Research On Control Strategy Of Micro-grid Inverter Based On Virtual Synchronous Generator

With the aggravation of energy and environmental pollution,microgrid with distributed generation(DG)as the main energy source is widely concerned.The dynamic of traditional power system is dominated by synchronous generator.Because of the inertia of synchronous generator,it can restrain the rapid frequency fluctuation and maintain the safe and stable operation of power grid.At present,more and more DG are connected to the power grid through power electronic grid connected inverter,compared with synchronous machine,although the inertia and damping are low,but the response speed is fast,which is also the biggest advantage of grid connected inverter.due to the lack of inertia,it will lead to the rapid response of power parameters,which brings new challenges to the safe and stable operation of power system.Therefore,the control technology of virtual synchronous generator(VSG)is proposed to simulate the new control strategy of rotary inertia and damping characteristics similar to synchronous generator.However,there are still some shortcomings in traditional VSG control,for example,only achieved the primary frequency modulation and the fixed rotational inertia cannot take account of the power oscillation and frequency fluctuation under load disturbance.Thus,in view of the shortcomings of traditional VSG control,improving and optimizing the VSG control method is of great significance for improving the new energy penetration,based on the existing literature research,realized the secondary frequency and adaptive control of rotary inertia.The main contents of this thesis is as follows:First,this thesis analyzes the operation control of microgrid inverter and the research status of VSG at home and abroad,sorts out the relevant control strategies of micro grid inverter,and summarizes the advantages and disadvantages of different control strategies and their application.The mathematical model of synchronous generator is established,and the realization method of VSG control is obtained according to the rotor motion equation,stator electrical equation,primary frequency modulation and voltage regulation equation of synchronous generator.The basic principle of VSG is elaborated in detail,and the selection of different feedback variables in voltage and current double loop control is analyzed,the results show that the control based on capacitive current feedback has better anti-interference performance.Second,aiming at the limited ability of traditional VSG control frequency regulation,improved the power frequency controller structure of VSG,that is,integral module is used instead of droop coefficient module in active power frequency control loop.Through simulation,compared with droop control and traditional VSG control,the improved VSG realized the frequent deviation-free control in micro-grid,reduces the frequency deviation of the system,and makes the frequency recover quickly.The grid connected control principle of VSG is studied,establishes a single machine infinite bus system,analyzes the characteristics of grid connected VSG,and verifies the grid connected stability of the system through simulation.Third,according to the two control strategies of VSG,the equivalent circuit diagram and small signal model in off grid mode and on grid mode are established,using eigenvalue analysis,by drawing the influence trend of main parameters on each root locus curve,analyzes the corresponding dynamic performance and system stability performance.And the effects of different rotary inertia and damping coefficient on frequency change are analyzed.Fourth,according to the advantage of flexible and variable of rotary inertia in VSG,a new adaptive control strategy is proposed.Adaptive inertia control is formed by feeding back deviation value and change rate of angular frequency to the rotational inertia.The function relationship between the angular frequency offset and the rate of change is established to realize the real-time dynamic adjustment of the rotary inertia.This control strategy mainly improves the power frequency controller,and the correctness and effectiveness of the proposed control strategy are verified by simulation.Furthermore,the stability of the system and the response characteristics of the frequency are improved.