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ANALYSIS AND MODELLING OF CYCLIC LOADING POWER TRANSFORMER

Thye Hun Shen

Master of Engineering 2010

Jl a . .. .. . '''k.lUW· , tU.ademi~ m MALAY rA SARAWAK

ANALYSIS AND MODELLING OF CYCLIC LOADING POWER TRANSFORMER

P.KHIDMAT MAKL.UMAT AKADI!MIK

111I11~JIOCm1J1H III

THYE HUN SHEN

A thesis submitted in fulfillment of requirements for the degree of Master of Engineering

Faculty of Engineering UNIVERSITI MALAYSIA SARAWAK

2010

ACKNOWLEDGEMENT

First and foremost I would like to express my appreciation to everyone who had

given me assistances throughout this project. My heartfelt thanks to Ir. Dr. Mohammad

Shahril Osman and Mr. Ngu Sze Song as my academic supervisors. Special thanks to

Mr. Leslie Chili and Mr. Kee Song Khai for being my industrial supervisors and giving

me endless guidance along the completion of this thesis.

I woutd like to express my deepest thanks to my family for their love,

encouragement and support throughout the whole process.

Not forgotten to thank my colleagues and friends, Hung Tze Mau, Lai Koon

Chun, Chai Choung Jung, James Tan, Kong Tzer Sin, Charlie Sia and Voon Chun Yung

for their helping hands to solve the project difficulties.

Finally, my deepest gratitude goes out to all the lecturers in Faculty of

Engineering who kindly share their knowledge and experience. Not forgetting my

dearest parents, brothers and sisters who always give me moral support, encouragement,

assistance and comfort. To them, I wish the best of luck and may the future hold bright

for you all.

11

ABSTRACT

Transformers can transfer the required electricity to the residential, commercial and

industrial are~ to fulfil the consumption demand. This study targets to determine the

permitted overloading conditions and to maximize the electricity supply without

reducing the lifespan of the transformer by setting the necessary transformer parameters.

This study concludes that the loading curve pattern and power usage are very much

dependent on the distribution network in each substation. The permitted overloading

conditions for Planned Load Beyond Nameplate (PLBN), Long-Time Emergency Load

(LTE) and Short-Time Emergency load (STE) are estimated. The parameters of the

transformer, i.e. loss of life, permitted overloading conditions and maximum load

capacity were examined in this study. The evaluated results reveal that the transformer's

loss of life correlates to the capacity loading curve pattern, power factor and ambient

temperature. Besides, the transformer's loss of life is highly dependent on the capacity

loading curve pattern. Other information, namely loading curve pattern, power factor

and factory test acceptance (F AT) reports are referred as well. Results reveal that the

loss of life for each transformer is approximately 0.0 % over 30 years of lifespan. The

results imply that the lifespan of every transformer in the substations can last for over

30 years.

----~,------------------------------

11l

~

ABSTRAK

Tranformer-transformer boleh menghantar bekalan elektrik kepada kawasan

kediaman, perdagangan dan kawasan perindustrian bagi memenuhi permintaan

mengunakan kuasa elektrik. Kajian ini mensasarkan bagi menentukan keadaan

pembekalan lebih muatan yang dibenarkan untuk memaksimurnkan bekalan elektrik

tanpa menjejaskan hayat transformer dengan menetapkan parameter tertentu. Kajian ini

menyimpulkan bahawa pola lengkung pemuatan dan penggunaan kuasa adalah banyak

bergantung pada rangkaian pengagihan kuasa. keadaan pembekalan lebih muatan yang

dibenarkan untuk Planned Load Beyond Nameplate (PLBN), Long-Time Emergency

Load (LTE) dan Short-Time Emergency load (STE) telah dianggarkan. Parameter bagi

transformer seperti kehilangan kehidupan, keadaan pembekalan lebih muatan yang

dibenarkan dan maksimum bekalan elektrik telah dikaji. Keputusan mendedahkan

bahawa kehilangan kehidupan transformer berhubung kait dengan pemuatan keupayaan

pola lengkung, faktor kuasa dan suhu ambien. Selain itu, kehilangan kehidupan

transformer adalah amat bergantung kepada pemuatan keupayaan pola lengkung.

Maklumat lain daripada pemuatan melengkok pola, faktor kuasa dan laporan-Iaporan

ujian penerimaan kilang telah d iruj uk. Keputusan .mendedahkan bahawa kehilangan

kehidupan bagi setiap transformer adalah lebih kurang 0.0 % lebih dari hayat 30 tahun.

