Understanding Generator Set Ratings
Rating gen sets for standby, prime
or continuous power uses is based on industry and engineering standards.
Architectural engineers, contractors and even building owners and maintenance
personnel have common questions about CAT gen set rating criteria.
Following are answers to commonly
asked questions:
Q. What is the meaning of standby
and prime ratings?
A.
Standby power ratings for specific gen set models are determined under the
following parameters:
- Peak cylinder pressure
- Exhaust temperature
- Smoke level
- Turbocharger speed (if applicable)
It is further defined as the output
available with varying load for the duration of the interruption of the normal
source of power. Ratings are based on fuel stop power in accordance with ISO
3046/1, AS 2789, DIN 6271 and BS 5514. The fuel stop is fixed for each engine
model/configuration. It represents the maximum safe horsepower the engine
model/configuration will be able to produce with without exceeding the limits
set according to the parameters.
Prime ratings are defined as the
output available with a varying load for an unlimited time. It is set
approximately 10 percent below the standby power ratings for each
model/configuration.
Q. Are there any other ratings?
A.
Continuous rating is output available with constant or near-constant load for a
limited amount of time. These are determined in accordance with ISO 8528 as
well as ISO 3046/1, AS 2789, DIN 6271 and BS 5514.
Electrical
Power Generation Rating Guidelines
|
Rating
|
Standby
|
Prime
+ 10%
|
Prime
|
Continuous
|
|
Typical
Load Factor
|
60% or less
|
60% or less
|
60-70%
|
70-100%
|
|
Typical
Load
|
Varying
|
Varying
|
Varying
|
Non-Varying
|
|
Typical
Hours/Year
|
100
|
500
|
No Limit
|
No Limit
|
|
Typical
Peak
Demand
|
80% of standby rated kW with 100%
of rating available for duration of an emergency outage
|
80% of prime +10% rated kW with
100% of rating available for duration of an emergency outage
|
100% of prime rated kW used
occasionally, but for less than 20% of operating hours
|
100% of continuous rated kW for 100%
of operating hours
|
|
Typical
Application
|
- Building service standby
- Enclosure / sheltered
environment
|
- Standby
- Rental
- Power module
- Unreliable utility
- Interruptible rates
|
- Industrial
- Pumping
- Construction
- Peak shaving
- Cogeneration
|
- Base load
- Utility
- Cogeneration
|
|
For conditions outside the above
limits, please contact us. Typical load factor is the typical loads applied
to the generator set divided by the engine operating hours under those loads.
|
Q. How do you apply these rating definitions?
A.
Unfortunately there are no hard and fast rules to determine what generator set
to utilize in any installation. However, the following guidelines can help you
select the kW rating that best suits your application. Your Caterpillar dealer
can also help make judgments based on your load profile.
Typically, standby generator sets
are used in situations requiring less than 100 hours of use per year, with a
variable load of 60 percent or less. The typical peak demand would be 80
percent of standby rated kW with 100 percent of rating available for the
duration of an emergency outage. Applications with these limits include
building service standby where the unit(s) is in an enclosure or other
sheltered environment.
Prime rated gen sets are divided into
two areas of use.
Prime plus 10 percent-rated gen sets
are best utilized in standby, rentals or power modules, or where there is
unreliable utility power and/or interruptible rates. These are generally used
less than 500 hours per year with a typical load factor of 60 percent or less;
peak demand is 80 percent of prime plus 10 percent rated kW with 100 percent of
rating available for the duration of an emergency outage.
Prime-rated gen sets are most common
where the gen set will be the only power source. At the prime rating, a gen set
can be used 24 hours per day, 365 days per year. In most installations, they
are specified to meet the demands of a variable load that is 60 to 70 percent
of the gen set rating, but can supply 100 percent of its prime kW rating for
less than 20 percent of operating hours. These prime gen sets are used
industrial, pumping, construction, peak shaving, and cogeneration applications.
Continuous-rated gen sets are sized
to operate at 70 to 100 percent of maximum load that is non-varying and has no
hour-use limit. Typical peak demand is 100 percent of continuous rated kW.
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