SF6 Circuit Breaker: Working Principle, Construction, and Types

SF6 circuit breakers are employed for their 

• High interrupt capacity, 
• Compact design, and 
• Reliable performance, 

all of which are important factors in modern electrical power systems, particularly in high voltage applications. 

Additionally, SF6 gas is a potent greenhouse gas, and efforts are being made to reduce its use and improve its environmental impact. 

In this post, we will go over the SF6 Circuit Breaker & its operation in extensive detail.

What is an SF6 Circuit Breaker?

SF6 – Sulphur Hexafluoride

The SF6 circuit breaker, also known as Sulphur Hexafluoride, is one of a type of circuit breaker (CB) that prevents an arc using pressurized SF6 gas. 

SF6 is a dielectric gas with features such as arc quenching and high insulation. 

It is mostly utilized to quench the arc within the high voltage CBs in the electrical grids & power plants up to 800 kV.

The Sf6 circuit breaker operating time varies according on the manufacturer, however the normal time is between 18 and 20 seconds. 

SF6 circuit breaker has a voltage range of 800 kV in the power grid (electrical grids) & 35 kV in the distribution systems. 

• When an arc forms across a circuit breaker’s contacts, it utilize free electrons & converts them into negative ions. 
• When compared to electrons, they are quite heavier. 

Because of their substantial weight, the ions’ mobility might be reduced. 

The charge’s mobility is reduced in SF6 gas, increasing the medium’s dielectric power since charge movement is responsible for current flow.

Working Principle of SF6 Circuit Breaker

In normal operation, the breaker’s contacts are closed. When a failure develops in electric system, the contacts get pulled apart, creating an arc between them. 

The movable contacts are synced to a valve that enters the arc-interrupting chamber with 16kg/cm² SF6 gas.

The SF6 gas absorbs free electrons in arc path & produces ions that do not behave as charge carriers. These ions boost the gas’s dielectric strength, extinguishing the arc. 

This method reduces the pressure of SF6 gas up to 3kg/cm² and stores it in the low-pressure reservoir. This low-pressure gas is redirected back to high-pressure reservoir for reuse.

Nowadays, puffer piston pressure is employed to generate arc quenching pressure over an opening operation using a piston attached to moving contacts.

Construction of SF6 Circuit Breaker

SF6 circuit breakers are made up of two parts: 

1. Interrupter unit and 
2. Gas system.

1). Interrupter Unit 

The interrupter unit is made up of moving and stationary contacts, a set of current-carrying elements, and an arcing probe. 

It’s connected to SF6 gas reservoir. This unit includes sliding vents in the moving contacts that let high-pressure gas into main tank.

2). Gas System

SF6 circuit breaker utilize a closed circuit gas system. SF6 gas is expensive, hence it is recycled after each operation. 

This device consists of low and high-pressure chambers, a low-pressure alarm, and warning switches. 

When gas pressure is low, dielectric strength diminishes, and arc quenching capacity of the breakers is threatened, this mechanism produces a warning alarm.

What is Arc Quenching in Circuit Breaker?

What is Arc?

In a circuit breaker, when the current-carrying contacts are opened, the medium in the middle of the contacts becomes extremely ionized, allowing the interrupting current to flow through a less resistive lane even when the current-carrying contacts are physically separated. When current runs from a contact to another, the conduit becomes so hot that it burns. This is referred to as an arc.

What is Arc Quenching?

Arc quenching is also known as Arc Interruption (or) Arc Extinction, and it occurs when the arc and metal transfer halt. 

If you want to prevent high voltages, you must limit the amount of current flowing at the time the contact is opened in order to reduce metal transfer. So decreasing the current flow will extend the life of the contacts.

Types of SF6 Circuit Breaker

SF6 circuit breakers are categorized into three categories.

1). Non-Puffer Piston Circuit Breaker

2). Single- Puffer Piston Circuit Breaker

3). Double-Puffer Piston Circuit Breaker

1). Non-Puffer Piston Circuit Breaker

First invented circuit breaker (CB) is Non puffer type SF6 without puffer cylinder. This circuit breaker works like air blast CB.

