Research On Control Strategy Of Microgrid Inverter Under Unbalanced Load

As a flexible Distributed Generator(DG)organization,Microgrid has the advantages of high efficiency in energy utilization and good economic performance.It has been favored by various countries in the field of new energy research.However,because of the three-phase asymmetry of the microgrid impedance and the random imbalance of the single phase load on the user side,the voltage imbalance in the microgrid is particularly common.It is one of the important problems to maintain the voltage stability in the isolated island mode.Microgrid inverter is the basic interface unit of most micro source access to.microgrid.Reasonable and effective control of microgrid is directly related to the safe and stable operation of microgrid.This paper focuses on two aspects of unbalanced voltage control of inverter port and unbalanced voltage coordination of microgrid bus voltage.Based on the comprehensive analysis of the background and current research situation,starting from analyzing the mechanism of unbalanced voltage caused by unbalanced load,This paper reveals the cause of voltage unbalance.Next,we discuss several commonly used unbalanced voltage control strategies for microgrid inverters,and establish corresponding system simulation models for simulation comparison.Then,based on improving the output voltage imbalance of the microgrid inverter and controlling the dynamic performance,a control method based on negative sequence voltage feedforward is proposed.The negative sequence voltage compensation is calculated by detecting the current dynamic current of the microgrid inverter,and the negative sequence voltage drop is compensated by the potential of the inverter to compensate the negative sequence voltage drop on the output impedance,thus the fast compensation for the unbalanced voltage of the microgrid inverter port is realized.The proposed control method is simulated by MATLAB/Simulink,and compared with the double PQ control method,the validity and correctness of the proposed negative sequence voltage feedforward control method is verified.Many loads in the microgrid system are mounted on the common bus(PCC),and the imbalance of the load and impedance parameters of the common bus will also lead to the imbalance of the PCC voltage,which affects the power supply quality of the load.Aiming at the problem of three-phase unbalanced load at the common bus in the microgrid system,this paper,starting with the study of the PCC voltage imbalance mechanism,proposes a kind of microgrid voltage unbalance coordination compensation control strategy.The method includes four aspects:the unbalanced compensation loop,the droop control loopy the virtual impedance loop and the voltage current loop.In order to improve the voltage balance of microgrid inverter port,we can take into account the imbalance of PCC voltage control.To maintain local load and bus load voltage balance stability.Finally,through the simulation of the parallel model of two microgrid inverters in the MATLAB/Simulink environment,the effectiveness of the coordinated compensation control method for the parallel system of multi machine microgrid inverter with unbalanced load is verified.