Unsteady voltage with AC8C Exciter during dynamic simulation
I'm performing fault-ride-through simulations, but I experience issues with the stability of the Exciter. We have run successful simulations with ST7C, and AC8B, but not when using the AC8C. During dynamic simulations with no fault applied, the voltage should be steady, but it is not. It starts to oscillate, and the rotor angle is "dragged" out of sync.
The hydropower generator has a brushless excitation system, which is why we believe AC8C is the correct excitation to use. AC8C should just be a more detailed version of AC8B, I believe, and AC8B provides a stable dynamic simulation. I have even tried "recommended parameters" for the model, not the parameters the client provided me, for the model, which is still unstable.
So I guess my question is, is there a known error with the AC8C model in PSSE v35? Or may the issue be an error in the load flow case set-up itself?
The system is modelled as a simplified 4 bus system with the grid in one end (swing bus), and the generator in the other, with a transformer and a line connecting them.
Can you show the DYR record for the model?