4 tests of current transformer

4 tests of the current transformer are discussed

The device that is most noticeable in front of the eyes when you hear the word substation is the current transformer. A current transformer is a special measuring transformer that can be used to measure a large ampere of a substation. So 6 tests of this current transformer are very important.

1. Ratio test
2. Polarity test
3. Excitement test
4. Insulation resistance test

Ratio test

CT ratio is defined as the ratio of primary input current to secondary load current at full load. For example, a CT of 300:5 ratio will generate 5 amps of secondary current while 300 amps of current will flow to the primary side. If the primary current changes, the secondary output current will change accordingly. For example, if 150 amps flow through a 300amp rated primary, the secondary current output will be 2.5 amps.

The CT ratio is specified in different taps of a multitap city. The ratio is tested to prove the accuracy of this ratio and to verify that the ratio is correct. The turn ratio is equivalent to the voltage ratio of the potential transformer and can be expressed as:

N₂/N₁=V₂/V₁

N1, N2 = number of primary and secondary turns

V1, V2 = potential of the primary and secondary side

Polarity test

The direction in which the coils are winded (clockwise or counterclockwise) around the core of a city is determined by how the leads are drawn from the city case. All current transformers are generally of substrate polarity.

How to test polarity

• All power connections must be disconnected before the test and an analog voltmeter connected to the City's secondary terminal for testing. The positive terminal of the meter will be connected to terminal X1 of the city whereas the negative terminal will be connected to X2.
• Patch a piece of wire at the top of the CT window and attach the positive edge of the 9volt battery to the H1 side and the H2 side to the negative edge.
• When the polarity is correct, the transient contact causes a slight distortion in the analog meter towards the positive. If the deviation is negative, the polarity of the current transformer is reversed. The test can also be done by reversing terminals X1 and X2.

Excitement test

The main purpose of the CT excitation test is to verify how much current is increasing with a small amount of voltage. This test is basically done by applying AC voltage to the core.

Insulation resistance test

Insulating resistance can usually be measured by applying a 1000volt DC voltage with the help of a mega. Its continuity can be verified by measuring the resistance repeatedly.