Generator Circuit Breaker

Out of phase in generator circuit breaker (GCB)

An out-of phase condition occurs when a making operation of the GCB is performed at the instant when there is no synchronism between the voltage phasors of the generator at one side of the GCB and the external grid at the other side.
Another example is when a generator is running out-of-phase as a result of system instability and the GCB has to be tripped.
The severity of this interruption depends on the out-of-phase angle δ. Since the generator is at risk for values of δ larger than 90◦, the protection relay trips around δ=90◦. The standardized out-of-phase TRV values are based on a 90◦ out-of-phase angle at rated voltage. For smaller generator units, a large out-of-phase angle can, however, occur.
In the case of an out-of-phase angle of δ=90◦, the current is about 50% of the fault current supplied by the system. On the voltage side, the GCB experiences a TRV with a RRRV roughly of the same order as in the system-source fault, but with a peak value nearly two times higher.
The out-of-phase current specified in the standard is half the system source-fault current.
Fig shown standardized TRV shapes of various generator faults compared with TRV of a 100% fault
for a 24 kV GCB.


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