General Conditions Relating To Supply And Use Of Electrical Energy

Classification Of Voltage a) Low voltage ------ Not exceeding 250 volts b) Medium Voltage ------ Not exceeding 650 volts c) High Voltage ------ More than 650 volts & upto 33kV

Permissible Voltage Variation ( I.E. Rule - 54 )a) Low & Medium Voltage ------ ( - ) ( + ) 6 % b) High Voltage ------ ( + ) 6 % or ( - ) 9 %c) Extra High Voltage ------ ( + ) 10 % or ( - ) 12.5 %

Permissible Frequency Variation ( I.E. Rule - 55 ) Supplier is not permitted to exceed the frequency 3 % more than the declared one. In India generally attempt is made to keep the frequency within 48.5 to 51 cps.

Standard Electrical Clearances

Line Voltage

Phase Difference ( in mm.)

Mid-Span Clearance

Ground Clearance

Live Metal Clearance(no swing condition)

( kV ) Horizontal Vertical ( in mm.) ( in mm.) ( in mm.)66 3500 2000 3000 5500 915132 6800 3900 6100 6100 1530220 8400 4900 8500 7000 2130400 9000 8000 9100 8400 3050

Permissible Minimum Clearance Above Ground ( I.E. Rule - 77 )

Line Voltage (in kV)

Across theStreet (in m.)

Along theStreet (in m.)

Other Areas (in m.)

0.650 5.8 5.5 4.6 11 6.1 5.8 4.6 33 6.1 5.8 5.2 66 6.1 6.1 5.5 132 6.1 6.1 6.1 220 7.0 7.0 7.0 400 8.8 8.8 8.8

N. B. :- For extra high voltage lines the clearance above ground shall not be less than 5.2 mtrs. Plus 0.3 mtr. For every 33kV or part thereof

by which the voltage of the line exceeds 33kV.

Permissible Minimum Clearance From Building ( I.E. Rules - 78&80 )

Line Voltage (in kV)

Vertical From Highest Object (in m.)

Horizontal From Nearest Point (in m.)

0.650 2.5 1.2 11 3.7 1.2 33 3.7 2.0 66 4.0 2.3 132 4.6 2.9 220 5.5 3.8 400 7.3 5.6

Clearance Of Overhead Lines Crossing Each Other ( I.E. Rule - 87 )

Line Voltage (in kV)

11kV (in m.)

33kV (in m.)

66kV(in m.)

132kV (in m.)

220kV(in m.)

400kV (in m.)

0.250 2.44 2.44 2.44 3.05 4.58 6.00 0.650 2.44 2.44 2.44 3.05 4.58 6.00 11 2.44 2.44 2.44 3.05 4.58 6.00 33 2.44 2.44 2.44 3.05 4.58 6.00 66 2.44 2.44 2.44 3.05 4.58 6.00 132 3.05 3.05 3.05 3.05 4.58 6.00 220 4.58 4.58 4.58 4.58 4.58 6.00 400 6.00 6.00 6.00 6.00 6.00 6.00

N. B. :- # Suitable guarding arrangement should be provided to guard against possibility of coming in contact with each other. # No guarding is required when an extra high voltage line crosses over another extra high voltage / high voltage / medium voltage line. # Crossing shall be made as nearly at right angles, as near the support of the upper line. Support of the lower line shall not be erected below the upper line.

Railway Crossing Clearances

Line Voltage

Broad Gauge & Narrow Gauge ( in mtrs. )

Up to and including 11kV Normally by CableAbove 11kV and up to 66kV 14.10Above 66kV and up to 132kV 14.60Above 132kV and up to 220kV 15.40Above 220kV and up to 400kV 17.90

En-route Tree Clearance From Over Head Lines

Line

On Either Side Of The Line ( in mtrs. )

Extra High Voltage Line 12.19High Voltage Line 6.095Low & Medium Voltage Line 0.914

Clearance Over The River Clearance must be minimum of 3.048 metres over highest flood level ( in case of non-navigable river ). In case of navigable river clearance must be decided in relation to the tallest mast of the ship passing through the river.

Clearance Between Power And Communication Lines a) Low and medium voltage line ----- 1380 mm ( 4’6” )b) H.V. lines up to & including 7.2 kV ----- 1525 mm ( 5’0” )c) H.V. lines up to 12 kV ----- 2130 mm ( 7’0” )

Clearance between communication and ground wires will not be less than 1070 mm ( 3’6” ). The minimum clearance between the guard wires and telecommunication lines shall be 600 mm. If the guards are fastened to the same supports as the power line, then the minimum distance will be 900 mm.

Line Clearance In WBSEB System

Line Voltage ( kV )

Ph - Ph (mts.)

Ph - E (mts.)

Ground Clearance (mts.)

