Why measure the polarity of current transformer T？ The polarity measurement of current transformers must be carried out before and after handover and major repairs. In addition, when differential protection, power direction protection misoperation, or reverse operation of the electricity meter, CT polarity check should also be performed. This is because if the polarity of the current transformer is incorrectly connected during the wiring process, the following dangers may occur:

If current transformers are used in relay protection circuits, it can cause misoperation or malfunction of the relay protection device, which can also affect the operation monitoring and accident handling of the power system, and even endanger the safety of equipment and personnel.

If current transformers are used in the instrument measurement circuit, the indications and energy measurements of various instruments will be incorrect.

If the polarity of any phase of a current transformer with incomplete star connection is reversed, the current of the phase without a current transformer (usually the middle phase) will increase several times compared to the other phases.

For current transformers with incomplete star connections, if the two phases are reversed, the three-phase currents on the secondary side can still maintain balance, but the phase angle difference with the corresponding primary side current is 180 °, and the instrument will reverse when the power is low.

Therefore, it is very important to correctly determine the polarity of the current transformer.