Generators Sizing Calculations – Part Ten




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
 

 
Today, we will continue explaining other Selection factors used for Generators Sizing Calculations.
 

 
Fourth: Applicable Procedures For Generators Sizing Calculations
 

  

 
Introduction
 

  

 
1- Generator Power Demand VS. Generator Connected Load
 
The performance required is often a more significant factor when sizing the generator than the calculated site rating - which is directly related to the steady-state power demanded by the connected load, the power demanded by the connected load will be calculated by using the following factors:
 
  1. Generator Load Factor,
  2. Load Demand Factor,
  3. Load Diversity Factor.
 

 

 
1.1- Generator Load Factor
 
Load factor of a generator set is used as one criterion for rating a generator. It is calculated by finding the product of various loads:
 
Load Factor = % of time x % of load
% of time = time at specific load / total operating time
% of load = specific load / rated load
 
Note:
Extended idling time and the time when the generator set is not operating does not enter into the calculation for load factor.
 
Example#1:
 
A facility has a generator rated at 550 kW and runs it two hours a week. During those two hours, it runs at 400 kW for 1.5 hours. Find the load factor?
 
Solution:
 
The formulas reveal the following:
% of load = 400 kW / 550 kW = 0.73
% of time = 90 min. / 120 min. = 0.75
Load Factor = 0.73 x 0.75 = 54.75%
 
This load factor would indicate that the generator could be used as a standby rated generator because it meets the load factor and other criteria of standby as shown in Table-1.
 
 
Load factor for different generator power ratings:
 
Power rating definitions for generator sets are based on typical load factor, hours of use per year, peak demand and application use.
Load factor for different generator power ratings are listed in table-1.
 
Rating
Standby
Prime + 10%
Prime
Continuous
Typical Load Factor
70% or less
60% or less
60-70%
70-100%
 
Table-1: Load Factor for Different Generator Power Ratings
 

 

 
1.2- Load Demand Factor
 
  • The load requirements must be defined as accurately as possible to determine the best size of generator needed.
  • The “maximum demand” or “demand factor” is the highest demand which is placed on the supply within a specified period of time.
  • Another way to describe demand factor is the mathematical ratio of the operating load divided by the connected load.
 
Demand Factor = (Total Operating kW x 100) / Total Connected kW
 
So, Demand factor is always less than one. 
 
Table-2 shows a range of common demand factors for different apparatus.
 
Range of Common Demand Factors
 
Apparatus
 
Total Connected Load
Motors for pumps, compressors, elevators, blowers, etc …
 
20 to 60 Percent
 
Motors for semi-continuous operations, such as process plants and foundries
 
50 to 80 Percent
 
Arc welders
 
30 to 60 Percent
Resistance welders
 
10 to 40 Percent
Heaters, ovens, furnaces
 
80 to 100 Percent
 
Table-2: Range of Common Demand Factors
 
Example#2:
 
We have a system where three motors connected to a generator set. The motors data are as follows:
 
  • The first motor: rated at 50 hp, operated at 100% capacity and expected to produce 54% of its total capacity,
  • The second motor: rated at 20 hp, operated at 100% capacity and expected to produce 100 % of its total capacity,
  • The third motor: rated at 100 hp, operated at 87% capacity and expected to produce 87% of its total capacity,
 
What is the demand factor of the above system?
 
Solution:
 
The first motor Rated KW will be calculated as follows:
The 50 hp motor were operated at 100% capacity, its total connected electrical load would be 37.3 kW. However, the 50 hp motor is only expected to produce 54% of its total capacity. Hence, the operating electrical load is 20 Kw.
 
