High-voltage circuit breakers may seem unfamiliar, and you might not think you'd encounter them in daily life. In short, however, they are crucial control and protection devices, primarily used in power plants, substations, and power systems.
Given their vital role, how do you choose the right high-voltage circuit breaker? Below, we'll explain.
General principles for ensuring the safety of high-voltage electrical equipment during normal operation, maintenance, short circuits, and overvoltage conditions:
Based on normal operating conditions, including voltage, current, frequency, and mechanical load.
Based on short-circuit conditions, including short-time withstand current, peak withstand current, making and breaking current.
Based on environmental conditions, including temperature, humidity, altitude, and earthquake resistance.
Based on overvoltage withstand capability, including insulation level.
Different types of high-voltage circuit breakers are selected based on the characteristics of various high-voltage electrical appliances, including the operating performance of switches, the protective characteristics of fuses, and the load and accuracy class of instrument transformers. Oil circuit breakers are classified into two types according to the amount of oil used: high-oil and low-oil.
In high-oil circuit breakers, the oil serves both insulating and arc-extinguishing functions; in low-oil circuit breakers, the oil is used solely as an arc-extinguishing medium.
When the circuit breaker trips, an electric arc is generated. The arc is quickly extinguished by the combined effects of the horizontal and vertical blowing of the oil flow and the mechanical blowing caused by the oil movement. High-oil circuit breakers are now obsolete due to their high oil volume, large size, high steel consumption, slow operating speed, heavy maintenance workload, inconvenient installation and transportation, large footprint, and susceptibility to fire. However, a small number of 35kV substations in my country still use high-oil circuit breakers.
Low-oil circuit breakers have the advantages of simple structure, low oil consumption, small size, relatively safe operation, and significant savings in oil and steel. Currently, the SN10-10 type oil-less circuit breaker is mainly used. It is classified into three types according to its breaking capacity: 300MVA, 500MVA, and 750MVA, respectively.
Due to its poor breaking performance and the flammability and explosiveness of oil, oil-less circuit breakers are gradually being phased out. New substations generally use vacuum circuit breakers or sulfur hexafluoride (SF6) circuit breakers. SF6 circuit breakers utilize sulfur hexafluoride (SF6) gas as the insulating and arc-extinguishing medium. They are a new type of circuit breaker developed after the advent of Haohua Electric in the 1950s. Due to the excellent insulation and arc-extinguishing properties of SF6 gas, its development has been rapid. It can be used in 6–500kV systems, and is currently mainly used in 110kV and above power systems.
The arc-extinguishing chamber structure of circuit breakers includes compressed air type, self-extinguishing type (rotary arc type, thermal expansion type), and hybrid arc-extinguishing type. The LN1 and LN2 type SF6 circuit breakers produced in my country use a pneumatic arc-extinguishing structure, while the LW3 type outdoor SF6 circuit breaker uses a suspended arc-extinguishing structure. The operating mechanism of SF6 circuit breakers mainly adopts a spring and hydraulic operating mechanism.