Principles For The Use Of Current Transformers

- Aug 15, 2018-

1. The wiring of the current transformer shall be in accordance with the series principle: that is, the one winding resistance should be in series with the circuit under test, while the two winding resistance is in series with all instrument loads.

2. According to the measured current size, select the appropriate variable ratio, otherwise the error will increase.

At the same time, two side end must be grounded to prevent insulation once damaged, one side high pressure into two low-pressure side, resulting in personal and equipment accidents.

3. Two times the side is absolutely not allowed to open the road, because once the open circuit, the primary side current I1 all become magnetized current, causing φm and E2 surge, resulting in excessive saturation magnetization of the core, serious heat and even burn the coil, at the same time, the magnetic circuit after excessive saturation magnetization, so that the error increases. When the current transformer is working normally, the two sides are used in series with current coils such as measuring instruments and relays, and the impedance of current coils such as measuring instruments and relays is very small, and the two sides are similar to short circuits. The size of CT two current is determined by a single current, and the magnetic potential generated by two currents is a balance of the magnetic potential of a single current.

If it is suddenly opened, the excitation EMF from a very small value of a very large value, the flux in the iron core presents a severely saturated flat top wave, so the two side winding will be in the magnetic through zero to sense a high peak wave, its value can reach thousands of or even tens of thousands of volts, endangering the safety of staff and the insulation performance of the instrument. In addition, two side open road to make two side voltage up to hundreds of volts, once touched will cause electric shock accident. Therefore, the current transformer on the two side are equipped with short-circuit switch, to prevent two side open circuit. In the use of the process, two times once the opening should be immediately removed from the circuit load, and then, and then power outage treatment.Everything is handled before it can be used again.

4. In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, in the generator, transformer, outlet, busbar segmented circuit breaker, Busbar Circuit breaker, bypass circuit breaker and other circuits are equipped with 2~8 two times around the current transformer.

5. The installation location of the protection current transformer shall be set according to the non-protected area of the main protection device as far as possible.For example, if there are two sets of current transformers, and the location allows, 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 busbar failure caused by the socket flashover of the pillar current transformer, the current transformer is usually arranged on the outlet or transformer side of the circuit breaker.

7. In order to reduce the damage in the internal fault of the generator, the 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 to detect internal faults before the generator is incorporated into the system, the current transformer used to measure the instrument should be installed on the neutral point side of the generator.