Principles of use of current transformers

1) The wiring of the current transformer should comply with the principle of series connection: that is, the primary winding should be in series with the circuit under test, while the secondary winding should be connected in series with all instrument loads

2) According to the size of the measured current, select the appropriate transformation ratio, 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 penetrating into the secondary low voltage side once the insulation is damaged, causing personal and equipment accidents

3) The secondary side is absolutely not allowed to open the circuit, because once the circuit is opened, the current I1 on the primary side all becomes the magnetization current, causing a sudden increase in φm and E2, resulting in oversaturation magnetization of the core, serious heating and even burning the coil; At the same time, the magnetic circuit is oversaturated and magnetized, which increases the error. When the current transformer is working normally, the secondary side is used in series with the current coil such as the measuring instrument and relay, and the impedance of the current coil such as the measuring instrument and relay is very small, and the secondary side is approximated as a short circuit. The magnitude of the CT secondary current is determined by the primary current, and the magnetic potential generated by the secondary current is balanced by the magnetic potential of the primary current. If it is suddenly opened, the excitation electromotive force changes from a very small value to a large value, and the magnetic flux in the iron core presents a severely saturated flat top wave, so the secondary side winding will induce a high peak wave when the magnetic passes through zero, and its value can reach thousands or even tens of thousands of volts, endangering the safety of staff and the insulation performance of the instrument.
In addition, the secondary side open circuit causes the secondary side voltage to reach several hundred volts, and once touched, it will cause electric shock accidents. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from being open. During use, once the secondary side is open, the circuit load should be removed immediately, and then the power should be cut off. Everything can be used again after everything has been processed.

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, 2~8 secondary winding current transformers are set in generators, transformers, outlets, bus section circuit breakers, bus circuit breakers, bypass circuit breakers and other circuits.

5) For the installation site of the current transformer for protection, it should be set according to the non-protection zone that eliminates the main protection device as much as possible. For example, if there are two sets of current transformers, and the position allows, they 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 the busbar fault caused by flashover of the pillar 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 in case of internal failure of the generator, the current transformer used to automatically adjust the excitation device should be arranged on the outlet side of the stator winding of the generator. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the current transformer used for the measuring instrument should be installed on the neutral side of the generator.