Derating Factor

It's got more to do with atmospheric pressure, which at +4000m is <62% of the pressure at sea level, according to The International Standard Atmosphere table contained within Thermodynamic and Transport Properties of Fluids, Mayhew & Rogers, published by Oxford Basil Blackwell, 1976.

If the genset cannot breathe, then it cannot burn fuel and if it cannot burn fuel, it cannot generate electrical power!

Efficiency and derating factor are two different concepts.

Efficiency is a measure of the work out from the device divided by the work available from the fuel in.

Derating factor is an operating constraint created by the manufacturer to account for differences in operating conditions versus the design, factory and/or testing conditions.

Not sure I can agree that they are separated as the derating factor negatively affects the machine's efficiency rating.

The higher the derating factor, the higher the loss of generation capacity, hence the lower the efficiency of the machine.

A very important aspect to consider when choosing a generator for any application especially when it comes to the altitude of mounting location.

The de-rating information may be in the manufacturer's gen-set manual or literature. Also try their website.

Alternatively you could contact them directly for application-specific answers.

Dear Mr.naren326,

DE-RATING FACTOR: The Correction Factor to be applied for the actual performance in relation to the design operating parametres.

EFFICIENCY: The ratio of the output to the input.

Hence they are 2 Different Parametres or Elements and cannot be compared or equated and independent of each other.

DHAYANANDHAN.S

If derating factor for 82.5 KVA DG set at 4000 m altitude is available to be 0.85.Then,for obtaining same output the capacity should be increased to 97.06 KVA.It means,if I am not wrong I have to go through 100 KVA DG set. Since,using derating factor shows the loss in power. Also the efficiency involves the loss in power.Does efficiency also decrease along with th e rating factor?Does the fall in efficiency more or less than the derating factor? Since,air density is very low at high altitude and without sufficient air,ignition of fuel is not possible resulting difficulty in starting the DG set? If there is any mistake in understanding,please make a clearification.

De-rating factor has to do with heat dissipation. At higher altitudes, as the density of air is lesser, less mass of coolant is available in a given volume and thus the de-rating.

Whereas efficiency is output/input.

A DG set can have lesser efficiency even when not de-rated.

But you have valid point.

If you go for 100kVA DG set and operate it at lesser altitudes, you would load it for, say, 100kVA and it would give a certain UPL (units per litre). But, if the same DG set is used at a higher altitude, you can load it only up to 85kVA (i.e.) 85% loading. Then, definitely, the UPL would be lesser.

Dear Mr.naren326,

You have mentioned in your original post "If 82.5KVA DG set (Jackson genset) is to be used at 4000m height where the atmospheric temperature is about 18°C,what will be its effect on the performance"

While you have mentioned DG set is 82.5 KVA, and DE-RATING FACTOR is 0.85, the out put will be 82.5 x 0.85 = 7.125 and not 97.06. You have devided the capacity by DE-RATING FACTOR, instead of multiplying.

Reduction in output will be more on account of DE-RATING FACTOR than the Drop in Efficiency.

DHAYANANDHAN.S

Mr. Dhayanandan, I am afraid you are a wee-bit incorrect. Mr. Naren has calculated correctly only. If you want 82.5kVA at 4000m Altitude, what would be the MSL capacity of the genset is the question. For this you only have to DIVIDE the required genset rating at the actual altitude by the De-rating factor to get the MSL genset rating.

Alternately, a 100kVA DG set at MSL, when to be used at 4000m altitude, when multiplied by the given de-rating factor of 0.85, would operate satisfactorily at 85kVA loading.

Genset Rating Calculation and Feasibility - CR4 Discussion Thread (globalspec.com)