Subject of Previous Article
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Article
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Glossary of Generators – Part One
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Glossary of Generators – Part Two
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Today, we will start explaining the Generator Sizing Calculations.
Generator
Sizing Calculations
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Note:
These
articles will explain the design of stationary generators. They will not
cover the design of stationary–designed commercial generators used as mobile
generators, which are generally considered to be an unintended application.
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First: Reasons for having on-site generators
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There
are Mandatory reasons for having generators in many installations as follows:
A-
To meet building code requirements:
Loss
of the normal power supply would introduce life safety or health hazards
B-
To meet economic requirements:
To
mitigate of the risk of loss of services, data, or other valuable assets due
to loss of electric power
To
reduce load rates offered by the electric utility
C-
To meet the loads’ specific requirements:
A
wide range of specific requirements will result in the need for on–site electric
generators. Some common needs are:
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Second: Applicable performance standards for
generator sets
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Applicable
performance standards for generator sets include:
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Third: Selection Factors Used For Generators
Sizing Calculations
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Here
we will describe preliminary factors for selecting a generator for certain
project, which will be as follows:
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1- Generator Power Ratings
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There
are four different types of ratings that must be considered when specifying a
generator set for an application:
1-
Industry Standard ratings (ISO-8528-1:2005)
This
is an industry standard that defines the performance parameters required in
various onsite power applications.
ISO-8528-1:2005
defines basic generator set rating categories based on four operational categories:
2-
Manufacturers’ ratings
Most
manufacturers follow the ratings detailed in ISO-8528-1. However, some
manufacturers have slight exceptions to these ratings in order to best fit
their customer requirements, performance capabilities, or maintenance
schedules.
3-
EPA emission ratings (Governmental regulations on engine Emissions)
The
U.S. Environmental Protection Agency has environmental ratings and
regulations for generator sets that vary by drive-engine horsepower and
application.
The
EPA began to enforce limits on off-road engines use in generator sets in 2006
and began phasing in regulations by Tier levels and engine type. Some
definitions one needs to be aware of are the differences between:
4-
Custom industry ratings
Various
organizations and industry segments have created custom ratings to fit
particular operational needs. For example, One mission-critical organization
called the
Uptime
Institute has set design standards for data centers, with the intention that
data centers are designed with similar practices to ensure data safety.
Note:
The
proper understanding of the standards will ensure the best rating selection
for the purpose the generator set serves, which is the basic foundation to customer
satisfaction.
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Here
we will focus on the Manufacturers’ ratings as follows:
Power
ratings for generator sets are based on fuel stop power in accordance with ISO
3046/1, AS 2789, DIN 6271 and BS 5514.
The
fuel stop power is fixed for each engine model/configuration. It represents the
maximum safe horsepower the engine model/configuration will be able to
produce without exceeding the limits set according to the parameters. They
are published by the manufacturers.
It is
further defined as the output available with varying load for the duration of
the interruption of the normal source of power.
Table-1:
Generator Power Ratings
Note: Typical
load factor are the loads applied to the generator set divided by the engine
operating hours under those loads
A-
Standby Power Rating
Fig(1): Standby Power
Rating
This
rating is only applicable to variable loads with an average power output of
70 percent of the standby rating over 24 hours of operation for a maximum of
200 hours of operation per year. In installations where operation will likely
exceed these limits, the prime power rating should be applied.
The
standby rating is only applicable to emergency and standby applications where
the generator set serves as the back up to the normal utility source.
No
sustained overload capability is available for this rating (Equivalent to
Fuel Stop Power in accordance with ISO3046, AS2789, DIN6271 and BS5514 as
well as emergency standby power (ESP) per ISO 8528).
No
sustained utility parallel operation is permitted with this rating. For
applications requiring sustained utility parallel operation, the prime power
or base load rating must be utilized.
B-
Prime Power Rating
Prime
ratings are defined as the output available with a varying load for an
unlimited time. The prime power rating is applicable when
supplying electric power in lieu of commercially purchased power. Prime
rated gen sets are divided into two areas of use:
1- Unlimited
Running Time Prime Power
Prime
power is available for an unlimited number of annual operating hours in
variable load applications. Applications requiring any utility parallel
operation at constant load are subject to running time limitations. In
variable load applications, the average load factor should not exceed 70
percent of the Prime Power Rating.
