The high vacuum has high dielectric strength and the current interruption in high vacuum occurs at the first current zero. These two properties of high vacuum make the vacuum circuit breakers more efficient, less bulky, and cheaper in cost.
We know that during contact separation of the circuit breaker arc is formed due to ionization of particles in the medium between the contacts. Whereas in the vacuum circuit breakers, this medium between the contacts is eliminated. A medium which has a pressure below atmospheric pressure (i.e. 760 mm of Hg) is known as vacuum and the pressures below about 10-5 torr (1 torr = 1mm of Hg) is considered as high vacuum.
The gas pressures from 10-4 to 10-6 torr are used to serve the purpose insulation in vacuum circuit breakers. At such pressures of gas, breakdown voltage becomes independent of pressure.
Vacuum Circuit Breaker Construction
It has a very simple construction in comparison to an air circuit breaker or oil circuit breaker. In a vacuum circuit breaker (VCB) current carrying contacts are separated in the vacuum. The separation of contacts and interruption of arc occur in a vacuum chamber in the VCB which is known as vacuum interrupter.
The outer envelope of vacuum interrupter is normally made of glass because the glass envelope facilitates the examination of breaker from outside. A sputter shield, usually made of stainless steel is placed between the contacts and the envelope in order to prevent the metal vapors reaching the envelop because it reduces the breakdown strength between contacts. Inside the sputter shield, VCB has one fixed and one moving contact. The distance between contacts is kept between 5 to 10 mm depending upon operating voltage.
The metallic bellows made of stainless steel is used for moving the lower contact. Copper-bismuth, copper-lead and copper-chromium are some of the alloys used as contact material.
The lower end of the breaker is fixed to a spring operated or solenoid operated mechanism so that the metallic bellows inside the vacuum interrupter chamber can be moved upward and downward during closing and opening operations respectively.
Vacuum Circuit Breaker Working Principle
It is very necessary to understand the arc of VCB to understand the working principle of 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 vapors 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.
It is very necessary that the current should 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 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 vapor. The contact surface is so designed that 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
- Current interruption takes place at the first current zero after contact separation with no restriking makes it very suitable for capacitor and cable switching.
- Pollution free, non-explosive and silent operation.
- Larger number of operations on load or short circuit.
- Suitable for repeated operating duty.
- Least requirement of maintenance and long life.
Disadvantages of Vacuum Circuit Breakers
- High tech plants are required for production and its production is economical only on large scale.
- Rated voltage of a single interrupter is limited to about 36kv. If we want to use the VCB at more than this voltage then we have to use more vacuum interrupters in series accordingly. Thus it becomes uneconomical for rated voltage more than 36kv.
Applications of Vacuum Circuit Breakers
- Due to the short gap and excellent recovery characteristics, these are very suitable for high speed switching industrial applications.
- For low fault interrupting capacities the cost of VCB is comparatively lower than the other interrupting devices.
- Very suitable and economical for medium voltage range. These are generally used at the voltage levels up to 72 kV. Above this voltage, it becomes very expensive to use them.
- Due to least requirements of maintenance, these breakers are very suitable for rural areas.
Thanks for reading about vacuum circuit breaker working principle and construction.