A PRELIMINARY STUDY OF LOSS FACTOR (tan ) IN DIELECTRIC OF CABLE PRODUCED IN THAILAND
MR.ANUCHIT AURAIRATCH
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN ELECTRICAL POWER ENGINEERING THE SIRINDHORN INTERNATIONAL THAI-GERMAN GRADUATE SCHOOL OF ENGINEERING (TGGS) GRADUATE COLLEGE KING MONGKUT’S INSTITUTE OF TECHNOLOGY NORTH BANGKOK ACADEMIC YEAR 2006 COPYRIGHT OF KING MONGKUT’S INSTITUTE OF TECHNOLOGY NORTH BANGKOK
Name
: Mr.Anuchit Aurairatch
Thesis Title
: A Preliminary Study of Loss Factor (tan ) in Dielectric of Cable Produced in Thailand
Major Field
: Electrical Power Engineering King Mongkut’s Institute of Technology North Bangkok
Thesis Advisors : Dr.Teratam Bunyagul Assistant Professor Srawut Kleesuwan Academic Year : 2006
Abstract
The thesis studies the influence of temperature, voltage levels and moisture on cables made in Thailand. The dissipation factor (tan
)
is used to indicate the
properties of new and old cables (5-10 years old). The results show that the temperature affects the tan tan
of the old cables. cables. The moisture moisture affects the change of
of both old and new cables. Nevertheless, the value of tan
is still less than
-4
80x10 . (Total 69 pages)
Keywords : Dissipation Factor, Loss Factor, tan , XLPE Cable
______________________________________________________________Advisor ii
ชื ่ ่ชือ ชื ่ ่ชือวิ อวิทยานิ ทยานิพนธ พนธ
นายอนุชิชติ อุไรรั ไรรัตน ตน : การศึกษาองค กษาองคประกอบการสู ประกอบการสูญเสี ญเสียในฉนวนของสายเคเบิ ยในฉนวนของสายเคเบิลที ่ ่ ลทีผลิ ผลิตใน ตใน ประเทศไทย : วิศวกรรมไฟฟ สาขาวิชา ชา ศวกรรมไฟฟากํ ากําลั าลัง สถาบันเทคโนโลยี นเทคโนโลยีพระจอมเกล พระจอมเกลาพระนครเหนื าพระนครเหนือ ที ่ ่ปรึ ปรึกษาวิ กษาวิทยานิ ทยานิพนธ พนธ : อาจารย ดร.ธีรธรรม รธรรม บุณยะกุ ณยะกุล ผูชชวยศาสตราจารย ว ยศาสตราจารยศราวุ ศราวุฒิฒิ คลี ่ ่ คลีสุสวรรณ วุ รรณ : 2549 ปการศึ การศึกษา กษา :
บทคัดย ดยอ
วิทยานิ ทยานิพนธ พนธนี ้ ้นีนํนาเสนอการวิ าํ เสนอการวิจัจัยอิ ยอิทธิ ทธิพลของอุ พลของอุณหภู ณหภูมิมิ แรงดันไฟฟ นไฟฟาและ าและ ความชื ้ ้ ความชืนที ่ ่ นทีมีมผลต ีผลตอสาย อสาย เคเบิลที ่ ่ ลทีผลิ ผลิตในประเทศไทยโดยใช ตในประเทศไทยโดยใชคคาตั า ตัวประกอบกํ วประกอบกําลั าลังสู งสูญเปล ญเปลาไดอิ าไดอิเล็เล็กทริ กทริก (tan ) เปนตั นตัวชี ้ ้ วชีวัวัด คุณสมบั ณสมบัติตโดยใช โิ ดยใชเคเบิ เคเบิลใหม ลใหมและเก และเกา (5 ป และ 10 ป) เปนตั นตัวอย วอยางทดสอบ างทดสอบ ผลการวิจัจัยพบว ยพบวาอุ าอุณหภู ณหภูมิมิ มีผลต ผลตอการเปลี ่ ่ อการเปลียนแปลงของ ยนแปลงของ tan ของเคเบิลเก ลเกา ความชื ้ ้ ความชืนมี นมีผลต ผลตอการเปลี ่ ่ อการเปลียนแปลงของทั ้ ้ ยนแปลงของทังเคเบิ งเคเบิล ใหมและเก และเกาเมื ่ ่ าเมือแรงดั อแรงดันเปลี ่ ่ นเปลียนแปลง ยนแปลง แตคคาของ า ของ tan ยังคงมี งคงมีคคาน านอยกว อยกวา 80x10 ทยานิพนธ พนธมีมจํีจานวนทั ้ ้ ํานวนทังสิ ้ ้ งสิน 69 หนา) (วิทยานิ -4
คําสํ าสําคั าคัญ : องคประกอบการสู ประกอบการสูญเสี ญเสีย, tan , สายเคเบิลชนิ ลชนิด XLPE
อาจารยที ่ ่ทีปรึ ปรึกษาวิ กษาวิทยานิ ทยานิพนธ พนธ
_______________________________________________________
iii
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Dr.Teratam Bunyagul and Asistant Professor Srawut Kleesuwan for their helpful guidance, suggestions and encouragement throughout this study. I am also indebted to MEA team for supporting me the XLPE cables and the dissipation factor record. I would like to thank my mother, teachers, family, friends and the staff of the Electrical Engineering Department, King Mongkut’s Institute of Technology North Bangkok, for their helpful suggestions and valuable assistance throughout the entire research. Finally, I would like to thank Rajamangala University Institute of Technology Rattanagosin Wangklaikangwon Campus for providing financial support from the research fund.
