Unconventional CT and VT Connections and How to Get Them 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 basic conventional current transformer connections for a two-winding-delta wye power transformer differential relaying is presented. The unconventional scenarios presented are: 1) one set of current transformers (CTs) being reversed or "rolled out"; 2) the other set reversed; and 3) both sets reversed from the conventional connection. Proper connection of these unconventional connected CTs is then explained. Also, the effect of reversed voltage transformer (VT) connections on directional overcurrent and directional power relay elements is presented. An example of how symmetrical component metering screens of a numeric relay were used to diagnose an incorrect VT 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.