High Altitude Considerations of Electrical Power Systems and Components

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High Altitude Considerations need to be taken in the design and application of electrical systems at eleva­tions in excess of 1000 m (3000 ft). It requires knowledge of the effects of atmospheric conditions on each particular component. Failure to understand adequately and include the effects of high altitude in the design and application of the equipment may result in its poor per­formance, premature aging, and/or failure. The relationship of rela­tive air density and altitude is discussed, followed by the effects of altitude on electric power system components. Along with the dis­cussion of the effects of high altitude on each component are sug­gestions or solutions to the high-altitude problem.

Although the subject deals with High Altitude Considerations of equipment, the per­formance of equipment from sea level to 1000 m may be affected by the relative air density. Since the relative air density decreases at a rate of approximately one percent per 100 m above sea level, the operation of any piece of equipment which is dependent on the air density will be different at 3300 ft compared to sea level. This subject is discussed so that independent conclusions may be drawn.

Temperature and barometric pressure together determine the relative air density, and the fact exists that, on the average, the relative air density decreases with an increase in altitude. In other words, the air is thinner at high altitudes, and it is the lack of air which adds the constraint to the electrical system design. As will be shown later, it is the combination of temperature and barometric pressure parameters, and to a certain extent humidity, which needs to be included in the actual design calculations. However, general rules or approxi­mations are given in the standards based on altitudes. The proper use of these approximations will greatly reduce the effort required in designing the system, but the engineer should be aware of the generalities and assumptions made by the standards in order to be more cognizant of the limita­tion of design.

Air is probably the most commonly and widely used of all insulating mediums. The majority of electrical distribution lines and practically all transmission lines are built above ground using air as the dielectric for both phase to phase and phase to ground insulation.

Another dielectric which is commonly used is that of a vacuum. Therefore, a seemingly good analogy is that if air is a good insulator and a vacuum is even better, then the thinner the air the better the insulator. The fallacy to this is easily shown in Fig. 1 by the relative dielectric strength of air as a function of pressure. The dielectric strength of air varies directly with the pressure until the pressure is low enough that a "good" vacuum is created. The better the vacuum, the higher the dielectric strength.