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MV Vacuum Circuit Breakers compared to SF6 Circuit Breakers

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Jubailcity

Electrical
May 27, 2003
6
Gentlemen,

in the medium voltage range, vacuum circuit breakers seem to have the largest share of the market nowadays. May I have your opinion about their performance compared to SF6 circuit breakers, in terms of switching and maintenance characteristics? Do anyone know of any 'independent' comparison list made about 'pros' and 'cons' of the mentioned technical performances?

Thank you for your inputs.

Regards,

GAC
 
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Vacuum breakers are more compact and economical. The only disadvantage is, while switching low kW motors rated at high voltage the VCBs are prone to give rise to high TRVs.

Provision of surge protection equipment near motors would take care of this.
 
A few related references -

thread242-12011


Vacuum and SF6 interruption techniques-a manufacturer's experience, Makin, P.; Klaus, D.W.
SF6 and Vacuum Switchgear at Distribution Levels (Digest No. 1996/185), IEE Colloquium on an Update in 10/16/1996 Nottingham, UK

Practicalities of using SF6 and vacuum switchgear
Balnaves, D., Midlands Electr. plc;
SF6 and Vacuum Switchgear at Distribution Levels (Digest No. 1996/185), IEE Colloquium on an Update in
10/16/1996 Location: Nottingham , UK
 
Although there may be really slight differences of behaviour in specific applications e.g. capacitor switching or motor starting (better done by a contactor up to 12kV though !!), SF6 and vacuum are equivalent in terms of performance, reliability and cost for general purpose.

Beware of suppliers having only one technology to offer...the best and cheapest of course !!

Just make sure that you get either SF6 from a large brand to make sure you get the QA and QC that SF6 requires,
or get vacuum from the latest technology, as vacuum interruption has improved a lot in the last 5-10 years.
 
Certainly vacuum seems to be on the up.

These days capacitor switching is no longer a problem with the restrike free vacuum interrupters on the market (provided that the interrupter is rated for the TRV - this is especially important when considering lower order filters, 3rd harmonic for example). The problems of switching motors and furnaces with vacuum is easily overcome with the installation of a low power RC network at the motor or furnace transformer terminals.

SF6 gives a 'softer' switch than vacuum and therefore is not as big a mechanism for generation of transients.

Vaccum is almost a perfect breaking medium and does not require regeneration of the dielectric during arc extinction. Contact travel is therefore shorter (less energy from the mechanism) and there are fewer parts. This implies better reliability.

Certainly when looking at indoor MV switchgear, it appears that the large manufacturers are favouring vacuum.

Possibly of more importance is the explosion proofing technology employed.

 
Comparative Information I have gleaned that might explain movement towards vacuum for <50kV

It seems vacuum interrupter transients (due to current chopping prior to the zero point of wave) are very much reduced on present day breakers by materials (copper alloys) of the arcing surface. Also at least some manufacturers use a technique called early opening pole that also helps - one phase opens slightly before the other two.

SF6 seems to have some environmental downside, mainly I think the materials that accumulate in the arc chamber are toxic/carcinogenic and may need special disposal at end of life.

SF6 is also a potent Greenhouse Gas - 23,900 times greater than CO2 . Some countries are imposing limits on it's use and while it is needed for HV it may not be justified for MV. U.S. See...
 
Suggestion: Reference:
1. Donald G. Fink, H. Wayne Fink &quot;Standard Handbook for Electrical Engineers,&quot; 14th Edition, McGraw-Hill, 2000,
1a. Vacuum Circuit Breaker section on page 10-89
1b. SF6 Circuit Breaker section on page 10-93
 
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