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Explain the principle and structural design of current transformer and voltage transformer in detail

by:Transmart     2022-12-20

Transformer, also known as instrument transformer, is a general term for current transformer and voltage transformer. It can change high voltage into low voltage and high current into small current for measurement or protection system. Its function is mainly to convert high voltage or high 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. change. At the same time, the transformer can also be used to isolate the high-voltage system to ensure the safety of people and equipment.

Voltage transformers can be used to measure voltage and power in high-voltage and ultra-high-voltage power systems. The protective current transformer mainly cooperates with the relay device to provide a signal to the relay device to cut off the fault circuit when a fault such as short circuit or overload occurs in the line to protect the safety of the power supply system. The working conditions of the protective miniature current transformer are completely different from those of the measuring transformer. The protective transformer only starts to work effectively when the current is several times or tens of times larger than the normal current.

Basic structure: It is a special transformer to expand the voltage range, and the basic structure is the same as that of ordinary transformers. Therefore, the voltage relationship between the primary side of the voltage transformer and the power supply is U1/U2=W1/W2.

Measuring current transformers are mainly used in conjunction with measuring instruments to measure current, voltage, power, etc. under normal working conditions of the line. Main requirements for miniature current transformers for measurement:

① Reliable insulation;

② High enough measurement accuracy;

③When the measured line fails and a large current occurs, the transformer should be saturated in an appropriate range (such as 500% of the rated current) to protect the measuring instrument.

Current Transformer

The current transformer can be used in the measurement of the exchange current, the measurement of the exchange electric degree and the protection in the electric drag line. It is made by using the characteristic that the primary and secondary currents of the transformer are proportional. Its working principle and equivalent circuit are the same as those of ordinary transformers, except that the 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, and is approximately short-circuited. The primary current (that is, the measured current) and the secondary current depend on the load of the line under test, and have nothing to do with the secondary load of the current transformer. Since the secondary side is close to a short circuit, the primary and secondary side voltages U1 and 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 open. Because once the circuit is opened, the primary current becomes the excitation current, which makes the magnetic flux and the secondary voltage greatly exceed the normal value and endangers the safety of people and equipment. Therefore, no fuse is allowed in the secondary circuit of the current transformer, and it is not allowed to remove the ammeter, relay and other equipment without bypassing during operation. The wiring mode of the current transformer is determined according to the operating requirements of the load it is connected to. The most commonly used wiring methods are single-phase, three-phase star and incomplete star.

Basic structure: The current transformer is a current transformer used to expand the measuring range.

When in use, the primary side of the main side is connected in series with the load of the circuit under test, and the secondary side is connected in series with a 5A ammeter and a protective relay current coil. Since the primary side uses a thick wire, the number of turns is only one or several turns, and its electric cathode It is small and the voltage is very low. After being connected in series to the circuit under test, it does not change the supplementary measurement current. The secondary wire is very thin and has many turns, but its induced potential is not high (only a few volts). The secondary ammeter and relay coil resistance are both Very small, when working, the auxiliary side can be regarded as a short circuit state. According to the operating principle of the transformer: the ratio of the current transformer primary current should 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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