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JohnnieE (Electrical) (OP)
2 Apr 03 9:37
I have an aplication were I have to use a 2 VFD's at 8000 feet, one is a 5 HP and the other is 7.5 HP.  Normally we do not ventitlate the cabinet, in this case I was planning to add a fan/filter kit to increase the cooling, both drives normally run at about 55% on full load current.  does anyone have similar experience and suggestions

thanks
CB2 (Electrical)
2 Apr 03 13:29
Urge you contact the VFD manufacturer for installation advice at this elevation.  Above about 6000' there are electrical insulation considerations and additional cooling concerns that may need to be addressed. Do-able but confirm.
RickOzone (Electrical)
2 Apr 03 14:28
At 8000 feet the air pressure is about 74% of sea level pressure (see:http://www.cleandryair.com/AltitudePressure.htm).  Therefore the cooling is likely to be about 26% less than at sea level.  If you are only running at 55% load then it may not be necessary to add additional cooling fans but it wouldn't hurt to add them anyway.  As for the insulation question, if you have any uninsulated terminals or conductors that are in close proximity to each other, then it could be an issue.  The minimum free air clearances would need to increase at lower pressure.
peebee (Electrical)
2 Apr 03 14:45
"The minimum free air clearances would need to increase at lower pressure."

Is that true?  It could well be.  But I was under the impression, though, that a vacuum was a better insulator than air, and I figured that lower air pressure would therefore insulate better than at 1atm.  Can someone verify this?
CB2 (Electrical)
2 Apr 03 15:21
Air is an insulator, less air, less insulation.  Distance between terminals etc. becomes an issue.

Have not seen a VFD in a vacuum, but I nave seen them well over 12,000'(Chile copper mines) and 690V rated VFD was used on a 480V system.

Also experianced motors operating in a argon gas enviroment. 460V motors survive about 6 months, 240V motors do just fine.
peebee (Electrical)
2 Apr 03 16:44
"Air is an insulator, less air, less insulation.  Distance between terminals etc. becomes an issue."

We're talking electrical insulation here.  If less air = less electrical insulation, then why in the world would anyone build a vacuum circuit breaker?  Seems like it'd just be a big mechancial pain in the butt, and all you end up with is a breaker that doesn't work as well as an air breaker.
cuky2000 (Electrical)
2 Apr 03 17:13
It is good idea double-checking with the VFD manufacturer for more specific details regarding derating at unusual operating conditions and dielectric design limits.

The HP (KW) derating capacity at 8000 ft of altitude could loss 5% to 7 % of the rating power capacity at sea level

That could be OK if the motor will not be loaded continuously above this limit. Other potential allowance to consider is the ambient temperature. At high altitude the ambient temperature is normally lower than the sea level. The enclose graph could help to estimate the derating factor with better accuracy.

http://www.marathonelectric.com/generators/faq_data/derate.pdf


For dielectrict strength purposes use ANSI Std C37.40 or as an alternative use the following relations:

At this voltage the dielectric losses may not be significant at 8000 ft above sea level. The dielectric design limits typically are as follow:
                                          Up to 230 V…………… 2, 000 V
                                    >230 V & < 600 V………….2,500 V
                                        
 If    (2xLL Volt + 1000 V)/Derating Factor > Dielectric Limit the unit will be OK
 
Ex.   For 460 V  operating at 8000 ft of altitude ASL.( Dielectric Correction Factor CF = 0.86)
                    
                             (2x460V +1000)/0.86 = 2,233V < 2500V  (OK)
CB2 (Electrical)
2 Apr 03 18:17
chuky2000, my documentation on elevation derate for cooling a VFD is 1% per 100 meters above 1000 meters.  Good job on dielectric strength of air.

peebee, a note on vacuum contactors and breakers, three advantages over air devices are much easier to extinguish the arch on opening, the metal that is ionized will redeposit on to the contacts, and no external contamination.
peebee (Electrical)
2 Apr 03 19:23
Well, I finally got off my butt and decided to Google a bit to see if I could answer my own question on air vs. vacuum.  I seem to have found some conflicting information, but at last I found one or two sites that maybe tie everything together into a single cohesive theory.

