A novel PLL for grid synchronization of power electronic converters in unbalanced and variable-frequency environment

Grid synchronization is one of the most important issues of the distributed power generation system (DPGS) connected to utility network. This paper proposes a novel phase-locked loop (PLL) for synchronization of power electronic converters in unbalanced and variable-frequency environments. The method utilizes double rotating reference frames to transform the input signal and detects the positive fundamental component and negative sequence component simultaneously. It's based on a synthesis circuit which generates the orthogonal and in-phase signals to decouple input signal. This method detects the phase without distortion even in variable-frequency environment comparing with existing synchronization methods. Structural simplicity of the method greatly simplifies its implementation in digital software and/or hardware environments. This paper gives a description and derivation of the method. Its excellent performance is verified by simulation.