So, the first motor Rated KW = 50 x 0.746 x 54% 20 KW
The second motor Rated KW = 20 x 0.746 x 100% 15 KW
The third motor Rated KW = 100 x 0.746 x 87% 65 KW
 
Operating kW Load = 20 + 15 + 65 = 100 kW
 
Total Connected Load = 50 + 20 + 100 = 170 hp = 170 x 0.746 = 127 KW
 
To find demand factor use the formula:
 
Demand Factor = (Total Operating kW x 100) / Total Connected kW = (100 KW x 100) / 127 kW = 79%
 

 
 
 
1.3- Load Diversity Factor
 
  • It is very unlikely that individual loads’ maximum demands will coincide at anyone point in time. The maximum demand on the system will always be less than the sum of the maximum demand s of the individual loads.
  • Diversity factor is the mathematical ratio of a system’s individual maximum demands divided by the maximum demand of the system as a whole.
  • The formula used to calculate diversity factor is total maximum demand divided by total incoming kW times 100.
 
 
Diversity Factor = (Total Max. Demand kW x 100) / Total Incoming kW
 
 
So, Diversity factor is usually more than one.
 
Typical diversity factors are shown in Table-3.
 
Typical diversity factors
 
Lighting feeders
 
1.10 to 1.50%
Power & light feeders
 
1.50 to 2.00% or higher
 
Table-3: Typical diversity factors
 
Note:
 
The connected loads to a generator should be interlocked so that they cannot all be impressed upon the generator set at the same time. If the loads are not interlocked, the generator set rating could be exceeded. Despite this precaution, you must always assume that total time-current characteristics of all motors and other loads starting at the same time will not exceed the generator set rating.
 
Example#3:
 
A system has individual loads which are connected to three load centers. The load centers have 20 kW, 15 kW and 65 kW loads each, the total connected load of the three load centers is routed to an 80 kW meter. What is the diversity factor of the above system?
 
Solution:
 
Total connected load = 20+15+65 =100 kW which will be the Total Max. Demand kW of the system.
Total Incoming kW for the system will not exceed the KW meter rating = 80 KW
 
To find the diversity factor use the formula to solve:
 
Diversity Factor = (Total Max. Demand kW x 100) / Total Incoming Kw = (100 x 100) / 80 = 125%
 
 
Example#4:
 
A system with connected loads of 300 kVA, 100 kVA and 500 kVA to e feed from one generator. Calculate the proper generator rating in the following two cases:
 
  • The diversity factor is 1.0
  • The diversity factor is 1.4
 
Noting that the demand factor for theses loads are 80%, 100% and 70% respectively.
 
Solution:
 
The total demand load on the system = 300x0.8 + 100x1 + 500x0.7 = 690 KVA
 
If the diversity factor is 1.0:
 
The total incoming KVA = the total demand load on the system / diversity factor = 690 / 1 = 690 KVA
 
From the manufacturer catalogs, to meet this load, a generator set rated at a standard size of 750 kVA is needed.
 
If the diversity factor is 1.4:
The total incoming KVA = the total demand load on the system / diversity factor = 690 / 1.4 = 492 KVA
 
From the manufacturer catalogs, a generator set rated at 500 kVA would satisfy the load.
 
Hence, a different diversity factor on the same system will change the total kVA needed.
 

 

 
2- Advantages Of Choosing The Right Size Generator
 
Here’s just a few of the benefits obtained by Choosing The Right Size Generator:
  1. No unexpected system failures,
  2. No shutdowns due to capacity overload,
  3. Increased longevity of the generator,
  4. Guaranteed performance,
  5. Smoother hassle-free maintenance,
  6. Increased system life span,
  7. Assured personal safety,
  8. Much smaller chance of asset damage.
 

  

 
Applicable Procedures For Generators Sizing Calculations
 

 

 
 
 
Now we will start listing the required steps for generator set sizing calculation and we will indicate the common errors done by designers when selecting the right generator set for an application.
 
Also, the required steps will differ according to the type of the installation; new or existing.
 
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.
 
 

 
In the next article, we will start explaining in detail the applicable procedures for Generators Sizing Calculations for Existing Installations. So, please keep following.

 

 

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