We shall cover the constructional details of a vacuum interrupter along with its function, how it facilitates arc quenching, etc.
Introduction to Vacuum Circuit Breakers
Before entering into the detailed discussion about the vacuum interrupter, let us have a look at the basic concept of vacuum circuit breakers.
A vacuum interrupter, which we also call a VI in short, is the key component of a vacuum circuit breaker or VCB. Actually, in a vacuum circuit breaker or VCB, the arc quenching takes place in a vacuum. No medium like air, oil, or SF₆ gas is used for the purpose. In a vacuum, there is no medium present, hence no ionized particles in the gap between breaker contacts.
Principle of Operation
There is a fixed contact and a moving contact. Suppose the moving contact is moving back, means both contacts get separated. If this separation is for the interruption of current, the atoms between the contacts get ionized due to the high voltage gradient between these opening contacts. Therefore, a channel of ionized particles forms and helps to continue the current even though the contacts are physically separated. This is because, as we know, an ionized medium is a good conductor of electric current.
Obviously, you can think that due to the absence of a medium, there should be no such phenomenon. Hence, the current is interrupted without any arc formation—that means the interruption of current is absolutely instantaneous.
It may be problematic for the system, because a sudden fall in current is nothing but current chopping. You know that the current chopping is dangerous to an electrical system.
But fortunately, vacuum arcing phenomena also occur, albeit in a different manner. In a vacuum, when two contacts are separated, a hot spot is formed at the final point of contact surface separation.
Arc Formation in Vacuum
Let us elaborate on the concept for better understanding. When any two surfaces get detached from their face-to-face contact, the separation is not instantaneous. Rather, the contact surface areas touching each other reduce gradually to a point and then finally get separated. At that last moment of detachment, the entire current tries to flow through that narrowest path. Therefore, a hot spot is easily generated there.
As the surrounding is a vacuum, the metal from the contact surfaces at that hot spot gets vaporized easily. This metal vapor, due to its free electrons, helps to continue the current by forming arc plasma. During the zero-crossing point of the current wave, the current becomes zero, hence the metal vapor cools down and gets condensed. Hence the current gets finally interrupted. Although practically, due to restriking voltage, the arcing may continue for one or two more cycles.
Let us see the waveform. Say, the moving contact gets separated from the fixed contact. The current still continues due to the metal vapor arc, as we mentioned. Here the current becomes zero at the the first zero crossing, and metal vapor gets condensed. For the next half cycle, current will not get any metal vapor to continue because it has already been condensed. As the contacts are already separated, the phenomenon of creating a hot spot, as occurred in the previous cycle, will no longer occur. Although practically, due to restriking voltage, the arcing may continue for one or two more cycles.
Vacuum Interrupter Components and Construction
A Vacuum Interrupter or VI is a vacuum chamber in the vacuum circuit breaker in which the entire phenomenon we just discussed takes place during operation of the circuit breaker. A VI is the loose part of the CB, it can be replaced as and when required by a new one.
A vacuum interrupter has to maintain a vacuum inside,
it has to help the smooth movement of the moving contact while keeping the vacuum intact,
it has to maintain insulation between contacts when the CB is open, and
it has to handle metal vapor during live operations.
Each part of a VI has its own purpose. Let us explain one by one.
Ceramic Insulation Cylinder
This is an insulation cylinder made of hard ceramic. It has two parts. These two parts are fixed in the middle with a stainless steel contact shield. It forms the main container for the VI bottle. Hence, it has to withstand the vacuum, therefore it has to be quite strong. Also, it has to withstand the basic insulation level of the system—meaning it has to withstand power frequency withstand voltage and lightning impulse voltages. So, this part of the vacuum interrupter must also be mechanically strong enough.
Contacts
The fixed contact is held by a metal cover flange, which is hermetically sealed at the joint between the contact and the cover. This cover, typically made of hardened steel, is also attached to one end of the ceramic cylinder, with the joint similarly hermetically sealed.
The fixed contact is composed of a copper–chromium alloy. Although the addition of chromium slightly reduces electrical conductivity, it is essential for controlling the generation and maintaining the quantity of metal vapor during arcing.
The moving contact is made from the same copper–chromium alloy and is attached to a steel bellow. The bellow is hermetically sealed to a steel cover located on the opposite end of the interrupter. This steel cover is also joined to the ceramic cylinder, forming a vacuum-tight enclosure.
All components—including the bellow, contacts, steel covers on both ends, and the ceramic cylinder—are hermetically sealed to ensure the integrity of the vacuum within the interrupter.
Steel Bellow
The bellow, constructed from a thin steel sheet, serves the critical function of enabling the axial movement of the moving contact while maintaining the vacuum seal. To protect the bellow from metal vapor deposition during arcing events, its lower portion is enclosed with a steel cap. This cap acts as a protective barrier, thereby enhancing the durability and reliability of the bellow during the operating life of the vacuum interrupter.
Contact Shield
Next, we consider the contact shield, which is typically fabricated from a non-magnetic material such as stainless steel. During each operation of the circuit breaker, a portion of the metal vapor generated can deposit and condense on the inner wall of the ceramic cylinder. Over time, this deposition degrades the insulating properties of the ceramic surface. To mitigate this issue, a contact shield is employed. It serves as a barrier that captures the metal vapor produced during contact movement, preventing it from reaching the ceramic wall and thereby preserving the dielectric integrity of the vacuum interrupter.
Contact Types
The contacts displayed are basic in design and are referred to as butt-type contacts. However, to ensure an even distribution of the electric arc across the contact surfaces, specialized contact designs have been developed. These include spiral-type, radial slot-type, axial magnetic field-type, radial magnetic field-type, and axial-radial hybrid-type contacts. Such designs facilitate uniform arc distribution, preventing localized arcing, which can cause contact erosion and consequently reduce the operational lifespan of the vacuum interrupter.