faculty of engineering - university of nigeria nsukka · engineering, faculty of engineering, ......
TRANSCRIPT
INVESTIGATION AND EVALUATION OF LOCAL
TRANSFORMER INSULATION
Ebere Omeje
CHUKWU IFEANYI MIRACLE
PG/M.ENG/11/59419
INVESTIGATION AND EVALUATION OF LOCAL LIQUID DIELECTRIC FOR POWER
TRANSFORMER INSULATION: A CASE STUDY OF
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL
ENGINEERING
Ebere Omeje Digitally Signed by: Content manager’s Nam
DN : CN = Webmaster’s name
O= University of Nigeria, Nsukka
OU = Innovation Centre
i
CHUKWU IFEANYI MIRACLE
INVESTIGATION AND EVALUATION OF LOCAL LIQUID DIELECTRIC FOR POWER
: A CASE STUDY OF
DEPARTMENT OF ELECTRICAL
Digitally Signed by: Content manager’s Name
DN : CN = Webmaster’s name
O= University of Nigeria, Nsukka
ii
DEPARTMENT OF ELECTRICAL ENGINEERING
UNIVERSITY OF NIGERIA NSUKKA
A THESIS SUBMITTED IN PARTIAL FULFILMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Engineering
TOPIC
INVESTIGATION AND EVALUATION OF LOCAL LIQUID DIELECTRIC FOR POWER TRANSFORMER INSULATION: A CASE
STUDY OF SOYBEAN OIL.
BY
CHUKWU IFEANYI MIRACLE
PG/M.ENG/11/59419
SUPERVISOR: PROF. T.C. MADUEME
DECEMBER, 2015
iii
APPROVAL
The contents of this report are true reflection of the project undertaken by Chukwu Ifeanyi
Miracle (PG/M.ENG/11/59419). It is hereby accepted by the Department of Electrical
Engineering, Faculty of Engineering, University of Nigeria, Nsukka in partial fulfillment of the
requirement of for the award of master of engineering in Electrical Engineering (M.ENG.) of
University of Nigeria, Nsukka.
………………………………… ……………..
Chukwu Ifeanyi Miracle Date
Student
………………………………… ……………..
Engr. Prof. T.C. Madueme Date
Project Supervisor
……………………………….. …………….
Engr. Prof E.C Ejiogu Date
Head of Department
……………………………… …………….
Engr. Prof. A. O. Ibe Date
External Examiner
………………………………… ……………..
Engr. Prof. E. S. Obe Date
P.G. Faculty Rep.
iv
CERTIFICATION
This is to certify that this project work titled “INVESTIGATION AND EVALUATION OF
LOCAL LIQUID DIELECTRIC FOR POWER TRANSFORMER INSULATION: A
CASE STUDY OF SOYBEAN OIL” was carried out by CHUKWU IFEANYI MIRACLE,
with Reg. No.: PG/M.ENG/11/59419 in the department of Electrical Engineering, University of
Nigeria Nsukka and meets the regulations governing the Award of Degree of Master of
Engineering (M.ENG) of the University of Nigeria Nsukka.
………………………………… ……………..
Engr. Prof. T.C. Madueme Date
(Project Supervisor)
……………………………….. …………….
Engr. Prof E.C Ejiogu Date
(Head of Department)
……………………………… …………….
Engr. Prof. A. O. Ibe Date
External Examiner
………………………………… ……………..
Engr. Prof. E. S. Obe Date
Faculty PG Rep.
v
DEDICATION
This work is dedicated to the Almighty God and to my Parents; Mr and Mrs Amos Chukwu
vi
ACKNOWLEDGEMENTS
My sincere gratitude goes to my supervisor Prof T.C. Madueme, for his care, guidance,
encouragement, and total support throughout the course of this thesis. It was an extremely useful
learning experience for me to be one of his students. From him I have gained not only extensive
knowledge, but also a careful research attitude. To Prof E.C. Ejiogu; who taught me that hard
work and persistence is an important research instrument. I also admire the motivation you gave
me during my research period. I am also grateful to Prof. L. U. Anih and Dr. D. O. Anyaka for
their kindness, love, and support during my course work.
I also want to appreciate Prof S. E. Obe who has been my guidance and a counselor during this
research period. Moreover, I thank Dr. C.U. Ogbuka, Dr. B.N Nnadi and Engr. Dr. C. A. Nwosu
for your persistent advice as regards my research work. I appreciate all staff of electrical
engineering department and my colleagues in the division of power electronics group, electrical
machines and all post graduate students in general for their support.
God bless you all.
vii
ABSTRACT
This work investigates and evaluates local liquid dielectric for insulation of power transformers.
Considering the importance of transformer insulation the investigation is borne out of the need to
develop the capacity for local production of transformer dielectric. Natural esters of vegetables
origin have been found to have the suitable dielectric properties for them to be used as
replacement dielectric fluids for mineral oil. However vegetable oil has poor resistance to
oxidation, hence the need for inhibiting the vegetable oil for use as insulation fluid. In this work
inhibited locally extracted soybean oil (soybean oil treated with antioxidant) is subjected to
accelerated ageing in order to determine its suitability as dielectric fluid. The properties of the
soybean oil is investigated against standard and compared with those of mineral oil. The
investigated soybean meets acceptability specifications since the measured flash point is 2570C,
the pour point is -150C, the acidity is low at 0.0027mgkoH/mg, its dissipation factor after ageing
is 12.11 x 10-3 (which is within recommended units) and showed a high dielectric breakdown
voltage (59.08kV). Results obtained showed that the inhabited soybean oil has properties
comparable to those of mineral oil and is suitable for use as transformer dielectric fluid.
viii
TABLE OF CONTENTS Pages
Title Page i
Approval ii
Certification iii
Dedication iv
Acknowledgements v
Abstract vi
Table Of Contents vii
List Of Figures ix
List Of Tables x
List Of Symbols And Abbreviation xi
CHAPTER ONE: Introduction 1
1.0. Background of the Study 1
1.1. Statement of the Problem 5
1.2. Objectives of the Study 5
1.3. Scope of the Study 5
1.4. Significance of the Study 6
CHAPTER TWO: Literature Review 8
2.1 General overview of local liquid dielectric concepts. 8
2.1.1 History of Ester Fluids as Dielectric Coolants 8
2.2 Basic concept of power transformer insulation level 9
2.2.1Transformer insulation 9
2.2.2 Ageing of oil 10
2.2.2.1. The Bathtub Curve Relationship 10
2.2.2.2 The accelerated thermal ageing process 13
2.2.3Dielectric testing (break down voltage) 20
2.3.4 Properties of liquid dielectric 21
2.3 Locally available oil base for liquid dielectric production 22
2.3.1. List of available local liquid 22
2.3.2. Comparative analysis of the existing conventional liquid and the proposed
Liquid dielectric on power transformer insulation
23
ix
2.3.3 Properties of vegetable oil 24
2.4 Antioxidants for Soy Beans Oil 26
2.4.1The Nature and Mechanisms of the Operation of Antioxidant 26
2.4.2Oxidation in lubricant base oil 28
2.4.3Measuring oxidation in lubricant base oils 30
2.5.4Oxidation stability index of vegetable oil 32
CHAPTER THREE: Methodology Of Experiment Design 34
3.1. Inhabitation of the Soybean Oil: Material and Method 34
3.2. ACID VALUE (MgKOH/g) 35
3.3. VISCOSITY 36
3.4. Flash and Fire Point 37
3.5. Pour Points Tests 38
3.6. Subjecting the oil to Accelerated Ageing 39
3.7. Measurement of the Oil Breakdown Voltage 41
3.7.1. Breakdown Voltage Detection 41
3.7.2. Electrode Gap Setting 44
3.7.3. Breakdown Voltage Test 44
3.7.4. Assessing Validity of the Test Result 44
3.8. Measurement of the Oil Sample Dielectric Losses (tan �) 48
3.9. Measurement of the Oil Sample Permittivity 50
CHAPTER FOUR 55
4.0. Result Data and Analysis 55
4.1. Result Data on the Physical and Chemical Properties of the Soybean Oil 55
4.2.2. Dielectric Dissipation Factor (Tan�) 56
CHAPTER FIVE: Conclusions And Recommendations 66
5.1. Conclusions 66
5.2. Recommendations 67
5.3. Suggestion for Further Studies 67
REFERENCES 68
APPENDIX 72
x
LIST OF FIGURES
Figure 2.1 Degradation mechanisms in oil 11
Figure 2.2 Bathtub curve of transformer lifetime 12
Figure 2.3. Basic thermal model in transformer winding 15
Figure 2.4. The 10 degree rule application on oil’s neutralization number 16
Figure 2.5. ECT vs. ageing time in mineral oil 18
Figure 2.6. Acidity before and after ageing 19
Figure 2.7. Viscosity before and after ageing 19
Figure 2.8. Diagram showing the oxidative degradation in lubricant base oils
Figure 3.1: Acidity test at UNN Energy Center
30
36
Figure 3.2. Viscosity measurement using digital viscometer at UNN Energy Centre 37
Figure 3.3 (a) Flash and Fire Point Tester showing oil sample in the test cup at UNN
Energy Centre
38
Figure 3.3 (b) Flash and Fire Point Tester showing test cup in the machine at UNN Energy Centre
38
Figure 3.4. Pour Points Tests at UNN Energy Centre 39
Figure 3.5(a): Megger dielectric breakdown voltage test sets showing oil sample in it
before test at PHCN Transcom Enugu
43
Figure 3.5(a): Megger dielectric breakdown voltage test sets showing reading after test at
EEDC Enugu
43
Figure 3.6. Picture of oil breakdown voltage test setup 43
Figure 3.7 Tan � tell cell dimension 45
Figure 3.8 Circuit diagram for tan � measurement 46
Figure 3.9(a) parallel circuit and (b) phasor diagram 49
Figure 3.10. Digital oven with oil sample in it. 51
Figure 3.11 Flow chart of the oil characteristics measurement
54
xi
LIST OF TABLES
Table 2.1. The minimum periods at accelerated hot spot temperatures 15
Table2.2 Dielectric properties of some liquid dielectric 22
Table 4.1: Physical and Chemical Properties of the Treated Soybean Oil 55
Table 4.2: Result of Breakdown Voltage (BDV) measurements for the mineral oil and
treated soybean oil
57
Table 4.3. Tan � results for the soybean oil 60
Table 4.4. Tan� results for mineral oil 61
xii
LIST OF ABBREVIATIONS
US: Untied States
IEC: International Electrotechnical Commission
ASTM: American Standard of Testing Materials
PCB: Polychlorinated Biphenyls
PDMS: Polydimethylsiloxane
DGA: Dissolved Gas Analysis
R&D: Research and Development
EIS: Electric Insulation System
ANSI: American National Standards Institute
IEEE: Institute of Electrical and Electronics Engineers
ECT: Electrostatic Charging Tendency
GB/T: Voluntary National Standard of Testing in China
BDV: Breakdown Voltage
TAN: Total Acid Number
FTIR: Fourier Transformation Infrared Spectroscopy
QSA: Quantitative Spectrophotometer Analysis
RULER: Remaining Useful Life Evaluation Routine
RBOT: Rotating Bomb Oxidation Test
BHA: Butylhydroxyanisole
BHT: Butylhydroxytoluene
TBHQ: Test-Hydroquinone
PG: Propyl-Gallate
xiii
AV: Acid value
UNN: University of Nigeria, Nsukka
ISO: International Standard Organization
PHCN: Power Holding Company of Nigeria
DDF: Dielectric Dissipation Factor
EIS: Electric Insulation System
COP21: 21st Conference of Parties
PF: Power Factor
14
CHAPTER ONE
INTRODUCTION
1.0. Background of the Study
Electrical faults that occur in power transfers accounts for over fifty percent of transformer
failure’s expenditures . Fifty percent of transformer failure’s expenditure is caused by insulation,
dielectric, and oil-related faults [1]. The need for better dielectrics and transformer oils for
insulation is unequivocal. In a liquid-filter transformer, the insulator liquid plays an important
function by providing both the electrical insulation and the means of transforming the thermal
losses to the cooling system. Insulating oil in a transformer must ensure the transfer of heat. This
function is realized both by thermal conductivity and convection [2].
