CURRENT TRANSFORMERS

Polarity and Orientation

CT Polarity■The polarity of a current transformer is

determined by the direction of the coils wound around the core of the CT (clockwise or counter clockwise) and by the way the secondary leads are brought out of the transformer case.

CT Polarity■It is very important to observe correct polarity

when installing and connecting current transformers to power metering and protective relays.

■CT polarity is also sometimes indicated with an arrow. These CTs should be installed with the arrow pointing in the direction of current flow.

CT’s Polarity

TestMarkings on current transformers have been occasionally misapplied by the factory. You can verify the polarity of a CT in the field with a 9V battery with the following test procedure.

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CT’s Polarity

TestDisconnect all power prior to testing and connect an analog voltmeter to the secondary terminal of the CT to be tested. The positive terminal of the meter is connected to terminal X1 of the CT while the negative terminal is connected to X2.

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CT’s Polarity

TestRun a piece of wire through the high side of the CT window and momentarily make contact with the positive end of the 9-volt battery to the H1 side (sometimes marked with a dot) and the negative end to the H2 side. It is important to avoid continuous contact, which will short circuit the battery.

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CT’s Polarity

TestIf polarity is correct, the momentary contact causes a small deflection in the analog meter in the positive direction. If the deflection is negative the polarity of the current transformer is reversed. The terminals X1 and X2 need to be reversed and the test can be carried out.

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180° Out of Phase

When does polarity matter?

■The polarity doesn’t matter in Single phase metering because you only get the peak value of the sinusoidal wave, but it does matter in 3-phase metering because vector sum of three CT's should be zero when connected together. Phase failure may occur.

References:

■ http://cr4.globalspec.com/thread/21663/Current-Transformer-CT-polarity

■ http://www.ccontrolsys.com/w/CT_Installation_and_Wiring■ http://

testguy.net/content/190-Current-Transformer-Basics-Understanding-Ratio-Polarity-and-Class

■ http://www.prosoundweb.com/article/print/polarity_and_phase_explained