Protection of transformer-ended feeders using multifunction relays

Modern digital multifunction relays provide protection, control, monitoring, metering, and communications functions at a fraction of the cost of old electromechanical relays. Because of their versatility, reliability, and lower cost, digital relays are becoming increasingly attractive to distribution power utilities in applications to solve specific protection and control problems. One such application is the protection of transformer-ended feeders (TEFs), which are very popular in some utilities. In this paper, we describe an application of digital multifunction relays for the protection of a feeder that ends with a distribution transformer. In this application, we attempt to provide as many protection functions as possible with the fewest relays. The application we describe is similar to that used at ENERGEX (a distribution power utility in Brisbane, Australia), in which two directional overcurrent relays protect both a feeder and a transformer. The relay located at the low-voltage (LV) side also provides restricted earth fault (REF) transformer protection and breaker failure protection as well as backup overcurrent, ground, and sensitive earth fault protection for a distribution network. In the application we describe, the LV relay also combines with existing feeder relays to provide LV busbar protection. We also present the results of simulations performed on a Real Time Digital Simulator (RTDS®), particularly regarding the selection of appropriate settings during transformer inrush conditions.