Single Circuit

Double Circuit

Single Circuit

Double Circuit

Single Circuit

Double Circuit

400 11 8 9.26 9.30 8.84 8.84220 7.55 7.8 4.9 4.9 7.015 7.015132 5.37 5.63 4.0 4.0 6.10 6.1066 4.8 3.44 5.49

Total Number Of Disc Insulators In A String

Line Voltage ( kV ) Suspension String Tension String66 5 6132 9 10220 14 15400 22 23

Switchyard Parameters

Phase Clearance (Outdoor) Bus

Bus Voltage (kV) Ph - Ph (mts.) Ph - E (mts.) Bay Width (mts.)33 1.3 1.9 6.166 1.7 2.2 7.7132 2.8 3.4 12.2220 4.5 4 17400 7 6.5 27

Bus Height

Bus Voltage (kV) Low Bus (mts.) Main Bus (mts.) Jack Bus (mts.)132 5.5 8.42 12.85220 6.25 10.95 16.5400 8.2 15.5 23

Earthing Resistance (Ideal Value)

Generating Station and Big Sub-Station : 0.5 132 kV Sub-Station : 1 66 kV Sub-Station : 2 - 4 33 kV Sub-Station : 4 - 6

Current Carrying Capacity Of Underground Cable

Conductor 6.6 & 11 kV 6.6 & 11 kV insulated 6.6 & 11 kV XLPE

Size (sq. mm)

P.I.L.C. armoured, served belted, 3 core Aluminium Conductor (Amps)

armoured, screen sheathed, Aluminium Conductor (Amps)

Cable Aluminium Conductor

(Amps)

Single Core Three Core Single Core Three CoreIn Ground

In Air In Ground

In Air

In Ground

In Air

In Ground

In Air

In Ground

In Air

16 58 50 - - - - - - - -25 72 68 73 69 73 69 90 110 86 9035 84 80 90 87 88 84 110 135 100 10550 105 100 115 105 105 105 135 160 125 13570 130 125 140 145 130 130 165 205 155 16595 155 155 170 180 155 155 195 250 185 205120 170 175 195 210 180 185 220 285 200 230150 190 200 215 245 200 210 250 330 225 265185 220 230 240 285 230 240 285 375 260 300225 240 260 255 320 255 270 - - - -240 250 275 265 335 260 285 320 445 300 360300 280 310 325 395 295 320 360 500 335 410400 320 365 360 455 330 380 420 610 385 480500 360 415 410 530 365 435 465 710 - -625 385 470 450 580 430 520 - - - -

Assumptions :- 1. Maximum Conductor Temperature - 6.6 kV cable - 800C 11 kV single core - 70 0C 11 kV 3 core belted - 65 0C 11 kV 3 core screened - 70 0C

2. Ambient temperature - 400C3. Ground temperature - 400C

4. Depth of laying - 90 cm. (for 6.6 & 11 kV cable)

Important Data Of All Aluminium Conductor ( A.A.C. )

Code Ward Strand Size (mm.)

Cu. Eq. SWG No.

Nominal Copper Area (sq. mm)

Nominal Aluminium Area (sq. mm)

Max. Current Carrying Capacity At 400C Ambient (Amp.)

Resistance at 200C (Ohms/KM)

Approx. ultimate Tensile Strength (Kg)

Approx. Weight (Kg/KM)

Canops 7 / 1.96 8 13 20 105 1.362 385 58Gnat 7 / 2.21 7 16 25 125 1.071 485 73Weevil 7 / 2.44 6 20 30 145 0.879 580 89Ant 7 / 3.10 3 30 50 200 0.544 892 144 Important Data Of Aluminium Conductor Steel Reinforced ( A.C.S.R. )

Code Ward Nominal Copper Area (sq. mm

Calculated Eq. Area Of Aluminium (sq. mm)

No. Of Wires

Al. St.

Dia. Of Wires (mm)

Al. St.

Overall Dia Of Conductor (mm)

Max. Current Carrying Capacity At 400C Ambient (Amp.)

Resistance at 200C (Ohms/KM)

Approx. ultimate Tensile Strength (Kg)

Approx. Weight (Kg/KM)

Squirrel 13 20.71 6 1 2.11 2.11 6.33 115 1.374 771 85Weasel 20 31.21 6 1 2.59 2.59 7.77 150 0.9116 1136 128Rabbit 30 52.21 6 1 3.35 3.35 10.05 200 0.5449 1860 214Raccoon 48 77.83 6 1 4.09 4.09 12.27 270 0.3656 2746 318Dog 65 103.6 6 7 4.72 1.57 14.15 324 0.2745 3299 394Panther 130 207.0 30 7 3.00 3.00 21.00 520 0.1375 9127 976Deer 260 419.3 30 7 4.27 4.27 29.89 806 0.06786 18230 1977Zebra 260 418.6 54 7 3.18 3.18 28.62 795 0.0680 13316 1623Moose 325 515.7 54 7 3.53 3.53 31.77 900 0.05517 16250 2002

Recommended Size Of Fuses

Fuses are overcurrent devices and must have ratings well above the maximum transformer load current in order to carry without ‘blowing’ during the short duration overloads that may occur because of motor starting. Also the fuses must able to withstand the magnetising inrush current drawn when the power transformers are energised. POWER TRANSFORMERSl. No.

Transformer Capacity

Voltage Ratio

High Voltage Side (33 kV)

Low Voltage Side (11 0r 6.6 kV)

(MVA) (kV) Full load current (amps.)

Size of fuse wire (SWG)

Full load current (amps.)

