This document discusses transformer sizing using ETAP software. It explains that ETAP takes into account factors like ambient temperature, altitude, cooling type and expected future growth to determine the proper transformer size. The document provides details on ETAP's 2-winding transformer sizing module, which calculates the rated MVA, maximum MVA and impedance based on loading, installation factors and short circuit requirements. It also discusses how ETAP can be used to check transformer regulation during motor starts. The document concludes that transformer sizing calculations can be standardized using ETAP due to the harmony between ETAP's sizing module and manual calculations using formulas.

1. TRANSFORMER MVA SIZING USING ETAP
By Waqar Hussain
Electrical Dept.

2. INTRODUCTIONINTRODUCTION
 In power systems, transformer is one of manyp y , y
devices whose proper size is critical to the
design of a power delivery system.
 ETAP has complied the information containedp
on standards ANSI – IEEE C57, IEC 60076-2
and IEC 60726 into a program that can easily
determine the proper size of a power
transformer

3. FACTORS FOR TRAFO MVA SIZING IN ETAPFACTORS FOR TRAFO MVA SIZING IN ETAP
ETAP takes into consideration the following factorsETAP takes into consideration the following factors
for trafo MVA sizing
Ambient temperatureAmbient temperature
Altitude
Cooling StageCooling Stage
Type (dry or liquid fill)
Expected future growth
Short circuit requirement (trafo impedence and BIL)

4. 2 WINDING TRAFO MVA SIZING2 WINDING TRAFO MVA SIZING
The two winding transformer MVA sizing calculation sizes the following
parameters of a transfomer
Rated MVA
Maximum MVA
Percentage impedance
Maximum MVA is 1.25 times the
rated MVA for ONAN – ONAF
trafo.

5. 2 WINDING TRAFO MVA SIZING (CONTD )2 WINDING TRAFO MVA SIZING (CONTD.)
Transformer Loading
 This section allows you to find the operating load
( l l t d f l d fl l i ) t d(calculated from load flow analysis) or connected
load to this transformer and then use either load
for sizing purposesg p p
 Also you can manually enter the loading for the
transformer
 While sizing transformer based on connected load,
spares can also be included

6. 2 WINDING TRAFO MVA SIZING (CONTD )2 WINDING TRAFO MVA SIZING (CONTD.)
Load Variation
 Growth Factor – For allowance for future growth. Growth
factor is used for calculating the rated MVA of afactor is used for calculating the rated MVA of a
transformer. If “Use growth factor for max MVA” is
checked, then it is used for calculating the max MVA
 Load Factor – is 100 % if the transformer carries
required load continuously all the time

7. 2 WINDING TRAFO MVA SIZING (CONTD )2 WINDING TRAFO MVA SIZING (CONTD.)
Installation
 ETAP adjusts the calculated required Rated MVA
based on trafo’s altitude value. For example for Liquidp q
Immersed Forced Air Cooled trafo, the MVA rating is
derated by factor 0.5 % for every 330 ft above 3300
ftft.
 Ambient Temp. ETAP adjusts the required Rated MVA
based on ambient temperature valuebased on ambient temperature value

8. 2 WINDING TRAFO MVA SIZING (CONTD )2 WINDING TRAFO MVA SIZING (CONTD.)
Impedancep
 Basic Impulse Level (BIL) Limit - ETAP utilizes thisp ( )
value for determining the trafo minimum impedance
according to ANSI – IEC standards
 Limit Short Circuit kA – If this option is checked, then
ETAP calculates % impedance based on short circuit
kA, trafo type and BIL LimitkA, trafo type and BIL Limit

9. 2 WINDING TRAFO MVA SIZING (CONTD )2 WINDING TRAFO MVA SIZING (CONTD.)
ResultsResults
Any one of the three results can be selected:Any one of the three results can be selected:
Larger size, Required size or smaller size

10. OTHER REQUIRED DATA FOR TRAFO SIZING ON ETAPOTHER REQUIRED DATA FOR TRAFO SIZING ON ETAP
 Standard (IEC or ANSI) Standard (IEC or ANSI)
 Type (Liquid fill or Dry)
 Sub type (mineral oil synthetic liquid etc ) Sub type (mineral oil, synthetic liquid etc …)
 Class (ONAN, ONAN-ONAF, OFAF etc …)
 Temperature rize Temperature rize
 Primary Winding kV rating

