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Proper use/dimensioning of extractor fans for GIS building

Proper use/dimensioning of extractor fans for GIS building

Proper use/dimensioning of extractor fans for GIS building

Right now I’m working on projects for different a client that involves the installation of a GIS inside a building; each of them uses have different guidelines for HVAC of the building. Some like to use extractor fans, some have asked for air conditioned (money is no barrier), some doesn’t use anything. I need some guidance about the principles/standards that determine the need of installment of extractor fans on the building. I’ve heard the following arguments:

• Keep the temperature inside the building below the dew point, in order to avoid condensation on the inside GIS enclosure.
• Need for heat extraction produced by the GIS, in order to ensure operator´s comfort. This would be calculated using Joule’s law but to be fair, the GIS enclosures I’ve encountered feel fairly cold.

I’d appreciate any guidance you could provide.


RE: Proper use/dimensioning of extractor fans for GIS building

Quote (OP)

• Keep the temperature inside the building below the dew point, in order to avoid condensation on the inside GIS enclosure.
You may want to keep the temperature above the dew point.
For equipment with thermal mass such as motors and generators the iron should be kept above the dew point. When the temperature is rising the iron temperature will take some time to warm up to the new temperature.
In the event that a weather front with higher temperature and higher relative humidity moves in, you may experience condensation on the cooler parts of the iron.
When this is a concern small heaters are used to keep the iron temperature a few degrees above the ambient temperature.
It is common to use 240 Volt rated heaters supplied with 120 Volts for long life and energized 24/7.
These may be directly mounted to the outside of the stator or mounted inside the end bells with a small air space.

"Why not the best?"
Jimmy Carter

RE: Proper use/dimensioning of extractor fans for GIS building

I would reach out to the manufacturer to get their recommendations. You might need to pay more for a larger humidity range, but you could cut down on HVAC costs. GIS normally has a high operating temperature range (-25C to +40 (or higher!)). There is a lot of heat generated per bay (on the order of x kW, usually). It might feel cool to the touch, but keep in mind the aluminum is a good heat conductor!

RE: Proper use/dimensioning of extractor fans for GIS building

Quote (Excerpt for GIS building Spec:
1.0 GIS hall electric unit heaters shall be furnished with remote thermostats and installed 5’ above finished floor to provide perimeter heating and spot heating around rolling doors. GIS hall maximum temperature required to be maintained in the winter is approximately 60 Degrees F. Provide adequate ventilation in summer to maintain the temperature below GIS specified temperature limit.
2.0 GIS hall ventilation shall be provided from wall mounted axial fans mounted in sheet metal plenums with motor operated dampers. Air shall be introduced through vertical storm louvers into the fan plenums and relieved through motor operated dampers and vertical storm louvers. Manual ventilation controls shall be provided. Fans shall be prewired and factory tested. Fans shall be provided with combination starter/disconnect switches with 120V step down transformer prewired to damper actuators. Dampers shall power open and fail closed.)

Since most of the GIS are designed to operated in a wide range of temperature, the fan inside the building is to preclude that the indoor temperature do not exceed the rated value to derate the GIS capability. Up to some degree, some utility meet union requirement for comfort during O&M of the unit. A/C usually is not needed in the GIS deck and only reserved for the P&C room.

Beware that beside the HVAC in case of overpressure caused by a sudden failure of the GIS vessel, a pop-pop automatic opening is recommended to be considered to avoid destruction of the building.

The substation can be rated for indoor only or Indoor & outdoor rated to operate in a wide range of temperature and humidity. Typical values are as follow:
• Ambient temperature range [°C]-30 … +40.
• SF6 High liquefaction temp, -25/30°C for 6.3/5.5 bar @ 20°C
• Installation: indoor / outdoor
• Humidity: Up to 100% (Humidity does not impact GIS operation).

RE: Proper use/dimensioning of extractor fans for GIS building

Keeping the enclosures on the mechanism areas of the GIS is, in my view, very important; so is not compromising the R value of the insulation on the covers, even within insulated and heated buildings...the heat developed within in the mechanism boxes generally keeps the contents within enough above ambient that condensation with changes in weather conditions is pretty well precluded.

You might think the foregoing was obvious...but it isn't always; one crew practiced the false economy of just leaving the covers off since that would optimize working on the equipment the next time around...unfortunately they did not take into account the fact that that particular SF6 building was mounted on stilts, in other words, above grade, and both uninsulated and unheated. To add insult to injury there were ventilation openings in the floor [with machinery-rated grate in them, of course] to allow minor amounts of released SF6 to flow out of the building by gravity, as well as manually started fans to rapidly scavenge the building of SF6 and arc by-products in the event of a burn-through fault. To this day the inside of that building has its own little micro-climate; fog in the air, leading to condensation on exposed electrical equipment, is not, unfortunately, an unknown occurrence.

Not only that, but this station, as well as others like unto it, is notorious for SF6 leaks from its GIS, due almost certainly to the interminable cycling of temperature. This can be contrasted with one of our sites where the building is insulated and heated [colder climate]; it literally took years and years for the GIS in the latter instance to develop its first low SF6 density condition. Indeed, one of the more junior operators, hired a little over a year after that site was commissioned, was on shift the day that first L/D alarm came in, and told the senior, "I've never seen one of those come in! What do we do about it?"


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

RE: Proper use/dimensioning of extractor fans for GIS building


I'll be straightforward: I appreciate your input, but this is what I find one of the biggest problems in today's older engineer demographic (and what sometimes translates to non-sense specs). You raise some good points about SF6 leakage and climate, but that's about it.

You should have submitted a report to the manufacturing company on the temperature cycles and the effect on the loss of SF6. HV switchgear manufacturers like to boast about their relative low losses, if they are still in business (or even still manufacture switchgear), they would be very much happy to help you out.

Everything else is an anecdotal story that shouldn't take form on a forum such as this.

Please try to keep things factual. I appreciate your experience, but it's not helping as younger engineers might read this and immediately assume that what you said applies to all switchgear types.

RE: Proper use/dimensioning of extractor fans for GIS building


I appreciate your frankness.

I submit that if a younger engineer reads this and immediately assumes that what was said applies to all switchgear types, they either haven't been trained very well or ignored the advice of their mentors.

I did not wish to needlessly bloat the"anecdote" with details, but in the case of the non-insulated and non-heated building my company in its wisdom chose to go its own path and cheap out on the building instead of following the switchgear manufacturer's recommendations, meaning the swgr mfr rightly washed their hand of the whole sorry mess. To assume a priori that the operational issues arising stemmed from problems with the switchgear design or manufacture would indicate failure to establish a complete and correct set of first principles before investigations even commence...and alas, I've seen this happen far too often.

For what it's worth, I'm not an engineer [see my profile] but, as you acknowledged, I have been blessed with 40+ years of operational experience, and as near as I can tell that's why my contributions to these fora are indulged and/or tolerated, and even occasionally welcomed.


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

RE: Proper use/dimensioning of extractor fans for GIS building

Below is an example of a GIS heat load used to estimate the fan size inside the building.

I hope the help

RE: Proper use/dimensioning of extractor fans for GIS building

crshears: Some of us enjoy (and also find very useful) the 'anecdotes.' There's a reason most people learn significantly more in their first couple years of working than their many years of education. Exposure to the experiences of others who have already walked the path is invaluable. New engineers will discover this, sometimes the hard way ;)

RE: Proper use/dimensioning of extractor fans for GIS building

marks1080, I'm humbled; thank you.


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

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