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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

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 steadystate power demanded by the
connected load, the power demanded by the connected load will be calculated
by using the following factors:

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 Table1.
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 table1.
Table1:
Load Factor for Different Generator Power Ratings

1.2 Load Demand Factor
Demand Factor = (Total Operating kW x
100) / Total Connected kW
So, Demand factor is always less than one.
Table2 shows
a range of common demand factors for different apparatus.
Table2:
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:
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
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 Table3.
Table3:
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
timecurrent 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:
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:

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 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. 
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