Size of fuse wire (SWG)

1. 0.50 33/6.6 8.75 28 48 182. 1.00 33/6.6 17.5 23 96 143. 3.00 33/6.6 52.5 17 288.7 OCB4. 0.50 33/11 8.75 28 26.3 215. 0.63 33/11 11.0 24 33.0 216. 1.00 33/11 17.5 23 52.5 157. 1.60 33/11 28.0 21 84.0 148. 3.15 33/11 55.0 17 165.3 2 X 149. 5.00 33/11 87.5 OCB 262.4 OCB10. 6.30 33/11 110.2 OCB 330.6 OCB

DISTRIBUTION TRANSFORMERSl. No.

Transformer Capacity

Voltage Ratio

High Voltage Side (11 0r 6.6 kV)

Low Voltage Side

(MVA) (kV) Full load current (amps.)

Size of fuse wire (SWG)

Full load current (amps.)

Size of fuse wire (SWG)

1. 25 6.6/0.4 2.4 38 36 222. 63 6.6/0.4 6.0 35 91 173. 100 6.6/0.4 9.6 28 149 124. 25 11/0.433 1.31 39 33.3 225. 63 11/0.433 3.30 38 84.0 176. 100 11/0.433 5.25 35 133.3 1 X 147. 200 11/0.433 10.5 28 266.6 HRC 2508. 250 11/0.433 13.12 25 333.3 HRC 320

Construction Of Transmission And Distribution Lines

Transmission means conveyance of electrical power at Extra High Voltage from the generating stations to the grid Sub-stations or between Grid Sub-Stations. Distribution is the term used for conveyance of electrical power from the Sub-stations to the actual consumers at high or medium or low voltage.

Transmission & distribution of power can be done with the help of -

i) Overhead linesii) Underground cables.

Type Of Power Transmission

Advantages Disadvantages

Overhead lines Cheaper Easy to maintain

Prone to disturbances from weather, lightning strokes etc.

Underground cables Easy for power distribution in congested urban areas, factories, residences, power houses, Sub-stations etc.

Takes longer time for breakdown repair.

Costlier

1. The main items in an over head line are :a) Conductor, b) Supports, c) Insulators, d) Metal Hardware

a) Conductor The principal materials used as conductors in construction of overhead lines are -

i) Hard drawn copper, ii) All Aluminium Conductor (AAC)iii) Aluminium Conductor Steel Reinforced (ACSR) iv) Cadmium Copperi) Steelv) All Aluminium Alloy Conductor (AAAC)i) Aluminium Conductor Alloy Reinforced (ACAR)vi) Aluminium Alloy Conductor Steel Reinforced (AACSR)

b) Supports i) Wood polesii) Steel Tubular polesiii) Rails and R.S. Joistsiv) Lattice type polesv) Steel Towersvi) Reinforced cement concrete poles (RCC)vii) Pre-stressed cement concrete poles (PCC)

c) Insulators

i) Pin insulatorsii) Shackle insulatorsiii) Disc insulators iv) Strain insulatorsi) Post insulators

d) Metal Hardware i) Strain clampii) Suspension clampiii) Twisting joint sleeveiv) Repair sleevev) Bolted clipvi) Tubular compression jointvii) Parallel Groove clamp (P.G.)viii) Vibration damper

2. Other Factors For Line Construction

i) Bracket or cross armii) Earthing systemiii) Stay and strutsiv) Foundationv) Jointingvi) Armoringvii) Dumperviii) Guard and safety deviceix) Anti climbing devicex) Danger noticexi) Pole numbering

Functions Of Transmission (O&M) Sub-Division

1. Attending breakdown of lines and Sub-Stations.

2. Preventive maintenance worki) Winter maintenance program.ii) Pre-puja maintenance work.iii) Pre-norwester maintenance work.

3. Procurement of spare equipment and equipment’s spares for both breakdown replacement and preventive maintenance work.

4. Up-keepment of control room building switchyard by periodical maintenance through annual maintenance program.

5. Ensuring round the clock vigilance for maintenance of power system.a) Through the duty roaster of operational staff, arrangement of necessary

availability of maintenance staff, vehicle for attending breakdown jobs at the shortest possible time.

b) Maintaining proper communication system with other Sub-station from which power is drawn and other distribution Sub-Stations and bulk consumer through which power is distributed.

6. Means of communication :i) P & T telephoneii) VHF communicationiii) PLCC ( power line carrier communication )iv) Future communication - VSAT communicationi) Maintaining walkie-talkie sets for small distance communication.v) Allotment of staff quarters for emergency maintenance & operational staff.

7. Operation of the Sub-station : Switching instruction are displayed in all Sub-stations for switching operation of different equipment during faulty condition or shutdown operation or interchanging of source of supply, shedding of power in case of scarcity in availability as directed by Central Load Despatch and also to save the system from total disaster.

Functions Of Transmission Construction Sub-Division Construction Of Sub-Station :-

1. Selection of site and acquisition process of the land through land acquisition department, Govt. of West Bengal with the assistance of the land acquisition cell of WBSEB, after issuance of work order by the CP & ED wing.

2. Soil testing work to facilitate CP & ED wing to prepare the design of foundation of structures and equipment and also control room building, staff quarters etc.

3. Preparation of estimate for boundary wall, foundation of equipment and structures, control room building on the basis of soil testing report for tender call.4. Preparation of estimate for erection of structures and equipment as per lay out drawing submitted by CP & ED wing.5. Preparation of list of materials and equipment like - isolators, C.Ts., L.As., OCBs etc. are to be prepared and requisition of materials are to be placed to CP & ED wing through proper channel for procurement action.6. Estimate for -

a) earthmat arrangement for earthing of equipment.b) cable trenches are also to be prepared as per layout drawing.c) similar estimate for land filling work, surface drain work, construction of store shed,

staff quarters, children park, recreation room etc. should also be prepared for inviting tenders for construction work.

