Unconventional connections and how to get it right

Abstract

This paper describes current and voltage instrument transformer connections that are not physically conventionally connected as portrayed in ANSI Standards and Guides or relay manufacturers' literature. A three-line diagram of a basic conventional current transformer connection for a two winding delta wye power transformer differential relaying is presented. Then, we present scenarios of (1) one set of CT's being reversed or 'rolled out', then (2) the other set reversed and, finally, (3) both sets rolled out from the conventional connection with proper connection of the unconventional connected CT's. Also, the effect of reversed voltage transformer connections on directional overcurrent and directional power are presented. An example of how symmetrical component metering screens of a numeric relay were used to diagnose an incorrect voltage transformer connection is also presented. This paper is a primer on analyzing unconventional instrument transformer polarity connections and how to obtain the right connection to achieve the desired direction of operation or polarity as indicated in protective relay manufacturers' literature.