Anuchit Aurairatch
iv
TABLE OF CONTENTS
Page Abstract (in English)
ii
Abstract (in Thai)
iii
Acknowledgements
iv
List of Tables
vii
List of Figures
viii
Chapter 1 Introduction
1
1.1 Introduction
1
1.2 Diagnostic techniques for insulating material
1
1.3 Insulation high voltage power cable
2
1.4 Dielectric properties of insulation
3
1.5 Dissipation factor
4
1.6 XLPE Cable
4
1.7 The literature survey of the cable test
7
1.8 Conclusion of the literature survey of the cable test
10
1.9 The research project
11
Chapter 2 The principle of dielectric loss and measurement
13
2.1 Dielectric loss
13
2.2 Dielectric loss measurement
21
2.3 The test result interpreted
26
2.4 The standard for XLPE dissipation factor
27
Chapter 3 The methodology of cable test
29
3.1 Introduction
29
3.2 Cable setup
29
3.3 Stress control
30
3.4 Partial discharge protection
31
3.5 Testing transformer
32
3.6 Heating current setup
33
3.7 Schering bridge setup
33
3.8 Experimental detail
35
Chapter 4 Experimentation results
37
v
TABLE OF CONTENTS (CONTINUED)
Page 4.1 The dissipation factor (tan ) of new cable and used cable
37
4.2 This dissipation factor (tan ) of new cable and used cable under wet condition
43
4.3 The comparison of the dissipation factor test result
51
4.4 The conclusions of the test result
58
Chapter 5 Conclusions and recommendations 5.1 The conclusions of the new cable and the used cable test
59 59
5.2 The conclusions of the new cable under wet condition and the 10-year used cable under wet condition
59
5.3 Conclusions
59
5.4 Recommendations
61
References
62
Appendix A Water tree
64
Appendix B Dielectric losses
67
Biography
69
vi
LIST OF TABLES
Table
Page
1-1 Typical Electrical Properties for crosslink Polyethylene Insulation Used in Transmission Cable Applications
5
4-1 The dissipation factor of 12/20 (24 kV) XLPE new cable
38
4-2 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable
40
4-3 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable
42
4-4 The dissipation factor of 12/20 (24 kV) XLPE new cable under wet condition
44
4-5 The dissipation factor of new cable under wet condition
46
4-6 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable under wet condition
48
4-7 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable under wet condition
50
vii
LIST OF FIGURES
Figure
Page
1-1 The cable’s tan
4
1-2 Cable Construction
5
2-1 The cable equivalent circuit
17
2-2 Schering bridge
21
2-3 Vector diagram parallel circuit of Cx and Rx
23
2-4 Very low frequency
25
2-5 The example of VLF test
25
2-6 The tan loss of new and aged 15kV XLPE cable
27
3-1 Cable setup
29
3-2 Sliding discharge
30
3-3 Uncontrolled cable end-potential distribution
30
3-4 Stress control
31
3-5 External partial discharge
31
3-6 Partial discharge protection
32
3-7 Testing transformer setup
32
3-8 Heating current setup
33
3-9 Schering bridge
34
3-10 Null indicator
34
3-11 The dissipation factor measuring setup
35
3-12 Cable under wet condition
36
4-1 The dissipation factor of 12/20 (24 kV) XLPE new cable
38
4-2 The dissipation factor of 12/20 (24 kV) XLPE new cable versus temperature with four different voltage (0.5U0 to 2U0)
39
4-3 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable versus voltage with nine different temperature (20°C to 100°C)
40
4-4 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable versus temperature with four different voltage (0.5U0 to 2U0)
viii
41
LIST OF FIGURES (CONTINUED)
Figure
Page
4-5 The dissipation factor of 12/20 (24kv) XLPE 10-year used cable versus voltage with nine different temperature (20°C to 100°C)
42
