Research On Protection Schemes For Distribution System With Distributed Generation

As the problems of environment pollution, energy shortage and power supply mode of centralized bulk power grid happened, the distributed generation technology based on renewable energy sources is developed quickly. After distributed generators connected, traditional current protection suitable for radial distribution system may not meet the demands, leading to maloperation. To solve the problems caused by the connection of distributed generators, the protection schemes for the distribution system with distributed generation are studied in this dissertation.Firstly, the fault analysis method of distribution system with distributed generators is investigated, and the expression of the short circuit current is derived theoretically. After so, the effect on traditional protection and reclosing is analyzed.Due to the change of output power of inverter interfaced distributed generators (IIDG), the value setting of the traditional current protection becomes very difficult. Based on the fault characteristic of the distribution system under this circumstance and the traditional adaptive current instantaneous trip protection scheme, the adaptive current instantaneous trip protection scheme for the distribution system with IIDG is studied. The results show that the value of the relay whose dorsal side is connected to IIDG can still be set according to the form of traditional scheme, with some parameters defined newly. In order to clear the fault quickly, the study on application of non-communication protection in distribution system with distributed generators is carried, and a fast current protection configuration scheme is presented. With this scheme, the line can be cutoff quickly from two ends without changing the configuration of circuit breakers.A new protection scheme based on the coordination of zoned pilot protection and overcurrent protection is introduced. According to the location of distributed generators, the feeder is divided into different zones. Directional pilot protection is configured at the upstream side of distributed generator, and the overcurrent protection is retained for the whole feeder. In order to be convenient for value setting and clear the fault quickly, definite-time or inverse-time overcurrent protection is adopted for the feeder based on different locations of distributed generators. No matter how the output power of the distributed generator changes, the fault can be cleared reliably with this protection scheme. According to the situation of potential transformer uninstalled on the feeder of distribution system, the practical criterion for the fault direction of the nodes with multi-branch is presented, based on the relationship between the current amplitude and phase in the downstream branch of the node. In addition, a new fault location scheme based on limited time current instantaneous trip protection is introduced, using the operation information of the protection in the relevant zone and the feature of the limited time current instantaneous trip protection.The protection schemes above can solve the problems caused by connection of distributed generators effectively, which can create the condition for further devemoment of distributed generation technology.