Working Principle and Features of the Vacuum Circuit Breaker

A vacuum circuit breaker (VCB) uses vacuum as the arc-extinguishing and insulating medium to interrupt and close the current within a vacuum container. Since its invention in the early 1960s, with the improvement of key processes and the development of new arc-extinguishing chambers and operating mechanisms, the technical parameters of vacuum circuit breakers have continuously improved. They have rapidly developed due to their excellent performance and remarkable advantages. Currently, vacuum circuit breakers dominate the 10kV voltage level circuit breaker market in China, with a utilization rate exceeding 96%. In the 35kV voltage level circuit breakers, the utilization rate of vacuum circuit breakers exceeds 62%. Vacuum circuit breakers have become the most widely used circuit breakers in the medium-voltage field below the 35kV voltage level.

(1) Working Principle of Vacuum Circuit Breaker

① Generation of Vacuum Arc 

Vacuum, in this context, refers to a relatively low-pressure space with gases that are less dense than normal atmospheric pressure. The vacuum degree of the arc-extinguishing chamber is generally required to be 1.33 x 10^-7 ~ 1.33 x 10^-1 Pa. The gas density in the arc-extinguishing chamber is very low, the free path of the gas molecules is large, and the chances of collision and ionization are minimal, leading to a high breakdown voltage. When a vacuum gap is broken down and an arc is generated, it is not the result of collision ionization of the gas, but the result of metal vapor released from the contact electrodes. When the current is interrupted, as the contacts separate, the contact area rapidly decreases, leaving only one or a few small contact points. The current density at these points becomes extremely high, causing the temperature to rise sharply. This results in the melting and vaporization of the contact metal, and due to the high temperature of the metal vapor and the strong electric field, field emission and ionization of the metal vapor occur, thereby forming the vacuum arc. The characteristics of the vacuum arc mainly depend on the materials and surface conditions of the contacts, as well as the types of residual gases, gap distances, and the uniformity of the electric field.

② Extinguishing of Vacuum Arc 

Because the charged particles and metal vapor generated in the vacuum arc have a high diffusion speed, when the arc current crosses zero and the arc temporarily extinguishes, the dielectric strength of the contact gap can quickly recover, leading to the extinction of the arc. The speed at which the dielectric strength of the vacuum circuit breaker’s contact gap recovers depends on the diffusion speed of the charged particles, the magnitude of the breaking current, and factors like the contact area, shape, and materials. When a transverse and longitudinal magnetic field is applied to the arc region, the arc is driven to diffuse quickly, which can increase the recovery speed of the dielectric strength and reduce the wear of the contacts, thereby increasing the service life.

(2) Features of Vacuum Circuit Breaker

① High Insulation Strength of Vacuum Medium 

The insulation strength of vacuum is high, and the contact gap inside the arc-extinguishing chamber is small (around 10mm for 10kV contacts), so the size of the arc-extinguishing chamber is compact. Due to the short stroke of the contacts during switching, the switching operation is fast, and the power requirement for the operating mechanism is relatively small, allowing the mechanism structure to be simpler. This makes the overall unit compact, light in weight, and suitable for frequent operations.

② Strong Breaking Capacity

Vacuum circuit breakers have strong breaking capabilities, capable of interrupting large currents. The arc extinction time is short, the arc voltage is low, the arc energy is minimal, and the contact wear is small. They can operate many times with low maintenance costs and a long service life (typically up to 20 years).

③ Reliable Operation and Safe Use 

The contact part does not use oil, and it has a completely sealed structure, unaffected by moisture, dust, or harmful gases. There is no risk of fire or explosion, making it suitable for various environments, especially hazardous locations.

④ Measures to Limit Overvoltage 

When breaking inductive or capacitive loads, overvoltages may arise due to current chopping, oscillations, or re-ignition. Measures should be taken to limit overvoltage, such as using parallel capacitors, RC damping devices, or metal oxide arresters (MOA) to suppress overvoltage.

⑤ High Sealing and Manufacturing Requirements 

Vacuum circuit breakers have high requirements for sealing processes and manufacturing techniques, leading to higher costs.

