Research On Hybrid Filtering Method Based Phase-Locked Loop For Three Phase Grid Synchronization

Grid synchronization technique is an important aspect of distributed power generation systems and renewable energy integration applications.It has also been widely used in islanding detection of microgrids and power quality control technique.Proper grid synchronization with the utility grid is an issue of high importance for grid-connected power converters.With the proliferation of nonlinear electrical load and distributed power generation systems,many non-ideal grid voltage conditions often occur to grid voltage,which reduce the detection accuracy of grid phase and grid frequency of grid synchronization technique.To improve the detection accuracy of phase locked loop(PLL)and dynamic performance under non-ideal grid voltage conditions,four novel hybrid filtering stage based three phase PLLs are proposed in this paper.The main contribution of this dissertation is as follows.Firstly,the conventional SRF-PLL structure is studied.Based on the analysis of many SRF-PLL structure based PLL,a common model of three phase PLL is established as a research tool for the design procedure of PLL and analysis method.Then,the voltage sequence components analysis of non-ideal grid voltage is carried out,which support the design procedure of filtering stage in proposed PLLs.Secondly,a complex notch filter based three phase PLL is proposed.The filtering stage of several advanced PLLs are analyzed in complex domain.A complex notch filter is proposed and integrated with MAF as a cascaded hybrid filtering stage.Then,a novel PLL is proposed by integrate the hybrid filtering stage into the inner loop of QT1-PLL.According to the model of conventional SRF-PLL,the small-signal model of the proposed PLL is presented and used for control parameter design procedure and stability analysis.The accuracy of the small-signal model is examined by simulation results.The effectiveness of the proposed PLL is confirmed by experimental results.Next,an adaptive notch filter based three phase PLL is proposed.An adaptive notch filter is designed for eliminating the fundamental frequency negative sequence voltage component and integrated with MAF in cascaded way as a hybrid filtering stage.The parameters of the hybrid filtering stage is designed.Moreover,the proposed filtering stage with dc-offset rejection capability is also presented.By integrating the hybrid filtering stage into the inner loop of QT1-PLL,a novel three phase PLL is proposed.The design guidelines of the proposed PLL is given.The stability analysis of system is also presented.The accuracy of the small-signal model and effectiveness of the proposed PLL are confirmed by simulation and experimental results.A modified DSC based three phase PLL is proposed in this paper.The conventional DSC based filtering technique is studied at first.Then,a modified DSC(MDSC)filtering unit is proposed and integrated with MAF in cascaded as a hybrid filtering stage.A novel PLL is proposed by integrating the hybrid filtering stage into QT1-PLL.Small signal model of the proposed PLL is presented.Parameters design guidelines are given.The stability analysis of the proposed PLL is also examined.The model accuracy of the proposed PLL is examined through simulation.The effectiveness of the proposed PLL is confirmed by experimental results.At last,a three phase PLL with dc offset rejection capability is proposed.Several existing dc offset rejection method is studied at first.Then,a novel dc offset filter based on MDSC is proposed and integrated into the conventional SRF-PLL to remove dc voltage component.Parameters design guidelines and stability analysis of the proposed PLL are given.The experimental results confirm that the proposed PLL can remove dc offset completely and the dynamic performance is satisfactory.