Instrument transformer is a general term of current transformer and voltage transformer. It can change high voltage to low voltage, high current to small current, used for measuring or protecting system. Its function is mainly to convert high voltage or high current into standard low voltage (100V) or standard small current (5A or 1A, all refer to the rating) in order 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 separate the high voltage system to protect the safety of people and equipment.

What is work principle and characteristics of Voltage transformer?

Voltage transformers can be used for voltage and power measurement in HV and UHV power systems.

Working principle: Voltage transformers can be divided into electromagnetic type and capacitive voltage divider type. When the voltage level is 220kV and below, it is mostly electromagnetic type, so electromagnetic type is introduced. The voltage transformer uses the principle of electromagnetic induction. There are two mutually insulated coils with different turns around the closed iron core. The primary coil N1 is connected to the power supply side, and the secondary coil N2 is connected to the output side.

When voltage is applied to the primary coil, AC current will pass through the coil, and alternating magnetic flux 1 with the same frequency as the power supply will be generated in the iron core. Since the primary winding and secondary winding are on the same iron core, according to the law of electromagnetic induction, induced electromotive force E2 with the same frequency and different value will be generated in the secondary coil. Because the number of turns is different, the induced electromotive force of the two coils is different. The specific numerical relationship is as follows: N1/N2=U1/U2. The output voltage of the secondary side of the voltage transformer is 100V.

Characteristics: The primary voltage of the voltage transformer is not affected by the secondary load. The voltage coil impedance of the instrument or relay on the secondary side of the voltage transformer is very large and the current passing through is very small, so the voltage transformer is close to the no-load state when it works normally. The secondary side of the voltage transformer should not be short-circuited, because a large short-circuit current will be generated on the secondary side after short-circuit, which will burn the voltage transformer. Therefore, fuses are generally installed on the primary and secondary sides of the voltage transformer for short-circuit protection.

What is work principle and basic structure of current transformer?

Current transformers can be used for measurement of exchange current, measurement of exchange intensity and protection of electric drive lines. It is made of the characteristic that the primary and secondary side currents of transformer are proportional.

Its working principle and equivalent circuit are the same as the general transformer, but its primary coil is in series in the circuit under test, and the number of turns is very small; The secondary coil is connected to ammeter, relay current coil and other low impedance load, approximate short circuit. The primary side current (the measured current) and secondary side current depend on the load of the line under test and are independent of 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 are very small, and the excitation current I0 is also very small. When the current transformer is in operation, the secondary side is not allowed to open. Because once the open circuit, the current of the primary side will become the excitation current, so that the magnetic flux and the voltage of the secondary side will greatly exceed the normal value and endanger the safety of people and equipment. Therefore, fuses are not allowed in the secondary circuit of the current transformer, and devices such as ammeter and relay are not allowed to be removed without bypass during operation. The connection mode of the current transformer shall be determined according to the operation requirements of the load connected to it. The most common wiring modes are single phase, three phase star and incomplete star.

Basic structure: Current transformer is a converter used to extend the range of time.

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