What is the principle of use of current transformers?

- Jul 03, 2020-

1) The wiring of the current transformer should comply with the series criterion: that is, 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 external loads.

2) Select the appropriate transformation ratio according to the size of the measured current, otherwise the error will increase. Together, 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, forming a personal and equipment incident.

3) The secondary side must never be open circuit, because once the circuit is opened, the primary side current I1 becomes the magnetizing current, causing φm and E2 to suddenly increase, forming an iron core oversaturation magnetization, severe heating and even burning the coil; together, after the magnetic circuit is oversaturated magnetization, Increase the error. When the current transformer is in normal operation, the secondary side is used in series with current coils such as measuring appearance and relays. The impedance of the current coils such as measuring appearance and relays is very small, and the secondary side is similar to a short circuit. The size 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 small value to a large value, and the magnetic flux in the iron core will appear a severely saturated flat-top wave, so the secondary winding will be induced when the magnetic flux passes through zero. Very high peak wave, its value can reach thousands or even tens of thousands of volts, endangering the safety of workers and the insulation function of the appearance.

Otherwise, the secondary side open circuit makes the secondary side voltage up to several hundred volts. Once touched, it will form an electric shock end. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from opening. During operation, once the secondary side is open, the circuit load should be removed immediately, and then the power will be cut off. It can be reused after all treatment.

4) In order to meet the needs of measuring appearance, relay maintenance, circuit breaker failure identification and fault filtering, etc., it is set in the generator, transformer, outlet, bus section breaker, bus breaker, bypass breaker and other circuits. 2~8 secondary winding current transformers.

5) The installation location of the maintenance current transformer should be set to eliminate the unmaintained area of the main maintenance device as much as possible. For example: if there are two sets of current transformers, and the orientation is agreed, they should be set on both sides of the circuit breaker, so that the circuit breaker is in the scope of interleaving maintenance.

6) In order to prevent the busbar fault of the strut-type current transformer bushing from forming the busbar fault, the current transformer is usually placed on the outlet of the circuit breaker or the transformer side.

7) In order to reduce the damage in the generator internal faults, the current transformer used to actively adjust the excitation device should be placed on the outlet side of the generator stator winding. In order to facilitate analysis and find internal defects before the generator is incorporated into the system, the current transformer used to measure the appearance should be installed on the neutral side of the generator.https://www.ctsensorducer.com/