7. List of equipment to be installed for inside the control room like control pannel, battery charger pannels and control cable for connecting equipment in the switchyard with the control pannel should also be prepared for procurement action by the CP & ED / Central purchase wing. Construction Of Over head Transmission Line:-1. By preliminary route survey, alternative routes or alignment are to be prepared avoiding

congested areas, railway crossings, roads, rivers, as far as practicable.2. Gazette notification & newspaper publication will be necessary mentioning the names of

Mouzas through which the line will pass for general information of public in terms of section 29 & 42 of I.E. Act, 1948.

3. Soil resistivity test is to be conducted along the route alignment and after that, schedule of towers involving the angle at different points are to be prepared.

i) ‘A’ type tangent tower tolerable angle up to 20

ii) ‘B’ type tower tolerable angle 20 to 300 iii) ‘C’ type tower tolerable angle 300 to 600

iv) ‘D’ type tower above 600 and dead-end tower. The foundation of the towers are designed according to the condition of the soil over which the route alignment is drawn -

i) dry soilii) semi-submerged soiliii) fully submerged soil.

4. After getting the preliminary survey report, specification for supply of different kind of towers and required foundation are to be prepared for tendering purpose.

5. Permission for Rly. Crossing & forest deptt. & clearance for environment deptt. & Airport Authority, National High way Authority are to be taken.

TRANSFORMER PROTECTION

Protection Is Provided To Minimise -a) Cost of repair of damage.b) Possibility of spreading & involving other equipment.c) Timely out of service of the equipment.d) Loss in revenue and of course the strained public relations.

Protection System Should Be -a) Very fast - Operate with correct speed i.e. fast clearance of fault to minimise damage

and increase power system stability.b) Selective - Able to discriminate between faulty & healthy equipment.c) Sensitive - Can operate under minimum generating condition andd) Stable - Stabilise under external fault condition and should not result in undesired

tripping when there is no fault in the equipment protected.

TRANSFORMER IS VIRTUALLY AN IMPEDANCE CONNECTED TO THE SYSTEM.

Faults In Transformer

Sl. No.

Fault Causes Effect Occurrence

1. Phase Fault(Phase to Phase)

Mostly ground faults in 2 phases, flashover, insulation failures.

High current, Mechanical Stress.

Rare

2. Ground Fault Insulation failure. High current in grounded neutral operation.

Common

3. Inter Turn Fault

Insulation failure. Short circuit current is high but line terminal current is low.

Common

4. Inter Winding Fault

Insulation failure between windings - primary to secondary.

Over voltage leading to developing ground fault.

Rare

5. Core Fault Laminations getting bridged, core bolt insulation failure.

Eddy current heating increases,

Increase of noise.

Common

6. Radiator Fault, Cooling duct Fault

Choking of pipes by sludge in oil, cooling ducts also may be affected.

Abnormal heating, Winding damage, Oil break down and gas

formation.

Common

In our system transformer ratings up to 3 MVA are generally protected only by fuse. Fuses are over-current devices and must have ratings well above the maximum transformer load current in order to carry, without blowing, the short duration over loads that may occur because of such as motor starting, also the fuses must withstand the magnetising inrush current drawn when power transformers are energised.

The protection provided for large capacity transformers are described hereunder –

1) Temperature Relay :The temperature indicator is fitted with mercury switches fixed on the pointer, so that on temperature rise, the switch tilts and makes contact through mercury between two electrodes which are connected to electric to initiate proper action. Detection of over-heating is normally done by –a) Oil Temperature Indicator (OTI)b) Winding Temperature Indicator (WTI) – The winding temperature is indirectly obtained

by measuring the top oil temperature by a Bourdon liquid expansion indicator mounted in a pocket, which also contains a heater element energised from a phase CT. The thermometer thus measures top oil temperature plus an increment proportional to load current.

Settings of WTI & OTI in WBSEB for several actions –

Protection System Cooling SystemAlarm Trip Fan ON Fan OFF Pump ON Pump OFF

O.T.I. 80oC 90 oC - - - -W.T.I. 90 oC 95 oC 65 oC 55 oC 70 oC 60 oC

2) Oil and Gas Devices :a) Boucholz Relay / Protective Surge Relay – When a transformer is fitted with conservator,

the formed gas within the transformer, flows towards the conservator where atmospheric pressure exists. Boucholz Relay is mounted in the pipe which has a slope between main tank and conservator. If the fault is of very minor nature, gases are liberated slowly and stream of gas bubbles flow towards conservator. But if there is violent evolution of gas, a sudden surge of oil flow towards the conservator followed by the gaseous products.

Bucholz relay has two floats with mercury switches attached –i) The upper float (for alarm) moves down when gas slowly accumulates on the upper

part of the chamber (result of incipient fault, failure of lamination insulation / core bolt insulation / interturn fault).

ii) A surge of oil however deflects the lower float (for trip) and closes mercury switch (indicating heavy fault / short circuits). In this case gas may not accumulate in relay.