Keputusan menunjukkan bahawa hayat bagi setiap transformer dalam stesen pencawang

boleh tahan lama selama lebih 30 tahun.

IV

t'Us.al K.JIidm 1 1HutUumill A"'~ucU\~ UNlVERSm MALAY IA ARAWAk

T ABLE OF CONTENTS

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

ABBREVIATIONS

NOMENCLATURES

CHAPTER 1: INTRODUCTION

1.1 Background

1.2 Problem Statement

1.3 Modelling Application

1.4 Objectives

1.5 Outline of the Study

CHAPTER 2: LITERATURE REVIEW

2.1 Cyclic Loading Transformer

2.1.1 Nameplate

2.1.2 Oil-Cooled Systems

2.1.3 Loading Curve Pattern (CP)

2.1.4 Power Factor (PF)

2.2 Ambient Temperature

2.2.1 Measurement

11

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IV

V

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XlIl

XIV

2

3

3

5

6

6

7

9

10

10

V

2.2.2 Computation 11

2.3 Transformer Thennal Response 12

2.3.1 Top-oil Temperature (TOT) 15

2.3.2 Winding Temperature Rise (WTR) 18

2.3.3 Hottest-spot Temperature (HST) 19

2.4 Loss of Life (LOL) 19

2.4.1 Daily Loss Rate of Loss of Life 20

2.4.2 Thennal Aging 22

2.5 Pennitted Overloading Conditions (POC) 23

2.5.1 Planned Load Beyond Nameplate (PLBN) 24

2.5.2 Long-Time Emergency Load (L TE) 24

2.5.3 Short-Time Emergency Load (STE) 22

2.6 Maximum Loading Capacity (ML) 25

2.7 Summary 26

CHAPTER 3: METHODOLOGY

3.1 Introduction 27

3.2 Methodology 27

3.3 Engineering Analysis 29

3.3.1 Study on Cyclic Loading Transformer 29

3.3.2 Significant Information 31

3.4 Substation Selection 32

3.5 Transformer Selection 35

3.5.1 Transfonner Types 35

3.6 On-Site Examination 37

Vi

3.6.1 Internal Inspection 37

,....

3.6.1.1 Loading Measurement 37

3.6.2 External Inspection 39

3.7 Modelling Development 41

3.7.1 Modelling on Top-Oil Temperature (TOT) 41

3.7.2 Modelling on Transformer Thermal Response, Loss of Life and 42 Permitted Overloading Conditions

3.7.3 Modelling on Maximum Load Capacity ofTransformer 46

3.8 Modelling Analysis 49

3.9 Summary 49

CHAPTER 4: RESULT AND ANALYSIS

4.1 Introduction 50

4.2 Power Factor 50

II4.3 Load Characteristic 53 II

4.3.1 Weekly and Daily Load Curve Characteristic 53

4.3.2 Power Usage on Weekly Load 60

4.4 Ambient Temperature 61

4.4.1 Ambient Temperature Computation using Weather Temperature 62 i r

4.4.2 Ambient Temperature Measured 63

4.5 Thermal Response Analysis by Using the Model 64

4.5.1 Top Oil Temperature (TOT) 65

4.5.2 Winding Temperature Rise (WTR) 68

4.5.3 Hottest-Spot Temperature (HST) 70

4.6 Loss oflife (LOL) 71

Vll

4.7 Estimated Patterns of Overloading Condition 73

4.8 Optimum Transformer Capacity 75

4.9 Summary 77

CHAPTER 5: CONCLUSION AND FURTHER WORK

5.1 Introduction 79

5.2 Conclusions 79

5.3 Further Work 80

REFERENCES 82

APPENDICES

Appendix A: Transformer Nameplate Value 87

Appendix B: Loading Computational 89

Appendix C: Thermal Response Simulation 101

Appendix D: Ambient Temperature Programming 105

Appendix E: Thermal Response, LOL and Overloading Condition 106 Programming

Appendix F: Recorded data for New Priok Substation, Kuching, 121 Sarawak

Appendix G: Recorded data for Airport Substation, Kuching, 127 Sarawak

Appendix H: Weather Temperature Data 129

Appendix I: Differences Equations between IEC and IEEE Standard 134

Appendix J: Estimation of Optimum Load and Thermal Response for 137 Oil-Nature-Air Nature Cooling Type Power Transformer

Appendix K: Prediction of the Overloading Condition by Using the 146 Nameplate Rating Value

