Network Partition And Island Identification Method Studies Of Smart Distribution Grid

Distribution generation (DG) technology has been rapidly developed in recent years, in order to improve their security and reliability; distributed generators are normally integrated into power grids. The direction and distribution of the short circuit current in distribution power system has changed a lot due to the introduction of DG, as a result, the existing feeder protection there may be reduced sensitivity, tripping failure, malfunction, and so on, which gives distribution network relay protection many problems. Therefore, it is necessary to carry out related research on protection principle and put forward a protection algorithm, which are of great significance for the smart distribution network in the development and promotion.When distributed generations integrate into the distribution power system, the traditional single power network changes into a multi-source network. The direction and distribution of the short circuit current in distribution power system of normal operation has changed a lot. The cooperation relationship in traditional distribution protection equipments have been broken, the automatic reclosing in distribution network and the island detection method of smart distribution network will be affected. Although some automation reclosing and island detection methods for distribution power system have been put forward, but they can only improve certain aspects with limitations, and can not satisfy the protection requirements of smart distribution network containing DG.On the basis of previous research and longitudinal comparison principle, a new protection scheme which is a kind of substation area longitudinal protection with master-slave structure is presented for smart distribution grid, with the objective to protect the whole distribution substation and all of its outgoing feeders. Every switch in the distribution power system that can cut off short-circuit current is equipped with slave protection device. A host computer of system is set in the substation, which can achieve the interaction of information with slave device by the optical fiber communication network quickly and reliably. When the distribution power system is failure, the fault direction can be obtained from the slave device by calculating local electrical information, which is collected through the communication system and used by fault detection and isolation algorithm. By data operation processing, the host computer can find out the fault section and the breakers needed to be opened, and then send the trap instruction to the slave device so that the fault can be isolated finally.Due to a large number of distributed generations (DG) or micro-grid (MG) integrating into smart distribution network, the automatic reclosing in distribution network meets new challenges. When faults occur in smart distribution network, in order to make the instantaneous fault arc disappear as soon as possible and improve the success rate of automatic reclosing, the main switches, DG branch switches or MG switches and load switches of the fault region should be tripped, then according to the strategy reclose tripped switches sequentially. In this paper a method of circuit breaker classification for smart distribution network is proposed, after fault occurs relay protection system accomplish fault detection and isolation. According to the current network topology the host computer forms the types of circuit switches and corresponding incidence matrix which marks each switch in distribution network, finally complete automatic classification of tripped switches with the calculation of the matrix and transmit the classification results and reclosing strategy to smart terminals in order to realize the automation reclosing strategy.The node types in distribution network are different, all kinds of DG switch and trip frequently, results that smart distribution network operation is flexible, especially when some switches are tripped, the grid will be divided into different and independent subnets, if the loads in subnet power are supplied by DG or MG, the subnet will run in the form of an island. On one hand the island brings great influence on grid dispatching, automation system, relay protection and automation reclosing in smart distribution system, on the other hand, many problems for security and stability of the island exist. In this paper, a network partition and island identification method based on reachability matrix has been put forward, using the switches position information based on the network topology and reachability matrix, so each regional property can be quickly judged, whether the island exists and the number and range of islands. The research in this paper will lay the foundation for island operation control and protection.