The principle of current transformer is based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. Its primary winding has a few turns, which are stringed in the line where the current needs to be measured. Therefore, it often has all the current flowing through the line. The secondary winding has more turns and is connected in series to the measuring instrument and the protection circuit. When the current transformer is working, its secondary side loop is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to short circuit. The current transformer is used to convert a large current on the primary side into a small current on the secondary side. The secondary side cannot be opened.
The function of the current transformer is to convert the primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.
1) The wiring of the current transformer should follow the series principle: that is, the primary winding should be connected in series with the circuit under test, and the secondary winding should be connected in series with all instrument loads
2) According to the measured current, choose 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 voltage side once the insulation is damaged, causing personal and equipment accidents
3) No open circuit is allowed on the secondary side. Once the circuit is opened, the primary current I1 will all become the magnetizing current, causing φm and E2 to increase sharply, causing the core to become over-saturated and magnetized, causing serious heating and even burning the coil; at the same time, the over-saturation and magnetization of the magnetic circuit increases 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, and the magnetic flux in the iron core presents a severely saturated flat top Therefore, the secondary winding will induce a very 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 workers and the insulation performance of the instrument.
In addition, an open circuit on the primary side causes the secondary side voltage to reach several hundred volts, which will cause an electric shock if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening. In the course of use, once the secondary side is open, the circuit load should be removed immediately, and then stop processing. It can be used after everything is handled.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., the generators, transformers, outgoing wires, bus section circuit breakers, bus circuit breakers, bypass circuit breakers and other circuits are all set 2 to 8 secondary winding current transformers. For large-current grounding systems, generally three-phase configuration; for low-current grounding systems, two-phase or three-phase configuration according to specific requirements
5) The installation location of the current transformer for protection 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 location allows, they should be installed 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 the 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 caused by the internal fault of the generator, the current transformer used for automatically adjusting 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 the measuring instrument should be installed on the neutral side of the generator.