6.04 Assessing the need for additional surge arresters in ...

Electricity Supply to Africa and Developing Economies: Challenges and Opportunities

Assessing the need for additional

surge arresters in an uprated high

voltage substation

Author: Peet Schutte

Co-Author: Chris van der MerweEskom Holdings SOC Ltd

Paper Number 6.04

Session 6

16 November 2017


Presentation outline

• Introduction

• Back-flashover simulation

• Simulation results

• Conclusion

• Acknowledgements


Introduction

• When a substation system voltage is uprated, surge arresters with higher protective levels are installed.

• Protective levels of arresters will determine the maximum over-voltages at the arrester terminals.

• Internal and external insulation may possibly be stressed to a point of failure.

• Insulation coordination studies are valuable to determine over-voltage stress on both internal and external insulation.

• In 88 kV substations, phase-to-earth clearance should not be less than 1000 mm and relates to an estimated 530 kV BIL.

• For internal insulation, 132 kV equipment insulation level (BIL) selected 550 kV.


Insulation types

Non-recoverable or internal insulation

type: Current transformer, power

transformer

Recoverable or external insulation type: Air gap


0

60

120

180

240

300

360

420

480

0.0001 0.001 0.01 0.1 1 10 100 1000 10000 100000

Max

imum

Res

idua

l Vol

tage

(kV

peak

)

Resistive current (A peak)

Maximum Current / Voltage characteristic curve 20 °C

Surge arrester characteristic

72 kV rated arrester used in 88 kV substation

108 kV rated arrester used in 132kV substation

10 kA Residual Voltage = 196 kV peak

10 kA Residual Voltage = 294 kV peak


Surge arrester and separation distance

10 kA Residual Voltage= 196 kVpk

10 kA Residual Voltage= 294 kVpk

Internal insulation 550 kV BIL

External insulation 1 m

phase-to-earth 530 kV BIL

Separation Distance from Arrester


Fast front transients entering a substation


Substation single line diagram


Back-flashover simulation: ATP model


Back-flashover simulation: Current source

Ramp Type Current Source

Current Peak (kA)

Probability of occurrence (%)

TAN-G Steepness

(kA/µs)

Front Rise Time (µs)

Time to half value

(µs)Source 1 10 95 13.83 0.72 75Source 2 30 52 25.32 1.185 75Source 3 80 7.8 43.42 1.842 75Source 4 160 1.4 63.58 2.5 75

0 10 20 30 40 50 60 700

50

100

150

Source 1 = 10 kA PeakSource 2 = 30 kA PeakSource 3 = 80 kA PeakSource 4= 160 kA Peak

Source 1 = 10 kA PeakSource 2 = 30 kA PeakSource 3 = 80 kA PeakSource 4= 160 kA Peak

Lightning ramp type source currents.

Time (us)

Cur

rent

(kA

)


Back-flashover simulation: Transient

The steepness of the transient over-voltages were:

• Source 1 (10 kA): Flashover not occurring, no surge on phase conductors.

• Source 2 (30 kA): Peak of 710 kV with a steepness of 1250 kV/µs.

• Source 3 (80 kA): Peak of 1.706 MV with a steepness of 1076 kV/µs.


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.5

1

1.5

230 kA Tower Top Voltage30 kA Phase A Voltage80 kA Tower Top Voltage80 kA Phase A Voltage160 kA Tower Top Voltage160 kA Phase A Voltage

30 kA Tower Top Voltage30 kA Phase A Voltage80 kA Tower Top Voltage80 kA Phase A Voltage160 kA Tower Top Voltage160 kA Phase A Voltage

Backflash Voltages

Time (us)

Vol

tage

(M

V)


Back-flashover simulation: Transient

The steepness of the transient over-voltages were:

• Source 1 (10 kA): Flashover not occurring, no surge on phase conductors.

• Source 2 (30 kA): Peak of 710 kV with a steepness of 1250 kV/µs.



0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.5

1

1.5

2

2.5

3

3.530 kA Phase A Voltage80 kA Phase A Voltage160 kA Phase A Voltage

30 kA Phase A Voltage80 kA Phase A Voltage160 kA Phase A Voltage

Backflash Voltages

Time (us)

Vol

tage

(M

V)


Simulation results: Evaluation of stress

Voltage NodePeak voltage

(kV)Safety margin for LIWL (%)

VLINE 335 64VTRFR1 330 66VBCCB 343 60VTRFR2CB (*CB DIST =60 m)

392 40

Airgap withstand 530kV/m

392 33


Simulation results: Evaluation of separation distance, stress

and separation distance

CB DIST (m) VTRFR2CB (kV) Safety margin (%)60 392 4080 395 39100 465 18.2120 492 11.7140 550 0


Simulation results: Surge arrester energy

0 10 20 30 40 50 60 70 80 90 100 110 1200

100

200

300

400

500

600

Line entrance arrester energy without busbar arrestersLine entrance arrester energy with arresters at bus couplerLine entrance arrester energy with busbar arresters on both end551 kJ maximum energy rating

Line entrance arrester energy without busbar arrestersLine entrance arrester energy with arresters at bus couplerLine entrance arrester energy with busbar arresters on both end551 kJ maximum energy rating

Arrester Energy

Time (us)

Ene

rgy

(kJ)


Conclusion

• Uprating the system voltage in a substation will require higher rated surge arresters.

• With higher protective levels, safety margins between withstand levels of external insulation are reduced.

• The evaluation of over-voltage stress caused by a back-flashover close to the substation results in the following :– Increasing the separation distance to 140 m will result in breaking through

the BIL of both internal and external insulation.

– This scenario is highly unlikely, but additional busbar arresters will improve the safety margins throughout the substation.

– The standard Eskom practice to install MOV arresters at the line entrance and power transformers reduces the probability of large separation distances.

– Optimising the over-voltage protection for non-recoverable insulation will result in decreasing the probability of recoverable insulation failing.

– Busbar arresters are to be installed to increase the safety margins where long conductor distances separate equipment from installed arresters.


Acknowledgements

• Mr. Chris van der Merwe

• Eskom

• CIGRE

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