Research On Control Strategy Of Microgrid Inverter Based On Virtual Synchronous Generator

With the increasingly prominent environmental problems,renewable energy power generation has been applied on a large scale.Microgrid is an important form of distributed generation organization.It can not only give full play to the advantages of renewable resources,but also reduce the adverse effects of distributed power sources on the power grid.However,the grid-connected generation control of microgrid mainly adopts the mode of power electronic grid-connected inverters.Although this mode has the characteristics of flexible control and fast response,it also has the disadvantages of small moment of inertia and lack of necessary damping.When the number of distributed generators increases,it will seriously threaten the stability of grid voltage and frequency.Synchronous inverters are voltage-source inverters based on virtual synchronous generator technology,which can provide necessary inertia and damping for microgrid and improve system stability.In this thesis,the control strategy of virtual synchronous generator based on DC/AC inverters in photovoltaic power generation system is studied on the basis of AC microgrid.Firstly,the concept of microgrid and the research status at home and abroad are briefly introduced,and the concept of virtual synchronous generator control for microgrid inverters is emphatically expounded.Secondly,taking the second-order typical model of synchronous generator as the control algorithm of virtual synchronous generator,the control structure of virtual synchronous generator is designed.Then,according to the characteristics of line impedance of low-voltage micro-grid,virtual impedance control is introduced to enable virtual synchronous generator control to be used in low-voltage micro-grid.The simulation proves the correctness of the control strategy and related theoretical analysis of the virtual synchronous generator.The effects of inertia parameters and damping parameters on the output frequency and active power of the system are analyzed.Based on the adaptive control of inertia,a hybrid control strategy of inertia damping for multi-machine parallel operation in island mode is proposed,and a small signal model of virtual synchronous generator parallel operation is established,which provides a theoretical basis for the selection of control parameters.At the same time,the MATLAB simulation model of AC microgrid is built and the proposed control method is simulated.The results show that the frequency mutation caused by load is effectively reduced and the stability of the microgrid system is improved by introducing the control strategy.Aiming at the contradiction between active steady state and dynamic characteristic regulation of virtual synchronous generator,a control strategy of virtual synchronous generator with power differential term is proposed and applied to grid-connected generation mode for analysis.By analyzing the influence of differential coefficient on the system,the setting principle of its parameters is given.The simulation results show that the introduction of power differential term can improve the steady-state characteristics of the system.