SF6 Circuit Breakers: Features, Operation & Applications

Introduction

Many modern designs of circuit breakers for use at transmission voltages utilize sulphur hexafluoride (SF₆) gas both as an arc-interrupting/arc-quenching and a dielectric medium. At distribution voltages SF₆ designs of circuit breakers are also used, however vacuum and bulk-oil circuit breakers still find a wide use as alternatives.

The SF₆ is an electron-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF₆ gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up sufficient insulation strength to extinguish the arc. The SF₆ circuit breakers have proven to be very effective for high power and high voltage service.

SF₆ Circuit Breaker Design Types

There are three basic design types of SF₆ interrupter/circuit breaker:

• Gas-blast interrupter/Gas circuit breaker.
• Puffer interrupter/Puffer circuit breaker.
• Rotating-arc interrupter/Rotating-arc circuit breaker.

Gas-blast interrupter tends to have a higher performance capability than other interrupters. This design is used in transmission circuit-breakers. Gas-blast interrupters cause a flow of pre-pressured gas across or a long the opening circuit breaker contacts.

In puffer circuit breakers, the gas is compressed during the initial part of the opening stroke and prior to separation of the circuit-breaker arcing contacts. This requires the circuit breaker to have a long operating stroke and a powerful operating mechanism to pre-compress the gas. Nonetheless, this type of configuration is the one that is commonly utilized on transmission SF₆ circuit breakers.

To overcome the requirement of a large powerful operating mechanism on puffer circuit-breakers, heat of the arc itself is used to pressurise the surrounding gas and induce arc extinction; this breaker is commonly termed to as self-pressurisation interrupter. While clearance at high values of fault current can readily be accomplished, it is still necessary to apply a small piston to assist in arc extinction at very low values of fault current. This design of circuit breaker is often used in distribution circuits and also utilized in circuit breakers for use at transmission voltages.

The rotating arc- design principle, used mostly in circuit breakers at distribution voltages, the arc is induced to rotate very rapidly under the influence of magnetic fields set up by a series coil inserted into the current path during the opening of the circuit-breaker contacts. Very rapid movement of the arc causes a flow of cool SF₆ gas across the arc to achieve the arc extinction.   

The Structure of SF₆ Circuit Breaker

The figure below illustrates key components of a typical SF₆ circuit breaker.

A typical SF₆ circuit breaker consists of fixed and moving contacts enclosed in a chamber (referred to as arc interruption chamber) containing SF₆ gas. This chamber is linked to SF₆ gas reservoir. When the contacts of the circuit breaker are opened, the valve mechanism allows a high pressure SF₆ gas from the reservoir to flow towards the arc interruption chamber.

The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to allow the SF₆ gas to be let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are typically coated with copper-tungsten arc resistance material. SF₆ is reconditioned and reclaimed by appropriate auxiliary system after each operation of the breaker.

Operation of the SF₆ Circuit Breaker

In a closed position of the circuit breaker, the contacts remain surrounded by SF₆ gas at a pressure of about 2.8 bars. When the circuit breaker operates, the moving contact is pulled apart and arc is struck between the contacts. The movement of the moving contact is synchronized with the opening of a valve which allows SF₆ gas at around 13.72 bar pressure from the reservoir to the arc interrupting chamber. The high pressure flow of SF₆ rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers, and consequently the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the arc extinction, the valve is closed by the action of a set of springs.

Advantages of SF₆ Circuit Breakers over Oil or Air Circuit Breakers

Because of the arc-interrupting/quenching properties of sulphur hexafluoride (SF₆) gas, the SF₆ circuit breakers have the following benefits:

• Due to the dielectric strength of SF₆ gas being 2 to 3 times that of air; such circuit breakers can interrupt larger currents.
• They have very short arcing time because of the better arc quenching property of SF₆.
• They have noiseless operation due to its closed gas circuit and no exhaust to atmosphere in contrast to the air blast circuit breaker.
• The closed gas enclosure keeps the interior dry so that there is no moisture problem.
• SF₆ are principally a good choice for areas where explosion hazard exist such as coal mines because they are totally enclosed and sealed from atmosphere.
• There is no risk of fire in SF₆ circuit breakers because gas used (sulphur hexafluoride) is non-inflammable.
• There are no carbon deposits, therefore tracking and insulation problems are eradicated.
• SF₆ circuit breakers have low maintenance cost, light foundation needs and minimum auxiliary apparatus.

Shortcomings of SF₆ Circuit Breakers

SF₆ circuit breakers have the following disadvantages:

• Because SF₆ gas to be recondition after every operation of the circuit breaker, additional apparatus is needed for this purpose.
• They are expensive due to high cost of SF₆ gas.

Applications of SF₆ Circuit Breakers

The development of SF₆ circuit breakers has seen their capability increased rapidly for example a single interrupter can achieve a typical rating of 40 kA at 420 kV. Two interrupters can be used per phase for a typical circuit-breaker rating of 55 kA three-phase at 420 kV. Generally a number of units are connected in series as per the system voltage.

Related articles:

• Switchgear
• 5 Causes of High-Voltage Switchgear Failures
• Types of Busbar Arrangements in Grid Stations and Substations
• Overhead vs. Underground Power Distribution Systems
• Key Factors to Consider In Substation Design
• AC Transmission and Distribution Systems