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LIST OF FIGURES

Figure 2.0: Loading curve (IEEE standard C57.91, 1995) 7

Figure 2.1: Approximation method (Perera, et. ai, 2001) 9

Figure 2.2 : Simplified diagram of an ONAN type transformer (lEC 76-2, 13 1993)

Figure 2.3: Temperature distribution model (lEC 76-2, 1993) 14

transformers

andPOC

Figure 4.2: Comparison of power consumption patterns for domestic and

Figure 2.4: The maximum and saturation stages 16

Figure 3.0: Flow chart ofthe methodology 28

Figure 3.1: The power triangle 30

Figure 3.2: Loading patterns for Astana, New Priok, Satok and Airport 34

Figure 3.3: Oil-Natural-Air-Natural (ONAN without fan) power transformer 36

Figure 3.4: Oil-Natural-Air-Forced (ONAF with fan) power transformer 36

Figure 3.5: Smart meter inside the substation monitoring room 38

Figure 3.6: The experimental setup of the thermocouples 40

Figure 3.7: Flow chart for the computation of the thermal response, LOL 44

Figure 3.8: Equivalent loading for the transformer 47

Figure 3.9: Maximum load capacity and load curve multiplier flow chart 48

Figure 4.0: Weekly loading pattern for New Priok Substation transformer 54

Figure 4.1: Weekly loading pattern for Airport Substation transformer 55

56 commercial consumers (Kho et aI., 2008)

Figure 4.3: New Priok substation 33/11 kV distribution network with three 57 shared 30/35MV A transformers

Figure 4.4: Airport substation 33/11 kV distribution network with two 58 shared 20/25MV A transformers

IX

,......

Figure 4.5: Computation nominal ambient temperature from 2004 to 2006 62

Figure 4.6: Ambient Temperature on August 2007 and January 2008 64

Figure 4.7: Top oil temperature at 8 August 2007 in New Priok substation 65

Figure 4.8: Overloading pattern on transformer 2 when transformer 1 is 67 turned off at Airport's substation on 17 Jan 2008

Figure 4.9: Top-oil temperature on 17 Jan 2008 for Airport substation 68

Figure 4.10: Winding temperature rise for the New Priok substation 69

Figure 4.11: Winding temperature rise on Airport substation 69

x

J

I

LIST OF TABLES

Table 2.0: Common exponents used in the temperature determination 18 equations

Table 2.1: Normal insulation life of a well-dried, oxygen-free 65°C average 21 winding temperature rise insulation system at the reference temperature of 110°C

Table 2.2: Suggested maximum temperature limits for four types of loading 23

Table 3.0: Types and number oftransformer in four substations located in 33 Kuching

Table 3.1: Loading capacities for the substations 33

Table 3.2: The rated power range of three category transformer 35

Table 4.0: Daily Power Factor for Transformer 1, Transformer 2 and 51 Transformer 3 in New Priok Substation

Table 4.1: Daily Power Factor for Transformers 1 and 2 in Kuching Airport 52 Substation

Table 4.2: Time for the peak load on each transformer in New Priok substation 59

Table 4.3: Time for the peak load on both transformers in Airport substation 59

Table 4.4: Power usage of Airport and New Priok Substation 60

Table 4.5: Loading Capacity of Airport and New Priok Substations 61

Table 4.6: Mean Square Error (MSE) for Models A and B for New Priok 66 substation

Table 4.7: The maximum and minimum HST in New Priok substation 70

Table 4.8: The maximum and minimum HST in Airport substation 71

Table 4.9: Accumulative aging factor and loss oflife for the New Priok's 72 transformers

Table 4.10: Accumulative aging factor and loss of life for the Airport's 72 transformers

Table 4.11: Overloading characteristics for New Priok substation 73

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1.

--,....

Table 4.12: Overloading characteristics for Airport substation 74

Table 4.13: Limits duration and maximum load per unit for different types of 75 loading

Table 4.14: Daily maximum loading capacity for each of the transformers 76

Table 4.15: Daily load curve multiplier for each of the transformers 77

XII

ABBREVIATIONS

Notation Description

ANSI America National Standard Institute CP loading curve pattern

FAT factory test acceptance FOA forced-oil-air FOW forced-oil-water HST hottest-spot temperature LCM load curve multiplier LOL loss oflife LTE long-time emergency load ML maximum loading capacity

ONAF oil-forced air-natural ONAN oil-natural air-natural

PF power factor PLBN planned loading beyond nameplate POC permitted overloading conditions

SCADA Supervisory Control and Data Acquisition STE short-time emergency load TOT top-oil temperature WTR winding temperature rise HST Hottest-spot temperature

