Download Transformer Basics

Current
Transformer
Testing
Outline
 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)
Current Transformer
 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.
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.
Megger Test
 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.
Excitation 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
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