Nowadays, transformers can be filled with three basic types of insulating liquids: (i) mineral oils,
(ii) synthetic oils or (iii) natural esters [3]. The use of each type is fortified by the application.
However, in the face of increasing demand for the use of environmentally friendly products in
the industry, more and more companies are working towards developing the use of esters and
specifically natural esters for use in the majority of their products.
Transformers have been filled with mineral oil for more than one century. This type of oil is a
petroleum based product, essentially composed of hydrogen and carbon atoms. Carbon and
hydrogen are assembled in different structures: napthenic (CnH2n), paraffinic (CnH2n+2) and
aromatic (CnH2n-6)[3]. The distribution of carbons in napthenic and paraffinic structures define
the type of mineral oil. This distribution is controlled by the crude oil and the refining processes
used. Because of its wide availability, good properties and low cost, mineral oil is the fluid most
used in the electric power transformer industry. New mineral oils have to be in accordance with
the IEC 60296 or ASTM D3487. As mineral oil has been used for such a long time, a large data
15
base of information is available to enable interpretation of changes to its characteristics and
thereby predict the possible malfunction of a transformer filled with it. IEC 60422 is a good tool
to evaluate the quality of insulating oils in operational transformers.
There are two main synthetic oils that can be used in transformers: silicone oil and ester of
pentaerythritol (Synthetic ester). These oils were developed in the 1970’s to replace askarels
(Polychlorinated Biphenyls) (PCB), which became outlawed because of their toxicity [3]. As
PCB was a non-flammable liquid (no fire point), the main peculiarity of these new oils was their
high fire resistance (higher fire point than mineral oil). Until now, their use was essentially
restricted to distribution and traction transformers, partly because of their price (three to eight
times more expensive than mineral oil) but also because a better fire resistance was required for
these applications. Their better thermal stability was also a positive point for their use and
especially for traction transformers. Esters of pentaerythritol also called tetra-ester are obtained
in an esterification reaction between a tetra alcohol (pentaerythritol) and mono-carboxylic acids.
These oils are composed of hydrogen, carbon and oxygen. New synthetic esters have to be in
accordance with the IEC 61099 and a maintenance guide that is published in the IEC 61203.
Silicon oil is a polymer based on silicon, which also includes carbon, oxygen and hydrogen
atoms. Specifically, the final product is obtained by the polymerization of polydimethylsiloxane
(PDMS). This oil presents the advantage to have a high thermal stability but on the other hand is
not biodegradable at all.
The third insulating liquid is vegetable oils also known as natural esters (tri-ester) as opposed to
synthetic esters. This work is channeled towards the investigation and evaluation of these natural
esters as a transformer dielectric fluid. These oils are naturally synthesized from living organisms
and come in particular from soybean, palm, groundnuts, sun flower, rape seed etc. Specifically,
16
natural esters are created from an esterification reaction between a tri-alcohol and fatty acids.
Other processes allow the final product to be obtained by the trans-esterification reaction (mono-
ester) or mixture of mono and tri-esters [4].
On the other hand, the application of vegetable oils and animal fats for industrial purposes and
especially application has been exploited. Inherent disadvantages and the availability of
inexpensive options have however brought about low utilization of vegetable oils for industrial
lubrication[5]. When applied in the science of tribology, vegetable oils fall order the class known
as fixed oils[6]. They are so named because they do not volatilize without decomposing. Natural
ester (vegetable oil) dielectric fluids have advantages over mineral oil in terms of fire safety,
environmental risk, and thermal performance [7]. These high fire points fluids are self-
extinguishing (non-propagating) and quality in many applications as an equivalent safeguard to
space separation, fire barriers, and extinguishment systems, all of which mitigate a fire but do not
prevent it. The thermal characteristics and interactions with cellulose insulation (i.e. transformer
paper insulation) give longer transformer paper insulation life or allow higher or extended
overheads without abnormal loss of insulation life.
However the major drawback in the application of vegetable oils as transformer oil or for
industry uses in general is the fact that in their natural forms, they lack sufficient oxidative
stability. By definition, the oxidative stability of oil is a measure of the length of time taken for
oxidation, deterioration to commence on a general level, the rates of reactions in auto-oxidation
schemes are dependent on the hydro carbon structure, oxygen concentration, and temperature
[8]. The untreated (i.e. uninhibited) oils from vegetable origin oxidize during use and polymerize
to a plastic life consistency [5]. Even when they are not subjected to the intense conditions of
industrial applications, fats and oils are liable to rancidity [9, 10]. Oil oxidation is an undesirable
17
series of chemical reactions involving oxygen that degrades the quality of the oil. Oxidation is
not one single reaction, but a complex series of reactions. In general terms, oxidative rancidities
in oil occurs when heat, metals or other catalysts cause unsaturated oil molecules to convert to
free radicals. These free radicals are easily oxidized to yield hydroperoxides and organic
compounds, such as aldehydes, ketones, or acids which give rise to the undesirable odors and
flavor characteristic of rancid fats [9]. The oxidative rancidity eventually affects the transformer
it reduces its thermal efficiency and weakens its dielectric strength.
Combating the issue of oxidative instability in vegetable oils for industrial use is a continuing
research area [11]. Three avenues are being pursued. These are:
i. Genetic modifications of oils to give higher mono unsaturated compounds
ii. Chemical medication
iii. The use of antioxidants (i.e. additives) and property enhancers[12]
Genetic modification has been made possible by recent advances in biotechnology. DuPont
technology has developed a soybean seed that presents 83% oleic acid as against having the
more unsaturated linolenic acid as the major constituent. This new seed provides oils that show
about 30 times the oxidative stability and viscosity stability of the conventional oil.
Chemical modifications involve the partial hydrogenation of the vegetable oil and a shifting of
its fatty acids.
The use of additives known as antioxidants to control the development of oxidative rancidity has
been applied in the US since 1947[13]. They still remain one of the most efficient and cost
effective ways to improve the oxidative stability of oils in both domestic and industrial
conditions. Hence the use of antioxidants to combat oxidative rancidity is adopted in this work.
18
This work investigates and evaluates local liquid dielectric for power transformer insulation.
Considering the issue of local availability and renewability, in this study the use of natural esters
is considered in place of local mineral liquid dielectric for power transformer insulation.
1.1. Statement of the Problem
This dissertation focuses on the evaluation of locally extracted soybean oil (a natural ester) for
the production of liquid dielectric for power transformer insulation. However, whether applied
for lubrication purposes or as transformer dielectric, one of the major challenges in the utilization
of the more environmentally friendly soy bean oil is its poor oxidative stability. Hence the main
problem to be addressed by this research is the setup and measurement of the dielectric losses
and breakdown voltage of soybean oil under accelerated thermal ageing.
1.2. Aim/Objectives of the Study
The aim of this study is to investigate and evaluate local liquid dielectric for power transformer
insulation. To this end, this work will realize the following specific objectives:
i. To obtain a dielectric fluid for use in the insulation of power transformers by the
oxidative inhibition of locally extracted soybean oil.
ii. To setup and carryout accelerated aging as per the IEC 61125 standard in which the
dielectric losses is carried out.
iii. To increase the inhibited soybean oil breakdown voltage. Evaluation based on the IEC
60156 standard employing the 2.5mm gap. Furthermore other properties of the inhibited
soybean oil are measured against the IEC 60247 for possible use as a dielectric fluid for
power transformers.
1.3. Scope of the Study
This work covers the evaluation of soy bean oil as a transformer liquid dielectric. The chemical
procedures covered include antioxidation. Test covered include acidity test, viscosity test,
19
dielectric withstand test, flash point and pour point test. However the test does not include the
use of the popular Dissolved Gas Analysis (DGA) for the evaluation of the liquid dielectric for
partial discharge.
1.4. Significance of the Study
Transformers are important component or equipment in power systems. Huge amount of money
is spent every year on maintenance of transformer. This has big impact on the electrical industry
and on the economy of the nation, since fifty percent of the transformer failure expenditure are
said to be caused by insulation, dielectric and oil related faults. This project which is focused on
investigation and evaluation of local liquid dielectric for power transformer insulation then has
significance for the Nigerian electricity industry and to the country’s economy.
There has been the push by international community to erect legislation for the safety of the
environment. The issue relating to the use of more environmentally friendly substances that are
biodegradable and pose little or no negative impact to the environment is becoming a strong one.
This is reinforced by the gathering of (this year, 30th November 2015) world leaders for the
COP 21 conference in France. Hence, a study such as this that focused on exploring more
environmentally friendly alternative to mineral oil for transformer becomes significant. The
significance of using the soybean oil in transformer insulation are as follows:
i. Nigerian farmers will benefit from this study and will concentrate more on soybean oil
production.
ii. With respect to the use of soybean oil as alternative to mineral oil, the cost of running the
transfers in Nigeria power system will be reduced since it is cheaper and locally available
20
than the conventional oil. Thus, it will reduce the huge amount spent in running and
maintaining power stations especially in developing country like Nigeria.
iii. Due to the biodegradability nature of soybean oil, it will reduce the hazard posed by the
conventional oil to the environment during oil linkage or spillage.
iv. It offers a lower transformer circle cost and no hazardous-waste disposal cost required, it
also has a potential for recycling value of used soybean oil.
v. It is safer than mineral oil because of its higher flash point (ignition temperature) and
reduces the impact of transformer fire.
vi. Its performance slow down the ageing of rate of insulation system.
21
CHAPTER TWO
LITERATURE REVIEW
2.1 General overview of local liquid dielectric concepts.
2.1.1 History of Ester Fluids as Dielectric Coolants
In 1892, experiments with liquids other than mineral oils included ester oils extracted from
seeds. None made operational improvement over mineral oil, and none were commercially
successful. A particular problem with seed oil based coolants was their high pour points and
inferior resistance to oxidation relative to mineral oil [14].
Except for occasional applications in capacitors and other specialties, renewed interest in ester–
based coolants did not occur until after the infamous issue of the PCB arose in the 1970’s,
coupled with the oil crises, which warranted the need of renewable transformer oil. By then,
there was the emergence of a mature synthetic organic ester industry serving the markets.
Depending on the type of acid and alcohol precursors, a variety of synthetic ester was possible.
