​Wiring principle of current transformer

1) Open circuit is not allowed at the secondary side of current transformer. The secondary open circuit may have serious consequences. First, the iron core is overheated, and even the transformer is burned; Second, due to the large number of turns of the secondary winding, dangerous high voltage will be induced, endangering the safety of people and equipment.

(2) The secondary side of high-voltage current transformer must have a point of grounding. Since the primary side of the high voltage current transformer is high voltage, when the high voltage of the outgoing line between the primary and secondary coils is broken down due to insulation damage, the high voltage will enter the low voltage. If the secondary coil is grounded at one point, the high voltage will be introduced to the earth, which can ensure the safety of personnel and equipment. However, it should be noted that only one point of grounding is allowed for the secondary circuit of the current transformer, and no grounding is allowed, otherwise it may cause shunt and affect the use.

The secondary coil of low-voltage current transformer should not be grounded. Due to the low voltage of the low-voltage transformer, the insulation desire between the primary and secondary coils is large, and the possibility of primary and secondary coil breakdown is small. In addition, the non grounding of the secondary coil will make the secondary return

The improvement of insulation capacity of circuits and instruments can also reduce the accidents of instrument burnout caused by lightning. In addition, the differential protection is composed of differential relays (such as BCH-2, etc.), and the current transformers on both sides of the differential protection can only be grounded at one point. Generally, the grounding point is set at the protection screen. When the differential protection adopts microcomputer protection device, the current transformers on both sides should be grounded respectively.

(3) The measurement level and protection level of current transformer shall not be connected incorrectly. Due to the different thickness of the iron core design of the measurement and protection windings, if the connection is wrong, one is to reduce the accuracy of the measurement in normal operation and make the electric energy measurement inaccurate; Second, in case of short-circuit fault, the iron core of metering winding is designed to ensure that the iron core is saturated when the short-circuit current exceeds a certain multiple of the rated current, which limits the growth of secondary current to protect the instrument. The iron core of relay protection winding is unsaturated, and the secondary current increases correspondingly with the short-circuit current, so as to make the relay protection act accurately. If the connection is wrong, the relay protection action is not sensitive, and the measuring instrument may be burnt out.

(4) Since the secondary winding of current transformer cannot be open circuit, the unused winding of current transformer needs to be short circuited. However, for current transformers with multiple taps, the unused taps should be empty and cannot be short circuited. For example, a current transformer has 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, 1S1 and 1S2 should be connected for use, and 1S1 and 1S3 should not be short circuited, otherwise the measurement accuracy of using taps will be affected.

(5) When wiring the metering winding of the current transformer and the relay protection winding involving the direction, master two points to determine the wiring. One is to see the installation position of the current transformer, that is, to determine which side the L1 of the current transformer is installed on; Second, the secondary wiring of current transformer can be determined according to the winding function or relay protection type.