Principles of use of split core current transformers

- Nov 13, 2019-

1) The wiring of the split core current transformers shall comply with the series principle: that is, the primary winding should be connected in series with the circuit under test, and the secondary winding should be connected in series with all instrument loads.

2) According to the magnitude of the current to be measured, select the appropriate ratio, otherwise the error will increase. At the same time, the secondary side must be grounded to prevent the insulation from being damaged when the primary side is immersed in the secondary low-voltage side, causing body and equipment accidents.

3) The secondary side is not allowed to open the circuit. Because once the circuit is open, the primary current I1 becomes the magnetizing current, causing the φm and E2 to increase suddenly, causing the core to be oversaturated and magnetized, causing severe heat or even burning the coil. At the same time, the magnetic circuit is excessively saturated and magnetized. To increase the error. When the current transformer is in normal operation, the secondary side is used in series with a current coil such as a measuring instrument and a relay. The current coil of the measuring instrument and the relay has a small impedance, and the secondary side approximates a short circuit. The magnitude of the CT secondary current is determined by the primary current, and the magnetic potential generated by the secondary current is the magnetic potential that balances the primary current. If it is suddenly opened, the field electromotive force suddenly changes to a large value from a small value, and the magnetic flux in the core exhibits a severely saturated flat-top wave, so the secondary winding will be induced when the magnetic flux passes through zero. Very high apex waves, which can reach thousands or even tens of volts, endangering the safety of workers and the insulation of the instrument.

In addition, the secondary side open circuit causes the secondary side voltage to reach several hundred volts, which may cause an electric shock accident if touched. Therefore, the secondary side of the current transformer is provided with a short-circuit switch to prevent the secondary side from opening. In the process of use, once the secondary side opens, the circuit load should be removed immediately, and then the power is cut off. It can be reused after everything is processed.

4) In order to meet the needs of measuring instruments, relay protection, breaker failure judgment and fault filtering, etc., in generators, transformers, outlets, busbar section breakers, busbar breakers, bypass breakers, etc. 2 to 8 secondary winding current transformers.

5) The installation location of the split core current transformers for protection shall be set as far as possible to eliminate the unprotected area of the main protection device. For example, if there are two sets of current transformers and the position is allowed, they should be placed on both sides of the circuit breaker to make the circuit breaker in the cross protection range.

6) In order to prevent the busbar fault caused by the strut-type current transformer casing flashover, the current transformer is usually arranged on the outlet or transformer side of the circuit breaker.

7) In order to mitigate the damage during the internal fault of the generator, the current transformer for automatically adjusting the excitation device should be arranged on the outgoing side of the stator winding of the generator. In order to facilitate analysis and to find internal faults before the generator is incorporated into the system, the current transformer for the measuring instrument should be mounted on the neutral point of the generator.