This allowed the industries to produce "designer" ester molecules. Synthetic aliphatic poly esters
were selected for askarel substitution in transformers because of their favourable viscosities/fire
point ratios, and excellent environmental and dielectric properties. They are members of the
same family of esters used for decades as jet engine lubricants. The mid nineteen eighties saw
the emergence of equipment and appliances using vegetable oils as insulating fluids. In 1984, the
first transformer applications of these synthetic esters were railroad rolling stock transformers
with very high duty requirements. Due to their compact dimensions, such transformers had
forced circulation flow to remote heat exchangers. Therefore excellent lubricity, very low pour
point temperature, and a high fire point were important fluid characteristics for their application.
22
Market acceptance of synthetic esters has limited to specialty applications, primarily due to their
high cost compared to other dielectric fluids. As a result of environmental regulations and
liability risks involving non-edible oils, an extensive R&D effort begun in 1990’s, led to
revisiting the natural esters. They share many of the excellent dielectric and fire safety properties
of synthetic poly esters and they are classified as edible oils. In addition, they are biodegradable
since they have organic composition and most importantly, they are much more economical than
synthetic esters [14].
2.2 Basic concept of power transformer insulation level
2.2.1Transformer insulation
Transformer and even other electrical equipment generate heat during operation; therefore a
coolant is necessary to dissipate this heat. The insulating oil fills up pores in fibrous insulation
and also the gaps between the coil conductors and the spacing between the windings and the
tank, and thus increases the dielectric strength of the insulation. Transformer in operation
generates heat in the winding, and that heat is transferred to the oil. Heated oil then flows to the
radiators by convection. Oil supplied from the radiators, being relatively cool, cools the winding.
There are several important properties, such as dielectric strength, flash point, viscosity, specific
gravity and pour point, to be considered when specifying certain oil as transformer oil. The
quality of the oil is very important. At high voltages, highly loaded transformers demand better
quality oils. While at low voltages, lightly loaded transformers, the demand for high quality oils
is not critical. For more than a century, petroleum-based mineral oils purified to "transformer oil
grade" have been used in liquid filled transformers. Synthetic hydrocarbon fluids, silicone, and
ester fluids were introduced in the latter half of the twentieth century, but their use is limited to
23
distribution transformers. Several billion liters of transformer oil are used in transformers
worldwide. The popularity of mineral transformer oil is due to availability and low cost, as well
as being an excellent dielectric and cooling medium. Petroleum-based products are so vital in
today’s world that the consequence of its unavailability cannot be imagined transformers and
other oil filled electrical equipment use only a tiny fraction of the total petroleum consumption,
yet even this fraction is almost irreplaceable.[14]
2.2.2 Ageing of oil
In general, insulating materials will deteriorate under normal operating conditions. The ageing
rate of any material is influenced by several external ageing stresses: thermal, electrical,
mechanical, and ambient stresses [15]. Thermal stress can be recognized by the temperature
gradient of insulation during the long-term operation and especially the presence of hotspots.
Electrical stress may lead to partial discharge, treeing, and dielectric heating due to a lossy
insulation or highly conductive contaminants. For mechanical stress, the ageing process relies on
the torsion, compression, tension or bending factors. Ambient stress is closely related to the
environmental factors such as corrosive chemicals, petroleum fluids, high humidity, and
ultraviolet radiation from the sun. The composition and molecular structure of insulating
materials can be decomposed by all of the stresses, leading to a situation in which the material
eventually can no longer fulfill its insulation functions.
2.2.2.1. The Bathtub Curve Relationship
The transformer reliability depends largely on the properties of the electric insulation system
(EIS) due to the fact that the highest percentage of transformer failures results from the
insulation degradation. Thus, ageing of the insulation has been recognized as one of the main
24
cause of transformer failures. Insulating oil plays an important role in the transformer insulation.
The degradation of the insulating oil can result in an increase of humidity, an enhancement of the
possibility of partial discharges in the insulation system, and a decrease of the breakdown
strength of the oil-paper insulation system. The ageing process of the insulating oil can be
illustrated by Figure 2.1, which is determined by the thermal and electrical stresses on it [16]. In
Figure 2.1, the oxygen comes from dissolved air and the thermal stress can decompose the oil
and form sludge or mud. Finally, the aged oil affects the solid insulation ultimately leading to a
transformer failure.
Figure 2.1 Degradation mechanisms in oil [16]
Several transformer life assessment methods have already been established to find out an optimal
life time prediction. The transformer’s predicted life time is determined by the failure rates of its
insulation. The relation of transformer’s failure rate and the predicted life time is represented in
the bathtub curve in Figure 2.2 [17].
Oxygen is provided from dissolved air and
hot cellulose
Oil begins to produce mud
Main cause of overheat and decay of solid
insulations
Oil becomes decomposed and
oxidized
Oil gradually loses its stability
Insulating oil in transformer operation
Thermal and electrical stresses
25
Figure 2.2 Bathtub curve of transformer lifetime [17]
Generally, the operating life of a transformer is divided into three phases [16, 17]:
- Phase A
This is the phase of a new transformer, which consists of the first 1-3 years after commissioning.
Most failures are caused by the production side due to poor materials, design or imperfect
installation.
- Phase B
During this phase, a transformer works in normal operation between 20-30 years. Any random
failures can happen such as switching surges, lightning, or operator’s faults which are not related
to the early failures.
26
- Phase C
The failure rate significantly increases due to the degradation of insulation system and it may
start after 20 years of operation time.
2.2.2.2 The accelerated thermal ageing process
Insulating oils in the transformer deteriorate during its service life. The ageing rate of the
insulating oils depends on the magnitude of the electrical, thermal, and mechanical stresses. It is
also determined by the composition and molecular structure of the oils. The thermal ageing
principle is derived from the thermal condition of the insulating oil itself.
Long term elevated temperatures caurse internal chemical effects on the insulating oil. It is
mostly related to oxidative degradation due to the interaction between hydrocarbon in the oils
and the dissolved oxygen from air, which generates oxidation by-products such as acids and
sludge.
Many researchers have tried to calculate the loss of transformer functional life to predict how
long the transformer would optimally operate. Transformer insulation life defines the total time
between the initial state for which the insulation is considered new and the final state for which
dielectric stress, short circuit stress, or mechanical movement, which could occur in normal
service and would cause an electrical failure [18]. The loss of transformer functional life was
actually the same as the loss of transformer insulation life.
Montsinger [19] in 1930 conducted a research on ageing and life time of a kind of varnished
cambric, placing the varnished cambric tape insulation into a series of oil-filled test tubes, getting
them heated up, then measuring the insulation’s tensile strength. He concluded that every 8°C
increase in continuous temperature will reduce the life of the varnished cambric by fifty percent.
27
In 1944, Montsinger revised his research by stating that the 8°C rule can not be properly applied
at lower temperatures.
Later, Dakin modified the Arrhenius law to calculate the transformer insulation deterioration
[20]. The Dakin-Arrhenius law is formulated as
The remaining life = �. ����
� …………………………….…. 2.1
Where A is the initial life, B is a constant, depending on the properties of the material studied,
and T is absolute temperature in K. The simple insulation life curve was developed to relate the
insulation’s life and its operating temperature.
According to life test protocol for distribution transformers which is mentioned in ANSI/IEEE
C57.100-1986 [21, 22], the life expectancy of oil-immersed distribution transformers is
formulated as:
�� �����ℎ� = ����.�
���� �− 11.269 …………………………….…. 2.2
Where h is the life time and " is the hot spot temperature of the winding. The formula is valid for
accelerated hot spot temperature, which has higher temperature rising than the conductor at the
top of the winding [20] as shown in Figure 2.3.
28
Figure 2.3. Basic thermal model in transformer winding[20]
The example of minimum ageing periods at accelerated hot spot temperatures [21] are shown in
the following table:
Table 2.1. The minimum periods at accelerated hot spot temperatures
To compensate for the statistical uncertainty, it is specified that the life testing be conducted five
times the minimum period shown above. It is customary to round off the final numbers to 10000,
2500, and 720 h, and this interval is divided into ten equal periods for the ten end-point tests. A
test period is a series of ageing temperature cycles which consist of a specified time at a
specified hottest-spot temperature followed by a return to approximately ambient temperature
29
[22]. The end-point test is considered as the particular test on specimen when the thermal
degradation has progressed to a point such that the specimen cannot withstand any one of a series
of tests (tensile strength, degree of polymerization).
A theory by Moses [23] about accelerated aging processes in oil is known as the ten (10) degree
rule which states “a 10 degree decrease in temperature cuts the reaction in a half, while the 10
degree increase doubles the reaction rate” [24]. The situation can be explained through Figure
2.4, where the time for the oil to oxidize versus a neutralization number is compared at three
temperatures: 70°C, 60°C, and (60-X)°C.
Figure 2.4. The 10 degree rule application on oil’s neutralization number [24]
30
According to standard IEC 61125 [25] , the oxidation stability in insulating liquids is measured
by accelerated thermal ageing of the liquid sample and injecting bubbled oxygen or air, and
maintaining the sample for a given period at a given temperature, 100°C or 120°C, in the
presence of solid copper. There are three different methods that can be selected:
a. Method A.
This method is applied at a temperature of 100°C ± 0.5°C for 164 h ageing duration and using a
piece of copper wire as a catalyst.
b. Method B
This method is applied at a temperature of 120°C ± 0.5°C without specific ageing duration and
using a piece of copper wire as a catalyst.
c. Method C
This method is applied at a temperature of 120°C ± 0.5°C for 164 h ageing duration and using a
piece of copper wire as a catalyst. For the insulating liquids with improved oxidation stability, it
requires longer test periods.
An experimental research [26] mentioned about an accelerated thermal ageing process on
mineral oil at 120°C for 675 h. The oil samples were tested in the ministatic tester for the
electrostatic charging tendency (ECT). Other parameters such as dissipation factor, volume
resistivity, and acid number were also measured with different standard methods. Dissipation
factor and volume resistivity have been measured according to IEC standard 60247-2004,
whereas measurement of acid number followed the Chinese standard GB/T264-1983. The
31
experiments discovered an increasing tendency of ECT along the ageing time as shown in Figure
2.5.
Figure 2.5. ECT vs. ageing time in mineral oil [26]
Perrier and Beroual [27] performed a similar experiment as Liu et al. on different types of oils.
The accelerated ageing process was performed at 120°C for several samples of mineral oil,
vegetable oil, silicone oil, and synthetic ester oil for duration of 14 days. They determined the
influence of accelerated thermal ageing on the insulating oil properties. Figure 2.6 shows that the
acidity level of all insulating oil samples increased due to the aging process. A similar behaviour
was found for the viscosity of vegetable oil, but the viscosity of mineral oil remained constant
(shown in Figure 2.7).
32
Figure 2.6. Acidity before and after ageing [27]
Figure 2.7. Viscosity before and after ageing [27]
33
Based on the result of these two researches, in this thesis, a similar accelerated thermal ageing
procedure is performed on two types of insulating oil, i.e. the mineral oil and synthetic ester oil.
The oil samples will be put into an oven to carry out the thermal ageing process at 100°C.
According to the 10 degree rule and 675 hours aging duration at 120°C, the ageing duration
carried out in this thesis project is determined to be 1440 hours.
2.2.3Dielectric testing (break down voltage)
The term dielectric strength (can as well mean voltage breakdown) has different meanings
depending on the primary focus, with reference to an insulating material it refers to the
maximum electric field strength that the material can withstand continuously without breaking
down; to be specific, without experiencing failure of its insulating properties.
