Introduction to the Principles of Use of Open Current Transformers

Ⅰ. The wiring of the open current transformer should comply with the principle of series connection

That is, the primary winding should be connected in series with the circuit under test, while the secondary winding should be connected in series with all instrument loads. Select the appropriate transformation ratio according to the measured current, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the high voltage of the primary side from entering the low voltage side of the secondary side once the insulation is damaged, causing an accident.

The secondary side of the open current transformer is excellent and not allowed to open circuit, because once the circuit is opened, the primary side current I1 will all become the magnetizing current, causing φm and E2 to increase sharply, resulting in excessive saturation magnetization of the iron core, serious heat generation and even burning of the coil; at the same time, the magnetic circuit is excessively saturated after magnetization, the error increases.

When the current transformer is working 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 secondary current of the open current transformer is determined by the primary current, and the magnetic potential generated by the secondary current balances the magnetic potential of the primary current.

If the circuit 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. Therefore, the secondary winding will induce a very high peak wave when the magnetic passes through zero, and its value can reach thousands or even tens of thousands of volts, which endangers the safety of the staff and the insulation performance of the instrument.

In addition, the open circuit of the secondary side makes the voltage of the secondary side reach several hundreds of volts, which will cause an electric shock accident if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from being open.

During the use of the current transformer, once the secondary side is open, the circuit load should be removed immediately, and then the power failure should be processed. It can be reused after everything is disposed of.

Ⅱ. Troubleshooting of open current transformers

1. In order to prevent the busbar fault caused by the bushing flashover of the pillar-type current transformer, the current transformer is usually arranged on the outgoing line or the transformer side of the circuit breaker.

2. In order to reduce the damage caused by the internal fault of the generator, the current transformer used for automatic adjustment of the excitation device should be arranged on the outgoing side of the stator winding of the generator.

3. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the current transformer used for measuring instruments should be installed on the neutral point side of the generator.

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