Research On Frequency Stability Analysis Of Virtual Synchronous Generator System Under Weak Grid

With the development of renewable energy technologies such as wind power generation and photovoltaic power generation,more and more renewable energy sources are connected to the grid through grid-connected inverter devices.However,due to the special geographical environment where China’s renewable energy power generation base and load center are inversely distributed,and as the proportion of renewable energy generation in the power grid increases gradually,the low inertia,low damping,and discretization characteristics of power electronic devices have become non-negligible.As a result,the instability of system voltage and frequency may occur under external disturbances,which seriously affects the safe and stable operation of power grid.Virtual synchronous generator(VSG)can simulate inertia,damping and other characteristics of synchronous generators,which is a good solution to improve the stability of renewable energy grid-connected systems.However,VSGs are power electronic device,which means that its anti-disturbance capability is far less than that of the traditional synchronous generators.Therefore,the study of the frequency stability of the single-VSG system and multi-parallel VSG-based systems is of great significance to the safety of the VSG system.Firstly,this paper introduces the mathematical model of synchronous generators and several traditional control strategies of inverters.The disadvantages of traditional inverter control strategies and the advantages of VSG in the weak grid and island scenarios are both analyzed.The basic principles and implementation methods of the VSG control strategy are given,and the frequency-power regulation characteristics of the VSG system under grid-connected and off-grid conditions are verified by the Matlab/Simulink simulation.Secondly,the full-order small-signal model of the single VSG-based grid-connected system is derived,and the characteristic roots of the state matrix of the system are obtained at the same time.The influence of factors such as virtual damping coefficient,virtual inertia coefficient,droop coefficient,and transmission line impedance on the frequency stability of the system is analyzed by the root locus method.The correctness of the analysis results is verified by simulations.Based on the small-signal model derivation method of single VSG-based gridconnected systems,the general small-signal model of multi-parallel VSG-based gridconnected system is established by ignoring the state variables that have less influence on the VSG frequency.The three-parallel VSG-based grid-connected system is used as the study case.Combined with the root trajectory analysis method,a participation factor trajectory analysis method is proposed to study the frequency characteristics of the VSGs and power grid.The simulation is used to verify the correctness of the proposed analysis method and the validity of the obtained analysis results.Finally,the three-parallel VSG-based experiment platform was built,and the effects of parameters such as the virtual inertia and droop coefficient of the VSG on the frequency stability of the system were analyzed through experiments.At the same time,the experiments also verified the automatic power sharing characteristics of the VSG control strategy and the frequency instability conditions of the multi-parallel VSG-based systems.