SF6 Circuit Breaker : Working & Its Applications

Modern power systems deal with huge power & numbers of associated electrical devices. If any fault in the circuit, short circuit, cable fault, high current flow occurs in power systems then the equipment & power network may damage. To save these systems from damage, the faults should be cleared as soon as possible. So to disconnect the parts from faults and to reconnect them, a special type of switching device like a circuit breaker is required. The circuit breaker or CB is an electrical device, used to protect electrical systems from fault current, overload, and short circuit. There are different types of circuit breakers available in the market like oil, vacuum, air blast, SF6, indoor, spring operated, pneumatic, etc. So, this article discusses one of the types of CB like the SF6 circuit breaker and its working with applications.

What is SF6 Circuit Breaker?

The SF6 circuit breaker or Sulphur Hexafluoride is one type of circuit breaker (CB) that uses pressurized SF6 gas to quench the arc. SF6 is a dielectric gas that has the properties like arc quenching & superior insulating. It is mainly used for quenching the arc within high voltage CBs in electrical grids and power stations up to 800 kV.

The Sf6 circuit breaker operating time mainly differs based on the manufacturers, but the typical operating time is about 18 -20 seconds. Sf6 circuit breaker voltage range is 800 kV in electrical grids and 35 kV in distribution systems. The sf6 circuit breaker diagram is shown below.

Once an arc is struck among the contacts of a circuit breaker, then it absorbs the free electrons from it and changes into negative ions. As compared to electrons, these are heavier. Because of the heavy weight of the ions, their mobility can be decreased. In SF6 gas, the mobility of the charge has less mobility which increases the dielectric power of the medium because the movement of the charge is responsible for the flow of current.

What is Arc Quenching in Circuit Breakers?

First, we have to know what is arc before going to discuss an arc quenching. In a circuit breaker, while opening the current-carrying contacts, the medium in the middle of opening contacts turns into extremely ionized throughout which the interrupting current will get less resistive lane & continues to supply throughout this lane even the current-carrying contacts are separated physically. When current flows from one contact to another, the lane will become so heated that it blazes. This is known as an arc.

The arc quenching is also called Arc Interruption or Arc Extinction, so it is the stoppage of the arc & metal transfer. If you want to break high voltages, then care must be taken to reduce the amount of flow of current at the time of opening contact to reduce the metal transfer. So reducing the current flow will increase the contacts life.

SF6 Circuit Breaker Construction

The SF6 circuit breaker includes two essential parts mainly the interrupter unit and the gas system.

Interrupter Unit

In the SF6 circuit breaker, the interrupter unit includes fixed & moving contacts that are enclosed within a chamber. Here, the fixed contacts are not movable while the moving contacts will move back & forth through an arm activated by a mechanism.

Here, this chamber is known as the arc interruption chamber which includes SF6 gas. The connection of this chamber can be done directly connected to the SF6 gas reservoir where a valve mechanism allows the SF6 gas toward the arc interruption chamber.

The fixed contact is a hollow cylindrical contact that is connected through an arcing horn whereas the moving contact can also be a hollow cylinder through rectangular holes within the sides. These holes allow the SF6 gas to let out throughout them once flowing along &across the arc.

The contacts like fixed and moving & arcing horn are covered through a copper-tungsten material. SF6 gas is expensive; it is cleaned & reclaimed through the appropriate auxiliary system after every circuit breaker operation.

Gas System

The SF6 gas is very costly & its product gases emission is dangerous for the surroundings. So, a closed gas system is utilized, wherever the utilized SF6 is recombined for reprocessing. Its pressure is also maintained as its dielectric strength greatly depends upon it. When the gas pressure is very high, then care should be taken to keep away from the gas leakage at joints by providing ideal scaling.

When the pressure of a gas is extremely high, then more care should be taken to avoid the leakage of gas at joints by offering ideal scaling.

The connection of both the low & high-pressure systems can be done through low-pressure-based alarms & several lockout switches which give a notice when the gas pressure decreases under a certain value otherwise the dielectric strength is reduced & arc quenching capacity of the circuit breaker will be endangered.

The SF6 gas can be stored within the high-pressure chamber at 16 ambiances whereas the pressure of the gas on the less pressure side is three ambiance

The 20◦C temperature is used to avoid liquefaction of SF6 gas within the high-pressure chamber. A heater is connected at less temperature within the high-pressure chamber. Once the ambient temperature drops under 16 degrees Centigrade, then a thermostat is connected to switch ON.

