Research On Control Strategy For Microgrid Inverter Under Unbalanced Load

As the basic interface of microgrid,microgrid inverter converts the power generated by distributed power to three-phase alternating current to provide power for local load.In low-voltage microgrid,due to the universal existence of unbalanced load,the output voltage of microgrid inverter will be unbalanced.This paper has studied the control strategy of single microgrid inverter and microgrid inverters in parallel based on modeling analysis of VSI.For single microgrid inverter under unbalanced load,this paper has analyzed the output voltage of inverter in dq coordinate system and found that output voltage exists double power frequency pulsation which is the negative sequence voltage component.Then,this paper briefly introduces the traditional voltage balance control strategy and points out that the control structure and parameter adjustment of traditional voltage balance control strategy are complex.Therefore,this paper has studied the voltage balance control strategy based on resonant controller.After adding resonant controller to voltage outer loop,the gain at double power frequency of voltage loop is increased,this can reduce the negative sequence components of output voltage.For microgrid inverters in parallel,this paper first has studied the droop control,virtual inductance technology and presynchronization technology for realizing parallel operation of inverters without communication.Then,the method of positive and negative sequence separation and power calculation are introduced.This paper has studied the voltage balance control strategy based on unbalance compensation block on the basis of the above techniques and methods.The factors that generate unbalance compensation reference are unbalance compensation gain,negative sequence reactive power and negative sequence voltage.The negative sequence voltage is reduced by injecting the unbalance compensation reference into the output voltage reference.This control strategy can realize autonomous voltage unbalance compensation of microgrid inverter.In addition,the simulation model of microgrid inverters is set up in Matlab/Simulink,and the experimental platform of microgrid inverters is set up in the laboratory.The effectiveness of the above control strategy is verified through simulations and experiments.