Clearance and creepage in oil

[TMD]

I have read few standards (only one I can think of) that specifies clearance and creep distances in oil filled equipment. All the standards I have read on medium voltage equipment (15 kV), both NA and European, specify spacing in air with no mention of oil (or other fluids). Anybody have any comments or references that will help? Seems to me both clearance and creep can be reduced, but by how much? An impulse test will be teh pass/fail for a design, but I'd like to pass the test on the first try.

 

[electricpete]

I don't know the answer. But since you have no answers I'll throw in one thought.

There is a routine test performed on oil called dielectric strength. Two different standards change the shape of the electrode.

It seems to me typical numbers are 35kv - 45kv. What is the spacing? If it is one cm, this seems like not much more than dielectric strength of air, which is 30kv. Maybe this suggests that clearance should be comparable to air?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[TMD]

Dielectric strength for oil is usually measured in accordance with ASTM D877. Spacing is 0.1". For air, we use 8 kV for this distance for design. So dielectric strength is much higher for fluids (35-45 kV). I'm more concerned about creep than clear actually.

I'm more concerned with designing to a standard, or if none are applicable, then designing based on rules of thumb or engineering experience such that I can pass electrical tests (dielectric, insul resistance, impulse and maybe PD)

 

[cuky2000]

In insulated medium such as oil, FS6 or vacuum, the creep distance may not be as significant as in air where is required because of the interaction of contaminant and humidity conducting bands. Sample of that are the transformer bushings with strait insulator inside the oil and irregular configuration to increase the leakage distance in contact with air as show in the enclose figure

On the other hand, clearance of static equipment such as transformers, depend primarily of the dielectric strength of the oil and geometric of the conducting parts (electrodes). If the electrode will create arching such interrupting devices, additional consideration and clearance reduction should apply. Humidity in oil and contaminating particles also contribute to reduce the insulating strength of the medium.

Here is a graph that may be use to determine the clearance in insulated oil.

 

[TMD]

cuky2000: Thanks for that. It'll take some time to put it to work, but it's a good, sensible start. I'm thinking these are not your drawings. If they are...triple thanks. Can I have a doc that I can reference?

 

[davidbeach]

First line of cuky2000's post should have SF6 in lieu of FS6. cuky, you may want to red flag your post and have the Eng-Tips folks correct that. If so, you can also have this message deleted.

 

[stevenal]

Is creepage really an issue at all under oil, SF6, within a nitrogen blanket, or even dry air inside equipment? Don't believe I've ever seen a creepage value specified for that part of a bushing below the flange.

 

[scottf]

I agree with Stevenal. I don't think creepage is even a relavent parameter under oil or basically any system not exposed to outdoor-like environmental conditions.

As for the space requirements under oil, it very much depends on the type of oil being used. Some oils have better insulating properties than others.

 

[electricpete]

cuky - you da man! Good stuff.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.