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Current transformer use matters needing attention
Apr 07, 2018

Principle of use

1) The wiring of the current transformer shall comply with the principle of series connection [8]: that is, one winding resistance should be connected in series with the circuit under test, and the second winding resistance should be connected in series with all the load of the meter.

2) According to the measured current size, select the appropriate ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the primary side high pressure from entering the secondary low pressure side once the insulation is damaged, causing personal and equipment accidents.

3) The open circuit is absolutely not allowed on the secondary side. Once once opened, the primary current I1 becomes a magnetizing current, causing a sudden increase in φm and E2, causing the core to become oversaturated and magnetized, causing serious heat generation and even burning the coil; at the same time, the magnetic circuit is excessively saturated and magnetized. , increase the error. When the current transformer is in normal operation, the secondary side is used in series with current coils such as measuring instruments and relays. The resistance of current coils such as measuring instruments and relays is small, and the secondary side is similar to a short circuit. The secondary current of the CT is determined by the primary current. The magnetic potential generated by the secondary current balances the magnetic potential of the primary current. If it suddenly makes it open, the excitation electromotive force will suddenly change from a very small value to a very large value. The magnetic flux in the iron core will show a severely saturated flat top wave, so the secondary winding will induce the magnetic flux to pass through zero. The high spire wave, whose value can reach thousands or even tens of thousands of volts, endangers the safety of the staff and the insulation performance of the instrument.

In addition, the secondary side of the open circuit makes the secondary voltage reach several hundred volts, once touched will cause electric shock accidents. Therefore, the short circuit switch is provided on the secondary side of the current transformer to prevent the secondary side from opening. In use, once the secondary circuit is opened, the circuit load should be immediately removed, and then the power should be cut off. It can be reused after everything is handled.

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

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

6) In order to prevent flashover of pillar-type current transformer bushings from causing bus faults, current transformers are usually placed on the outgoing or transformer side of the circuit breaker.

7) In order to reduce the damage in the internal fault of the generator, the current transformer for automatically adjusting the excitation device should be arranged on the outgoing 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 transformers used for the measuring instruments should be mounted on the neutral side of the generator.