SIZING OF DISTRIBUTION TRANSFORMERS 
BASED ON " L I F E - C Y C L E - COST" 
Wellana Gamage Pawithra 
(128776N) 
Dissertation submitted in partial ftilfilment of the requirements for the Degree 
Master of Science in Electrical histallations 
University' of Moratuwa 
TH3375 
7^ 
Department of Electrical Engineering 
University of Moratuwa 
Sri Lanka 
May 2017 
D E C L A R A T I O N O F T H E C A N D I D A T E & S U P E R V I S O R 
I declare that this is my own work and this dissertation does not incorporate 
without acknowledgement any material previously submitted for a Degree or 
Diploma in any other University or institute of higher learning and to the best 
of my knowledge and belief it does not contain any material previously 
published or written by another person except where the acknowledgement is 
made in the text. 
Also, I hereby grant to University of Moratuwa the non-exclusive right to 
reproduce and distribute my dissertation, in whole or in part in print, electronic 
or other medium. I retain the right to use this content in whole or part in future 
works (such as articles or books). 
W.G. Pawithra 
The above candidate has carried out research for the Masters Dissertation 
under my supervision. 
Signature: 5 ^ Date: Soli- 0£-24, 
Signature of the supervisor: 
Prof. J.R. Lucas 
DEDICAT IO N 
To my loving parents and husband 
who always pick me up on time and encourage me to go on every adventure, 
especially this one 
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A C K N O W L E D G E M E N T 
Firstly, I would like to express my sincere gratitude to my advisor Prof. J. 
Rohan Lucas for the continuous support of my MSc. study and related 
research, for his patience, motivation and immense knowledge. His guidance 
helped me all the time of research and in writing of this thesis. I could not have 
imagined having a better advisor and mentor for my MSc. study. 
Besides my advisor, I would like to thank Dr. Asanka S. Rodrigo and Eng. 
W.D. Anura S. Wijayapala not only for their insightful comments and 
encouragement, but also for the hard questions which convinced me to widen 
my research from various perspectives. 
Further, I would like to thank my parents and husband for their encouragement 
and support. 
Lastly, I should thank many individuals, friends and colleagues who have not 
been mentioned here personally in making this educational process a success. 
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ABSTRACT 
Many energy audits in Sri Lanka have identified the installed presence of oversized 
transformers and excessive number of transformers. No proper study has been done on 
the effect of the no load losses added to the system due to oversized transformers. 
Thus, transformers have not always being properly sized at the design stage. 
In this research, a sample of bulk consumers from the North Western Province was 
selected and their installed capacities were compared with the properly sized 
transformers. Analysis shows that the no load losses can be reduced, on the average by 
about 40% with the proper sizing. Having identified the importance of proper sizing of 
transformers, software has been developed to select the optimum transformer capacity 
comparing the total owning costs of the transformers. The effect of the load pattern, 
transformer life time, discounting factor and the tariff rate has been considered in the 
optimization. Also, the software permits the determination of the optimum transformer 
for a few different load curves as well. The software tool has been validated in 
comparison with manual calculations. 
This software is a useful tool for the bulk consumer as well as electrical consultants to 
determine the optimum transformer capacity for a given load curve based on total 
owning cost. The consumer can select the optimum size at the initial stage of the 
project so that unnecessary future costs due to losses can be minimized. Moreover, 
this would not only be a blessing to the consumer, but would mean that the overall 
distribution loss in the total electrical network will be reduced. 
Keywords: 
Life Cycle Cost, Transformer, Optimization, Load Losses, No load losses. Total 
Owning Cost 
I V 
TABLE OF CONTENTS 
D E C L A R A T I O N O F T H E C A N D I D A T E & S U P E R V I S O R I 
D E D I C A T I O N I I 
A C K N O W L E D G E M E N T H I 
A B S T R A C T I V 
L I S T O F F I G U R E S V I I 
L I S T O F T A B L E S V I I 
L I S T O F A B B R E V I A T I O N S V I I I 
1 I N T R O D U C T I O N 1 
1.1 B A C K G R O U N D , - l 
1.1.1 Adverse impacts of oversized transformers 1 
1.1.2 Sri Lankan Scenario 1 
1.2 P O W E R S Y S T E M T R A N S F O R M E R S 2 
1.2.1 Distribution transformers 4 
1.2.2 Transformer losses 4 
1.3 T O P I C O F R E S E A R C H 6 
1.3.1 Research Motivation 6 
1.3.2 Problem Statement - 6 
2 L I T E R A T U R E R E V I E W 7 
2.1 B A C K G R O U N D 7 
2.2 T E C H N I C A L PAPERS R E V I E W E D 7 
2.3 R E S E A R C H G A P 12 
