Grid-forming Control Strategy Of Virtual Synchronous Generator Under Isolated Microgrid With Photovotaic And Storage System

Microgrid is a product of the transition from traditional power grids to smart grids of the future with the goal of providing diverse green energy sources,low-carbon environmental protection,high efficiency and reliability,and meeting the diverse power needs of different users.However,with the continuous increase of the penetration rate of distributed energy in the microgrid,the inertialess inverters connected by the traditional control algorithm will affect the stable operation of the microgrid.The virtual synchronous generator(VSG)has inertia and damping characteristics and can improve the inertialess characteristics of the inverter.This article takes the inverter in the microgrid with photovotaic and storage system as the research object and studies its grid-forming control strategy controlled by VSG.The main research contents are as follows:(1)For a certain situation that the VSG control strategy can reduce the control voltage and frequency deviation under load disturbance,but can not eliminate the frequency and voltage deviation,so this paper proposes a centralized control method of secondary frequency regulation and voltage regulation.Firstly,using the real-time sampling of the AC bus voltage,frequency and rated AC bus voltage,frequency to get the deviations,and then obantaing the frequency,voltage adjustment after PI adjusting,which assigned to each VSG active frequency and reactive voltage controller in accordance with the capacity ratio to achieve no difference compensation of frequency and voltage.Secondly,considering the pre-synchronization problem of parallel operation of VSG,a pre-synchronization method based on the elimination the modular sum of voltage deviation is proposed.By adding the frequency compensation amount of the voltage deviation modulus value after PI adjusting to achieve the phase tracking,which can achieve rapid synchronization between the VSGs and reduce the active and reactive power shocks that the VSG directly merges into the microgrid.(2)In order to accurately identify the inertia of VSG under different operating conditions,this paper proposes a fuzzy control method of inertia of VSG.Firstly,the principle of fuzzy control is introduced,and the influence of the moment of inertia on the active power and frequency of the VSG is analyzed.On this basis,the optimization problems of the active oscillation and the frequency response curve are considered.A fuzzy inertia controller is designed to realize the different moments of inertia's real-time tracking under different operating conditions,which can avoid active oscillations when the VSG is incorporated into the microgrid and increase the ability of the frequency to withstand load disturbances and photovoltaic output fluctuations.In this paper,a microgrid simulation system containing a set of photovoltaic and two sets of energy storage is built in the Matlab/Simulink environment.The simulation results show that:(1)the secondary frequency regulation method can achieve the recovery of frequency and voltage under load fluctuations,and realize no-difference compensation;(2)the parallel pre-synchronization method can realize the rapid pre-negotiation between VSGs and reduce the active and reactive power shocks of the VSG directly incorporated into the microgrid,which does not affect the stable operation of the original system.(3)the fuzzy control of inertia can avoid the active oscillation when the VSG is incorporated into microgrid,slow down the frequency change during the photovoltaic output fluctuations and load disturbance,and then speed up the frequency recovery after disturbance.