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Transformers - Auxiliary current transformers

Auxiliary current transformers are used in protective relaying circuits for a multitude of applications. They are often used to adjust the secondary current to the proper level for the relay using one primary auxiliary CT input winding and one secondary winding feeding the relay. Auxiliary CT's can also be used to feed a single phase relay with the vector sum of three phase to neutral currents from main CT's. Generally any two or more AC currents can thus be summed and transformed to one secondary current via auxiliary CT's for many different relaying and measuring applications.

The auxiliary CT provides isolation between primary and secondary current circuits. This feature can be used to increase the insulation level between current circuits. The standard insulation level for the SLCE and SLXE auxiliary CT's is factory tested at 2,5 kV for 1 minute between windings and also between windings and ground.

With the RADHL pilot-wire differential relay, a version of the SLCE 8 auxiliary CT with 15 kV insulation is available for the pilot wires. Another version of the SLCE 8 is used with RADHL as a three-phase summation CT. The dimensions of the SLCE 8 can be found in the data sheet for RADHL.

Multi-tapped SLCE 12 auxiliary CT's are available to match CT ratios for example for the RADSB or other transformer differential relays. SLCE 16 and SLXE 4 are larger and used when more iron is needed to handle the relay circuit load.

The shown standardized winding versions of the auxiliary CT's have been tested for use with certain products such as the RADSB, RADSC and RADSS and REB 100 series of transformer and bus differential relays.

On request, auxiliary CT's can be obtained with other ratios or specified characteristics. 

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