Research On Distributed Fixed-time Secondary Control Algorithm For Islanded Microgrid Under Economic Operation

Distributed Generation(DG)based on renewable energy sources such as photovoltaic and wind power,is booming to meet the increasing demand for electrical energy and reduce environmental pollution.So as to realize flexible and efficient coordinated control of various distributed power generation units,the concept of microgrid emerges as time.As a solution to integrate renewable energy into the main power grid flexibly and reliably,microgrid has been widely researched and applied.The microgrid can be connected to the large grid,operated in the grid-connected mode or in the isolated island mode,and can smoothly switch between the two modes when a random fault occurs.However,the output power of distributed power generation unit based on renewable energy such as wind power and photovoltaic in microgrid,which has the characteristic of randomness and intermittence,which poses a challenge to control stable operation of the microgrid,when microgrid is operating in islanded mode,loses the support of large power grid,how to ensure its voltage and frequency stability,and to satisfy the power supply and demand balance and economic operation of the microgrid system is an urgent problem to be solved.Using hierarchical control structure of microgrid,the economy and robustness of the system can be enormously enhanced,this thesis focuses on the problems of economic operation of microgrid and design of controller algorithm,and provides distribued fixed-time control and power optimal dispatch mehod.The main research contents of this work are as follows:(1)The primary control layer is considered in this paper,and the mathematical model of DG with inverter as interface is exploited.A multi-loop control strategy of inverter based on droop control is represented,utilizing droop control,virtual impedance control and voltage and current control,which aims to realize power allocation and frequency and voltage stability control.(2)Regarding to the voltage and frequency recoveration problems in microgrid secondary control,the recoveration problems are converted into leader-follower consensus control problem.By adopting distributed control theory,fixed-time theory,event-triggered mechanism,distributed fixed-time controllers based on event-triggered is established to restore the voltage and frequency to the respective nominal set values and realize power averaging simultaneously,which makes the system more robust.Meanwhile,by adding event-triggered mechanism,the amount of calculation of controllers is greatly reduced,and the efficiency of conrol system is rised.Finally,Lyapunov function is used to verify the stability of the system,and it is proved that there is no Zeno behavior in the system.(3)In terms of economic dispatch problem in microgrid,by selecting total generation cost of microgrid system viewed as the dispatch objection,a distributed fixed-time economic dispatch algorithm is proposed,considering supply-demand balance constraint and generator constraint.It is theoretically proved that the proposed algorithm enables to realize optimal power scheduling.Then,choosing the optimal active power as reference value,modifying the active power droop coefficient,and combining secondary control strategy,distribtued fixed-time optimal dispatch is implemented,and the economic operation of microgrid is guaranteed.The disigned control strategy improves the economy and robustness of isolated microgrid.