4-6 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable versus temperature with four different voltage (0.5U0 to 2U0)
43
4-7 The relationship of dissipation factor versus voltage of new cable under wet condition with ten different time (100 hours t o 1000 hours)
45
4-8 The relationship of dissipation factor versus the test time of the new cable under wet condition with four different voltage (0.5U 0 to 2U0)
45
4-9 The relationship of dissipation factor versus voltage of new cable wet condition with eight different temperature (20°C to 100°C)
46
4-10 The dissipation factor versus temperature of new cable under wet condition
47
4-11 The relation ship of dissipation factor versus voltage of new cable under wet condition with ten different time (100 hours to 1000 hours)
48
4-12 The relation ship of dissipation factor versus the test time of the 10-year used cable under wet condition with four different voltage (0.5U0 to 2U0)
49
4-13 The relation ship of dissipation factor versus voltage of 10-year used cable under wet condition with eight different temperature (20°C to 90°C)
50
4-14 The dissipation factor versus temperature of 10-year used cable under wet condition
51
4-15 The comparison of the dissipation factor of the new cable and the used cable
52
4-16 The comparison of the dissipation factor of the new cable and used cable
53
ix
LIST OF FIGURES (CONTINUED)
Figure
Page
4-17 The comparison of dissipation factor of the new cable and the new cable under wet condition
54
4-18 The comparison of dissipation factor of the new cable and the new cable under wet condition
54
4-19 The comparison of dissipation factor of the new cable under wet condition and the 10-year used cable under wet condition
55
4-20 The comparison of dissipation factor of the new cable under wet condition and the 10-year used cable under wet condition
56
4-21 The comparison of dissipation factor of the new cable, 5-year used cable, 10-year used cable, new cable under wet condition and 10-year used cable under wet condition
57
4-22 The comparison of dissipation factor of new cable, 5-year used cable, 10-year used cable, new cable under wet condition and 10-year used cable under wet condition
57
A-1 The water tree
65
A-2 The water tree
65
A-3 The extent of water tree damage in the cable
66
B-1 Diagram of currents in a lossy dielectric
68
x
CHAPTER 1 INTRODUCTION 1.1 Introduction
The underground cables of distribution and transmission systems are the major investment for electrical utilities. These cables must be highly reliable in order to decrease lost of revenues suffered from a premature failure. From an operator point of view, it is of great importance to be continuously informed about the degradation in electrical strength. Electrical utilities are faced with decisions to maintain, repair, refurbish, or replace their cable systems. In order to make these decision, the condition of
the
cable
system
needs to be
determined by knowing [1]: 1. How do the different components of the system age, i.e. aging factor ? 2. The system operating conditions 3. How to measure the aging by diagnostics 4. Criterias for repair, refurbish or replace The
cause
of
cable
failures
stem
from
electrical, thermal
and
environmental stresses. Temperature, age, humidity, water content, load and usage contribute to the deterioration of underground cables [2]. A common cause of cable failures is the wearing and breakdown of the insulation. Insulating materials are dielectrics, which prevent the flow of current when the input voltage is applied. The major insulation used in cables are Cross Llinked Polyethylene (XLPE).
