SF6 Circuit Breaker: Working Principle, Types, Applications, Technical Issues

SF6 circuit breakers are widely used in power transmission and distribution systems to protect electrical equipment from overcurrents and short circuits. The remarkable properties of sulfur hexafluoride (SF6) gas make it an ideal arc-quenching medium for high-voltage circuit breakers.

In this article, we will delve into the working principle, applications, technical problems, and solutions of SF6 circuit breakers. The full form of SF6 in SF6 circuit breakers is “Sulfur Hexafluoride.”

What are SF6 Circuit Breakers?

SF6 circuit breakers are a type of circuit breaker that use sulfur hexafluoride gas as the arc extinguishing medium. They are commonly used in high-voltage electrical systems to protect electrical equipment and power transmission lines from short circuits and overloads.

SF6 circuit breakers are known for their high reliability, fast interruption capability, and compact design, making them suitable for various applications in the electrical industry.

Working Principle of SF6 Circuit Breaker

SF6 circuit breakers work on the principle of quenching the arc formed when the contacts of the circuit breaker separate. The SF6 gas is used as an arc-quenching medium due to its high dielectric strength and excellent thermal stability. When the circuit breaker contacts open, a high-voltage electric arc is generated, and SF6 gas is released to cool and extinguish the arc. The rapid quenching ability of SF6 gas ensures safe interruption of the current flow.

SF6 Circuit Breaker Diagram

The basic construction of an SF6 circuit breaker consists of fixed and moving contacts enclosed in a gas-tight chamber filled with SF6 gas. The contacts are connected to the control mechanism and operate based on the electrical signals received. Upon receiving a trip signal, the contacts open, and the SF6 gas is released to extinguish the arc.

SF6 Circuit Breaker Construction

The SF6 circuit breakers are an essential component in electrical power systems, and they are designed with two main parts: the Interrupter Unit and the Gas System.

Interrupter Unit:

The Interrupter Unit is a critical part of the SF6 circuit breaker and is responsible for interrupting the current flow when a fault occurs. It consists of two types of current-carrying contacts: the fixed contact and the movable contact. The fixed contacts, as the name suggests, remain stationary, while the movable contact moves back and forth using an arm actuated by a mechanism.

During normal operation, the fixed and movable contacts are in contact with each other, allowing the current to pass through the circuit breaker. However, when a fault is detected, the mechanism triggers the arm, moving the movable contact away from the fixed contact, and thus interrupting the current flow.

To ensure effective interruption of the current, the SF6 gas is utilized in the interrupter unit. When the contacts open, an arc is formed between them due to the ionization of the gas. This arc generates a significant amount of heat and ionized gases. To cool and extinguish the arc, there are vents in the interrupter unit that allow the pressurized SF6 gas to flow in and out.

Gas System:

The SF6 gas used in the circuit breaker is not only expensive but also harmful to the environment when released into the atmosphere. To address this concern, a closed gas system is employed in SF6 circuit breakers. In this system, the used SF6 gas is collected, recombined, and recycled for reuse, minimizing the emission of SF6 product gases.

Maintaining the appropriate pressure of the SF6 gas is crucial for the circuit breaker’s efficient operation. The dielectric strength of the SF6 gas significantly depends on its pressure. Hence, the gas system is designed to maintain the correct pressure, ensuring optimal performance and safety.

By employing a closed gas system, SF6 circuit breakers not only reduce environmental impact but also demonstrate their sustainability by recycling and reusing the SF6 gas.

SF6 circuit breakers consist of an Interrupter Unit responsible for interrupting the current flow and a Gas System designed to control the SF6 gas used for arc quenching. The combination of these two parts ensures the reliable and efficient operation of SF6 circuit breakers, making them a vital component in modern electrical power systems.

Current Rating of SF6 Circuit Breaker

SF6 circuit breakers are available in various current ratings, ranging from a few hundred amperes to several thousand amperes. The selection of the appropriate current rating depends on the specific application and the maximum fault currents that the circuit breaker is expected to interrupt.

Applications of SF6 Circuit Breakers

SF6 circuit breakers find widespread applications in power transmission and distribution systems, including substations, power plants, and industrial facilities. Their ability to interrupt high fault currents and withstand high voltages makes them ideal for protecting electrical equipment and ensuring grid stability.

