Study On The Control Strategy Of Inverter In Microgrid Based On Virtual Synchronous Generator

Microgrid is an effective approach to integrate the advantages of all kinds of distributed resources,to take less impact on grid and improve the economic profits of distributed generations.It has become a hot topic and important direction in distributed generation area.However,the converter-based Microgrid systems have no rotary inertia which is beneficial to maintain the grid stability and also a inherent characteristic of the synchronous generator which dominates in conventional power system.Therefore,with the growing permeability of Microgrid systems in power system,the safety and stability of power system would face new challenges.Virtual Synchronous Generator(VSG)is a control scheme applied to the inverter to support power system stability by imitating the behavior of the synchronous generator.This thesis takes inverters in Microgrid based on VSG as object of study and makes research on their operation performance and control strategy.Implementation algorithm of VSG is studied according to the simplified model,and the function of rotary inertia and topology of VSG is analysed in detail.The working principles of speed governor and excitation controller has been analyed,and on this basis the math model of active power loop and reactive power loop is established.The impacts of each parameter on the system dynamic and static response and stability is analysed and the method of parameter design is given.For the realization of the seamless switch between the islanding mode and grid-connected mode,presynchronization technology is studied in this thesis.A new presynchronization method based on virtual power is proposed,which avoids the over-complication in conventional presynchronization technology.The admittance model of VSG is established,and the influence of grid harmonics on the grid current of VSG is invesgated from the admittance standpoint.And a grid voltage feedforward strategy based on notch filter is proposed,which can suppress current harmonics by remodeling the admittance at the harmonic frequencies.The impact of transmission impedance on reactive power distribution is analysed.In this papar,and a method by adding integrator in reactive power droop-control loop is proposed which can eliminate the impact and achieve the goal of reactive power sharing.In order to verify the correctness of related principles,two 5kW VSG experimental platforms have been built.Experiments of the operation of grid-connected mode,islanding mode,mode transfer between the islanding/grid-connected modes,current harmonic suppression mode and reactive power sharing mode have been implemented.The results verify the correctness and effectiveness of the adopted VSG control strategy in this thesis.