Research Of Control Strategies Of Grid-connected Photovoltaic Inverter Under Unbalanced Faults

In recent years,new energy power generations such as photovoltaic(PV)power generation have got attention all over the world and have been vigorously developed.Because of the advantages of clean,no-pollution,convenient operation and installation,photovoltaic power generation has become one of the most important power generations.When unbalanced faults occurs,there will emerging problems such as power fluctuations,output current distortion and excessing peak current in PV inverter,which seriously affect the stability of PV system.This paper first analyzed the mathematical model of PV inverter and current control structure under synchronous reference frame and stationary reference frame are obtained.The reasons of power fluctuations and excessing peak current are analyzed in theory.Several typical reference current algorithms of PV inverter have been analyzed,the advantages and disadvantages have been summed up.On this basis this paper proposed an improved reference current algorithm contains three parameters concerning power fluctuations,output harmonic currents and peak current,the relationships of harmonic current,power fluctuations,peak current and parameters are analyzed,based on the sequential quadratic programming algorithm,the determination principle of adjusting parameters are proposed.Then an improved reference current algorithm is proposed thinking about power fluctuations and peak current suppression without the harmonics,power fluctuations and peak currents can be controlled.Based on the improved reference current algorithm,a flexible active and reactive power coordinate control strategy is proposed to expand the active power ability of PV inverter when unbalanced faults occurs,and it can be in strict accordance with grid codes.Aimed at problem of excessing peak current of inverter under unbalanced faults,this paper proposed a flexible active power adjusting control strategy considering peak current control for distributed PV inverters.Through coordination between different control objectives,the output active power is increased,while the power fluctuation is decreased.The simulation results validate the effectiveness of the proposed method.