Improved Phase Lock Loop system for grid connected renewable energy sources under distorted conditions

Under distorted grid conditions with high bandwidth, synchronous reference frame (SRF) Phase Lock Loop (PLL) is able to work properly but increased bandwidth introduces harmonics which affect the PLL output and SRF PLL is also not able to produce accurate phase information under distorted grid conditions. This paper suggests a new phase detection technique based on cosine look-up table (cLUT) using sliding discrete Fourier transform (SDFT) for grid interfaced renewable energy sources (RES) under non-ideal grid conditions. The cLUT offers precise locking by acting as an integral controller such that the competence of the SDFT PLL is improved, steady state error in the output frequency of the voltage controlled oscillator (VCO) is reduced, and the input signals are also synchronized with generated sine and cosine reference signals. The proposed improved PLL system is mathematically analyzed and final enhancement to the system performance is studied under amplitude deviation and three phase voltage unbalance in the grid voltage and the results are compared with predictable SDFT PLL system.