A Method to Improve PLL Performance Under Abnormal Grid Conditions

Phase tracking system or phase locked loop (PLL) is an important part of the control unit of a grid connected power converter. Popular methods such as zero crossing detection (ZCD) and synchronous reference frame (SRF) PLL do not produce accurate phase information when grid is non-ideal. In this work a method to obtain accurate phase information when the grid voltages are unbalanced by adding a moving average filter to a SRF PLL has been proposed. The performances of the PLL have been experimentally verified for abnormal grid conditions such as unbalance, harmonics, voltage sag and swell, faults etc. Generally, during a fault phase voltages become unbalanced so this method can give better fault ride through capability to a power converter