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NOMENCLATURES

Notation Description II Unit I daily average ambient temperature hourly 8ad(h) °C

8adm(h) average daily maximum temperature hourly °C

8adn(h) average daily minimum temperature hourly °C

8ay yearly average temperature °C

ambient temperature 8a °C

8al ambient temperature measured °C

8a2 ambient temperature computed °C

A amplitude of yearly variation ofdaily mean -ambient temperature

B amplitude ofdaily variation for ageing-rate -calculation

hottest day ofa day dayDX

hottest hour of a day TX hour

day day number -

hour the hour of the day hour

top oil temperatures rise !18to °C

initial oil temperature rise at t = 0!18oi °C

ultimate oil temperature rise !18ou °C

top oil temperatures rise at rated kVA !18or °C I

t time interval for the loading hour

oil time constant hour'fro

oil time constant at rated kV A -'fTO,R

K ratio of the actual load to the rated load at the -interval time .

XIV

n cooling type exponent parameters

m

C

cooling type exponent parameters

thermal capacity of the transformer watt-hours/ °C \

'

Pr.R total loss at rated load watts

t-.e H winding hottest-spot temperature rise over top-oil temperature

°C

t-.eH.i initial hot-spot temperature rise at t = 0 °C

t-.eH ,u ultimate hot-spot rise over top oil °C

t-.eH.R hot-spot temperature rise at rated kVA °C

Tw interval time for the loading hour

eh hot-spot temperature °C

V relative rate of thermal ageing

L relative ageing rate

T total time interval of application hour

FAA accumulative aging factor

%LOL percent loss oflife per day

LOL loss of life

FEQA equivalent insulation aging factor for a total time period

FAA,n insulation aging acceleration factor for the time interval

PF power factor

L, ,L2 , LN various loads in percentage

t"t2 , tN durations ofthe load hour

R(t) function of the resistance

xv

a winding oil average temperature rise °C

b winding gradient °C

Ay,n monthly average weather temperature which n is °C the month number

8 max,,, maximum weather temperature °C

8 min, 1I minimum weather temperature °C

V voltage V

MWAR mega volt-amp-reactive MVAR

MW mega watt MW

XVI

CHAPTERl

INTRODUCTION

1.1 Background

Power transfonner is a device that transfers electricity energy from one circuit to

another through inductively coupled conductor. The electricity energy is transferred by

stepping up or stepping down the quantity of voltage through the internal winding. The

purpose of transferring the electricity energy by using power transfonner is to reduce

losses in transmission and distribution power system. Thus, power transformer becomes

an important medium in to connect the distribution and transmission system.

1.2 Problem Statement

Large amount of electricity is required to fulfil the demand of consumer daily

consumptions. The continuous flows of electricity supply to industrial and commercial

areas must be properly sustained. Thus, power transformer is required to supply

continuous flow of electricity in order to avoid electricity supply breakdown.

In most cases, the electricity supply breakdown is caused by the transformer failure

despite human mistakes. Transfonner failure is mainly due to overheat generation by

the internal elements of the transformer such as the core and winding. Various ways had

been implemented and studied to improve the reliability of the power transformer. For

--

instance, the backup system was installed every substation to reduce the frequency of

the transformer failure (Giverlberg et ai., 1996). Therefore, there is a crucial need to

understand the performance of the existing transformers and to maximize their loading

capacity with minimum risk of transformer failure.

In fact, the energy provider company have to overload the power transformer from

time to time in order to prevent the in short supply electricity. However, inconsistency

overloading process configurations will degrade transformer's lifespan rapidly.

According to McLyman (1988), the induced temperature rise by overloading process

must be controlled in order to prevent damage or breakdown to the inner wire insulation.

Thus, the relationship between the transformer lifespan (loss of life) and the permitted

overloading conditions should be investigated. A large amount of investment cost could

be saved by extending the transformer's lifespan without exceeding the critical point of

operation stated on the transformer's nameplate rated value.

1.3 Modelling Application

A mathematical modelling is carried out in this study to investigate the methodology

in prolonging the transformer's lifespan. The lifespan, or known as loss of life (LOL) in

this thesis, is modelled based on the ambient temperature and the thermal response

variables. These variables such as top-oil temperature (TOT), the winding temperature

rise (WTR) and the hottest-spot temperatures (HST) are relevant in studying. Other

relevant modelling inputs include the factory acceptance test (FAT) parameters, loading

curve and power factor. This model is beneficial to estimate the permitted overloading

2

I

conditions and increase the electricity supply without reducing the lifespan of the

transfonner. This model is able to calculate the maximum loading capacity for the

transfonner before reaching the plateau point ofoperation.

