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Principles Of Operation Of Current Transformers
Aug 14, 2018

1) The connection of the current transformer shall comply with the principle of series connection [8]: that is, the primary winding resistance shall be in series with the circuit under test, and the secondary winding resistance shall be in series with all instrument loads

Current transformer

Current transformer

Series connection

2) choose the appropriate ratio according to the magnitude of the measured current, 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 breaking 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 circuit, because once open circuit, the primary side current I1 all becomes magnetization current, causing a sudden increase of_m and E2, resulting in excessive saturation magnetization of the core, serious heating and even burning of the coil; at the same time, the magnetic circuit after excessive saturation magnetization, so that the error increases. When the current transformer works normally, the secondary side is connected in series with the current coils such as measuring instruments and relays. The impedance of the current coils such as measuring instruments and relays is very small, and the secondary side is close to short circuit. The magnitude of the secondary current of CT is determined by the primary current. The magnetic potential produced by the secondary current balances the magnetic potential of the primary current. If it is suddenly opened, the excitation EMF suddenly changes from a very small value to a very large value, and the magnetic flux in the core presents a seriously saturated flat-top wave. Therefore, the secondary side winding will induce a very high peak wave when the magnetic field passes through zero, which can reach thousands or even tens of thousands of volts, endangering the safety of the staff and the insulation of the instrument. Yes.

In addition, the two side open circuit will cause the two side voltage up to several hundred volts. Once it touches, it will cause electric shock. Therefore, there are short circuit switches on the two side of the current transformer to prevent the two side open circuit. During the operation, the circuit load should be removed immediately after the two side of the circuit is opened, and then the power will be cut off. Everything is handled well before it can be reused.

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, 2-8 secondary winding current transformers are installed in generator, transformer, outlet, bus segment circuit breaker, bus circuit breaker, bypass circuit breaker and other circuits.

5) The installation location of protective current transformer should be set up according to the non-protective zone of eliminating the main protective device as far as possible. For example, if there are two sets of current transformers, and the position permits, should be located on both sides of the circuit breaker, so that the circuit breaker in the cross protection range.

6) In order to prevent bus failure caused by bushing flashover of pillar current transformer, current transformer is usually arranged on the outlet line of circuit breaker or transformer side.

7) In order to reduce the damage caused by internal faults of the generator, the current transformer used in the automatic regulating excitation device should be arranged at the outlet side of the generator stator winding. In order to facilitate the analysis and find the internal fault before the generator is merged into the system, the current transformer used for measuring instruments should be installed at the neutral side of the generator.


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570066
E-mail:
sales@great-relay.com