Transformer Sizing Using ETAP

TRANSFORMER MVA SIZING USING ETAP By Waqar Hussain Electrical Dept.

INTRODUCTION 

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

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ETAP has complied p the information contained 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

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

2 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.

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

2 WINDING TRAFO MVA SIZING (CONTD.) (CONTD ) Load Variation

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Growth Factor – For allowance for future growth. Growth 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

2 WINDING TRAFO MVA SIZING (CONTD.) (CONTD ) Installation

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ETAP adjusts the calculated required Rated MVA based on trafo’s altitude value. For example p for Liquid q Immersed Forced Air Cooled trafo, the MVA rating is derated by factor 0.5 % for every 330 ft above 3300 ft. ft Ambient Temp. ETAP adjusts the required Rated MVA based on ambient temperature value

2 WINDING TRAFO MVA SIZING (CONTD.) (CONTD ) p Impedance

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

2 WINDING TRAFO MVA SIZING (CONTD.) (CONTD ) Results

Any one of the three results can be selected: Larger size, Required size or smaller size

OTHER REQUIRED DATA FOR TRAFO SIZING ON ETAP

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Standard (IEC or ANSI) Type (Liquid fill or Dry) Sub type (mineral oil oil, synthetic liquid etc …)) Class (ONAN, ONAN-ONAF, OFAF etc …) Temperature rize Primary Winding kV rating

REGULATION 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 motor start.

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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

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This voltage drop is proportional to the regulation of trafo during motor start

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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

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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

REGULATION OF TRAFO ON MOTOR START pp g p y Approximate voltage drop on the transformer secondary   can be calculated by the following formula

Where D = % Voltage drop on secondary side D  % Voltage drop on secondary side P = Peak Load on trafo in kVA other than largest Motor  M = Starting kVA of largest Motor M = Starting kVA of largest Motor S = Short circuit capacity of trafo in kVA

EXAMPLE

UCH2 Unit Auxiliary Transformer sizing using ETAP

INPUT DATA – TRAFO PARAMETERS Standard: IEC

Type: Liquid Fill Sub type: mineral oil

Cl Class: ONAN - ONAF

T Temp. rise: i 65

Primary V = 14.5 kV Sec. V = 6.6 kV

INPUT DATA – UCH2 ETAP MODEL 

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

Unit Auxiliary transformer to be sized u p of o 13869 3869 kVA Lump1 represents the UCH2 operating BOP load other than 1 No. circulation water pump motor. The lump load contains all the plant cable losses and loading factors of different loads

SIZING MODULE 

Based on Operating transformer load, t f l d 120 % growth factor (for max MVA or ONAF) and 28 degree C p , the ambient temp. recommended size by ETAP is 20-25 MVA at ONAN-ONAF

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

UCH2 LOAD FLOW AT 3 DIFFERENT UNIT AUX. TRAFO RATINGS – WORST CASE LOADING 16‐20 MVA trafo % Z = 10, X\R = 22.2

20‐25 MVA trafo % Z = 10, X\R = 22.2

25‐31.5 MVA trafo % Z = 10, X\R = 22.2

Trafo Input kVA

16198

16046

15932

Trafo Output kVA

15217

15286

15339

Increase in trafo output kVA as trafo ratingg is increased. ? 20 % of the lump load in UCH2 ETAP model is static

For good voltage regulation at trafo sec. the static load consumes more kVA

Decrease in trafo input kVA as trafo ratingg is increased. ? Trafo % Z is taken as contant at 10 %

For ONAN – ONAF rated trafo, ETAP performs Load flow at ONAN rating

ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFO RATINGS – NORMAL CASE LOADING Motor Starting Analysis For Circulation Water  Pump Motor

Trafo Input MVA Load Flow before  Motor start

Load Flow at  Motor start

Load Flow after Load Flow after Motor start (steady  state)

16‐20 MVA trafo % Z = 8, X\R = 22.2

13 607 13.607

20 865 20.865

15 277 15.277

20‐25 MVA trafo % Z = 8, X\R = 22.2

13.655

21.345

15.333

20‐25 MVA trafo % Z = 10, X\R = 22.2

13.632

20.865

15.277

If Normal loading is considered (i e lump 1 = 13600 kVA ), (i.e ) then the sizing module recommends 16 – 20 MVA trafo

ETAP MOTOR START STUDY AT 3 DIFFERENT TRAFO RATINGS – NORMAL CASE LOADING ….. CONTD Motor Terminal  Motor Starting Analysis Trafo sec. Voltage at  For Circulation Water  Voltage at Motor  For Circulation Water Voltage at Motor Motor Start Pump Motor Start

Motor Terminal  Voltage at Steady  Voltage at Steady State

16‐20 MVA trafo % Z = 8, X\R = 22.2

88.35%

92.09%

98.38%

20‐25 MVA trafo % Z = 8, X\R = 22.2

90.34%

94.18%

99.49%

20‐25 MVA trafo 20 25 MVA trafo % Z = 10, X\R = 22.2

88.35%

92.09%

98.38%

As per IEEE Standard 399-1997, IEEE Recommended Practice for Industrial and Commercial Power System Analysis (Brown Book), the Voltage at terminal of Motor g duringg startingg shall not fall below 80 % of rated voltage.

Hence this condition is satistfied for all the three different trafo ratings

Motor 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 For ONAN – ONAF rated trafo, ETAP performs Motor Start at ONAN rating

CALCULATION OF TRAFO SEC. VOLTAGE AT MOTOR START USING FORMULA …… COMPARISON Motor Starting Analysis For Circulation Water  For Circulation Water Pump Motor

Trafo sec. Voltage at Motor Start calculated using ETAP

calculated using  calculated using Formula

16‐20 MVA trafo % Z = 8, X\R = 22.2

88.35%

87.36%

20‐25 MVA trafo % Z = 8, X\R = 22.2

90.34%

89.89%

20‐25 MVA trafo % Z = 10 X\R = 22 2 % Z = 10, X\R = 22.2

88.35%

87.35%

Almost same results using formula and ETAP, confirms the validity of the formula used in UCH2 trafo sizing calculations

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

SUMMARY ……. UCH2 UAT SIZING

ETAP Transformer Sizing Module

Normal load for GT, ST, HRSG

Worst load for GT, ST, HRSG

Growth Factor at ONAF

16 – 20 MVA

Growth Factor at ONAN

20 – 25 MVA

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 MVA Before 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 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 MVA trafo at 8 % Z

STANDARDIZATION ON ETAP FOR TRAFO SIZING 

Havingg witnessed the harmonyy between ETAP transformer sizing module and manual sizing through formulae, the sizing calculations can be standardized on ETAP

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Following documents can be issued if sizing is done using ETAP ETAP Load Flow analysis Report

Snapshot of ETAP sizing Module

ETAP Motor Start Study Report

THANK YOU OU Q&A