Derating of Electrical Equipment 2

[skm]

I understand we need to derate electrical equipment rating when installed at higher altitude due to reduction in the density of air. What is the standard derating factor used for various equipment? How are the clearance and the crrepage get affected by this altitude and to what extent?

 

[dpc]

This will vary depending on the type of equipment and the standards it was designed to. Transformers will be different from motors; US may be different than IEC, etc.

You really need to do this on a case by case basis. The manufacturer can give you information on appropriate derating for altitude.

Just as an example, the ANSI C37 standard for metal-clad switchgear requires that the basic specifications be met up to an elevation of 1000 meters. Above that, derating is permitted. At 3600 meters, the continuous current rating is 95% of nominal and the rated voltage is 75% of nominal, per GE book I'm looking at.

Your mileage may vary.

 

[jraef]

This is from a PE Utility Engineer I know (his page)

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

Lots of other good charts and stuff from him if you go back to his home page link at the bottom of this page.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[skm]

dpc,

can you let me know which GE book you are referring to? One method to arrive at the voltage derating is given in IEC60071-2 which is e^(m*H/8160), where H is the altitude above MSL and m is factor which depends on whether we are calculating the factor for LI, SI or PF withstand voltage. I could not get anything regarding current rating derating.

 

[dpc]

It's just the application guide for GE metal-clad switchgear. As I mentioned, you have to go by the standards used for rating the switchgear in the first place.

At very high altitudes, the continuous current rating can be less because the air is thinner and has less ability to carry heat away.

If you looking for information on US standards, this is covered (somewhere) in ANSI C37 for switchgear, C57 for transformers, and probably NEMA MG-1 for motors.

 

[cuky2000]

Below is a data to determine the altitude derating factors for a partial list of electrical equipment based on ANSI/IEEE Stds.

The ANSI Std, IEC or VDE altitude voltage or BIL derating factor are comparable in values. See also thread238-106701

Beware that not derating factor is required for part of equipment not exposed to air such as transformer winding immersed in fluid, breaker interrupting mechanisms in oil or SF6, etc. However, derating apply for bushings and any other exposed electrical parts.

Most equipment insulation are build with enough margins for altitude up to 1000 m (3,300 ft). However, equipment such as surger arresters are good up to 6000 ft without derating. Other such as insulators, derating start above sea level.

 

[scottf]

BIL is derated 1% for every 100m over 1000m.