Hi Can anyone explain me why no ad

[hhhansen]

Hi
Can anyone explain me why no adjustment should be made for the permissible limit of winding temperature rise for indirect cooled machines when the difference in test temperature and site temperature is less than or equal to 30 K. This is stated in IEC34-1 (ed. 10.2)table 10.

As and example we have a machine (IC611,4700 kW) tested at 21 deg. C and 6 m. a.s.l. The temperature at site is 50 deg. C and the machine is placed at 130 m a.s.l. The density of air at the test room is 1,264 kg/m3 and 1,075 kg/m3 at site. Thus one would expect the cooling circuit efficiency to decrease or what ?

 

[aolalde]

hhhansen;

I think you are making two questions.

Why no correction is made if test and site ambient temperatures are into 30 K?
How elevation affects the cooling process?

My oppinion is that into the limits of your example, the temperature rise due to the air change properties is not affected. ( review change of air densities with 130 m change)

I don’t know how air cooling properties are affected with temperature; I assume temperature reduces the air ability to absorb heat. The standard should consider that 30 K will not affect air properties appreciable.

For height over see level NEMA MG1 states the following adjusting factor.

Trsl = Tra *{ 1 – (Alt-1000)/10,000} were:

Trsl = expected test temperature rise at sea level °C
Tra = temperature rise limit per the insulation system used
Alt = altitude above sea level in meters at which machine is to be operated.

For example considering 80 °C temperature rise limit and 1,130 m ASL for location:

Trsl= 80 * (1-(1130-1000)/10000) = 0.987*80 = 78.96°C

So For 130 meters asl, no change is expected.