DG set sizing

You need to arrive at the rating of the set first after detailed study of the load and using the diversity factor etc. Derating is done depending upon the engine and alternator's manufacturers recomendation like higher altitude > 1000 m above sea level, higher ambient temp etc.So deratng is done depending upon other conditions and not while arriving at the rating.

By the way, what is the difference between standby and continuous power rating?

Continuous rating is 100% of the gen set ( for both engine and alternator), Standby is 110% for one hour in 12.

Derating is required for engine and a little bit also for alternator for altitude, temperature and a small bit for humidity (not much). Approximately 3% per each 300 m above 300 m for small units about 100 kW (125 kVA) or less and larger units may be rated for 1500 m before derating occurs. There is no standard supplier trade name for their ratings so you must ask the supplier for the " (Rated) output at the generator terminals with all parasitic losses and derating factors for X * C, X % RH and X altitude considering recommended coolant for - x * C freeze protection and / or + x*C boiling point rise with 20% fouling factor for water-cooled components". To fill in the (rated) space above consider the following:

Rating trade name terms are varied between supplier and confusing so you can't easily compare. Use one of the 3 following generally recognised terms to fill in the (Rated) space above:

* Continuous uninterrupted duty

* Prime power base load

* Emergency standby

These are defined below after you read the "understanding" comments.

The key is to get the supplier's definitions of their ratings and apply it to your load profile. Rating Definitions are standardised by ISO 3046 and others. Here is small generator set example. Here is a large generator set example (http://ringpower-systems.com/3412%2050_60Hz%20600_725.pdf - sorry, link no longer available). Confusing, but not when you put your load definition in writing and have the supplier explain their terminology. Just ask for ISO 3046 definitions to be submitted with the offer.

Understand these things first:

Engines need oxygen and there is less oxygen at high altitude so one cannot put as much fuel with the thin air and hence power is less requiring a larger engine for the same power at sea level.

Most larger engines have turbochargers which is a type of air pump that helps overcome altitude losses. The turbocharger compressed air is bloody hot so it must be cooled by an aftercooler. So some engines have models followed by T (turbo) and A (aftercooled).

Air density is less at altitude and fan cooled things are less efficient by 3% per each 300 m such as the cooling radiator fan and also the generator cooling fan. This is often overlooked. If the engine is air cooled this must be considered also.

Air is less dense with high temperatures and the oxygen content is slightly less so there is minor power and cooling deratings for high ambient temperature.

Humidity displaces oxygen but this derating is very very minor and not normally a factor. It is more of a factor for fungus growth on generator windings and junction boxes.

Engine cooling systems need corrosion protection, freeze protection and boiling point raised by use of chemicals, normally ethylene glycol. Glycol doesn't remove heat and give it off as efficiently as straight water so radiator derating and aftercooler derating is required if the aftercooler is using engine water as a coolant. Do not ever run engines with straight water, ever. The inside will corrode and the water will boil. 3 mm of corrosion has as much insulation as 40 mm of solid cast iron so heat will not arrive to the water if there are scale deposits inside.

Large DG engine pistons are cooled by lube oil sprayed under the crown, so high load heats up the piston and also the lube oil. Some rating allow overload for an hour followed by an hour at lesser load. This is to give the lube oil a chance to cool off.

Generators (alternators) have temperature rise ratings; i.e. how hot will the windings get at that load. The lower the rise, the more copper, the most money, the longest life. So a generator rated for 500 kW and 80* C rise above 50* C ambient will have more copper than a generator rated 500 kW and 120* C rise above 50* C ambient. In USA we tend to rate DGs in kW. Everywhere else they use kVA. The power factor is considered as 0.8 average so outside USA those suppliers assume 0.8 PF and report the kVA in this way. So a 100 kW at 0.8 PF is a 125 kVA machine. The definition of power factor is too complicated for this already long post.

Rating definitions:

ISO 3046 makes things complicated for common people to understand. They give engineering conditions not to exceed. Basically:

"Continuous Uninterrupted" is that load for 24/7 not one drop in load below the rating. This rating is so conservative one can overload the machine by up to 120% for a few minutes, and 110% for an hour. The generator will likely be rated for 80* C rise (more copper)

"Prime Base Load" is that load for 24/7 but the load will fluctuate lower and allow the oil to cool off. So if you know your building load is 1000 kVA worst case with all the ACs switched on and the kitchen fully switched on, etc., but, in reality the load is 80% one day and 90% the next and during each day it fluctuates up and down you do not need a 'Continuous Duty' machine. For prime base load the generator will likely be a 105* C rise ( a little less copper than above). Overload of 110% for an hour is allowed.

"Emergency Stand-By or Intermittent" This is the highest rating and has no reserve. It is meant for support of power failures that do not exceed a few hours in length. Most people do not try to run the whole building when the power fails and the suppliers know this. These machines have very few hours on them in a year so the customer receives good value for the intended purpose. If you used this rating for a 24/7 continuous uninterrupted duty the engine and generator would need overhaul every 6 months, instead of every 10 years. The generator will likely be a 130* C rise unit. The smallest amount of copper compared to above.