A transformer is an electronic component used to detect current, voltage and power, and is an indispensable device in the power system. It works based on the electromagnetic induction phenomenon, converting the high voltage and high current of the primary system into low voltage and low current for use by measuring instruments, relay protection and automatic devices in the secondary system.

According to the measurement object, the mutual inductor can be divided into current mutual inductor and voltage mutual inductor; according to the working principle, it can be divided into electromagnetic type, Hall type and photoelectric type; according to the structure, it can be divided into dry type, cast type, oil-immersed type, etc. In addition, there are electronic mutual inductors, which can convert electronic signals or electric forces into another signal or energy form.

The main function of the current transformer is to convert the large current of the primary circuit into the standard value of the secondary circuit, so as to standardize and miniaturize the measuring instruments and relays, and at the same time achieve electrical isolation to ensure the safety of people and equipment. In addition, it can also obtain zero-sequence current for use by the relay protection device that responds to ground faults.

The function of the voltage transformer is to convert the high voltage of the primary circuit into the standard value of the secondary circuit, and also standardize and miniaturize the measuring instruments and relays. In addition, it can also provide line voltage and phase voltage for general measurement and protection, and can form an open triangle through the auxiliary winding to reflect the sum of the three-phase voltage to ground, that is, the zero-sequence component of the voltage to ground, for ground monitoring.

The working principle of the mutual inductor is similar to that of the transformer, both of which are based on the electromagnetic induction phenomenon. When in use, its primary coil is directly connected in series to the primary circuit with very few turns, so the magnitude of the primary current depends entirely on the magnitude of the load current in the measured circuit and has nothing to do with the magnitude of the current in the secondary coil. The current coil impedance of the instrument and relay connected to the secondary coil is very small, so the mutual inductor operates in a state close to a short circuit.

The secondary circuit of the current transformer in operation is not allowed to be open. Otherwise, high voltage will be generated at both ends of the open circuit, endangering the safety of personnel and equipment, or causing the current transformer to heat up. Because in normal operation, the impedance of the secondary winding is very small, and most of the magnetomotive force generated by the primary current is compensated by the magnetomotive force generated by the secondary current. When the secondary circuit is open, the impedance increases infinitely, the secondary current becomes zero, and the magnetomotive force of the secondary winding also becomes zero, while the primary winding current does not decrease with the secondary circuit opening, and the compensation effect of the magnetomotive force of the secondary winding is lost. The primary magnetomotive force is very large and is all used for excitation. The synthetic magnetic flux suddenly increases many times, making the magnetic circuit of the iron core highly saturated. At this time, the primary current is all converted into excitation current, generating a very high electromotive force in the secondary winding, threatening personal safety or causing damage to the secondary insulation of the instrument, protection device, and transformer.

The current passing through the secondary side of the voltage transformer in operation is determined by the size of the secondary circuit impedance. When the secondary side is short-circuited, a large short-circuit current will be generated, which may damage the voltage transformer. In order to protect the voltage transformer, a fuse or a fast automatic air switch is generally installed at the secondary side outlet for overload and short-circuit protection.