Design Considerations of Mounting Bushings at an Altitude More Than 1000 Meters

Mounting transformer bushings above 1000 meters above mean sea level (MSL) introduces unique dielectric and environmental challenges:

• As altitude increases, air density decreases, lowering air breakdown strength. At sea level, the dielectric strength of air is around 30 kV/cm, although it decreases substantially at higher altitudes. As a result, insulating coordination margins are reduced, increasing the risk of flashovers.

• The arcing distance must be increased by an appropriate amount to ensure that the bushing's external withstand voltages are adequate at altitudes greater than 1000 meters.

• Adjusting the radial thickness of insulation or the clearance of the oil-end of the bushing is not required. The puncture strength and the flashover voltage in the immersion medium of a bushing are not affected by altitude.

• For installations of bushings at an altitude higher than 1000 meters, the required arcing distance under the standard reference atmospheric conditions is to be determined to withstand the voltages obtained by multiplying the withstand voltages required at the service location by factor kₐ.

Calculation of correction factor - kₐ

Where,

H = altitude in meters from MSL, not exceeding 4000 meters as per IEC 60137:2017 and 4500 meters as per IEEE C571900:2023.

m = 1 for lightning impulse withstand voltage

m = 1 for power-frequency withstand voltage where Uₘ ≤ 245 kV

Example: Determine corrected test voltage for the bushing (245 kV 2000 A 300 CT) suitability for operation at 2500-meter altitude ‘H’ from MSL with a verified arcing distance of 3000 mm.

For calculating of correction factor kₐ -

Considering m = 1 for lightning impulse withstand voltage and power-frequency withstand voltage

Considering m = 0.76 for switching impulse withstand voltage as per the figure above,

Option A: Increase in arcing distance.

Minimum arcing distance at ‘H’ meter altitude of operation

= Correction f factor kₐ X verified arcing distance

= 2400 X 1.27

= 3048 mm

Note: If the increased arcing distance is based on a verified arcing distance, it can be considered valid.

Option B: If the arcing distance is not increased, it can be verified by testing the bushing at the corrected dielectric test voltage.

Corrected test voltage = standard test voltage X correction factor kₐ