Embed Size (px)
Citation preview
THE EFFECT OF THERMAL AGEING ON LOW VOLTAGE CROSS-LINKED
POLYETHYLENE INSULATED CABLE
NOOR FAEZAH BINTI HAMDI
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Forensic Engineering.
School of Civil Engineering
Faculty of Engineering
Universiti Teknologi Malaysia
JANUARY 2019
iii
DEDICATION
This thesis is dedicated to my parents
for their love , endless support
and encouragement.
iv
ACKNOWLEDGEMENT
"In The Name Of Allah, The Most Beneficient,The Most Merciful"
I would first like to thank my main supervisor, Dr. Mohd Azreen bin Mohd
Ariffin for his continuous support, motivation and encouragement. I am forever
grateful for his kind advice and guidance in helping me for the completion of this
thesis.
My research would have been impossible without the aid and support of Ir.
Dr. Lau Kwan Yiew from School of Electical Engineering. His guidance and
immerse knowledge helped me in all the time of research experiment.
My sincere thanks also goes to the Jabatan Perkhidmatan Awam for the study
opportunity and providing the financial support for me to further my studies at the
Universiti Teknologi Malaysia, Johor. Not to forget, my current employer, Jabatan
Kerja Raya Malaysia for the opportunities and providing me the access, data and
sample for the this research. Financial assistance from the Universiti Teknologi
Malaysia for my research work is highly appreciated.
Finally, to my families and friends, thanks you for being there with your
unconditional love and continuous support.
v
ABSTRACT
This research presents some results and discusses the investigation on thermal
ageing of low voltage cross-linked polyethylene (XLPE) insulated cable rated at
0.6/1(1.2) kV. This type of cable was found to experience premature degradation
within three to five years of service. The objective of this investigation is to study the
characteristic of XLPE insulated cable under thermal conditions. A number of tests
were conducted to investigate the chemical properties, electrical properties and
mechanical properties of unaged and aged XLPE insulated cable. Samples were
thermally aged using air ventilated oven at temperature of 90 oC. The breakdown
strength of XLPE materials were measured for each samples. The structural changes
due to the formation of carbonyl groups were observed using Fourier Transform
Infrared (FTIR) spectroscopy. The application of Differential Scanning Calorimetry
(DSC) revealed the effect of thermal ageing on the crystallinity variation on XLPE
insulated cable. Mechanical properties were observed through tensile strength test.
Experimental findings in this study suggests that thermal ageing induces significant
degradation on XLPE insulated cable.
vi
ABSTRAK
Kajian ini menerangkan hasil siasatan dan ujian terhadap kabel voltan rendah
jenis cross-linked polyethylene (XLPE) dengan kadaran penebat pada 0.6 / 1 (1.2)
kilo volts. Kabel jenis ini didapati semakin kerap mengalami kerosakan akibat
pereputan awal dalam tempoh operasi antara tiga hingga lima tahun. Objektif
siasatan ini adalah untuk mengkaji ciri-ciri kabel jenis XLPE apabila diletakkan
dalam keadaan berhaba tinggi. Beberapa siri ujian telah dibuat untuk mengkaji sifat
kimia, elektrikal dan mekanikal kabel jenis XLPE dalam keadaan baru dan lama.
Kabel dipanaskan pada suhu 90 OC di dalam ketuhar sebagai kaedah penghasilan
kabel lama / tua. Kekuatan dielektrik untuk setiap sampel XLPE diukur. Perubahan
struktur bahan akibat pembentukan kumpulan karbonil turut diteliti menggunakan
teknik spektroskopi Fourier Transform Infrared (FTIR). Penggunaan teknik
Differential Scanning Calorimetry (DSC) membolehkan kesan penuaan haba
terhadap perubahan pada penghabluran bahan XLPE. Ciri mekanikal bahan diuji
melalui ujian kekuatan tegangan bahan. Hasil dari kajian ini mendapati bahawa
kesan penuaan haba telah menyebabkan kemerosotan jangka hayat kabel jenis voltan
rendah XLPE.
