Altitude Derating
Altitude Derating
(OP)
All,
Can anyone explain the logic (physics) behind the need to derate at altitide.
To my twisted way of thinking, it is the opposite to what I would have thought.
At altitude, air is less dense, and I would have thought that this would have made it a better insulator - apparently it does not, it is the opposite - why??
Also, there is the issue of different derating requirements between LV and MV. For LV, derating only applies above 2,000 metres, while for MV, derating applies above 1,000 metres - why is this so??
I haven't been able to find a suitable explantion why - any explanation would be appreciated (because for my poor old mind, if you use this logic, a vacuum should be a good conductor).
Cheers!!
Can anyone explain the logic (physics) behind the need to derate at altitide.
To my twisted way of thinking, it is the opposite to what I would have thought.
At altitude, air is less dense, and I would have thought that this would have made it a better insulator - apparently it does not, it is the opposite - why??
Also, there is the issue of different derating requirements between LV and MV. For LV, derating only applies above 2,000 metres, while for MV, derating applies above 1,000 metres - why is this so??
I haven't been able to find a suitable explantion why - any explanation would be appreciated (because for my poor old mind, if you use this logic, a vacuum should be a good conductor).
Cheers!!






RE: Altitude Derating
One common type of derating I am familiar with is power derating of a motor of transformer due to altitude. If the air is less dense it is less effective at removing heat.
It sounds like you're talking about reducing voltage due to altitude. I haven't heard of that, but I do believe that the breakdown voltage reduces as air pressure decreases. As to why a vacuum interruptor works well.... breakdwon strenght is no a monotonically increasing or decreasing function of pressure. As we decrease pressure from atmospheric the breakdwon strength starts to go down, then hits a minimum and starts to go down. Similar to Paschen's curve.
RE: Altitude Derating
RE: Altitude Derating
Yes, i am talking about voltage derating. The rated one-minute power frequency withstand voltage, the impulse withstand voltage and the maximum voltage rating must be corrected (derated) for altitudes above 1,000 metres for MV applications.
The continuous current rating must also be corrected.
For the site I am at, in the Andes at 3,600 metres, the altitude correction factors for MV are 0.75 for voltage and 0.95 for current.
As you say, as the air becomes less dense, it becomes less effective at removing heat - doesn't this make it a better insulator - why does this not apply electrically.
We use the correction factors as per the code, but I am just curious as to why - what are the physics involved.
RE: Altitude Derating
http://home.earthlink.net/~jimlux/hv/paschen2.htm
RE: Altitude Derating
Blacksmith
RE: Altitude Derating
generate a .1 inch spark/discharge. Around 1mm hundreds
are enough ( see: neon/ glow discharge lamps)
<nbucska@pcperipherals.com>
RE: Altitude Derating
You mention that PFWV, BIL, Maximum Voltage, and continuous current ratings are affected by altitude.
In my experience (mainly instrument transformers and distribution transformers) only the BIL rating is generally affected by altitude, mainly because it is more dependant on strike distance than the others and since the strike distance is through air...
The altitude (outside air pressure) has no affect on anything under oil (i.e. inside a tank or housing) in sealed type apparatus. Therefore, I do not see why it would affect max voltage or continuous current rating (at least not at any reasonable altitude).
RE: Altitude Derating
The voltage derating only applies for anything exposed to air - power line insulators, t/f bushings, switchgear busbar support insulators, etc.
At altitude, air becomes a less effective insulator electrically (as I have just discovered), hence the voltage derating.
Also, at altitude air becomes a more effective thermal insulator, hence current derating.
There is quite a good article on:
http://hot-streamer.com/TeslaCoils/OtherPapers/NorthReport/north7.pdf
RE: Altitude Derating
However, I still don't understand the need to derate operating voltage. I can tell you that in equipment I deal with, operating/maximum voltage rating is not reduced with increased altitude. The air distance/strike distance should only affect the insulation levels and in that regard the transient ratings, i.e. BIL. If you have an apparatus where the rated voltage is affected by the strike distance/air distance, then I would question the integrety of that design!
IEEE C57.13 also gives a dielectric correction factor of 1% for every 100m over 1000m. This is the same as in the IEC44-XX specifications for instrument transformers. Neither of these specifications mention the need to de-rate the operating voltage rating as a function of altitude.
RE: Altitude Derating
I've seen the effects of the lower insulation qualitis of air at higher altitudes. If you're not expecting it, you stand around scratching your head until you figure it out. It IS real. A 35 kV large interface elbow will not work in Rock Sprngs, Wyoming, even though along the east coast they use them all the time without any problems. You get a flash about at least 10% of the time when you operate them.
Mark in Utah
RE: Altitude Derating
believe the answer you're looking for has to do with the pressure-altitude-voltage
derating related to corona discharge, which increases (gets worse) with higher
altitude and peaks at around 80-100,000'
Typically this is considered to occur at votlages above 300 V peak, but be sure
you are accounting for all transients and internal voltages.
RE: Altitude Derating
There are several graph in ANSI C37.30 and IEC/DIN DVE 0111 that describe this factor in similar maner as follow:
Altitude Derating Factor
Altitude: Dielectric(pu) Current (pu)
>3,000 ft (1000 m) 1.00 1.00
4,000 ft (1200 m) 0.98 0.992
5,000 ft (1500 m) 0.95 0.980
10,000 ft (3000 m) 0.80 0.96
12,000 ft (3600 m) 0.75 0.95
14,000 ft (4200 m) 0.70 0.935
EX. Determine the equipment rating at sea level to operate at 3,600 m (12,000 ft) if the design insulation is 900 kV BIL.
Per the above table, the dielectric derating factor for 3,600 m is 0.75.
External Insulation: 900kV/0.75=1200 kV BIL.
The internal insulation for appartus not direct exposed to the air could such as circuit breaker in SF6 or transformer inmerse in oil, there are not need for external derating. In this case only the bushing could be affected by the altitude.
Note that cables, surge arrester, creepadge distance and safety requirement are ruled with similar but different requirements.
Other factor such as temperature, moisture, contamination level, should be take in consideration for determine the appropiate dielectric strength.
RE: Altitude Derating
http://www.saftronics.com/pdf_fils/library/tn_vfd_gen002.pdf
for more info
RE: Altitude Derating
RE: Altitude Derating
RE: Altitude Derating
For the lighter air, you have less number of particles in the air. To expalin in a very simple way, I can you an exmple of a needle to be inserted in a soft material and a hard material. Say foam: it is easier to pierce with lower force. For dense foam, you need more force to pierce.
Now replace foam with say air (or for that purpose any insulating gas), the needle with electron. Apply same analogy. You will need more force (electro motive)as you have dense air (gas).
Now that we are on this subject, I give some mild socks to your old mind. More humidity (water vapour, a gas, will increase breakdown voltage of air (gas). Keep im mind only condensed water is killer not the humidity! For higher humidity, the voltage correction factor is +ve.
Hope this will explain you the science of breakdown of gaps.