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Temperature rise inside the switchgear enclosure due when placed outdoors (Solar loading)

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ArcEnergy

Electrical
Aug 25, 2015
8
Hello all.
I have posted this question in Thermodynamics forum and I am posting here again to get some opinions by electrical engineers who might have experience with similar situation as mine.

My question is to estimate the amount of temperature rise inside the enclosure due to solar loading. I am writing in two Parts. the PART 1 is what I am trying to achieve and PART2 is what I have already calculated.

PART 1:

I have a Switchgear enclosure (made of aluminum) of LxWxH of 2.4mx1.15mx1.97m , Thickness would be around 0.01m
The internal power Dissipation is 900W .. it is because of a current of 12.5kA flowing in a conductor of resistance 6x10^-6 Ohms
The ambient temperature (Tamb) is 40°C
I am considering the Solar Radiation of 1100W/m^2. The enclosure is painted with light Grey color with Absorption coefficient of 0.5 and emittance coeff of 0.87 (correct me if I am wrong)
h for the Aluminum as 2 W/m^2K (not sure if this value make sense)

I would like to know how much would be the temperature rise inside the enclosure (Tenc).

PART 2:

I have done the calculations based on just current and temperature rise values as explained below

The switchgear is tested with 12.5kA of rated current at 40°C of ambient temperature. according to standards we are allowed to have a 65K of rise inside the switchgear cubicle. this value is mainly because the current carrying conductor has only tested to withstand 105°C. as you see the allowed temperature rise is 105°C-40°C = 65K

I have then calculated the effect of different ambient temperatures on conductor current by keeping the conductor temperature of 105°C constant using the formula

Inew/Irated = sqrt[(105°C - Tamb new)/Temp rise allowed]

Eg: If the Tamb new is 50°C, then Inew = Irated x sqrt [55/65) gives us the new Current of 11.5kA .. That means when the out side temp is 50°, I am only allowed to pass 11.5kA in the conductor.

Now considering the situation of placing the switchgear in outdoors. The additional effect is solar radiation. According to the standard "C37.24-2003 - IEEE Guide for Evaluating the Effect of Solar Radiation on Outdoor Metal-Enclosed Switchgear"

for grey painted enclosures, the absorption is 0.50 which increases a temperature rise of 7.7K when the solar radiation is 1044 W/m2... So in my new calculation, I have added this 7.7K to my allowed 65K temp rise and calculated the Inew for the amb temp of 40°C. I get the value of 11.64 as allowed current in the conductor when placed my switchgear in outdoors at 40°C.

I don't know if my above calculations make sense because they didn't consider the enclosure dimensions or heat convection and radiation etc.
Can someone tell me if the calculations based on IEEE standard are trustable.. If yes then why? or is there any other way to estimate the temperature rise considering the all the parameters that I mentioned in the first part.

Thanks in advance

regrads,
ArcEnergy
 
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50dC ambient is very high for anyone to live, so where is this transformer located?
 
This is not transformer.. It is a switchgear that is placed outside and the 50°C is just a value that is assumed. But some areas in middle east has been recorded around 50-55°C of ambient temperature for some days.
 
We designed for 55 degree C for Kuwaiti power plants.
 
I don't see any calculations as being very valid.. It would be VERY hard to determine the rise, using any kind of math, since there are probably two dozen variables that would all matter. You might as well come up with an equation to predict the stock market 5 years in advance, it would be a lot easier!

Stratification alone will make the air in an enclosure range over 30C. Then there's the color, the orientation, the size of each face, the specific angle of each face to the sun at each moment of the sun's arc during the day, month, and year, the radiant temperatures of the surfaces the enclosure's faces are facing and how their temperatures change with the sun's arc also matters. Any air flows and how they play on each face of the enclosure. The conduits entering the enclosure and what drafts they may experience. How dirty the faces are. The convective situations on each side of each face during the sun's arc. How the inside faces transfer radiatively their heat to the other faces across the internal spaces.

It can't reasonably be done. You might as well recognize that the inside of a sun drenched enclosure in a 50C environment would probably see well over 100C and anything you place in it had better not be bothered by operation at anything below 125C.

Prudent designers don't let friends install enclosures that aren't under shade structures, which completely nullifies all aspects of the solar gain caused by the sun's arc. It now becomes a box immersed only in a temperature with no solar gains, which is still daunting to do the math on. But at least you can set up a test space fairly easily and just freaken measure it empirically. And then don't forget that once there is any dissipation inside, all the heat will stratify to an astonishing extent.

If you stick to no shade then ultimately you will need to measure the temp in your installed gear to get anywhere close to reality.

Keith Cress
kcress -
 
I think you should start thinking about using a cooling system inside of the switchgear.
If you are using MV-105 cables, I suggest to using 173% insulation. MV-105/173% is a lot better for high temps. I was having a cable failure about once every six months or so with 133% cables and changed to 173% level and had no failures for 15+ years. The problem was not the load as much as the heat where entered the vault right next to a 14" concrete wall. The other side of the wall is 6 feet from the 76MVA arc furnace tap hole and tapping about 55 minutes 275 ton of steel. That wall goes way 100C in the summer. The tap was moved after furnaces where operational. It was not a design fault found that metal stirs better going CCW. don't ask

4 - 1250kcmil cables per phase of 1010 feet long 34.5kV for two furnaces that is 24 cables (a lot of money) in cables and labor
Hope this will help,
Dave

ps: there is large-large water cooled air conditioner blowing on cables
 
My error about mentioning transformer, however,Bbfore we talk about cooling or forced ventilation systems, or radiation effects or even high humidity as experienced in some Arabian countries where is the switchgear used and located?
 
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