Stability Analysis and Design of Volt-VAR Controller for Grid Connected PV Systems with Consideration of the Impact of Voltage Feedforward

The high penetration of PV systems in distribution networks might give rise to overvoltage. A solution is controlling the reactive power output of the PV inverters based on V-Q droop control method. In this method, when the PV terminal voltage exceeds a threshold, the PV inverter absorbs reactive power. The reactive power absorption produces a voltage drop across the distribution transformer and the lines, hence mitigating the overvoltage. In order to ensure stability, the V-Q droop controller is accompanied by a low-pass filter. The cut-off frequency of the reactive power filter has a significant impact on the system dynamics. In this paper, the impact of the filter's bandwidth on the small signal stability of the system and the system's dynamic response is analyzed. It is shown that while adopting a lower bandwidth for the V-Q droop filter can ensure the stability, it might cause transient voltage overshoots when the active power output undergoes fast changes due to cloud passing. This issue is partly due to the unwanted interaction between V-Q droop controller and the voltage feedforward control. By utilizing a lowpass filter in the voltage feedforward path, the stable operation and fast dynamic response can be achieved simultaneously.