Current transformer (CT) is an important electrical equipment in the power system. It is responsible for the isolation between high and low voltage systems and the conversion of high voltage to low voltage. Whether the wiring is correct or not is of great significance to the normal operation of the protection, measurement, metering and other equipment of the system. When new CT is installed and CT secondary cable is put into operation or replaced, measuring the correctness of CT polarity is already an essential work procedure for relay protection workers. Next, HV Hipot will introduce the CT polarity measurement in detail:
HV Hipot GDHG-201A transformer comprehensive CT/ PT characteristic tester
1. What is the polarity of CT?
The polarity is the electromotive force induced by the primary coil and the secondary coil under the action of the same magnetic flux of the iron core. The two ends that reach a high potential at the same time or the end that is a low potential at the same time are called the same polarity end.
The so-called current transformer (CT) polarity refers to the relationship between the current direction between its primary winding and secondary winding. According to regulations, the first end of the CT primary winding is marked as P1, and the tail end is marked as P2; the head end of the secondary winding is marked as S1, and the tail end is marked as S2. In wiring, P1 and S1, P2 and S2 are called the same polarity end. Assuming that the current I1 of the primary winding flows in from the head end P1 and flows out from the tail end P2, the induced current I2 in the secondary winding flows out from the head end S1 and flows in from the tail end S2. At this time, the magnetic flux generated in the iron core In the same direction, such a CT polarity sign is called depolarization. On the contrary, it is called adding polarity. Commonly used current transformers, unless otherwise specified, use depolarization.
2. Why measure the polarity of CT?
The polarity of the current transformer must be tested before and after handover and overhaul. In addition, when the differential protection in operation, the power direction protection malfunctions or the watt-hour meter is reversed, the polarity of the CT must also be checked. It is because if the polarity of the current transformer is wrongly connected during wiring, the following hazards will occur:
(1) If the current transformer is used in the relay protection circuit, it will cause the relay protection device to malfunction or refuse to operate, and at the same time, it will affect the operation monitoring and accident handling of the power system, and even endanger the equipment and personal safety in severe cases.
(2) If the current transformer is used in the instrument measurement circuit, it will make the indication of various instruments and meters and the measurement of electric energy incorrect.
(3) If the current transformer with incomplete star connection is used, if the polarity of any phase is reversed, the current of one phase (usually the middle phase) of the unconnected current transformer will be several times higher than that of other phases.
(4) If the current transformer with incomplete star connection is used, if the two phases are reversed, although the three-phase current on the secondary side can still maintain balance, the phase angle difference with the corresponding primary side current is 180°, so that will reverse the meter.
Therefore, it is a very important task to correctly judge whether the polarity of the current transformer is correct or not.
Post time: Dec-06-2022