Arc quenching occurs in the interrupter section of non-puffer sf6 circuit breakers.

It has two contacts:

• Fixed contact and 
• Movable contact. 

Both contacts are the hollow cylinders.

Arc horns on the fixed contact. Arc horns prevent flashover component switching.

Rectangular gas ports on the moving contact. After arc extinction, gas leaves these rectangular holes.

Copper-tungsten arc-resistant substance coats arc horn tips and contacts.

The major arcing process occurs in the arc chamber. A valve connects the arc chamber to an SF6 gas reservoir.

Valve movement matches circuit breaker contact. If the moving contact separates from fixed contact when a failure, the SF6 gas tank valve will open and gas will enter the arc chamber.

In the starting point, the connections are closed and surrounded by SF6 at 2.8 kg/cm². 

System faults cause the moving contact to detach from fixed contact.

The arc interruption theory states that the medium ionizes and the contacts strike an arc.

The valve connecting to the SF6 gas tank will open, releasing pressurized gas into the arcing chamber at 14 kg/cm².

As previously stated, SF6 gas will pass across the arc and quench it quickly.

The gas leaves the gas outlets once the arc and current stop, and it is recombined and reconditioned using appropriate processes.

2). Single- Puffer Piston Circuit Breaker

The changeable hollow puffer cylinder dominates this SF6 CB. The stable connections are bridged by this cylinder. 

To make and break contact between the two stable contacts, it slides both axially upward and downward.

This CB compresses SF6 gas through a rotating cylinder and releases it through a nozzle while extinguishing the arc. This circuit breaker is used at voltages from 13.6 kV to 760 kV.

Puffer cylinders are placed. The puffer cylinder has a fixed piston which is between the mobile cylinder and fixed piston is SF6 gas.

Gas starts at normal pressure. Gas passes through two nozzles during circuit breaker activation.

A malfunction in the system causes the moving contact to move away from fixed contact, creating an arc.

The contact and movable cylinder move together. The fixed piston compresses SF6 gas as it moves.

Gas pressure will open the movable cylinder nozzle eventually. SF6 gas will travel through the arc from these nozzles.

As we explained before, SF6 gas arc quenching applies here. Gas flow reduces arc cross-section and cools it. This increases dielectric strength and ends the arc.

3). Double-Puffer Piston Circuit Breaker

For arc quenching, double pressure puffer SF6 circuit breakers use compressed SF6 gas in cylinders. 

A nozzle releases SF6 gas from high-pressure system to low-pressure system during arc extinction in this CB. The design is old and no longer used.

In this SF6 circuit breaker, the 2 nozzles cover the current-carrying contacts. Because of their design, these nozzles accelerate gas flow.

Both sides have differing pressures of SF6 gas. When pressure P1 exceeds pressure P2, gas flows from P1 to P2.

When a fault occurs, the contacts separate & an arc is struck, so gas flows via the arc and the nozzles speed up the flow.

As mentioned, Sulphur hexafluoride gas extinguishes arcs.

Gas passing across the arc reduces its cross section, and resistance as it has inversely proportional to it. Arc resistance will increase & current will cease to flow.

Properties of SF6 Gas Circuit Breaker

The following physical chemical and electrical properties make SF6 a better arc quenching medium.

Physical Properties

• Gas is nonflammable.
• Gas is odorless and colorless.
• Very good heat conductivity.
• High density & heavier than air.
• It liquefies at pressure-dependent low temperatures.

Chemical Properties

• Gas SF6 is stable & inert.
• Its products are harmful but its pure form is not.
• Its high electronegativity means it loves free electrons.
• It recombines easily soon after the arc quenching for reuse.
• Not corrosive.

Electrical Properties

• Pressure directly affects its enhanced dielectric strength.
• It quenches arcs almost 100 times more effective than air.
• Voltage frequency does not alter dielectric strength.

