Altitude and derating

When equipment is expected to work at high altitudes de-rating factors should be applied when choosing rating of the components. At high altitudes, where there is a reduced atmospheric pressure, the cooling of electrical equipment is degraded due to reduced ability of the air to remove the heat from the cooling surfaces of the machine or transformer or heat-sink of the converter. Different equipment providers give slightly different derating factors. One possible collection of factors is presented in tables 1 and 2. In DriveConstructor derating is done automatically when the user sets the right altitude for the components.

Power derating

The coefficient of power derating with altitude $k_h$ is defined as permissible power output in % of rated power.

Table 1. Power derating $k_h$ as function of altitude.

When the component needs to be de-rated for both temperature and altitude, the de-rating factors given in the respective tables should be multiplied together. When a component, like liquid-cooled FC, has both air- and water-cooled parts, derating should be calculated for both parts of the respective ambient- and coolant temperatures. In addition derating for altitude (if needed) should be implemented. Smallest of the products $k_h*k_a$ and $k_h*k_w$ should be taken as the overall overload.

Temperature of the coolant may help to solve the high-altitude problem for liquid-cooled equipment - it is possible to compensate high altitude with lower temperature of the coolant.

Voltage derating

The coefficient of voltage derating with altitude $k_v$ is defined as permissible voltage in % of rated voltage.

Table 2. Voltage derating $k_v$ as function of altitude.

As the cable between the machine and the converter can be long (in special cases up to several km) the altitude can be quite different for the machine and the converter.