Creepage Altitude Derating Factor

[cuky2000]

Hello all,

Does any one knows were to find information derating factor for altitude for insulator creepage distance?

I found some reference publication related with insulation coordination without clear reference associated with any standard. The IEEE or IEC standard may cover the BIL derating factor for altitude but there is not a lot of information for creepage distance.

Thanks.

 

[jraef]

http://www.geocities.com/thomas_b_34654/altit1.html

Tom is a PE friend of mine in Florida and sits on a number of IEEE committees. If you click back to his home page you can send him an email if you want to know where he derived his formula from. He's a very congenial guy and very open about sharing his knowledge.

 

[scottf]

I've never heard of derating creepage distance for altitude.

Perhaps you mean strike distance?

 

[cuky2000]

Thanks JRaef, I sent an email to Tom Blair as you suggested.

Scottf, I am agree with you that there is not much information in this subject. I am not sure where to find support reference from recognize standards.

I would like to share with you some references in this subject associated with pollution/creepage, BIL & SIL that may be found in the enclose link.

http://cuky2000.250free.com/Altitude_Derating_Factors3.jpg

COMMENTS:
1- Power apparatus and electrical equipment tested according with IEEE or IEC have allowance up to 1000 m without derating. (DF=1)

http://cuky2000.250free.com/Altitude_Derating_Factors1.jpg

http://cuky2000.250free.com/Altitude Derating factor 2.jpg

2.-In contrast for insulators, the encloses curves suggesting derating factor different than 1 for altitude under 1000 m.
3- There are different derating factor for BIL and for switching surge as also suggested on T. Blair web site.

 

[stevenal]

After discussing various creepage versus contamination methods, IEEE 1313.2 says all the methods are applicable at sea level. Then multiply the number required at sea level by a factor of e^(A/14) where A is in km to get the number required at altitude.

 

[cuky2000]

Stevenal, thanks for the input.

Jraef, you were right, Tom Blair is not only a nice person but also very knowledgeable in the subject. I would like to share his input as described below

===========================================================
IEEE is working on a draft standard for "IEEE C37.100.1 - Standard of Common Requirements for High Voltage Power Switchgear Rated Above 1000 V" that addresses Altitude insulation derating methods. Historically, there was one method here in states, and one method for international. IEEE is proposing a modification of the two methods to make a third method that is a combination of the historical two methods.
I might recommend IEEE PC37.100.1, appendix B as a reference for further research. This addresses the current three methods of altitude insulation level derating.
The short version is that, method 1 which mimics the IEC 60071-2 standard methods, derates from 0 feet altitude by the correction factor Ka where;

Ka = exp(m*(H/8150)

Where: H = altitude in meters and
m = 1 for power-frequency, lightning impulse and phase-to-phase switching impulse voltages.
m = 0.9 for longitudinal switching impulse voltage (i.e. across the isolation gap)
m = 0.75 for phase-to-earth switching impulse voltage.

And the new insulation rating at the actual altitude is the insulation rating at sea level, divided by Ka. This method would have a derate (for m=1) of about 13% for switchgear at an altitude of 1000 meters.
Note that, while this method defined by IEC 60071-2 derates from 0 feet above sea level, the historical method (we will call method 2) that is used in the USA, derates starting at 1000 meters (3300 feet) above sea level. In USA, there has been no derate from 0 - 1000 meters.

For this method (Method 2, historical method), the above correct factor for the method 2 (historical method) would be;

Ka = exp(m*((H-1000)/8150)

In draft standard PC37.100.1, IEEE has taken a combination of the above two methods, we shall call method 3 and is placing info in their new standard (currently in draft and review phase) The new standard will be IEEE C37.100.1 - Standard of Common Requirements for High Voltage Power Switchgear Rated Above 1000 V. It uses the derating factor of method 1 (the IEC method), but allows for no derate between 0 and 1000 meters above sea level. Therefore, between 0 to 1000 meters, IEEE C37.100.1 will have no derate, but at 1001 meters there is a derate of 13% (for m=1) and derate increases above this at a value defined by;

Ka = exp(m*(H/8150)

Where H = altitude in meters and
m = 1 for power-frequency, lightning impulse and phase-to-phase switching impulse voltages.
m = 0.9 for longitudinal switching impulse voltage (i.e. across the isolation gap)
m = 0.75 for phase-to-earth switching impulse voltage.

This is a new approach and my web site still shows the historical method. Once the IEEE standard C37-100.1 gets approved, I suspect I will need to update my altitude derate page for the new IEEE method.
You might also consult UL for their standards on altitude derating methods.

http://www.ul.com

I hope the above info is helpful to you. Contact me anytime I can be of assistance.

 

[scottf]

cuky2000-

Correct me if I'm wrong, but all of the derating you've listed is applicable to strike distance.

I understood the original post to have been about creepage distance. I'm of the understanding that creepage distance is not affected by altitude....that is leakage current due to pollution does change as a function of altitude.

Certainly, we never take altitude into consideration for creepage distance for the units I work with (instrument transformers up to 800 kV).

 

[cuky2000]

Hi Scott,
It is true that most altitude derating factor is applicable to BIL. In most practical application, the BIL is the dominant factor. If the BIL increases the creepage distance became larger automatically. The creepage distance is also a function of the power frequency voltage that may have small impact with altitude.
The following is an excerpt from a publication from the University of Quebec: "In the presence of contamination, the surface resistance is reduced more drastically. Altitude or the elevation above sea density, hence, weakening the surface insulation strength".
This and the link above are indication that the creepage distance may decrease with altitude. However, I could not find references in the standards or any recognized publication directly applicable to creepage distance variation with altitude.