Sf6 Circuit Breaker Working

Generally, the circuit breaker contacts are closed in normal operating conditions. Once the error or fault takes place within the system, then circuit breaker contacts will be pulled separately & an arc is struck among them. The moving contacts displacement can be synchronized through the valve which enters the high-pressure gas within the interrupting chamber of arc at 16kg/cm^2 pressure.

The SF6 gas attracts the free electrons within the arc lane to form ions where ions do not perform like a charge carrier. These Ions will enhance the strength of the dielectric of SF6 gas, thus the arc can be extinguished. So, this procedure will decrease the force of the gas up to 3kg/cm^2 so; it is simply stored within the less pressure reservoir.

This less pressure SF6 gas is pulled back to the reservoir to use again. Now, puffer piston force is utilized mainly for producing the pressure of arc quenching throughout an opening process besides a piston connected to the moving contacts.

SF6 Circuit Breaker Vs Vacuum Circuit Breaker

The difference between the SF6 circuit breaker and the vacuum circuit breaker is discussed below.

Types of SF6 Circuit Breaker

There are three types of SF6 circuit breakers which include the following.

• Non-Puffer Type
• Single Pressure Puffer Type
• Double Pressure Puffer

Non-Puffer Type

The first invented circuit breaker or CB is Nonpuffer type SF6 that does not contain the puffer cylinder. The working principle of this circuit breaker is similar to air blast CB.

Single Pressure Puffer Type

This type of SF6 CB mainly contains the puffer cylinder, which is a variable hollow cylinder. This cylinder works like a bridge between the two stable contacts. It slides axially upward & downward through the contacts to make & break the contact among the two stable contacts.

In this type of CB, the SF6 gas can be compressed through the moving cylinder & the gas can be released throughout a nozzle while the arc is extinguished. This type of circuit breaker is very popular for a wide range of voltages which ranges from 13.6 kV – to 760 kV.

Double Pressure Puffer Type

Double pressure puffer type SF6 circuit breaker includes compressed SF6 gas within a cylinder for quenching the arc. In this CB, the SF6 gas can be released from the high-pressure system to the low-pressure system using a nozzle throughout the process of arc extinction. This type of design is outdated, so not used anymore.

Advantages

The advantages of the SF6 circuit breaker include the following.

• The property of SF6 gas is arc quenching that is 100 times effective as compared to air.
• Very short arcing time.
• Dielectric strength is high because of electronegativity.
• Large current can be interrupted because of high dielectric strength.
• Design is compact
• It has a compact design.
• It handles all types of switching.
• This CB includes a closed circuit gas system through no leakage. So, it can be connected in any type of severe environment.
• Noiseless operation
• SF6 gas is not flammable and nontoxic
• Requires low maintenance

Disadvantages

The disadvantages of the SF6 circuit breaker include the following.

• As compared to oxygen, the SF6 is heavier.
• These CBs are costly
• SF6 leakage must be monitored.
• It needs special transportation.
• Reconditioning & recombination of this gas needs extra equipment.

Applications

The applications of SF6 circuit breakers include the following.

• These CBs are used to protect very high voltage-based circuits up to 800 kV.
• It can depower & break a high voltage-based circuit for any type of maintenance & inspection.
• These CBs protect distribution systems & power transmission. These are used in power grids & power generation plants.
• Power distribution & control.
• Used in power circuits for Line protection.
• Used in rectifier and capacitor circuits
• Used in transformer protection.

Why SF6 is used in Breaker?

The reasons to use SF6 gas in circuit breakers are non-flammability, high electrical insulating properties, non-toxicity, competitive cost & low maintenance.

Is SF6 flammable?

SF6 is non-flammable with high chemical stability and it is obtained through chemical reactions in between sulfur & fluorinated gas.

When would you use a vacuum circuit breaker?

Vacuum CBs are mainly used where system voltage levels are up to 72 kV.

What is the purpose of SF6 gas in circuit breakers?

The purpose of SF6 gas in the CB is to extinguish the arc. It is a dielectric gas available with arc quenching & superior insulating properties.

What is the pressure of SF6 gas in the circuit breaker?

The pressure of SF6 gas in CB is about 2.8 kg/cm2

Thus, this is all about an overview of an SF6 circuit breaker and its working with applications. The main properties of SF6 circuit breakers are toxicity, thermal conductivity, dielectric strength, arc quenching, etc. Here is a question for you, what is SF stands for in the SF6 circuit breaker?