Principles of using Power Current Transformer

1) The wiring of the Power 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 magnitude of the current to be measured, select a suitable 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 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, the magnetic circuit will be over-saturated and magnetized. , Make the error increase. When the Power 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 very small value to a very large value. The magnetic flux in the iron core presents 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, an open circuit on the secondary side causes the secondary side voltage to reach several hundred volts, and once touched, it will cause an electric shock accident. Therefore, the secondary side of the Power Current Transformer is equipped with a short-circuit switch to prevent the secondary side from opening. During 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 generators, transformers, outlets, bus section circuit breakers, bus circuit breakers, bypass circuit breakers and other circuits are all set 2～8 Power Current Transformers with secondary winding.

5) The installation location of the Power 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 Power Current Transformers, and the location permits, they should be installed 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 Power Current Transformer from causing busbar faults, the Power 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 current transformer used for the automatic adjustment of 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 current transformer used for the measuring instrument should be installed on the neutral side of the generator.