Maintenance of SF6 Circuit Breaker

• Due to the relevance of circuit breakers in both the safety of equipment and the switching of routines, it is necessary to perform maintenance on them.
• In case any of the circuit breakers fail to function correctly as a result of a lack of precautionary maintenance, the electric transmission system will be damaged, and there is also the possibility that equipment may be damaged.
• For circuit breakers that have been inactive for a period of at least six months, it is necessary to make them open and close multiple times in sequence. 
• This is done to ensure that they are operating appropriately and to remove any dust that may have accumulated on the contacts and moving parts.

Advantages of SF6 Circuit Breaker

• SF6 gas provides good insulating, arc extinguishing, and other features that are significant advantages of the SF6 circuit breakers.
• The gas is non-flammable and chemically stable. Their breakdown products are non-explosive, thus there is no danger of fire (or) explosions.
• Electric clearance is greatly limited due to SF6’s high dielectric strength.
• Its performance is unaffected by atmospheric conditions.
• It operates quietly and has no overvoltage issues since the arc is extinguished at the natural current zero.
• There is no decrease in dielectric strength since no carbon particles are generated during arcing.
• It needs less maintenance & eliminates the need for a pricey compressed air system.
• SF6 performs a variety of tasks flawlessly, including resolving short-line faults, opening unloaded transmission lines, switching, & transformer reactors.

Disadvantages of SF6 Circuit Breaker

• SF6 gas suffocates to a certain extent. In the event of a leak in breaker tank, the SF6 gas is heavier than air and hence settles in the surroundings, causing breathing difficulties of the operating staff.
• The presence of moisture in SF6 breaker tank is extremely detrimental to the breaker and causes numerous failures.
• Internal parts must be cleaned during periodic maintenance in a clean and dry atmosphere.
• The particular facility required for transportation and gas quality maintenance.

Applications of SF6 Circuit Breaker

• Protecting circuits that are based on extremely high voltages up to 800 kV is the purpose of these CBs.
• When performing any kind of maintenance or inspection, it is able to depower and break a circuit that is based on high voltage.
• These CBs are responsible for protecting power transmission and distribution systems. Both electricity grids and power producing plants make use of these resources.
• For the purpose of protecting transformers.
• In power circuits, used for the purpose of line protection.
• Electrical power distribution and management.
• Utilized in rectifier & capacitor circuits

Difference between SF6 Circuit Breaker and Vacuum Circuit Breaker

SF6 Circuit Breaker vs Vacuum Circuit Breaker

Characteristics	SF6 Circuit Breaker	Vacuum Circuit Breaker
Definition	An SF6 circuit breaker (CB) is one that uses SF6 pressurized gas to quench the arc.	A vacuum circuit breaker is one that uses a vacuum to quench an arc.
Type	The circuit breakers are old.	The circuit breakers are new.
Operating Energy	Operating energy requirements are significant.	Operating energy requirements are modest.
Arc Energy	Arc energy is reduced	Arc energy is really low.
Contact Erosion	It has minimal contact erosion.	It has very small contact erosion.
Short Circuit Operation	The total number of short circuit operations is 10 to 50 times.	The total number of short circuit operations is 30 to 100 times.
Dielectric Strength	High	Low
Reliability	High	Low
Material & Labour	Material costs are low, whereas labour costs are high.	Material costs are high & labour costs are low.

Frequently Asked Questions 

1). Is SF6 Flammable?

SF6 is non-flammable and has great chemical stability; it is formed through chemical interactions between sulfur and fluorinated gas.

2). Why SF6 is used in Breaker?

The advantages of using SF6 gas in circuit breakers are its non-flammability, strong electrical insulating qualities, non-toxicity, competitive pricing, and low maintenance.

3). What is the purpose of SF6 gas in circuit breakers?

The function of SF6 gas in CB is to extinguish an arc. It is a dielectric gas that can be used to quench arcs and provides excellent insulation.

4). When would you use a vacuum circuit breaker?

Vacuum CBs are mostly employed in systems with voltage levels of up to 72 kV.

5). What is the pressure of SF6 gas in the circuit breaker?

With a pressure of approximately 2.8 kg/cm2, SF6 gas is present in CB.