Basic Structure of Vacuum Circuit Breaker

The vacuum circuit breaker mainly consists of three parts: the vacuum arc-extinguishing chamber, the support, and the operating mechanism. The vacuum arc-extinguishing chamber is the core component of the vacuum circuit breaker and performs the functions of interrupting, conducting, and insulating the current. It mainly consists of an insulating shell, movable and stationary contacts, a shielding cover, and a bellows. The performance of the vacuum arc-extinguishing chamber mainly depends on the materials and structure of the contacts, the design of the shielding cover, the materials of the arc-extinguishing chamber, and the manufacturing process.

(1) Insulating Shell 

The insulating shell of the vacuum arc-extinguishing chamber serves as both the vacuum container and the insulator between the movable and stationary contacts. Its function is to support the movable and stationary contacts and the shielding cover, and it is welded to these parts in a hermetically sealed manner to ensure the high vacuum inside the arc-extinguishing chamber. It is generally required that the vacuum inside the arc-extinguishing chamber should not drop below the specified value for at least 20 years, so it must be strictly sealed. The insulating shell is usually made of hard glass, aluminum oxide ceramics, or microcrystalline glass.

(2) Contacts 

The contacts of the vacuum circuit breaker are the current-carrying components during closing and the arc-extinguishing components during opening. The material and structure of the contacts directly affect the breaking capacity, electrical life, dielectric strength, closing ability, current-limiting overvoltage, and long-term current-carrying capability.

The materials for the contacts are required to have good electrical conductivity, thermal conductivity, and mechanical strength, as well as resistance to welding, good arc extinguishing performance, low current interruption, and low gas content. In practice, alloy materials can solve these contradictory requirements. Commonly used contact materials internationally are copper-bismuth and copper-chromium alloys. Copper-chromium alloys are the most widely used due to their excellent comprehensive properties.

The breaking capacity of the vacuum circuit breaker largely depends on the contact structure. The contacts usually adopt a butt-type structure, with commonly used types being transverse magnetic field contacts and longitudinal magnetic field contacts. Both types utilize magnetic field forces to move the vacuum arc quickly, preventing the formation of high-temperature areas on the contacts that would require extended cooling.

The transverse magnetic field contact uses the transverse magnetic field generated by the current passing through the contacts to drive the arc to move along the contact surface. The typical designs include cup-shaped and spiral-shaped contacts. The longitudinal magnetic field contact uses the longitudinal magnetic field in the gap between contacts to improve the breaking capacity. Longitudinal magnetic fields constrain the charged particles, reducing arc voltage and evenly distributing the arc energy across the entire contact surface, preventing localized melting of the contact. These contacts are suitable for breaking high currents, with breaking currents reaching up to 70kA. Longitudinal magnetic field contacts include coil-type and cup-type contacts.

(3) Shielding Cover 

The commonly used shielding covers in the vacuum arc-extinguishing chamber include the main shielding cover, bellows shielding cover, and voltage equalization shielding cover. Shielding covers are typically made of copper or steel and are required to have high thermal conductivity and excellent condensation capability.

The main shielding cover is installed around the contacts, typically fixed in the middle of the insulating shell. Its main functions are: 1) preventing the large amounts of metal vapor and metal particles generated during arcing from splashing onto the inner walls of the insulating shell, which could reduce the insulating strength or cause flashover; 2) improving the uniformity of the electric field inside the arc-extinguishing chamber, lowering local electric field strength, and improving insulation performance; 3) absorbing some arc energy, cooling and condensing arc products, and improving the recovery speed of the dielectric strength after the arc is extinguished, thereby enhancing the breaking capacity of the arc-extinguishing chamber.

The bellows shielding cover is wrapped around the bellows to prevent metal vapor from splashing onto the bellows, which could affect its operation and reduce its lifespan.

The voltage equalization shielding cover is installed near the contacts to improve the electric field distribution between the contacts.

(4) Bellows 

The bellows ensure that the movement of the movable contact within a certain stroke range does not break the sealing of the arc-extinguishing chamber. The bellows are typically made of stainless steel and can be hydraulically formed or welded in a diaphragm structure. Every time the vacuum circuit breaker operates, the bellows undergoes mechanical deformation. Long-term frequent and intense deformation can lead to fatigue failure of the bellows, causing leakage in the arc-extinguishing chamber, which would render the breaker unusable. The bellows is the most vulnerable component in the vacuum arc-extinguishing chamber, and its fatigue life determines the mechanical life of the vacuum arc-extinguishing chamber.