Generators Sizing Calculations – Part Thirteen


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 de-rate 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,
  2. By using generator manufacturers’ performance data.
 
1- By using equations
 
  • The transient reactance (X'd) is used to describe generator performance during transient events such as large load applications. reactances provide an agreed standard and consistent way to compare one generator to another, regardless of the manufacturer.
  • The magnitude of the voltage dip at a generator's terminals, following load switching, is a direct function of the sub-transient and transient reactances of the machine. It can be calculated from the following equation:
 

 
Or by this equivalent equation:
 
 
 
 
Notes:
 
  • The equation above can be used to show that for a given machine rating (same kVA, same voltage, same frequency), the lower the X'd, the lower the amount of voltage dip for an applied load. This is based on physics and holds true for all generator set manufacturers.
  • Many manufacturers’ software programs calculate the voltage dip easily.
 

 

 
The Maximum Allowable Voltage Dip
 
The maximum allowable voltage dip is 30%.
 
Notes:
 
  1. Choosing lower allowable voltage dip requires a larger generator set. 
  2. As you reduce the maximum allowable frequency dip, you increase the size of the generator set needed.
  3. To reduce the dip, reduced voltage starters can be used. Engine transient response will add to this dip, but the engine response is usually much slower than the generator. This assumes a very low starting power factor.
 
The relation between the generator size and voltage dip% is shown in Fig.1:
 
 

Fig.1: Generator Size vs. Voltage Dip%
 

 

 
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 de-rating percentage will differ from a manufacturer to another. However, an average de-rating 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 de-rate chart. (TMI: Technical Marketing Information).
 
 

Fig.2: TMI Sample
 
 
The following Altitude / Ambient De-rate Chart can be used to determine the de-rating factor for Altitude / Ambient approximately:
 
 
 
Fig.3: Altitude / Ambient De-rate Chart
 

  

 
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 de-rating.
 
The standard de-rating formula states that for every 1000 ft above sea-level, the following de-rating must be applied:
 
  1. Gasoline, diesel, or liquid propane generators usually should be de-rated by 2–3% of its standard output.
  2. Natural gas generators, the de-rating factor is typically closer to 5% of its standard output.
 

  

 
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:
 
  1. Oversizing the generators or
  2. Leaving room for another generator to be installed at a later date.
 
However, the projected load growth for any application should never be less than 10%.
 
Table-1 shows typical load growth over a period of 10 years for various applications.
 
 
Load Growth Over 10 Years
Application
Typical Load Growth Factor (Kf)
Bank
30 – 50%
Medical Center
30 – 40%
Church
10 – 30%
School
50 – 80%
Hospital
40 – 80%
Warehouse
10 – 30%
 
Table-1: 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 Three-phase generator sets are rated for 0.8 PF loads and single-phase generator sets for 1.0 PF loads.
 
For example, the following data in Table-2 for the Siemens AG type 1FC6 generators typify the de-rating factors for power factor that need to be applied.
 
 
Lagging power factor
De-Rating factor
0.8 to 1.0
1.0
0.7
0.97
0.6
0.91
0.5
0.89
0.4
0.87
0.0
0.84
 
Table-2: Typical derating factors
 
The following generator’s kW and kVA vs % lagging PF Chart can be used to determine the de-rating factor for lagging PF approximately:
 
 
 
Fig.4: generator kW and kVA vs % lagging PF Chart
 

 
 
For more information about Power Factor de-rating rule, please review our articleGenerators Sizing Calculations – Part Seven”.
 

 

 
Step#12: Calculate the Adjusted Generator Rating
 

 
 
From step#5, articleGenerators 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 G-effective = Σ KW effective of all Load categories
 
Based on this KW G-effective, 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 G-accepted
 
After that, Site Conditions de-rating factors, Fuel Type de-rating factor, Future Needs factor and Power Factor de-rating have to be applied as explained in steps# 8, 9, 10 & 11 respectively.
 
Finally, we can say that the Adjusted Generator Rating KW G-adjusted can be calculated as follows:
 
KW G-adjusted = KW G-accepted 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 G-adjusted 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:
 
  • In a three-phase system, the Phase voltage unbalance must be ≤ 2%
  • Single phase loads must be ≤ 10% of the generator set three-phase kVA capacity
 
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:
 
  • Case#1: If the percentage of block load is under the first load step curve, the block load can be accomplished in one step
  • Case#2: If the percentage of the block load is greater than the first load step curve but smaller than the second load step curve, it will take two load steps to reach desired power.
  • Case#3: If the percentage of block load is greater than the first two load step curves but less than the third load step curve, it will take three load steps to reach desired power.
 

 
 
For more information about The Brake Mean Effective Pressure (BMEP), please review our articleGenerators 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 on-site generators

Second: Applicable performance standards for generator sets

Third: Selection Factors Used For Generators Sizing Calculations
  1. Generator Power Ratings
  2. Application type
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#5-Existing Installations: Calculate the peak load of the installation
Step#5-New Constructions: Calculate Connected Loads to Generator
Step#6- Existing Installations: Check for transients or harmonics by using power analyzers and de-rate the peak load value.
Step#6-New Constructions: Calculate Effective Load to Generator,
 

 


 

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