Research On Control Method Of Virtual Synchronous Generator Under Unbalanced Grid

With the depletion of non-renewable energy sources and the increasingly prominent environmental problems,it is an inevitable trend for the future development of power grid to vigorously promote large-scale friendly integration of distributed power sources such as photovoltaic.As the permeability of distributed power generation such as photovoltaic power generation increases in the grid,how to realize friendly access,effective regulation and stable operation of distributed power generation is an urgent problem to be solved,and also an important challenge to be faced at present.This paper takes the three-phase grid-connected inverter based on virtual synchronous generator(VSG)as the research object and the three-phase unbalanced power grid as the research background to study the control method of VSG under unbalanced grid,which can improve the operation performance of photovoltaic power generation system under unbalanced power grid.Firstly,this paper reviews the research status of VSG and the inverter control method under unbalanced grid,and analyzes the necessity of VSG control method under unbalanced power grid.Taking the three-phase inverter as the research object,based on the basic principle and mathematical model of VSG,the realization method of VSG synchronous reactance is analyzed,and on this basis,the output impedance characteristics of VSG under different synchronous reactance implementation methods are analyzed.Furthermore,the quantitative method of the unbalanced system is given,and the influence of the unbalanced grid on the VSG system is analyzed.In order to further analyze the stability and the parameter design method of the VSG system,the dynamic small-signal model of the system is established based on the concept of dynamic phasor.On the basis of analyzing the stability of the system,considering the idea of power decoupling and synchronous resonance suppression,the parameter design method based on virtual impedance is given.In order to extract the positive and negative sequence components accurately,a positive and negative sequence separation method based on cross-decoupling complex coefficient filter is adopted.Aiming at the power fluctuation and current imbalance under unbalanced grid,the relationship between negative sequence current and unbalanced current,active power double frequency fluctuation and reactive power double frequency fluctuation is derived.On this basis,a composite VSG control method without changing the VSG voltage source property is proposed,which improves the power and current quality during grid-connected operation and improve the capacity of the system with unbalanced load.The VSG simulation model is built in the PLECS environment to verify the correctness of the composite VSG control method proposed in this paper.Finally,based on the three-phase grid-connected inverter experimental platform,the relevant verification work is carried out.The experimental results show that the composite VSG control method proposed in this paper can achieve unified control of current balance and constant power when the grid voltage is unbalanced.Meanwhile,the inverter can output stable symmetrical three-phase voltage when off-grid,providing stable voltage support for load(including unbalanced load).