Classification of High Voltage, Medium Voltage, and Low Voltage Levels

High voltage typically refers to voltage levels of 110kV and above. Low voltage is defined as AC 1kV or DC 1500V and below. Medium voltage lies between high and low voltage levels, commonly at 3kV, 6kV, 10kV, 20kV, 35kV, and 66kV for system voltages, and 3.6kV, 7.2kV, 12kV, 24kV, 40.5kV, and 72.5kV for switchgear. These are the standard voltage levels in China. For high-speed railways, which use two-phase power supply, the voltage levels are 27.5kV/55kV.

Consistent Control and Protection Functions

Regardless of whether it is high, medium, or low voltage, the functions of power control and protection are consistent. Circuit breakers are needed for short-circuit protection in all cases. Switchgear maintenance and inspection require isolating switches and grounding switches. Lightning protection involves the use of arresters, current measurement is done using current transformers, and voltage measurement is performed with voltage transformers (low voltage systems may measure directly from the power source without transformers).

High Voltage Circuit Breakers

Due to the high voltage, high voltage circuit breakers require substantial electrical clearances, particularly in Air-Insulated Switchgear (AIS). For instance, the safe distance for 110kV is 1.5 meters. To meet environmental impact requirements, such as pollution, the external insulation of the breaker must satisfy specific creepage distance requirements, resulting in large physical dimensions. As voltage levels increase, spatial requirements grow. However, Gas-Insulated Switchgear (GIS) can significantly reduce dimensions due to the excellent insulating properties of SF6 gas. In GIS, switches, circuit breakers, and transformers can be integrated into a compact, tank-type design.

Low Voltage Circuit Breakers

Most low voltage circuit breakers are air circuit breakers, which interrupt short-circuit currents within an air arc chamber. The arc is extinguished by cutting and cooling it using an arc chute. The size of low voltage circuit breakers is determined by the current rating. Given the low voltage, an insulation distance of mere millimeters suffices. Currently, low voltage circuit breakers can handle up to 8000A. The structure typically comprises an operating mechanism at the front, a transmission mechanism connecting the moving contacts in the middle, and connection units at the rear. Low voltage circuit breakers can be either draw-out or fixed type.

Protection Trip Units

Low voltage circuit breakers are generally equipped with protective trip units, such as microprocessor-based trip units or thermal trip units. This feature distinguishes low voltage circuit breakers from medium and high voltage ones. Medium and high voltage circuit breakers are controlled by external relays or microprocessor-based protection systems, functioning primarily as mechanical devices.