vii
TABLE OF CONTENTS
TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF ABBREVIATIONS xiii
LIST OF SYMBOLS xiv
LIST OF APPENDICES xv
CHAPTER 1 INTRODUCTION 1
1.1 Problem Background 6
1.2 Problem Statement 6
1.3 Research Objectives 7
1.4 Research Scope 7
1.5 Significance of the Study 5
CHAPTER 2 LITERATURE REVIEW 7
2.1 Introduction 7
viii
2.2 Characteristics of XLPE Material 9
2.3 Thermal Ageing of the XLPE Material 9
2.4 Oxidation in XLPE Insulation 11
2.5 Electrical Properties of XLPE Insulation 12
2.6 Mechanical Properties of XLPE Insulation 13
2.7 Crystallinity of XLPE Insulation 13
CHAPTER 3 RESEARCH METHODOLOGY 15
3.1 Overview of Research Design 15
3.2 Materials and Preparation 16
3.2.1 Materials 16
3.2.2 Preparation of Materials 19
3.2.3 Thermal Ageing Procedure 19
3.3 Experimental Techniques 20
3.3.1 Breakdown Strength Test 20
3.3.2 Fourier Transform Infrared Spectroscopy 23
3.3.3 Tensile Strength Test 24
3.3.4 Differential Scanning Calorimetry 26
CHAPTER 4 RESULT 29
4.1 Breakdown Strength Test 29
4.2 Fourier Transform Infrared Spectroscopy 33
4.3 Tensile Strength Test 37
4.4 Differential Scanning Calorimetry 42
ix
CHAPTER 5 DISCUSSION AND CONCLUSION 45
5.1 Discussion of the Results 45
5.1.1 Breakdown Strength Test 45
5.1.2 Fourier Transform Infrared Spectroscopy 46
5.1.3 Tensile Strength Test 51
5.1.4 Differential Scanning Calorimetry 52
5.2 Conclusion 54
REFERENCES 57
APPENDICES 61
x
LIST OF TABLES
TABLE NO. TITLE PAGE
Table 4.1 Breakdown strength data of cable A and cable C. 23
Table 4.2 Breakdown strength data of cable B and cable C. 24
Table 5.1 Carbonyl compound group frequencies 42
Table 5.2 Functional group found in XLPE Fresh A1 43
Table 5.3 Functional group found in XLPE Fresh B1 44
Table 5.4 Functional group found in XLPE Aged A2 45
Table 5.5 Functional group found in XLPE Aged B2 46
Table 5.6 Functional group found in XLPE Degraded 47
Table 5.7 Crystallinity variation from DSC measurement
(cooling) for XLPE material of cable A and cable C.
51
Table 5.8 Crystallinity variation from DSC measurement
(cooling) for XLPE material of cable B and cable C.
51
xi
LIST OF FIGURES
FIGURE NO. TITLE PAGE
Figure 1.1 A single core low voltage XLPE insulated cable. 2
Figure 1.2 A multi-core low voltage XLPE insulated cable. 3
Figure 1.3 Degraded red core XLPE cable found in switchboard. 4
Figure 1.4 Degraded XLPE insulated underground cable. 5
Figure 2.1 The ATR principle 9
Figure 3.1 Experimental works 13
Figure 3.2 25 mm2 low voltage XLPE insulated cable from
manufacturer A.
14
Figure 3.3 25 mm2 low voltage XLPE insulated cable from
manufacturer B.
14
Figure 3.4 25 mm2 low voltage XLPE insulated cable from
manufacturer C.
15
Figure 3.5 Oven (Constance Model FC-9023A) for heating
process.
16
Figure 3.6 Breakdown strength test configuration. 17
Figure 3.7 Sample immersed in transformer oil. 18
Figure 3.8 Damaged surface on sample due to breakdown voltage
at 38 kilovolts.
20
Figure 3.8 Shimadzu FTIR Spectrophotometer Model IRTRacer-
100
20
Figure 3.9 LRX 2.5kN Lloyd Tensile Tester. 21
Figure 3.10 Dumb-bell shaped sample piece for tensile test. 21
Figure 3.11 DSC 1 STAR System equipment. 22
Figure 4.1 Weibull plots comparing the AC breakdown strength 24
xii
of XLPE insulation layer of fresh and aged for cable
by Manufacturer A, and degraded cable.
Figure 4.2 Weibull plots comparing the AC breakdown strength
of XLPE insulation layer of fresh and aged for cable
by Manufacturer B, and degraded cable.
