how to choose the right insulators (high altitude)

[aherbas]

A question.

How do I specify the correct number or characteristic of insulators for a transmission line, at 3700 meters (12200 feets) over the sea, working at 69 kV.

The catalogs of insulators show the Critical FlashOver and not the BIL.

I understand that, to specify the insulation of the line I choose 350 kV of BIL at sea level and correct it at the actual atmospheric conditions, but the catalog do not show the BIL, just CFO.


Thanks.

 

[cuky2000]

The BIL of insulators require to be adjusted by altitude in accordance with guidelines of standards and accepted empirical engineering practice.
Beware that most electrical apparatus derating start after 1000 m (~3000 ft). However, for insulator some literature suggests start at sea level while others suggest start at 1500 ft.
Some of the most popular recommendations for altitude correction factors for insulators are described below:

a)Empirical Rule For Disk Insulators, a practical rule a 3% increases in clearance or insulator strings’ length per 1000 ft as elevation increase.

b) IEEE Std 1312: "Standard for Preferred Voltage Ratings for Alternating-Current Electrical Systems and Equipment Operating at Voltages Above 230 kV Nominal."

ACF = e^-Ht/8600
_{Where: ACF= Altitude correction Factor; Ht = the altitude in meters.}


c) DIN EN 60071-1 (VDE 0111Part 1):

Ka = e^m.H/8150

_{Where: Ka= Altitude correction factor, H =altitude in meters and m= factor for contamination (1=clean insulator, 0.5<m<0.8 for contaminated insulator)}.