Grid Integration of Three-phase Inverter using Decoupled Double Synchronus Reference Frame PLL

Distributed generation system (DG) coupled to utility grid are in need of proper grid synchronization which is one of the most significant issues to be addressed. This study suggests a phase-locked loop (PLL), decoupled double synchronous reference frame (ddsrfPLL) for synchronization of grid connected inverters in unbalanced grid and variable frequency conditions. The concept uses two reference frames to convert the input voltage so as to recognize the positive and negative sequence component simultaneously. A synthesis circuit generating the quadratic and in-phase signals is utilized for decoupling the input voltage. This technique identifies and tracks the frequency and phase angle without distortion compared to synchronous reference frame PLL (dqPLL) even in variable-frequency environment. This paper addresses description and derivation of the both dqPLL and ddsrfPLL method. Its good performance is demonstrated by simulation and comparison results are presented.