SF6 Circuit Breaker Technical Problems and Its Solutions in Field

SF6 circuit breakers, while highly reliable and widely used in power systems, can encounter certain problems in the field. Here are some common issues and their corresponding solutions:

1. Gas Leakage:

Problem: SF6 gas leakage from the circuit breaker’s gas chamber can reduce its dielectric strength and interrupting capability, compromising its performance and safety.

Solution:

• Regular Gas Leak Detection: Implement a routine gas leak detection program using specialized equipment to identify and locate any gas leaks promptly.
• Tightening of Connections: Regularly inspect and tighten the connections to ensure proper sealing and prevent gas leakage.
• Prompt Repairs: In case of gas leakage, conduct immediate repairs or replace damaged components to maintain the integrity of the circuit breaker.

2. Overheating of Contacts:

Problem: Overheating of the contacts due to excessive current or poor alignment can lead to contact erosion and failure.

Solution:

• Regular Inspection: Periodically inspect the contacts for signs of overheating, such as discoloration or deformation.
• Correct Current Ratings: Ensure that the circuit breaker’s current rating matches the system requirements to avoid overheating due to excessive current.
• Proper Contact Alignment: Align the contacts correctly to minimize contact resistance and heat generation.

3. Arcing:

Problem: Arcing during the opening and closing of contacts can cause erosion and damage to the contact surfaces.

Solution:

• Regular Cleaning: Regularly clean the contacts to remove any accumulated dust or contaminants that could contribute to arcing.
• Low-Chopping Contacts: Use low-chopping contacts that are specifically designed to minimize the effects of arcing during operation.

4. Mechanical Failures:

Problem: Mechanical failures in the operating mechanism can lead to circuit breaker malfunctions.

Solution:

• Lubrication: Ensure that all moving parts are adequately lubricated to reduce friction and wear.
• Regular Maintenance: Implement a comprehensive maintenance program to identify and address potential mechanical issues proactively.
• Timely Replacement: Replace worn-out or damaged components promptly to prevent mechanical failures.

5. Dielectric Strength Degradation:

Problem: Over time, the dielectric strength of SF6 gas can degrade, reducing the circuit breaker’s performance.

Solution:

• Periodic Testing: Conduct regular dielectric strength tests on the SF6 gas to monitor its condition and identify any degradation.
• Gas Replacement: If the dielectric strength is below acceptable levels, consider replacing the SF6 gas to restore the circuit breaker’s performance.

By implementing a robust maintenance program and promptly addressing any issues that arise, SF6 circuit breakers can continue to deliver reliable performance in power systems. Regular gas leak detection, proper lubrication, and timely repairs are vital for extending the service life of SF6 circuit breakers and ensuring their continued efficiency and safety in the field.

Types of SF6 Circuit Breaker:

Following are the types of SF6 Circuit Breaker:

1. Non-Puffer Type SF6 Circuit Breaker:
   In a non-puffer type SF6 circuit breaker, the SF6 gas is used for insulation purposes only and does not play a role in arc quenching. Instead, the arc quenching is achieved by using vacuum interrupters. The contacts in the interrupter unit are opened in a high vacuum, which creates a high dielectric strength and allows for effective arc interruption. These circuit breakers are compact, require low maintenance, and are suitable for medium voltage applications.
2. Single Pressure Puffer Type SF6 Circuit Breaker:
   In a single pressure puffer type SF6 circuit breaker, the SF6 gas is compressed by the moving piston during the opening of the contacts. As the contacts start to open, the piston compresses the SF6 gas and generates a pressure wave that creates turbulence in the gas. This turbulence helps in breaking the arc by stretching it and extinguishing it. The pressure inside the circuit breaker remains constant during both opening and closing operations. These circuit breakers are commonly used for medium and high voltage applications.
3. Double Pressure Puffer Type SF6 Circuit Breaker:
   In a double pressure puffer type SF6 circuit breaker, the SF6 gas is compressed by the moving piston during the opening of the contacts, similar to the single pressure puffer type. However, in this type, the pressure inside the circuit breaker is varied during the opening and closing operations. During opening, the pressure is increased to enhance arc quenching, while during closing, the pressure is reduced to minimize the compression of gas. This variation in pressure improves the performance and reduces the mechanical stresses on the circuit breaker. Double pressure puffer type circuit breakers are commonly used for high voltage applications.

