Generator Synchronizing Issue

J

Thread Starter

Jim

Friend called me with this issue. He is overseeing testing of a new generator stator that was re-wound by GE, we will call it G2. Installed the stator. G1 and G2 already independently checked with a phase rotation meter, check out good. Turned on G1 and closed the G1 CB, load is a resistive only load bank. Fired up G2. Turned on sync scope of G2. Sync scope is rotating slow in the CCW direction. Attempted to increase speed on G2. Sync scope rotates fast in the CCW direction.
 
Jim,

When the speed of G2 was increased, what did the governor say happened to the prime mover speed--did it increase, or did it decrease?

Is your friend certain that the operator was increasing the speed of G2--because if the speed of G1 was actually being increased then the synchroscope, indeed, slow down (presuming the speed of G2 did not change). I presuming you were trying to synchronize G2 to G1 and the load bank--which wasn't clear.

Usually, the synchroscope only uses a single phase of each source (generator (incoming), and bus (running)), so this is another reason why misconnected phase leads wouldn't result in frequency decreasing when speed was increased (with the governor).

You didn't say what the generator prime mover was, and you didn't say if any work was done on the prime mover when the generator was being re-wound. Also, was any work done on the governor when the generator was being re-wound?

I've seen this a couple of times, and both times it was related to the governor driving the control valve in the wrong direction.

Even if the phases were somehow misconnected, frequency is frequency. So, if the rotation were somehow C-B-A instead of A-B-C if the speed of the generator rotor is increased the frequency will increase, and if the speed of the generator rotor is decreased the frequency will decrease.

Older steam turbines had DC motors to change the steam flow-rate (easily reversed by changing polarity of the applied voltage). And, both of the instances I observed this was immediately after a planned outage where the speed changer DC motor leads had been reconnected improperly.

The synchroscope looks at the difference between bus (running) and generator (incoming) frequency (which is directly related to speed). Even if the potential transformers (PTs) feeding the synchronizing circuit were misconnected after the outage, it's still unlikely something associated with the rewind is causing this problem.

Please write to let us know how your friend resolves the problem.
 
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