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Principle and structure design of current transformer and voltage transformer

Principle and structure design of current transformer and voltage transformer


Transformer, also known as instrument transformer, is a general term 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 function is mainly to convert high voltage or large current into standard low voltage (100V) or standard small current (5a or 1a, both refer to rated value) in proportion, so as to realize the standardization and miniaturization of measuring instruments, protection equipment and automatic control equipment. At the same time, the transformer can also be used to isolate high voltage system to ensure the safety of personnel and equipment.

Voltage transformer

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 signal to the relay device to cut off the fault circuit in case of short-circuit overload and other faults of the line, so as to protect the safety of the power supply system. The working conditions of micro current transformer for protection are completely different from those for measurement. The protective transformer only starts to work effectively when the current is several times and dozens of times greater than the normal current.

Basic structure: it is a special transformer to expand the voltage range. The basic structure is the same as that of an ordinary 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 is mainly matched with the measuring instrument to measure current, voltage, power, etc. under the normal working state of the line. Main requirements of micro current transformer for measurement:

① Reliable insulation;

② High enough measurement accuracy;

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

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 working 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. Since 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 is running, the secondary side is not allowed to be open circuit. Once open circuit, the primary side current becomes the excitation current, resulting in large magnetic flux and secondary side voltage 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 mode of current transformer shall be determined according to the operation requirements of the connected load. The most commonly used wiring methods are single-phase, three-phase star and incomplete star.

Basic structure: current transformer is a converter used to expand the measuring 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 does not change. The secondary side wire is very thin and there are many turns, 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 state. According to the transformer operation principle: 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.

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