Study Of Fast Fault Detection And Isolation Technology For Smart Distribution Grid

One of the development goals of smart distribution grid is to solve the operation problems of a scale of dispersed generators being installed and operated in distribution network. In order to improve their security and reliability, distributed generators are normally integrated into power grids. Because they can supply power to important loads continually when accidents occur, some of the distributed generators are allowable to keep on running rather than exiting, which causes the conversion of distribution system from single-source network into multi-source network. The cooperative relationship between protection equipments in the traditional distribution system will be broken, which has great influence on the action behavior and performance of protection equipments. As a result, the existing feeder protection may be reduced sensitivity, tripping failure, malfunction, and so on, and no longer suitable for use. 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.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. Therefore, the performance verification on power direction relay, the research on fault detection and isolation algorithm, the development of software based on the algorithm and implementation of device and communications programs are the key technology of the master-slave regional pilot protection system, which are also the main research contents of this paper.Using wind turbines as distributed generators and 90-degree phase-comparison directional element to detect the fault direction, the model of distribution network was built based on DigSILENT to verify the performance of power direction relay in the master-slave regional pilot protection system. The line-line voltage of each network node and its relevant phase current can be obtained through simulations of a typical short-circuit fault, which will be applied to verify the accuracy and reliability of power direction relay after data processing in MATLAB.The direction and distribution of the short circuit current in distribution power system has changed a lot due to the introduction of DG, which result in the inapplicability of intrinsical fault detection and isolation algorithm. For the smart distribution system with distributed generation, a new fault detection and isolation matrix algorithm based on the fault overcurrents and their directions is proposed. In order to improve the practicability, a modified algorithm suitable for network topology changes, communication systems failure and fault information lost or wrong is proposed, which is appropriate not only for the network with DG, but also for the traditional network. And compared to the original algorithm, it solves the problem of the limitations effectively. Based on the above algorithm, a software system for fault detection and isolation of smart distribution grid is developed, using VS.NET as platform, and adopted SQL server database and the object-oriented programming design thought, which contains functions of fault simulation, query of fault results, communication between master and slave device and so on. This paper developed a detailed communication strategy and built a reliable communications platform, to ensure the system communication quickly and the reliable communication between master and slave device in the processing of fault detection and isolation. Finally, we set up the test platform and developed the corresponding test program to complete the entire system testing, which examined the master-slave regional pilot protection scheme in the distribution network with DG could also accomplish the fault detection and isolation correctly and rapidly and verify the applicability of the approaches proposed.