How to Use Current Transformers?

A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current that needs to be measured. This article will introduce the principles and precautions that should be paid attention to in the actual application of current transformers.

• Principles of Using Current Transformers:

1. The principle of series connection
   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, and the secondary winding should be connected in series with all instrument loads.
2. Choose a suitable transformation ratio
   According to 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 high voltage on the primary side from entering the low voltage side of the secondary when the insulation is damaged, causing personal and equipment accidents.
3. No open circuit on the secondary side
   The secondary side is absolutely not allowed to open circuit, because once the circuit is opened, the primary side current I1 will all become the magnetizing current, causing φm and E2 to increase rapidly, causing the iron core to be over-saturated and magnetized, causing serious heating and even burning the coil; at the same time, after the magnetic circuit is over-saturated and magnetized, it will cause The error increases. When the current transformer is working normally, the secondary side is used in series with current coils such as measuring instruments and relays. In addition, an open circuit on the secondary side causes the secondary side voltage to reach hundreds of volts, which will cause an electric shock if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from opening. 
4. Set up 2～8 secondary winding current transformers
   In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., 2~ are set in the circuits of generators, transformers, outlets, bus section circuit breakers, bus circuit breakers, bypass circuit breakers, etc. 8 current transformers with secondary winding.
5. Installation location
   The installation location of the current transformer for protection should be set according to the protection area that eliminates the main protection 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
6. protection range
   Outgoing wire or transformer side of the circuit breaker
   In order to prevent the bushing flashover of the pillar-type current transformer from causing a bus failure, the current transformer is usually arranged on the outgoing line or the transformer side of the circuit breaker.
7. Outgoing side of generator stator winding
   In order to reduce the damage caused by the internal fault of the generator, the current transformer used for the 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 the measuring instrument should be installed on the neutral side of the generator.

• Matters Needing Attention:

The secondary side is not allowed to open circuit when the current transformer is running. Once the circuit is opened, the primary side current becomes the excitation current, so that the magnetic flux and the secondary side voltage greatly exceed the normal value and endanger the safety of people and equipment. Therefore, it is not allowed to connect a fuse in the secondary circuit of the current transformer, and it is also not allowed to remove the ammeter, relay and other equipment without bypassing during operation.