Pole Discrepancy Protection Working Principle:
Pole discrepancy protection is used to protect the generator/alternator from single phasing when the circuit breaker is failured to close his all three pole. Pole Discrepancy protection is little bit different from the local breaker backup protection. when the local backup protection operates when the circuit breaker failured to operate all three pole either closing or opening, the relay trips the upstream breaker. But, In case Pole Discrepancy Protection trips the same breaker during the delayed operation of breaker poles when we going to close the breaker. It may be due to circuit breaker internal mechanism failure hence the breaker poles will be closed unequally or delayed.
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Pole discrepancy is detected if the magnitude of the lowest phase current is lower than the fraction CurrUnsymPU(%) of the highest phase current.
Also, pole discrepancy protection is implemented in two ways..
- Breaker’s pole closing contact logic
- All three phase current the circuit breaker is measured using current transformer. If the difference between the phase currents is larger than a CurrUnsymPU this is an indication of pole discrepancy, and the protection will operate.
Consequences:
- Pole discrepancy cause unsymmetrical current in the power system.
- Negative sequence currents that will give stress on motors and other electrical loads.
- Unnecessary or Nuisance tripping of standby earth fault relay due to the flow of Zero sequence currents in the power system.
Pole Discrepancy Protection ANSI Code: 52PD
Relays acted:
- 52 PD Flag operation in protection panel
- 86 Master trip relay opted.
Note: if the breaker trips on the pole discrepancy protection, Resynch the generator. Even after two to three attempts, the machine is tripping on the same protection, probably the generator breaker is faulty. Inform to maintenance staff for rectification of the same.