With major focus on transformers, dielectric strength applies to transformer oil. The dielectric
strength is determined taking note of at what voltage, sparks between two electrodes immersed in
oil and a specific gap between them. Low value of dielectric strength shows presence of moisture
and maybe other conducting substances in the oil. This is done with a break down voltage
measuring kit in which one pair of electrodes is fixed and has a gap measuring about 2.5mm
apart. Then voltage shall be applied to the pair of electrodes.
With careful increasing of voltage on both electrodes, critical observation should then be taken
on when the sparks between electrodes happen; exactly when the dielectric strength of
transformer oil between electrodes has been broken down. [1]
34
2.3.4Propertice of liquid dielectric
Liquid dielectrics, because of their inherent properties, appear as though they would be more
useful as insulating materials than either solids or gases. This is because both liquids and solids
are usually 103 times denser than gases and hence, from Paschen's law it should follow that they
possess much higher dielectric strength of the order of 107 V/cm. Also, liquids, like gases, fill
the complete volume to be insulated and simultaneously will dissipate heat by convection. Oil is
about 10 times more efficient than air or nitrogen in its heat transfer capability when used in
transformers. Although liquids are expected to give very high dielectric strength of the order of
10 MV/cm, in actual practice the strengths obtained are only of the order of 100 kV/cm.[28]
Liquid dielectrics are used mainly as impregnants in high voltage cables and capacitors, and for
filling up of transformers, circuit breakers etc. Liquid dielectrics also act as heat transfer agents
in transformers and as arc quenching media in circuit breakers. Petroleum oils (Transformer oil)
are the most commonly used liquid dielectrics. Synthetic hydrocarbons and halogenated
hydrocarbons are also used for certain applications. For very high temperature application,
silicone oils and fluorinated hydrocarbons are also employed. In recent times, certain vegetable
oils and esters are also being tried. However, it may be mentioned that some of the isomers of
poly-chlorinated diphenyls (generally called askerels) have been found to be very toxic and
poisonous, and hence, their use has been almost stopped. In recent years, a synthetic ester fluid
with the trade name 'Midel' has been developed as a replacement for askerels.
Liquid dielectrics normally are mixtures of hydrocarbons and are weakly polarised. When used
for electrical insulation purposes they should be free from moisture, products of oxidation and
other contaminants. The most important factor that affects the electrical strength of an insulating
35
oil is the presence of water in the form of fine droplets suspended in the oil. The presence of
even 0.01% water in transformer oil reduces its electrical strength to 20% of the dry oil value.
The dielectric strength of oil reduces more sharply, if it contains fibrous impurities in addition to
water.[28]
Table 2.2 shows the properties of some dielectrics commonly used in electrical equipment.
Table2.2 Dielectric properties of some liquid dielectric
Source:[28]
2.3 Locally available oil base for liquid dielectric production
2.3.1. List of available local liquid
Vegetable oil that can by found in Nigeria market are as follows: groundnut oil, cotton seed oil,
shea butter oil, sesame oil, butter oil, palm oil, palm kernel oil, olive oil, oil from animal fat,
sunflower oil, coconut oil, mustard oil, soya beans oil, Mazola oil, almond oil and galic oil.
36
2.3.2. Comparative analysis of the existing conventional liquid and the proposed Liquid
dielectric on power transformer insulation
Mineral oil can be obtained from petroleum and it is a good insulating material because of its
good electrical properties. Other characteristics such as high BDV also transformer operation.
The most important thing is that mineral oil is already produced worldwide and offered at a low
cost. But the biggest problem will occur when there is a transformer leakage and the mineral oil
will endanger the environment because of its poor biodegradability.
The vegetable oil has nearly the same dielectric constant as that found in mineral oil. The
mineral oil mainly consists of refined petroleum which contains non polar alkane molecule. So
we can compare the two types of different oils by looking at their experiment results.
Vegetable oil can be obtained from fish oil, animal fats, palm fruits or seeds with different
processes. Several typical oil seeds which are obtained and processed before they are ready to be
used as the transformer insulation are represented.
When we mention comparison between mineral oil and vegetable oil characteristic, we have to
mention several Criteria between both of those types:
For example; Key properties for mineral oil characteristic: produced from petroleum crude and
non-renewable power and scarce sources.
Key properties for vegetable oil characteristic: Produced from domestically grown and from
renewable sources, such as soybeans and corn.
Environmental Properties for the mineral oil: Contains compounds that do not readily
biodegrade. It may also contain traces of a confirmed carcinogen.
37
Environmental Properties for the vegetable oil: Highly biodegradable; non‐toxic; does not
contain petroleum, silicone, or halogens.
Leaks and Spills for the mineral oil: The latest findings of the scientists of genetic engineering to
eliminate this problem. Some bacteria have the ability to absorb these substances’ toxic oils and
synthetically convert it into a food items. It could be formed by hybridization for more than one
type of bacteria found in nature. This could possibly cause a large number of exchanges between
different genes to reach the desired qualities to produce a new type of bacteria that does not exist
in nature.[1]
2.3.3 Properties of vegetable oil
Vegetable oils are obtained from oil containing seeds, fruits, or nuts by different pressing
methods, solvent extraction or a combination of these .Crude oils obtained are subjected to a
number of refining processes, both physical and chemical. These are detailed in various texts and
articles.
There are numerous vegetable oils derived from various sources. These include the popular
vegetable oils: the foremost oilseed oils - soybean, cottonseed, peanuts and sunflower oils; and
others such as palm oil, palm kernel oil, coconut oil, castor oil, rapeseed oil and others. They also
include the less commonly known oils such as rice bran oil, tiger nut oil, patua oil, ko÷me oil,
niger seed oil, piririma oil and numerous others. Their yields, different compositions and by
extension their physical and chemical properties determine their usefulness in various
applications aside edible uses. Cottonseed oil was developed over a century ago as a byproduct
of the cotton industry. Its processing includes the use of hydraulic pressing, screw pressing and
solvent extraction. It is classified as polyunsaturated oil, with palmitic acid consisting 20 – 25%,
38
stearic acid 2 – 7%, oleic acid 18 – 30%, and linoleic acid 40 – 55%. Its primary uses are food
related – as salad oil, for frying, for margarine manufacture, and for manufacturing shortenings
used in cakes and biscuits. Palm oil, olive oil, cottonseed oil, peanut oil, and sunflower oil
amongst others are classed as Oleic – Linoleic acid oils seeing that they contain a relatively high
proportion of unsaturated fatty acids, such as the monounsaturated oleic acid and the
polyunsaturated linoleic acid. They are characterized by a high ratio of polyunsaturated fatty
acids to saturated fatty acids. As a consequence of this, they have relatively low melting points
and are liquid at room temperature. Iodine values, saponification values, specific compositions
and melting points in addition to other physical properties have been determined and are widely
available in the literature . Other oils fall under various classes such as the erucic acid oils which
are like the oleic linoleic acid oils except that their predominant unsaturated fatty acid is erucic
acid (C22). Rapeseed and mustard seed oil are important oils in this class. Canola oil is a type of
rapeseed oil with reduced erucic acid content . It is stable oil used in salad dressings, margarine
and shortenings. Soybean oil is important oil with numerous increasing applications in the
modern day world. It is classed as linolenic acid oil since it contains the more highly unsaturated
linolenic acid. Other oils include castor oil (a hydroxy-acid oil) which contains glycerides of
ricinoleic acid. Also worthy of note is that coconut oil, which unlike most vegetable oils, is solid
at room temperature due to its high proportion of saturated fatty acids (92%) particularly lauric
acid. Due to its almost homogenous composition, coconut oil has a fairly sharp melting
point.[11]
39
2.4 Antioxidants for Soy Beans Oil
2.4.1The Nature and Mechanisms of the Operation of Antioxidant
The word anti-oxidant is used in a general sense to refer to any type of chemical agent which
inhibits attack by oxygen or ozone. As applied to vegetable oils, anti-oxidants are compounds
which interrupt the oxidation process by preferentially reacting with the fat radical to form a
stable radical which does not quickly react with oxygen. When the reference is to food uses, they
are grouped as a food additive which has the effect of increasing the shelf life of foods by
protecting them against deterioration caused by oxidation which leads to rancidity and colour
changes.
Antioxidants function either by inhibiting the formation of free alkyl radicals in the initiation
step or by interrupting the propagation of the free radical chain. In truncating the propagation
step, the antioxidants function as hydrogen donors.
Generally, the most popular antioxidants are hydroxylphenol compounds with various ring
substitutions. They are characterized by possessing low activation energies for the hydrogen
donation process. The antioxidant radical which results is stabilized with its local electrons
delocalized; hence antioxidant free radicals do not readily initiate other free radicals. They rather
even react with lipid free radicals to form stable and complex compounds. In investigating
phenolic antioxidants, it is found that their antioxidative capabilities bear a relationship to the
number of phenol groups occupying 1,2 or 1,4 positions in an aromatic ring as well as to the
volume and electronic characteristics of the ring substituent’s present. In elucidating the
mechanism of oxidative inhibition, it is generally established that anti-oxidants function as
oxygen interceptors in the oxidative process thereby breaking the chain reaction that perpetuates
40
the process. Common phenolic antioxidants include Butylated Hydroxyanisole (BHA), Butylated
Hydroxytoluene (BHT), Propyl Gallate (PG) and Tertiary Butyl Hydroquinone (TBHQ). The
donation of hydrogen to free radicals followed by the formation of a complex between a lipid
radical and the antioxidant radical formed as a result of the hydrogen loss. Here the antioxidant
radical functions as a free radical acceptor. The general scheme is presented below [11]:
Mention must be made of synergists – substances which increase the effectiveness of a primary
antioxidant. Certain metallic ions such as copper and iron act as pro oxidants, catalyzing the
oxidation process. Such metal ions can be sequestered by certain organic acids.
They effectively contribute to lower transition metal activity. Synergists are not as effective
when used alone; rather, they work best when combined with an antioxidant. Examples of such
compounds are citric acid, phosphoric acid and some of their derivatives . Synergism has been
studied, not just in relation to antioxidants alone, but in relation to combinations of antioxidants,
41
anti wear and other additives. An example of a synergist used in conjunction with phenolic
antioxidants is citric acid.[11]
2.4.2Oxidation in lubricant base oil
The ageing of the oil
Oxidation is a common problem in many industries. When the oil is being used in a machine, it
oxidizes over a period of time depending on the type of oil, the operational conditions and the
environment. Oxidation can have negative effects on lubrication and hydraulic system, all of
which may have serious consequences for the performance of the machinery.
The base oil consists of a mixture of many different hydrocarbons. The oxidation process occurs
when these hydrocarbons react with oxygen. It is a self-propagating process that occurs via free
radicals. Temperature is of great importance in oxidation. At lower temperatures (up to about
150 ºC) the pattern of oxidation reactions may differ substantially from those at high
temperatures. In a general case it is assumed that oxidation of oil proceeds in three stages:
initiation stage, propagation stage and termination stage. The following model shows the
oxidative degradation in lubricant base oils [28].
Initiation stage:
The hydrocarbons react with oxygen forming hydrocarbon free radicals. The reaction is
catalysed by traces of transition metal ions like copper, iron, cobalt, chromium, etc.
