Transformer, also called instrument transformer, is the general term of current transformer and voltage transformer. It can change high voltage into low voltage and large current into small current for measurement or protection system. Its main function is to transform high voltage or high current into standard low voltage (100V) or standard low current (5a or 1a, both refer to rated value) in proportion, so as to complete the standardization and miniaturization of measuring surface, protection equipment and automatic control equipment. All transformers can also be used to separate high voltage systems to ensure the safety of people and equipment.

Current transformer

The current transformer can be used in the measurement of exchange current, the measurement of exchange electricity and the protection of electric drive circuit. It is made of the characteristics that the primary and secondary current of the transformer are in proportion. Its operation principle and equivalent circuit are the same as the general transformer, only 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 with low impedance load such as ammeter, relay current coil, etc., which is approximately short circuited. The primary side current (i.e. the measured current) and the secondary side current depend on the load of the line under test, but are independent of the secondary side load of the current transformer. As the secondary side is close to the short circuit, the voltage U1 and the excitation current I0 of the primary and secondary sides are very small When the current transformer is working, the secondary side shall not be open circuited. Once the circuit is open, the primary side current becomes the excitation current, which makes the flux and the secondary side voltage greatly exceed the normal value and endangers the safety of people and equipment. Therefore, it is not allowed to connect fuse in the secondary circuit of current transformer, or dismantle ammeter, relay and other equipment without bypass during operation. The wiring method of current transformer shall be confirmed according to the working requirements of the connected load. The most commonly used wiring methods are single-phase, three-phase star and incomplete star.

Substructure: current transformer is a converter used to expand the range.

When in use, the primary side of the main circuit is connected in series with the load of the circuit under test, and the secondary side is connected in series with 5A ammeter and protection relay current coil. Because the primary side of the current coil uses a thick wire, its turns are only one or several turns, but its cathode is small, and the voltage is very low. After being connected in series with the circuit under test, the supplementary current will not be changed. The secondary side wire is very thin, and the turns are many, but its induced potential is not high (only a few volts ）The resistance of the secondary ammeter and relay coil 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 primary side current of the current transformer shall be inversely proportional to the number of turns: L1 / L2 = W2 / W1 or L1 = W2 / w1l2 W2 / W1 = K2 transformation ratio

Voltage transformer can be used to measure voltage and power in high voltage and ultra-high voltage power system. The current transformer for protection shall cooperate with the relay device first. In case of short-circuit overload and other defects of the line, it shall provide the relay device with signal blocking circuit to protect the safety of the power supply system. The working conditions of the protective miniature current transformer are totally different from those of the measuring transformer. The protective transformer only starts to work effectively when the current is several times and dozens of times larger than the normal current.

Bottom substructure: it is a special transformer with extended voltage range, and the bottom substructure is the same as the general transformer. Therefore, the voltage relationship U1 / U2 between the primary side of the voltage transformer and the power payment is W1 / W2.

The current transformer for measurement is mainly used in cooperation with the measuring surface to measure the current, voltage, power, etc. under the normal operation condition of the line. Primary requirements for measuring miniature current transformers:

① reliable insulation;

② high enough measurement accuracy;

③ when there is a large current in the defect of the tested line, the transformer shall be full (such as 500% rated current) in the proper range to protect the measuring surface.