Relay Burden - Abb j2d31

ABB Distribution Automation & Components News Thursday, February 4, 2010

The Importance of Burden in Metering and Relay Applications

Pictured is an ABB SCR current transformer typically used inside of switchgear. Here at ABB Pinetops, we are often asked about the burden of a current transformer being used in both metering and relay applications. The burden is actually the load on the secondary, but is referred to as burden to avoid confusion with the primary load. Current transformers are usually designed for either metering or relay applications. An example of this would be our 15kV CTs - the KON-11 for metering and the KOR-11 for relay applications. The KON11 has a higher rating factor and the KOR-11 has a greater relay accuracy. Let's take a look at metering burden first. Metering CTs typically operate at 80-90% power factor. IEEE C57.13, which governs instrument transformer standards, defines standard burdens according to the following: 2.5VA @ 90% PF (B-0.1) 5.0VA @ 90% PF (B-0.2) 12.5VA @ 90% PF (B-0.5) 22.5VA @ 90% PF (B-0.9) 45.0VA @ 90% PF (B-1.8) The burden on a B-0.1 is 0.1 ohms. Likewise, the burden on the secondary of a B-1.8 is 1.8 ohms. You will see a metering CT identified as a 0.3B-0.9. This means that the CT meets the ANSI metering standard of 0.3% accuracy and has a burden of 0.9 ohms on the secondary. Relay accuracy is very similar to metering accuracy, with different nomenclature. Relay burdens are usually designated with a C (for Calculated) or a T (for Tested). Relay CTs are typically operated at 50% power factor. IEEE C57.13 defines relay burdens as: 12.5VA @ 50% PF - 0.5 Ohms or C50 25VA @ 50% PF - 1 Ohm or C100 50VA @ 50% PF - 2 Ohms or C200 100VA @ 50% PF - 4 Ohms or C400

200VA @ 50% PF - 8 Ohms or C800 A relay CT will be labeled C200 which means that it is capable of a maximum of 2 ohms on the secondary. If you have any questions about your instrument transformer performance, please an ABB Sales Engineer in Pinetops, NC at 252-827-2121