Correct use method of current transformer

- May 08, 2020-

1. The wiring of the current transformer should abide by the principle of series connection: the primary winding resistance should be connected in series with the circuit under test, and the secondary winding resistance should be connected in series with all instrument loads

2. According to the size of the measured current, select the appropriate change, 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 side once the insulation is damaged, causing personal and equipment accidents

3. Open circuit is absolutely not allowed on the secondary side. Once the circuit is opened, the primary side current I1 becomes magnetizing current, causing φm and E2 to increase suddenly, causing excessive saturation magnetization of the iron core, severe heating and even burning the coil; meanwhile, after the magnetic circuit is over saturated magnetized To increase the error. When the current transformer is working normally, the secondary side is similar to a short circuit. If it is suddenly opened, the excitation electromotive force will suddenly change from a small value to a large value. Wave, so the secondary winding will induce a very high peak wave when the magnetic passing zero, 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 primary side open circuit makes the secondary side voltage reach several hundred volts, which will cause an electric shock accident once touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening. During use, once the secondary side is open, the circuit load should be removed immediately, and then stop processing. After everything is processed, it can be reused.

4. In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., it is set in the generator, transformer, outlet, bus section breaker, bus circuit breaker, bypass circuit breaker 2 ~ 8 secondary winding current transformers. For high current grounding systems, generally three-phase configuration; for small current grounding systems, two-phase or three-phase configuration according to specific requirements

5. The installation location of the protection current transformer 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 current transformers, and the position permits, they should be set on both sides of the circuit breaker, so that the circuit breaker is in the cross protection range

6. In order to prevent the busbar failure caused by the flashover of the pole-type current transformer bushing, the current transformer is usually arranged on the outlet or transformer side of the circuit breaker

7. In order to reduce the damage during generator internal faults, the current transformer used for 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 measuring instruments should be installed on the neutral point side of the generator.