For a loss of oil condition also, both the floats make contacts of the corresponding mercury switch (low oil level condition).I.S.S. (Indian Standard Specification) 3637 – 1966 sets down the following figures relating to a Bucholz relay –

Nominal Pipe Bore (mm.) Gas volume for alarm at 5 o pipe angle (cc)

Steady oil flow for trip at 1-9 o pipe angle (cm/sec)

25 90 – 165 70 – 13050 175 – 225 75 – 14080 200 – 300 90 – 160

N.B. – 1. Often after initial energisation of transformer, the trapped air get released by vibration & warming up of the oil and operate the Bucholz Relay. To prevent this proper release of air from bushing turrets, radiators, tank-tops are required.

2. Oil surges also occur when the transformer feeds external short circuits and there is dynamic stress in the windings. Thus if Bucholz Relay is made very sensitive, there is chance of operation for fault outside the transformer.

Type of Relay -

Relay No. Name Of The Relay2 Time delay relay3 Checking or interlocking relay21 Distance protection relay25 Synchronising / Syn. Check relay27 Under voltage check relay30 Annunciation relay32 Directional power relay37 Under current or under power relay40 Field failure relay46 Reverse phase or phase balance current relay49 Machine or transformer thermal relay50 Instantaneous over-current relay50 N Instantaneous earth-fault relay51 IDMT (Inverse Definite Minimum Time) O/C relay51 N IDMT (Inverse Definite Minimum Time) E/F relay52 Circuit Breaker52 a Circuit Breaker auxiliary contact N/O52 b Circuit Breaker auxiliary contact N/C55 Power factor relay56 Field application relay59 Over voltage relay60 V / I balance relay64 REF (Restricted Earth Fault) Relay67 Directional O/C & E/F relay68 Blocking relay74 Supervision relay79 AC reclosing relay80 DC fail relay86 Lock out / Trip87 Differential relay

Protective Relays

Over Current Relay Excessive current flow through an electrical circuit due to a fault in any part of the network or due to abnormal operating condition in the system, is most conveniently detected by overcurrent relays which operate when the magnitude of current through it exceeds a set value. These are again of the following types -

i) Instantaneous - time a few cycles onlyii) Definite time - fixed intentional time delay independent of current magnitudeiii) Inverse time - operating time decreases as actuating current increases.

According to the characteristics, these are classified as -a) Inverse, b) Very Inverse, c) Extremely Inverse.

The most commonly used type of relays work on induction (electromagnetic) principle and develop torque proportional to I 2. Hence the torque increases rapidly with current, but beyond a certain value of current depending on the core construction, saturation sets-in and the induction decreases so that further increase of current does not increase the torque and the relay operating time levels out to a definite time. Such characteristic is known as Inverse Definite Minimum Time (IDMT).

Fault Calculation

For determining the settings of relays a knowledge of the fault current that can flow through the network into the fault is necessary. Hence the data required for the setting study are :a) Single line diagram of the system with ratings and impedance of Generators,

Transformers, Feeders with details of CTs and protective relays shown.b) Maximum and minimum of short circuit current expected to flow through each protective

device.c) Characteristic curve of relays.d) Maximum peak load current through the protective device including starting current of

motors, if supplied.

The basic principle followed in relay settings is to allow shortest operating time for maximum fault current and then recheck the time co-ordination at minimum fault current. For the calculation of the fault current, the data of %IZ or the line impedance in ohms must refer to a common base MVA and base voltage level. With the above the network may be reduced to a single source with series impedance for ease in fault current calculation.

Base MVA is the 3-phase powerBase voltage is line voltage in kV

We know, MVA = 1000 . kV . I = 1000 . kV . ( V / Z )

= 1000 . kV . { ( kV / 1000 ) / Z } = ( kV )2 / ZSo, Base Impedance = ( kV )2 / MVAPer unit impedance = (Actual Impedance) / (Base Impedance)

= Z / {( kV )2 / MVA } = Z . (Base MVA) / (Base kV )2 Z p.u. (new base MVA) = Z p.u. (given base MVA) X [(new base MVA) / (given base MVA)]Z p.u. (new base kV) = Z p.u. (given base kV) X [(new base kV) / (given base kV)]2

Example :

The h.v. source is assumed to have negligible source impedance. Converting all the impedance to a common base MVA of say 100 MVA,Tr. at D, % impedance on 100 MVA = 7 x (100 / 10) = 70 % = 0.7 p.u.Line BC, % impedance = (5 x 0.6 x 100) / 332 = 0.2754 p.u.Line AB, % impedance = (10 x 0.6 x 100) / 332 = 0.55 p.u.50 MVA Tr., % impedance on 100 MVA = 10 x (100 / 50) = 20 % = 0.2 p.u. 2 x 50 MVA Tr. in parallel, % impedance on 100 MVA = 10 % = 0.1 p.u

Fault LevelsAt Bus D = 100 / (0.7 + 0.2754 + 0.55 + 0.1) = 61.35 MVAAt Bus C = 100 / (0.2754 + 0.55 + 0.1) = 107.53 MVAAt Bus B = 100 / (0.55 + 0.1) = 153.85 MVAAt Bus A, Maximum = 100 / 0.1 = 1000 MVA Minimum = 100 / 0.2 = 500 MVAConsidering only one 50 MVA Tr. in service, the minimum fault levels are -At Bus D = 57.9 MVAAt Bus B = 97.5 MVAAt Bus C = 133.3 MVAAt Bus A = 500 MVA

