Research On Microgrid Protection Strategy

The prominence of energy crisis and environmental issues has drawn widespread attention to microgrid systems which utilize new energy and renewable source as the main form of power generation.Microgird not only can improve the distribution reliability of large grid,but also alleviate the negative impact of energy crisis and environmental pollution effectively.In this paper,microgrid protection technology is taken as the research object.On the basis of summarizing and analyzing the present theoretical development and engineering practice of microgrid at home and abroad,two basic development ideas are pointed out: one is to judge the occurrence of faults and to determine the fault area by collecting local fault information,that is,the protection scheme based on local information;another is a communication-based protection scheme that determines and locates faults by collecting global or regional information.Based on the above ideas,this paper has carried out targeted research on the microgrid protection scheme.The main work and conclusions are as follows:The fault characteristics of typical micro sources are analyzed.The fault models of inverter interfaced micro sources are established.Meanwhile,the characteristics of asymmetric and symmetric short-circuit fault of inverter interfaced micro sources under PQ and V/f control mode are analyzed respectively.The mathematic model of control system and the formula of fault current are built.The microgrid protection strategy based on local information is studied.The idea of implementing protection by designing microgrid network topology is put forward and four topologies suitable for local information protection schemes are extracted,which are switch station,star,hand-in-hand,the area combination of micropower and load are extracted,and their protection configurations in microgrid are analyzed respectively.A protection scheme based on local information is proposed.The problems of adaptive current quick-break protection applied to microgrid feeder protection are pointed out,and the solutions are given.The setting strategy of the protection device is analyzed,and the setting scheme of one-stage adaptive current quick-break protection and two-stage adaptive time-limited current quick-break protection are established.The microgrid protection strategy based on communication is studied.In this paper,a distributed protection scheme for microgrid is proposed by collecting regional information to determine the occurrence and location of faults.Aiming at the microgrid with complex topology,a graphical model of microgrid is established.Based on the graphical model,the microgrid is divided into several regions,and the fault characteristics of microgrid are analyzed in the view of graphical model.A distributed edge state protection algorithm based on graph model is proposed,including edge direction protection and edge current protection.For the short circuit in the non-terminal region,the fault occurrence and location are judged according to the power direction of the adjacent line in the region.For the short circuit in the end area,the fault occurrence and location are judged according to the current size and direction of the adjacent line in the region.This paper studies the impact of protection configuration with microgirds connecting to traditional distribution network and the corresponding strategies.The influence of microgrid accessing on the current protection and distance protection of distribution network should be dealt with by means of timely exit of microgrid,limitation of access location and capacity of microgrid,introduction of advanced communication technology and directional components,which are working without changing the traditional protection configuration.The influence of microgrid accessing on automatic reclosure of distribution network is analyzed.Without changing the reclosure configuration of distribution network,a scheme of accelerating before reclosure in coordination with the timing for microgrid's on/off-grid is proposed.In order to deal with the unplanned isolated problem caused by the instantaneous grounding fault of low voltage distribution network,the topology structure of micro-grid with low voltage traversing ability is designed.When an asymmetric ground fault occurs in the distribution network,the microgrid can operate through connected grid and provide certain support for the fault recovery of distribution network.