The use of current Transformer | GETWELL

From the wire passing through the high current, the small current is induced according to a certain proportion for measurement, and it can also provide power for relay protection and automatic devices.

Give examples to illustrate

For example, there is a very thick cable, and its current is very large. If you want to measure its current, you need to disconnect the cable and connect the ammeter in series in this circuit. Because it is very thick and the current is very large, a large ammeter is needed. But in fact, it is not such a large ammeter, because the specifications of current meters are less than 5A. What are we going to do? At this time, we need the help of current transformer.

First select the appropriate current transformer, and then pass the cable through the current transformer. At this time, the current transformer will induce the current from the cable, and the induced current is just reduced by a certain multiple. The induced current is sent to the instrument for measurement, and then the measured results are multiplied by a certain multiple, and the real results can be obtained.

Some friends may say that the clamp meter can also achieve this purpose, in fact, there is a current transformer inside the clamp meter, the principle is similar.

Mode of use

1) the wiring of the current transformer should follow the principle of series connection: 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) choose the appropriate change according to the measured current, 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 escaping into the secondary low voltage side once the insulation is damaged, resulting in personal and equipment accidents.

3) the open circuit on the secondary side is absolutely not allowed, because once the circuit is opened, the current I1 on the primary side becomes the magnetization current, causing a sudden increase in φ m and E2, resulting in excessive saturation magnetization of the core, serious heating and even burning of the coil; at the same time, after the excessive saturation magnetization of the magnetic circuit, the error increases. When the current transformer is working normally, the secondary side of the current transformer is similar to a short circuit. if it is suddenly opened, the excitation EMF will suddenly change from a very small value to a very large value, and the magnetic flux in the iron core presents a seriously saturated flat-top wave. therefore, the secondary side winding will induce a very high peak wave at 00:00, whose value can reach thousands or even tens of thousands of volts, which is at risk to the safety of workers and the insulation performance of the instrument. In addition, the secondary side open circuit makes E2 up to several hundred volts, once touched, it causes an electric shock accident. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening the circuit. Such as K0 in figure l, in the process of use, once the secondary side opens the circuit, the circuit load should be removed immediately, and then stop processing. Don't use it until everything is taken care of.

4) in order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault recording, current transformers with 2 to 8 secondary winding are installed in generator, transformer, outlet, bus section circuit breaker, bus circuit breaker, bypass circuit breaker and other circuits. For large current grounding systems, it is generally configured according to three-phase configuration; for low current grounding systems, according to specific requirements, according to two-phase or three-phase configuration.

5) the installation location of the current transformer for protection shall be set according to the non-protection zone of the main protection device as far as possible. For example, if there are two groups of current transformers and the position permits, they should be located on both sides of the circuit breaker so that the circuit breaker is in the cross protection range.

6) in order to prevent busbar fault caused by bushing flashover of pillar current transformer, the current transformer is usually arranged on the outlet line or transformer side of the circuit breaker.

7) in order to reduce the damage caused by the internal fault 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 the analysis and find the internal fault 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.

The above is the introduction of the use of the current transformer, if you want to know more about the inductor, please feel free to contact us.