BIL Question.

[clfrozzty]

Hi all:

Im new in foro and i have some query about BIL.

Why BIL are derrating in high altitude (over 1000 masl)?

I have the impression that the distances are increasing at a higher altitude and not "material quality" (eg porcelain insulator).

I hope we can discuss and present different points of view.

 

[LiteYear]

One reason is probably Paschen's Curve: at pressures near 1 atmosphere, breakdown voltage of air falls with falling pressure.

For example, at 2000 masl, air pressure is about 0.8 atm (608 torr). According to this chart breakdown voltage across a 0.1" air gap falls by about 10kV.

 

[clfrozzty]

Hi LiteYear, thank for you reply...

Now, the Pashen´s curve exposes what im saying (distance need increasing over 1000 masl), but i understand for BIL which is the basic insulation level for one material, for example:

For epoxy dry tranformer, IEC 60076-11 indicate for transformer for use at high altitudes (12.2), shall be increased by 1% for each 100 m above 1000 m, but, the material are the same (have the same material at 1000 m). No indicate derating the BIL of material (epoxy).

Someone else has another point of view?

 

[scottf]

The BIL "performance" inside material is not affected by altitude. The derating comes from the "external" performance, i.e. the dielectric strength of the air. Essentially, the strike distance must be increased as altitude increases to maintain the same external insulation level.

So a device with a BIL rating of 550 kV would have to be de-rated to 495 kV at 2000 masl (10% derating = 1% per 100m above 1000m).