Generators Sizing Calculations – Part Twelve


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#5  - Existing Installations: Calculate the peak load of the installation
 


 
 
We can calculate the peak load of an existing installation by one of the following methods:
 
  1. Measurement Method,
  2. Billing History Method,
  3. Load Summation Method,
  4. NEC Load Calculation Methods,
  5. Estimate based on square footage Method.
 Then we can size the generator(s) of an existing installation based on the calculated/estimated peak load.
 



 
1- Measurement Method
 
Use a clamp-on Amp meter or power analyzer to measure facility load levels. Clamp each leg separately and take the measurement during peak usage levels.
 
1.A- For 240V 1ø Applications:
 
To determine peak usage in kW, add the highest Amp readings from the two legs then multiply by 120 and divide by 1,000.
 
Step#1: Peak Amps = (L1 + L2)
 
Step#2: Peak kW = (L1 + L2)120 / 1000
 
 
1.B- For 3ø Applications:
 
Add the peak Amp readings from all three legs and divide by 3 to determine peak Amps. Multiply peak Amps by volts, multiply the result by 1.732 (square root of 3), then divide by 1000 to convert Amps to kW.
 
Step#1: Peak Amps = (L1 + L2 + L3) / 3
 
Step#2: Peak kW = [(Peak Amps x Volts) x 1.732] / 1000    (Assumes power factor of 1.0)
 


  
 
2- Billing History Method
 
Many commercial customers have a utility rate structure that has a peak demand charge. Using a year's worth of electric bills, find the Peak Demand. Then Verify motor and UPS load compatibility.
 
Step#1: Peak Demand = largest peak demand from Billing History
 



 
3- Load Summation Method
 
Step#1: Enter running kW for all motor loads (except the largest) expected to run during peak load levels into Table-3. Refer to Table-1 for typical motor load sizes and electrical requirements.
 
 
 
Table-3
Motor Load Table (refer to Table 1)
Device
HP
RA
LRA
kW Running (= HP)
Starting kW
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 Table-4 below.
 
Table-4: How To Calculate kW for loads with no listed HP
120 V 1ø
Amps x 120/1000 = kW
240 V 1ø
Amps x 240/1000 = kW
208 V 3ø
(Amps x 208 x 1.732 x PF) /1000 = kW
240 V 3ø
(Amps x 240 x 1.732 x PF) /1000 = kW
480 V 3ø
(Amps x 480 x 1.732 x PF) /1000 = kW
PF is the load power factor - Typical application power factor is 0.95.
 
 
Step#2: Enter kW for all non-motor loads expected to run during peak load levels into Table-5. Refer to Table-2 for typical residential loads and rules of thumb.
 
 
 
Table-5
Non-Motor Load Table (refer to Table-2)
Device
Amps
Kw
 
 
 
 
 
 
 
 
 
 
 
 
 
Step#3: Add the running motor load kW, non-motor load kW, and the starting kW of the largest motor load.
 
From Table-3, the Motor load running total - minus largest motor = X kW
From Table-5, Non-motor load total = Y kW
From Table-3, Starting load from largest cycling motor = Z kW
 
Total electrical loads = X+Y+Z kW
 



 
4- NEC Load Calculation Methods
 
In our course “EE-3: Basic Electrical design course – Level II”, we explain how to calculate the  total load by using NEC Load Calculation Methods for both dwelling and non-dwelling buildings as follows:
 
 
4.A- For Dwelling Buildings
 
You can calculate the total load For Dwelling Buildings by using our electrical load calculators explained in the following articles:
 
 
 
Dwelling Buildings Calculator
 
4.B- For Non-Dwelling Buildings
 
You can calculate the total load For Non-Dwelling Buildings by using our electrical load calculators explained in the following articles:
  


 
5- Load Estimation Method
 
In our course “EE-2: Basic Electrical design course – Level I”, we explain how to estimate the total load by using (3) Electrical Load Estimation methods which are:
     
  1. Space by space (functional area method), 
  2. Building Area method,
  3. Area method. 

All these methods are explained in the following Articles:
 
 
Or , you can download our PDF Course for Preliminary Electrical Load Estimation from the following link: Electrical Load Estimation Course
 
 
 
Preliminary Electrical Load Estimation Course 
 

 

  
Step#5- New Construction: Calculate Connected Loads to Generator
 
  


 

It is important to note that if some non-linear loads are present in the system, generator rating is not being just sum of the loads. The effective generator load will be the sum of the effective loads on generator which must be calculated properly to avoid generator over sizing.

