A Study On The Protection Scheme For Distribution Network Including Distributed Generators

Nowdays, traditional fossil energy depletion and environment pollution are increasingly serious. The distributed generation technology characterized with high energy efficiency, good environmental compatibility, using renewable energy is developed quickly. Distributed generators connected to the distribution network can perfect power quality and improve the security and reliability of grid. But at the same time, changes of the network topology, uncertainty direction of power flow, variation of fault current will bring serious influences to traditional current protection for radial distribution system. Distribution network connected with multi-DGs is the development direction of the future, so that it is necessary to find a suitable protection scheme to meet actual running needs.Firstly this paper makes a summary of research status about distribution network protection considering the connection of DGs in home and abroad. After so, the conclusion is that protection schemes basing on communication technology and using wide area information is the future directions. At the same time, this paper makes a brief introduction of DG, including types of DGs, interface modes, control strategies and fault characteristics. The definition, structure and characteristics of microgrid basing on distributed generation technology are also elaborated.Then this paper emphasizes the analysis on the differentia of fault current between synchronous machine DG and IIDG when short-circuit fault occurs. The relationship between fault current and DG capacity, DG location and fault location are derived theoretically. On this basis, the effects on traditional protection and reclosing are analyzed. Simulation results from PSCAD/EMTDC verify the correctness of the conclusion.Finally a new protection scheme for distribution network considering the introduction of multi-DGs has been put forward. Main protection is regional protection. Firstly the distribution network should be divided into distinct regions. The protection algorithm is composed of two steps:first and secondary fault location to discover the fault section and achieve protection. According to Kirchhoff’s current law, current positive-sequence fault component value is largest in the fault direction. We can utilize current positive-sequence fault component information from Bus-IED in substation and A-IEDs in feeder during fault periods to achieve first fault location, then take advantage of current variable amount from A-IEDs or B-IEDs after the corresponding breaker action to realize secondary fault location. Time delay instantaneous overcurrent protection is reserved. Backup protection will clear the fault with a delay while A-IED can not determine fault direction. Considering the situation of potential transformer uninstalled on the feeder of distribution network, the protection scheme in this paper uses current data only. In this scheme, adaptive region division has features of simple algorithm, small storage space, easy realization and good visibility. The advantages of fault location are little information, high reliability and good adaptability. Finally, simulation results from PSCAD/EMTDC verify the correctness of the method.