 

[scottf]

BIL and creepage distance are not directly related.

Taking my product as an example, we have all kinds of insulator options. I can use an insulator twice the strike distance, but 75% of the creepage distance of a comparable insulator, depending on the shed design and profile.

For most types of apparatus, the creepage distance plays no part in the ability of the apparatus to withstand a BIL or dry power frequency withstand test.

For example, we could take our standard 230 kV voltage transformer and make it with an insulator with no sheds. It would still meet a 900 kV BIL test and a 395 kV dry PFWV test.


I've never heard of surface resistance being reduced as a function of altitude. I wonder what the theory behind that assumption is. I've never heard of increased tracking problems as a function of altitude.

 

[cuky2000]

Scott,

Please check if the curve for pollution and contamination in the enclose link have any relevance for this discussion.

http://cuky2000.250free.com/Altitude_Derating_Factors3.jpg

Thanks.

 

[stevenal]

Consider a flashover event that begins as leakage along the damp surface of a contaminated insulator. As leakage current increases, I believe that at some point the arc will leave the surface. Maybe the current follows the surface halfway down the isolator, then jumps from the halfway point to ground. In other words the too low creepage has reduced the effective strike distance. Even if the overall strike distance is increased for the altitude, it is still necessary to increase the creep to avoid this occurrence.

 

[jghrist]

Basically, contamination flashover is a function of the surface contaminant, not air. The air gets less dense at higher elevation, but the surface contaminant doesn't change. One thing is for sure, the higher you are above sea level, the less sea salt contaminant you will have.

 

[cuky2000]

Hi jghrist,
This is getting better. Appears there are two conflicting position regarding the surface contaminant resistance if weakening or not with altitude. We probably need more support research in this area.
In addition of the fault by the pollution in the surface of the insulator, the flashover mechanism could also happen bridging by drips or cascades and air flashover as shown in the enclose link.
Probably in any bridging in the air, the altitude may have an impact reducing the insulator dielectric strength.
Any comment is welcome

http://cuky2000.250free.com/Pollution and Contamination 2.jpg

 

[stevenal]

jghist,

For a contamination initiated flashover to be unaffected by the dielectric strength of air, the entire event must be confined to the surface. Do you have evidence that this is how these flashovers occur?

 

[cuky2000]

Below are two links showing pictures of insulator flashover. The second link is an excerpt of a simulated test showing that the contamination flashover of insulators depends not only on the severity of the contamination, but also on the altitude at witch these insulators are situated.

http://cuky2000.250free.com/Insulator Flashover.jpg

http://cuky2000.250free.com/Altitude Creepage Derating Factor 2.jpg

 

[prc]

Cuky, I dont think the links that you put up shows that creepage distance and altitude has a relationship.Of course,arcing distance and altitude has a link and standards have clearly specified this .Eg. Bushings (IEC 60137) require that arcing distance has to be increased by 1 % for every 100 metres that the altitude exceeds over 1000 metres.I have never seen any such stipulation in any standard (eg IEC 60815 Guide for selection of insulators in respect of polluted conditions)with respect to creepage distance of insulators.
As Scottf said if the equipment is indoor, petticoats are not provided and creepage distance has not much significance.Problem starts with pollution deposit with moistute from rains.Once it was thought that increasing the creepage length by providing more petticoats with deep corrugation underside ( anti-fog sheds) will improve the performance.Studies done during 60's&70's (esp by NGK ) showed that it is not the correct solution and easy clean shed profile came in to practice (IEC 60815)Protected creepage distance requirement also changed from more than 50 % to less than 50 % !

 

[stevenal]

I didn't expect that when I posted my example scenario on the 20th, that a picture would should show up illustrating it almost perfectly. See Cuky's upper left hand insulator picture. But they all show that the lack of dielectric strength of the insulator surface combined with that of the air combine for these flashovers to occur.

 

[jghrist]

prc,

It seems clear to me that the highlighted sentences in cuky's second link shows a relationship between creepage distance and altitude. If the flashover is lower at higher altitudes for a given creepage distance, then it follows that the creepage distance requirement to meet a particular flashover voltage would increase at higher altitudes.

There may not be any stipulation in standards, but some consideration could be made based on the Chinese study results.

 

[prc]

Jghrist,My submission was that Chinese paper merely says that contamination flashover depends not only on the severity of contamination,but also on altitude.It does not show any experimental results.The issue is with altitude -arcing distance is to be increased with higher altitude.But it does not require to increase creepage length in mm with higher altitude.But with higher arcing distance gap between sheds is automatically increased there by increasing the flashover with stand strength.These issues are well discussed in IEC 815 based on the studies by NGK and others.

Stevenal is correct.Flashover is not happening by creeping over the porcelain surface.That is exactly why we went away from deep corrugated, anti-fog sheds to easy clean sheds with reduced creepage length but with new stipulations on shed pitch Vs shed hangover etc.

So the position as on today in Industry is creepage length is a factor of equivalent salt deposit density X Rated Votage (In IEC, system voltage,in ANSI phase to earth voltage)X a constant depending on shed dia (when dia increases ,this factor goes up).There nowhere altitude is coming in to and I think the logic is correct .

 

[cuky2000]

PRC, a few observation to your post.
• Presently IEC or ANSI standards do not address the creepage variation with altitude.
• Your discussion appear to be based in personal opinion with emphasis in BIL derating with altitude. The Chinese paper is based in lab. experiment.
• The paper addresses no only the increase in creepage with altitude but also with ESDD for different type of insulators.
• Other publications including ABB, university of Quebec,IEEE, etc support the correlation between additional creepage distances for high altitude.