Vacuum interrupter technology was first introduced in the year of 1960. A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in a vacuum. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called a vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the center symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10- 6 bar. Every medium that has a pressure below atmospheric (760 mm of Hg) is known as a vacuum.
Introduction to Vacuum Circuit Breaker
The material used for current-carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. The vacuum circuit breaker is today recognized as the most reliable current interruption technology for medium voltage switchgear. For higher voltage vacuum technology has been developed but not commercially viable. It requires minimum maintenance compared to other circuit
breaker technologies. Low pressure is measured in torr, where,
1 torr = 1 mm of Hg. = 0.001333 bar = 0.001315 atm = 0.001359 kg/sq cm = 133.322 pascal
High vacuum properties
The high vacuum has two outstanding properties:
- Vacuum is the superior dielectric medium in comparison to other insulating media. But air and SF6 gas are generally employed at high pressure.
- When an ac circuit is opened by separating the contacts in a vacuum,interruption occurs at the first current Zero with the dielectric strength across the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers.
The breakdown voltage of vacuum compared to air for one pair of 9.4 mm diameter tungsten are as follows;
The above properties make the vacuum circuit breakers more efficient, less bulky, and cheaper in cost.
Construction of Vacuum Circuit Breaker
Vacuum circuit breakers are simple in construction compared to air or oil circuit breakers. It consists of
– Outer envelope
– End cap
– Fixed contact
– Moving contact
– Contact tip
– Sputter shield
The outer envelope is normally made of glass and metallic end caps are connected to it. A sputter shield, usually made of steel, is placed between the contacts and the envelope in order to prevent the metal vapor from reaching the envelope. Inside the sputter shield the breaker has two contacts
– fixed contact
– moving contact.
The moving contact moves through a short distance of 5 to 10 mm depending upon the operating voltage. The metallic bellows made of stainless steel are used to move the lower contact. The design of the below is very important because the life of the vacuum breaker depends upon the ability of this component to perform repeated operations satisfactorily.
The periphery of the end cap is sealed to the envelope and the fixed contact stem is an integral
part of the end cap. One end of the fixed as well as moving contact is brought out of the chamber for external connection.
A newer technology is now being used in the vacuum interrupter. It involves using spiral-shaped copper-chrome contacts inside the vacuum tube. They provide a self-induced magnetic effect that moves the arc root around the contact periphery. This very efficient arc control method prevents hot spots, minimizing contact erosion.
Operating of vacuum circuit breaker
The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within a very small contact gap. For short contact gap, low contact mass, and no compression of medium the drive energy required in a vacuum circuit breaker is
When two face-to-face contact areas are just being separated from each other, they do not be separated instantly, the contact area on the contact face is reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of a microsecond it is a fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts is concentrated on that last contact point on the contact surface and makes a hot spot.
As it is a vacuum, the metal on the contact surface is easily vaporized due to that hot spot and creates a conducting media for the arc path. Then the arc will be initiated and continued until the next current zero. At current zero this vacuum arc is extinguished and the conducting metal vapor is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of the contact surface, for the next cycle of current. That means, the arc cannot be reestablished again.
In this way, the vacuum circuit breaker prevents the re-establishment of the arc by producing high dielectric strength in the contact gap after the current is zero.
The lower end of the breaker is fixed to an operating mechanism so that the metallic bellows inside the chamber are moved upward or downward during closing and opening operations.
Advantages of vacuum circuit breaker
They do not need an auxiliary air system or oil handling system.
• Rapid recovery of very high dielectric strength.
• Current interruption occurs at the first current zero after contact separation with no re-striking
• Very high power frequency and impulse withstand voltages with small contact spacing.
• Pollution free
• Compact construction, less space required
• Suitable for repeated operating duty, long life.
• Usable on any voltages up to 230 kV.
• Constant contact resistance, no oxidation
Disadvantages of vacuum circuit breaker
• Requirement of high technology for the production of vacuum interrupters.
• Costly. Uneconomical to produce in a small volume.
• Rated voltage of a single interrupter is limited to about 36 / √3 i.e. 20 kV. Above 36 kV two interrupters are required to be connected in series. Thus, above 36 kV, these breakers become uneconomical.
• Entire interrupter needs to change in case of interrupter failure.
Application of vacuum circuit breaker
Vacuum breakers can be used for all duties which include the following:
– distribution networks
– Switching of long transmission lines connected to very important load areas which require extremely fast and multi-shot auto reclosure.
– Switching of transformers. The magnetizing current values must however be carefully checked.
– Switching of motors.
– Switching of capacitors
– Shunt reactor switching
– Very suitable in rural areas