Principle of using miniature current transformer

- Aug 17, 2020-

Open circuit operation is strictly prohibited

Open circuit is not allowed on the secondary side. Once the circuit is opened, the primary current I1 becomes magnetizing current, causing φm and E2 to increase suddenly, causing excessive saturation magnetization of the iron core, severe heating and even burning the coil; meanwhile, after the magnetic circuit is oversaturated magnetized, the error Increase. When the current transformer is working normally, the secondary side is used in series with 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 similar to a short circuit. The magnitude of the CT secondary current is determined by the primary current. 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 very small value to a large value, and the magnetic flux in the iron core will show a severely saturated flat-top wave, so the secondary winding will be induced when the magnetism 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 performance of the instrument.

In addition, the secondary side open circuit makes the secondary side voltage up to several hundred volts, which will cause an electric shock accident if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from opening. During use, once the secondary side is open, the circuit load should be removed immediately, and then the power will be cut off. After everything is processed, it can be reused.


The wiring of the current transformer should observe the principle of series connection: 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 instrument loads.

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 primary side high voltage from entering the secondary low side once the insulation is damaged, causing personal and equipment accidents.

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

The installation location of the protection 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 sets of current transformers, and the position permits, they should be set on both sides of the circuit breaker, so that the circuit breaker is in the cross protection range.

In order to prevent the busbar fault caused by the flashover of the pole-type current transformer bushing, the current transformer is usually arranged on the outlet or transformer side of the circuit breaker.

In order to reduce the damage during generator internal faults, the current transformer used for automatic adjustment of 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 measuring instruments should be installed on the neutral point side of the generator.