Vacuum Circuit Breaker (VCB) – Construction and Working

VCB – Vacuum Circuit Breaker and Vacuum Interrupters

Circuit breakers play a crucial role in protecting electrical systems from damage caused by overcurrents and short circuits. Among the various types of circuit breakers available, the vacuum circuit breaker (VCB) stands out for its reliability, efficiency, and performance. This article delves into the working principles, construction, different types, advantages, and applications of vacuum circuit breakers.

What is a Vacuum Circuit Breaker “VCB”?

A vacuum circuit breaker (VCB) is a type of circuit breaker that uses a vacuum as the arc quenching medium to interrupt the flow of electrical current in a circuit.

Vacuum is a superior dielectric and the best medium for arc extinction in circuit breakers. For medium voltage (e.g., up to 38 kV), the vacuum circuit breaker is considered the closest to an ideal circuit breaker. The illustration on the right shows the formation of an arc when the contacts of the VCB open.

When the current-carrying contacts are separated in a vacuum, an arc is formed between them. Hot spots are formed intensely during the separation of the contacts, from which metal vapors rapidly emerge, leading to the establishment of plasma. The amount of vapor in the plasma is suitable for the rate of emission of vapor from the electrodes, i.e., the amount of arc discharge depends on the amount of interruption current.

The arc extinguishes at the zero position of the AC wave with the next current. Immediately after the nipple current becomes zero, metal vapors condense. Dielectric strength builds up rapidly, and the arc re-striking stops. This principle is used in vacuum circuit breakers. In a vacuum circuit breaker, two contacts (a fixed piece and a moving one) are placed inside an insulating envelope.

The air inside the insulating envelope is completely removed, and the envelope is sealed. This envelope is an important part of the VCB, and it becomes a vacuum during operation. The movement of the moving contacts inside the interrupter unit is facilitated by metal bellows.

One human benefit of a vacuum circuit breaker is that during separation, the moving contact has to cover a very short distance (a few millimeters). Stopping the current is very easy and fast. Vacuum circuit breakers typically have one or more cylinders per pole consisting of interrupters.

For voltages up to 36 kV, single interrupter VCBs are usually used, and for voltages of 725 kV and above, multi-unit VCBs are made. In addition, a commercial manufacturer of 36 kV circuit breakers has developed a breaker that uses SF gas insulation and vacuum interrupters for arc extinction. This joint operation will greatly help in making an ideal circuit breaker.

As a result, such circuit breakers are being manufactured in the United States and Japan. Vacuum circuit breakers have several advantages, such as rapid and silent operation, rapid restoration of dielectric strength after operation, suitable for repeated operations, simple operating mechanism, safe from fire, and designed to be compact, light, and easy to maintain.

High vacuum has the following two characteristics:

• Highest Insulating Strength: It offers very high insulating strength, making it suitable for interrupting currents.
• Instantaneous Interruption: When an AC circuit opens due to the separation of the contacts in a vacuum, interruption occurs at the first zero current, at which point the dielectric strength between the contacts is restored at a rate thousands of times higher than that of a conventional circuit breaker.

These characteristics of vacuum circuit breakers make them more efficient, less bulky, and more reliable than conventional devices. Their service life is also much higher than that of traditional tools, and they require minimal maintenance. Field experience has proven that vacuum equipment has unparalleled performance and reliable properties.

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Construction of VCB

The construction of a vacuum circuit breaker typically includes the following components:

• Upper Support and Heat Sink
• Terminal of Pole (Copper)
• Insulator Support Rod
• Fixed Contact
• Moving Contact
• Insulator
• Metallic Bellows
• Lower Terminal (Copper)
• Lower Heat Sink
• Mechanism Linkage
• Glass Fiber Rod for the Breaker Pole
• Opening Spring

Each pole of a Vacuum Circuit Breaker is supported on insulators in the mechanism chamber with two cross arms mounted on the post insulators, forming a sturdy frame. The frame protects the housing against switching forces and electrodynamic forces.

Moving contacts are connected to the breaker terminals with shiny methods. A special three-way cross-sectional view of a particular vacuum circuit breaker is shown in fig below.

The main parts inside a typical VCB each playing a vital role in its operation are as follows:

1. Vacuum Interrupter: The vacuum interrupter is the core component of a VCB. It consists of a fixed contact and a moving contact enclosed in a vacuum-sealed ceramic or glass envelope. When the circuit breaker is closed, the contacts are in contact, allowing current to flow. When the circuit breaker is tripped, the contacts separate, creating an arc. The vacuum interrupts the arc, extinguishing it quickly and efficiently.
2. Operating Mechanism: The operating mechanism controls the opening and closing of the circuit breaker contacts. It can be manual, spring-operated, or motor-operated, depending on the application. The operating mechanism must provide sufficient force to separate the contacts and maintain them in the open position.
3. Insulating Enclosure: The vacuum interrupter is housed in a sturdy insulating enclosure that protects it from environmental factors and maintains the vacuum integrity. The enclosure is typically made of high-quality insulating materials such as ceramic or glass.
4. Arc Chutes: Arc chutes are used to facilitate the interruption of the arc and to cool and deionize the arc gases. They are typically made of metal plates or fins that are arranged to create a path for the arc to follow, allowing it to extinguish quickly.
5. Control and Protection System: Modern VCBs are equipped with advanced control and protection systems that monitor the electrical parameters and actuate the circuit breaker when abnormal conditions are detected. These systems help to ensure the safe and reliable operation of the circuit breaker.
6. Auxiliary Contacts: Auxiliary contacts are used to provide feedback to the control system and to indicate the status of the circuit breaker (open or closed). They are also used for remote operation and monitoring of the circuit breaker.

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Construction of Vacuum Interrupters

The vacuum interrupter is the core component of a Vacuum Circuit Breaker where the process of arc quenching and interruption take place in  the pole assembly . It consists of a fixed contact and a moving contact enclosed in a vacuum-sealed ceramic or glass envelope. The construction of a vacuum interrupter is as follows:

1. Fixed Contact: The fixed contact is typically made of copper or a copper alloy and is mounted inside the vacuum interrupter. It is designed to withstand high currents without overheating and maintains contact with the moving contact during normal operation.
2. Moving Contact: The moving contact is also made of a copper alloy and is connected to the operating mechanism of the circuit breaker. During normal operation, the moving contact is in contact with the fixed contact, allowing current to flow through the circuit breaker.
3. Vacuum Enclosure: The fixed and moving contacts are housed inside a vacuum-sealed enclosure made of ceramic or glass. The vacuum inside the enclosure is crucial for the interruption of the arc. It prevents the arc from re-striking after current zero, ensuring reliable interruption of fault currents.
4. Insulating Envelope: The vacuum enclosure is typically made of high-quality non-porous insulating (i.e. ceramic or glass) material that can withstand the high temperatures generated during arc interruption. The envelope also provides electrical insulation between the contacts.
5. Stem Shields or Metallic Bellows: Stainless steel bellows are used to allow for the movement of contacts inside the VCB.
6. Vapor Condensing Shields: As metal vapors are released from the surface of contacts during arcing, it is important to use metal shields to prevent them from freezing on the insulating enclosure inside the interrupter.
7. Shielding Structure: To enhance the arc quenching capabilities of the vacuum interrupter, a shielding structure is often used. This structure helps to control the movement of the arc, ensuring that it is quickly extinguished.
8. End Flange or End Shields: The end flange enclosure, made of non-magnetic material, supports the protective core of the fixed contact, vapor-condensing shields, and bellows.
9. Contacts Stem & Material: The prior face of contact or the rod attached to the moving contact is known as stem. The contacts of a vacuum interrupter are made of materials that can withstand high temperatures and mechanical stress. Copper and copper alloys are commonly used due to their excellent conductivity and mechanical properties.

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Working Principle of Vacuum Circuit Breaker

The fundamental principle behind a Vacuum Circuit Breaker is the use of a vacuum as the arc quenching medium. When an overcurrent or short circuit occurs, the circuit breaker’s contacts are forced to open, creating an arc due to the current flow. In a VCB, this arc is quickly extinguished as it enters a vacuum environment.

The vacuum offers several advantages for arc interruption. It provides a clean and stable medium for the arc, preventing re-ignition after current zero. Additionally, the absence of a gas medium eliminates the risk of gas-related issues such as decomposition or ionization, ensuring reliable performance.

The Operating Mechanism of VCB

In a vacuum circuit breaker,

1. No human medium is required to extinguish the arc.
2. The weight of the moving contact is very low.
3. The contact has to cover a very short distance during opening.
4. The switching velocity is low.

All these reasons require very little energy for the operating mechanism of the VCB. This energy is immediately available from the relay’s interlock signal. These requirements for the operating mechanism are provided through in-built motion action or pre-loaded operating springs.

With the circuit breaker, pre-existing latches, nozzles, contact interlocking arrangements, and protective devices are provided to keep the contacts in the closed position. The operation single break type. The operating mechanism is simple and does not require any special maintenance other than normal.

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Types of Vacuum Circuit Breakers (VCBs)

Vacuum circuit breakers (VCBs) are categorized based on their application, construction, and operating mechanism. Here are the main types:

• Tank Type Vacuum Circuit Breaker (SF6 Gas Insulation):

This type of VCB utilizes a tank filled with sulfur hexafluoride (SF6) gas as the insulating and arc-quenching medium. It is commonly used in high-voltage applications due to its excellent insulating properties and high interrupting capability.

• Dry Air Insulated Dead Tank Vacuum Circuit Breaker:

Unlike SF6 gas-insulated VCBs, this type uses dry air as the insulation medium. It offers environmental benefits by eliminating the use of SF6 gas, which has a high global warming potential. These breakers are suitable for medium to high-voltage applications.

