.378 2710.065 3363.918 4063.937 4810.122 5602.473 6440.99 7325.673 8256.522850 569.64 867.85 1127.06 1347.27 1528.48 1670.69 1773.9 1838.11 1863.32 1849.53 1796.74 1704.95900 506.962 845.658 1136.938 1380.802 1577.25 1726.282 1827.898 1882.098 1888.882 1848.25 1760.202 1624.738950 489.585 838.561 1138.121 1388.265 1588.993 1740.305 1842.201 1894.681 1897.745 1851.393 1755.625 1610.4411000 1643.15 1671.4 1699.65 1727.9 1756.15 1784.4 1812.65 1840.9 1869.15 1897.4 1925.65 1953.9

Table 7-53: 50% Flashover Voltage (kV) as the Function of Gap Size and Time to Crest Based on Table 7-7

Appendix 196Presented Equations for different gap size in Table 7-55 are suggested for calculation of

the U50 of a gap as the function of time-to-crest. Each U50 is obtained individually to

match the insertion points along the U-curve where the time to crest meets the 50%

sparkover voltage.

Time to Crest (µsec) Estimated Formula for the gap size D (m)

50 U50 (kV) = -1.96D3 + 17.85D2 + 243.08D +189.47

100 U50 (kV) = -2.45D3 + 26.27D2 + 180.26D + 222.7

150 U50 (kV) = -1.02D3 + 5.14D2 + 245.59D + 192.56

200 U50 (kV) = -0.77D3 + 1.43D2 + 247.09D + 210.8

250 U50 (kV) = -0.47D3 - 2.34D2 + 246.72D + 242.74

300 U50 (kV) = -0.33D3- 4.59D2 + 248.01D + 278.58

350 U50 (kV) = 0.18473D3- 14.597D2 + 295.73D + 244.35

400 U50 (kV) = 1.12D3 - 32.54D2 + 386.18D + 163.1

450 U50 (kV) = 1.40D3 -38.22D2 + 418.82D + 132.23

500 U50 (kV) = -10.452D2 + 253.95D+ 427

550 U50 (kV) = -10.95D2 + 260.83D + 419.16

600 U50 (kV) = -10.38D2 + 254.34D + 451.73

650 U50 (kV) = -12.12D2 + 277.77D + 401.5

700 U50 (kV) = -9.63D2 + 240.93D + 516.19

750 U50 (kV) = -9.42D2 + 240.48D + 513.95

800 U50 (kV) = 23.08D2 + 399.94D1+ 133.29

850 U50 (kV) = -19.5D2 + 356.71D + 232.43

900 U50 (kV) = -23.70D2 + 409.82D + 120.85

950 U50 (kV) = -24.70D2 + 423.1D + 91.193

1000 U50 (kV) = 28.25D+ 1614.9

Table 7-54: Estimated formulae for Calculation of U50 Voltage as the Function of GapSize for Each Transient Time to Crest

Appendix 197

List of Publications

The core sections and findings of this PhD thesis are published or submitted to IEEE and

CIGRE. One paper is submitted with one paper already published with the details

provided in below:

1. Martini, Pietro and Cotton, Ian. “Evaluating the Risk of Live-Line Working on a

400kV Transmission Line” 2015 CIGRÉ Canada Conference, Winnipeg,

Manitoba, August 31- September 2, 2015.

2. Martini, Pietro and Cotton, Ian. “Influence of Fault Type on Minimum Approach

Distance in Live-Line Working”. IET International Conference on Resilience of

Transmission and Distribution Networks (RTDN) 2015, Birmingham, 2015.

LIVE-LINE WORKING AND EVALUATION OF RISK ON 400kV TRANSMISSION LINE

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