Choosing Current Transformers for Relaying Use March 28, 1995 Current transformers used for relaying under high current conditions must have a relay rating. This rating is expressed as the letter "C" or the letter "T" followed by a number. The number represents the voltage output at the CT terminals with 20 times normal secondary current flowing (100A for a typical 5 A secondary CT), without exceeding 10% ratio error. Standard values are 10, 20, 50, 100, 200 and 400. The "C" or "T" describes the method used for determining this voltage. The voltage for CT's with a "T" designation has been determined by test, the usual method for wound primary CT's. Recent work by a working group of the IEEE Power Systems Relaying Committee has shown that lowratio CT's applied to systems with high fault current levels may saturate under fault conditions and cause improper or no operation of the overcurrent relays. Extreme saturation of the CT's can result in very narrow pulses of secondary current, only a few electrical degrees wide. Although the magnitude of the pulse may be above the pickup setting of the relay, the current does not flow for long enough at a time to operate the relay. To be sure that the CT's do not saturate, the relay rating of the CT should be twice the voltage necessary to drive the maximum fault current through the connected burden. For example, assume a bus with 24 kA available fault current and a CT rated 400/5 A (80:1 ratio). Divide 24,000 A by the CT ratio, 80, to determine the secondary current, 300 A, under maximum fault conditions. If the secondary burden is 0.33 ohms, the voltage required is 300x0.33, or 100 V. A CT with a relay rating of C200 or T200 should be chosen for this application. Where low-ratio CT's are needed on a system with a high available fault current, it may be very difficult to find a CT with the necessary relay rating. Several possible ways to improve the situation are to use higher ratio CT's, to use higher accuracy CT's, to use lower burden relays, to divide the burden between two sets of CT's, or to use separate instantaneous overcurrent relays connected to a separate set of high-ratio CT's. If you want to review the entire report of the PSRC working group, you can find it published in two different IEEE Transactions. The report title is "Relay Performance with Low Ratio CT's and High Fault Currents". It may be found in IEEE Transactions on Power Delivery, Vol. 8, No. 3, July 1993, pp. 884897, and in IEEE Transactions on Industry Applications, Vol. 31, No. 2, March/April 1995, pp. 392-404. Baldwin Bridger, P.E. Technical Director Powell Industries, Inc. PO Box 12818 Houston, TX • 77217 ©2005 Powell Industries, Inc. • All rights reserved. Tel: 713.944.6900 • Fax: 713.947.4453 www.powellind.com info@powellind.com
