Creep

[stevenal]

No, I'm not talking about Busbar.

I've been trying to firm up some of our specs, and am getting a little hung up on creep distance. Seems like creep should relate to contamination level and voltage, and possibly orientation and material. But there seems to be little consistency between the various IEEE and ANSI committees. Should there be a device dependency? I thought the transformer bushing people and the breaker people had their stuff together when both C57.19.100 and C37.06 showed the same mm/kV for the same contaminant levels. On further inspection, C57 applies the number to the nominal voltage level while C37 applies it to the maximum voltage rating. Do I need higher creepage on the breaker bushings than on the transformer bushings in the same substation?

Other devices don't specify a contaminant level, just a standard value that may be increased if needed.

Some information is given in IEEE 1313.2, but it doesn't seem to correspond to any of the device standards.

Heres an example for 115kV 550 BIL in a medium contamination area: transformer - 91", breaker - 98", station post - 99", MOV arrester - 90", instrument transformer - 79". Twenty % from one extreme to the other.

Does this make sense, or do we just have committees that don't talk to each other? Should this be device dependent? And if so, why shouldn't the highest value be at the high dollar transformer rather than the inexpensive station post? What's your practice? Thanks.

 

[scottf]

Just as a note, most manufacturers follow the bushing standard, which is based on line-to-ground voltage.

FYI, the instrument transformer standard, IEEEC57.13, does not list 79" for medium contamination. Rather, the IT standard only lists minimum creepage distances per voltage class. This seems to be a function of the bushing standard "light" level, which is 28 mm/kv. I believe C57.13 uses the maximum line-to-ground voltage and comes up with 79" for the 115 kV class as a minimum requirement.

The "medium" level of the bushing stand call for 35mm/kV, so based on max. line-to-ground voltage, that would be approx. 98".

What I believe you will find is that most equipment manufacturers, have a standard creepage distance between the "medium" and "high" levels. Most of the IT's that my company produces have a creepage distance of approx. 115" for the 115 kV class.

From my perspective, creepage distance should not be device specific. You should set a minimum level for all equipment in a given station, if you are moved to do so by evironmental conditions.

 

[stevenal]

scottf,

Yeah I used medium in my example because most of the non-breaker/transformer equipment seemed to be geared toward that level. The other extreme, the station post, also had only one rating.

Station posts are probably the most puzzling. In going from 15 KV to 123 kV maximum ratings, mm/kV goes from 31 to 44 and back down to 35, spanning the three levels of contamination of the breaker and transformer bushing standards.

I appreciate the feedback. Anyone else?

 

[jbartos]

Comment: The creepage distance can make the product life-cycle longer, mean time to maintenance longer, etc. Therefore, the manufacturer may exceed inductry standards in creepages in return for the better business.

 

[dbaird]

FYI.....
Online Creepage Calculator per 60950 standards:

http://www.creepage.com/

I believe UL, NEMA and IEEE all have creepage standards. You can get a free download of NEMA Standard ICS 1-2000, Industrial Control and Systems: General Requirements, which has examples of how to measure creepage in Annex C:

http://www.nema.org/index_nema.cfm/1427/9D33CEFC-89BD-4710-AB874C9405A1C635/

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[jbartos]

Comment: IEEE Std 100-2000 Dictionary defines:
Creep Distance (Outdoor Apparatus Bushing) as the distance measured along the external contour of the weather casing separating the metal parts which have the operating line-to-ground voltage between them.
Also:
The shortest distance measured along the external contour of the insulating envelope that separates the metal part operating at line voltage and the metal flange at ground potential.

 

[stevenal]

Seems to be some confusion regarding my question. I know the definition, and don't care about measuring creepage (I'm happy to let the manufacturers deal with that part.) The question deals with specifying creepage in an outdoor substation subject to contamination. Thanks.

 

[cuky2000]

The values below are minimum recommended by IEEE C57.19.100. Those valuses are also are in agreement with IEC standard.

Contamination VL-G VL-L
Light 28 mm/kV 16 mm/kV
Medium 35 mm/kV 20 mm/kV
Heavy 44 mm/kV 25 mm/kV
Extra Heavy 54 mm/kV 31 mm/kV

Derating of the above values should be corrected for altitudes above sea level.

 

[dpc]

You will run into limitations on the maximum bushing voltage ratings that you can get on transformers and breakers due to length limitations inside the equipment. I generally specify "extra-creep" at a specific voltage for coastal or dirty areas. If client wants something more, you can specifiy a higher voltage rating, subject to physical size limitations.

 

[cuky2000]

The major factors that affect the creepage distance are:

a- Insulator Position (Vertical, inclined, horizontal, V, etc). This could vary for transformer, breaker and other power apparatus bushing or post insulators.
b- Diameter of insulator or average diameter for none uniform section.
c- Phase to ground and pole clearance.
d- Shed minimum distance.
e- Shed configuration (inclination, form, alternation, etc)
f- Surfacing coating
g- Material (porcelain, semi-conducting, polymer, etc)

Insulator for most standard BIL typically covers the requirement for medium or heavy polluted environment. This perhaps contributes to have wide spread of creepage distance for different apparatus.

There are few insulator performance indicator related with creepage distance:
- Profile Factor
- Insulator Form Factor = Integral [d(cd)/C(cd)]
_{Where:
.cd = Creepage distance
C(cd)= Insulator circumference as a function of the creepage distance.}

In addition to the above factors, design economics and the random characteristic of dielectric failure/strength made virtualy impossible to standardize the creepage distance for all electrical apparatus.

To answer specifically your questions an insulation coordination study is required. This is time consuming, costly, and many times the accuracy could be questionable if is no done properly. Most practical approach is to determine the history of insulator failure in the area and use the higher creepage distance that can be afforded for each apparatus.

 

[jbartos]

Comment: The content of IEEE C57.19.100 may need to be revisited. Namely, the distance/kV relationship between the VL-G and VL-L is related by sqrt3. It means that the VL-G surface resistance or impedance is sqrt3 times VL-L related resistance or impedance. Therefore, the creep distance is in direct proportion to the surface resistance or impedance.

 

[stevenal]

The mm/kV values given in both C57.19.100 and C37.06 are based on voltage across the insulation which is line to ground for the dead front equipment the standards cover. The difference is that one uses maximum voltage while the other uses nominal. 123kV versus 115kV in my example case above.