What is the role of current transformer

- Sep 14, 2020-

The current transformer is based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. The primary winding has very few turns and is stringed in the line that needs to be measured. Therefore, it often has all the current of the line flowing. The secondary winding has a large number of turns. It is connected in series in the measurement appearance and maintenance loop. The current transformer is in When it is working, its secondary loop is always closed, the impedance of the coil in series with the measuring appearance and the maintenance loop is very small, and the working state of the current transformer is close to short circuit.

The function of the current transformer is to convert the primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio for maintenance and measurement purposes.

1. The wiring of the current transformer should follow the series rules: 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 external load current transformers.  

2. Select the appropriate change according to the size of the measured current, otherwise the error will increase. At the same time, one end of the secondary side needs to be grounded to prevent insulation. Once damaged, the primary side high pressure rushes into the secondary low pressure side, causing a personal and equipment incident.  

3. The secondary side does not allow an open circuit. If once the circuit is opened, the primary current I1 will all become the magnetizing current, causing φm and E2 to increase sharply, resulting in excessively full magnetization of the iron core, severe heating and even burning of the coil; at the same time, after the magnetic circuit is over-full magnetized, Increase 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 small value to a large value, and the magnetic flux in the iron core presents a severe and full flat top. Therefore, the secondary winding will induce a very high peak wave when the magnetism passes through zero, and its value can reach thousands or even tens of thousands of volts, which threatens the safety of workers and the insulation performance of the exterior.

In addition, an open circuit on the primary side causes the secondary side voltage to reach several hundred volts. Once touched, an electric shock will occur. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to avoid an open circuit on the primary side. In the process of use, once the secondary side is open, the circuit load should be removed immediately, and everything can be used after handling.

4. The installation location of the maintenance current transformer should be set up as far as possible to eliminate the non-maintenance area of the main maintenance equipment. 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 maintenance range.

5. In order to avoid the bushing flashover of the pillar-type current transformer, the current transformer is usually installed on the outlet or transformer side of the circuit breaker.

6. In order to meet the needs of equipment such as appearance measurement, relay maintenance, circuit breaker failure judgment and fault filtering, etc., circuit breakers such as generators, transformers, outgoing wires, bus section circuit breakers, bus circuit breakers, and bypass circuit breakers are all set 2 to 8 secondary winding current transformers. For large-current grounding systems, three-phase equipment is generally used. For small current grounding systems, two-phase or three-phase equipment is required according to specific requirements.

7. In order to reduce the damage caused by the internal fault of the generator, the current transformer used for the automatic conditioning of the excitation equipment should be placed on the outlet side of the generator stator winding. In order to facilitate the analysis and find internal faults before the generator is incorporated into the system, a current transformer used to measure the appearance is installed on the neutral side of the generator.https://www.ctsensorducer.com/