Research On Control Strategy Of Parallel Operation Of Multiple Virtual Synchronous Generators

With the increasing consumption of non-renewable resources and the increasing emphasis on environmental protection,it has become a trend to increase the development and utilization of renewable clean energy.Renewable and clean energy such as wind and solar energy has been rapidly developed.Compared with traditional synchronous generators,distributed power generation system based on new energy lacks inertia support ability and frequency and voltage automatic adjustment ability,which will cause large fluctuations during operation and reduce system stability.To solve this problem,the concept of Virtual Synchronous Generator(VSG)is proposed by experts and scholars.VSG,as a new type of "grid friendly" power electronic grid-connected device,improves the stability of the system by simulating the synchronous generator’s primary frequency modulation and voltage regulation ability.With the continuous increase of power consumption,the capacity of a single VSG in operation is not enough to meet the load demand,so it is necessary for multiple VSGS to share the load in parallel.The VSG parallel system can increase or decrease the machine at any time according to the load demand to meet the increasing demand of the load.But the parallel system under off-grid conditions also faces some problems such as unreasonable distribution of circulating current and power.Therefore,based on VSG,this paper studies how to run a multi-VSG parallel system stably under off-grid conditions.Specific research contents are as follows:Firstly,the background of microgrid is summarized,and the single VSG technology and parallel system operation technology are introduced so far.The principle of VSG is analyzed,including the active power ring,reactive power ring,and voltage and current double ring structure is described,the research model is built on the Matlab platform,and the simulation analysis verifies its feasibility.Secondly,aiming at the problem of instant impulse current when VSG is incorporated into the system under off-grid conditions,a parallel resynchronization strategy based on secondary frequency compensation is proposed on the basis of traditional synchronization to realize the smooth integration of VSG into the system to form multiple VSG parallel operation,reduce the impulse current in parallel,reduce power shock and improve stability.Then,the topological structure of the VSG parallel operation system is analyzed,and the reasons for the possible circulation are deeply studied.An improved reactive power loop control strategy based on voltage compensation is proposed,which can greatly reduce circulation and reduce the influence on the system’s stability.Small signal modeling is carried out to verify the feasibility of the improved control strategy.Then the Matlab simulation platform is used to verify its effectiveness.Finally,by analyzing the output power characteristics of the VSG parallel system,the relationship between circulation suppression and power distribution is found,and the influence of various parameters on the power distribution accuracy is discussed to improve the power distribution accuracy.Two and three VSG parallel models were built in Matlab/Simulink to verify the effectiveness of the proposed control strategy.