Principles of Using Electrical Current Transformer

1) The wiring of the Electrical Current Transformer should follow 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 measured current, select the appropriate transformation ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the primary side high voltage from entering the secondary low voltage side once the insulation is damaged, causing personal and equipment accidents.

3) No open circuit on the secondary side is allowed. Once the circuit is opened, the primary side current I1 will all become magnetizing current, causing φm and E2 to increase sharply, resulting in excessive saturation magnetization of the iron core, severe heating and even burning of the coil; at the same time, after the magnetic circuit is over-saturated and magnetized , Make the error increase. When the Electrical Current Transformer works normally, the secondary side is used in series with current coils such as measuring instruments and relays. The impedance of the current coils such as measuring instruments and relays is very small, and the secondary side is similar to 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 balances the magnetic potential of the primary current. If it is suddenly opened, the excitation electromotive force will suddenly change from a small value to a large value, and the magnetic flux in the iron core will present a severely saturated flat-top wave, so the secondary winding will induce when the magnetic passes through zero. The very high peak wave, its value can reach thousands or even tens of thousands of volts, endangering the safety of workers and the insulation performance of the instrument.

In addition, the open circuit on the secondary side makes the voltage on the secondary side reach hundreds of volts, and once touched, it will cause an electric shock accident. Therefore, the secondary side of the Electrical Current Transformer is equipped with a short-circuit switch to prevent the secondary side from opening. In the course of use, once the secondary side is open, the circuit load should be removed immediately, and then the power failure should be dealt with. It can be reused after everything is handled.

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., the generator, transformer, outgoing line, bus section circuit breaker, bus circuit breaker, bypass circuit breaker and other circuits are all set Electrical Current Transformer with 2 to 8 secondary windings.

5) The installation location of the electrical current transformer for protection should be set as far as possible to eliminate the unprotected area of the main protection device. For example: if there are two sets of Electrical Current Transformers, and the location permits, they should be located on both sides of the circuit breaker so that the circuit breaker is in the cross protection range.

6) In order to prevent the bushing flashover of the pillar-type Electrical Current Transformer from causing bus failure, the Electrical Current Transformer is usually arranged on the outlet or transformer side of the circuit breaker.

7) In order to reduce the damage caused by the internal fault of the generator, the Electrical Current Transformer used to automatically adjust the excitation device should be arranged on the outlet side of the generator stator winding. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the Electrical Current Transformer used for measuring instruments should be installed on the neutral side of the generator.