Study On Small Signal Stability Of PV Gird-Connected System Considering Negative Sequence Control And Reactive Power Support

When the grid is weak,the voltage at the grid-connecting point of the photovoltaic is prone to fluctuate and the grid voltage imbalance is ubiquitous.Under the unbalanced grid voltage,inverter adopts negative sequence control strategy to suppress inverter output power fluctuation or negative sequence component in grid current.In addition,in order to stabilize grid-connected voltage,PV power plants must have some reactive power supporting ability.Negative sequence control and reactive power supporting are the important control strategies and methods in PV grid-connected system,which will inevitably affect the stability of the system while achieving the corresponding function.This paper analyzes the influence of negative sequence control and reactive power supporting on system stability from the perspective of small interference stability.The main contents are as follows:1.Analyzing system oscillation risk considering of negative sequence control.Establishing the generalized impedance model of photovoltaic inverter considering of negative sequence control and generalized impedance model of power gird based on generalized impedance method.Impedance analysis method is used to reveal the instability mechanism of photovoltaic grid-connected systems considering the negative sequence control under weak power grid.The influence of the medium quasi-resonance coefficient on the stability of the system indicates that the greater the resonance coefficient,the worse the system stability.In order to improve the system stability,an additional phase compensation method based on voltage feedforward is proposed from the viewpoint of improving the phase angle of the PV inverter impedance.A method of parameter design for additional phase compensation is proposed.The proposed method can effectively improve the compensation.The phase angle at the corner frequency increases the stability margin of the system and reduces the oscillation risk of the photovoltaic grid-connected system under the weak grid.Finally,MATLAB/Simulink simulations are provided to verify the analytical results.2.Research on system oscillation risk when PV Inverter and SVG provide reactive support.First of all,a model of single photovoltaic access to the infinite power grid and photovoltaic grid-connected system considering SVG access are established in MATLAB/Simulink respectively.Secondly,a linearized model of the system is obtained based on software's own linearization tool.Based on previous work,when PV inverters provide reactive power and SVG with different control strategies provide reactive power support,the risk of system oscillation under different power grid strength are analyzed by eigenvalue analysis.Finally,MATLAB/Simulink simulation verifies the correctness of the analysis.