Research On Drooping Control Of Microgrid Inverter

Microgrid is a small power system which integrates various distributed generation.Since distributed generation usually use power electronics to access the microgrid,the inverter control method has always been the research focus of microgrid technology.For "plug and play",flexible operation and other advantages,Droop control which based on peer control structure becomes an important method in inverter control.However,droop control in low voltage microgrid has various problems such as power decoupling,power distribution of parallel inverters and smooth switching of the microgrid operation mode.We focus on the above issues to research the droop control scheme of microgrid inverters.The research background and development status of microgrid were simply investigated in this paper.The basic structure and characteristics of microgrid were introduced,and droop control strategy of inverter that based on peer control structure was selected as the research object.After analyzing the inverter's typical control method of constant power control(PQ)and constant voltage constant frequency control(V/f),the design principles of the droop control were deduced in detail.Considering the transmission line as perceptive,droop control power transfer model wasbuilt.The mathematical models for each link were established in a rotating coordinate system and drooping control structure was built.The small-signal model for multi-inverter parallel microgrid systems wasbuilt.The influences on dynamic performance and stability of microgrid system were analyzed with the method of root locus,which caused by system parameters such as drooping coefficient,line impedance and load.For power allocation problem of droop control,power sharing conditions were derived.In order to realize the rational allocation of reactive power by capacity,the virtual impedance was used to satisfy the reactive equalization equation.We analyzed the effect of virtual impedance on the output characteristics of the inverter,and used virtual inductance to improve the sensing characteristics of equivalent transmission line impedance to meet power decoupling conditions.Reactive power sharing equations,power decoupling conditions,and system stability requirements were taken as restrictions,range of virtual impedance has been got in this paper.As the above scheme adopts approximate means to ignore the inverter output impedance,reactive power cannot be accurately distributed.We redesigned reactive distribution equation,added average reactive droop to correct voltage reference,and solved the problem of accurate distribution of reactive power.Under the hierarchical control structure,secondary control was utilized to restoring voltage and frequency offsets caused by droop control.Power quality of microgrid system was increased and primary node voltage and frequency could Keep at rated value when the microgrid operates steadily.Pre-sync control strategy which adjust the inverter grid point voltage and frequency to follow the main network was proposed to reduce grid-connected inrush current and realize the smooth switching of the microgrid operating mode.We improved reactive power droop equation in grid-connected mode by increasing reactive power feedback,to achieve that output power of the inverter connected to the microgrid to be rated and the control method can be equivalent to PQ control.Various control Strategies and working conditions were simulated by using simulation tools.The effectiveness of the above control strategy was successfully verified.