SF6 over pressure 1

[Mack_Attack]

Hi all, new to this forum but found the site recently and it's full of good information. I'm hoping to tap into some of that if I may.

We have a Merlin Gerin FG1 11kV breaker that faulted. We've had to scrounge spares from site and use an old unused truck that was pilfered for parts. So far it's back on its feet but we have been having a debate over the SF6. Basically it was tested and found to have passed dew point, purity etc. What's got us scratching our heads is the pressure. We've had to find tolerances from various sources but can only find a minimum pressure, no maximum. The arcing chamber is detailed at minimum 1.5bar, ideally 2.5bar relative (or gauge whichever you prefer). The issue lies in that it was tested and readings taken in absolute. It was found to be 4.2bar absolute. We did some calcs using atmospheric pressure on the day and it translated the 4.2 bar to around 3.2 bar relative. The discussion lies on whether it is actually over its rated pressure. Discussions with schneider have only provided the recommended 2.5bar and 1.5 bar relative minimum.

Anyone have any thoughts on if we're barking up the wrong tree or would this be something to worry about?

Thanks for any help!

 

[crshears]

I've been involved in the operation of SF₆ switchgear by numerous manufacturers over a time span of decades, and based on experience I'd say as long as the compartment stays above minimum pressure you're fine. I've seen pressures as much as double the minimum that didn't cause anyone to bat an eye.

That being said: packing in more SF₆ than needed only imposes unnecessary internal mechanical forces that, with thermal cycling over time, will inevitably accelerate failure, even if said acceleration is only slight to moderate. Additionally, the more gas there is in a compartment, the more will eventually escape to atmosphere and the greater will be the greenhouse effect.

Incidentally, some switchgear manufacturers choose to monitor SF₆ density rather than pressure; this allows for operating no more above minimum pressure than required, thus saving money by not filling compartments with any more gas than necessary.