Propagation stage:
The hydrocarbon free radicals react with oxygen forming peroxide radicals, that are highly
reactive, and react further with hydrocarbons from the oil. This leads to hydroperoxides and
hydrocarbon free radicals, which can react with oxygen.
42
Termination stage:
The hydroperoxides, formed in the propagation stage, cleave homolytically and form oxygenated
compounds like aldehydes, ketones, alcohols and water. These compounds can react further
forming organic acids and high molecular weight polymeric products. Further polycondensation
and polymerisation of these products lead to insoluble products called sludge, which may
precipitate as a thin film forming lacquers or varnish deposits on hot or cold metal surfaces.
Consequences of the oxidative degradation in lubricant base oils
Corrosion and wear
Acid formation will cause corrosion and wear in the internal surfaces of the machine. Water,
which is also formed under the oxidation process, increases the corrosive effect of the acids.
Increase in oil viscosity
The oil viscosity increases due to the formation of oxidation products e.g. polymeric compounds
and sludge. It will result in friction, wear and loss of energy.
Sludge and varnish
Sludge and varnish can result in valves blocking and orifices clogging. Once varnish deposits are
formed on metal surfaces, it is very difficult to dissolve them even at high temperatures. These
deposits are like a sticky material that will trap hard contaminants creating a “sandpaper
surface”, which can cause accelerated wear in the components. In addition to this, varnish acts as
an insulator, reducing the effect of the heat exchangers which again means higher temperatures
and accelerated rates [28].Figure 2.8 shows the stages of oxidation degradation in lubricant base
oil.
43
Figure2.8. Diagram showing the oxidative degradation in lubricant base oils [28]
2.4.3Measuring oxidation in lubricant base oils
Oxidation by-products (aka soft particles) are compounds of molecular sizes less than 1 micron,
which cannot be measured by conventional particle counting methods.
TAN analysis (total acid number) and viscosity testing can be used as indicators of oxidation but
they cannot determine whether or not the oil is oxidized. Dark colour, and sour, putrid odour
usually indicate oxidation. There are several method to monitor oxidation in the oil. Generally,
these methods are used to either determine oil degradation products from the oil or the
consumption of anti-oxidants in the oil. The latter is used to predict when the onset of oxidation
will occur. The following are examples of methods to monitor the oxidative degradation[28]:
44
i. FTIR analysis (Fourier Transformation Infrared Spectroscopy)
This analysis is based on the principles of molecular spectroscopy spectroscopy. It can verify
oxidation in lubricants by identification of functional groups (e.g. ketones, carboxylic acids) in
molecules.
ii. QSA (Quantitative Spectrophotometer Analysis)
The oxidation by-products are isolated from the oil sample and a quantitative spectrophotometer
analysis of the isolated contaminants is carried out in order to determine the degree of oxidation.
iii. Ultracentrifuge test
This test uses the gravitate forces to extract and settle the contaminants of the oil. The sediment
is compared with a sedimentation rating system to determine oxidation.
iv. Gravimetric analysis
This analysis examines oxidation by measuring the weight of residual components.
The oil sample is treated with the solvents pentane (hexane), toluene and pyridine to determine
the oxidation products.
v. Colorimetric Analysis:
This analysis examines the colour of contaminants collected on membrane filters by a
colorimetric device.
The following are examples of methods to monitor the consumption of anti-oxidant additives:
vi. FTIR analysis (Fourier Transformation Infrared Spectroscopy)
45
This analysis, mentioned above, can monitor the additive depletion.
vii. RULER test (Remaining Useful Life Evaluation Routine)
It measures the remaining anti-oxidants by voltametric analysis.
viii. RBOT test (Rotating Bomb Oxidation Test)
It measures the oil’s resistance to oxidation under prescribed conditions.[28]
2.4.4 Oxidation stability index of vegetable oil
Numerous experimental works have established the positive effect of anti-oxidants on the
oxidative stability of vegetable oils for both edible uses and industrial uses. An important class of
anti-oxidants consists of the phenolic compounds Butylhydroxyanisole (BHA),
Butylhydroxytoluene (BHT), Propyl Gallate, and Tert-butyl Hydroquinone (TBHQ). Their use in
vegetable oils meant for domestic and industrial processes is widespread. Vegetable oils in their
natural form possess constituents that function as natural antioxidants. Amongst them are
ascorbic acids, α-tocopherole, β-carotene, chlorogenic acids and flavanols. Tests conducted to
investigate the effectiveness of natural anti-oxidants contained in red pepper oil added to
soybean and sunflower oils indicate that they provide variable protection against light induced
auto-oxidation. Measuring fatty acid formation and the measurement of peroxide values as a
means of monitoring oxidation, results indicate an inhibitive effect on oxidation . In another
study which monitored the inhibitive action of tocopherols on rapeseed and palm kernel oils by
measuring the presence of the oxidation product, monoaldehyde, indicate some measure of
protection provided by these natural anti-oxidants.
46
In the above mentioned study on the inhibitive effect of natural antioxidants contained in red
pepper oil, it was additionally observed that the phenolic anti-oxidant Butylated Hydroxytoluene
(BHT) shows more effectiveness generally than natural anti-oxidants.
The common phenolic anti-oxidants were tested for their effectiveness in improving the
oxidative stability of biodiesel obtained from soybean oil. The general conclusion is that up to a
point, increased loading of antioxidants is beneficial in terms of improving oxidative stability
[11].
47
CHAPTER THREE
METHODOLOGY OF EXPERIMENT DESIGN
The overall strategy used in this study has three main components
1. The extracted soybean oil is first inhibited (i.e. treated with antioxidant, and with metal
deactivators (additives to inhibit transition metals).
2. The inhibited oil is then subjected to accelerated thermal ageing.
3. Test and evaluation is carried out for the acceptability of the proposed local oil as a
transformer insulation oil.
3.1. Inhabitation of the Soybean Oil: Material and Method
The antioxidant used is Butylated hydroxytoluene (BHT). BHT, also known as
dibutyldroxytoluene, is a lipophilic organic compound, chemically a derivative of phenol, that is
useful for its antioxidant properties [29]. Industrially, BHT is prepared by the reaction of p-cresol
(4-methylphenol) with isobutylene (2-methylpropen) catalyzed by euphoric acid [30].
CH��C�H%�OH + 2CH� = C�CH��� → ��CH���C��CH�C6H�OH
Sample Preparation
120ml of soybean oil was put into a conical flask. The flask containing the oil was heated to
700C and agitated vigorously. While agitation was going on, BHT the antioxidant was added to
the first conical flask, 0.36g of BHT was added to the conical flask. The antioxidant was added
slowly and allowed to dissolve properly. The conical flask was vigorously agitated for an
additional 20 minutes to ensure complete dissolution.
48
Note: For the required concentration of the inhibitor, the reference [31] recommends 0.3%.
Hence for the 120ml of soybean oil, the weight of BHT used is then 0.36g.
3.2. ACID VALUE (MgKOH/g)
Introduction
The acid value is a measure of the extent to which the glycerides in the oil, have been
decomposed by lipase or other reactions. The decomposition is accelerated by heat and light. As
rancidity is usually accompanied by free fatty acid formation, the determination is often used as a
general indication of the condition and edibility of oils. A known quantity of soy bean oil to be
analysed is dissolved in a neutral ethanol and titrated with a standard KOH(0.1N) to a faint pink
colour. Figure 3.1 shows the set up for acid value test at UNN energy center.
Materials
i. Ethanol
ii. 1% phenolphthalein indicator
iii. 0.1N KOH
Procedure
5 g of the sample was weighed into a flask. 50 ml of neutralized ethanol was poured into the
flask. The contents were mixed together and boiled. It was then titrated with 0.1N KOH to a faint
pink colour that persisted for at least 15 seconds.
AV = 56.1 × N × T × 100
1000 × G
49
Where:
AV= Acid Value
N = Normality of standard KOH used
T = Titration volume
G = weight of sample
Figure 3.1. Acidity Test at UNN Energy Centre
3.3. VISCOSITY
Viscosity of the soybean oil was done with a digital viscometer made by Searchtech
instruments, England. The appropriate spindle is selected and fixed on the instrument. The
spindle is inserted in the oil sample to be analyzed till the level mark on the spindle reach the
50
surface of the sample. Enter button on the instrument is pressed and the viscosity of the oil
sample is displayed on the screen. Figure 3.2 shows viscosity test set up using digital viscometer.
Figure 3.2. Viscosity measurement using digital viscometer at UNN Energy Centre
3.4. Flash and Fire Point
The flash and fire point of the soy bean oil samples was determined as per ISO 2719(Appendix
A). A Pensky Martin Flash Point (closed) apparatus was used to measure the flash and fire point
of the oil samples. The sample was filled in the test cup up to the specified level and was heated
and stirred at a slow and constant rate. At every 10 C temperature rise, flame was introduced for
a moment with the help of a shutter. The temperature at which a flash appeared in the form of
sound and light was recorded as flash point. The fire point was recorded as the temperature at
51
which oil vapour catches fire and stays for minimum of five seconds. The temperature was
measured with the help of a thermometer. Figure 3.3a shows the flash and fire point test set with
the test cup on the table, containing the soybean oil sample in it. Figure 3.3b shows the test cup
in the machine during the test.
3.5. Pour Points Tests
The Pour point of soy bean oil samples were determined as per ISO 3016 (Appendix B), using
the Pour point apparatus. The apparatus mainly consists of 12 cm high glass tubes of 3 cm
diameter. These tubes are enclosed in an air jacket, which is filled with a freezing mixture of
crushed ice and sodium chloride crystals. The glass tube containing soy bean oil sample is taken
out from the jacket at every 10 0C interval as the temperature falls, and is inspected for pour
point. The pour point was taken to be the temperature 10 0C above the temperature at which no
motion of soy oil was observed for five seconds on tilting the tube to a horizontal position.
Figure 3.3 (a) Flash and Fire Point Tester showing oil sample in the test cup at UNN Energy Centre
Figure 3.3 (b) Flash and Fire Point Tester showing test cup in the machine at UNN Energy Centre
52
Figure 3.4 shows the pour point test set up at UNN energy center.
Figure 3.4. Pour Points Tests at UNN Energy Centre
3.6. Subjecting the oil to Accelerated Ageing
According to standard IEC 61125p[ 32 ], the oxidation stability in insulating liquids is measured
by accelerated thermal ageing of the liquid sample and injecting babbled oxygen or air, and main
training the sample for a given period at a given temperature, 1000C or 1200C, in the presence of
solid copper.
Insulating oil plays an important role in the transformer insulation. Thus, ageing of the insulation
has been recognized as one of the main cause of transformer failures. The ageing rate of the
insulating oils depends on the magnitude of the electrical, thermal, and mechanical stresses [33].
53
The loss of transformer functional life was actually the same as the loss of transformer insulation
life.
First, the sample bottles are cleaned before they are filled with oil. Solid copper is added to the
oil sample (i.e. pile of copper wire acting as a catalyst). The oil samples are put into an oven to
carry out the thermal ageing process. The accelerated thermal ageing process is arranged for
approximately 14 days or 336 hours at an ageing temperature of 1200C. The schedule used for
each dielectric measurement is described in table 3.1.