1.2 Diagnostic Techniques for Insulating Materials [3]
In the high voltage industry today, a major problem is determining the life expectancy of
insulator in an underground power cable. The properties of
insulating materials are subjected to physical and chemical deterioration. The deterioration processes are contributed by moisture content, environmental stresses, thermal and electrical aging. The information about the condition of the
2 insulation can be obtained using non-destructive diagnostic methods. The voltage and current are measured to determine the polarization and conduction process. Therefore the deterioration of insulations can be investigated. The non-destructive diagnostic methods used to determine the condition of
insulation include:
1. Direct Current (DC) High Voltage Method 2. Polarization Spectrum 3. Voltage Response Method 4. Return Voltage Method 5. Interface Diffusion Method
1.3 Insulation – High Voltage Power Cable [4]
In order to protect the conductors and ensure they carry the full capacity of the input load, insulation must be physically and chemically in good condition. The type of electrical insulation used in the cable distinguishes all underground cables. There are basically three kinds of cable insulation: tape insulation, solid insulation and gas insulation. The tape insulation is generally oil-impregnated cellulose paper.
This
insulation can be pressurized. The pressured type represents the best and most stable insulating system. The pressured type is then broken into two categories, self-contained
oil-filled
cables
and
pipe
type
oil-filled
cables.
Oil-paper
insulation is still used in the industry today. Solid insulation is usually extruded on to the conductor, with many different materials used. These materials include polyethylene, butyl rubber, ethylene-propylene copolymer, or ethylene-propylene-dien terpolymer and crosslinked polyethylene. Gas insulated cables are mainly used for a long distance. Compressed sulfur fluoride is usually the preferred type of gas. The thickness of insulation in a power cable is determined by the cable ability to withstand steady state alternating current (AC) voltage and transient lightning impulses as well as surge voltage. It has been established that the
3 performance of the insulation is determined by the electrical stress place on it.
1.4 Dielectric Properties of Insulation [4]
Dielectric or electrical insulator is a material that withstand the electrostatic field and resist
the flow of electric current. Ideally
the
resistivity (specific
resistivity) of electrical insulation should be infinitely high. Each dielectric’s magnitude of resistivity is not definite and depends on a number of factors such as humidity, temperature, impurities and applied voltage. Moisture is one of the biggest problems associated with insulation breakdown. All substances are more or less hygroscopic. They can absorb moisture when wetted
with water
or air
containing water vapors. Moisture is also directly
proportional to humidity. The resistance of a dielectric is reduced by a small amount of water content. This can be explained by the fact that the impurities in the water dissociate into ions. Water can aid in the dissociation of the molecules in the dielectric matter. When the
magnitude of
the applied
voltage changes, the insulation
resistance is invariable, usually resistance drops with an increase in voltage. This circumstance is of great practical importance since it follows the idea that when the resistance of insulation (i.e. cable) is measured at the voltage and temperature below the
working values, then an
excessive
value of
this
resistance can be obtained.
1.5 Dissipation Factor [4, 5]
The tan δ, also called Loss Angle or Dissipation Factor test, is a diagnostic method of test cables to determine the quality of the cable insulation. This is done to try to predict the remaining life expectancy and in order to prioritize cable replacement. It does work if the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cable approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material.
4 In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be less than 90 degrees. The extent to which the phase shift is less than 90 degrees
is
indicative
of
the
level
of
insulation
contamination, hence
quality/reliability. This “Loss Angle” is measured and analyzed. Figure 1-1 is a representation of a cable. The tangent of the angle δ is measured. This will indicate the level of resistance in the insulation. By measuring IR /IC, we can determine the quality of the cable insulation. In a perfect cable, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning contamination. The greater the angle, the worse the cable.
