Dec 8, 2018

Auxiliary Current Transformer?


EARTfelt Welcome dear friends of protection and control engineering, due to today's generation of digital protection devices and the concomitant existence of numerical ratio and vector group calculation, switchable current transformers for adaptation within differential protection circuits are becoming increasingly rare.

With their help, many problems were solved at the time of electromechanical and also analog-static device technology.

These include:

🌐 Adaptation of the ratio for transformer protection

🌐 Adaptation of the vector groups for transformer protection

🌐 Zero sequence elimination using a star-delta circuit

🌐 Galvanic isolation of circuits

🌐 Reduction of the burden for bigger main CT's

Since auxiliary current transformers are still in operation in older systems and are also helpful in special cases, we present below the typical wiring of these small versatile transformers.

The transmission ratios of the phases are set via simple jumpers on the CT. A classic auxiliary CT has 8 partial windings with 16 terminals for this purpose.

These can be connected both in primary and secondary in series to achieve a desired ratio. In order to get the wanted ratio, the windings can be changed in polarity to perform either an addition or a subtraction of the respective winding.

Let's look at the basic principle. The following ratio is to be realized:

With the existing number of turns, all digits from 1 to 52 can be combined. With the help of a small spreadsheet, we quickly find the ideal ratio of 5:18.

We can realize the number of 5 by subtracting 7 minus 2, and we get the number of 18 from the sum of 16 and 2. So we can do the wiring, which finally looks like this:

The auxiliary current transformers of the three phases can now be connected via the known transformer vector groups, analog to realize zero-sequence blocking and vector group variations.

HEARTfelt Greetings Alexander Muth