The Research Of Control For Grid-connected/Islanded And Smooth Switching Of Micro-grid

The existence of micro-grid has a great significance to the integration of wind energy,solar energy and other new energies.However,the grid-connected and islanded operation of micro-grid is closely related to its control strategy,especially the problem of transient shock caused by inconsistent output state of the controller during the mode switching.Therefore,this thesis aims to study the grid-connected,islanded and smooth switching control strategies of micro-grid.Firstly,the models and output characteristics of photovoltaic cells,wind turbines and energy storage unit in the micro-grid are analyzed.For the problem of low efficiency and easy fluctuation of particle swarm optimization(PSO)in photovoltaic cell's maximum power point tracking(MPPT),the inertia weight and the learning factor in the process of its optimization are dynamically changed and the chaotic local search algorithm(CLS)is added,and proposes an adaptive particle swarm chaotic local search algorithm.According to the topological structure of the micro-power inverter,its mathematical model in the d-q coordinate system is studied and reasonable LC filter parameters are designed.When the micro-grid is connected to the grid,the PQ control mode of the main inverter is designed to realize the constant power output of the micro-power supply.When the micro-grid is islanded,V/f control mode and Droop control mode are designed to ensure a stable operating environment required for islanded operation of the micro-grid.In addition,for the voltage and frequency fluctuation problem of traditional Droop controller in the operation of micro-grid,a V/f Droop controller with adaptive droop coefficient based on voltage and frequency variation is designed.At the same time,aiming at the problems of voltage fluctuation,current shock and frequency change in the process of grid-connected and islanded switching of micro-grid,this thesis adopts a switching mode combining state compensation andpre-synchronization control.It includes two parts: when the micro-grid is transferred from grid-connected to islanded,a compensating negative feedback module is added to ensure that the improved V/f controller and the PQ controller have the same output state;When the micro-grid is transferred from islanded to grid-conneced,voltage and phase signals of the micro-grid and distribution network are collected,and the pre-synchronization module is used to realize synchronous operation of voltage and ensure smooth transition of grid-connected.In addition,because of the influence of alternate current disturbances on power quality in traditional islanding detection methods,an adaptive detection method combining alternate current disturbances and active frequency drift is proposed.The two methods detect the island phenomenon every 0.5s cycle,effectively reducing the power quality problems caused by alternate current disturbances.Finally,the model of the micro-grid was built in Matlab/simulink and the operation characteristics of the micro-grid in the state of grid-connected,islanded and smooth switching were analyzed to verify the rationality and effectiveness of the control strategy designed in this thesis.