The Relay Studies Of Distribution System With Micro-grid

Distribution generation (DG) technology has been rapidly developed and the DG penetration level is becoming higher and higher in the distribution system in recent years. The magnitude, direction and distribution of the short circuit current in distribution power system has changed a lot due to the introduction of much more DG, as a result, the traditional overcurrent protection zones can't work exactly .To satisfy the requirements of the relay protection of the distribution systems under a high DG penetration level, this paper introduces a new scheme to resolve the existing problems of the relay protection. The new protection scheme is based on longitudinal comparison principle. With the foundation of communication technology, the new scheme is a kind of substation area longitudinal protection with master-slave structure, for the purpose of protecting the substations of the distribution system with more DG and their feeders. This paper will build some different models of the distribution network by using the PSCAD / EMTDC simulation software, as a result, the effective time of direction components can be verified. In addition, the paper proposed a communication program to satisfy the protection scheme.Because the DG is connected with the distribution network, the traditional single-supply radial network becomes a multi-source network. The distribution of the normal power flow will be affected. And the amperage,direction,and distribution of short-circuit current in the existing distribution network will also change. As a result, the operation and control of the distribution network will become unacceptable. After the DG interconnection, the impact of the relay protection is mainly manifested in the following areas: (1) The sensitivity of the line protection is played down and even trip failure. (2) The line protection happens malfunction. (3) The instantaneous overcurrent protection of the neighbor line occurs malfunction to lose the sensitivity. (4) DG may make the reclosure unsuccessful. Although some improved protection schemes for the DG have been introduced, many schemes can only improve some aspects of the traditional relay protection, and they have a lot of limitations. Under the low DG penetration level, many of them can not meet the requirements of relay protection.Under high DG penetration level, the advantages of DG can be fully shown and it will be the future development trend of the power grids. This paper introduces a new pilot protection scheme to solve the problem with DG connected under high DG penetration level. Different from the traditional "peer-to-peer" longitudinal comparison protection, the new scheme introduces a master-slave structure. The kernel of the new scheme is that a station-level master protection device (MPD) is equipped at the protected distribution substation and several field slave protection units (SPU) are installed at the position of different circuit breakers. The MPD communicates with the SPU through communication network. Each SPU has a directional relay inside it, which measures the voltage and current of its position and determines the fault direction. The fault direction message will be sent to the MPD when a query command is received by the SPU. The MPD processes the all of messages from different SPU and decides the fault section using a fault location algorithm to trip correlative breaker. The protection scheme is a fault detection algorithm based on the hierarchical order. Through the hierarchical comparison, it can easily and clearly identify the location of fault, and the failure equipment will be rapidly, sensitively, selectively and reliably removed without affecting the normal operation of other equipments.In this paper, the direction information of fault from the slave is very important for the protection scheme, so the accuracy of the results from the fault direction elements is strictly demanded. When fault happened, the results of fault direction from the fault direction elements may be impacted by DG, and sometimes misjudge may arise. In order to ensure the accuracy of the results from the fault direction elements, This paper will build some different models of the distribution network by using the PSCAD / EMTDC simulation software, as a result, the results of the direction components can reflect correctly fault direction within 200ms after fault .Because the required time of the relay action is a little less, the results from the direction of components can ensure the feasibility of the scheme. In order to ensure the speed of relay, a powerful communication system will be need to removal the fault timely. In view of the characteristics of the transmission speed and free from interference with other factors of the optical fiber communications, the information between the master and slave protection device will be exchanged by the optical fiber communication in this paper...