Research On Control Strategy Of Micro-grid Inverter Based On Virtual Synchronous Generator

Wind,light and other renewable energy have become the focus of research and development,with the worldwide energy shortage,environmental pollution,rising sea level and other crises,which are significant difficulties human beings are facing.Renewable energy has the characteristics of randomness,intermittence and volatility.Generally,they operate by connecting with the inverter to the grid or network operation,on the one hand,it solves the problems of energy shortage and environmental pollution;on the other hand,it brings new challenges to the output power quality of the inverter and stability operation of the grid.Therefore,searching for the grid-friendly inverter control technology is imperative.This will have great significance for the new energy power generation itself(photovoltaic power plants,wind farms)and the stable operation of power grid.Firstly,the traditional control strategies(PQ,VF and Droop)of micro-grid inverter are analyzed,and the features are pointed out that they cannot provide the inertia and damping effect for the system.This results in poor output power quality of inverter while in island operation mode,and having struck to the grid while in grid-connected mode,the dissertation proposes a control strategy based on virtual synchronous generator.On the bases of droop control,the mathematical model of VSG is established by introducing the rotor inertia equation and stator electromagnetic equation of mathematical model;the dissertation establishes the P-f controller and P-u controller model referred to the principle of governor and the excitation regulator of synchronous generator.Secondly,in order to test and verify the influence of the parameters perturbation on the VSG system,the small signal model of grid-connected VSGs and network VSGs are established respectively,and the influence of the parameters perturbation on the stability of the system are analyzed.On the bases above,the appropriate parameters are setted for the system.The double closed loop(the inner loop of current and the outer loop of voltage)control of the inverter based on VSG is used,and the control parameters of VSG control system are determined.Thirdly,based on the small-signal analysis and parameters design,the physical simulation models of VSG are established,which used to verify the P-f control,P-u control,inertia and damping characteristics respectively in the grid connected and isolated island mode.The dissertation compares the response of the inverter under the traditional control strategies(PQ,VF and Droop)and the VSG control strategy under same power fluctuation.The result shows that the VSG control strategy has obvious superiority compared with other strategies.Aiming at the output allocation problem of multiple inverters,the dissertation proposes a principle of active power output based on consistency algorithm.The simulation results show that the multiple active power based on consistency algorithm can allocated according to capacity ratio to meet the control requirements.Finally,aiming at the low accuracy of voltage and current double loop PI control of VSG under nonlinear and unbalanced loads,a sliding mode voltage control of VSG is proposed based on the double loop PI control.The simulation results show that the control precision of VSG has improved controlled by the sliding mode voltage control compared with the traditional PI control.Under the nonlinear and unbalanced load,the VSG both can output better power quality.