Overpressure device on SF6 equipment 3

[marks1080]

Has anyone ever seen an over pressure device used on SF6 gear? Specifically, something like a high power SF6 VT (100kva per phase)?

It's being suggested for a project I'm working on but I don't have experience doing this. The functional analogy that is wanted would be like a Bucholtz relay for an oil filled transformer. I don't know how SF6 behaves under fault. I've never heard of an over pressure device being used for protection for say a SF6 HV breaker... The device isn't included by the manufacturer and some are asking to add it in after the fact (by drilling into the tank and plugging the hole with the pressure device). I can only assume the people suggesting this believe that the over pressure device can pick up in time to save the unit for an internal fault (as you hope happens for a regular 63 gas trip), but again, I have no idea how the SF6 behaves or if it would be useful for this function. I haven't gotten any explanation so far and was hoping someone here may have had some experience with this.

 

[crshears]

Fluid mechanics ain't my specialty, but it seems to me the pressure wave through SF6 [or any other gas] would not propagate nearly as quickly as in a liquid, meaning there'd be enough inevitable time lag to render the whole idea pointless. If it was desired to try this anyway, it would also be necessary to first consult the appropriate pressure vessel regulations via the AHJ to see whether boring a hole in the native metal would even be permissible without voiding the vessel's certification, or if it would be necessary to weld in a fitting...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[JG2828]

I believe they already have rupture disks built into them. So you would need to make sure that your over-pressure relay trip is set under the rupture disk failure pressure.

 

[marks1080]

crshears - I've been pondering all the same points. It's also concerning when you google "SF6 over pressure device" and find nothing... Pretty sure we are working in the same system, just FYI you may see folks trying to in service this gear without technical documentation to go along with it.

JG2828 - Good point, thanks!

 

[unclebob]

Maybe its already installed, but I would prefer to have an under pressure on a SF6 insulated apparatus.

 

[crshears]

Low density alarms and trips are pretty well standard on all of our HV SF6 apparatus; we also have high SF6 pressure alarms in select cases.

I'm not aware of us using a pressure wave trip [63?] on any of our SF6 equipment.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[marks1080]

The device in question comes with duel stage low SF6. I'm using stage 1 to alarm, and stage 2 to trip the zone.

The over pressure device, as far as I know, is being added after the fact at our request. So my assumption is a hole will be drilled through the tank of the VT and an over pressure device used to plug up the hole. The intent is this over pressure device will activate a trip signal for an internal fault.

crshears - can you give me an example of where we've used over pressure devices for SF6 alarming? Sounds like a pretty standard thing to have for alarming, but I'd like to check the instrumentation providing the signal. There's a big difference between over pressure due to environmental temperature or simply putting too much gas in the device vs using it for 'protection' trips.

 

[crshears]

Haefley air-to-gas bushings are the first example that comes to mind; some are of a two-pressure design and high low-side pressure alarms are used as an indication that inter-compartmental leakage has occurred.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[marks1080]

Ahhh yes... I wasn't thinking about GIS. Sounds like different functionality vs what is intended for these VTs. Have you ever seen a trip signal from an SF6 over pressure or just alarms?

 

[crshears]

crshears wrote: We [also] have high SF6 pressure alarms in select cases.I'm not aware of us using a pressure wave trip [63?] on any of our SF6 equipment.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cuky2000]

Rupture discs are typically installed onto circuit breaker, gas-insulated switchgear and instrument transformers that contain SF6 gas to protect the apparatus for unusual overpressure.

In the event of an internal short circuit or external fire, the disc removes the risk of an overpressure before reaching an explosion. Metallic vessel for dead-tank breaker are ASME certified and usually UM stamped. Live tank breaker, on the other hand, made with nonmetallic material (porcelain or polymer) and protected for overpressure in a similar manner than the dead-tank breaker design.

There is a less issue of overpressure for outdoor AIS substation. However, for indoor applications, an additional precaution is recommended for overpressure on buildings. For illustration, see the links below.

https://www.youtube.com/watch?v=ITVp7gCNp4E

thread238-450529

 

[ElecEE8]

Just wanted to share what we have specified in standards of GIS regarding overpressure device requirements. You may get some idea on this.
"Each gas compartment shall have its own pressure relief device (automated external pressure relief device) to provide instant and safe discharge of accidental overpressure during internal arc. Rupture diaphragms shall be preferably used as pressure relief mechanisms. Such gas discharge shall be directed away from the normal operation position of the switchgear so that persons standing there are not endangered. If necessary, diverter nozzles shall be provided to ensure that the gas is directed away from this position. Internal relief devices between adjacent compartments are not acceptable."

 

[crshears]

My take on the OP was that it was clearly regarding overpressure or pressure wave trip initiating devices; given that context, rupture disc info isn't relevant to the question being posted, other than for the co-ordination issue raised by JG2828.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cuky2000]

[highlight #FCE94F]Tampering making hole in the tank is not recommended.[/highlight] Think about the equipment warranty tampering the apparatus adding untested devices, system reliability degradation, liability issue, O&M, etc.
SF6 static gas overpressure helps to maintain or improve the dielectric strength of the equipment. There is a small risk of increasing SF6 leak rate and gasket issues that could be detected during periodic routine maintenance.
Do you really need to trip under gas overpressure?

I believe in the relevance of mechanical overpressure protection vs. electrical trip initiation for the following reason:
a) There is not a good reason to overpressure the device beyond the static pressure recommended by the manufacturer.
b) If static overpressure happened, still the equipment can be in service with minimum issues.
c) Sudden overpressure mainly happens during internal SC fault and this will be detected by the protective device.
d) Mechanical disk rupture is the ultimate backup protection to avoid a catastrophic overpressure the create permanent damage or explosion.

So far, the SF6 apparatus has a good performance in term of overpressure performance. I do not recall seeing any overpressure tripping devise In the past 50 years working for HV equipment manufacturer, utility, field and consulting firm. In fact, it is doubtful if adding tripping feature will add any benefit to improve protection or reliability of the system.