Evaluation of 13kV Dry-Type Shunt Reactor Protection following Near-

MissGary L. Kobet, P.E.

Presented to the 71st Annual TAMU Conference for Protective Relay Engineers

March 28, 2018

1

Overview

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• Shunt reactors at TVA• Event sequence• Protection questions• Post-event actions• Conclusion

Shunt reactors at TVA• Compensate lightly loaded

500kV lines• 45MVAR three-phase• Dry type (air core)• Connected to tertiary

windings of 500/161/13kV transformer banks

• Neutral-switched – reactors remain energized even with circuit breaker open and zero current flowing!

3

500kV

161kV13kV

Reactor circuit breaker

Shunt reactor – neutral-switched

4

Circuit breaker

A

C

B

45MVAR (3ph)

1. Circuit breaker open, reactors energized but zero current

2. Circuit breaker closed, reactors energized, load current flowing

2000A

2000A

2000A

Circuit breaker

A

C

B

45MVAR (3ph)

So:With the circuit breaker open:Is this a fault?

Standing open

Original TVA Standard – Split Winding Shunt Reactor Protection

5

301887

46

5187

87T

500kV

161kV

13kV

45MVAR (three-phase) split-winding shunt reactor

Trip & lockout 500kV & 161kV PCBsAUX

96LOR

Trip 3018

Alarm

3017

Three-line diagram

6

3018

Standing open

87A

87B

87C

51

46

Trip 3018

Alarm

Trip 96 xfmrbank LOR

46

5146

A3017

3017C

3017B

51

To xfmrbk diff

Event initiation

• A hawk wingspan could easily bridge the outer phases of the neutral breaker bushings

7Remains of bird

3018

15:44 Bird lands on 3018 bridging phases between reactor and breaker

8

3018

Standing open

87A

87B

87C

51

46

Trip 3018

Alarm

Trip 96 xfmrbank LOR

46

5146

A3017

3017C

3017B

51

To xfmrbk diff

2000A

2000A

2000A

2000A current flowing in all three phases –uncontrolled arcing

From 15:44 to 15:58:53 Split winding differential relays operate

9

3018

Standing open

87A

87B

87C

51

46

Trip 3018

Alarm

Trip 96 xfmrbank LOR

46

5146

A3017

3017C

3017B

51

To xfmrbk diffBreaker already open!!

By 16:00:50 (16 minutes later) Split winding CTs and buswork destroyed

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3018

Standing open

87A

87B

87C

51

46

Trip 3018

Alarm

Trip 96 xfmrbank LOR

46

5146

A3017

3017C

3017B

51

To xfmrbk diff

750A

750A

A- and C-phase (one each phase) stingers laying on grounded support steel

Operator information

11Next step?

C-phase

A-phase

18:21:38 Operator attempts to open 3017 beginning with A-phase

12

3018

Standing open

51

Trip 96 xfmrbank LOR

51

3017

3017C

3017B

51

To xfmrbk diff

750A

750A

Switch flashes over, arc creates 3-phase fault (82kA)

A

Bank differential trips bank, deenergizes 13kV yard

13

Damaged CTs/buswork

PCB 3018

Location of operator during switching

3017 switches

46’

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Appears to be head of hawk

Left wing

Right wing

Protection questions

1. Why didn’t bank differential operate before the fault occurred?

– Relay pickup/slope characteristic

2. Why did tertiary bus ground detection alarm not assert?– All components tested

good– Scheme will not detect

double-line-to-ground fault (only single)

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6817pu

2.5pu

2.5pu V0 = 0.5pu

Operational practice – changes• De-energize 13kV yard

by de-energizing 500kV transformer prior to:– Any switching in 13kV

yard, routine or emergency

– Inspecting yard after tertiary bus ground fault alarm

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Industry Standard Review• IEEE Std C37.109:

– Turn-to-turn faults:• Voltage unbalance (single

winding)• Split winding differential

– Phase-phase faults, unbalance current, open circuits on neutral side:

• Negative sequence relays – TRIP

– Tertiary bus ground detection – alarm (?)

• TVA (original design):– Turn-to-turn faults:

• Same as C37.109

– Phase-phase faults, unbalance current, open circuits on neutral side:

• Negative sequence relays – ALARM

– Tertiary bus ground detection – alarm (?)

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Revised TVA Standard – Split Winding Reactor Protection

Element Original Design Revised Design46 negativesequence

Alarm only Trip reactor breaker & arm reactor breaker failure

51 phase overcurrent

Trip transformer bank

Trip reactor breaker & transformer bank

87 split-winding differential

Trip reactor breaker

Trip reactor breaker & arm reactor breaker failure

50BF reactorbreaker failure

Not installed Trip transformer bank

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Revised TVA Standard – Split Winding Shunt Reactor Protection

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3018 87 5187

87T

500kV

161kV

13kV

45MVAR (three-phase) split-winding shunt reactor

Trip/lockout 500kV & 161kV PCBs

AUX96LOR

Trip 3018 & BFI

3017

46

BFBFI

Any relay trip to 3018

62

Tertiary bus ground detection• Previous action: Alarm only

– With no guidance given to operators on what to do

– Next ground fault results in fault currents between 60kA to >200kA

• Revised action: Trip transformer bank after 15 cycles– Field to inspect tertiary

buswork, transformer tertiary bushings, etc

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Equipment must NOT remain energized with current flowing after a protective relay operation!!!

Conclusion• Protection application to

neutral-switched equipment unique

• Event illustrated danger to personnel & catastrophic equipment damage

• Protection MUST ensure equipment is deenergized on faults & abnormal conditions

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(remains of copper stinger)

Questions?

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