Today, we will continue explaining 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: Adjust
the Generator Rating According To Power Factor,
Step#12: Calculate the Adjusted Generator
Rating,
Step#13: Select Generator Rating from
Standard Sizes/Manufacturers Catalogs,
Step#14:
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: Adjust
the Generator Rating According To Power Factor,
Step#12: Calculate the Adjusted Generator
Rating,
Step#13: Select Generator Rating from
Standard Sizes/Manufacturers Catalogs,
Step#14:
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#7: Adjust the Generator Rating According To Transient Voltage Dip

Generator Voltage Dip Calculation
We
have two methods to calculate/determine the Generator Voltage Dip as follows:
1 By using equations
Or by this equivalent equation:
Notes:

The
Maximum Allowable Voltage Dip
The maximum allowable voltage dip is 30%.
Notes:
The relation
between the generator size and voltage dip% is shown in Fig.1:

If
calculated
voltage dip ˃ Maximum Allowable Voltage Dip
You must increase the value of the
generator effective load used in the calculation of the voltage dip.
Then
Generator
KW _{adjusted } =
K x Generator KW _{effective}
_{ }
where factor K ˃1 by a percentage
sufficient to minimize the voltage dip below the Maximum Allowable value_{}
Example:
2000 kW
@ 0.8 PF with X’D = 18.2% starts a 750 HP code F motor – What is the
approximate voltage dip?
Solution:
Rated
kVA = 2000 kW / 0.8 PF = 2500 KVA
Starting
kVA = 750 HP × 5.6 kVA / HP = 4200 KVA
% dip =
100 x (0.182) / (0.182 + (2500 / 4200)) = 23.4%
Calculated
voltage dip (23.4%) ˂ Maximum Allowable Voltage Dip (30%)
Then
the size of the selected generator is suitable for the application.

For more information about Transient
Voltage Dip, please review our article” Generators
Sizing Calculations – Part Six”.

Step#8: Adjust the Generator Rating According To Site Conditions

1 Ambient Temperature
Where the temperature of the ventilating
air to the generator exceeds 40°C (104°F), derating of the generator may be
necessary.
The derating percentage will differ from
a manufacturer to another. However, an average derating percentage can be as
follows:
An engine’s power rating assumes a nominal
altitude of less than 1000 feet, ambient temperature less than 104°F, and
humidity less than 75%. Manufacturers detail the percentage reduction in
available power for ambient conditions that exceed those assumed for the
nominal rating.
2
Altitude
The higher the altitude, the lower the
air density. The altitude/temperature
derating chart found in TMI can be used for proper derating information for
generators and each specific engine. Figure
.2 shows a sample engine TMI derate chart.
(TMI: Technical Marketing Information).

Step#9: Adjust the Generator Rating According To Fuel
Type

The
selection of natural gas, diesel, or LPG fuel will affect generator set
availability and sizing. Often, generator sets running on
gas or LP must be oversized due to derating.
The standard
derating formula states that for every 1000 ft above sealevel, the following
derating must be applied:

For more information about Site
Conditions & Fuel Type selection, please review our article” Generators
Sizing Calculations – Part Four”.

Step#10: Adjust the Generator Rating According To
Future Needs

The customers’ future needs are to be
taken into account when sizing the generator set. If the customer anticipates
growth in their application due to increased volume or expanded needs, one of
two design solutions can be applied:
However, the projected load growth for
any application should never be less than 10%.
Table1 shows typical load growth over a period of 10 years for various
applications.
Table1: Load Growth over 10 Years for various
applications

Step#11: Adjust the Generator Rating According To
Power Factor

Lower
PFs require larger alternators or generator sets to properly serve the load.
Usually Threephase generator sets are rated for 0.8 PF loads and
singlephase generator sets for 1.0 PF loads.
For example, the following data in
Table2 for the Siemens AG type 1FC6 generators typify the derating
factors for power factor that need to be applied.
Table2: Typical derating factors
The following generator’s kW and
kVA vs % lagging PF Chart can be used to determine the derating factor for lagging
PF approximately:
Fig.4: generator kW and
kVA vs % lagging PF Chart

For more information about Power
Factor derating rule, please review our article “Generators
Sizing Calculations – Part Seven”.

Step#12: Calculate the Adjusted Generator Rating

From step#5, article “Generators
Sizing Calculations – Part Twelve”,
after application of demand and diversity factors to each category’s
effective load, we calculate the Effective Load to Generator in step#6 in the
same article as follows:
Effective Load to
Generator KW _{Geffective} = Σ KW _{effective }of all Load
categories
Based on this KW
_{Geffective}, we can make a preliminary selection of the generator.
Then, we must adjust the selected generator
rating to comply with Transient
Voltage Dip rule explained in step#7 to get at the end the KW _{Gaccepted}
After
that, Site Conditions derating factors, Fuel Type derating factor,
Future Needs factor and Power Factor derating
have to be applied as explained in steps# 8, 9, 10 & 11 respectively.
Finally,
we can say that the Adjusted Generator Rating KW _{Gadjusted }can be
calculated as follows:
KW _{Gadjusted }=
KW _{Gaccepted} x K _{site }_{conditions} x K
_{fuel type} x K _{future needs }x K _{pf}

Step#13: Select Generator Rating from Standard
Sizes/Manufacturers Catalogs

We can use the different Manufacturers Catalogs to assign
the proper generator standard rating that will be next higher to KW _{Gadjusted}_{
}calculated above.
You can download the most famous Manufacturers Catalogs from the
following links:
After selecting the standard rating from
above step, the following rules must be taken into consideration:
Otherwise, you will need to move to the
next higher size till you comply with the above two rules.

Step#14: Assign Required Number Of Steps/Starting
Sequence

The
Brake Mean Effective Pressure (BMEP) charts are used for load step analysis and for developing a
starting sequence of a generator.
Fig.5:
BMEP Curve from TMI
For example, Using Figure.1, the number of load steps needed for
desired power can be determined.
First, find the BMEP of the engine at
rated speed; BMEP levels are shown for each rating in the performance data of
the TMI (Technical Marketing Information).
Second, compare the percentage of block
load with BMEP Curve, we will have 3 cases:

For more information about The
Brake Mean Effective Pressure (BMEP),
please review our article “Generators Sizing Calculations –
Part Nine”.

In the next article, we will explain
the rules of thumbs and solved examples for Generators Sizing Calculations. 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


Fourth: Applicable
Procedures For Generators Sizing Calculations
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,


Fourth: Applicable
Procedures For Generators Sizing Calculations
Step#5Existing Installations: Calculate the peak load of the
installation
Step#5New Constructions: Calculate Connected Loads to Generator
Step#6 Existing Installations: Check for transients or harmonics by using power analyzers and derate the
peak load value.
Step#6New Constructions: Calculate
Effective Load to Generator,

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