SF6 Circuit Breaker

KHALID JAMAL SHAHID KHANNIT WarangalEEE

Introduction Why SF6?? History Physical Properties Electrical properties Working principle SF6 Decomposition Advantage and Disadvantage Application Conclusion Reference

Outline:

SF6 switchgear is currently used world-wide. It is estimated that an average of about 80 % of HV equipment manufactured now has an SF6 content.

Introduction

Sulfur hexafluoride (SF6) is an inert gas with excellent insulating properties

exceptional thermal and chemical stability

it insulates 2.5 times better than air

arc quenching capability 100 times better than air

better heat dissipation capacity than air;

Why is SF6 used in electric power equipment?

1938- First patent on use of SF6 as an interrupting medium were field in Germany by Vitaly Grosse

1951-Later in United States by H J Lingal 1953-First Industrial Application of Sf6 for current interruption with a

breaking capacity of 600A 1956-First High Voltage cicuit breaker built by Westing House, that could

interrupt a current of 5kA under 115kV 1957-Puffer-type technique was introduced for SF6 C.B 1959- First HV SF6 C.B with high short circuit capability was introduced (up

to 41.8kA under 138k) 1970-High interrupting capabilities SF6 C.B were developed, up to 800kV Presently, SF6 has been adopted for use in ring main units, contactors ,

bus bar coupler and circuit-breakers, GIS, covering all the needs of the electrical distribution industry

History:

Physical Properties of SF6

SF6 is one of the heaviest known gases

The variation of pressure with temperature is linear and relatively small in the range of service temperatures (-25 to + 50°C)

volumetric specific heat of SF6 is 3.7 times that of air.

Electrical Properties of SF6

The excellent dielectric properties of SF6 are due to the electronegative character of its molecule . It has a pronounced tendency to capture free electrons forming heavy ions with low mobility making the development of electron avalanches very difficult.

The dielectric strength of SF6 is about 2.5 times higher than that of air under the same conditions.

For non-uniform fields a maximum breakdown voltage is obtained at a pressure of about 0,2 Mpa

Because of its low dissociation temperature and high dissociation energy, SF6 is an excellent arc quenching gas.

Electrical Properties of SF6

Working Principle:

Interrupting Unit of SF6 Circuit Breaker (1/2)

The interrupting unit filled with pressurized SF6 gas is placed at the top of the pole and contains Stationary Contact, Nozzle, Moving Contact, Puffer Cylinder and Fixed Piston (Fig.1).

During opening operation (Fig.2), the Moving Contact along with the Puffer Cylinder is pulled down. The Fixed and Moving contacts get separated and arc is formed between fixed contact and moving contact.

Motion of the Puffer Cylinder compresses SF6 against the Fixed Piston thus generating a powerful SF6 blast through the Nozzle over the arc. After travelling through some distance, the dielectric strength of the gap is raised sufficiently to extinguish the arc.

Working Principle:

Working Principle:

Interrupting Unit of SF6 Circuit Breaker (2/2)

SF6 DECOMPOSITION PRODUCTS

SF6

SF4 + 2F

SOF2 + 2HF MFn

SiF4 + 2 H2O

SPARK

H2O

H2O SiO2

M

SO2 + HF

Electrical clearances are very much reduced because of high dielectric strength of SF6

Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to the air-blast circuit breaker and minimum oil breaker.

SF6 circuit breaker is simple, comparatively cheaper in cost, maintenance free and compact

Its performance is not affected due to variation in atmospheric conditions

It gives noiseless operation it does not make sound like air-blast circuit breaker during operation

Minimum maintenance. The breaker may require maintenance once in four to ten years

The SF6 gas circuit breaker can perform various duties like clearing short line faults, opening unloaded transmission lines, capacitor switching, transformer reactor switching etc without any problem

Advantages of SF6 Circuit breakers?

Imperfect joints leading to leakage of the SF6 gas. Continuous monitoring devices are required

Arced SF6 gas is poisonous and should not be inhaled

SF6 gas being heavier than the air settles in the surroundings and may lead to suffocation of the operating personnel. However it is not poisonous

it may contribute to the man made greenhouse-effect, if it is released into the atmosphere.

Disadvantages of SF6 Circuit breakers?

SF6 Circuit breakers are mostly employed for High voltage applications. Because of the above said features and properties of SF6 gas they are mostly employed in switching duties related to high voltage and extra high voltage application

SF6 Circuit Breakers Applications:

The adoption of SF6 in switchgear for all operating conditions has brought advantages in performance , size, weight, global cost and reliability.

Conclusions

IEC 60056 – High-voltage alternating-current circuit-breakers, Amendment 3, 1996-09.

A. Plessl, L. Niemeyer, F. Perdoncin: Research for medium-voltage SF6 circuit-breakers. ABB Review 2/89, 3–10.

D. Braun, W. Heilmann, A. Plessl: Application criteria for SF6 and vacuum circuit-breakers. ABB Review 4/89, 25–33.

http://www.openelectrical.org/wiki/index.php?title=File:40.5_kV_SF6_Circuit_Breaker.jpg&limit=20

http://ww2.wapa.gov/sites/western/newsroom/NewsFeatures/Documents/2011/APPATeamaward.aspx

www.schneider-electric.co.in/documents/technical-publications/en/shared/electrical-engineering/breaking-techniques-switchgear/general-knowledge/ect188.pdf sf6 circuit breakers properties

http://www.capitalelectric.com/main.cfm

References: