Why do we need to derate (lower the rating) the air cooled turbo-generators which are to be comissioned 1000 metres above mean sea level? Is it because of mechanical or electrical constraints? (By mechanical I mean that does the heat dissipating capacity of air decrease with inclease in altitude?) What is the derating factor for siemens make of generators? Is there a curve for this purpose? Any help in this regard will be appreciated.

 

Abhay, derating for altitudes above 1,000 mt is required for an electrical reason. At elevated altitudes, atmospheric pressure decreases, reducing air-density. As a result heat dissipation is curtailed.

Although rather simple, the USA and German approaches are different. However, both result in a linear equation. The US approch is covered in NEMA Std MG 1, while the German approach can be found in VDE 0530.

If you are unable to access either document, let me know!

Regards, Phil Corso, PE, ([email protected])

 

Another reason for the deration factor for elevation as well as the main reason, is that the oxygen is thinner as you get higher up so the prime mover doesn't operate as efficiently. Temperature deration is also because of both the prime mover and the generator. The prime mover because too much heat causes the difference in temperature ratio to become smaller therefore decreasing the capability. For example, 2 identical generators, one in Iraq @ 140F, the other in Germany where it's 70F. The cooling system is not supposed to go higher than 190F or an alarm and then a shutdown occurs. There's only a 50 degree tempo difference in Iraq, so it get's to max temp with a lot less work being done than the one Germany. This is essentially the same reason for the generator itself, the wiring operate best below a certain temp so the higher the temp is the less current that can pass through the windings and wiring.