Vaccum circuit breaker construction, working principle, advantages and disadvantages.

 Vacuum Circuit Breaker

A breaker which used vacuum as an arc extinction medium is called a vacuum circuit breaker . In this circuit breaker, the fixed and moving contact is enclosed in a permanently sealed vacuum interrupter. The arc is extinct as the contacts are separated in high vacuum. It is mainly used for medium voltage ranging from 11 KV to 33 KV.

Vacuum circuit breaker has a high insulating medium for arc extinction as compared to the other circuit breaker . The pressure inside the vacuum interrupter is approximately 10-4 torrent and at this pressure, very few molecules are present in the interrupter. The vacuum circuit breaker has mainly two phenomenal properties.

1. High insulating strength: In comparison to various other insulating media used in circuit breaker vacuum is a superior dielectric medium. It is better than all other media except air and SF6, which are employed at high pressure.
2. When an arc is opened by moving apart the contacts in a vacuum, an interruption occurs at the first current zero. With the arc interruption, their dielectric strength increases up to a rate of thousands time as compared to other breakers.

The above two properties make the breakers more efficient, less bulky and cheaper in cost. Their service life is also much greater than any other circuit breaker , and almost no maintenance are required.

Construction of Vacuum Circuit Breaker

It is very simple in construction as compared to any other circuit breaker. Their construction is mainly divided into three parts, i.e., fixed contacts, moving contact and arc shield which is placed inside the arc interrupting chamber.

The outer envelope of vacuum circuit breaker is made up of glass because the glass envelope help in the examination of the breaker from outside after the operation. If the glass becomes milky from its original finish of silvery mirror, then it indicates that the breaker is losing vacuum.

The fixed and moving contacts of the breaker are placed inside the arc shield. The pressure in a vacuum interrupter at the time of sealing off is kept at about 10-6 torr. The moving contacts of the circuit breaker are move through a distance of 5 to 10 mm depending upon the operating voltage.

The metallic bellows made of stainless steel is used to move the moving contacts. The design of the metallic bellows is very important because the life of the vacuum circuit breaker depends on the ability of the component to perform repeated operations satisfactorily.

Vacuum Circuit Breaker Working Principle

It is very necessary to understand the arc of VCB to understand the working principle of a vacuum circuit breaker. In other circuit breakers arc is formed due to ionization of particles in the medium between the contacts. But a vacuum arc is formed by evaporating its surface material and continued to until the next current zero instant.

At current zero instant, the arc is quenched, and contact metal vapours are condensed again on the contact surface. Due to the high dielectric strength of vacuum, vacuum arc can be quenched within very small contact gap.

The current must be interrupted at zero current instant; otherwise, high voltage transients will be induced due to current chopping. Therefore, for successful arc interruption, it should be stable before current zero instant.

The arc stability depends upon the contact material very much. Thus arc extinction process in a VCB is related to the material and shape of the contacts and the method used in condensing the metal vapour. The contact surface is so designed that the arc root keeps on moving so that temperature at one point on the contact does not attain a very high value.

Advantages of vacuum circuit breakers are as follows

    (1) The dielectric strength of the vacuum medium is high, the contact gap is small, the volume of the arc extinguishing chamber is small, and the power requirement of the operating mechanism is small.

 (2) Strong arc extinguishing ability, large breaking current, short arc burning time, low arc voltage, small electrical wear of contacts, many breaking times, long electrical life, generally up to 20 years.

(3) The medium strength recovery speed is fast, the inertia of the moving conductive rod is small, it is suitable for frequent operation and fast cut-off occasions, and has multiple reclosing functions.

4) The medium does not age and does not need to be replaced. In the service life, the vacuum interrupter and the contact part do not need to be repaired, the maintenance workload is small, and the maintenance cost is low.

 (5) Safe to use, small size and light weight.

(6) Environmental pollution is small. The opening and closing is carried out in a closed container. Arcs and hot metal vapors will not splash out and pollute the surrounding environment. There is no serious noise during operation, no flammable and explosive media, and no danger of explosion and fire.

(7) The arc chamber is an independent component, which is easy to install and adjust. 

Disadvantages of vacuum circuit breakers: 

(1) When the inductive load or the capacitive load is switched off, it is easy to cause overvoltage due to interception, oscillation and reignition.

 (2) Docking contact structure and spring energy storage operating mechanism are easy to produce closing bounce and opening bounce. Closing bounce will not only produce high over-voltage and affect the stable operation of the power grid, but also cause contact burnout and even welding, especially when the inrush current is generated in the capacitor bank and the short circuit is closed. Opening bounce will reduce contact spacing and cause heavy breakdown.

(3) High requirements on sealing technology and manufacturing technology, and relatively high prices.