Optimization Of Microgrid Droop Controller Based On Virtual Impedance And Nonlinear Control Strategy

Microgrid is a novel form of network which utilizes all kinds of Distributed energy generation in it, and it is complementary with the public grid. As an important part of a smart grid, micro-grid has been gotten more attention gradually. Inverter is the core component of micro-grid, and the control of micro grid are mainly concentrated in the inverter control, therefore, the control of the inverter is the research emphasis in this paper.There are three mainly inverter control methods in micro-grid power, namely droop control、V/f control、PQ control. Nowadays, the droop control is mostly adapted in the micro-grid inverter. Some characteristics of synchronous generator are imitated in the droop control strategy, and the characteristics make that the micro-grid is of the FM (frequency modulation) function. However, this kind of frequency modulation is a primary frequency modulation that can’t modulate frequency without difference, and droop control also has shortcomings in the micro-grid operation. With the fluctuations of the load, the frequency of power system would change rapidly, and the low frequency load shedding devices do not have enough time to cut load, thus it is not conducive to stable operation of power system. Therefore, this paper makes the improvement to the deficiency of the droop control method in the microgrid.Firstly, the distributed generation system and the concept and research status of micro grid is introduced. The micro power grid inverter control strategy and micro grid system run control strategy is discussed in detail.Secondly, on the basis of the droop control theoretical analysis, the droop controller was designed and the parameters of the controller were determined. The simulation model of droop controller was established by using Matlab, and the validity of the model was testified by carrying simulation experiments. And the operations of power system were also studied based on the droop control strategy. However, some shortcomings of the droop controller are found by analyzing the results of simulations. On the basis of droop control, the operation characteristics of microgrid are studied as the peer to peer control strategy is adapted.Finally, the shortcomings of the droop controller are improved:the virtual impedance is introduced in the active power-frequency droop control method, and the nonlinear control strategy is used in reactive power-voltage droop control. By modeling in the Matlab, the simulation results verify the validity of the improved method. With the improved droop control strategy, the system frequency can slowly transition to a steady value, and there are enough time to make the low frequency load shedding device work when the load changes random, therefore, the frequency stability of system can be maintained. At last, the power oscillation was analyzed in the state of multimachine parallel, and it is concluded that the power oscillation of system can be restrained by setting the controller parameters.