In Article “Power and Distribution Transformers Sizing Calculations – Part One”, we indicate that the contents of
our articles for Power and Distribution Transformers sizing calculations will
include the following points:
 Glossary of Sizing Power and Distribution Transformers,
 Power and distribution transformer components,
 Power and distribution transformer classification: construction and application,
 Threephase power and distribution transformer connections,
 Power and Distribution Transformers sizing calculations.
The following points were
explained before (or will be explained) in our course “EP3:Electrical Procurement – Transformers Course”:
 Power and distribution transformer components,
 Power and distribution transformer classification: construction and application,
 Threephase power and distribution transformer connections,
So, we will not go through these
points here, we will focus only on the following two points:
 Glossary of Sizing Power and Distribution Transformers,
 Power and Distribution Transformers sizing calculations.
Also, in Article “Power and Distribution Transformers Sizing Calculations – Part Two” , we indicate that Our study for the
Power and Distribution Transformers sizing calculations will include the
explanations of the following points:
 Resources used to calculate basic ratings of power and distribution transformers,
 Selection Factors,
 Calculations procedures For Sizing of Power and Distribution Transformers,
 Special cases.
Today we will explain the
calculations procedures for sizing of Power and Distribution Transformers.
Note: I’d like from all of you
to review our course “EP3: Electrical Procurement – Transformers Course” to be more familiar with the
contents of our new articles about the Power and Distribution Transformers
sizing calculations.
Calculations
Procedures For
Sizing Of Power And Distribution Transformers. 
In power systems, there are many devices whose proper size
is critical to the design of a power delivery system. One of the most important
is the power transformer.
There are several factors
involved in the process of sizing a transformer which were explained in article
“Power and Distribution Transformers Sizing Calculations – Part Three”, these factors can be used to
determine if a transformer can handle its required operating load or not.
Inadequately sized transformers may shorten the equipment's operating life or
cause overloading failures.
Now, we will explain step by step the accurate sizing
calculations of Power and Distribution Transformers which will include the
following:
 Applicable calculations procedures for sizing of power and distribution transformers,
 Applicable procedures for calculating power and distribution transformer ratios,
 Applicable procedures for calculating power transformer efficiency,
 Applicable procedures for calculating power transformer voltage regulation.
1
Applicable Calculations Procedures
For
Sizing Of Power And Distribution Transformers.

Summary
Of Selection/Calculation Procedures


Step#
1
Selected data:

Step#
2
Selected data:

Select the
primary and secondary voltage ratings from Table1 and the given
information.
For
threephase transformers, assume 3Ø, 3wire, deltaconnected primaries
and 3Ø, 4wire,
wyeconnected secondaries.
Table1:
Standard Voltage Ratings

Step#
3
Selected data:

Select the
BIL Rating for the transformer from Table2 for liquidfilled
transformers, or from Table3 for drytype transformers.
Table2:
Transformer BIL Ratings for liquidimmersed transformers
Table3
BIL Ratings for DryType Transformers

Step#
4
Calculated Data:

A Calculate
the load kW for each load or combination of loads at rated utilization
voltage.
Note: Include
any projected (known) future loads.
kW_{load}
= 3 x kV x I x p.f. or
kW_{load }=
kVA_{load} x p.f.
B Calculate
the load kVAR for each load or combination of loads at rated utilization
voltage.
Note: Include
any projected (known) future loads.
kVAR_{load}
= 3 x kV x I x sin (cos^{1} p.f.) or
kVAR_{load}
= kW_{load} x tan (cos^{1} p.f.)
C Calculate
site kVA.
kVA_{(site)} = (kW^{2} _{total}
+ kVAR^{2} _{total})^{1/2}
D Calculate
the adjusted site kVA after derating for temperature from Table4.
Table4:
Ambient Temperature DeRating
Example1:
Use a design
ambient temperature of 40^{0}C.
Then:
kVA _{(adjusted)}
= kVA _{(site)/}0.85 for liquidfilled transformers,
kVA _{(adjusted)}
= kVA _{(site)/}0.94 for drytype transformers with 150^{0}C
insulation,
kVA _{(adjusted)}
= kVA _{(site)}/0.96 for drytype transformers with 220^{0}C
insulation.

Step#
5
Selected data:

Select the
next standard size kVArated transformer from Table5 for
liquidfilled transformers, or from Table6 for drytype transformers.
Note:
For
selfcooled transformers (OA or AA only) add 10% for growth.
Table5:
Standard Transformer kVA Ratings (LiquidFilled)
Table6:
Standard Transformer kVA Ratings (DryType)

Step#
6
Selected data:

Select the cooling
class kVA ratings as follows:
Table7:
The Cooling Class kVA Ratings For LiquidFilled Transformers (750  12,500
KVA)
Table8:
The Cooling Class kVA Ratings For LiquidFilled Transformers Greater Than
12.5 MVA
Table9:
The Cooling Class kVA Ratings for DryType Transformers
Notes:

In the next article, we will explain the following calculations procedures:
 Applicable procedures for calculating power transformer efficiency,
 Applicable procedures for calculating power transformer voltage regulation.
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