Research On Comprehensive Control Strategy For Frequency And Voltage Of Micro-grid Based On Virtual Synchronous Generator

With the continuous development of micro-grid technology,the control of distributed generation(DG)of micro-grid has become the focus of researchers.In micro-grid,as the interface of energy conversion between DG and load and microgrid,the controller of inverters affects the power quality and stability of micro-grid directly.As a grid-friendly control method proposed in recent years,virtual synchronous generator(VSG)control technology has attracted more attention than traditional control methods such as V/f,P/Q and droop control because of its inertia and damping characteristics like rotational synchronous generator(RSG).However,traditional VSG control still has many shortcomings.For example,it can only achieve primary frequency and voltage control,the parameter design method rely on too many parameters and the process is complex.Therefore,in view of the defects of traditional VSG control,this thesis improves the VSG control based on the existing research,realizing the secondary control of frequency and voltage and stable parallel operation with multiple VSGs.The main work of this thesis are as follows:(1)The working principle of photovoltaic battery and lithium iron phosphate battery energy storage system is analyzed,the key parameters are given,and the commonly used equivalent circuit model is introduced.(2)The topology and performance differences of three VSG frequency loop controllers are compared and analyzed,and the damping part reference value is selected as nominal angular frequency.The schematic diagram of the source-load structure of the system is given,and the applicability of the droop control idea in the micro-grid with transformers is proved by an impedance example.Aiming at the limited frequency regulation ability and complex parameter design program of traditional VSG control,an improved VSG frequency loop controller is proposed.The primary control objective is recongnized and the influence of integrator on frequency stability is analyzed.The controller parameter design method is optimized to reduce the complexity of parameter design and dependence on external circuit parameters.The simulation results show that improved VSG can achieve secondary frequency control compared with traditional VSG.(3)Aiming at the defect that the traditional excitation controller can only adjust the voltage of output terminal,the voltage amplitude estimator is introduced to estimate the voltage at the point of common coupling(PCC)timely,an improved voltage self-recovery controller is proposed.The logarithmic frequency characteristic of the output impedance proves that the integrator will not change the fundamental output impedance.By analyzing the influence of the change of parameters on output impedance,it is obtained that the change of proportional gain K and droop coefficient have obvious but opposite effects on the output im-pedance.The simulation results verify the effectiveness of the proposed control strategy.(4)The parallel operation simulation analysis of the improved VSG control is conducted.The simulation results show that each VSG can share the load equally,realizing the stable parallel operation with multiple VSGs,thus enhance the stability,reliability and scalability of the micro-grid.