Vacuum Circuit Breakers: Features, Operation & Applications

In vacuum circuit breakers, vacuum typically at pressures ranging from 10^-9 to 10^-6 bar is used as the quenching medium. At such pressures, high dielectric strength can be achieved. The contact separation needed at such low pressures is only 0-20 mm and low energy mechanisms may be employed to operate the contacts through expendables bellows.

Since vacuum provides the highest insulating strength, it has far superior arc quenching properties than any other medium. For instance, when contacts of a breaker are opened in vacuum, the interruption happens at first current zero with dielectric strength between the contacts building up at a rate of thousands of times higher than that achieved with other circuit breakers.
The figure below shows a cutaway view of a typical vacuum circuit breaker:

As illustrated in the figure above, a typical vacuum circuit breaker consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of a leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield/sputter shield or metallic screens prevents the deterioration of the internal dielectric strength by preventing metallic vapors falling on the inside surface of the outer insulating cover. The shield also protects the envelope from thermal shock.

The Principle behind Vacuum Circuit Breaker Operation

The interruption of a short circuit current involves the initial formation of a conductive path between the contacts which very rapidly becomes a high grade insulator normally after the first current zero. The conductive path comprises of contact metal vapor. The arc is extinguished when the current falls to zero. The conducting metal vapor condenses on metallic screens or sputter shields inside the vacuum tube walls within a few µs and the dielectric strength is restored to form an open circuit. The shields prevent metal vapor deposits from reducing the overall dielectric strength of the insulated vacuum interrupter casing.

Arcing times are typically of the order of half a cycle (10 ms at 50Hz). To prevent overheating, the contact system must be designed to allow the arcing to move to different points over the contact surface area by utilizing its own magnetic field and by using special contact materials. In this way, current chopping and the related transient overvoltages are avoided except at the lowest levels of current interruption (a few amperes)

The Operation of a Vacuum Circuit Breaker

When the breaker operates, the moving contact separates the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of the contacts.  A very careful selection of contact material is thus necessary for vacuum circuit breakers. At high current interruption, metal vapors are produced which assist in arc extinction and, therefore, for long contact life a hard contact material is the ideal choice. However, under low-current interruption conditions, hard contacts produce very little metallic vapor and very rapid arc extinctions and ‘chopping’ of the current waveform results, which produce high overvoltages. A softer contact material is better for low-current interruption, however erodes too rapidly at high currents. Therefore, contact material must be a compromise between the two extremes and contact designs have evolved to ensure that the arc is kept in motion to minimize contact erosion. Important contact material criteria are: vapor pressure, electrical conductivity, heat conductivity and melting point. Commonly used contact materials include: copper-bismuth or copper-chrome alloys.

At or near zero current the arc extinguishes, vapor production ceases and very rapid re-combination and de-ionization of the metal vapor occurs.  The metal vapor products are deposited on the shield thus ensuring the clean conditions necessary for withstanding transient recovery voltage across the open contacts.

Applications of Vacuum Circuit Breakers

Vacuum interrupters, by virtue of their construction, are single-phase devices. They are usually mounted in air in a three-phase circuit-breaker with air and solid insulation between the phases. The maximum voltage rating of a vacuum interrupter is about 36 kV and maximum short-circuit currents may, in extreme cases, be as high as 100 kA, with rated currents of up to 4000 A.

The main use of vacuum interrupters is in circuit breakers for use on distribution systems. They are also being used in generating station auxiliary supply applications instead of the free-air circuit breakers.

Vacuum circuit breakers are rarely used at transmission system voltages as their maximum voltage limitations would require a large number of series connected interrupters which would be uneconomical.

The vacuum circuit breaker can be used in a much lighter construction, as a contactor for motor switching applications at voltages of up to 12 kV

The lifespan of typical vacuum circuit breakers is very long (usually 20,000 switching and a hundred short circuit operations) before replacement is required. The upper voltage range of vacuum interrupters is extended in some designs by surrounding the vacuum tubes and busbars with SF₆ circuit breakers.

Advantages of Vacuum Circuit Breakers

The advantages of the vacuum circuit breaker or contactor are:

• Complete isolation of the interrupter from atmosphere and contaminants.
• Absence of oil reduces the risk of fire.
• There is no generation of gas during and after operation.
• They have low arc energy.
• Maintenance requirements are low and involve attention to the operating mechanism.
• They have low inertia and hence require smaller power for control mechanism.
• Very compact metal-enclosed designs are available. Where necessary the designs may incorporate a series fuse to enhance short circuit capabilities and still render the units safe should the contact surfaces weld under loss of vacuum conditions.

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