Each type of SF6 circuit breaker has its own advantages and is suitable for specific voltage levels and applications. The choice of the circuit breaker type depends on factors such as the voltage rating of the system, the operating conditions, and the required performance characteristics.

List of spare parts of SF6 Circuit Breaker

Replacement can only be carried out by trained personnel and/or in our workshops:

1. Opening spring.
2. Closing spring.
3. Complete pole.
4. Basic operating mechanism.
5. Spring charging geared motor.
6. Operating mechanism auxiliary contacts.
7. Additional transmitted contacts.
8. Under-voltage release.

Replacement/use possible directly by the customer:

1. Geared motor limit switch contact.
2. K63 instantaneous relay.
3. K163 instantaneous relay.
4. Anti-condensation heater.
5. Shunt opening release.
6. Shunt closing release.
7. Thermostat.
8. SF6 gas refilling kit.
9. SF6 gas cylinder.

SF6 circuit breakers play a vital role in ensuring the reliability and safety of power transmission and distribution systems. Their unique properties, such as high dielectric strength and arc-quenching capability, make them indispensable in various applications. Regular maintenance and troubleshooting are essential to address technical issues promptly and ensure optimal performance. With ongoing advancements in circuit breaker technology, SF6 circuit breakers continue to evolve, providing more efficient and reliable solutions for electrical power systems worldwide.

Properties of SF6 Circuit Breaker

Sulphur hexafluoride (SF6) circuit breakers possess several remarkable properties that make them highly effective for electrical applications. These properties are:

1. Safety: SF6 gas is colourless, odourless, non-toxic, and non-inflammable, making it a safe choice for use in electrical equipment.
2. High Density: SF6 gas is much denser than air, providing excellent insulation and arc quenching capabilities.
3. Effective Cooling: With its high thermal conductivity, SF6 gas helps in efficiently cooling the current-carrying parts of the circuit breaker.
4. Electronegativity: SF6 gas is strongly electronegative, meaning it readily captures free electrons, creating negative ions that aid in the interruption of electrical discharges.
5. Fast Recombination: After an electrical spark is extinguished, SF6 gas rapidly recombines, making it 100 times more effective as an arc quenching medium compared to other gases.
6. High Dielectric Strength: SF6 gas exhibits 2.5 times greater dielectric strength than air, allowing for effective insulation and voltage withstand.
7. Pressure-Dependent Strength: The dielectric strength of SF6 gas increases with higher pressure levels, enhancing its performance under varying operating conditions.

However, SF6 circuit breakers are susceptible to the presence of moisture. When SF6 gas combines with humidity, hydrogen fluoride can be formed during the interruption of an arc. This can lead to the corrosion of circuit breaker components, potentially affecting the overall performance and reliability of the equipment. Therefore, proper measures must be taken to prevent moisture ingress and ensure the long-term effectiveness of SF6 circuit breakers.

SF6 Circuit Breaker Advantages & Disadvantages:

Sure, here’s a table summarizing the advantages and disadvantages of SF6 circuit breakers:

Advantages	Disadvantages
1. High dielectric strength.	1. High global warming potential (GWP).
2. Excellent arc quenching capability.	2. Expensive and costly to produce.
3. Compact and lightweight design.	3. Release of harmful SF6 gases into the atmosphere.
4. Low maintenance requirements.	4. Special handling and disposal of SF6 gas.
5. Long service life.	5. Risk of SF6 gas leakage.
6. Suitable for high voltage applications.	6. Limited availability in some regions.
7. Fast and reliable operation.	7. Potential health hazards if exposed to SF6.
8. Wide range of applications.

It’s important to note that while SF6 circuit breakers offer numerous advantages, the environmental concerns and potential health hazards associated with SF6 gas usage have led to increasing efforts to find alternative solutions and promote more sustainable practices in the power industry.

Source: This article is referenced from international standard of IEC 62271-100.

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