Study On Control Strategy Of Micro-grid In Island Mode

In case of large grid failure,the micro grid needs the ability of island operation.In islanding mode,the micro grid system is prone to generate voltage and frequency fluctuations.The appropriate control strategy can make the system run stably.Therefore,this paper aims to study the control strategy of micro grid in island mode.Firstly,this paper introduces the structure of AC microgrid and analyzes the output characteristics of different distributed power sources.This paper mainly studies the maximum power tracking of photovoltaic cells in the local shadow.Aiming at the problem that the traditional particle swarm optimization(PSO)is easy to fall into the local maximum in the tracking process,the compression factor is introduced into the PSO algorithm to form the particle swarm optimization algorithm with compression factor.Secondly,the paper introduces two operation modes of grid connected / island connected micro grid,and studies the island detection method.For active frequency drive positive When the feedback(AFDPF)algorithm detects the capacitive load system,the detection speed is slow and the distortion current has great disturbance to the system.The frequency difference function in the AFDPF algorithm is improved,the cubic root function is introduced into the frequency difference function,and the piecewise function is designed to adjust the feedback coefficient adaptively.An improved adaptive feedback coefficient AFDPF algorithm is proposed.After the completion of islanding detection,the PQ control strategy and V/f droop control strategy of the master-slave micro grid in islanding mode are mainly studied.Firstly,the mathematical model of three-phase AC inverter is established and the filter parameters are designed.Then PQ control strategy is designed to track the system power instructions.Then,aiming at the voltage and frequency fluctuations in the process of frequent load switching in the microgrid in island mode,the V/f droop control strategy with adaptive droop coefficient is adopted in this paper.The relationship between load power demand instruction and droop coefficient function isimproved.The droop coefficient is dynamically adjusted by using the stability of the function.A feedforward control link is added to the power outer loop of droop control link to compensate the voltage and frequency deviation,so that the main controller can provide stable voltage and frequency support for the system in island state.Finally,a model is built in Matlab/simulink to verify that the proposed improved V/f droop strategy can effectively suppress the fluctuation of voltage and frequency in the process of frequent load switching,and provide stable voltage and frequency support for the micro grid system in island state.