132 kVSource

132 / 33 kV 50 MVA, 10%

%%%%%%

10 Miles 0.6 / m

5 Miles 0.6 / m

132 / 33 kV 50 MVA, 10%

%%%%%%

33 / 11 kV 10 MVA, 7%

%%%%%% A B C D

Location Fault MVA Fault Current, Amps.Maximum Minimum Maximum Minimum

A 1000 500 17540 8770B 154 133.3 2700 2340C 108 97.5 1900 1710D 61.5 57.9 1080 1010Alternative Method : The same calculation of fault current could as well be done from ohmic values of the impedance in the circuit.Z in ohm = Z p.u. x kV2 / MVAFor 50 MVA Tr. at 33 kV, Z = 0.1 x 332 / 50 = 2.18 For the transformers in parrel at 33 kV side, Z s = 1.09 Line AB, ZAB = 6 Line BC, ZBC = 3 For 10 MVA Tr. at 33 kV, Z T = .07 x 332 / 10 = 7.623 Fault at A at 33 kV side, I FA = (33 / 3) / 1.09 = 17480 amps.Fault at B, I FB = (33 / 3) / (6 + 1.09) = 2687 amps.Fault at C, I FC = (33 / 3) / (3 + 6 + 1.09) = 1888 amps.Fault at D at 33 kV side, I FD = (33 / 3) / (7.623 + 3 + 6 + 1.09) = 1075 amps. at 11 kV side, I FD = (11 / 3) / (7.623 + 3 + 6 + 1.09) = 3225 amps.

Selection Of CT Ratios : The CT ratios for each section of the feeders are selected from the data of maximum load current flowing into the section. The settings of the protective relays should be safely above the maximum load current and also the transient peak load but well below the minimum fault current.

Co-ordination For Phase Fault Protection : For the system considered let us assume, Load from Bus C - 10 MVA Load from Bus B - 20 MVA Load from Bus A - 30 MVA

10 MVA load at 33 kV = 175 amps., CT Ratio at C 200 / 510 MVA load flowing through BC, CT Ratio at B 200 / 510 + 20 MVA load flowing through AB, CT Ratio at A 600 / 550 MVA Transformer, 33 kV side, CT Ratio 1200 / 5

132 kVSource

10 Miles 0.6 / m

5 Miles 0.6 / m

33 / 11 kV 10 MVA, 7%

%%%%%% A B C

132 / 33 kV 50 MVA, 10%

%%%%%%

D

30 MVA30 MVA 20 MVA

300 / 5 1200 / 5

600 / 5 200 / 5

200 / 5

50 MVA Transformer, 132 kV side, CT Ratio 300 / 5

Depending on the amount of fault current the CT Ratio may have to be increased to prevent saturation.

Selection Of Relay Characteristic : The relay at C possibly has to co-ordinate with fuses provided in the 11 kV circuits which generally have extremely inverse time-current characteristic, as such an extremely inverse relay is chosen.

Time Grading- Phase Fault Relays

Relay at Sub-Station C This relay has to co-ordinate with possibly fused sub-circuits from sub-station D i.e. it must allow time for faults in the 11 kV system to be cleared by fuses.

For a fault very close to a sub-circuit with a fuse of 200 amps., the current may be assumed to be the maximum fault current at Bus D at 11 kV. IFD =3225 amps. This current will result in 33 kV side, fault current of 1075 amps. Which will flow through CT at C in the transformer circuit.Typical characteristic of a 200 amps. Fuse is :

Operating Current (amps.) Operating Time (sec.)500 300.01000 3.01200 1.02000 0.23000 0.076000 0.01

The operating time may vary by about 30% whereas the relay operating time is assumed to vary by 10%. These are made additive for security and a further factor of safety of 0.15 sec. Is applied. Thus at the fault level of around 3000 amps., Operating time of fuse = 0.07 + (0.4 x 0.07) + 0.15 = 0.248 sec. 0.25 sec.Thus this time must elapse before the relay at C clears the fault at Sub-station D.

Relay C SettingsMaximum fault current through C for a fault at D = 1075 amps. So, CT Ratio 200 / 5. As a general guide for discrimination relay setting of approximately 3 times the fuse rating reffered to same voltage base is satisfactory. Hence 200 amp. Fuse at 11 kV is (1/3 x 200 ) amp. At 33 kV and ( 3 x 1/3 x 200 ) i.e. 200 amp. Is a satisfactory setting for relay at C. So, 100% current or plug setting is chosen i.e. 5 amp tap plug.The fault current is 1075/200 = 5.37 times tap value.Multiple of relay tap or plug setting multiplier = 5.37

33 / 11 kV 10 MVA, 7%

%%%%%% D

200 / 5

C

33 kV

11 kV

Fuse

For extremely inverse relay characteristic at Time Dial 1.0, relay operating time = 2 sec at 5.37 times tap.So, required time dial setting = 0.25/2 = 0.125, say 0.15Minimum Current The pick up of relay C is set for 3 times the fuse rating referred to same voltage base. Let us check the operating time at 5 times fuse rating i.e. 1000 amp.At 1000 amp fuse operating time is 3 sec. 1000 amp at 11 kV corresponds to 333.3 amp at 33 kV.So, plug setting multiplier = 333.33/200 = 1.57At this multiple of relay tap the operating time of the relay at time delay 1.0 is 20 sec.Hence, at time delay 0.15 it is 3 sec. So, there is sufficient co-ordination.

