Research On Hybrid Filtering Method Of Three-Phase Grid-Connected Phase-Locked Loop

As a important technology in the grid-connected devices application field,grid synchronization technique is widely used in distributed generation and power quality control.The Synchronous Reference Frame Phase-locked Loop(SRF-PLL)has become a widely used method in grid synchronization technique for its simple structure and strong stability.In recent years,with the increased penetration of renewable energy sources to the power grid and the proliferation of domestic and industrial nonlinear loads,the power quality problems caused by harmonic pollution have become more and more serious,which make the accurate grid-connected devices synchronization with the grid under non-ideal grid voltage conditions become a challenging task.In order to improve SRF-PLL filter capability and dynamic response performance without jeopardizing its stability under non-ideal power grid conditions such as grid distortion and mixed DC offset,this paper proposes four new hybrid filters based three-phase grid-connected phase-locked loops.The main contribution of this paper is as follows:Firstly,the principle and mathematical model of the standard SRF-PLL are analyzed,and the mathematical models of the advanced SRF-PLL with a variety of additional filters are established as a mathematical model basis for the hybrid filters design.Then the voltage sequence components of the non-ideal grid voltages is analyzed,and the corresponding relationship between the grid voltage sequence components and frequencies that need to be filtered under the ?? frame and the dq frame is obtained,and the complex filter principle is also is analyzed,which provides a theoretical basis for the design of filtering stage in proposed hybrid filters.Then,to overcome the defect that the conventional Dual Second-Order Generalized Integrator(DSOGI)based PLL cannot suppress the harmonic voltage and eliminate the DC offset voltage,a dual modified novel second-order generalized integrator based three-phase PLL is proposed.A novel SOGI(NSOGI)structure is designed,and a modified NSOGI(MNSOGI)with DC offset rejection capability and a corresponding dual MNSOGI(DMNSOGI)filtering stage are proposed.Incorporating the DMNSOGI and a moving average filter(MAF)in series to form a hybrid filter,and a new three-phase PLL is formed using the proposed hybrid filter.The mathematical modeling,parameter design and model accuracy verification of the proposed PLL are performed.The effectiveness of the proposed PLL under complex grid voltage is verified by simulations and experiments.Next,to sovle the problem that the conventional Dual Third-Order Generalized Integrator(TOGI)does not have DC offset rejection capability and the conventional Delayed Signal Cancellation(DSC)filtering method has obvious delay to seriously affect PLL dynamic performance,a dual modified TOGI and enhanced DSC based three-phase PLL is proposed.Based on the conventional TOGI structure,a modified TOGI(MTOGI)structure for rejecting DC offset components and a corresponding dual modified TOGI(DMTOGI)filter are designed.At the same time,an enhanced DSC(EDSC)operator which can reduce the number of DSC operators based on modified DSC approach is proposed to effectively eliminate harmonic components.The DMTOGI is transformed to the dq frame and cascaded with EDSC to form a new hybrid filter which is incorporated into the SRF-PLL control inner loop.The parameter design and stability analysis of the new three-phase PLL are carried out.The simulation and comparison experiments verify that the proposed PLL has excellent dynamic performance and filtering capability.To improve the filtering performance and DC offset rejection capability of Complex-Coefficient Filter(CCF)based PLL and speed up the dynamic response speed-of the PLL under non-ideal grid voltage,a dual second-order complex coefficient filter(DSOCCF)based three-phase PLL is proposed.The first-order complex coefficient filter and higher-order complex coefficient filter are analyzed,and a dual second-order complex coefficient filter(DSOCCFdc)with DC offset rejection capability is designed.It is transformed into in the dq frame and cascaded with MAF to form a new hybrid filter Incorporating it into the SRF-PLL proposes a hybrid filter-based PLL.The mathematical model of proposed PLL is established,and the control parameters are designed to verify the stability of the control system.The validity of the proposed PLL is verified by simulation and experiments.Finally,to overcome the defect that existing PLL filters have obvious delay to reduce system bandwidth and have not DC offset rejection capability,a complex notch filter(CNF)and cascaded dqDSC based three-phase PLL is proposed.Based on the transfer function of the conventional first-order complex coefficient filter,a new complex notch filter(dcCNF)is proposed to eliminate the DC offset component.At the same time,the conventional dqDSC operator is analyzed and dqDSC4 and dqDSC24 are cascaded to form dqCDSC4,24 filter which is used to eliminate the fundamental voltage negative-sequence and other harmonic components of the grid voltage.Combining dcCNF with dqCDSC4,24 as a series hybrid filter and incorporating it into the inner loop of SRF-PLL to propose a new hybrid filter based PLL.The PLL mathematical model is established,the dynamic performance and filtering performance of the method are verified by simulation and experiment.