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Explain the function and application of current transformer and voltage transformer in detail

Explain the function and application of current transformer and voltage transformer in detail


Transformer, also known as instrument transformer, is the general name of current transformer and voltage transformer. It can change high voltage into low voltage and large current into small current for measurement or protection system. Its main function is to convert high voltage or large current into standard low voltage (100V) or standard small current (5a or 1a, all refer to rated value) in proportion, so as to complete the standardization and miniaturization of measuring surface, protection equipment and automatic control equipment. All transformers can also be used to isolate high voltage system to ensure personal and equipment safety.

Current transformer

Current transformer can be used in the measurement of exchange current, the measurement of exchange degree and the protection of electric drive line. It is made of the characteristics that the current of the primary and secondary sides of the transformer is proportional. Its operating principle and equivalent circuit are also the same as those of general transformers, except that its primary winding is connected in series in the circuit under test, and the number of turns is very small; The secondary winding is connected to low impedance loads such as ammeter and relay current coil, which is approximately short circuited. Primary side current (i.e. the measured current) and secondary side current depend on the load of the measured line, but have nothing to do with the secondary side load of the current transformer. Because the secondary side is close to short circuit, the primary and secondary side voltages U1 and I0 are very small, and the excitation current I0 is also very small. When the current transformer works, the secondary side is not allowed to be open circuit. Once open circuit, the primary side current becomes the excitation current, so the magnetic flux and secondary side voltage are large Exceeding the normal value and endangering personal and equipment safety. Therefore, it is not allowed to connect the fuse in the secondary side circuit of the current transformer, and it is not allowed to remove the ammeter, relay and other equipment without bypass during operation. The wiring method of current transformer shall be confirmed according to the working requirements of the connected load. The most commonly used wiring methods are single-phase, three-phase star and incomplete star.

Substructure: current transformer is a converter used to expand the range.

When in use, the primary side of the main circuit is connected in series with the load of the circuit under test, and the secondary side is connected in series with 5A ammeter and protective relay current coil. Because the primary side uses thick wire, the number of turns is only one or several turns, while its cathode is small and the voltage is very low. After being connected in series with the circuit under test, the supplementary test current is not changed. The secondary side wire is very thin and the number of turns is many, However, the induced potential is not high (only a few volts). The coil resistance of the secondary ammeter and relay is very small. During operation, the secondary side can be regarded as a short circuit. According to the working principle of the transformer, the ratio of the primary side current of the current transformer shall be inversely proportional to the number of turns: L1 / L2 = W2 / W1 or L1 = W2 / w1l2. W2 / W1 = K2 transformation ratio

Voltage transformer can be used for voltage and power measurement in high voltage and ultra-high voltage power system. The current transformer for protection mainly cooperates with the relay device to provide signals to the relay device to block the defective circuit in case of short-circuit overload and other shortcomings of the line, so as to protect the safety of the power supply system. The operating conditions of the miniature current transformer for protection are completely different from those for measurement. The protective transformer starts effective operation only when the current is several times and dozens of times greater than the normal current.

Base substructure: it is a special transformer for expanding voltage range. The base substructure is the same as that of general transformer. Therefore, the voltage relationship between the primary side and the secondary side of the voltage transformer U1 / U2 = W1 / W2.

The measuring current transformer mainly cooperates with the measuring surface to measure current, voltage, power, etc. under the normal operation condition of the line. Primary requirements for micro current transformer for measurement:

① Reliable insulation;

② High enough measurement accuracy;

③ In case of large current of the tested line, the transformer shall be full within the appropriate range (such as 500% rated current) to protect the measuring surface.

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