Research On Control Method Of AC Microgrid And Its Converter

As the depletion of fossil energy and the disadvantages of traditional centralized power generation,more and more domestic and foreign scholars pay attention to the distributed generation for its typical advantages,such as rich resources,low pollution,flexible installation site and so on.However,the great capacity of distributed generations connected to the traditional grid will effect the operation and scheduling of the traditional grid.The distributed generations connected to the grid by means of AC microgrid,not only make full use of renewable energy,but also manage the distributed generation conveniently.As a autonomous network,converters are core units of microgrid.It is important to research the control of microgrid and its converters for the full use of renewable resources and the stable operation of microgrid.Firstly,the definition,topological structure and operating modes of AC microgrid are introduced,and the common types of micro sources and their converters are expounded.Especially,the mathematical model of DC/AC converter is analyzed in the paper.Meanwhile,the common control method of DC/AC converter and control mode of AC microgrid,as well as the advantages and disadvantages of each method are analyzed.Secondly,the control method of converter,droop control based on virtual impedance is introduced.The reason of traditional droop control can not be directly applied in microgrid system are analyzed.Then virtual impedance are introduced to the traditional droop control to make the droop control apply to AC microgrid more better.Droop controller is designed and the control parameters of voltage and current loop have been set.Then the simulation model of droop control based on virtual impedance is established,and the simulation results demonstrate that the control strategy is correct.Meanwhile,the control strategies of microgrid are introduced,including the accurate allocation of active power and reactive power between micro sources,the stability of voltage and frequency when microgrid is island,and the smoothing switchover between operation modes with energy storage by using hierarchical control.The simulation model of hierarchical control is established and the simulation results demonstrate that hierarchical control is effective.Finally,the stability of microgrid is discussed.The effects on the microgrid stability by the control parameter of converter,including the active and reactive droop coefficient and line impedance are analyzed by the small signal model of microgrid.The simulation model of microgrid based on MATLAB/SIMULINK is established and the simulation results verified the effect of converter control parameters on the stability of microgrid.