Current Transformer

Testing

Current Transformer CT Test Setup Tap, Ratio and Polarity Test Megger Test Excitation Test Testing CT’s in a Circuit Breaker Testing CT’s in a Three Phase Delta-Wye Transformer Digital Current-transformer Testers (EZCT-10)

Outline

Transformer is a transformer, no matter whether it is a voltage or current transformer.

A Current Transformer circuit must be grounded; however, it should only be grounded in one location.

A Current Transformer circuit secondary must never be opened when load is flowing in the Primary.

Current Transformer

CT TEST SETUP

Tap, Ratio and Polarity Test

This test is to verify the correct CT polarity, full CT ratio, and taps.

It also verifies taps are correctly brought out to the CT terminal blocks.

Voltage applied may be determined by the CT ratio per the next page chart.

Reduce the voltage to zero before switching Off the AC source to prevent magnetizing of the CT core.

This test is to verify that adequate insulation exists between the CT winding and ground.

With one Megger lead connected to ground, touch the other Megger lead to one CT tap. The CT should typically read 75 mega ohms or more.

If the readings are not within this limit, then check the wires, terminal blocks, or CTs for moisture, condensation, or anything else that could contribute to the low resistance readings

Note that the 75 mega ohms guideline is only applicable for testing individual CT’s, not parallel CT circuits or long cable runs connected to the CT.

Megger Test

• This test is to verify that the CT does not have any shorted secondary turns and to compare the measured exciting current against the manufacturer’s CT test curves.

• The best test method is to determine what voltage is required to approach the saturation point (knee of the curve) of the CT for the full winding and apply a voltage slightly below this value and read the current.

• Measured currents greater than those shown on the CT curves would indicate trouble within the CT.

• If the CT curves are not available and the current increases quickly with small increases in voltage there is trouble within the CT.

Excitation Test

• If the voltage source available for testing is not high enough to approach the saturation point, the test can be run with a lower voltage and or fewer secondary turns.

• The lowest voltage that we would want to use would be 0.50 volts per turn connected from X1 to X5.

• You may also use the minimum number of turns calculation as shown below to determine test values.

Excitation Test

Excitation Test

Testing CT’s in a Circuit Breaker

Testing CT’s in a Single Phase Transformer

Testing CT’s in a Three Phase Delta-Wye Transformer

With all bushings, not being tested, shorted together and connected to non-polarity

Digital Current-transformer Testers (EZCT-10)

This test set can perform the current transformer (CT) excitation, CT current-ratio, and winding polarity tests.

EZCT Cable Connections

Connection to CT’s on Y Transformer

Connection to CT’s on Delta Transformer

Typical EZCT Connection to Multiple CT’s