11. REGULATION OF TRAFO ON MOTOR STARTREGULATION OF TRAFO ON MOTOR START
 One of the factors to be considered while sizing a trafo is the voltage drop on
the terminals of the largest motor (connected to trafo secondary) during motorthe terminals of the largest motor (connected to trafo secondary) during motor
start.
 According to IEEE Brown Book, the voltage at terminals of starting motor
should not fall below 80% of rated voltage for typical NEMA design B motors
 This voltage drop is proportional to the regulation of trafo during motor start
 The voltage drop at trafo secondary (regulation) during motor start is inversely
proportional to trafo short circuit kVA and directly proportional to motor
starting kVA
 The Transformer MVA Sizing Module of ETAP does not take into account this
factor. The regulation of trafo can be checked by Motor Starting Study on ETAP

12. REGULATION OF TRAFO ON MOTOR STARTREGULATION OF TRAFO ON MOTOR START
Approximate voltage drop on the transformer secondary pp g p y
can be calculated by the following formula
Where
D = % Voltage drop on secondary sideD % Voltage drop on secondary side
P = Peak Load on trafo in kVA other than largest Motor
M = Starting kVA of largest MotorM = Starting kVA of largest Motor
S = Short circuit capacity of trafo in kVA

13. EXAMPLEEXAMPLE
UCH2 Unit Auxiliary Transformer sizing using ETAP

14. INPUT DATA – TRAFO PARAMETERSINPUT DATA TRAFO PARAMETERS
Standard: IEC
Type: Liquid Fill
Sub type: mineral oil
Cl ONAN ONAF T i 65Class: ONAN - ONAF Temp. rise: 65
Primary V = 14.5 kV
Sec. V = 6.6 kV

15. INPUT DATA – UCH2 ETAP MODELINPUT DATA UCH2 ETAP MODEL
 Plant ETAP model
is required tois required to
evaluate connected
and operating load
on a transformer
Unit Auxiliary
transformer to
be sized
Lump1 of 13869 kVAu p o 3869
represents the UCH2
operating BOP load other
than 1 No. circulation
water pump motor. The
lump load contains all
the plant cable lossesthe plant cable losses
and loading factors of
different loads

16. SIZING MODULESIZING MODULE
 Based on Operating
t f l d 120 %transformer load, 120 %
growth factor (for max MVA
or ONAF) and 28 degree C
ambient temp. , thep ,
recommended size by
ETAP is 20-25 MVA at
ONAN-ONAF
 Here worst case load is
considered for GT, HRSG
and ST MCC as providedand ST MCC, as provided
by HET. (This load is
included as “lump 1” in
ETAP model)

17. UCH2 LOAD FLOW AT 3 DIFFERENT UNIT AUX. TRAFO
RATINGS – WORST CASE LOADING
16‐20 MVA trafo 20‐25 MVA trafo 25‐31.5 MVA trafo
% Z = 10, XR = 22.2 % Z = 10, XR = 22.2 % Z = 10, XR = 22.2
Trafo Input kVA 16198 16046 15932
Trafo Output kVA 15217 15286 15339
Increase in trafo
output kVA as
trafo rating is
Decrease in trafo
input kVA as trafo
rating is increased. For ONAN – ONAFg
increased. ?
20 % of the lump
load in UCH2 ETAP
g
?
Trafo % Z is taken
For ONAN ONAF
rated trafo, ETAP
performs Load flow
at ONAN rating
load in UCH2 ETAP
model is static
For good voltage
as contant at 10 %
For good voltage
regulation at trafo
sec. the static load
consumes more
kVA

18. ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFO
RATINGS – NORMAL CASE LOADING
Motor Starting Analysis
Trafo Input MVA
Load Flow after
For Circulation Water
Pump Motor
Load Flow before
Motor start
Load Flow at
Motor start
Load Flow after
Motor start (steady
state)
16‐20 MVA trafo
13 607 20 865 15 277
% Z = 8, XR = 22.2
13.607 20.865 15.277
20‐25 MVA trafo
% Z = 8, XR = 22.2
13.655 21.345 15.333
20‐25 MVA trafo
% Z = 10, XR = 22.2
13.632 20.865 15.277
If Normal loading is considered
(i e lump 1 = 13600 kVA ) then(i.e lump 1 = 13600 kVA ), then
the sizing module recommends
16 – 20 MVA trafo

19. ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFO
RATINGS – NORMAL CASE LOADING ….. CONTD
Motor Starting Analysis
For Circulation Water
Trafo sec. Voltage at
Motor Terminal
Voltage at Motor
Motor Terminal
Voltage at SteadyFor Circulation Water
Pump Motor
Motor Start
Voltage at Motor
Start
Voltage at Steady
State
16‐20 MVA trafo
% Z = 8, XR = 22.2
88.35% 92.09% 98.38%
20‐25 MVA trafo
% Z = 8, XR = 22.2
90.34% 94.18% 99.49%
20 25 MVA trafo20‐25 MVA trafo
% Z = 10, XR = 22.2
88.35% 92.09% 98.38%
Motor Rated Voltage:
As per IEEE Standard 399-1997, IEEE Recommended Practice
for Industrial and Commercial Power System Analysis (Brown
Book), the Voltage at terminal of Motor
during starting shall not fall below 80 % of rated voltage.
Please note that the motor rated
voltage is 6.3 kA where as the MV bus
and trafo sec. rated voltage is 6.6 kV
g g g
Hence this condition is satistfied for all the three
different trafo ratings
For ONAN – ONAF rated
trafo, ETAP performs
Motor Start at ONAN
rating

20. CALCULATION OF TRAFO SEC. VOLTAGE AT MOTOR
START USING FORMULA …… COMPARISON
Motor Starting Analysis
For Circulation Water
Trafo sec. Voltage at Motor Start
calculated usingFor Circulation Water
Pump Motor
calculated using ETAP
calculated using
Formula
16‐20 MVA trafo
% Z = 8, XR = 22.2
88.35% 87.36%
Almost same results using
formula and ETAP, confirms
the validity of the formula
20‐25 MVA trafo
% Z = 8, XR = 22.2
90.34% 89.89%
20‐25 MVA trafo
% Z = 10 XR = 22 2
88.35% 87.35%
used in UCH2 trafo sizing
calculations
% Z = 10, XR = 22.2
Starting kVA of motor = Full Load kVA x 6 = 11412 kVA
Peak load on trafo excluding motor load = 13869 kVA

21. SUMMARY UCH2 UAT SIZINGSUMMARY ……. UCH2 UAT SIZING
Growth
16 20
ETAP
Normal load
for GT, ST,
HRSG
Factor at
ONAF
Growth
Factor at
16 – 20
MVA
20 – 25
ETAP
Transformer
Sizing Module
Worst load
Factor at
ONAN MVA
20 25Worst load
for GT, ST,
HRSG
20 – 25
MVA
AT Normal Load for GT, ST, HRSG – Total BOP Load including UAT losses = 15.9 MVA approx
AT Worst Load for GT, ST, HRSG – Total BOP Load including UAT losses = 15.3 – 15.6 MVA approx
The Motor starting Condition is satisfied for both transformer ratings as indicated in previous slides

22. SUMMARY ……. UPSIZING OF UCH2 UAT FROM 16 -20
MVA TO 20 – 25 MVAMVA TO 20 25 MVA
Before addition of 1 MW
colony feeder and two
For 16 – 20 MVA trafo, at
motor start the voltage at
825 kW HSD Electric
Heaters
MV bus was greater than
90 %
After addition of 1 MW
colony feeder and two
825 kW HSD Electric
Heaters
For 16 – 20 MVA trafo, at
motor start the voltage at
MV bus became lesser
than 90 %
Mushtaq Sb. (from PMT)
required the voltage to
be greater than 90 %
To incorporate this
comment, UAT rating was
increased to 20 – 25increased to 20 25
MVA trafo at 8 % Z

23. STANDARDIZATION ON ETAP FOR TRAFO SIZINGSTANDARDIZATION ON ETAP FOR TRAFO SIZING
 Having witnessed the harmony between ETAPg y
transformer sizing module and manual sizing
through formulae, the sizing calculations can
be standardized on ETAP
 Following documents can be issued if sizing is
done using ETAP
ETAP Load Flow Snapshot of ETAP ETAP Motor StartETAP Load Flow
analysis Report
Snapshot of ETAP
sizing Module
ETAP Motor Start
Study Report

24. THANK YOUTHANK YOUOU
Q & A
OU
Q & AQ & AQ & A