Study on soft phase locked method to solving the synchronization problem of active power filter in stand-alone power grid

Traditional LC filters can´t work stably in small rating stand-alone power grid. So active power filter (APF) is becoming an important tool to solve the power quality problem in small rating stand-alone power grid. In most current detection algorithm of APF, it needs a synchronizing signal. Firstly, the influence of the synchronizing signal error on current detection of APF is analyzed in this paper. Voltage distortion, voltage unbalance and frequency fluctuation are frequent and severe in small rating stand-alone power grids, so traditional synchronizing signal obtaining method--phase locking based on zero-cross detection can´t work effectively in small rating stand-alone power grid. Then a soft phase locked loop with additional filter is proposed. 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 grid. 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.