Research On Islanding Detection And Seamless Reconnection Technology Of Microgrid Based On UPQC

With the rapid development of electric power system and the continuous improvement of industrial and agricultural automation level,the types and quantity of load in the power grid are increasing day by day,and a large number of important loads are constantly introduced,which puts forward higher requirements for the power quality of the power system.At the same time,due to the development of power generation form,a large number of micro grids emerging,and the grid structure of power system becomes more and more complex because of the grid connected operation.The power electronic switch equipment such as non-linear load and rectifier and inverter introduced in the process of micro grid connected to the power system are continuously connected to the power system,which seriously pollutes the power quality of the power system.As a kind of power quality control device with comprehensive functions,the unified power quality controller(UPQC)has clear research significance and practical value in purifying power quality by applying it to the composite power system of main grid and micro grid.Firstly,Based on the existing micro grid UPQC system,a more flexible system(grid micro grid UPQC: G-M-UPQC)structure is proposed by adjusting the location of UPQC between the main grid,micro grid and load.The system can not only realize the power quality compensation during the operation of the system,but also realize the islanding detection and seamless reconnection of microgrid,which improves the flexibility of operation.Secondly,by analyzing the advantages and disadvantages of typical voltage and current detection methods,combined with the actual needs of the system,the detection method based on Park transform is the basis of the detection method of the system.In order to improve the detection error caused by PLL and the delay caused by low-pass filter in the detection process,a detection method based on Park transform without PLL and average filtering(WPLL-AF)is proposed,which improves the response speed of detection method and the detection accuracy of abnormal variables.Thirdly,after comparing the advantages and disadvantages of the typical UPQC control strategy,combined with the application and work of the system,the idea of hierarchical control of the system is proposed based on the indirect control strategy.The controller is divided into the main controller and the sub controller,and the system hierarchical control strategy based on islanding detection and seamless reconnection is proposed.The system can not only play the function of UPQC comprehensive management of power quality,but also perform islandingdetection and seamless reconnection after islanding,which greatly improves the flexibility of the whole system operation.Finally,in order to verify the effectiveness of the proposed system,the simulation model of the system is built.By setting different voltage and current transformation conditions to the model,the fault of the power grid is simulated to verify the effectiveness of the proposed system.