FIGURE 1-1 The cable’s tan δ
1.6 XLPE Cable [4, 5]
1.6.1 Introduction Crosslink polyethylene (XLPE) was first introduced in the late 1950s as an insulation for medium voltage cables up to 35 kV, and today it is in use in Europe and Japan up to 500 kV. The most common method for crosslink polyethylene is by the peroxide system. The resultant XLPE insulation contains at least 98% polyethylene. 1.6.2 Electrical properties Typical electrical properties determined on crosslink polyethylene
5 insulated cables are listed in Table 1-1.
TABLE 1-1 Typical Electrical Properties for crosslink Polyethylene Insulation
Used in Transmission Cable Applications
Dielectric Constant
2.3
Dissipation Factor,% at 20°C
<0.03
90°C
<0.03
Volume Resistivity , Ωm Short – term AC Breakdown on Medium Voltage Cable, kV/mm
1.6.3 Cable component and construction The illustration of the cable construction is shown in Figure 1-2.
FIGURE 1-2 Cable construction
1016
48
6 1.6.3.1 Conductor The conductor can be copper concentric strand, compressed strand or compacted strand type. The conductor size is specified in square millimeters. The cross sectional area of the conductor cannot be too small for a certain cable voltage rating due to predetermined criteria. As an example, the cross sectional area of a 15 kV. Cable will not be smaller than 35 mm 2. 1.6.3.2 Conductor shield The conductor shield is an extruded layer of semi-conducting vulcanizable compound applied in tandem with and firmly bonded to the insulation. In some cases, the manufacturer may choose to supplement the conductor shield with a semiconducting tape applied spirally between the conductor and the compound layer. The conductor shielding system is the cable component having the function of assisting the voltage stress vector to align regularly along the cable cross-section radial and thereby precluding excessive voltage stress in possible voids between the conductor and the insulation. 1.6.3.3 Insulation The insulation is an unfilled cross-linked polyethylene meeting the requirements of ICEA Publication S-66-524 [6]. Since the insulation is one of the most important components in the cable assembly. Manufacturing precautions must be made during the insulating process. In short, the insulating compound will be delivered to the extruder through a metal detector in order that any possible metallic particles be eliminated. During the starting of the extrusion process, the steam temperature and pressure will be adjusted in proper sequence and manner apart from other activities to acquire dimensional requirements. In a successful insulation process, at least 70% of the thermoplastic polyethylene (sampled from the part adjacent to the conductor shield) must be converted
to a thermo-setting material. Voids of any kind in the insulation and
between layers of the insulation and the conductor shield must be within the quality control limits in both size and number. The outer surfaces of the insulation and the conductor shield must be smooth cylindrical and concentric to each other. Errors, if
7 any, must be within the specified tolerance. 1.6.3.4 Insulation shield The insulation is shielded with a layer of semi-conducting tape applied directly over the insulation, and one or more bare copper tapes applied with a lap over the semi-conducting tape. In a particular system where other configurations of the insulation shield are required, the manufactured can be consulted for satisfactory cable performance. One example is copper shield versus tape shield which reduces weight, improves bending performance and improves shield splicing. The insulation shield has the function of confining the dielectric field within the cable and providing symmetrical distribution of the voltage stress. When such function is attained, the insulation shield has inherent advantages of limiting induced voltage to the cable, limiting the cable interference to communication systems and reducing the hazard of shock if the shield is grounded. 1.6.3.5 Jacket The jacket can be polyvinyl chloride or polyethylene to suit the cable application. Underneath the jacket a layer of compound filled fabric tape may be provided, if required. For the choice of jacketing material, polyvinyl chloride is usually selected for outdoor installations due to its flame retardant properties while polyethylene is selected for aerial installation because of its weather-proof characteristics. In other applications, judgment has to be made between the two materials based on end usage.