25
Figure 4.3 ATR spectrum for sample XLPE Fresh A1. 26
Figure 4.4 ATR spectrum for sample XLPE Fresh B1. 27
Figure 4.5 ATR spectrum for sample XLPE Fresh A2. 28
Figure 4.6 ATR spectrum for sample XLPE Fresh B2. 29
Figure 4.7 ATR spectrum for sample XLPE Degraded. 30
Figure 4.8 The stress-strain curve of sample XLPE Fresh A1
(kept at room temperature).
31
Figure 4.9 The stress-strain curve of sample XLPE Fresh B1
(kept at room temperature).
32
Figure 4.10 The stress-strain curve of sample XLPE Aged A2
(after heated at temperature 90 OC for 50 hours )
33
Figure 4.11 The stress-strain curve of sample XLPE Aged B2
(after heated at temperature 90 OC for 50 hours)
34
Figure 4.12 DSC thermogram for sample XLPE Fresh A1. 36
Figure 4.13 DSC thermogram for sample XLPE Fresh B1. 37
Figure 4.14 DSC thermogram for sample XLPE Fresh A2. 38
Figure 4.15 DSC thermogram for sample XLPE Fresh B2. 39
Figure 4.16 DSC thermogram for sample XLPE Degraded. 40
xiii
LIST OF ABBREVIATIONS
XLPE - Cross-linked polyethylene
PVC - Polyvinyl chloride
EPR - Ethylene propylene rubber
IEC - International Electrotechnical Commission
JKR - Jabatan Kerja Raya
EMAL - Electrical Material Approved List
FTIR - Fourier transform infrared
DSC - Differential scanning calorimetry
DCP - Dicumyl peroxide
ATR - Attenuated total reflectance
AC - Alternating current
Hz - Hertz
C-H - Carbon-hydrogen
OH - Oxygen hydrogen (Hydroxy)
xiv
LIST OF SYMBOLS
EB - Breakdown field
VB - Breakdown voltage
d - sample thickness
F(x) - Cummulative probability of failure
exp - exponential
x - Breakdown voltage
α - Scale parameter
β - Shape parameter
χc - Crystallinity
ΔH0 - Enthalpy corresponding to the melting of 100% crystalline
material
ΔH - Heat of fusion
xv
LIST OF APPENDICES
APPENDIX TITLE PAGE
APPENDIX A IEC 50602-1 (2004) 61
1
CHAPTER 1
INTRODUCTION
1.1 Problem Background
Cross-linked polyethylene (XLPE) is manufactured from polyethylene (PE)
plastic with a three-dimensional molecular bond that is created within a structure of a
plastic. Manufacturing process of XLPE is done by adding small amounts of
chemical additives (such as antioxidants) to the polymer. Additives help improve the
flow properties of a polymer that will be moulded. This process allows the molecular
chains to form a lattice formation by cross-linking the polymer under appropriate
treatment after extrusion (Reeves and Heathcote, 2003).
The process of cross-linking changes its original state from thermoplastic to
thermoset (Tamboli et al, 2004). As a thermoset polymer, it consists of long chain of
molecules that form bonds between the chains during polymerisation. This forms a
single network of polymer chains that are generally not straight, but a tangled mass.
XLPE is commonly used in electrical cable construction for a wide range of
voltage. A study by Dilip K and Das Gupt (1997) had shown the benefis of using
XLPE as insulation material in general are due to their great physical, chemical and
dielectric properties. XLPE is favoured over other insulation material, such as
polyvinyl chloride (PVC), ethylene propylene rubber (EPR) and silicone rubber due
to its toughness against high temperature. This material also has high insulation
resistance, excellent dielectric strength and a low dissipation factor at all frequencies.
Low voltage is a voltage between the range of 50 volts and 1,000 volts
alternating current, as defined by the International Electrotechnical Commission
(IEC). Low voltage XLPE insulated cable are used in low voltage (600/1000 volts)
applications. In Malaysia, this type of cable is designated for general use and for
2
underground cabling, where lightness and convenience of terminating are major
considerations. XLPE insulated cable also withstands smaller radius bending,
allowing for easy and reliable installation.
The design of low voltage XLPE insulated cable must conform to IEC 60502
(2004) and rated at 600/1000 (1,200) kilovolts. It has plan circular conductors made
of compacted stranded copper or stranded aluminium. The XLPE insulation layer is
rated at temperature of 90 OC. A typical construction of a low voltage XLPE
insulated cable is illustrated in figure 1.1 and figure 1.2.
