For a field oriented control startegy with fixed torque angle approach, used to control a permanent magnet synchronous generator for wind turbines.

The q-axis current refrences is determined as an output of the velocity controller. While simulating for a turbelent and stochastic wind, the velocity of the PMSG changes permanently leading to severe fluctuations in the q-axis current reference that changes signs rom negative to positive and vice versa throughout the simulation. Now, the PMSG torque is a function of the q-axis current in the adopted control approach. Thus, as the current changes signs the torque does too resulting in a bhaviour that oscilate between a generator mode and a mototr mode for the PMSG torque.

How can we explain that ? Is it caused by the velocity controller or the nature of the wind ?