1.7 The Literature Survey of The Cable Test
Many publications intend to study electrical characteristic of EthylenePropylene With Rubber Based Formulation (EPR) and XLPE cable. The dissipation factor (tan ) is one kind of electrical characteristic of the cable. This section describes the studies related to the electrical characteristic of the cable. 1.7.1 Performance characteristic of XLPE versus EPR as insulation for high voltage cable [7]
8 The publication intends to study the dissipation factor and the dielectric loss of EPR cable and XLPE cable. The dissipation factors was measured with an alternating voltage of 6 kV to 15 kV with the temperature of 20°C to 130°C. The conclusions are the dissipation factor and the dielectric loss of XLPE less than EPR cable.
The
dissipation factor of XLPE was not increased following the voltage raise. It differed with EPR cable that the dissipation factor was increased follow with voltage. 1.7.2 Electrical aging performance of tree retardant XLPE versus standard XLPE as insulation for distribution cable [8] The publication intends to study the breakdown voltage and the dissipation factor of Tree Retardant Crosslinked Polyethylene (TR-XLPE) and XLPE cable. The dissipation factor was measured with 25°C and 130°C with an alternating voltage of 13 kV-26 kV. In order to study, the cable was immersed in the liquid tank and the tap water was put into the two ends of insulator. The tested result, the dissipation factor of XLPE cable was less than the TR-XLPE cable. This shows the performance of XLPE cable is better than TR-XLPE cable. 1.7.3 Comparative laboratory evaluation of TR-XLPE and XLPE cables with super smooth conductor shield [9] The publication focus on electrical characteristic of TR-XLPE and XLPE with ac breakdown, impulse breakdown, moisture content and dissipation factor. It focuses on un-aged cable and aged cable. The cables were aged in the cylindrical insulating tanks, which were filled local tap water and replacing evaporated water with deionized water. The dissipation factors was measured with an alternating voltage of 5 kV to 20 kV with the temperature of 25°C and with the frequency of 0.1 Hz. The test result shows the dissipation factor of un-aged cable was not increased follow with voltage raise. In the other hand, the dissipation factor of aged cable was increased follow with voltage raise. The impulse breakdown and the ac breakdown of the aged cable was less than the un-aged cable. 1.7.4 Water treeing and dielectric loss of WTR-XLPE cable insulation [10] The publication focuses on two patterns, first is the water treeing in Water Tree Retardant Crosslinked Polyethylene (WTR-XLPE) and conventional XLPE. Then the dissipation factor of XLPE and WTR-XLPE are focused as a function of electrical
9 field and temperature. The dissipation factors was measured with an alternating voltage of 5 kV to 40 kV with the temperature of 20°C and 90°C. The test result shows that the dissipation factor of good XLPEs was not increased follow with voltage raise but it was increased follow with temperature raise. 1.7.5 The behavior of water in XLPE and EPR cables and its influence on the electrical characteristics of insulation [11] The publication focuses on the influence of moisture related with the performance of XLPE cable and EPR cable. In this test, the tap water was put in the cable conductors and the ends were properly closed with terminal boxes. The results indicate the relation between the cables with combined effects of water, water vapor, pressure and moisture. The test results show the dissipation factor of EPR and XLPE cable were increased follow with aging time and water content raise. The moisture is the cause of the dissipation factor increase. 1.7.6 Performance characteristics of dielectric in the presence of space charge [12] The publication focuses on the changing of the dielectric constant of EPR and filled XLPE cable with different temperature. The other is to study the dissipation factor of EPR, filled XLPE and XLPE cable with difference temperature. The dissipation factors was measured with an alternating voltage of 15 kV to 180 kV with the temperature of 20°C to 150°C. The test result shows the dielectric constant of the cable was decreased follow with temperature raise, the dissipation factor of EPR was increased quickly follow with temperature raise, exempt the dissipation factor of XLPE cable it was slowly increased follow with temperature raise and the dissipation factor of XLPE and EPR was decreased follow with frequency raise. 1.7.7 Accelerated aging of XLPE and EPR cable insulation in wet condition [13] The publication focuses on the aging effect in wet conditions on water absorption and density of steam and dry cured WTR-XLPE and EPR. In the first case water was injected in the cable conductor with the cable ends properly closed, in the second case water was injected in the steam cured WTR-XLPE cable conductor with the cable ends open. The tests of electrical characteristics of WTR-XLPE and EPR cable insulation were performed as a function of aging time at different aging temperature. The dissipation factors was measured with an alternating
10 voltage of 3 kV to 6 kV with the temperature of 20°C and 90°C and the time of 0 to 12000 hours. The dissipation factor was increased follow with aging time raise. This shows the beginning of the worse in quality of TR-XLPE and WTR-XLPE cable.
