The role and use of current transformers and voltage transformers
Transformer, also called instrument transformer, is a general term for current transformer and voltage transformer. It can turn high voltage into low voltage and large current into small current for measurement or protection systems. Its function is mainly to convert high voltage or large current into standard low voltage (100V) or standard low current (5A or 1A, all refer to rated value) in proportion, so as to complete the standardization and small size of measuring surface, protection equipment and automatic control equipment. change. All transformers can also be used to isolate high-voltage systems to ensure the safety of people and equipment. Current transformers Current transformers can be used in the measurement of exchanged current, the measurement of exchanged electrical degrees and the protection of power drag lines. It is made using the characteristic that the primary and secondary side currents of the transformer are proportional. Its working principle and equivalent circuit are the same as those of general transformers, only the primary winding is connected in series in the circuit under test, and the number of turns is small; the secondary winding is connected to low-impedance loads such as ammeter and relay current coil, which is approximate to short circuit. The primary side current (that is, the current under test) and the secondary side current depend on the load of the line under test, and have nothing to do with the secondary side load of the current transformer. Since the secondary side is close to a short circuit, the voltages U1 and the secondary side are very small, and the excitation current I0 is also very small. When the current transformer is working, the secondary side is not allowed to open circuit. Because once the circuit is open, the primary side current becomes the excitation current, which makes the magnetic flux and secondary side voltage greatly exceed the normal value and endangers personal and equipment safety. Therefore, it is not allowed to connect fuses in the secondary circuit of the current transformer, and it is also not allowed to remove the ammeter, relays and other equipment without bypassing during operation. The wiring method of the current transformer shall be confirmed according to the working requirements of the connected load. The most common wiring methods are single-phase, three-phase star and incomplete star. Basic structure: The current transformer is a current transformer used to expand the range. When in use, the primary side of the main 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. Because the primary side uses thick wires, the number of turns is only one or a few turns. It is small and the voltage is very low. After being connected in series to the circuit under test, it does not change the current of the supplementary measurement. The secondary wire is very thin and has many turns, but its induced potential is not high (only a few volts). Both the secondary ammeter and the relay coil resistance are It is very small. During operation, the secondary side can be regarded as a short-circuit condition. According to the working principle of the transformer: the ratio of the current on the primary side of the current transformer should be inversely proportional to the number of turns: L1/L2u003dW2/W1 or L1u003dW2/W1L2. W2/W1u003dK2 ratio voltage transformer can be used for voltage and power measurement in high-voltage and ultra-high-voltage power systems. The protective current transformer mainly cooperates with the relay device, and provides a signal to the relay device to cut off the faulty circuit in the event of short-circuit overload and other defects in the line to protect the safety of the power supply system. The working conditions of the protection miniature current transformers are completely different from those of the measurement transformers. The protection transformers only start to work effectively when the current is several times larger than the normal current. Basic structure: It is a special transformer with extended voltage range, and the basic structure is the same as that of general transformers. Therefore, the voltage relationship between the primary side of the voltage transformer and the power supply is U1/U2u003dW1/W2. The current transformer for measurement mainly cooperates with the measurement surface, and is used to measure current, voltage, power, etc. under normal operating conditions of the line. The main requirements of the miniature current transformer for measurement: ①Reliable insulation; ②High enough measurement accuracy; ③When a large current occurs in the line under test, the transformer should be full within the appropriate range (such as 500% of the rated current) Protect the measuring surface.
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