It’s not unusual for a power line to be rated at 400,000 to 750,000 volts. Current transformers convert primary current into the secondary current for use in control equipment, meters, and several other instruments. Essentially, current transformers play a vital role in protecting and monitoring electrical power systems.
Current transformer (CT) sets, especially those used for metering, must be highly accurate to guarantee precise billing. Similarly, current transformers for protection purposes must react adequately and quickly when a fault occurs.
Instrument transformers tests are very important. They prevent risks such as mixing up connections and confusing protection and metering equipment. current transformer testing before initial use helps reduce theses risks significantly. Also, current transformer testing help to identify electrical changes caused by aging insulation, among other causes. To ensure CT performs accurately and optimally, you should complete the following tests.
1. Ratio Test
CT ratio is the ratio of primary current input to secondary current output at full load. Changes in primary current changes will cause a change in secondary current output. The primary winding of the current transformer has one or a few turns, unlike voltage or power transformer. Ratio tests help to verify that the ratio is correct at each tap of the multi-tap current transformer. When performing the ratio test, a suitable voltage is applied to the secondary winding of the current transformer.
2. Polarity Test
The direction of coils around the transformer core determines the CT polarity. All current transformers have subtractive polarity. Polarity tests help to prove that the predicated direction of secondary CT current is correct according to the primary current direction.
It is crucial to observe the correct polarity when connecting or installing current transformers to protective relays and power metering. The polarity of CT is assumed correct if the direction of the primary current is opposite to the secondary current direction. It is important to test CT polarity especially when it is used together with three or single-phase applications.
Nowadays, there are current transformer testing devices that can use lead testing energy meters to perform ratio tests automatically. The transformer test instruments will show whether the polarity is correct or incorrect. A 9V battery and analog voltmeter can be used to verify the CT polarity.
3. Saturation or Excitation Test
A saturated CT makes the magnetic path in the CT to operate as a short circuit when flowing through the transmission line. Energy from the primary winding is pushed away from the secondary winding.It creates a magnetic field in the CT.
CT excitation tests identify the rated knee point against IEC or IEEE standards, which is the point at which the transformer is unable to produce output current that is in proportion to the specified ratio.
The saturation test involves applying an AC voltage to the CT’s secondary winding. Voltage is increased gradually until CT is saturated. To demagnetize the CT test, voltage is decreased slowly to zero.
4. Insulation Resistance Test
When undertaking a comprehensive current transformer testing, it is important to check the insulation between the winding of a current transformer and winding to the ground. Three tests are performed to determine the insulation condition of the current transformer under test. These tests include primary to secondary, primary to ground, and secondary to ground. The reading of insulation resistance should be constant over time.
5. Winding Resistance Test
Dc winding resistance determines the accuracy, state, and actual condition of the current test. CT winding resistance is bound to change depending on use, age, and other external conditions. It is important to measure the DC winding test regularly. The winding resistance is calculated by dividing the voltage drop with DC in the winding. After the test, the CT should be demagnetized.
6. Burden Test
Burden test is the total impedance in ohms on the terminals of the secondary output. The burdening test helps verify the current supplied by CT to a circuit within the burden rating. The total burden is impedance provided by relay current coils, terminal blocks, watt-hour meter coils, contact resistance, test switches, and wire resistance combined. Burden tests ensure that the CT is tightly connected to a single ground point. It also ensures that CT is not energized using shorting devices. Lastly, the burden test ensures that CT does not have an open circuit when not in use.
