Virtual Synchronous Generator Technology And Its Application In Microgrid

In recent years,distributed power generation and microgrid technology using clean energy have been widely used.However,the inertia and damping of power electronic devices are relatively small.As the penetration of distributed generators with power electronic converters as interfaces in power systems is increasing,the utility grid will be vulnerable to power fluctuations and lose stability easily since the inertia and damping is reduced.Virtual synchronous generator(VSG)technology gives power electronic inverters the characteristics of synchronous generators,which can effectively solve various problems caused by new energy connecting to utility grid.Based on this,the control strategy of virtual synchronous generator(VSG)technology and its application in microgrid are studied in this paper.The contents of this paper are as follows:(1)By improving the implementation of VSG control strategy,the active power loop and the reactive power loop both have damping and inertia,which can suppress the active and reactive fluctuations of the system.In this paper,the application characteristics of the proposed VSG control in islanded microgrid are analyzed.The control parameters to bear load according to rated capacity are obtained when multiple VSGs are running in parallel and the matching condition of equivalent output impedance and droop coefficient need not be satisfied.(2)The principle of synchronous generator synchronization mechanism is expounded.This paper presents a pre-synchronization method using virtual grid-connected power based on synchronization mechanism.This method can achieve self-synchronization before grid-connected without phase-locked loop and has little impact on load in phase tracking process.What's more,it doesn't need to consider the lead-time of grid-connected switch.The transfer functions of active and reactive power loops are established,and the grid-connected regulation characteristics of VSG technology are analyzed.(3)By establishing the dynamic mathematical model of power transmission,it is found that when the damp of the inductance on the grid side is small,the dynamic mathematical model of power transmission has conjugate poles close to the imaginary axis.The grid-connected power is prone to 50Hz oscillation,which is synchronous frequency resonance.A feedforward compensation control is proposed,which eliminates the conjugate poles and effectively suppresses the oscillation.(4)After the correctness of the theory is verified by simulation,the voltage and current signals of physical three-phase inverters are reduced according to a certain proportion,and the mathematical relationship between them is expressed by the basic circuit composed of operational amplifiers following the idea of hardware-in-the-loop.PCB board is made to replace the main circuit of three-phase inverters,and the analog simulation circuit experiment platform is built.The validity of the proposed virtual synchronous generator control and the pre-synchronous control strategy are verified on the test platform.