Research On Control Strategy Of Photovoltaic Grid-Connected Inverter Under Unbalanced Grid Voltage

In the actual grid,voltage imbalance is caused by accidental or normal faults of the grid,which lead to the deterioration of the PV grid-connected inverter.Moreover,equipment is burned in severe cases.Since the control strategy of the grid-connected inverter cannot meet the control requirements when the grid voltage is unbalanced,it cannot ensure the stable and reliable operation of the PV system.Therefore,it is important to study the control strategy of a three-phase PV grid-connected inverter under the grid voltage unbalance condition.In this paper,take the PV grid-connected inverter as the research object.Investigate the control problem of the inverter when the grid voltage is unbalanced.The research mainly includes the following aspects.Firstly,introduce the background and significance of the subject research,and the influence of voltage imbalance on PV power generation system is briefly described.On this basis,according to the analysis of the working mechanism of the PV grid-connected inverter,the mathematical models of the grid voltage balance and unbalance are derived.Secondly,the symmetrical component method is used to solve the idea of control in the case of unbalanced power grids,and the DDSRF-PLL is introduced to separate the positive and negative sequence of the unbalanced power grid voltage and lock phase.On this basis,to solve the problem of high delay caused by the low-pass filter in the DDSRF-PLL structure.And it combined with the DSOGI-PLL,the ROGI-PLL is introduced.Not only the DSOGI-PLL has the characteristics of small delay,but also simple structure,which can provide voltage/current positive and negative sequence components and phase-frequency information for the control of the inverter.Thirdly,a two-stage PV grid-connected inverter system is the research object.Through the aforementioned analysis of the mathematical model when the grid voltage is unbalanced,the current command calculation method is used to calculate the reference current command in order to suppress the negative sequence current on the grid side of the grid-connected inverter.Aiming at the problem that the traditional PV grid-connected inverter control strategy cannot meet the control requirements when the grid voltage is unbalanced,a control strategy based on PI controller is designed,and then QPR controller and PI-ROR controller control strategy to solve the problem of three-phase current imbalance.Through the analysis of the two control strategies,it can be seen that the ROR controller in the PI-ROR control strategy can accurately control the negative sequence component.So the current positive and negative sequence reference values can be directly added to obtain the current reference value and then controlled in the same current loop,which simplifies the control structure and avoids the separation of positive and negative sequence current calculations.Therefore,the PIROR control strategy has more obvious advantages than the QPR control strategy and the effect of balancing the three-phase grid-connected current is better.Finally,a simulation model of the 6k W PV grid-connected inverter system is built in MATLAB/Simulink software.In this paper,the proposed control strategy of suppressing the negative sequence current on the grid side is simulated.The simulation results show that the proposed control strategy can effectively suppress the negative sequence component in the grid-connected current when the grid voltage is unbalanced,improve the quality of the grid-connected current,and make the current follow the phase change of the grid voltage to achieve rapid grid-connection.