Electric Power Applications, Engine and Generator Sizing – Caterpillar

Introduction to Genset Rating

An engine rating is primarily limited by structural and thermal limits. These limits include maximum cylinder pressure during combustion, turbocharger speed, exhaust gas temperature and in natural gas engines, fuel type. Where an engine operates relative to these limits will determine the maximum altitude and ambient temperature for a given rating.
When an engine exceeds the maximum altitude or ambient temperature, the engine must be derated. The environment or site conditions also impact an engine or generator rating, thus impacting the rating of the entire genset. Site conditions which may affect a rating include altitude, temperature, corrosive atmospheres, humidity, and dust. Altitude and temperature most heavily influence engine ratings.
The higher the altitude, the lower the air density. Clean dense air is needed for efficient combustion. Likewise, an increase in temperature lowers air density.

Therefore, a derate of the engine must occur in high altitude and/or high temperature conditions in order for the genset to meet performance expectations.

Altitude

Generators operating at altitudes above 1000 M (3281 ft.) require temperature rise reduction of 1% for every 100 M (328 ft.) above base (1000 M or 3281 ft.). A derate chart is available in TMI for generators and each specific engine.
Table 1 shows a sample engine TMI derate chart.

Altitude Capability Data (Corrected Power Altitude Capability)

Ambient Operating Temp.	50 F	68 F	86 F	104 F	122 F	Normal
Altitude
0 F	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp
984 F	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp
1,640 F	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,876 hp
3,281 F	2,876 hp	2,876 hp	2,876 hp	2,876 hp	2,808 hp	2,876 hp
4,921 F	2,876 hp	2,876 hp	2,816 hp	2,726 hp	2,642 hp	2,876 hp
6,562 F	2,835 hp	2,737 hp	2,647 hp	2,563 hp	2,484 hp	2,763 hp
8,202 F	2,663 hp	2,572 hp	2,488 hp	2,407 hp	2,333 hp	2,624 hp
9,843 F	2,500 hp	2,415 hp	2,335 hp	2,261 hp	2,190 hp	2,492 hp
10,499 F	2,437 hp	2,353 hp	2,277 hp	2,203 hp	2,135 hp	2,441 hp

*** The powers listed above and all the powers displayed are Corrected Powers.

Impact of site conditions on genset performance

Heat

Where the temperature of the ventilating air to the generator exceeds 40 C (104 F), derating of the generator may be necessary. The altitude/temperature derating chart found in TMI can be used for proper derating information.

Corrosive Atmospheres

Salt and other corrosive elements can cause damage to the winding insulation which can lead to failure of the generator. Protection from these elements includes additional coatings of insulation on the windings during the manufacturing process and epoxy compounds as a final winding overcoat.

Humidity

Condensation resulting from humidity will present a problem for all generators unless they are fully enclosed.
Temperature rise of the machine and the circulation of cooling air with sufficient load operation will usually prevent condensation. Space heaters should be used to raise the temperature to 5 C above the ambient temperature to prevent condensation in high humidity areas.

Dust

Conductive or abrasive dust drawn in through the cooling fan can be very harmful to the generator.

Examples of abrasive dust are: cast iron dust, carbon dust, sand, powdered graphite, coke dust, lime dust, wood fiber, and quarry dust.

When these foreign particles blow through the generator, they act as sandpaper scraping away the insulation. These abrasions can cause an electrical short within the generator. An accumulation of these materials in the crevices of the insulation system will act as an insulator or as a moisture attractor.
Filters which fit over the unit’s intake air openings or enclosure ventilation openings can prevent damage.
When using filters, it is important that they be regularly changed so as not to impede airflow. The use of a generator air filter will cause the generator to be derated due to higher temperature rise resulting from reduced cooling airflow. Differential pressure switches may be available as an option on many generator sets.
Resource: Electric Power Applications, Engine and Generator Sizing – Caterpillar