Use of current transformers

1) The wiring of the current transformer should comply with the series connection principle: that is, the primary winding should be connected in series with the measured circuit, and the secondary winding should be connected in series with all instrument loads

2) According to the size of the measured current, select the appropriate change, 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 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 not allowed to be open. Once the circuit is open, the primary side current I1 will all become magnetizing current, causing a sudden increase in φm and E2, causing the core to be over-saturated and magnetized, causing severe heat and even burning the coil; at the same time, the magnetic circuit is over-saturated and magnetized, which 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 very small value to a very large value, and the magnetic flux in the core will present a severely saturated flat-top wave. Therefore, the secondary winding will induce a very high peak wave when the magnetic field passes through zero, and its value can reach thousands or even tens of thousands of volts, endangering the safety of the staff and the insulation performance of the instrument.

In addition, the primary side open circuit makes the secondary side voltage reach several hundred volts, which will cause electric shock accidents once touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening. During use, once the secondary side opens, the circuit load should be removed immediately, and then the vehicle should be stopped for processing. It can be used again after everything is handled.

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, current transformers with 2 to 8 secondary windings are installed in the generator, transformer, outgoing line, busbar section circuit breaker, busbar circuit breaker, bypass circuit breaker and other circuits. For large current grounding systems, it is generally configured in three phases; for small current grounding systems, it is configured in two or three phases according to specific requirements

5) The installation location of the protective current transformer should be set to eliminate the non-protected area of ​​the main protection device as much as possible. 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 within the cross protection range

6) In order to prevent the flashover of the pillar current transformer bushing from causing busbar failure, the current transformer is usually arranged on the outgoing line or transformer side of the circuit breaker

7) In order to reduce the damage caused by internal faults of the generator, the current transformer used for automatic adjustment of the excitation device should be arranged on the outgoing line side of the generator stator winding. In order to facilitate analysis and detect internal faults before the generator is connected to the system, the current transformer used for measuring instruments should be installed on the neutral point side of the generator.