Transformer Sizing Using ETAP

TRANSFORMER MVA SIZING USING ETAP

By Waqar HussainElectrical Dept.

INTRODUCTIONINTRODUCTION

In power systems, transformer is one of many p y , ydevices whose proper size is critical to the design of a power delivery system.

ETAP has complied the information contained pon standards ANSI IEEE C57, IEC 60076-2 and IEC 60726 into a program that can easily determine the proper size of a power transformer

FACTORS FOR TRAFO MVA SIZING IN ETAPFACTORS FOR TRAFO MVA SIZING IN ETAP

ETAP takes into consideration the following factors ETAP takes into consideration the following factors for trafo MVA sizingAmbient temperatureAmbient temperatureAltitudeCooling StageCooling StageType (dry or liquid fill)Expected future growthShort circuit requirement (trafo impedence and BIL)

2 WINDING TRAFO MVA SIZING2 WINDING TRAFO MVA SIZING

The two winding transformer MVA sizing calculation sizes the following parameters of a transfomer

Rated MVAMaximum MVAPercentage impedance

Maximum MVA is 1.25 times therated MVA for ONAN ONAF trafo.

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


Load Variation

Growth Factor For allowance for future growth. Growth factor is used for calculating the rated MVA of a factor 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


Installation

ETAP adjusts the calculated required Rated MVA based on trafos altitude value. For example for Liquid p qImmersed 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


Impedancep

Basic Impulse Level (BIL) Limit - ETAP utilizes this p ( )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


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

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

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 onthe terminals of the largest motor (connected to trafo secondary) during motorthe terminals of the largest motor (connected to trafo secondary) during motorstart.

According to IEEE Brown Book, the voltage at terminals of starting motorshould 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 inverselyproportional to trafo short circuit kVA and directly proportional to motorstarting kVA

The Transformer MVA Sizing Module of ETAP does not take into account thisfactor. The regulation of trafo can be checked by Motor Starting Study on ETAP

REGULATION OF TRAFO ON MOTOR STARTREGULATION OF TRAFO ON MOTOR START

Approximatevoltagedroponthetransformersecondarypp g p ycanbecalculatedbythefollowingformula

Where

D = % Voltage drop on secondary sideD %Voltagedroponsecondaryside

P=PeakLoadontrafo inkVA otherthanlargestMotor

M = Starting kVA of largest MotorM=StartingkVA oflargestMotor

S=Shortcircuitcapacityoftrafo inkVA

EXAMPLEEXAMPLE

UCH2 Unit Auxiliary Transformer sizing using ETAP

INPUT DATA TRAFO PARAMETERSINPUT DATA TRAFO PARAMETERS

Standard: IECType: Liquid Fill

Sub type: mineral oil

Cl ONAN ONAF T i 65Class: ONAN - ONAF Temp. rise: 65

Primary V = 14.5 kVSec. V = 6.6 kV

INPUT DATA UCH2 ETAP MODELINPUT DATA UCH2 ETAP MODEL

Plant ETAP model is required to is required to evaluate connected and operating load on a transformer

Unit Auxiliary transformer to be sized

Lump1 of 13869 kVAu p o 3869represents the UCH2

operating BOP load other than 1 No. circulation

water pump motor. The lump load contains all the plant cable losses the plant cable losses and loading factors of

different loads

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. , the p ,recommended size by ETAP is 20-25 MVA at ONAN-ONAF

Here worst case load is considered for GT, HRSG and ST MCC as provided and ST MCC, as provided by HET. (This load is included as lump 1 in ETAP model)

UCH2 LOAD FLOW AT 3 DIFFERENT UNIT AUX. TRAFORATINGS WORST CASE LOADING

1620MVAtrafo 2025MVAtrafo 2531.5MVAtrafo%Z=10,X\R=22.2 %Z=10,X\R=22.2 %Z=10,X\R=22.2

TrafoInputkVA 16198 16046 15932

TrafoOutputkVA 15217 15286 15339

Increase in trafooutput kVA as trafo rating is

Decrease in trafoinput kVA as trafo

rating is increased. For ONAN ONAF gincreased. ?

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 trafosec. the static load

consumes more kVA

ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFORATINGS NORMAL CASE LOADING

MotorStartingAnalysisTrafoInputMVA

Load Flow afterForCirculationWaterPumpMotor

LoadFlowbeforeMotorstart

LoadFlowatMotorstart

LoadFlowafterMotorstart(steady

state)

1620MVAtrafo13 607 20 865 15 277

%Z=8,X\R=22.213.607 20.865 15.277

2025MVAtrafo%Z=8,X\R=22.2

13.655 21.345 15.333


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

ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFORATINGS NORMAL CASE LOADING .. CONTD

MotorStartingAnalysisFor Circulation Water

Trafosec.VoltageatMotorTerminalVoltage at Motor

MotorTerminalVoltage at SteadyForCirculationWater

PumpMotorMotorStart

VoltageatMotorStart

VoltageatSteadyState


88.35% 92.09% 98.38%


90.34% 94.18% 99.49%

20 25 MVA trafo2025MVAtrafo%Z=10,X\R=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 Motorduring 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

CALCULATION OF TRAFO SEC. VOLTAGE AT MOTOR START USING FORMULA COMPARISON

MotorStartingAnalysisFor Circulation Water

Trafo sec.VoltageatMotorStart

calculated usingForCirculationWaterPumpMotor calculatedusingETAP

calculatedusingFormula


88.35% 87.36%

Almost same results using formula and ETAP, confirms the validity of the formula


90.34% 89.89%

2025MVAtrafo% Z = 10 X\R = 22 2

88.35% 87.35%

used in UCH2 trafo sizing calculations

%Z=10,X\R=22.2

Starting kVA of motor = Full Load kVA x 6 = 11412 kVAPeak load on trafo excluding motor load = 13869 kVA

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 25 Worst 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

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 25 increased to 20 25 MVA trafo at 8 % Z

STANDARDIZATION ON ETAP FOR TRAFO SIZINGSTANDARDIZATION ON ETAP FOR TRAFO SIZING

Having witnessed the harmony between ETAP g ytransformer 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 Start ETAP Load Flow analysis Report

Snapshot of ETAP sizing Module

ETAP Motor Start Study Report

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Transformer Sizing Using ETAP