1.4 Objectives

The objectives ofthis study are stated as below:

I. To investigate transfonner's lifespan and transfonner's thermal response.

11. To determine the permitted overloading conditions for the transformer based on

the existing transformer loading curves.

111. To calculate the maximum load capacity for the transformer before reaching the

critical point of operation.

IV. To create a mathematical modelling to prolonging the transformer's life span

1.5 Outline of the Study

The fIrst chapter gives basic and overall views which cover the background of this

study. Besides that, it also revealed the objectives and some problems statement that

lead to the rationality on conduction the research. The general introduction and problem

statement are briefly stated out in this chapter. In Chapter 2, the literature reviews on the

3

transfonner parameters and the related formulae are explained. The methodology for

this study is elucidated in Chapter 3. There are four stages, namely engineering analysis,

transfonner selection process, modelling development and the data analysis invo lved in

the process flow. Chapter 4 evaluates the results obtained from the calculation and

modelling processes. Finally, the conclusions and some suggestions for future work are

narrated in Chapter 5.

4

,.. Pusat Khidlmu Ma umat r\k.tUlt'mi~ UNIVERSITI MALAYSIA SARAWA

CHAPTER 2

LITERA TURE REVIEW

2.1 Cyclic Loading Transformers

Cyclic loading transformers stabilize the electricity to fulfil the population demands

in daily activities. Generally, these transformers are designed to handle heavy load

conditions, which include emergency and cold load pickup scenarios. There are three

loading conditions for the transformers, namely underload, overload and emergency

load. In most cases, the cyclic loading power transformers have to be overloaded to

maintain the maximum electricity supply. However, with an increase in load, internal

transformer temperature increases and the normal life expectancy of the transformer

decreases accordingly. If the overload process continues and exceeds the stated critical

operating point, the transformers will experience major breakdown. The operation

fai lure can certainly reduce the lifespan and meanwhile increase the maintenance costs

of the transformers. Therefore, the international standards such as IEC standard 354

(1991), IEEE standard C57.91 (1995) and IEEE standard C57.91-1995/Cor I (2002) are

referred while operating the transformers. Besides, relevant parameters e.g. power factor

and loading curve pattern of the transformer should be studied prior to the operation.

5

2.1.1 Nameplate

Nameplate is a metal plate on the transformer' s body where the crit ical operating

conditions are stated on. The parameters supplied by manufacturer include oil types,

core materials and winding designs. The specific thermal capacity and oil-time constant

can be deduced for those parameters (Rummiya et ai, 2005). The rated values on the

nameplate must be strictly fol1owed before operating the transformer. For instance, the

average top oil temperature (TOT) should be limited to 55°C and the winding

temperature rise (WTR) should not over 65°C. The limitations help in protecting the

insulation layers of the transformers and thus minimizing the overheat issues. Details of

the thermal response will be further discussed in section 2.3.

2.1.2 Oil-Cooled Systems

The inner heat generated by the core and winding of the transformers can reduce the

power conversion efficiency. Thus, the cooling system is generally implemented in

order to reduce the amount of heat that circulates inside the transformer. In fact, oil is

commonly used as a cooling medium as it can serve various purposes such as a heat

dissipater. Thus, the windings and core that are the primary sources of the heat must be

fully immersed in the oil in order to disperse the he~t effectively inside the oil-cooled

transformers.

There are two types of the oil-cooled systems, namely oil-natural-air-natural

(ONAN) and oil-natural-air-forced (ONAF) transformers. The ONAN cooling type

transformer utilizes the natural air circulation to move the oil passing through the core

6

and winding for heat dissipation, whereas the ONAF cooling type forces the oil to move

by using the fans installed on the transformer heat sink.

2.1.3 Loading Curve Pattern (CP)

Patterns of the repeating load cycle for the transformers from day to day is known as

loading curve, as illustrated by Figure 2.0. The loading curve, recorded in a daily basis,

consists of the essential information i.e. initial load, actual load and peak load of the

operating transformers.

I I I

-~ I ~I HOUR

ACTUAL LOAD

o~------~----~~~----~~~--------~ 12 SAM 12,. 6PM

."CTUAt LO~D CV~L£

Figure 2.0: Loading curve (IEEE standard C57.91, 1995)

Fluctuation loss can be generated by a transformer while operating under a

fluctuated load. The total losses due to the fluctuation load are about the same as that of

an intermediate constant load for the same period of time (IEEE standard C57.91, 1995).

7