It is important to conduct current transformer testing that includes inspection and calibration on scheduled intervals. There are six CT tests that should be performed with CT test meters for the best accuracy and service reliability:
1. Ratio Test
CT ratio is the ratio of primary current input to secondary current output at full load. If there is a change in the primary current output, the secondary current output will adjust accordingly. Unlike the voltage or power transformer, the primary winding of the current transformer consists of only one or very few turns, being either a single flat turn, a heavy wire coil wrapped around the core, or a bar or conductor that has been positioned through a central hole.
2. Polarity Test
The polarity of a CT is determined by what direction the coils wrap around the transformer core, either clockwise or counterclockwise, and how the leads come out of the CT case. Current transformers are subtractive polarity, with the flow direction defined by the following marks:
These CT marks define the relative instantaneous directions of the currents’ instantaneous directions. Transformer test instruments for a polarity test proves that secondary CT current direction is correct for primary current.
3. Excitation or Saturation Test
Transformer test instruments are used for Excitation tests by applying an AC voltage to the secondary winding of the CT and increasing the voltage in steps until the CT is in saturation.
Demagnetizing the CT is performed with meter testing that slowly decreases test voltage to zero. The test results are recorded on a log-log graph and the transition period evaluated between normal operation and saturation.
4. Insulation Resistance Test
Three forms of CT testing is conducted to determine condition of the insulation of the CT:
1. Primary to secondary: Checks insulation condition between high to low.
- 2. Primary to ground: Checks insulation condition between high to ground.
- 3. Secondary to ground: Checks insulation condition between low to ground.
Insulation resistance readings should stay fairly consistent over a period of time.
5. Winding Resistance Test
Periodic DC winding resistance is recommended on a single tap or multi tap current transformer testing. The CT should be demagnetized after the completion of winding resistance test.
6. Burden Test
If a current transformer is not properly sized based upon secondary loop burden, it may result in a decrease in CT secondary current. Transformer test instruments for burden testing ensures that the CTs are:
Burden can be measured by injecting the rated secondary current of the CT from its terminals towards load side and isolating the CT secondary with all connected load, observing the voltage drop of the injection points, and observing every point of the circuit to ground.