Research On Control Strategy For Grid-connected Inverter Under Unbalanced Grid

In recent years,the penetration of the distributed energy source in the grid is becoming higher and higher.This phenomenon will have an effect on the safety and stability of the power system.Distributed generation technology should have the ability to regulate the voltage amplitude and frequency of power grid.Also should participate in power regulation of the power system to make power system operate safely and stably.Foreign scholars have proposed the concept of Virtual Synchronous Generator(VSG).The VSG can make the grid-connected inverter has the advantages of synchronous generator which can make distributed power friendly to the grid.The VSG will improve the safety and stability of distributed generation systems.However,unbalanced grid voltage will have a serious impact on PV systems.Therefore,the control problem of the inverter under unbalanced grid voltage has been increasingly emphasized.In this paper,the control strategy for grid-connected inverter under unbalanced grid is discussed.Firstly,the adverse effects of unbalanced grid are explained.The thesis analyzes the control method of VSG under unbalanced grid and summarizes their advantages and disadvantages.What's more,because of power oscillation and grid current distortion caused by unbalanced grid,the negative-sequence current controller is put into the traditional VSG control strategy.The coordinated control strategy for grid-connected inverter is proposed.The instantaneous power theory is employed to compute the reference negative-sequence current command,which suppress the active-power oscillation,reactive-power oscillation and unbalanced current respectively.Finally,the adjustment coefficient is introduced to establish a unified analytical formula of three controls.The control strategy can achieve the coordinate control of power and current.Considering the over-current problem will be appeared during the fault ride-through of the grid-connected inverter under unbalanced grid.It's easy to cause damage to electrical equipment.According to the photovoltaic array's output power characteristics and analyzing the P-u relationship between photovoltaic output power and photovoltaic voltage.This paper presents a maximum power and constant power tracking double-mode variable power tracking control strategy.Collecting photovoltaic voltage and current and comparing the constant power with the PV power for changing the voltage of PV.Under normal condition,the PV array on the basis of the MPPT(Maximum Power Point Tracking)and CPG(Costant Power Generation)control output;the CPG plays an important role while the unbalanced grid fault occurs.Lastly,simulation models are built in Matlab/Simulink to verify the validity of the improved strategy of VSG and variable power output control strategy.The RT-lab platform is used as the controller,verify the practicability of the proposed strategy.