The basic function of current transformer (1)

July 13 , 2020
Transformers are generally used in the maintenance process of high-voltage lines. After applying the transformer, the ultra-high voltage can be converted into a low voltage that is not harmful to us, so that we can carry out safety measurements.

The key role of the transformer is to reduce the large current of the current loop by a certain proportion to the value that is conducive to accurate measurement of the meter and relay, and then use the meter and relay connected in series in its secondary winding circuit. The meter can be done Delicate and precise. The secondary current of a common current transformer is 5A, so the connection between the secondary side and the load can use a small-section transmission line or control cable.

Current Transformer

When a short-circuit fault occurs, the short-circuit current is usually several times larger than the normal load current, or even dozens of times. Such a large short-circuit current will cause great mechanical stress and thermal effects, which is harmful to the current transformer. Therefore, when selecting the current transformer, in addition to the capacity and insulation performance should meet the requirements, the mechanical stress and thermal effect during the short-circuit fault should also be checked so that it does not exceed the rated control value of the current transformer.

Under all operating conditions, the secondary winding terminals of the current transformer cannot be opened, otherwise dangerous high voltages will be caused, resulting in personal safety and accidents of machinery and equipment.

This is because when the secondary winding of the current transformer is open, all the current in the primary winding of the transformer is the excitation current, which greatly increases the magnetic flux density in the iron core, and then induces thousands of volts in the secondary winding. Induced electric potential precipitates the insulation layer of the secondary winding and threatens the lives of workers. In addition, because the magnetic flux density is greatly increased, the eddy current and hysteresis loss in the iron core are greatly increased, causing a lot of heat, so that the temperature of the magnetic conductor of the transformer exceeds the maximum safe range. This high temperature Winding insulation is very harmful. Third, because of the relationship between residual magnetism, the magnetic properties of magnetic conductors are getting worse.

Transformers are mainly divided into two categories, one is called voltage transformers, the other is called current transformers, and its appearance is mainly used in the power industry.

In the power supply system, large currents and high voltages sometimes cannot be directly measured accurately with ammeters and voltmeters. It must be accurately measured after being reduced proportionally through the transformer.

The internal structure of the transformer is a transformer. It operates according to the principle of transformers.

The working principle of the voltage transformer is equal to the transformer with an open circuit on the secondary side. It is used for voltage transformation, and a voltmeter is connected to the secondary side to accurately measure the voltage (a number of voltmeters can be connected in parallel). The secondary side of the voltage transformer must not be short-circuited.

The working principle of the current transformer is equal to the transformer of the short-circuit fault on the secondary side. It is used for current conversion, and an ammeter is connected on the secondary side to accurately measure the current (it can connect several ammeters in series). The secondary side of the current transformer must not be open.

The voltmeter is equal to the accurate measurement equipment for the large load (large impedance) of the voltage transformer. The ammeter is equal to the current transformer's small load (small impedance) accurate measurement equipment.

In the normal operation of the voltage transformer, the secondary load impedance will be very large, the voltage transformer is a constant voltage source, the internal impedance is not large, the capacity is not large, the primary winding transmission line is very thin, when the transformer short-circuit fault occurs twice The primary current will be very large. If the secondary fuse is not selected properly and the fuse cannot be fused, it will cause the voltage transformer to be easily burned.

When the secondary side of the current transformer is open during operation, the primary side current will not change. The secondary side current is equal to zero, and the demagnetizing flux caused by the secondary current will also disappear.

At this time, the primary current becomes the excitation current, which saturates the transformer core and the magnetic flux is also very high, which will cause the following adverse effects:

(1) Because the magnetic flux is saturated, its secondary side will cause a high voltage of thousands of volts, and the wave shape changes, causing great harm to personal safety and machinery and equipment.

(2) Because the magnetic flux of the transformer core is saturated, the loss of the transformer core becomes large, resulting in high temperature, which will destroy the insulation layer.

(3) Residual magnetism will be caused in the transformer core, which will enlarge the ratio difference and angle difference of the transformer and lose accuracy. Therefore, the secondary side of the current transformer cannot be opened.

The working principle of the transformer and the transformer is the same. Both work with the principle of the magnetic effect of current. The role of the transformer is to convert one level of voltage into another level of the same frequency voltage. It only has the completed voltage The conversion of output cannot complete the conversion of output power. The transformer is divided into a voltage transformer and a current transformer. The role of the voltage transformer is to supply voltages such as detection instruments and relays, and then accurately reflect the various operating conditions of the primary electrical system. Isolate the secondary electrical systems such as testing instruments and relays from the primary electrical system to ensure the safety of workers and secondary machinery and equipment, and convert the high voltage of the primary electrical system into a low voltage value of the same standard (100 V, 100 /1.732 volts, 100 /3 volts). The role of power transformers is basically the same as that of voltage transformers. The difference is that current transformers convert a large current in the electrical system into a standard 5 A or 1 A. Supply the relay to detect the current coil of the instrument.
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