Vacuum Breaker or SF6 Breakers for Furnace Transformer 1

[Ajay1971]

On a current project we purchased slag cleaning furnace transformers rated 1500kVA and 2100kVA. The transformers have a 33kV delta connected primary and the secondary delta is closed by the elctrodes.

Prior to purchasing the 33kV switchgear, I had talked to manufacturer's of Vaccum Breakers and SF6 breakers.

#1. Vaccum Breaker manufacturer's recommended providing surge protection.

#2. SF6 breaker manufacturer's did not recommend surge protection.

#3. A few books of furnace transfomer design recommended locating the switchgear near the transformer (just a few feet away) and providing surge protection as well.

I am a little confused now...could somebody show me the path forward in specifying the right breaker. I would also appreciate any help in providing me reference to books or websites addressing this issue.

 

[jghrist]

This has been discussed in thread238-62554, Thread242-12011 and Thread238-62554.

 

[Ajay1971]

Thank you jghrist for referencing the previously discussed threads.

Can it be concluded that an SF6 breaker is recommended only because it hase lesser TRV's in comparison to a Vacuum Breaker while switching furnace loads?

Could there be any other reasons on making the choice of breaker due to the flicker and unbalanced loading created by an EAF?

 

[jghrist]

The flicker and unbalanced loading should have no influence on the choice of breaker.

 

[ARC12]

One factor you need to consider is switching frequency. Many EAF have a high switching frequency (>50 operations per day). If your furnace transformers are in this category then switch the transformers with a vacuum contactor installed between the transformer and the circuit breaker. See

http://www.joslynhivoltage.com/catVBTTransmaster.htm.

If your switching frequency is < 50 operations per day but > 20 I would use a vacuum circuit breaker. Under 20 operation per day you could use SF6 or vacuum.

Vacuum contactors will employ contact materials that differ from vacuum circuit breaker contact materials and will not generate as many overvoltage events. In addition a vacuum contactor operating mechanism will prove more reliable than a circuit breaker operating mechanism in a high switching frequency application.

A vacuum circuit breaker operating mechanism is inherently more suitable for frequent operation than a SF6 mechanism hence my recommendation for vacuum over SF6 in moderate frequency switching applications.

EAF switching is a trivial matter for either SF6 or vacuum as far as arc interruption is concerned as long as the device is applied with-in it's rating. Prudence dictates surge arrestors be applied with vacuum switchgear in most installations.

 

[dpc]

Generally, if there is a reasonable choice between vacuum and SF6, I recommend vacuum, due to greater simplicity, easier maintenance and fewer environmental issues to deal with.

The surge arrester issue is a minor one, in my view. Adding surge arresters is a relatively small cost and I would be putting them on either vacuum or SF6 gear anyway.

 

[simtricity]

I cannot offer any advice on furnace transformer design,but choosing sf6 switch gear can have large environmment issues in the future.
ie high cost disposal of sf6
high clean up costs if the switchgear should fail(under large fault conditions sf6 gas produces a highly corisive paste wich will require special waste/disposal procedures).

Arc12 mentioned vaccuum contactors ,investigate this possibility.

 

[jbartos]

Suggestion: Visit

http://www.wylelabs.com/vcb.html

for: Wyle Lab comments on Vacuum versus SF6 circuit breaker. It may be considered authoritative.

http://www.schneider-electric.ca/

http://www/en/techpaper/html/circbm.htm

etc. for more info