The Research On Circulation Control Strategy Of Low Voltage Microgrid

In order to solve the energy shortage problem,distributed generation(DG)have been developed rapidly and widely applied in recent years.However,a large number of DG connecting the grid will have a negative impact on the grid.A microgrid technology can effectively solve the impact problem.Different from the traditional power network,the microgrid has two operating modes,which not only can run in the grid-connected mode,supplying power for load with the cooperation of power network,can also run in the island mode,with disconnection from the power network,and offer clean electric energy for loads.When the microgrid is running in the island mode,due to the loss of the power network for the voltage and frequency support,if the line impedances of DG are not equal to each other or DG is of local load,then there is a larger circulation in the microgrid to affect a microgrid operation.the circulation control strategy is mainly studied in the microgrid under the island mode,and the main contents are as follows:Firstly,the control methods of different types of DGs in a microgrid are briefly introduced.On this basis,the operation control strategy of a microgrid is analyzed.The microgrids with droop control are used as the research objective.The reason of circulation emerging is analyzed in a microgrid and the relationship between line impedances and microgrid circulation is deduced.The simulation results show that when DG line impedances is not equal to each other or DG is of local load,the traditional droop control and the droop control based on virtual impedance are non-ideal for the suppression of a microgrid circulation.Secondly,based on the analysis of microgrid circulation,a control strategy based on adaptive virtual impedance is proposed to restrain the circulation of microgrid.The principle and control flow of control strategy are introduced.The difference of reactive power is used to adjust the virtual impedance in the control strategy,and three algorithms are designed to control the virtual impedance,so that the virtual impedance can follow the difference between the line impedances and realize the output reactive power according to the capacity allocation,thereby restraining the circulation of the microgrid.The voltage drop on the virtual impedance is analyzed.It is pointed out that when the value of the virtual impedance is too large,the voltage sag of DG is serious,and a simple voltage compensation method is designed to compensate the voltage drop.Finally,in the MATLAB software,the simulation model of microgrid is established,and the circulation control strategy and voltage compensation method proposed in this paper.The simulation results verify that the control strategy proposed can effectively restrain the circulation of microgrid,and even if the communication fault occurs during the virtual impedance adjustment,the control performance of circulation is better than the droop control strategy based on the virtual impedance.When the control strategy proposed is adopted in the microgrid,the voltage compensation method can compensate the voltage drop on the virtual impedance without affecting the reactive power distribution.