Checking the Polarity of Current Transformers

The Polarity of current transformers is extremely important. Just like a battery, a current transformer too has a polarity. The polarity determines the direction of the secondary current in relation to the primary current.

Wrong connection of the current transformers can cause false operation of the protection relays. Hence, it is vital to ensure that the current transformers are connected with the correct polarity.

The figure shows a setup to test the polarity of a current transformers.

A DC source is connected with the positive terminal to P1 and the negative terminal to P2. An analog voltmeter is connected to the secondary terminal of the CT. The positive terminal of the meter is connected to terminal S1 of the CT while the negative is connected to terminal S2.

A contact is momentarily made through the switch. The contact is made for a second and broken. This is important as continuous contact can short-circuit the battery. The momentary make-break contact causes a deflection in the analog multimeter in the positive direction, if the polarity is correct.

If the deflection is negative, it indicates that the polarity of the current transformer is reversed. The terminals S1 and S2 need to be reversed and the test can be carried out.