Improved robust phase locked loop for utility grid applications

A phase locked loop (PLL) is an essential part of power-electronic converter control in utility grid applications, which is used to synchronize the control reference signals with the fundamental components of the grid voltages. In order to cope with different grid conditions such as voltage harmonics, voltage dips and frequency variations, an improved robust PLL with frequency-adaptive ability for large grid frequency changes is presented. Three different filtering methods for determining the estimated grid frequency are introduced and compared. The effectiveness of the improvement of robust PLL is verified by simulation and experimental results.