Relays play a great role in power system protection and fault diagnosis. The present work deals with a retrofitting work on the feeder panel of 33 KV Kachumari sub-station of APDCL under Sivasagar Electrical Circle. The system is integrated with ABB REF615 class numerical relays in series with the older generation of electro-mechanical relays. The mode of operations of the old and new systems, their circuit diagrams, CT and PT connections, among others have been explored. The whole process of retrofitting has been discussed in details. Investigations shows that the sensitivity of the new numerical relay is better and it is able to successfully isolate various faults viz. over current and earth faults. The fault event history after the installation of the numerical relay has been recorded and analyzed. The fault propagation to upstream grid side has been found to be considerably minimized after the retrofitting work. Since inception and commissioning, it is in service and helping in an improved customer experience
