2 parallel generators sharing 3 GSU transformers fighting over voltage control

[9wearSuave]

We have two identical synchronous generators connected at low side 13.8 sharing 3 GSU- 2 identical 138kV GSU's and 1 69kV GSU. Each generator has its own Basler decs 300 AVR and utilizes reactive droop compensation of 5%. Following a disturbance, the generators will cause MVAR oscillations one AVR must be switched to manual to make it stop. Is there a special configuration for 2 generators sharing 3 GSU's?

 

[pwrengrds]

Better tuning or cross current compensation. Are they tuned the same way?

David

 

[bacon4life]

I do not see why the GSU arrangement would require anything unique. A few things to check:
-Is the droop is looking the correct direction? Are the droop CTs shorted?
-Are there overly aggressive power system stablizers?
-Are there any controld from SCADA or the plant control system that impact voltage control? I have seen oscillations at a plant when SCADA issues a new setpoint to only some units at a plant.
-It sounds like the 138 kV and 69 kV grids are tied through the 13.8 kV bus. Is there significant flow between the two transmission voltages via the generator bus?

 

[9wearSuave]

At normal operation of both units, MVAR flows into 1 138 GSU terminal and also the 69kV terminal, and MVAR flows out of the other 138GSU terminal. It seems as if VARs are circulating on the 13.8 bus