Improved Grid Synchronization Algorithm for DG System using and UH PLL under Grid disturbances

Distributed Generation (DG) System is a small scale electric power generation at or near the user's facility as opposed to the normal mode of centralized power generation. In order to ensure safe and reliable operation of power system based on DS, grid synchronization algorithm plays a very important role. Unbalanced Harmonic (UH) based phase locked loop (PLL) aimed to provide an estimation of the angular frequency and both the positive and negative sequences of the fundamental component of an unbalanced three- phase signal. The UH PLL does not require transformation of variables into synchronous reference frame coordinates. Therefore, the proposed scheme is not based on the phase angle detection. Instead the angular frequency is detected and used for synchronization purposes. The design of this PLL is based on a complete description of the source voltage involving both positive and negative sequences in stationary coordinates and considering the angular frequency as an uncertain parameter. Therefore UHPLL is intended to perform properly under severe unbalanced conditions and to be robust against angular frequency variations, sags and swells in the three-phase utility voltage signal. INDEX TERMS: Grid synchronization, phase locked loop, power quality, grid disturbances, positive and negative sequence detection, synthesis circuit, adaptive control, frequency estimation.