The Wiring Principle of Current Transformers

1. Open circuit is not allowed on the secondary side of the CT current transformer

The secondary open circuit may have serious consequences, one is that the iron core is overheated, and even the transformer is burned; the other is that due to the large number of turns of the secondary winding, a risky high voltage will be induced, endangering the safety of people and equipment.

Since the secondary winding of the current transformer cannot be opened, the windings that are not used by the current transformer need to be short-circuited. However, for a current transformer with multiple taps, the unused taps should be empty and cannot be short-circuited. For example, a current transformer has secondary taps 1S1, 1S2, and 1S3, of which 1S1 and 1S2 are 300/5A, and 1S1 and 1S3 are 600. /5A, when 300/5A is required, connect 1S1 and 1S2 for use, and should not short-circuit 1S1 and 1S3, otherwise it will affect the improved accuracy of the tap.

2. The secondary side of the high-voltage CT current transformer must be grounded at one point

Since the primary side of the high-voltage current transformer is high-voltage, when the high-voltage breakdown occurs between the primary and secondary coils due to insulation damage, the high-voltage will enter the low-voltage. Safety of persons and equipment. However, it should be noted that the secondary circuit of the current transformer is only allowed to be grounded at one point, and no more grounding is allowed, otherwise it may cause shunt and affect the use.

The secondary coil of the low voltage current transformer should not be grounded. Due to the low voltage of the low-voltage transformer, the insulation strength between the primary and secondary coils is high, and the possibility of breakdown of the primary and secondary coils is small. In addition, the ungrounded secondary coil will make the secondary circuit and the instrument insulation. The performance is improved, and the accident of the instrument being burned by the lightning strike can also be reduced.

In addition, differential maintenance is composed of differential relays (such as BCH-2, etc.), and the current transformers on both sides of differential maintenance can only be grounded at one point. Generally, the grounding point is set at the maintenance screen, and when differential maintenance uses During the maintenance and installation of the computer, the current transformers on both sides should be grounded separately.

3. The improvement level and maintenance level of the CT current transformer cannot be wrongly connected

Due to the different thicknesses of the winding iron core design for improvement and maintenance, if the connection is wrong, firstly, the improved accuracy during normal operation will be reduced, and the electric energy measurement will be inaccurate; When the short-circuit current exceeds a certain multiple of the rated current, the iron core is saturated, limiting the growth of the secondary current to maintain the meter. The core of the relay maintenance winding is not saturated, and the secondary current increases accordingly with the short-circuit current, so that the action of the relay maintenance is performed. If the connection is wrong, the relay maintenance action will be inflexible, and the meter may burn out.

When connecting the metering winding of the current transformer and the relay maintenance winding that is involved in the direction, control two points to confirm the connection. One is to look at the installation position of the current transformer, that is, to determine which side of the L1 installation of the current transformer is; the second is to look at the winding. Function or relay maintenance type, the secondary wiring of the current transformer can be determined by the above two points.

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