STATOR CURRENTS IN AC GENERATOR

[powergage]

We recently ran an URGE test on a steam turbine generator. We didn't experience any problems, but after reviewing some data, we realized that there is some variation in the stator currents for the different phases. We measured up to about a 3% difference between two of the phases. My question is: Is this a problem? I am not sure if this is typical of steam turbine generators or if this is something to be addressed and researched. Any input appreciated. Thanks!!

 

[ERaySir]

Phase unbalance could be a result of rotor alignment. The rotor should be aligned when it is warmed up and running at or close to rated output. When the unit is first synchronized it is common for there to be some unbalance as there is still some sag in the rotor. The alignment is an involved task; it must be done cold as per the manufacturer’s specifications and present temperature, then checked hot. One method of checking is machine vibration as compared to manufacture’s spec. Another crude option is a stroboscope to see where the end of the shaft ends up in comparison to the hosing. A millwright would have more accurate means.

System voltage unbalance is another factor that could cause phase unbalance.

Ray Micallef, P. Eng.
Power Generation/Utility Industry

 

[powergage]

Thanks, ERaySir.
We feel confident that our rotor alignment is good. However, we do have a voltage unbalance which is likely causing the problem.