Today, we will explain in detail the applicable procedures for Generators Sizing Calculations for Existing and New Installations.
Fourth: Applicable Procedures For Generators Sizing
Calculations

In the previous Article “Generators
Sizing Calculations – Part Ten”, we list the required steps for generator set sizing
calculation and we indicate that the required steps will differ according to
the type of the installation; new or existing as follows:
For
Existing Installations
the required steps for generator
set sizing calculation will be as follows:
Step#1:
Determine the Required Generator(S) Set Rating,
Step#2: Assign the System Voltage and Phase, Step#3: Segregate the Loads, Step#4: Match the System to the Load Profile, Calculate the Required Number of Generator Sets and Paralleling Requirement, Step#5: Calculate the peak load of the installation Step#6: Check for transients or harmonics by using power analyzers and derate the peak load value. Step#7: Adjust the Generator Rating According To Transient Voltage Dip, Step#8: Adjust the Generator Rating According To Site Conditions, Step#9: Adjust the Generator Rating According To Fuel Type, Step#10: Adjust the Generator Rating According To Future Needs, Step#11: Calculate the Adjusted Generator Rating, Step#12: Select Generator Rating from Standard Sizes/Manufacturers Catalogs, Step#13: Assign Required Number Of Steps/Starting Sequence.
For
New Constructions
the required steps for generator
set sizing calculation will be as follows:
Step#1:
Determine the Required Generator(S) Set Rating,
Step#2:
Assign the System Voltage and Phase,Step#3: Segregate the Loads, Step#4: Match the System to the Load Profile, Calculate the Required Number of Generator Sets and Paralleling Requirement, Step#5: Calculate Connected Loads to Generator Step#6: Calculate Effective Load to Generator,, Step#7: Adjust the Generator Rating According To Transient Voltage Dip, Step#8: Adjust the Generator Rating According To Site Conditions, Step#9: Adjust the Generator Rating According To Fuel Type, Step#10: Adjust the Generator Rating According To Future Needs, Step#11: Calculate the Adjusted Generator Rating, Step#12: Select Generator Rating from Standard Sizes/Manufacturers Catalogs, Step#13: Assign Required Number Of Steps/Starting Sequence.
Now, we will explain in detail the
above steps of Generators Sizing Calculations for Existing and New Installations.

Step#1: Determine the Required Generator(S) Set
Rating

Generator set(s) rating can be determined
based on the following factors:
Knowing one or more of the above factors can help
you in assigning the required generator set(s) rating from the following
ratings:
The
following guidelines can help you select the generator set rating that best
suits your application:
1 Standby
generator sets are used in applications where:
2 Prime
rated gen sets are divided into two areas of use.
2.1 Prime
plus 10 percentrated gen sets are used in applications where:
2.2 Primerated
gen sets are used in applications where:
3 Continuousrated
generator sets are used in applications where:
The following table summarizes the
conditions for selecting the proper generator(s) set rating for your
application.

Step#2: Assign the System Voltage
and Phase

Some
factors needs to be considered when selecting a generator for an
application like:

1 Number of Phases
Generators
are available in single or threephase as follows:
1 SinglePhase
Generators
For smaller
singlephase loads, these Generators usually do not go above 40 kW.
They are commonly used in residential environments and have a power factor of
1.0.
2 Three Phase Generators
Mainly for larger
industrial power generation, these Generators can provide both single
and 3phase power for running industrial motors with higher horsepower,
branch power out for separate lines, and are in general more flexible.
They are typically used in commercial environments and have a power factor of
0.8.
Note:
The
threephase selection permits singlephase loads to be connected.