Experimental Approach
For carrying steps 2 and 3, the experimental research was carried in the laboratory of
Transmission Company of Power Holding Company of Nigeria (PHCN) and Enugu Electricity
Distribution Company at Enugu. The experiments followed the following methods:
a. Oil sample for the acerbated ageing process was kept in the oven for approximately 14
days at 1000C based on IEC 61125.
b. Breakdown voltage measurement was carried out based on IEC 60156;
c. Dielectric losses measurement was carried out based on IEC 60247
Table 3.1. Schedule of Measurement
Test Breakdown voltage measurement
Ageing time New 30h 70h 102h 134h 168h 204h 246h 290h 334h
Test Tan � measurement
Ageing time New 40h 72h 104h 136h 170h 206h 248h 292h 336h
54
With the schedule of table 3.1, The breakdown voltage and the dielectric losses (i.e. Tan �)
measurement. These dielectric properties are measured along side the accelerated ageing process.
3.7. Measurement of the Oil Breakdown Voltage
Sampling
i. Sample bottles and syringes are used
ii. Procedure used ensured that the sample bottles seals are airtight
iii. The oil was poured into the vessel swiftly with minimum turbulence so as not to entrap
air.
iv. The sample was allowed to stand for 3 minutes before the test. This is in order to allow
bubbles to clear.
v. To prevent the sample from absorbing water from the air and reduce the breakdown
voltage, each sample was not left for more than 5 minutes.
The temperature of each sample is recorded in the test report. The trending of test results to
identify changes in breakdown voltage is only valid if the sample and ambient temperature for all
results have been taken into account. The breakdown voltage of an oil sample increases
significantly with temperature.
3.7.1. Breakdown Voltage Detection
It is important in this work to state how the oil test set used detects breakdown voltage. Efficient
breakdown detection is very important as it determines the ability of the instrument to correctly
register the breakdown voltage of the oil, and also to limit the amount of energy dissipated in the
arc in the oil [34]. For these reasons, the oil testing standards specify the precise conditions that
must be met to trigger the instrument’s breakdown detection system.
55
Modern test instrument detect breakdown directly on the instrument output, which improves
detection accuracy. Very few instruments are capable of breakdown detection that meets both
IEC and ASTM requirements. Instruments in the new Megger OTS60SX range are an exception
as they can detect breakdown in terms of current increase or in terms of voltage drop.
It is worth noting that some older instruments, such as the Megger OTS60SX model used for this
work, do not include automatic breakdown detection. However this limitation does not impact on
the quality of this work since the IEC 60156 standard states that the circuit applying the test
voltage may be opened manually if a transient spark (audible or visible) occurs between the
electrodes. This provision is included specifically to cover the use of these older instruments
where, if a spark is head or seen between the electrodes, a breakdown has occurred and the test is
complete.
The Megger OTS60SX at the PHCN test lab used for this study complies with the requirement of
the IEC 60156 standard. Though not exhaustive, appendix A gives the test standards and their
differences supported by the Megger test equipment. Figure 3.5.shows the Megger dielectric
breakdown voltage test sets.
56
The breakdown test set up for the oil is illustrated in figure 3.6.
Figure 3.6. Picture of oil breakdown voltage test setup
Figure 3.5(a): Megger dielectric breakdown voltage test sets showing oil sample in it before test at PHCN Transcom Enugu
Figure 3.5(b): Megger dielectric breakdown voltage test sets showing reading after test at EEDC Enugu
57
3.7.2. Electrode Gap Setting
According to IEC standard 60156, the gap between the electrodes is originally set to 2.5mm +
0.05mm. But due to limitation of output voltage of the test set up, the gap is adjusted to 1.5mm +
0.05mm. Appendix C shows the gap size of the electrode on the breakdown voltage tester
(Megger).Appendix D shows the name plate of the Megger.
3.7.3. Breakdown Voltage Test
i. First voltage application is started approximately 5 min after completion of the filling and
there should be no bubbles which are visible in the electrode gap.
ii. The applied voltage uniformly increases from zero at the rate of 2kV/s+0.2kV/s until
breakdown occurs.
iii. The measurements are carried out until 6 breakdowns on the same cell filling have
occurred, allowing a pause of at least 2 min after each breakdown before reapplication of
voltage or until there is no gas bubbles present within the electrode gap..
iv. The final result is calculated from the mean value of the 6 breakdowns in kV.
3.7.4. Assessing Validity of the Test Result
The IEC 60156 specifies conditions that must be met if the result of a test is to be accepted as
valid. IEC 60156 specifies that expected range of standard deviation/mean ratio as a function of
the mean is provided as a chart [34]. In IEC 60156, there is a graphical representation of
standard deviation otherwise known as the coefficient of variation versus the mean breakdown
voltage. IEC 60156 states that, for the test results to be considered valid, the following procedure
must be followed:
58
i. Perform six tests
ii. Calculate the mean of the results
iii. Divide the standard deviation by the average value, noting that scatter is expected and
acceptable.
iv. If the value is acceptable conclude testing.
v. If not, perform six more test
vi. Repeat the calculation using all 12 results.
The procedure used to calculate the standard deviation is to calculate the difference between each
of the six test results and the mean value of those test results. Square each of the differences and
add them together. Divide the figure obtained by 2, and then take the square root. The final
answer is the standard deviation for the set of test results.
3.8. Measurement of the Oil Sample Dielectric Losses (tan 1)
The dielectric dissipation factor (tan�) measurement in this project follows the standard IEC
60247 including preparation of measurement tool. Figure 3.7 and figure 3.8 show the design of
the test cell.
Figure 3.7 Tan 1 tell cell dimension
300
25mm
27mm
LV side
1.5mm
100mmHV side
59
This cell consists of two parts, high voltage (HV) and low voltage (LV) electrodes.
The cover on the top of LV electrode is made of epoxy as an insulator and equipped with a
potential guard which is put inside it. The potential guard has a function to minimize the effect of
any leakage current on the measurement.
The test cell was measured with empty condition and a capacitance value of 133.58pf and 3.60E-
4 of tan � value at 600v applied voltage were measured.
The tan � for the oil was measured using the Schering Bridge. Figure 3.8 gives the circuit
diagram for the measurement.
Figure 3.8 Circuit diagram for tan 1 measurement
The test cell is parallel connected to a standard capacitor 23 at the HV side which has a nominal
value of 100pf.
The measurement procedures are described as follows:
Protection relay
LV
C_ tan � bridge
HV
v
~ v AC
A
Test
object
Cx
Cn
60
i. The test cell and oil samples are measured at ambient temperature. The test cell is ringed
three times with a portion of oil sample. The filling process o foil sample is carefully
done to minimize the entrapment of air bubbles.
ii. The test AC voltage is applied to the liquid within the electric stress between
0.03kV/mm-1 kV/mm. The applied voltages are slowly increased to the test voltage, and
then tan � value can be determined from an adjustment of the Schering bridge variable
(capacitance, resistance).
iii. The final result is calculated as the mean of two consecutives value for tan � and agrees
to within 0.0001 plus 25% of the higher value of two values being compared.
There are Two Fundamental Parameters
Characterizing a dielectric material, the conductivity � and the real part of the permittivity or
dielectric constant 6 (or 67).
The conductivity 8 of a dielectric material is defined as the ratio of the leakage current density J1
(in A cm-2) to the applied electric field density E (in Vcm-1),
8 = 9:
;………………………………………………………………….. (3.1)
It is also determined in terms of the measured insulation resistance R (in Ω) as
8 = <
7=………………………………………………………………….. (3.2)
were d is the insulation thickness (in cm) and A is the surface area (in cm2).
61
The dielectric constant 6 is defined as the amount of electrostatic energy which can be stored per
unit volume per unit potential gradient. But it is also known as the real part of permittivity which
is determined as the ratio of
ε = ?
?@ ………………………………………………………………….. (3.3)
Were C is the measured capacitance (in f and C0 is the equivalent capacitance in vacuum. Co can
be obtained from the same specimen geometry of
C� = C� = ε�A
B ………………………………………………………… (3.4)
Were ε� represents the permittivity in vacuo with the value of 8.854 x 10-14 fcm-1. The value of
ε� in free space is essentially equal to that in a gas (εC of Air = 1.600536).
The relative permittivity definition from IEC 60247 is the ratio of capacitance C of a capacitor in
which the space between and around the electrodes are entirely and exclusively filled with the
insulating material, to the capacitance C0 of the same configuration of electrodes in vacuum.
εCJ =
?K
?@ ………………………………………………………………….. (3.5)
Dielectric losses may initially be caused by the movement of free charge carriers (electrons and
ions). Space charge polarization or dipole orientation. Most causes are influenced by the
temperature, electric field strength, and are frequency dependent. A complex permittivity ε is
defined as
ε = ε− jε" ……………………………………………………………….. (3.6)
62
Were ε" is the imaginary value of permittivity and means the dielectric loss contributed by the
leakage current and the polarizations, ε is the complex permittivity and equal to the ratio of
dielectric displacement vector NO to the electric field vector PQ. The losses determine the phase
angle � between vector NO and PQ. Based on the position in the vector, NO and PQ should love the
complex retention
D0 exp [j(wt-�)] and E0exp [jwt] with D0 and E0 as the vector magnitudes respectively. Thus, the
following relation result
ε = R@
;S 2�T �……………………………………………………………….. (3.7)
έ = R@
;S T�U �……………………………………………………………….. (3.8)
The dielectric dissipation factor (tan �) can be expressed as:
Tan � = V′′
V′=
V′′WSX� YZ
V′ ………………………………………………….. (3.9)
The behaviour of the losses of a dielectric can be determined from an equivalent electrical
circuit. The parallel circuit is commonly used for analysis as seen in figure 3.6(a).
Figure 3.9(a) parallel circuit and (b) Phasor diagram
V R
C
Ic
I
�
[\ (b) (a)
I1
v I
I1
63
When an applied voltage V passes through a dielectric material, it generates a leakage current
I1 = ]
7 and displacement current Ic = jwcv. From the phasor diagram in figure 3.6(b), it represents
the value of tan � = ^:
^_, then by applying the substitution on I1 and Ic, it will give a final
expression of tan � =
`7a ………………………………………… (3.10)
The definition of dielectric dissipation factor (tan �) according to IEC 60247 is the tangent of the
loss angle and the loss angle is described as the angle difference between applied voltage and the
resulting current deviating from b/2def.
3.9. Measurement of the Oil Sample Permittivity
The measurement procedures are described as follows:
i. Measurement of the capacitance of the empty test cell or capacitance of the cell with air
as the dielectric Ca.
ii. Measurement of the capacitance Cx of the cell filled with the liquid under test. The
relative permittivity 6g can be determined 6g = ah
ai ……………………… (3.11)
iii. The measurement was two consecutive results that differ by no more than 5% of the
higher value. The final result calculated from the mean of the valid measurements.
64
Figure 3.10 shows the digital oven for heating the oil sample, set at 100�C in the lab.
Figure 3.10. Digital oven with oil sample in it.
65
Figure 3.11 shows the flow chart of the oil characteristics measurement
The following flow chart summarizes the measurement procedure used in this study
Start
Sample preparation
The oil characteristics assessment i. Breakdown voltage ii. Dielectric losses
Breakdown voltage measurement of oil
Dielectric losses measurement
Sample (aged oil) is taken on schedule
Sample (aged oil) is taken on schedule
Follow the standard procedure of IEC 60156
Follow the standard procedure of IEC 60247
A B
What type of test to be conducted, Breakdown voltage or Dielectric
losses?