Relay at Sub-Station B This relay has to co-ordinate with the relay at C and must allow time for faults at the 10 MVA Tr. h.v. bushings also to be cleared by relay C. For a fault very close to the CT primary for relay C the fault current is the same as that at the bus C which is 1888 amps. The operating time for relay C at this current from extremely inverse characteristic is :-

10 Miles 0.6 / m

5 Miles 0.6 / m

33 / 11 kV 10 MVA, 7%

%%%%%% A B C

600 / 5 200 / 5

200 / 5

33 kV11 kV

P 5.0TD 0.3

P 5.0TD .35

P 5.0TD .15

20 MVA

IDMTEXTREME INVERSE

Economics Of Different Types Of Power Stations

Particulars Hydroelectric Thermal Nuclear Diesel (1) (2) (3) (4) (5)1. Site Ample quantity of water at

sufficient head and possibility of constructing a dam to store water in the catchment area are decisive factors. Transportation facilities should also be available.

Generally located near the load centre but other factors such as - transportation of fuel, enough water for cooling the condensate, cost of land are also kept in view. Present thinking is to install thermal stations near coal mines.

Located near the load centre. Easy transportation of nuclear fuel, availability of cooling water and disposal of radioactive waste determine the site for a nuclear power station.

Can be installed anywhere.

2. Costa) Initial cost

b) Fixed cost per annum as a percentage of initial costc) Variable costi) Cost of fuel

ii) Transportation of fuel

iii) Maintenance cost

iv) Transmission cost

Initial cost is very high because of dam construction and excavation work.

10 %

Particularly Nil

Nil

Comparatively low.

Very high

Initial cost is lower than those of hydroelectric and nuclear power plants.

13 %

High

Very high specially when the power stations are away from coal mines and railway sidings.

Higher as compared to hydro and diesel but comparable to nuclear stations.

Low except in the case of mine-head stations.

Initial cost is highest because of huge investment on building a nuclear reactor.

15 %

Low

Since quantity of fuel required is very small, transportation cost is low.

On the high side as skilled and well trained staff is required to handle the equipment.

Low

Initial cost is less as compared to other plants.

15 %

Very high

Higher than nuclear stations but lower than that for thermal stations.

Comparatively low.

Nil

3. Time required for completion

About 7 - 8 years 3 - 4 years 10 years 1 - 3 months

(1) (2) (3) (4) (5)4. Simplicity and cleanliness Simple and clean Causes air pollution. Disposal

of ash is another problem.Clear source of power generation. However, handling of equipment is complicated and nuclear wastes disposal is a major problem.

Simpler and cleaner than thermal plants.

5. Field of application Can be used to supply peak load or base load

Generally used to supply base load.

Used to supply base load as the reactors cannot be easily controlled to respond quickly to load changes.

Used as a standby to supply part of load in a power system when required. Also used to supply peak load demand in conjunction with hydro, thermal or nuclear stations which supply base load.

6. Reliability Simple, robust and most reliable

Reliable Reliable Less reliable.

7. Limit of source of power Water is the source of power which is not dependable because of wide variations in the rainfall every year.

Coal is the source of power which has limited reserves all over the world.

The source of power is the nuclear fuel which is available in sufficient quantity. It is because small amount of fuel can produce huge power.

Diesel is the source of power which is not available in huge quantity due to limited reserves.

8. Starting Can be started instantly. Requires a lot of time for starting

Can be started easily. Can be started quickly.

9. Overall efficiency Most efficient. Overall efficiency is about 85 %.

Least efficient. Overall efficiency is about 25 %.

More efficient than steam power station.

Efficiency is about 35 %.

10. Space Need a lot of space for civil engineering construction works such as dams. The building has to be much larger than that required for other types of plant.

Need much more space than Diesel electric stations but much less when compared with hydro stations. A huge space is required for storage of fuel (i.e. coal).

Need less space as compared to hydro and thermal stations.

Normally of small sizes and need less space.

Earthing

Earthing is separated into two separate and distinct group :-1. System and winding earthing2. Equipment, structure and enclosure earthing.

1. System Earthing :- System earthing is associated with the problems of circuit functioning & performance and is essential in system design & equipment application problem.

In an earthed system, the neutral point earthing is absolutely necessary to stabilise the neutral point of the system and hereby the system insulation is uniformly stressed under normal condition and the danger of overstressing under fault condition is reduced. In our system neutral point is solidly earthed.2. Equipment Earthing or safety earthing :- The second type of earthing deals

strictly with safety and accidental earthing of equipment structures and enclosure which could otherwise create hazard to personnel.

