Research On Optimal Control Of Micro-grid Power Quality Based On Synchronverter

As an intelligent management system of distributed power,micro-grid promotes the development of distributed renewable energy.Distributed power energy transmits power to the power grid or load by means of interface converters control technology.Considering the characteristics of small capacity,weak inertia and low damping of micro-grid,when the output of the distributed power generation oscillates,the nonlinear load increases,and the fault grid occurs,the power quality problems such as the frequency drift,the output three-phase unbalanced voltage/current and so on,make the micro-grid unable to meet the requirements of safe and reliable operation.The traditional control strategies of converters have fast dynamic response speed,but weak inertia,large disturbance,and a large number of grid connection will weaken the stability of the power grid.In order to solve this problem,many experts and scholars began to study the control technology of micro-grid interface converters to realize the auxiliary control of power quality.In recent years,the virtual moment of inertia has been designed for the synchronverter,which provides inertia and damping support for the micro-grid,and provides a new idea for the control technology of interface converters.Therefore,based on the synchronverter control technology,this paper studies the outstanding power quality problems in the grid-connected and island-connected micro-grid respectively,and puts forward the corresponding optimal control methods for the power quality of the micro-grid.The specific research work is as follows:(1)The control strategy of synchronverter and power quality analysis of Micro-grid.The control strategy of synchronverter and the power quality problems of voltage and frequency in micro-grid based on synchronverter are analyzed.And then,by analyzing the model and structure of the synchronous generator,the structure,principle and mathematical model of the synchronverter are introduced,the control strategy of the synchronverter is analyzed one by one,and a complete synchronous control system is established.The simulation model of grid-connected and island-connected micro-grid under the control of synchronverter is established respectively.The the problem of power quality of voltage and frequency in the two modes are qualitatively analyzed by simulation experiments.It provides a theoretical and experimental basis for the follow-up chapters to solve the related power quality problems.(2)A novel negative sequence current suppression method of synchronverter based on situation orientation.When the grid voltage is unbalanced,the output negative sequence current of synchronverter in grid-connected operation causes serious unbalance of three-phase grid-connected current,and this situation further aggravates the instability of power grid.The situation orientation method to suppress the negative sequence current is proposed in this paper.Firstly,the output current of synchronverter is predicted to foresee current situation information in advance.Secondly,the value of voltage that is calculated by the differential current value from situation prediction is imported to synchronous control system to realize real time compensation.The simulation results show that the proposed method can effectively restrain the negative sequence current,reduce current unbalance,and improve the dynamic performance of the control system.(3)Frequency modulation control strategies of isolated island micro-grid based on synchronverter.When the load fluctuation occurs,the frequency of the system is deviated or even exceeded the limit,which makes the system unable to operate normally in isolated island operation mode.This chapter presents a frequency modulation control strategy under the control of synchronverter.Based on the primary and secondary frequency modulation control strategies of a typical synchronous that is the generator-micro gas turbine,the primary frequency modulation and secondary frequency modulation control strategies of synchronverters are obtained by means of theoretical modeling and mathematical derivation,and the zero difference frequency modulation is realized.Moreover,the simulation model and experimental platform of frequency modulation of two synchronverters are built to verify the effectiveness of the proposed method.