When 2 out of 3 poles of the circuit breaker of a Generator are closed


Thread Starter

Santanu Das

I heard that during opening of a Circuit breaker of a generator connected with a bus (synchronized), if 2 poles gets open & one closed nothing so harmful will happen. but if 2 is closed & one open the massacre of generator will happen. Why so? plz explain with links if u have...
Santanu Das,

The answer to this is complicated to explain in words. The simple, but not detailed, answer is that normally a three phase synchronous machine would have constant torque as a result of the mmf produced in the air gap by the sinusoidal current on each of the three phases. If one phase of the machine opens, the remaining two phases will produce a sinusoidal torque that varies between zero and a maximum at the synchronous frequency (the torque is no longer constant). It is this varied torque that causes the problem for the machine. A three phase machine running on a single phase machine will not have this problem.
Santanu... if closure of the faulted-breaker at the instant both the generator-side PTs and the 330kV-side PTs (or those on both-sides of the faulted breaker) indicate they are "Synchronized" then nothing should happen!

But, as soon as armature-current increases because of an action by the fuel/speed command to the prime-mover, then unbalanced phase-currents should result in a trip-action by the negative-sequence protection relays!

If, trip-action is materially delayed then other consequences will evolve!

Regards, Phil Corso
Santanu... The disturbance question I replied to was just the opposite of what you described, and I apologize for the misunderstanding!

Addressing your situation; is the faulted-breaker you referred to the 220kV breaker connecting the GSU to the transmission-line?

Phil Corso