Therefore for construction of a Sub-Station, system and equipment earthing is done in a proper manner by designing an Earth-mat in the area where Sub-station switchyard will be constructed

Effectively Earthed System

A system is said to be effectively earthed if under any fault condition the line to earth voltage on the healthy phase will not exceed 80 % of the system ‘LINE TO LINE’ voltage. In general a system in which all the transformers have star connected windings with all neutrals solidly earthed (i.e. multiple earthed system) is regarded as effectively earthed. The system highest voltage is generally assumed as 110 % of the corresponding nominal system voltage. The co-efficient of earthing is defined as the “ ratio of the highest r.m.s. voltage to earth of sound phase at the point of application of an arrester during a line to earth fault to the highest line to line r.m.s. voltage expressed as a percentage of the latter voltage”. Therefore it follows that the highest r.m.s. voltage to earth of sound phase equal to the highest line to line r.m.s. voltage multiplied by the co-efficient of earthing, i.e., Voltage Rating Of Arrester = Highest Line to Line r.m.s. Voltage X Co-efficient of earthing For effectively earthed system, the co-efficient of earthing is 0.8. So that, in a 132 kV effectively earthed system,

Voltage rating of arrester = 132 X 1.1 X 0.8 = 116 kV

Soil Resistivity

The resistance to earth of an electrode of given dimensions is dependent upon the electrical resistivity of soil in which the electrode is installed. Type of soil largely determines it’s resistivity. Earth conductivity is however essentially electrolytic in nature and the same is affected by the moisture content of the soil and it’s chemical composition and concentration of salt dissolved in the contained water. Since soil resistivity rises sharply with fall of moisture content, it is essential to bury current carrying electrodes at such a depth that the surrounding soil is not affected by seasonal variations particularly drying out during dry weather. For measurement of resistivity of soil, WENNER’s four electrode method is mostly used. Four probes are driven into the depth along a straight line at equal distances ‘S’ apart, driven to a depth ‘B’. The voltage ‘V’ between the inner (potential) electrodes is then measured and divided by the current ‘I’ between the two outer electrodes. Then, = { 4 S ( V I )} [ 1 + {2S (S2 + L2 )} - {2S (4S2 + 4L2 )}]

Measurement Of Resistivity Of Soil And Earth Resistance

4 Point Megger

P1 P2

C1 C2

L S S S

= (4 S R ) [ 1 + {2S (S2 + 4L2 )} - {2S (4S2 + 4L2 )}] 2 S R [ when S L, atleast S is 20 times greater than L ]

where, = Soil resistivity in ohm-m R = Megger reading in ohm and is measure of V / I S = Electrode spacing in mts. L = Depth of burial of electrode in mts.

3 Point Megger

P1 P2

C1 C2

L S S

= { 4 S ( V I )} [ 1 + {2S (S2 + L2 )} - {2S (4S2 + 4L2 )}] 2 S R [ when S L, atleast S is 20 times greater than L ]

where, = Soil resistivity in ohm-m R = Megger reading in ohm and is measure of V / I S = Electrode spacing in mts. L = Depth of burial of electrode in mts. V = Voltage between two inner electrodes in Volts I = Current flowing through two outer electrodes in Amp.

Dimensions Of Tower Superstructure

h1

d1 h1

h2

d2 d1 h3 h2

L d3 d2

h h

Double Circuit Single Circuit

Tower (kV) L (mts) h1(mts) h2 (mts) h3 (mts) h (mts) d1 (mts) d2 (mts) d3 (mts)220 D/C 30.44 6.93 4.90 4.90 13.71 3.93 4.15 4.6132 D/C 25.5 5.19 4.0 4.0 12.31 3.15 3.40 3.60132 S/C 22.172 5.787 4.0 - 12.385 3.14 3.32 -66 D/C 18.22 3.81 2.44 2.44 9.53 2.14 2.14 2.2166 S/C 17.5 5.5 2.2 - 9.665 2.55 2.675 -

Datas Regarding E.H.V. Tower Lines A. Types Of TowerTower Suspension Tension (kV) ‘A’ Type ‘B’ Type ‘C’ Type ‘D’ Type132 0 - 2 30 60 60 - 90 220 0 - 2 30 60 60 - 90 400 0 - 2 15 30 60

B. Earth Footing Resistance Of Tower 400 kV line - 25 220 kV line - 25 132 kV line - 13.2 66 kV line - 6.6 Below 66 kV line - 4 - 6

Transformer Oil Characteristics

Diff. in Temp.

100 150 200 250 300 350 400 450 500

Factor K

1.65 2 2.6 3.2 4.2 5 6.6 7 10.5

Filter Machine at Kaliompong

NISHA Engineering Corporation285E B.B.Ganguly Street, Calcutta-12.Model – TOPL – 100Capacity – 100 lts./hr.Heater – 6 kWSl. No. – 87 / 10 / 05½ HP Motor Capacity (Inlet / Outlet / Vaccum)

Bus Pipe Specification

132 kV Bus 2 ½ “ dia. Bus Pipe 73 mm. Outside dia. 66 kV Bus 2 “ dia. Bus Pipe 57 mm. Outside dia. 33 kV Bus 1 ½ “ dia. Bus Pipe 45 mm. Outside dia.

Current Equivalent in Different System

In 132 kV System 1 MVA 4.35 amp.In 66 kV System 1 MVA 8.70 amp.In 33 kV System 1 MVA 17.4 amp.In 11 kV System 1 MVA 52.2 amp.In 6.6 kV System 1 MVA 87.0 amp.

Conversion for Compensation

1 katha 720 sq. ft.1 bigha 20 katha 1338.29 sq. m.1 acre 0.4047 hector1 hector 2.471 acres 10,000 sq. m.