With the increase of altitude, the atmospheric pressure decreases, and the air density and humidity decrease correspondingly, which is characterized by:
a, the air pressure or air density is lower;
b, the air temperature is low, the temperature change is large;
c, the absolute humidity of the air is small;
d, dayang radiation illuminance is high;
e, less precipitation;
f, the number of strong wind days;
1. The impact of reduced air pressure or air density
1) Influence on the strength of insulating medium
The reduction of air pressure or air density causes the reduction of external insulation strength. In the range of altitude to 5000m, when the average pressure is decreased by 7.7~10.5kPa per 1000m, the external insulation strength decreases by 8%~13%.
2) Impact on breakdown voltage of Electrical gap For products with fixed design, the breakdown voltage also decreases with the decrease of air pressure because the electrical gap has been fixed. In order to ensure the sufficient breakdown resistance of the product in the plateau environment, it is necessary to increase the electrical clearance. The electrical gap of highland electrical products can be corrected according to the following table.
| Attitude/M | 0 | 1000 | 2000 | 3000 | 4000 | 5000 |
|
Atmospheric pressure /kpa |
101.3 | 90.0 | 79.5 | 70.1 | 61.7 | 54.0 |
| electric gap/1000m | 1.0 | 1.13 | 1.27 | 1.45 | 1.64 | 1.88 |
| electric gap/2000m | 0.89 | 1.0 | 1.13 | 1.28 | 1.46 | 1.65 |
| correction factor | 0.78 | 0.88 | 1.0 | 1.13 | 1.29 | 1.47 |
3) On the cooling effect of the medium, that is, the influence of product temperature rise
The decrease of air pressure or air density leads to the decrease of cooling effect of air medium. For electrical products with natural convection, forced ventilation or air radiators as the main mode of heat dissipation, the temperature rise increases due to the decrease in heat dissipation capacity. In the range of altitude to 5000m, the temperature rise increases by 3%-10% for every 1000m increase, that is, the average pressure decreases by 7.7-10.5kpa.
A. The temperature rise of power transformer is related to the cooling method with the increase of altitude, and the increase rate per 100m is: oil-immersed self-cooling, 0.4% of the rated temperature rise; Dry self-cooling, rated temperature rise of 0.5%; Oil immersion forced air cooling, rated temperature rise of 0.6%; Dry forced air cooling, rated temperature rise of 1.0%;
Two、Dry Type Transformer
Epoxy cast resin transformer，For the transformer operating at an altitude of more than 1000M and testing at a normal altitude, its temperature rise limit shall decrease accordingly. For the part with an altitude of more than 1000M, the temperature rise limit shall be reduced by 2.5% for the self-cooled transformer and 5% for the air-cooled transformer. The rated short-time power frequency withstand voltage is also increased by 6.25%.
Three， Oil Immersed Transformer
In terms of external insulation, it is generally necessary to strengthen the outer insulation of the casing, increase the leakage distance along the surface and the discharge distance to the ground, and increase the casing and sleeve. The air gap size of the tube to the ground component to overcome the influence of thin air and low air pressure on the stability of the transformer external insulation.
Oil immersed power transformer outer insulation distance according to the altitude of 1 000 m above, every rise of 100 m for a level, each level increase the air gap 1%;
Oil immersed self cooling transformer ， The decrease rate is 2% for every 500 m elevation increase; Oil-immersed air-cooled and strong-oil air-cooled transformers reduce 3% per 500 m elevation.
