The role and characteristics of the transformer

The transformer is a special transformer, widely used in power distribution devices for AC electrical measurement and relay protection. The transformer is divided into two categories: a current transformer is also called an instrument transformer, the purpose is to transform the AC high current into a standard small current (5 amps or 1 amp), supply power to the current coil of the measuring instrument and protection device; the other is a voltage transformer is also called instrument transformer, the purpose is to transform the AC high voltage into a standard low voltage.

The role of the voltage transformer is: that the high voltage is proportionally transformed into 100V or lower levels of standard secondary voltage for protection, metering, and instrumentation devices.

At the same time, the use of voltage transformers can be high voltage and electric = staff isolation. Although the voltage transformer is also in accordance with the principle of electromagnetic induction I as a device, its electromagnetic structure of the relationship with the current transformer is compared to the opposite.

The voltage transformer secondary circuit is a high impedance circuit, the size of the secondary current is determined by the impedance of the circuit. When the secondary load impedance decreases, the secondary current increases, making the primary current automatically increases a component to meet the electromagnetic balance between the first and second sides. It can be said that the voltage transformer is a special transformer with a limited structure and use form.

A voltage transformer is an indispensable electrical apparatus for power plants, substations, and other power transmission and supply systems. Precision voltage transformers are used in electrical test laboratories to expand the limits of measurement, measuring voltage, power, and electrical energy of an instrument.

Voltage transformers are very similar to transformers in that they are used to change the voltage on the line. However, the purpose of transformer voltage conversion is to transmit electrical energy, so the capacity is very large, generally in kVA or MVA as the unit of calculation.

And the purpose of the voltage transformer transform voltage, mainly for measuring instruments and relay protection device power supply, used to measure the line voltage, power, and electrical energy, or used to protect the line in the event of a fault in the line of valuable equipment, motors, and transformers, so the voltage transformer capacity is very small, generally, only a few volt-amps, tens of volt-amps, the maximum is not more than one kilovolt-amps.

The function of the current transformer is to convert the large value of primary current into a smaller value of secondary current by a certain ratio, which is used for protection, measurement, and other purposes. For example, a current transformer with a ratio of 400/5 can convert a current of 400A into a current of 5A.

The purpose of a current transformer is to measure relatively large currents.

The characteristics of current transformers are:?

(1) secondary coil is in series in the circuit, and the number of turns is very small, so the current in the primary wire enclosure depends entirely on the load current of the circuit under test. And the secondary current has nothing to do with it.

(2) the current transformer secondary coil connected to the instrument and relay current coil impedance are very small, so under normal circumstances, the current transformer is in the near short-circuit state of operation.

Current transformer, the ratio of the secondary rated current, known as the current transformer rated mutual inductance ratio: kn=I1n/I2n

Because the primary coil rated current I1n has been standardized, the secondary coil rated current I2n unified for 5 (1 or 0.5) amp, so the current transformer rated mutual inductance ratio has also been standardized.

kn can also be approximated as a transformer, the number of turns of the secondary coil ratio, that is, kn ≈ kN = N1 / N2 formula N1, N2 for the number of turns of the second coil.

To be continued...