2 Voltage Ranges
To determine the size of the generator
needed, we must assign the applicable voltage range. There
are three voltage ranges; Low, medium, and high.
Figure.1
shows a voltage versus kVA
chart, indicating low, medium and high voltage selections.
Fig.1:
Voltage/KVA Chart
Common Voltages on
Generator Sets:
The most common voltages in North America are:
120/240V, 1 phase, 3W
This is the standard voltage for single phase systems. 120
V is common for office equipment and small singlephase motors. Motors are limited to 10HP maximum. Either 120 volts or 240
volts can be used to supply loads. 120 volt loads must be balanced across the
generator L1 to N and L2 to N. Disadvantages are that single phase motors are
more unreliable than three phase motors, especially capacitor start motors.
The voltage drop is higher in single phase systems for a given load for the
same wire size used in 3 phase systems.
120/208V, 3 phase, 4W wye
This is a good choice for a three phase system because you can
balance 120 volt loads around the wye to equally load the generator. 208V
single phase and 208V three phase loads can be used as well as 120V single
phase loads. Motors must be rated 200 volt operation; 240 volt resistance
heating equipment used on a 208V generator will only produce 75% of its rated
KW output.
277/480, 3 phase, 4W wye
This voltage is usually used on large systems to reduce incoming
service size, wire size and distribution equipment size. Fluorescent and
other discharge lighting can be used at 277 volts. 480 volt single phase,
480V three phase for large air compressors, motors,
chillers, and air handlers, and 277 volt single
phase loads can be used on this system.
Also good for minimizing voltage drops on long runs.
Disadvantages are that a step down transformer is required to get 120/208 or
120/240 volt power for lights and outlets. Motors should be started directly from
the generator buss, not from a step down transformer to minimize
voltage drop.
120/240V, 3 phase, 4 W Delta
This Voltage should not use on generator as it overloads 1 phase if there are
large 120 volt loads. This is the
least desirable voltage to use if there is a large amount of 120 volt load.
The generator cannot be balanced and may overheat the windings. Advantages
are 240 volt motors and equipment are more common than 200 volts.

Step#3: Segregate the Loads

Why
analysis and categorization of generator loads is very important?
Loads have different electrical
characteristics. When developing a load analysis, it is helpful to analyze
and categorize generator set loads into groups with common characteristics to
assure proper consideration of their power demand because A generator set is
a limited power source, sometimes referred to as a “limited bus”. The limited
bus does not have the reserve capability of a utility grid.
Note:
There
are no rigid standards for categorizing loads.

A For New Constructions
After accumulation of load list it
is necessary to segregate the loads in different application categories as
listed below:
B For Existing Installations
After accumulation of load list it
is necessary to segregate the loads in different application categories as
listed below:
Enter
running kW for all motor loads (except the largest) expected to run during
peak load levels into Table3.
Refer to Table1 for
typical motor load sizes and electrical requirements.
Notes:
For
HP < 7.5; starting kW = HP x 3
For
HP > 7.5; starting kW = HP x 2
Starting
kW for loads with no listed HP; calculate HP based on running amps in Table4
below.
Enter
kW for all nonmotor loads expected to run during peak load levels into Table5.
Refer to Table2 for
typical residential loads and rules of thumb.

Step#4: Match the
System to the Load Profile, Calculate the Required Number of
Generator Sets and Paralleling Requirement

This step is used to evaluate whether the standby power system should be composed of a single large generator set or several smaller generator sets operating in parallel.
To do this, we have to determine the facility’s load profile by grouping the loads according to their priority as follows:
Standby
power systems with multiple generator sets offer a number of advantages that
include:
So, the owner and designer must decide one of the following
two solutions:
To select the best solution, the following factors must be taken
into consideration:
However, the lowest cost should never be the sole consideration
when implementing a standby power system. Care must be taken to review the
total project requirements, capabilities, and cost tradeoffs as explained
above.

In the next article, we will continue
explaining the applicable procedures for Generators Sizing Calculations for
Existing Installations. So, please keep following.
The
previous and related articles are listed in the below table:
Subject of Previous Article

Article

Glossary of Generators – Part One


Glossary of Generators – Part Two


First: Reasons
for having onsite generators
Second: Applicable performance standards for generator sets Third: Selection Factors Used For Generators Sizing Calculations


Third: Selection Factors Used For
Generators Sizing Calculations
3 Location Considerations, 4 Fuel Selection Considerations, 5 Site Considerations, 

Third: Selection Factors Used For
Generators Sizing Calculations
6 Environmental Considerations, 7 System Voltage and Phase, 

Third: Selection Factors Used For
Generators Sizing Calculations
8 Acceptable percent of voltage & frequency dip, 9 Acceptable duration of the voltage & frequency dip, 

Third: Selection Factors Used For
Generators Sizing Calculations
10 Percent And Type Of Loads To Be Connected – Part One 

10
Percent And Type Of Loads To Be Connected – Part Two


Third: Selection Factors Used For Generators Sizing Calculations
11
Load step sequencing
12
Future needs


Fourth:
Applicable Procedures
For Generators Sizing Calculations
1.1
Generator Load Factor
1.2
Load Demand Factor
1.3
Load Diversity Factor

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