Wiring Principle of Through-core Current Transformer

Ⅰ. What is a through-core current transformer?

Feed-through current transformer is a common electrical device. Because of its simple wiring and convenient installation, it is widely used in metering, testing and protection circuits. However, if you are not careful in use, it can cause great errors. Inaccurate measurement, failure of protection, and even electrical accidents are related to the ampere-turn capacity of the current transformer.

Ⅱ. Principles of wiring of through-core current transformers

1. The wiring of the through-core current transformer should follow the series connection principle: the primary winding is connected in series with the circuit under test, and the secondary winding is connected in series with all instrument loads.

2. Select the appropriate change 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 jumping into the secondary low voltage side after the insulation is damaged, causing personal and equipment accidents.

3. The second side must not be opened. Once it is open, all the primary current I1 will become the magnetizing current, which will cause a sudden increase in φm and E2, causing the core to exceed saturation magnetization, seriously even heating and burning the coil. At the same time, when the magnetic circuit is over-saturated magnetization, the error will increase.

When the current transformer is working normally, the secondary side is close to a short circuit. If the circuit is suddenly opened, the excitation electromotive force will suddenly increase from small to large, and the magnetic flux in the iron core presents a flat-topped wave with severe saturation.

Therefore, when the magnetic flux passes through the zero point, the secondary winding will generate a high peak wave, the value of which can reach thousands or even tens of thousands of volts, which endangers the safety of workers and the insulation performance of the instrument.

In addition, the open circuit on the primary side makes the voltage on the secondary side up to several hundreds of volts, and once in contact, it will cause an electric shock accident. Therefore, the secondary side of the current transformer is provided with a short circuit switch to prevent the primary side from opening.

During the use of the feedthrough current transformer, once the secondary side is opened, the circuit load should be removed immediately, and then the processing should be stopped. It can only be reused after everything has been processed.

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