Power: For standardized transformers used at high altitudes a reduced value of power may be calculated, which from the point of view of cooling and temperature rise corresponds to service with rated power under normal ambient conditions.
External insulation: Altitude correction factor is applied to the coordination withstand voltage to account for the difference in dielectric strength between the average pressure corresponding to the altitude in service, and the standard reference pressure. As an example, the selection of a Um value higher than the next standard value of Um equal to or higher than Us may arise when the equipment has to be installed at an altitude higher than 1,000 m in order to compensate for the decrease of withstand voltage of the external insulation.
Name plate information: When a transformer is intended for installation at high altitude, the altitude, power rating and temperature rise at that altitude shall be indicated on the name plate. For more details refer to IEC 600076 & IEC 60071.
Testing at lab: If the installation site is more than 1,000 m above sea level but the factory is not, then the allowable temperature rises during the test in the factory shall be reduced as follows:
- for a naturally-cooled transformer (....AN), the limit of top-liquid, average and hot-spot winding temperature rises shall be reduced by 1 K for every interval of 400 m by which the installation's altitude exceeds 1,000 m;
- for a forced-cooled transformer (…. AF), the reduction shall be 1 K for every 250 m exceeding 1,000 m. A corresponding reverse correction may be applied in cases where the altitude of the factory is above 1,000 m and the altitude of the installation site is below 1,000 m.
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