Soft Phase Locked Loop for Active Power Filter Applied in Small Rating Stand-alone Power System

Traditional LC filters can't work stably in small rating stand-alone power system. So active power filter (APF) is becoming an important tool to solve the power quality problem in small rating stand-alone power system. In this paper the influence of the synchronizing signal error on current detection of APF is analyzed. Voltage distortion, voltage unbalance and frequency fluctuation are frequent and severe in small rating stand-alone power systems, so traditional phase locking method based on zero-cross detection can't work effectively in small rating stand-alone power system. Then a soft phase locked loop with additional filter is proposed to solve this problem. It can lock the phase angle on to the positive sequence of fundamental voltage accurately and rapidly. It ensures the performance of APF applied in the small rating stand-alone power system. Moreover, the soft phase locked loop is easy to be implemented in a digital signal processor (DSP). Simulation and experimental results validate that the soft phase locked loop has satisfactory performance.