We have 2 nos 11 kV, GE frame 6 GTGs with ALSTOM make AVRs. Normally when we run our machines in synchronisation with GRID. we keep one machine in V mode to maintain Voltage and other machine on Q control to maintain VAR or P.F.

Recently due to GRID voltage being down and only one GTGS in service and in voltage mode, the excitation of generator has gone in leading mode and we could not control the voltage as GRID voltage was low.

Can you give your opinion to run single machine with GRID in Voltage control mode OR Var control mode- which is better for system stability?
Supplier recommends Voltage control with GRID if single machine is running?

 

You can run your generator any way you like, or any way you are required to run it by the grid operator of the grid you are synchronized to, as long as you keep the operation of the generator within the limits defined by the generator reactive capability curve.

The generator reactive capability curve for the generator defines the limits of generation (watts and VArs) and that should be the guiding document for how the generator is ultimately operated.

Many times grid operators have requirements about maintaining system voltage or operating at certain VAr levels during certain times, or other limits/restrictions.

But, as long as you remain with the reactive capability curve supplied by the generator manufacturer, then you should be safe.

Some exciter regulators (AVRs) have adjustable limits to prevent operation outside of the generator reactive capability curve, mostly for under-excited conditions (leading VArs) but sometimes for over-excited conditions as well (lagging VArs).

If you choose voltage control, you are telling the exciter regulator to adjust the excitation to keep the generator terminal voltage (usually) equal to the generator terminal voltage setpoint you have specified. Grid voltage can change throughout the day, and presuming power output is relatively constant that would mean that the VAr meter would change as the grid voltage changed.

If you choose VAr control, you are telling the exciter regulator to adjust the excitation to keep the VAr value equal to the VAr setpoint you have specified. In this condition, the generator terminal voltage will change as the grid voltage changes in order to maintain the VAr setpoint.

You might look at this thread for some help in understanding the relationship between VArs and terminal voltage

http://control.com/thread/1264891627

Hope this helps!