Research On Control Strategy Of Flexible Multi-state Switch Under Unbalanced Grid Voltage

With the access of non-linear fluctuating loads such as distributed power sources and electric vehicles and the increasing use of electrical energy,the power quality problems of the distribution network are becoming more and more serious,and the three-phase imbalance is becoming more and more serious,which seriously affects the load.Uptime and safety.Facing the increasing challenges of the distribution network,flexible multi-state switch?FMSS?is proposed as an effective solution to improve the power quality of the distribution network.This article takes the three-port flexible multi-state switch with back-to-back structure under three-phase unbalance as the research object,first analyzes the working principle of the three-port flexible multi-state switch,and derives the synchronization of the flexible multi-state switch under the three-phase unbalance.Rotate the dq coordinates to analyze the power relationship of FMSS under imbalance.After introducing several methods of separating positive and negative sequence components,a new method of positive and negative sequence components is proposed,which has better dynamic performance and steady state.The U_dcQ control and PQ control of FMSS in the case of three-phase equilibrium are introduced,and the shortcomings of this control when facing three-phase imbalance are analyzed.Based on the analysis of the existing three-phase imbalance methods,the phase angle of the negative-sequence compensation current is adjusted and optimized with the unbalance of the point of common coupling?PCC?as the feedback amount,so that the PCC voltage meets the grid-connected index and the negative-sequence current The smallest.At the same time,for the asymmetrical voltage drop,using the characteristics of FMSS feeder resistance,a method for optimizing the active power and reactive power coordination voltage is proposed.At the same time,in order to solve the problem of double-frequency fluctuations of the DC-side voltage caused by active power fluctuations when the three-phase is unbalanced,the principle of the DC-side voltage fluctuation and the interference terms of the DC-side voltage are analyzed,and a new positive-negative sequence double-loop DC-side voltage is proposed.Control Strategy.The positive sequence uses power feedforward control to stabilize the steady-state error of the DC voltage.The negative sequence uses single sinusoidal signal resonance feedback control to suppress the periodic fluctuation of the DC voltage.This strategy can effectively suppress the DC side of the flexible multi-state switch under the simultaneous imbalance of multiple ports.The ripple of the voltage,the output current has no harmonic current and the current peak is limited within the rated range.Considering the problem of PCC voltage imbalance caused by suppressing DC-side voltage fluctuations,a method of selecting a port control mode is proposed,and the DC-side voltage control port is selected according to the actual situation of each FMSS port,so that the overall FMSS imbalance is low.Finally,a three-terminal FMSS simulation platform was set up in simulink,and simulation experiments were performed under imbalance in various cases.The simulation results verified the effectiveness and superiority of the proposed scheme.