Research On Virtual Synchronous Generator Control Under Power Grid Fault

Under the background of "two-carbon",new renewable distributed power generation systems represented by photovoltaic and wind power are connected to the grid on a large scale,resulting in a gradual decline in the proportion of synchronous generators,which directly affects the stability and security of the power system.Virtual synchronous generator(VSG)simulates the characteristics of synchronous generator to make distributed generation system with inertia and damping,which is gradually used in real power system.In practical operation,distributed generation system is generally connected to the end of the grid,but the end of the grid failures frequently,resulting in the inverter output performance reduction and equipment damage.Therefore,this paper takes VSG as the research object and power grid short-circuit fault as the research background to discuss the control method of VSG in the case of power grid fault and improve the operation performance of VSG.The main work of this paper is as follows:(1)The structure of distributed power generation system is introduced,several commonly used inverter control methods and their respective advantages and disadvantages are summarized,and the development process of VSG and the research status of improved control under grid faults are summarized.(2)The working principle and mathematical model of synchronous generator in traditional power system are summarized.Based on this,the traditional VSG control model is obtained and the grid-connection characteristics of VSG under grid failure are analyzed.By establishing the equivalent circuit model and solving the differential equation,the output current of VSG in grid-connected operation is deduced and calculated,and the control parameters most conducive to the output performance of VSG in the case of grid faults are selected by comparison.(3)The influence of power grid failure on the grid-connected operation of VSG is analyzed,and the reasons for the decline of VSG output performance in the case of power grid failure are obtained.When the grid is symmetric,the grid voltage drops instantaneously,and the traditional VSG response speed is much lower than the grid voltage drop speed,which causes the change of VSG power Angle and voltage,and then leads to the instantaneous increase of the output current and power.When the power grid is asymmetrical,the output current of VSG will be asymmetrical due to the difference of three-phase voltage amplitude and phase Angle,and the output power will fluctuate at the same time.(4)Improved the traditional VSG control strategy.The improved control method can suppress the steady-state component of the output current by changing the active power command value and the reference voltage command value under symmetrical faults,and introduce virtual impedance to suppress the transient component of the output current.The idea of positive and negative sequence independent control is adopted for asymmetric faults,and the negative sequence current control ring is added.The reference values of negative sequence current under three single objective control systems are obtained according to the power wave equation.On this basis,the multi-objective optimization expression is established to realize the multi-objective optimization control of VSG output performance.Finally,the effectiveness of the proposed control strategy is verified in Matlab/simulink.