Effect of Offshoot Branch Connected Rotary Machines on Line Selection Protection Relay

The purpose of this paper is to study an effect of offshoot branch connected rotary machines on a line selection protection relay. The motivation for this study stems from the fact that in a real power system with offshoot branch containing rotary machines like synchronous generators and induction motors, a case of mal-tripping of a line selection relay was observed. The main cause of this mal-tripping was thorough investigated through digital simulation followed by experimental verification carried out by connecting a real time simulator to the real world relay. It is found that the mal-operation of the relay was triggered by the combined dynamic characteristics of the rotary machines like synchronous generators and induction motors. Hence, the main result of this paper is that in case of a power system with an offshoot branch containing induction motors in addition to generators, it is very important to consider their combined dynamic characteristics in true sense, which hitherto has been more or less limited to generators only while conducting such relay settings/studies.