Figure 1.1 A single core low voltage XLPE insulated cable.
3
Figure 1.2 A multi-core low voltage XLPE insulated cable.
1.1 Problem Background
Previous studies and researches showed that XLPE cable has a great
performance over high temperature and could last 40 years or more (Anwar ul-
Hamid et al, 2015). However, there are increments in the number of complaints for
insulation failure in low voltage XLPE insulated cable within 5 years of service. This
leads to a temporary suspension in the usage of XLPE insulated cable. The approval
for this type of cable from sizes between 1.5 mm2 up to 25 mm2 has been withdrawn
from the Jabatan Kerja Raya (JKR) Electrical Material Approved List (EMAL).
During installation, cables are subjected to repeated bending or random
flexing causing its copper conductors to be stressed. Mechanical stresses during
installation are generally more severe than those encountered while in service.
Physical damage caused by mishandling of the cables may cause injury to the
4
insulation layer. Degraded cable can often be found in the electrical switch board is
as shown in figure 1.3
Figure 1.3 Degraded red core XLPE cable found in switchboard.
5
In the operating environment, constant abrasion of the cable by other cables
could cause failure in cable insulation. Environmental factors such as high humidity
and elevated temperatures can also degrade XLPE insulation layer. Moisture may
exist in cable surroundings and it could affect the basic characteristics of the XLPE
insulation and the realibility of XLPE insulated cables (Mecheri et al, 2010).
According to Jong-Il Weon (2009), the degradation of XLPE is also caused by
exposure to heat, ultraviolet radiation, discolouration among others. Figure 1.4 shows
the degraded XLPE insulated underground cable.
Figure 1.4 Degraded XLPE insulated underground cable.
6
1.2 Problem Statement
A reliable study on understanding how the degradation mechanism especially
on low voltage system is still lacking although there area high numbers of published
papers related to degradation of XLPE material. A reliable method of investigating
the insulation failure, failure analysis and failure origin must be studied. The
degradation on low voltage XLPE insulated cable is investigated using several
methods comprise of the study of chemical, electrical and mechanical properties of
the XLPE material.
1.3 Research Objectives
This work aims to fabricate a forensic investigation process for cable
insulation failure. Therefore, there is a potential for improvement in identifying the
cause that made the material fails to perform its intended function. The specific
research objectives are as follow;
i) To identify the primary degradation and failure mechanism of
low voltage XLPE insulated cable.
ii) To characterize and compare the properties of the XLPE materials
under different thermal conditions.
iii) To provide thermal ageing analysis for cable failure.
7
1.4 Research Scope
The scope of research for this experimental study includes;
i) Insulation layer of low voltage XLPE insulated cable was used in this
experiment to study the electrical, mechanical and chemical properties
for both fresh and aged conditions.
ii) Insulation layer of low voltage XLPE insulated cable that had
experienced degradation was also used in this study for reference and
comparison purposes.
iii) The properties and behaviour of each samples were compared and
characterized using breakdown strength test, tensile strength test,
Fourier infrated (FTIR) spectroscopy and differential scanning
calorimetry (DSC).
1.5 Significance of the Study
Eventhough many experimental works have been done to investigate the
degradation of XLPE insulated cable, not much research and publication have been
done particularly for cable degradation in the low voltage cable. Therefore, the
benefits expected from this research are;
i) To determine the condition of low voltage cable failure with the
application of forensic engineering in the investigation.
ii) The use of research information helps to provide forensic engineers
a proper method of doing forensic investigation on cable failure.
57
REFERENCES
E.A Reeves and Martin J. Heathcote (2003) Newnes electrical pocket book. (ed.)
Britain: Newnes Publication
S M Tamboli, S T Mhaske and D D Kale (2004) 'Crosslinked polyethylene', Indian
Journal of Chemical Technology, (11), 853-864
Dilip K. and Das-Gupta (1997) 'Conduction mechanisms and high-field effects in
synthetic insulating polymers', IEEE Transactions on Dielectrics and Electrical
Insulation, 4(2), 149-156
Ul-Hamid, A., Soufi, K. Y., Al-Hadhrami, L. M., and Shemsi, A. M. (2015). 'Failure
investigation of an underground low voltage XLPE insulated cable', Anti-
Corrosion Methods and Materials, 62(5), 281–287.
