Research On Parallel Operation Of Inverters Based On VSG

In recent years,people’s demand for clean energy has been increasing,and distributed new energy generation has become increasingly mature.With the large increase in installed capacity,it is increasingly difficult for a single large inverter to complete the networking task.Therefore,the method of connecting multiple inverters in parallel has been increasingly used.At the same time,the large-scale introduction of power electronic equipment has caused inertia and damping.In combination with new energy sources such as photovoltaics,the inertia and damping characteristics are even more inadequate,causing the system to be unable to perform as traditional large-scale thermal power generators deal with frequency fluctuations.Therefore,the Virtual Synchronous Generator(VSG)technology emerges at the historic moment.This method simulates the inverter’s operating characteristics of traditional synchronous motors through a control algorithm,and has the advantage of flexible parameter adjustment.Cooperate with the parallel operation mode to expand the operating capacity of the system.This paper studies the VSG control method and VSG-based inverter parallel operation.The specific research content is as follows:(1)This article briefly describes the development process of VSG technology,analyzes the mathematical model of VSG used in this paper,the control structure of active frequency loop and reactive voltage loop,and the circuit topology of the inverter unit.The setting methods of the moment of inertia and damping coefficient of the main control parameters of VSG are given.The difference between VSG control and droop control was analyzed,and a single-machine operation model of VSG was established in Matlab/Simulink to verify the effects of the two main control parameters J and D on the VSG control effect.And description.(2)For VSG parallel systems,this article takes two VSGs connected in parallel at the same node as an example,and performs small signal modeling and analysis.The active droop coefficient,reactive droop coefficient,moment of inertia,damping coefficient,equivalent output resistance,etc.The effect and stability region of the effective output reactance value are calculated and plotted in Matlab,and two VSG models running in parallel are built in Simulink.The above parameters are simulated and verified one by one.(3)The causes of circulating current problems,unbalance power distribution problems,and voltage drop problems in parallel systems are deduced,and specific influencing factors are specified.In the parallel operation of VSG,according to the characteristics of VSG,the parameter matching relationship between each VSG inverter branch is given.Aiming at the traditional virtual impedance algorithm to configure the virtual impedance as an approximate control method that is much larger than the line impedance,this paper analyzes and simulates the deficiency of its control effect.For the problem that it cannot fundamentally suppress the circulating current and accurate power distribution and cause a large power loss,this paper proposes an algorithm based on virtual impedance adaptive control.The specific value of the line impedance is obtained through reverse calculation,and only the total value is set.The equivalent output impedance is taken as the value,and the virtual impedance is configured according to the line impedance and the total impedance,so as to shield the effect of line differences.Through Simulink platform and experimental equipments,this paper designs 7 operating conditions,which are simulated and verified one by one from different capacity ratios,different load types,multiple VSGs in parallel,switching between traditional algorithms and adaptive algorithms,and a non-rated operating condition.The results prove that the control strategy proposed in this paper achieves the effects of circulating current suppression,accurate power allocation and reducing power loss.