Effect of non standard characteristics of overcurrent relay on protection coordination and maximizing overcurrent protection level in distribution network

Abstract

This research employs the capabilities of 7SJ602 model (overcurrent and earth fault relay) for better performance and faster response. The Semiens 7SJ602 model is connected to the real radial system of an Egyptian Substation to protect the feeder. Standard relay may solve the coordination problem. But due to the fault current behavior in the applied network, the coordination problem wasn’t solved by the standard characteristics of the 7SJ602 relay curves. In the study of fault current behavior, the light and heavy loading and fault locations are considered under the radial nature of the network. This paper produces three categories for coordination problem treatment. The first category proposes non standard model for 7SJ602 OC relay to familiarize with the network behavior and treat the coordination problem. The second category is to improve the first category such that the proposed model minimizes the relay tripping time by about 78.9%. The addition of third category is to build new back up protection in the relay for increasing the network protection level under relay failing. The new contribution of this work is the design of multicharacteristic relay for solving the coordination problem due to the fault current behavior of the network and maximizing the protection level by creating self back up protection where the relay can operate as a back up for itself under failing. If I≫ setting fails to operate, the setting I>>> will operate as back up. Appearance of self back up protection can reduce the tripping time of back up protection and its coordination time interval by about 56.85% and 69% respectively. The protection coordination due to earth faults is considerd in the study since this type of network is unbalanced in nature. The short circuit and coordination analysis are performed using Etap software.