3 R E S E A R C H M E T H O D O L O G Y 13 
3.1 I N T R O D U C T I O N 13 
3.2 I D E N T I F I C A T I O N O F R E S E A R C H G A P 13 
3.3 T H E O R E T I C A L A N A L Y S I S 13 
3.4 D E V E L O P A M O D E L T O C A L C U L A T E T H E T O T A L O W N I N G C O S T 14 
3.5 V A L I D A T E T H E M O D E L 14 
4 T H E O R E T I C A L A N A L Y S I S O F S I Z I N G O F T R A N S F O R M E R S 1 5 
4.1 E F F E C T O F T H E O V E R S I Z E D T R A N S F O R M E R S T O T H E S Y S T E M 15 
4 .2 F A C T O R S F O R PROPER S I Z I N G O F T R A N S F O R M E R S ( L I F E C Y C L E C O S T ) 16 
4.2.1 Technical factors 16 
4.2.2 Financial factors 17 
V 
r • • • ; 
4.2.3 Production/business related factors 17 
4.3 T Y P I C A L L O A D P A T T E R N S C O N S I D E R E D 19 
4.3.1 Type A Load Curve: Distribution Load Curve 19 
4.3.2 Type B Load Curve: 20 
4.3.3 Type C Load Curve: 20 
4.3.4 Type D Load Curve: 21 
4.4 A S S U M P T I O N S A N D S T A N D A R D S 2 2 
5 T O T A L O W N I N G C O S T O F T R A N S F O R M E R S 2 4 
5.1 B A C K G R O U N D 2 4 
5.2 T O T A L O W N I N G C O S T C A L C U L A T I O N S O F T W A R E 2 5 
6 R E S U L T S 2 8 
6.1 R E S U L T S O F T H E T H E O R E T I C A L A N A L Y S I S 2 8 
6.1.1 Scenario 1 - Effect of Load Pattern 28 
6.1.2 Scenario 2- Effect of discounting factor 29 
6.1.3 Scenario 3 : Ejfect of Transformer Lifetime 29 
61.4 Scenario 4: Effect of Tariff. , 30 
6.2 R E S U L T S O F T H E S O F T W A R E A N A L Y S I S 3 0 
6.2.1 Scenario 1 - Effect of Load Pattern 30 
6.2.2 Scenario 2- Effect of discounting factor 31 
6.2.3 Scenario 3: Effect of Transformer Life time 31 
6.2.4 Scenario 4: Effect of Tariff. 31 
1 C O N C L U S I O N S A N D R E C O M M E N D A T I O N S 3 2 
7.1 L I M I T A T I O N S 3 3 
7.2 R E C O M M E N D A T I O N S A N D F U R T H E R W O R K 3 3 
L I S T O F R E F E R E N C E S ' 3 4 
A N N E X U R E 3 6 
S O F T W A R E C O D E 3 6 
Main User Form 36 
Load Curve Method User Form 36 
Load Pattern User Form ^7 
Variable Confirmation User Form 46 
Output Display Form 47 
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L I S T OF F I G U R E S 
Figure 4-1: Type A load curve 19 
Figure 4-2: Type B load curve 20 
Figure 4-3: Type C load curve 20 
Figure 4-4: Type D load curve 21 
Figure 4-5: Equivalent two step load curve according to lEC 60354 22 
Figure 4-6: Overload curve according to lEC 60354 23 
Figure 5-1: Software development algorithm 25 
Figure 5-2: Interface of the software 26 
Figure 5-3: Variable inputs 27 
Figure 5-4: Results-Evaluated best transformers.... 27 
L I S T O F T A B L E S 
Table 2-1: Total Owning Cost Calculation Comparison 8 
Table 2-2: Monthly Energy Loss of Total Transformer Circuit 10 
Table 2-3: Rating Wise Transformer Losses for 1 Month 10 
Table 4-1: Tariff Rates 17 
Table 5-1: Cost of no load and load losses of 75 kVA transformer 24 
Table 6-1: Total owning cost for different load curves 28 
Table 6-2: Total owning cost for different discounting factors 29 
Table 6-3: Total owning cost for different transformer Ufe times 29 
Table 6-4: Total owning cost for different tariffs 30 
Table 6-5: Total owning cost from Software for different discounting factors 30 
Table 6-6: Total owning cost from software for different discounting factors 31 
Table 6-7: Total owning cost for different transformer life times 31 
Table 6-4: Total owning cost for different tariffs 31 
vii 
L I S T OF F I G U R E S 
Figure 4-1: Type A load curve 19 
Figure 4-2: Type B load curve 20 
Figure 4-3: Type C load curve 20 
Figure 4-4: Type D load curve 21 
Figure 4-5: Equivalent two step load curve according to lEC 60354 22 
Figure 4-6: Overload curve according to lEC 60354 23 
Figure 5-1: Software development algorithm 25 
Figure 5-2: Interface of the software 26 
Figure 5-3: Variable inputs 27 
Figure 5-4: Results-Evaluated best transformers ,. 27 
LIST O F T A B L E S 
Table 2-1: Total Owning Cost Calculation Comparison 8 
Table 2-2: Monthly Energy Loss of Total Transformer Circuit 10 
Table 2-3: Rating Wise Transformer Losses for 1 Month 10 
Table 4-1: Tariff Rates 17 
Table 5-1: Cost of no load and load losses of 75 kVA transformer 24 
Table 6-1: Total owning cost for different load curves 28 
Table 6-2: Total owning cost for different discounting factors 29 
Table 6-3: Total owning cost for different transformer life times 29 
Table 6-4: Total owning cost for different tariffs 30 
Table 6-5: Total owning cost from Software for different discounting factors 30 
Table 6-6: Total owning cost from software for different discounting factors 31 
Table 6-7: Total owning cost for different transformer life times 31 
Table 6-4: Total owning cost for different tariffs 31 
vii 
L I S T O F A B B R E V I A T I O N S 
CEB : Ceylon Electricity Board 
mc : International Electrotechnical Commission 
LKR : Sri Lankan Rupees 
TOC : Total Owning Cost 
DF : Discounting Factor 
Tf : Transformer 
Distn : Distribution 
I 
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