The working principle of voltage transformer and current transformer

The transformer is mainly divided into two types in the power supply and distribution system: voltage transformer and current transformer. In power supply and distribution systems, large currents and high voltages cannot sometimes be measured directly with ammeters and voltmeters, and must be scaled down by transformers to measure. The internal structure of the transformer is the transformer. Operate according to the principle of the transformer. The working principle of the voltage transformer is equivalent to the transformer with 2 side open circuit, used for voltage transformation, and the voltage is measured by connecting the voltmeter on the secondary side (multiple voltmeters can be connected in parallel). The secondary side of the voltage transformer cannot be short-circuited. The working principle of the current transformer is equivalent to the transformer with short-circuit on the 2nd side, which is used to change the current, and the current meter is connected to the secondary side to measure the current (multiple ammeters can be connected in series). The secondary side of the current transformer cannot be opened. The voltmeter is equivalent to a voltage transformer with a large load (high impedance) measuring device. The ammeter is equivalent to a current transformer small load (small impedance) measuring device.

The voltage transformer is in normal operation, the secondary load impedance is very large, the voltage transformer is a constant voltage source, the internal impedance is small, the capacity is small, the primary winding wire is very thin, when the transformer occurs twice When the short circuit occurs, the primary current is very large. If the secondary fuse is improperly selected and the fuse cannot be blown, the voltage transformer is easily burned out.

When the secondary side of the current transformer is open during operation, the primary side current remains unchanged, and the secondary side current is equal to zero, the demagnetizing flux generated by the secondary current also disappears. At this time, all the primary currents become the excitation current, the transformer core is saturated, and the magnetic flux is also high, which will have the following consequences: (1) Due to the saturation of the magnetic flux, the secondary side will generate several thousand volts of high voltage, and the waveform Change, causing harm to people and equipment. (2) Since the core flux is saturated, the core loss is increased, and high heat is generated, which may damage the insulation. (3) Remanence will be generated in the iron core, the transformer difference and angular difference will increase, and the accuracy will be lost. Therefore, the secondary side of the current transformer is not allowed to open.

The transformer and the transformer work in the same way, and they all work by the principle of electromagnetic induction. The function of the transformer is to convert one level of voltage into another level of the same frequency. The realization of voltage conversion, can not achieve power conversion. The transformer is divided into voltage transformer and current transformer. The role of voltage transformer is to supply measuring instruments, relays and other voltages, so as to correctly reflect the various operating conditions of an electrical system. Separate the secondary electrical system such as measuring instruments and relays from the primary electrical system to ensure the safety of personnel and secondary equipment, and convert the high voltage of the primary electrical system into a low voltage value (100 volts, 100/1.732 volts, which agrees to the standard). 100/3 volts. The role of the power transformer is basically the same as that of the voltage transformer. The difference is that the current transformer converts the large current of the primary electrical system into a standard 5 amp or 1 amp supply to continue the appliance. Current coil.

What is the role of current transformer?

The current transformer is a very important primary device in the power system. Its principle is based on the principle of electromagnetic induction. Its primary coil has few turns, usually a single turn coil, that is, a copper. Rod or a copper row. The secondary coil is mainly connected to the measuring instrument or the coil of the relay. The secondary side of the current transformer cannot be operated in an open circuit. When the secondary side is open, the current on the primary side is mainly used for excitation, which will be in the second The secondary side induces a high voltage, which jeopardizes the safety of the secondary equipment and the human body, and also causes the current transformer to burn out. Its main functions are: 1. Converting a large primary current into a standard 5 amp; The measuring device and the relay-protected coil provide current; 3. The primary device and the secondary device are isolated.

What is the difference between the voltage transformer and the current transformer?

The main difference is that the working state is very different during normal operation. The performance is as follows: 1) The current transformer can be used twice. Short circuit, but not open circuit; voltage transformer can open circuit twice, but not short circuit; 2) Relative to the load on the secondary side, the internal impedance of the voltage transformer is small or negligible, it can be considered that the voltage transformer is a The voltage source; while the current transformer has a large internal resistance, so that it can be considered as a current source with an infinite internal resistance. 3) When the voltage transformer works normally, the magnetic flux density is close to the saturation value, and the magnetic flux density decreases when the fault occurs; the magnetic flux density is very low when the current transformer works normally, and the magnetic current becomes very large due to the primary side short-circuit current during the short circuit. The pass density is greatly increased, sometimes even far exceeding the saturation value.