Research On Three-phase Load Imbalance Regulator

With the rapid development of the national economy,the scale of the power system is also expanding,and people’s requirements for the quality of power supply are increasing.In the three-phase four-wire low-voltage distribution network,the unbalanced three-phase load has a bad impact on the power quality.On the one hand,the use of a large number of single-phase loads and reactive loads causes the threephase load current to be asymmetrical and the power factor decreases which affect the power quality.on the other hand,the existence of nonlinear loads causes distortion of the load current waveform,and the generated harmonic current threatens the safe operation of the power grid.Aiming at the compensation and treatment of negative sequence,zero sequence,reactive power and harmonic currents caused by load imbalance in distribution network,this thesis studies and designs a parallel three-phase load imbalance regulator to improve power quality.This thesis first analyzes the power characteristics of several unbalanced loads,and then introduces the basic operating principle,mathematical model and several common modulation strategies of three-level grid-connected converter.In view of the neutral point voltage problem caused by the three-level topology,a mathematical model of the neutral point voltage is established based on SPWM,and the self-balancing of the neutral point voltage when the regulator is connected to the grid and outputs different forms of compensation current is analyzed.The analysis puts forward an active control method that can quickly balance the neutral point voltage when the neutral point voltage shifts.In order to realize the grid-connected compensation of the neutral point,this thesis studies the phase-locking technology under the unbalanced grid voltage and the extraction method of negative sequence,zero sequence,reactive power,and harmonic command.Finally,this thesis uses a double closed-loop control method to realize the grid-connected compensation operation of the regulator.The voltage outer loop uses PI adjustment to achieve DC side stability,and the current inner loop uses a deadbeat control strategy with the current prediction correction and error compensation algorithm to achieve the accurate control of three-phase current.The establishment of a simulation model and an experimental platform verify the feasibility and effectiveness of the proposed control algorithm in two aspects.