https://doi.org/10.1108/ACMM-02-2014-1352
International Electrotechnical Commission (2004) IEC 60502-1. Switzerland:
International Electrotechnical Commission
Y. Mecheri, M. Nedjar, A. Lamure, M. Aufray., and C. Drouet (2010), 'Influence of
moisture on the electrical properties of XLPE insulation'. 2010 Annual Report
Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), pp. 1-
4. http://dx.doi.org/10.1109/CEIDP.2010.5724017
Weon, J. Il. (2010). 'Effects of thermal ageing on mechanical and thermal behaviors
of linear low density polyethylene pipe'. Polymer Degradation and Stability,
95(1), 14–20. https://doi.org/10.1016/j.polymdegradstab.2009.10.016
58
Mo Shan-jun, Zhang Jun, Liang Dong and Chen Hong-yin (2013), 'Study on
pyrolisis characteristics of cross-linked polyethylene material cable'. Procedia
Engineering 52 (2013), 588-592. doi:10.1016/j.proeng.2013.02.190
Tetiana Salivon and Xavier Colin (2015), 'Degradatio of XLPE and PVC cable
insulators', 2015 Annual Report Conference on Electrical Insulation and
Dielectric Phenomena 18-21 October. Ann Arbor, Michigan: IEEE 656-659
L. Boukezzi, A. Boubakeur, and M. Lallouani (2010),'Oxidation evaluation of cross-
linked polyethylene under thermal degradation : FTIR study', 5th International
Symposium on Hydrocarbons and Chemistry (ISHC5). 23-25 May, Sidi Fredj,
Algiers:
M. Muhr, E. Neges, R. Woschitz, and Ch. Sumereder (2004), 'Aging behaviour of
cross-linked polyethylene (XLPE) as an insulating material for high (HV) and
extra-high voltage cables (EHV). 2004 Annual Report Conference on Electrical
Insulation and Dielectric Phenomena, Boulder, Colorado:
Nobrega, A. M., Martinez, M. L. B., & De Queiroz, A. A. A. (2013), 'Investigation
and analysis of electrical aging of XLPE insulation for medium voltage covered
conductors manufactured in Brazil'. IEEE Transactions on Dielectrics and
Electrical Insulation, 20 (2), 628–640.
https://doi.org/10.1109/TDEI.2013.6508767
L. Boukezzi, M. Nedjar, L. Mokhnache, M. Lallouani, and A. Boubakeur (2006).
'Thermal aging of cross-linked polyethylene'. Annales de Chimie: Science Des
Materiaux, 31(5), 561 – 569. https://doi.org/10.3166/acsm.31.561-569
59
Emna Khouildi, Rabah Attia, and Rafik Cherni (2017), 'Investigating thermal affect
on a cross-linked polyethylene power cable', Indonesian Journal of Electrical
Engineering and Computer Science, 5(1), 33-40.
https://doi.org/10.11591/ijeecs.v5.i1.pp33-40
Y. Mecheri, S. Bouazabia, and A. Boubakeur, and M.Lallouani (2013). Effect of
thermal Ageing on the Properties of XLPE as an Insulating Material for HV
Cables. International Electrical Insulation Conference, May 2013, Birmingham,
United Kingdom:
C.P Martin, A.S Vaughan and S.J Sutton (2003), 'The thermomechanical behaviour
of crosslinked polyethylene cable insulation material' 2003 Annual Report
Conference on Electrical Insulation and Dielectric Phenomena 88-91. 19-22
Oct, Alburquerque, New Mexico
Kwan Yiew Lau Structure and electrical properties of silica-based polyethylene
nanocomposites, PhD Thesis Universiti of Southampton, 2013
Weon, J. Il. (2010). 'Effects of thermal ageing on mechanical and thermal behaviors
of linear low density polyethylene pipe'. Polymer Degradation and Stability,
95(1), 14–20. https://doi.org/10.1016/j.polymdegradstab.2009.10.016
Geng, P., Song, J., Tian, M., Lei, Z., & Du, Y. (2018). Influence of thermal aging on
AC leakage current in XLPE insulation. AIP Advances, 8(2).
https://doi.org/10.1063/1.5017297
M. Blazso, B.Zelei, and E. Jakab (1995),'Thermal decomposition of low-density
polyethylene in the presence of chlorine-containing polymers', Journal of
