SRF-PLL with in-loop differentiator decouple filter for unbalanced three-phase systems

Abstract

It is vital to track voltage phase to synchronization control for grid-connected power converters. For unbalanced three-phase grid voltages, the PD output of conventional SRF-PLL contains disturbances oscillating at frequency of double fundamental, which introduced steady-state phase error for detecting positive sequence at fundamental frequency. To solve this problem, this paper presents a new differentiator decouple filter (DDF) applied in-loop of SRF-PLL. The DDF uses simple mathematical operation to separate the dc terms of PD output in synchronous reference frame The filter has been well addressed in details, especially the digital implementation of differentiator based on second order generalized integrator (SOGI). As a result, the loop filter parameter tuning makes it possible for high bandwidth of the SRF-PLL. The simulation of proposed SRF-PLL was shown using Matlab/Simulink, and test results show that the steady state error of detected phase angle is rather small, and faster transient response is achieved thanks for high loop bandwidth. Also, the experiments results based on TMS320F28335 are presented to verify the theoretical discussion.