Fig(2): Unlimited Running Time
Prime Power
2- Limited
Running Time Prime Power
Prime
power is available for a limited number of annual operating hours in constant
load applications such as interruptible, load curtailment, peak shaving and
other applications that normally involve utility parallel operation. The
total operating time at limited running time prime power must not exceed 500
hours per year. Generator sets may operate in parallel with the utility
source up to 500 hours per year at power levels not to exceed the Prime Power
Rating. Any application requiring more than 500 hours of operation per year
at the Prime Power Rating should use the Base Load Power Rating.
Fig(3): Limited Running Time Prime
Power
C-
Base load power rating (Continuous Power Rating)
This
rating is applicable for utility base load operation. In these applications,
generator
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 for 100 percent of the time. These are determined in
accordance with ISO 8528 as well as ISO 3046/1, AS 2789, DIN 6271 and BS
5514.
Typically,
this rating is used when the units are being operated in parallel with the
utility and the output will remain constant, such as a cogeneration
application.
No
sustained overload capability is available at this rating.
Fig(4): Base load power
rating
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2-
Application Type
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The
application of On–site power generation systems can be determined by system
type and generator set rating.
Table-2:
Generation System Types
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1- Standby System Types
A-
Emergency Systems
Emergency
systems are generally installed as required for public safety and mandated by
law. They are typically intended to provide power and lighting for short
periods of time for three purposes: to permit safe evacuation of buildings,
for life support and critical equipment for vulnerable people, or for
critical communications systems and facilities used for public safety. Code
requirements typically specify the minimum load equipment to be served.
B-
Legally–Required Standby
Legally–required
standby systems are generally installed as mandated by legal requirements for
public safety. These systems are typically intended to provide power and
lighting for short periods of time where necessary to prevent hazards or to
facilitate fire–fighting operations. Code requirements typically specify the
minimum load equipment to be served.
C-
Optional Standby
Optional
Standby systems are generally installed where safety is not at stake, but
loss of power could cause an economic loss of business or revenue, interrupt
a critical process, or cause an inconvenience or discomfort. These systems
are typically installed in data centers, farms, commercial and industrial
buildings, and residences. The owner of the system is permitted to select the
loads connected to the system.
In
addition to providing a standby source of power in case of loss of a normal
power supply, on–site generation systems are also used for the following
purposes.
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2-
Prime System Types
A-
Prime Power
Prime
power installations use on–site generation in lieu of a utility electricity
supply, typically where utility power is not available. A simple prime power
system uses at least two generator sets and a transfer switch to transfer
supply to the loads between them.
One
or the other of the generator sets runs continuously with a variable load, and
the second generator set serves as backup in case of a failure, and to allow
downtime for required maintenance. A changeover clock within the transfer
switch alternates the lead generator set on a predetermined interval.
B-
Peak Shaving
Peak
shaving installations use on–site generation to reduce or flatten peak
electricity use for the purpose of saving money on energy demand charges.
Peak shaving systems require a controller that starts and runs the on–site
generator at the appropriate times to flatten the user’s peak demands.
Generation installed for standby purposes may also be used for peak shaving.
C-
Rate Curtailment
Rate
curtailment installations use on–site generation in accordance with electric
energy rate agreements with the serving electric utility. In exchange for
favorable energy rates the user agrees to run the generators and assume a
specified amount of load (kW) at times determined by the utility, typically
not to exceed a specified number of hours per year. Generation installed for
standby purposes may also be used for rate curtailment.
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3- Continuous
System Types
A-
Continuous Base Load
Continuous
base load installations use on–site generation to supply a constant power
(kW) typically through interconnection equipment into a utility grid. These
installations are usually owned by electric utilities or under their control.
B-
Co–Generation
Often,
continuous base load generation is used in Co–Gen application. Simply put,
Co–Gen
is utilizing both the direct electricity generation and waste exhaust heat to
substitute for utility supplied energy. The waste heat is captured and either
used directly or converted to electricity.
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In the next article, we will
continue explaining other Selection factors used for Generators Sizing Calculations.
So, please keep following.
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