Classification of Electrical Equipment
There are many electrical equipments in the power system. They are usually divided into electrical primary equipment and secondary electrical equipment, depending on their role in operation.

Electric Power Primary Equipment
The equipments directly involved in the production, transformation, transmission, distribution and consumption of electricity are known as electrical primary equipments, including:
Equipments for the production and conversion of electrical energy, such as electric generators, electromotors, transformers, etc.
Switchgear for connecting and turn-off circuit, such as circuit breakers, disconnectors, contactors, fuses protector, etc.
Current-carrying conductor and gas-insulated equipment. Such as busbar, power cables, insulators, wall bushing and so on.
Limiting over-current or over-voltage equipment, such as current limiting reactor, arrester and so on.
Mutual Inductor Equipment: speaking of the reduction of the high voltage and high current in the primary loop for the use of measuring instruments and relay protection devices, such as voltage transformers, current transformers [1]

Electric Power Secondary Equipment
In order to ensure the normal operation of the electrical primary equipment, this device will make the measurement, monitoring, control and regulation to the running state of the primary equipment. So it is called as electrical secondary equipment. There are mainly: a variety of measuring meters, all kinds of relay protection and automatic devices, DC power supply equipment etc.