1.8 Conclusions of The Literature Survey of The Cable Test
The majority of the publication focuses on the electrical characteristic of the EPR cable, WTR-XLPE cable and the conventional XLPE cable such as ac breakdown, impulse breakdown, the dissipation factor with water content, aging time and temperature. Moisture is the major of the subject with the increased cable dissipation factor. It is also the major of the subject of the decreased of breakdown voltage. The majority of the thesis was to study the penetration of water into XLPE and EPR insulations by putting the water into the end of cable. The thesis is different from others
thesis because of it only studies the
dissipation factor of under ground XLPE cable with new cable and used cable. The thesis also studies the dissipation factor of new cable and used cable under wet condition test by immersing the cable directly in the water insulation tank. The water is not put into the ends of cables. This thesis focuses on the relationship of the dissipation factor versus electrical fields and temperatures of the new and used cables. The tests show the changing of the dissipation factor with related to the service time of the cable. It shows the performance of the XLPE cables produced in Thailand. If the dissipation factor is not increased with voltage raise, the cables are still in good condition. The wet condition was used to study the influence of moisture by observing the changing of the dissipation factor. If the dissipation factor increase follow with voltage raise, the moisture absorb thoroughly into the cable insulator and it increase the cable insulation conductance. The dissipation factor shows directly the cable, which is bad or good condition, is an advantage of the thesis.
11 1.9 The Research Project
1.9.1 Motivation As mentioned, a major hurtle in the power industry is estimating the lifetime of cable insulation. It would be a great advantage to electrical utilities to be informed a cable’s lifetime expectancy. Dielectric strength of insulation cannot be measured without damaging it or subsequently destroying it. Measuring the high-voltage dielectric loss (tan
δ
loss) by using the Schering Bridge is one method overcoming
this problem. Most of the tests are focused on XLPE insulation, as these are the newly and highly used method for insulating underground cables. 1.9.2 Objectives The main aim of this research is to diagnose a component of tan
δ
loss in
dielectric of power cable which is produced in Thailand, to analyze relationship of temperature, voltage, aging and moisture to tan
δ
loss of a cable. The major cause of
failures in high voltage equipment is the breakdown of the insulating material. Information regarding the condition of equipment would be invaluable because possible faults could be detected and predicted before they eventually occur. The major objectives are: 1. To diagnose tan
δ
loss of
XLPE
power cables of any voltages and
temperatures 2. To diagnose tan
δ
loss of aged XLPE power cable
3. To diagnose tan
δ
loss of XLPE power cable that used in wet condition
1.9.3 Structure of the thesis Chapter 1: Introduction. This chapter provides an introduction to dielectric property of insulation, diagnostic techniques for insulating materials, dielectric loss in power cable, the literature survey of the cable test.
That aims to introduce the
background problem that lead to the origin of the research project. The motivation, objective and structure of the thesis are also discussed. Chapter 2: The principle of dielectric and measurement. This chapter describes the tools for measuring dielectric loss (tan δ) of cable by using the Schering Bridge, the very low frequency (VLF).
