Grid synchronization algorithm for distributed generation system during grid abnormalities

Abstract

Distributed Generation (DG) System is a small scale (less than 20 MW) electric or shaft power generation at or near the user‟s facility as opposed to the normal mode of centralized power generation. DGs include wind energy, photo voltaic system, fuel cell and combined heat and power (CHP) generation etc. To ensure safe and reliable operation of power system based on DG system, usually power plant operators should satisfy the grid code requirements such as grid stability, fault ride through, power quality improvement, grid synchronization and power control etc. Number of synchronizing algorithms has been discussed in the literature to track the phase angle of the grid voltage or current to synchronize DG system with the grid. One of the earliest introduced methods for tracking the phase angle is Zero Crossing Detector (ZCD) and it gives better response during balanced grid conditions. But the occurrence of power quality problems, especially in weak grid, leads to malfunctioning of the controllers based on ZCDs. An alternative to ZCD is the Phase Locked Loops (PLLs) for grid synchronization which provides better response with accuracy. Different PLLs have also been proposed and are studied extensively in the available literature starting from Charge Pump PLL to Linear PLL and Synchronous Reference Frame (SRF) PLL which can very well track the phase angle and frequency during balanced conditions. But during abnormal conditions like frequency deviations, voltage unbalances and power quality problems such as voltage flicker, voltage dip, voltage sag, distorted grid voltages, these PLLs fails to track the phase angle and frequency with good accuracy. These shortcomings are overcome by Double Synchronous Reference Frame (DSRF) PLL and a PLL based on Second Order Generalized Integrator (SOGI).In this work the above mentioned PLLs are analysed for different abnormal grid condition which is followed by the comparative study between two advanced PLL techniques (DSRF PLL and SOGI based PLL). The analysis is carried out in MATLAB/SIMULINK environment and the obtained results are discussed for effectiveness of the study.