State Space Modeling And Stability Analysis Of Virtual Synchronous Generator

With the increasing popularity of renewable energy generation,the traditional central power generation mode is shifting to the distributed power generation mode.Distributed power sources such as photovoltaic and wind power are connected to the power grid in large numbers through power electronics grid-connected devices,which promotes the development of power electronic based power systems.However,distributed energy grid connection has the problems of low inertia and weak damping.Achieving friendly interaction between distributed power sources and power grids is an important issue in the global energy Internet process.In order to solve the problem of insufficient inertia and damping in power electronic power systems,virtual synchronous generator(VSG)control strategy has attracted much attention.The VSG technology,which simulates the primary frequency regulation characteristics,primary voltage regulation characteristics,inertia and damping characteristics of traditional synchronous generators,can support the grid frequency and voltage and maintain the system power balance.VSG can easily draw on the control strategy,analysis method and operating experience of traditional synchronous generators,which is helpful for the friendly access of distributed power.VSG simulates the external characteristics of traditional synchronous generators,but VSG is not completely equivalent to traditional synchronous generators.Therefore,it is of great engineering significance to study the mathematical model,dynamic performance and optimization of VSG.In order to obtain the control structure of virtual synchronous generator,the frequency adjustment and voltage adjustment mechanism of the traditional synchronous machine are firstly studied,the second-order electromechanical transient model of the synchronous generator is established,and the operation principles of the governor and exciter are analyzed.Based on this,two implementation schemes for the simulation of synchronous generators in power electronic systems are introduced,namely droop control strategy and VSG control strategy.Finally,the specific implementation scheme of VSG is deduced in detail,and the equivalent relationship between VSG and traditional synchronous machine is established.This paper introduces a small-signal modeling method based on state space,establishes a full-order small-signal state-space model of inverters based on VSG control,and verifies the correctness of the modeling method based on a three VSGs parallel system.The VSG control strategy relies on digital control technology,which has the inherent delay problem of digital control.This paper analyzes the formation mechanism of control delay in detail,and then introduces the delay link into VSG small signal state space modeling to form a delay correction model.Based on this,combined with the characteristic value trajectory and parameter sensitivity,the influence of multiple control parameters on the system stability is analyzed.Finally,in order to mitigate the influence of control delay on the stability and dynamic performance of the system,the Dead-Beat idea was introduced into the VSG control strategy,and a virtual synchronous machine strategy with Dead-Beat power predictive control was formed.This strategy is based on the system status at the current sampling time,predicts the system output power at the next sampling time,and then controls the system based on the predicted power.Simulation verifies the feasibility of the strategy.