1.  http://www.tu-bs.de/institute/htee/forschung/veroeffentlichungen/giere2001.pdf
This seems to indicate that at least up to 10 or 15mm of contact distance, vacuum is vastly superior to air and equal to or better than SF6.

2.http://newt.phys.unsw.edu.au/~epe/1250.L4/1250.L4.html
This seems to indicate that vacuum has an infinite breakdown resistance.  Strange, but I seem to remember hearing something like that a long time ago in my high school physics class, too.  Doesn't seem right, otherwise none of those tube amps would work.

3.  http://www.isi-seal.com/search/doc/dielectric_strength.htm
This seems to indicate that the dielectric strength of air drops roughly linearly (if I'm correctly visualising a "stretched" log scale) with pressure from 1 atm (760 torr) down to about 0.5 torr.  At 0.5 torr, it looks like air ionizes easily and acts like a neon light, it'd actually be a pretty good conductor.  Below that, the dielectric strength starts shooting way back up again.

4.http://www.powellelectric.com/ptb/ptb37.htm
Indicates that "Modern vacuum interrupters are evacuated to a pressure on the order of 10-7 Torr. . . [but] will still interrupt its rated interrupting current at a pressure <= 10-3 Torr."  At those pressures, vacuum would perform better than air according to site (3) above.

So it looks like everybody's right.  At high altitudes, the lower air pressure reduces dielectric strength.  But when you start pulling a real vacuum, the vacuum starts performing better than air.


Ya learn something new every day.
CB2 (Electrical)
2 Apr 03 19:50
peebee, this is too much fun.
cuky2000 (Electrical)
3 Apr 03 2:07
There are few interesting remarks applicable to electrical equipment operating at high altitude:

1-    Dielectric strength rating (BIL, power frequency voltage, etc.) of electrical components non exposed in direct contact to air  are not affected by the altitude. Example of this are transformer winding submersed in oil, vacuum or SF6 interrupter devices

2-    The correction factor for dielectric strength (CFd), continuous current(CFc) and temperature (CFt)are affected in different way by the altitude (H) 1000 m above sea level as specified per ANSI/IEEE Std C37.40 (similar to IEC Std 282-1.2.)

A close linear analytical approximation is proposed as follow:

           a-    Dielectric:       CFd = 1.077 – 2.646x10-5 (H)   (1% per 100 meters
           b-    Cont. Current:  CFc = 1.018 –5.994x10-6 (H)
           c-    Temperature:   CFt  = 1.04 –1.199x10-5 (H)

                
3-    Correction factor start to at different altitude accordance with ANSI/IEEE Std for different equipment as follow:
          a- Metal Enclosed DC LV.bkr, Pwr circ. bkr, flex insulated bus, LV AC pwr.,........ >2000 m
          b- Control switchboards  ....................                                                         >1500 m
          c- Relays and relay systems   ........................                                                >1500 m
          d- Surge Arrester..............................                                                             >3000 m

4-    Maximum altitude rating is as follow:
       1- Transformers (oil, cast or resin), Instrument transf., regulator, shunt reactors     <4500 m         
       2- HV power circ. bkr, low V pwr circ. bkr, Metal enclosed rigid bus                              <3900 m                     
       3- HV air switches, insulators and bus supports:                                                       <6000 m
       4- HV fuses, insulators and bus supports:                                                                   <6100 m
       5-  Metal Enclosed DC LV bkr, Pwr  bkr, flex insulated bus, LV AC pwr.,                        <3900 m
       6- Control switchboards,( C37.14-79.)                                                                         <3550 m          
       7- Reclosers, sectinalizers load interrupting sw. & pad mtd swgr for AC                        <4900 m
       8- Relays and relay systems                                                                                      <6000m                                              

5-    Creepage distance derating is different than the dielectric correction factor. Several investigators suggest start apply correction factor few meters above sea level.


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