Research On Control Strategy Of Microgrid Based On Power Electronic Transformer

Under the global background of traditional fossil energy in tense use and energy conservation,the distributed generation technology gets a rapid development. Microgrid technology gets moreand more attention, which contains the integration of distributed generation, power electronics, andpower system control technology. In order to realize effective complementary between microgridand utility grid, grid-connected technology is worth a further exploration. Many issues exist in thetraditional grid-connected mode, such as slow speed, large impact, low power supply reliability,uncontrollable power flow, in addition due to the absence of effective separation measures betweenthe two grids, the fault current, harmonic pollution and other problems having effect on each other,and the power quality will not be effectively guaranteed. So it is necessary to adopt a new way ofgrid-connection to improve the reliability and flexibility between microgrid and utility grid.In this paper, we design a new type of grid-connected device based on the power electronictransformer. The device consistes of power electronic transformer and energy storage system, whichreplaces the conventional power transformers and circuit breakers. It can not only transform voltagerapidly and accurately, control circuit switching, but also run asynchronously between microgridand distribution network.Based on the analysis of grid-connected device structure, a coordinated control strategy ofmicrogrid DGs and grid-connected devices is proposed: the output stage VSC of PET, as the masterunit, adopts the V/f control strategy; DGs of microgrid, as the slave unit, adopt PQ control. Thesimulation analasis of the microgrid model is completed by PSCAD/EMTDC. The simulationresults show that the grid-connected device can maintain the microgrid voltage stability, frequencystability and reactive power compensation during the grid-connected mode and islanding mode;There is no need to change the control strategy of PET output stage and DGs when it runs fromgrid-connected mode to islanding mode.The influence of microgrid’s accessing to the utility grid on the power flow, system loss, powerquality and the market economy is discussed, and the definition of friendly microgrid is confirmedwhose key is to mantian the stability of the power exchange beteeen microgrid and utility grid. Onthis basis, a grid-friendly microgrid control strategy based on the grid-connected device is putforward: the output stage of power electronic transformer adopts PQ control strategy, and theuncontrollable DGs also adopt PQ control, the controllable DGs and loads adopt droop controlstrategy. The simulation analysis for grid-friendly microgrid is completed by PSCAD/EMTDC. The simulation results show that the controllable DGs and loads can distribute the suddenly-changedpower reasonably, which guarantee the power balance and maintain the constant of power flow atPCC. When power flow changes at PCC, the controllable DGs and loads can also maintainmicrogrid power constant and relize the stable operation in the microgrid.