Principles of use of split current transformers

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, while the secondary winding should be connected in series with all instrument loads

2) According to the magnitude 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 on the primary side from entering the low voltage side of the secondary side once the insulation is damaged, causing personal and equipment accidents

3) The secondary side is absolutely not allowed to open circuit, because once the circuit is open, the current I1 on the primary side will become the magnetizing current, causing a sudden increase in φm and E2, resulting in excessive saturation and magnetization of the iron core, severe heat generation and even burning of the coil; at the same time, after the magnetic circuit is excessively saturated and magnetized , increasing the error. When the current transformer is working normally, the secondary side is used 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 similar to 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 balances the magnetic potential of the primary current. If it is suddenly opened, the excitation electromotive force suddenly changes from a 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 Very high peak waves, whose value can reach thousands or even tens of thousands of volts, endanger the safety of workers and the insulation performance of instruments.

In addition, the open circuit of the secondary side causes the voltage of the secondary side to reach hundreds of 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 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. It can be reused after everything is processed.

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., set 2 to 8 current transformers with secondary windings.

5) The installation location of the protective current transformer should be set to eliminate the unprotected area of the main protective device as much as possible. For example: if there are two sets of current transformers, and the location permits, 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 reduce the damage caused by the internal failure of the generator, the current transformer used to automatically adjust 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 integrated into the system, the current transformer used for measuring instruments should be installed on the neutral point side of the generator.