1
66
No
Sample Preparation: i. Test cell is almost filled with sample, leaving 3% of the container volume free of
air space ii. Temperature of the test liquid and ambient air do not differ by more than 50C iii. The gap between the electrodes is set at 2.5mm + 0.05mm
A
Before filling the test cell, the sample container is gently agitated and turned over several times to ensure homogenous distribution of any contained impunities
The test cell is drained and walls, electrodes and other components part are raised with the test sample
First application voltage is started approximately 5min after completion of filling and checking that no air bubbles are visible in the electrode gap
Voltage is applied to the electrodes and uniformly increased from zero at the rate of 2kV/s + 0.1kV/s until breakdown occurs
6 breakdowns is carried out on the same cell filling allowing a pause of 2min after each breakdown
The mean value of the 6 breakdowns in kV is calculate
The range of the Measurements must not exceed 10% of the mean
value
Stop
Yes
1
67
Sample Preparation: i. The oil is put into a sample vessel ii. The sample is moved to the oven and the temperature of the ageing set to
1000C
B
Test temperature: The oil sample is taken out of the oven according to the test time arrangement (i.e. the schedule)
Filling the cell i. The cell is refilled ii. The cell and content is brought to the required test temperature (ambient
environment)
Measurement
i. The oil is subjected to the test AC voltage between 0.03kVmm-1kV/mm and having a frequency of 50Hz
ii. Measurement is started as the voltage is turned on and slow increased to respective voltage and the Schering bridge variable (capacitances resistance to find the tan �value
iii. The result is recorded
6 breakdowns is carried out on the same cell filling allowing a pause of 2min after each breakdown
Does the result give 2 consecutive values for tan � agree to within 0.0001 plus 25% of the higher value of 2 values being compared?
Stop
Yes
No
Figure 3.11. Flow chart of the oil characteristics measurement
68
CHAPTER FOUR
4.0. Result Data and Comparison
The techniques laid out in the last chapter are used on samples of the inhibited soybean oil and
samples of mineral oil. The performance of both oils for the accelerated ageing last is presented
and analyzed in this chapter. The properties of the soybean oil are analyzed in this chapter
against standards.
4.1. Result Data on the Physical and Chemical Properties of the Soybean Oil
Test for the following properties of the inhibited soybean oil were carefully carried out at the
University of Nigeria energy centre:
i. Acidity
ii. Flash point
iii. Viscosity
iv. Pour point
The result of this test is shown in table 4.1
Table 4.1: Physical and Chemical Properties of the Treated Soybean Oil
Property Treated soybean oil Untreated soybean oil Mineral oil
Acidity 0.0027mgkoH/gm 0.030 mgkoH/gm 0.0072 mgkoH/gm
Flash point 2570C 2500C 176
Viscosity 400C 9.3mm2/s 8.78 mm2/s 10.4 mm2/s
Pour point -150C -14.30C -150C
69
The measured flash point of the treated soybean oil is 2570C. This value obtained for the flash
point shows that the inhibited soybean oil can safely be used even where the temperature is
expected to be very high. Though the reference [14] indicates that oil flash point limits for most
applications to be 1000C, the ISO 2719 standard recommends a minimum (i.e. limit) of 1350C for
transformer oil and 1000C for power switchgear soil. Based on operational acceptance
requirements, the obtained flash point value for the treated soybean oil exceeds the minimum
required based on ISO 2719 recommendation. Appendix A extracted from [35] gives limits for
most properties of dielectric fluid.
The measured value of the acidity of the soybean oil is 0.0027 mgkoH/sm. The standard IEC
62021-1 recommends a maximum value of 0.01kg koH/gm for acidity (refer to appendix A). The
obtained value of 0.0027mgkoH/gm is within the limit.
The measured viscosity at 400C and pour point of the inhibiter soybean oil are 9.3mm2/s and -
150C respectively. Based on ISO 3104 the treated soybean oil is within limit since the
recommended limit for viscosity is 12mm2/s maximum. The pour point value of -150C is below
the ISO 3016 recommended limit of -400C.
The compliance of the properties of the inhibited soybean oil, as measured, shows the suitability
of the oil for use as dielectric fluid.
Result data on the electrical properties of the oil 4.29 breakdown voltage (BDV).
Based on the experimental methodology as described in the last chapter, the BDV is measured
alongside the accelerated ageing process. The breakdown voltage is measured using the schedule
of table 3.1.
70
Mineral oil and the proposed inhibited soybean oil were put through the same ageing procedure
as described in chapter three. The results of breakdown voltage (BDV) measurements for the
mineral oil and the treated soybean oil are listed in table 4.2.
Table 4.2: Result of Breakdown Voltage (BDV) measurements for the mineral oil and
treated soybean oil
Ageing time (hours) Breakdown voltage (kV)
Treated
Soybean oil
Mineral oil Untreated
Soybean oil
0 59.08 58.32 58.55
38 56.03 54.58 55.41
70 49.63 47.57 48.70
102 43.02 41.03 42.67
134 37.94 38.76 37.73
168 42.56 43.03 42.65
204 38.58 37.64 38.55
246 38.37 37.17 37.90
290 38.42 37.67 37.51
334 36.88 36.43 36.73
For each ageing stage six breakdown tests were conducted on each oil sample. In this way there
six breakdown voltages obtained for each ageing stage. The results of each ageing time listed in
table 4.2 are the mean values of the six measurement values.
71
0 50 100 150 200 250 300 35035
40
45
50
55
60
Ageing Time(Hour)
Breakdow
n Voltage(kV)
The Inhibited Soybean oil vs Ageing TimeMineral oil vs Ageing Time
Soybean oil Before inhibition vs Ageing Time
Figure 4.1B: Breakdown Voltage vs Ageing Time
0 50 100 150 200 250 300 35035
40
45
50
55
60
Ageing Time(Hour)
Breakdow
n Voltage(kV)
The Inhibited Soybean oil vs Ageing Time
Mineral oil vs Ageing Time
A graphic presentation of the breakdown voltage comparison between mineral oil, inhibited
soybean oil and uninhibited soybean oil along the ageing time is described in figure 4.1a and
4.1b.
Figure 4.1a. Breakdown Voltage vs Ageing Time of Inhibited of Soybean and Mineral Oil
Figure 4.1b. Breakdown Voltage vs Ageing Time of Inhibited of Soybean and Mineral Oil Uninhibited of Soybean and Mineral Oil
72
The ageing time profile of the soybean oil before inhibition is of the same form as that after
inhibition. However, as can be seen from fig 4.1b, there are slit variations in their values of
breakdown voltage. As indicated in the plot, the different in their breakdown voltage as ageing
progresses can be said to be negligible.
From figure 4.1a, the breakdown voltage of the inhibited soybean oil (the red line) at the
beginning of the ageing (i.e. oil still new) is 59.08kV. This value is well over the standards
recommended minimum breakdown voltage for the acceptability of oil for use as a transformer
dielectric. Regarding minimum breakdown voltage, the IEC 60156 recommends 30kV as
minimum. This indication that the breakdown voltage of the inhibited locally extracted soybean
oil is adequate. Figure 4.1a. indicates that the breakdown voltage of the treated soybean oil was
higher than that of the mineral oil. But around 134hour it jumped down to 37.94kV. The soybean
oils breakdown voltage then increased to 42.56kV at around 168 hour from the beginning of
ageing. At the same time that of the mineral oil increased from 38.76kV to 43.03kV. For the
soybean oil the value of the breakdown voltage was reduced from 59.08kV (new oil) to 36.38kV
(end of ageing). For the mineral oil, the breakdown voltage value was reduced from 58.32kV to
36.43kV. However the 14 days accelerated ageing did not completely deteriorate the two oils.
Both types of oil show lower breakdown voltage value at the end of ageing if compared to the
initial breakdown voltage value.
Generally, by way of comparison, as the graphical representation of figure 4.1a. shows, the
inhibited soybean oil and the mineral oil indicated some similarity in their response to the
accelerated ageing. However there are slight variations in their response. This indicates that the
soybean oil is comparable to the mineral oil as a dielectric fluid.
73
The results comply with the theory of breakdown in oil by Naidu [36] who assumes that each
breakdown is an independent event due to the self healing ability of insulating oil, and a
consideration that there is sufficient time between measurement to expel the breakdown products
or gas bubbles.
Essentially can be observed from the slaps of the graphs of figure 4.1a, that breakdown voltage
values of the inhibited soybean oil is close to those of the mineral oil.
4.2.2. Dielectric Dissipation Factor (Tan1)
The Dielectric Dissipation Factor (DDF) or tan � of the proposed inhibited soybean oil and the
mineral oil along the ageing time are calculated from the mean of two consecutive measurements
for each applied voltage. Table 4.3 shows the tan � results for the soybean oil dielectric and table
4.4. gives the tan � results for the mineral oil for each applied voltage from 100V to 600V.
Table 4.3. Tan 1 results for the soybean oil
Ageing
time (hour)
Tan � results (x 10-4)
100V 200V 300V 400V 500V 600V
0 1.58 2.68 2.97 3.33 3.57 3.66
38 1.53 2.57 3.12 3.39 3.59 3.59
70 0.67 1.78 2.53 2.62 2.86 3.32
102 1.45 3.36 3.47 3.43 4.56 4.76
134 1.43 2.59 2.8 3.12 3.43 3.62
168 1.97 3.18 3.54 3.68 4.25 4.46
204 4.59 5.89 5.98 6.54 6.79 7.34
246 5.47 6.83 7.63 7.78 8.23 8.35
290 9.58 10.72 11.45 11.72 11.89 11.28
334 12.11 13.53 14.08 14.52 14.64 15.15
74
Table 4.4. Tan1 results for mineral oil
Ageing
time (hour)
Tan � results (x 10-4)
100V 200V 300V 400V 500V 600V
0 1.68 0.37 0.35 0.38 0.35 0.34
38 1.36 2.03 2.05 1.53 1.52 1.62
70 1.82 3.02 2.76 2.47 2.46 2.26
102 4.03 3.02 2.47 2.45 2.76 3.32
134 3.74 3.45 3.36 3.28 3.29 3.31
168 3.35 3.19 3.09 3.08 3.02 3.08
204 1.78 1.06 0.67 1.00 1.12 1.22
246 0.43 0.45 0.69 0.46 0.44 0.46
290 0.98 0.98 0.88 0.89 1.06 0.89
334 1.57 1.23 0.96 0.86 0.69 0.82
Graphical presentation of the tan � (i.e. dissipation factor) for the treated soybean oil and mineral
oil along the ageing time at 100V applied voltage and 600V applied voltage are illustrated in
figure 4.2 and figure 4.3 respectively. While the tan delta trend of the soybean oil is illustrated
more specifically in figure 4.4. to show that the dissipation factor values increased along the
ageing time and at elevated applied voltages.
75
Figure4.2: Tan delta vs. ageing time at 100V applied voltage.
0 50 100 150 200 250 300 3500
0.2
0.4
0.6
0.8
1
1.2
1.4x 10
-3
Ageing Time(Hour)
Tan
gen
t d
elta
(Dis
sip
atio
n F
acto
r)
The Inhibited soybean oil At 100V vs Ageing Time
Mineral oil At 100V vs Ageing Time
76
Figure 4.3: Tan delta vs. ageing at 600V applied voltage.