 

In step#3: Segregate the Loads which   explained in article “Generators Sizing Calculations – Part Elevenwe indicate the (5) categories of the loads are as in below table:

 


Category

Application

Required Data

Category-1

Linear loads

Rated Load in KW

 

Power factor PF

 

Category-2

Running highest motor or a group of motors to be started at a time

Rated Load in KW

 

Starting Load in KW

 

Running PF

 

Starting PF

 

Starting Time based on load type (ts)

 

Category-3

Running motor loads except VFD and soft started

Rated Load in KW

 

Overall PF

 

Category-4

UPS loads

Rated Load in KW

 

Overall PF

 

Category-5

Soft started motor

Rated Load in KW

 

Overall PF

 

 
So, the Effective loads to generator will be calculated based on load segregation done in step#3 as follows:
  

Connected Load for Category-1 = Σ kW catagory-1

Connected Load for Category-2 = Σ kW Starting- category-2

Connected Load for Category-3 = Σ kW Running- category-3

Connected Load for Category-4 = Σ K 1 x kW catagory-4

Connected Load for Category-5 = Σ K 2 x kW catagory-5

 
Where:
  


Factor

Value

Application

K 1

1.25 * 1.4

for 1 phase and 6 pulse UPS

1.25* 1.15

for 6 pulse UPS with input filters or 12 pulse UPS

Factor 1.25  is considered to take the battery charging after drain out with supplying connected loads

K 2

1.15

for without bypass contactor after start

1.0

for with bypass contactor after start

 
 

 
  Step#6 - Existing Installations: Check For Transients Or Harmonics By Using Power Analyzers And De-Rate The Peak Load Value.
 



 
Power Quality Analyzers
 
  • Several measurement tools are available for power quality measurement. Power quality analyzers are the most commonly used tools to observe real-time readings and also collect data for downloading to computers for analysis. While some are permanently installed in the distribution system, handheld analyzers are necessary for many applications, especially troubleshooting.
Power Quality Analyzers
  • Handheld power quality analyzers are fairly lightweight (generally 4 lb to 5 lb) and will measure a variety of parameters. The most typical include voltage, amperage, frequency, dips (sags) and swells in voltage values, power factor, harmonic currents, and the resulting distortion and crest factor, power and energy, voltage and current unbalance, inrush current values, and light flicker. If an analyzer measures and records such basic parameters, you can address most power quality issues successfully.
  • Power quality analyzers are available for both single phase and three-phase circuits. In addition to measuring harmonics, power quality analyzers measure and record total harmonic distortion THD.
  • Total Harmonic Distortion (THD) is the ratio of the sum of the interference from all harmonics to the fundamental signal.
 
How to measure harmonics using a power quality analyzer
 
Step#1: Measure with a clamp meter that is capable of indicating total harmonic distortion (THD). THD for voltage should not exceed 5 %. THD for current will run considerably higher.
 
Step#2: Use a power quality analyzer to further investigate the magnitude and effects of the individual harmonics.
 
Notes:
 
  • THD and harmonic levels should be measured at the point of common coupling (PCC) – the point at which the nonlinear loads suspected of causing the problem connect to the remainder of the distribution system. Look for THD for voltage approaching 5 % and check for the presence and the levels of different harmonic frequencies.
  • Total Harmonic Distortion (THD) is the measurement of the sum of all harmonics. Most loads will continue to operate with THD at 15 to 20%.
  • However, loads with sensitive electronic equipment can develop problems with THD greater than 5%.
 
 
Example for measuring harmonics by using power quality analyzer: See Figure-2.
 
 

Fig.2: Harmonics on power quality analyzer
 
In this power quality analyzer screenshot, the harmonic frequencies appear on the harmonic axis. The percent to which the specific harmonic frequency is a component of the fundamental 60 Hz frequency appears on the vertical axis. The cursor has been placed over the third harmonic frequency, and this third harmonic current appears to represent approximately 25 % of the 60 Hz frequency.
 

  

 
Generator Sizing Rule For Non-Linear Loads
 
In cases where non-linear loads cause increased generator heating and Total Harmonic Distortion (THD) exceeds 15%, two techniques are typically used to compensate for the increased generator heating:
 
Method#1: Using Deration factors while sizing the generator.
Method#2: Using a generator with oversized kVA requirement.


The following generator Rating vs. %Harmonics current De-rate Chart can be used to determine the de-rating factor for Harmonics and non-linear loads:

 
 
 
 

 
Step#6 - New Construction: Calculate Effective Load to Generator
 

  

 
In this step, we will calculate the Effective Load to Generator by calculating the effective loads for each load category which are calculated by applying demand and diversity factor to each category’s connected load as follows:
 
 
Category
Connected Load
KW effective
Category-1
Σ kW catagory-1
K demand x K diversity x Σ kW catagory-1
Category-2
Σ kW Starting- category-2
K demand x K diversity x Σ kW Starting- category-2
Category-3
Σ kW Running- category-3
K demand x K diversity x Σ kW Running- category-3
Category-4
Σ K 1 x kW catagory-4
K demand x K diversity x Σ K 1 x kW catagory-4
Category-5
Σ K 2 x kW catagory-5
K demand x K diversity x Σ K 2 x kW catagory-5
 
 
Where:
K demand is the demand factor & K diversity is the diversity factor.
 

  

 
1- Demand Factor
 
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. 
 
The below table 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
 
 
2- Diversity Factor
 
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 below table.

 
 
 
Difference between demand and diversity factor:

Most of the electrical engineers confuse between the demand and diversity factors, to solve this confusion, don't forget that:
  • The Demand factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load. 
  • The Diversity Factor is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board).
 

 


  

 
Effective Load to Generator of the generator will be
 
Effective Load to Generator = Σ KW effective of all Load categories
 
And The Effective KVA rating of the generator will be
 
KVA effective = KW effective / PF overall
 

  

In the next article, we will continue explaining the applicable procedures 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
 

 


 

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