Explain the functions and uses of current transformers and voltage transformers in detail -

Transformer, also known as instrument transformer, is the 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. All transformers can also be used to isolate high-voltage systems to ensure the safety of people and equipment. Current Transformer Current transformer can be used in the measurement of exchange current, the measurement of exchange electric degree and the protection in 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 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 similar to a short circuit. 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 working, the secondary side is not allowed to open circuit. 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, it is not allowed to connect the fuse in the secondary circuit of the current transformer, and it is not allowed to remove the ammeter, relay and other equipment without bypassing during work. The wiring method of the current transformer is confirmed according to the working 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 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, during operation, the auxiliary side can be regarded as a short circuit condition. According to the working 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 variable ratio voltage transformers 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 to provide a signal to the relay device to block the faulty circuit when the fault occurs in the circuit such as short circuit and overload, so as to protect the safety of the power supply system. The operating 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 dozens 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 a general transformer. Therefore, the voltage relationship between the primary side of the voltage transformer and the power supply is U1/U2=W1/W2. The current transformer for measurement mainly cooperates with the measurement instrument, and is used to measure current, voltage, power, etc. under the normal operation of the line. The primary requirements of the miniature current transformer for measurement: ①Reliable insulation; ②High enough measurement accuracy; ③When the large current caused by the defect occurs in the measured line, the transformer should be full within the appropriate range (such as 500% of the rated current) Protect the measurement surface.