• Insulator Type Vacuum Circuit Breaker:

In this design, the vacuum interrupter is enclosed within an insulating material, such as porcelain or resin. This construction provides both electrical insulation and mechanical support, making it suitable for medium-voltage applications where reliability is crucial.

• Insulator Type 2-pole Vacuum Circuit Breakers:

These VCBs feature a two-pole design, where each pole is enclosed within an insulating material. This configuration offers enhanced insulation and mechanical strength, making it suitable for medium-voltage applications requiring a higher level of reliability and safety.

• Outdoor Vacuum Circuit Breakers (OVCB):

Designed for outdoor installations, these VCBs offer weatherproof and robust construction, ensuring reliable operation in harsh environments.

• Indoor Vacuum Circuit Breakers (IVCB):

These VCBs are specifically designed for indoor applications, offering compact designs and efficient operation within controlled environments.

• Medium Voltage Vacuum Circuit Breakers:

Typically used in medium-voltage applications, these VCBs offer reliable performance and are suitable for a wide range of industrial and commercial applications.

• High Voltage Vacuum Circuit Breakers:

These VCBs are designed for high-voltage applications, providing excellent arc extinguishing capabilities and reliable performance in critical power systems.

• Fixed Vacuum Circuit Breakers:

These VCBs are permanently installed and offer a fixed configuration for specific applications, ensuring reliable operation and minimal maintenance requirements.

• Draw-out Vacuum Circuit Breakers:

Featuring a draw-out design, these VCBs offer easy maintenance and replacement capabilities, making them suitable for applications where downtime needs to be minimized.

• Vacuum Circuit Breakers with Embedded Poles:

These VCBs feature embedded poles, offering compact designs and efficient operation in space-constrained applications.

• Modular Vacuum Circuit Breakers:

These VCBs feature a modular design, allowing for easy expansion and customization based on specific application requirements.

• Generator Vacuum Circuit Breakers:

Specifically designed for generator applications, these VCBs offer reliable performance and protection for generator circuits.

• Railway Vacuum Circuit Breakers:

These VCBs are designed for railway applications, offering reliable performance and protection for railway signaling and traction systems.

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Advantages & Disadvantages

Advantages

The use of Vacuum Circuit Breakers offers several advantages over other types of circuit breakers:

• VCBs are known for their high reliability, with fast and efficient arc interruption capabilities.
• Due to their simple and robust design, VCBs require minimal maintenance, resulting in cost savings and increased uptime.
• As they do not use environmentally harmful gases, VCBs are considered more environmentally friendly.
• VCBs are typically more compact than other types of circuit breakers, making them suitable for applications where space is limited.
• With proper maintenance, VCBs can have a long service life, providing reliable protection for electrical systems.
• After the arc is extinguished, the contact piece rapidly deionizes, reducing the chances of the arc re-striking due to quickly restoring the dielectric strength.
• The weight of the moving parts is reduced, and the contacts need to cover less distance.
• They are cost-effective and perform better.
• There is no effect of gas composition during arcing, so they are clean from pollution.
• There is no need to fill gas or oil, and no need for any kind of auxiliary system.
• There is no risk of explosion in the enclosure.
• They are suitable for multiple operations on short circuit or load. Therefore, they are suitable for repeated operations.
• Circuit breakers create minimum disturbances and have a long service life.
• They support fast and silent operations.
• They require minimal maintenance.
• They occupy less space and can be installed at any desired location.
• There is no risk of contact oxidation in a vacuum.
• The contact resistance does not change during the working life of the VCB.
• They are suitable for copper welding cable switching and industrial load switching.
• They also have a beautiful appearance.

Disadvantages:

• Compared to other types of breakers, they are more expensive in terms of the initial cost, and manufacturing vacuum interrupters in small quantities is expensive.
• The use of single interrupters is limited to about 20. For series-connected above 36k, two interrupters are required. Thus, the breaker becomes unsuitable and expensive for voltages above 36kv.
• High technology is required for the production of vacuum interrupters.
• In case of failure or breakdown of the breaker, the entire interrupter is rendered useless and cannot be repaired, so it cannot be fixed again.
• For the interruption of multiple ranges of low magnitude or zero crossings of VCB, additional suppressors are required parallel to each pole of the VCB.

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Applications

Vacuum Circuit Breakers find widespread applications in various industries and sectors, including:

1. Power Generation and Transmission: VCBs are commonly used in power generation plants and substations to protect electrical equipment and systems from overcurrents and short circuits.
2. Industrial Applications: They are used in industrial facilities for the protection of motors, transformers, and other electrical equipment.
3. Railway Systems: VCBs are used in railway systems for the protection of overhead lines, traction systems, and signaling equipment.
4. Commercial Buildings: They are used in commercial buildings for the protection of electrical distribution systems.

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