Generators Sizing Calculations – Part Eleven


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

 
 
Generator set(s) rating can be determined based on the following factors:
 
  1. Typical load factor,
  2. Hours of use per year,
  3. Peak demand,
  4. Application use.
 
Knowing one or more of the above factors can help you in assigning the required generator set(s) rating from the following ratings:
 
  1. Standby rating
  2. Prime rating
  3. Continuous rating
 
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:
  • Application requires less than 100 hours of use per year, with a variable load factor of 70 percent or less.
  • The typical peak demand would be 80 percent of standby rated kW with 100 percent of rating available for the duration of an emergency outages.
  • Applications with these limits include building service standby where the units are in an enclosure or other sheltered environment.
 
2- Prime rated gen sets are divided into two areas of use.
 
  • Prime plus 10 percent-rated
  • Prime-rated
 
2.1- Prime plus 10 percent-rated gen sets are used in applications where:
 
  • Application requires less than 500 hours per year with a typical load factor of 60 percent or less;
  • Peak demand is 80 percent of prime plus 10 percent rated kW with 100 percent of rating available for the duration of an emergency outage.
  • They are best utilized in standby, rentals or power modules, or where there is unreliable utility power and/or interruptible rates.
 
2.2- Prime-rated gen sets are used in applications where:
 
  • They are most common where the gen set will be the only power source.
  • At the prime rating, a gen set can be used 24 hours per day, 365 days per year (no hour-use limit).
  • In most installations, they are specified to meet the demands of a variable load that is 60 to 70 percent of the gen set rating, but can supply 100 percent of its prime kW rating for less than 20 percent of operating hours.
  • They are used in industrial pumping, construction, peak shaving and cogeneration applications.
 
3- Continuous-rated generator sets are used in applications where:
 
  • They are specified to meet the demands of a is non-varying load that operate at 70 to 100 percent of maximum load of the gen set rating,
  • The application loads are non-varying and has no hour-use limit.
  • Typical peak demand is 100 percent of continuous rated kW for 100 percent of the time.
 
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,
  2. Voltage Ranges.
 

  

 
1- Number of Phases
 
Generators are available in single- or three-phase as follows:
 
1- Single-Phase Generators
 
For smaller single-phase 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 3-phase 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 three-phase selection permits single-phase 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.
 
Low voltage range
Medium voltage range
High voltage range
Low voltage is the voltage on a local level or part of a site.
Medium voltage is a low level distribution rating. This voltage is distributed to power residential sites and other campuses.
High voltage runs over regions and is the voltage at a utility or the national grid.
600 V or less with 800 kVA for less than 250 V is typical.
 
601 V-5000 V with 5-10 MVA is the range for the medium rating.
 
5001 V-15,000 V used with MVA’s greater than 10 are considered High Voltage.
 
 
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:
 
Single-Phase
120
240
120/240
 
3-Phase
208
120/208
220
240
440
480
277/480
600
2400
3300
4160
6900
11500
13500
 
 
 
The most common voltages in North America are:
  • 120/240V, 1 phase, 3W,
  • 120/208V, 3 phase, 4W wye
  • 277/480, 3 phase, 4W wye
  • 120/240V 3 phase, 4W Delta
 
120/240V, 1 phase, 3W
 
This is the standard voltage for single phase systems. 120 V is common for office equipment and small single-phase 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:

 

  • Category-1: Linear loads like lighting, heating etc.,
  • Category-2: Running highest motor or a group of motors to be started at a time or sum of auto accelerated motor,
  • Category-3: Running motor loads except VFD and soft started,
  • Category-4: UPS loads,
  • Category-5: Soft started motor.

 


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

 

 

 
B- For Existing Installations

 

After accumulation of load list it is necessary to segregate the loads in different application categories as listed below:

 

  • Category-1: Motor Loads,
  • Category-2: Non-Motor Loads.

  

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

Category-1: 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.

 

 
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

Category-2: Non-Motor Load Table (refer to Table-2)

Device

Amps

Kw

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 



 

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:

  1. Essential Or Critical Loads: such as life-safety systems; security, computer and data systems; communications; or certain critical manufacturing processes.
  2. Important, But Not Critical Loads: such as ventilation systems, general lighting and less-vital manufacturing processes.
  3. Non-Essential Loads For Business Continuity: an example might be the HVAC system, unless it is needed for cooling major computer or data facilities.

Standby power systems with multiple generator sets offer a number of advantages that include:

  • Enhanced reliability and availability,
  • Operational flexibility,
  • Fuel savings,
  • More convenient maintenance scheduling,
  • Easier system expansion.

 

 

So, the owner and designer must decide one of the following two solutions:

 

  1. Install a standby power system using a single large generator set or
  2. Install a system with multiple smaller, paralleled generator sets.

 

To select the best solution, the following factors must be taken into consideration:

 

  • The facility’s load profile and priorities,
  • The impact of enhanced power system reliability and availability,
  • The impact of enhanced operational flexibility on improved maintenance scheduling,
  • The potential fuel cost savings and electricity rates for participation in utility interruptible-rate programs,
  • The impact of load growth and reduction of future expansion costs.

 

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 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
 

 


 

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