The Control Strategy Research And System Realization Of Off-grid VSG Inverter

Microgrid can effectively use various distributed power sources,therefore,it has become a popular research point and hot topics in the field of distributed power generation.Since the inverter became an important part of the Microgrid system,the traditional inverter control technology can no longer meet the needs of stable operation of the Microgrid under the condition of high penetration of distributed energy.Virtual Synchronous Generator(VSG)control technology which can be seen as a new inverter control technology has attracted much attention in recent years.VSG simulates the moment of inertia and damping characteristics of the synchronous generator based on droop control technology that make the inverter has the voltage and frequency regulation characteristics,damping characteristics and inertia adjustment characteristics of the synchronous generator.This article conducts research on off-grid VSG inverter control strategies and system implementation.The main research contents are as follows:1.The mathematical model of VSG inverter and the control circuit model are established.Based on the droop control,the rotor motion equation and the stator electrical equation of the synchronous generator are introduced to establish the mathematical model of the VSG;the principle of the governor and excitation regulator of the synchronous generator is used to establish the active frequency of the VSG inverter controller and reactive-voltage controller models.2.The power-frequency control characteristics and excitation regulation characteristics of the off-grid VSG system is researched,and a small-signal model of the off-grid VSG system is built.The influence of key parameters of VSG on system stability was researched based on the small-signal model.The analyzing of impact of system performance and the summary of rules for selecting the main parameters of off-grid VSG systems Verification are certificated and analyzed through simulation by establishing the main control parameters of its VSG,which are the small-signal model expressions of the moment of inertia,damping coefficient,reactive voltage regulation coefficient.Emphasis is given to the superiority of the VSG control strategy through simulation and comparison experiments with droop control.3.Coordinated adaptive control strategy of off-grid VSG moment of inertia and damping coefficient.Based on the flexible and adjustable characteristics of VSGcontrol parameters,an improved adaptive control strategy for off-grid VSG inertia and damping coefficient coordination was proposed.The adaptive function relationship of the improved control strategy is established,the principle block diagram of the adaptive control is designed,and the simulation model of the improved adaptive control strategy is established.The simulation results show that the improved VSG control strategy can adaptively adjust the moment of inertia and damping coefficient according to the frequency change rate and deviation amount caused by the load disturbance.The frequency change rate and the absolute deviation of the frequency are significantly reduced which means that the frequency response of the inverter is improved.4.Off-grid VSG inverter experimental platform construction and system implementation.A stand-alone experimental platform for off-grid VSG inverters was established in order to verify the correctness of the design control scheme in this thesis.The experimental results can reflect the advantages of the VSG control strategy and enhance the performance of the VSG inverter system.It means that the platform can prove the effectiveness and advancement of VSG control strategy,and verify the correctness of theoretical analysis with experiments.