0 50 100 150 200 250 300 3500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6x 10
-3
Ageing Time(Hour)
Tan
del
ta(D
issi
patio
n F
acto
r)
The Inhibited soybean oil At 600V vs Ageing Time
Mineral oil At 600V vs Ageing Time
77
Fig 4.4: Tan delta vs. ageing time for the inhibited soybean oil at different applied voltages.
First at the beginning of the ageing, the dissipation factor (i.e. tan delta) for the soybean oil at
100V and 600V are 1.58 x 10-4 and 3.66 x 10-4 respectively. These values are adequate for the
acceptance of the oil as a liquid dielectric; since these values are well below the IEC 60247
recommended limit of 0.005. From the comparison of the graphics it can be inferred that the
trends are different for both types of oil. The inhibited soybean oil exhibited a significant
increase of dissipation factor (tan �) along the ageing time.
In comparison with the mineral oil, the treat soybean oil shows a higher increase of dissipation
factor from the inception of ageing towards the end of ageing. For the mineral oil at 100v, the
0 50 100 150 200 250 300 3500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6x 10
-3
Ageing Time(Hour)
Tan
del
ta(D
issi
patio
n F
acto
r)
The Inhibited soybean oil At 100V vs Ageing
The Inhibited soybean oil At 200V vs AgeingThe Inhibited soybean oil At 400V vs Ageing
The Inhibited soybean oil At 600V vs Ageing
78
dissipation factor increased from around 38hour to a maximum of 4.03 x 10j% then decreased to
a minimum of 0.43 x 10-4 then increased slightly towards the end of ageing. For the inhibited
soybean oil, the dissipation factor (tan delta) decreased slightly from 1.58 x 10-4 at the start of
ageing to 0.67 x 10-4 at around 70hours of the ageing. It later increased steadily to 12.11x10-4
towards the end of the ageing. However the value at the end of the ageing is still within the IEC
60247 recommended limit.
From the graphical comparison, it can be observed that the mineral oil has a better dissipation
factor in comparison with the soybean oil. Nevertheless, the dissipation factor (tan delta) of the
soybean oil is within the acceptable limit for its use a transformer dielectric.
79
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
5.1. Conclusions
The accelerator ageing test showed comparable performance of the inhibited locally extracted
soybean oil with that of mineral oil. The investigation of the electrical, physical and chemical
properties of the treated locally extracted soybean oil shows that the oil possesses good
properties that enable it to be used as a suitable dielectric fluid for power transformers.
The properties of the soybean oil have been investigated against standards, and the result shows
that the treated soybean oil has a very good potential to be used as an insulation and heat transfer
fluid. The breakdown voltage, which is of paramount importance to dielectric fluids, has been
found to be adequate. Results from the study include the following:
1. Both the mineral oil and the synthetic ester oil exhibited lower breakdown voltages at the
end of the ageing process than at the beginning. For the mineral oil, BDV value reduced
from 58.32 kV (new oil) to 36. 43 kV (end of ageing). For the soy bean oil, the BDV
value reduced from 59.08 kV (new oil) to 36.38 kV (end of ageing). Nevertheless, these
values are still within the IEC 60247 recommended limit.
2. The tan � of the inhibited soy bean oil increased significantly along the ageing process
from 1.58 x 10j4 (new oil) to 12.11 x 10j4(end of ageing).However, it is still within the
recommended limit of IEC 60247.
3. We were able to formulate an integrating accelerated ageing procedure to determine
dielectric loss test (tan�) and dielectric strength test (breakdown voltage) procedure
within an experimental schedule.
80
5.2. Recommendation
The following are recommended to ensure efficient operation of the Nigeria grid system:
1. In order to achieve more information about the effect of the accelerated thermal ageing
process on oil, we need to perform chemical measurements such as the anisidine value to
measure oxidation.
5.3. Suggestion for Further Studies
Some suggestions are made to improve the research in this field, which can resort to the new
measurement methods or analysis.
1. Further research work should be conducted on the effect of antioxidant to the inhibited soy
bean oil for transformer insulation, also, its effect on the environment.
2. Research work should be carried out on the design of power transformer that will run on
vegetable oil.
81
REFERENCES
[1] D .N. Tanteh, S .Y. Al-tiddawl, D. Ssekasiko “Properties of transformer oil that affect
efficiency”, Department of electrical engineering, bleking institute of technology, Jan.
2014.
[2] P. M. Paris. “Silicone oils for transformer (in French)”, E. O. F. Bulletin des e’tudes et
rechesche-series B, Reseanx elecfrigues, materials electrigues, N. pp 5-13. 1987.
[3]A. Darwin, C. P. Follist. “The use of natural ester fluids in transformers” mat past Avera T
and N, UK, 2007.
[4] Y. Bertrand, L. C. Hoang. “Vegetable oils as substitute for mineral insulating on in medium
voltage equipment”, Proceedings, CIGRE Conference, Paris, NO. D1-202, 2004.
[5] L. Honary . “Biodegradable/Biobased lubricants and greases.” Machinery lubrication wayazi
issues Number 200109 Nonia Corporation, 2004. www.oilmentenance.com
[6] R. C. Gunther .” Lubrication”. Bailey brother and swinfen ltd, folkestone.1971.
[7] J. Voukeletos, K. Arsitopoulos, Copper, P. Stenborg, J. Luksich. “Natural ester (vegetable
oil) dielectric fluid application in transformers”, cooper power systems, McGraw-
Edison Development Corp, Attens Greece, 2009.
[8]R. A. Ferrani, O. V. Veira, A. Scabio. “ Oxidative stability of biodiesel from soybean oil
fatty acid ethyl esters”. Sci. Agric (Piracicaba, Braz.), vol. 62, No3, pp 291-295, 2004.
[9] Eastman Chemical Company. “High performance additives.”2007.
www.eastman.comeastmanchemicalcompnay,kingsport,TN,USA.
[10] K. S. Morleza, M. Saeedi , S. Shashani. “Antioxidant activity of the methanslic extracts of
some species of plomis and studys on sunflower oil”. Afr. J. Biotechnol. Vol. 5, No.
24, pp 2428-2432, 2006.
[11] E. O. Aluyor, M. Ori-Jesu. “The use of antioxidants in vegetable oils- A review” African
Journal of Biotechnology Vol. 7, No 25, pp. 4836-4842, 29 Dec. 2008.
82
[12] S. Howell . “Promising industrial application for soybean oil in the US”, 2007.
[13] M. Bennion. “Introductory foods. 10th edition”. Prentle-Hall Inc., Upper Saddle River,
New Jersey, USA, 1995.
[14] S. M. Bashi, U. U. Abdullahi, R. Yonus and A. Nordin “Use of natural vegetable oils as
alternative dielectric transformer coolants” Journal of the institution of Engineers,
Welaysia, Vol. 67, No 2,pp4-9, June 2006.
[15] R. Bartnikas, "Dielectrics and Insulators," in the Electrical Engineering Handbook, Second
Edition, R. C. Dorf, Ed.: CRC Press, 1997.
[16] G. Daemisch, "Geriatrics of transformer.", Journal of Electrical System, vol. 13,pp 483-485,
2006.
[17] M. Mirzai, A. Gholami, and F. Aminifar, "Failures analysis and reliability calculation for
power transformers," Journal of Electrical System,vol.2 No 1,pp1-12, 2006.
[18] "IEEE Guide for Loading Mineral-Oil-Immersed Transformers," in IEEE Std C57.91, 1995.
[19] V. M. Montsinger, "Loading Transformers by Temperature," AIEE Transactions, vol. 49,
pp. 776-792, 1930.
[20] T. W. Dakin, "Electrical Insulation Deteriation Treated as a Chemical Reaction Rate
Phenomenon," AIEE Transactions, vol. 66, pp. 113-122, 1947.
[21 T.V. Oommen, "Vegetable Oils for Liquid‐filled Transformers", IEEE Electrical Insulation
Magazine,No18, Vol.1, pp 7-11, 2002.
[22] "IEEE standard test procedure for thermal evaluation of oil-immersed distribution
transformers," in IEEE Std C57.100, 1986.
[23] M. R. Yenchek, "Mechanical Performance of Thermally Aged Trailing-Cable Insulation "
IEEE Transaction on Industry Applications, vol. 25, pp. 1000-1005, 1989.
83
[24] M. Horning, J. Kelly, S. Myers, and R. Stebbins, “Transformer Maintenance Guide 3rd
edition”: Transformer Maintenance Institute, 2004.
[25] IEC, "IEC 61125 - Unused Hydrocarbon-based Insulating Liquids Test Methods for
Evaluating the Oxidation Stability," 1992.
[26] Q. Liu, J. Zhao, X. Wang, L. Zhong, Q. Yu, X. Chen, X. Cao, M. Hanai, and S. Mori,
"Experimental Research on the Streaming Electrification on Transformer Oil under
Ageing," IEEE International Conference on Condition Monitoring and Diagnosis, 2008.
[27] A. Bradwell, “Electrical Insulation. London”,: Peter Peregrinus Ltd., 1983.
[28] M. Naidu, V kamaraju. “High Voltage engineering second edition”, McGraw-Hill.pp: 49-
51, 1996.
[28] L C. Ancho “Oxidation in lubricant base oil”. The filter, No 4, February 2006.
[29] Yehye, Wagech, A. Rahman, N. Abdul, Ariffin, Azhar, Abd Hamid, S. Bee, Akadi,A.
Abeer , Kadir, Farkead, Yaeghoobi, Marzieh . “Understanding the chemistry behind the
antioxidant activities of butylated hydroytocluse (BHT): A review”. European journal
of medicinal chemistry.vol 101, pp 295-312 , 2015.
[30] H. Fiege, H. Voges, T. Hamamoto, S. Umemura, T. Iwata, H. Miki, Y. Fuyita, H.
Buysch, D. Garbe, W. Paulus . “A phenol denvatives in ollammn’s encyclopedia of
industrial chemistry”, wiley-VCH, Weinheim, 2002.
[31]A. Shkolnik “Oxidation inhibitor and rein habiting oil-filled transformer, Technical brief,
July, 2013.
[32] IEC “IEC 61125 – Unused Hydrocarbon-based insulating liquids test methods for
evaluating the oxidation stability,” 1992.
[33] Bartnikas, “Dielectrics and insulators”, in the Electrical engineering handbook, Second
Edition, R.C. Dorf. Ed: CRC Press. 1997.
84
[34] Megger “The Megger guide to insulating oil dielectric breakdown testing”,part No.2003-
149-v 01,2003.
[35] International standard IEC 60296 third edition 2003-11 reference number IEC 60296:
2003(E).
[36] H. L. Withis, G. V. Welch, and R. R. Schrieder, Ageing power delivery infrastructure oil.
12 New York: Marcel Dekker, Inc, 2001.
85
Appendix A:
86
Appendix B:
87
Appendix C: Showing Megger, Breakdown Voltage Testing Machine
88
Appendix D: Showing the Panel Label of Megger
89
1‐35, 40‐42, 44‐50, 52‐57, 59‐61, 65‐71
36‐39, 43, 51, 58, 62‐64, 74‐75