Generator Breaker Failure Applications · PDF file2 • Overview of Standards discussing...
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Generator Breaker Failure ApplicationsChristopher Dall, Dennis Tierney
Calpine Corporation
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• Overview of Standards discussing breaker failure for generator applications
• Case studies– CT location– Flashover protection– Noise and instantaneous re-trip– Breaker lockouts– Current detectors (exciters online or offline)
Contents
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• Two main standards to consider:– IEEE Std C37-102– IEEE Std C37-119
• Breaker failure has two classifications:– Failure to Trip (e.g. mechanical issues)– Failure to Clear (e.g. loss of dielectric, flashover)
Overview of BF for generator applications
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• Other situations not classified as Breaker Failure (C37.119)– Loss of dielectric– Contact flashover
• Given the potential damage to a generator these situations should be considered in a breaker failure scheme.
Overview of BF for generator applications
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Minimum fault current scheme (C37-119)
Overview of BF for generator applications
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Flashover protection scheme (C37-102)
Overview of BF for generator applications
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• Consider all possible fault locations…
Case 1 – Current transformer location
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Case 1 – Current transformer location
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• After the BFI has de-asserted necessary to re-enable breaker failure scheme for flashovers
Case 2 – Flashover protection
GSU High Side Currents
GSU Low Side Currents
GSU NeutralCurrent
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Case 2 – Flashover protection
*Current detector must be set sensitively enough to declare breaker failure
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• Consideration for flashover protection:
1. Use separate 52a and 52b contacts (i.e. do not invert 52a)
2. Implement a 52 contact disagreement scheme3. Ring bus and breaker and a half schemes should
require both 52b contacts to enable• Susceptible to tripping for transmission LG faults
Case 2 – Flashover protection
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• For latched BFI signals or instantaneous re-trips noise can cause nuisance tripping
Case 3 – Noise on Breaker Failure Initiate
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Case 3 – Noise on BFI and instantaneous re-trip
*62BFI + 62BF = total breaker failure time
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Case 4 – Circuit breaker lockouts
• Circuit breakers may have alarms which will block them from opening– Low spring charge– Low gas (SF6) pressure
• In the event of a protective trip we must declare a breaker failure if we know the circuit breaker is blocked from opening.
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Case 4 – Circuit breaker lockouts
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• Depending on the system conditions when the generator circuit breaker is required to open the currents can vary drastically– System fault– Problem at minimal load
• If the breaker fails and the turbine and exciter have been tripped the generator will be motored potentially causing serious damage to the rotor, stator and turbine.
Case 5 – Current detector pickup
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• Example of unbalanced currents on generator after breaker failure
Case 5 – Current detector pickup
IN101TRIP1OUT101
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Case 5 – Current detector pickup
Simplified Sequence network for two‐phase flashoverGSU High Side Currents
GSU Neutral Currents
GSU Low Side Currents
Ibsys – base current on system voltageIbase – base current on generator voltage
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• If the exciter remains online after the failed GCB is opened then the current magnitudes can be undetectable on the GSU high voltage side
Case 6 – Detection with exciter online
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Case 6 – Detection with exciter online
GCB Trip Signal SWYD CB Opens
0
25
50
75
100
125
0
500
1000
1500
2000
2500
Current (referred to 345 kV)
Current (referred to 18 kV)
Time
A (18 kV @gen)A (345 kV @SWYD)Min 50BF Pickup
Measured current
Minimum Pickup for 50BF in switchyard relay
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Case 6 – Detection with exciter online
GCB Trip Signal Exciter Tripped SWYD CB Opens
0
50
100
150
200
250
300
350
0
500
1000
1500
2000
2500
3000
3500
4000
Current (referred to 345 kV)
Current (referred to 18 kV)
Time
A (18 kV @gen)A (345 kV @SWYD)Min. 50BF Pickup
Measured current
Minimum Pickup for 50BF in switchyard relay
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Case 6 – Detection with exciter online
GCB Trip Signal
SWYD CB Opens
‐10
‐5
0
5
10
15
20
25
30Generator MW/M
VAr
Time
MWMVArRev. Power (MW) PUReverse power element
pickups
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Case 6 – Detection with exciter online
GCB Trip Signal Exciter Tripped SWYD CB Opens
‐60
‐40
‐20
0
20
40
60Generator MW/M
VAr
Time
MWMVArRev. Power (MW) Pickup
Reverse power element pickups
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Case 6 – Detection with exciter online
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• Locate Current transformers on the circuit breaker
• Desensitize the scheme to noise
• Special considerations for flashover protection
• Current detectors based on motoring events
• Use of reverse power element to detect failure
Conclusions
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• Power plant operators should have defined procedures in the event that a breaker failure of a GCB goes undetected by the protection.
Conclusions