The transformer is an electronic component used to detect current, voltage and power, and is an indispensable device in the power system. It works on the basis of electromagnetic induction phenomenon, and converts the high voltage and high current of the primary system into low voltage and low current, which is used by the measuring instrument, relay protection and automatic device of the secondary system.

Transformers can be divided into current transformers and voltage transformers according to measurement objects. According to the working principle, it can be divided into electromagnetic type, Hall type and photoelectric type. According to the structure can be divided into dry type, pouring type, oil immersed and so on. In addition, there are electronic transformers, which can transform an electronic signal or electrical force into another signal or form of energy.

The main function of the current transformer is to change the large current of the primary circuit into the standard value of the secondary circuit, so that the measuring instrument and relay are standardized and miniaturized, and at the same time, the electrical isolation is realized to ensure the safety of the person and the equipment. In addition, it can also obtain zero sequence current for relay protection devices that react to ground faults.

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

The working principle of transformers is similar to that of transformers, which are based on the phenomenon of electromagnetic induction. When used, the primary coil is directly connected to the primary circuit, and the number of turns is very small, so the size of the primary current completely depends on the size of the load current in the line under test, and has nothing to do with the size of the secondary coil current. The impedance of the current coil of the instrument and relay connected to the secondary coil is very small, so the transformer operates in a state close to short circuit.

Do not open the secondary circuit of the current transformer in operation. Otherwise, high voltage will be generated at both ends of the open circuit, endangering the safety of personal equipment, or heating the current transformer. Because the impedance of the secondary winding is very small during normal operation, the magnetomotive force generated by the primary current is mostly 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, the secondary winding magnetomotive force also becomes zero, and the primary winding current does not become smaller with the secondary open circuit, losing the compensation of the secondary winding magnetomotive force, the primary magnetomotive force is very large, all used for excitation, the synthetic magnetic flux suddenly increases many times, so that the magnetic circuit of the core is highly saturated. At this time, all the primary current becomes an exciting current, which generates a high electromotive force in the secondary winding, threatening personal safety or causing secondary insulation damage to instruments, protection devices and transformers.

The current passing through the secondary side of the voltage transformer in operation is determined by the impedance of the secondary circuit. 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 quick automatic air switch is generally installed at the secondary exit for overload and short circuit protection.