Research On Seamless Grid-connection/Island Switching Technology Of Synchronous Fixed Frequency Microgrid Inverter

Microgrid is an effective way to make use of distributed power flexibly and efficiently.The smooth and seamless switching between grid-connected microgrid and isolated island is the key to ensure the stable operation of the system and reliable power supply.This paper focuses on the operation control strategy of full-inverter synchronous constant-frequency micro-grid based on PMU-Phasor Measurement Unit,and proposes the seamless switching strategy of synchronously constant-frequency micro-grid grid-connection/island-connection to suppress voltage disturbance and current shock caused by switching.In this paper,the phase mutation,voltage fluctuation and current shock of the grid-connected and isolated microgrid systems are solved.The main work is as follows:firstly,this paper summarizes the control strategy of microgrid system-level control,inverter control strategy and the control strategy of double-mode seamless switching,analyzes the advantages and disadvantages of the existing control strategy,and puts forward the concept of synchronous fixed-frequency microgrid.Then,aiming at the synchronization problem among the inverters during the operation of the microgrid island,a global unified time Generation method based on PMU was proposed to provide the phase synchronization information for all the Distributed Generation(DG-Distributed Generation)inverters in the microgrid,so as to realize the global synchronous output of the microgrid inverter under the island state.At the same time,the voltage frequency of each inverter is fixed to 50Hz by taking advantage of the controllable inverter frequency.Based on the inverter voltage and current double-loop control strategy,the balanced power distribution among the distributed generation inverters is realized according to the PMU current measurement data and considering the influence of line impedance.Finally,the causes of possible disturbance in the transient switching process of microgrid island and grid-connected operation modes are analyzed,and an island detection method using PMU phase Angle information as auxiliary criterion is proposed to improve the accuracy and rapidity of island detection.According to the difference in structure and quantity between grid-connected and independent operation mode control strategies of the inverter,a seamless and smooth switching control method for grid-connected/isolated microgrid is put forward to solve the problems of voltage fluctuation,current shock and phase jump in operation mode conversion of microgrid.In order to verify the effectiveness of the proposed theory,the simulation model of microgrid is built by using Matlab/Simulink simulation software.The simulation results show that the proposed control strategy can effectively realize seamless switching between the two modes.The experiment results show that the proposed control theory is feasible in practical engineering application,which lays a solid foundation for further engineering application.