Research On Operation Control Of Unbalanced Three Phase Grid Connected Inverter

When analyzing the grid connected inverter,the grid is usually regarded as three-phase symmetrical.However,in some cases,there will be unbalanced faults in the power grid.At the same time,due to the failure,the power device can not work normally.Therefore,when the three-phase grid connected inverter is studied as an object,the undesirable situation should be taken into account.Firstly,this paper describes the harm of non ideal grid voltage to grid connected inverter and other power devices,and even has a bad impact on the system.The causes of three-phase unbalance are briefly described.Due to the existence of some harmonics,some filters are used,and two filter structures are briefly introduced: l filter and LCL filter.This paper is based on LCL filter.Firstly,according to the circuit diagram of LCL grid connected inverter,the mathematical models in three coordinate systems are calculated.Then for the unbalanced three-phase voltage,based on the symmetrical component method,some positive and negative sequence separation algorithms are briefly introduced.In this paper,two second-order generalized integrators(sogi)are used to separate the unbalanced system voltage,only the positive sequence component is extracted,and only the phase of the positive sequence component is selected as the system voltage phase,so as to avoid the influence of the negative sequence component.Then the principle of PLL is briefly introduced.In this paper,only the positive sequence component is used as the input of PLL,so as to realize the phase locking of power grid.Then,the parameters of LCL filter are selected mainly through four limitations.Due to the inherent resonance phenomenon of LCL filter,four passive damping methods are compared and analyzed.Although the resonance is suppressed,some losses will be accompanied.In this paper,the negative sequence current is suppressed as the goal,and the resonance is damped by the active control of the feedback capacitor current.The control strategy based on the double current loop in the DQ rotating coordinate system is adopted,and the PI control in the dq coordinate system is used to separate the positive and negative sequence of the grid current,so as to achieve the purpose of restraining the negative sequence current to zero,and eliminate the impact of the negative sequence component on the grid connected system.In this paper,the system simulation model of three-phase grid connected inverter is established on matlab/simulink,which verifies the feasibility of this method.