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.
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
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
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
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
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.
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
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
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
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