12 Chapter 3: The methodology of the cable test. This chapter describes the setting of high-voltage transformer, the tan
δ
measurements unit, the heating current unit, the
cables setup, the stress control and partial discharge protection. Chapter 4: Experimentation results. The cables are tested and measured with the component of temperature, voltage and moisture. The results obtained from the test and measurement Chapter 5: Conclusions and recommendations. The summary of the thesis is described. This includes the main achievements, conclusion and suggestions of future work. Appendix A: Water tree Appendix B: Dielectric loss
CHAPTER 2 THE PRINCIPLE OF DIELECTRIC LOSS AND MEASUREMENT 2.1 Dielectric Loss [4], [14]
Dielectric loss can be separated in two sections as resistance of dielectric less than infinity and the static dielectric constant that is an effect of polarization under dc (direct current) condition. When the applied field, or the voltage across a parallel plate capacitor, is a sinusoidal signal, then the polarization of the medium under these ac conditions leads to an ac dielectric constant that is generally different from the static case. 2.1.1 Physical basic for dielectric polarization The force an electric field on an electric charge is :
F = qE
Eq. 2-1
The force (F) tends to move the charge in the field direction. If the charge is free, it moves on average in the field direction. If the charge is constrained, the force displaces the charge generating an electrical dipole moment, which is dielectric polarization. Since all movements in a material are disturbed by the thermal movement, the polarization reaches an equilibrium state, which at moderate fields strengths, is in most cases linearly related to the field applied. 2.1.2 Polarization in the time domain In free space, the relationship between the displacement field vector (D(t)) and the electric field vector (E(t)) is:
D ( t ) = ε 0E ( t )
Eq. 2-2
14 Where the permittivity of free space constant ( ε 0 ) is 8.85x10-12. If the electric field is applied to a dielectric material, a polarization field vector (P(t)) is also added, and Eq. 2-2 becomes:
D ( t ) =ε 0 E ( t ) +P ( t )
Eq. 2-3
Maxwell’s equation, presented below :
J ( t ) =σ 0 E ( t ) +
∂D ( t ) ∂(t)
Eq. 2-4
defines the current density ( J ( t ) ) in terms of DC conductivity ( σ0 ) and displacement current density, expressed by ∂D ( t )
∂(t) .
Polarization (P(t) depends on (E(t)) and its history. In order to conduct an analysis of the time dependence of polarization, a dielectric response function (f(t)) could be defined. The function f(t) describes polarization time dependence, assuming the material is linear and isotropic. The relationship between f(t) and P(t) can be described by the time dependence of polarization under a delta function:
P ( t ) =ε 0 ( EΔt ) f ( t )
Eq. 2-5
The requirements on f(t) are the causality demand:
f (t)
≡ 0 for t 〈 0
Eq. 2-6
where f(t) tends toward zero at infinite time
lim f ( t ) = 0 t →0
and where the integral function is limited
Eq. 2-7
15 ∞
∫ f (t)e
-jωt
dt
〈∞
Eq. 2-8
0
Assuming that polarization can be described by f(t) the time dependent polarization can be expressed as:
∞
∫
P ( t ) = ε 0 f ( τ ) E ( t-τ )dτ
Eq. 2-9
0
2.1.3 Polarization in the frequency domain In the frequency domain, the frequency dependent susceptibility is defined as the Fourier transform of the response function f(t) expressed as:
∞
∫
∞
χ ( ω ) = χ ( ω ) -jχ '' ( ω ) = f ( t ) e '
-jωt
∫
dt = f ( t ) cos ( ωt ) dt-sin ( ωt )dt
0
Eq. 2-10
0
Since the susceptibility ( χ ( ω ) ) is a complex function, it provides information about both the amplitude and phase components of the polarization. The real part gives the amplitude of the polarization, in phase with the field, and the imaginary part
χ ''( ω ) gives the component in quadrature with the field. The real and imaginary parts of the complex susceptibility at zero frequency are:
∞
∫
χ ' ( 0 ) = f ( t )dt
Eq. 2-11
0
and
χ '' ( 0 ) = 0
Eq. 2-12
The inverted Fourier transform can be used for getting f(t) when χ ' ( ω ) or χ ''( ω) is know:
16
f (t) =
2 π
∞
2
∞
∫ χ ' ( ω) cos ( ωt ) dω = π ∫ χ ''( ω) sin ( ωt ) dω 0
Eq. 2-13
0
When the Fourier transform is applied to Maxwell’s equation (Eq. 2-4), it becomes:
J ( ω ) = σ0 E ( ω ) +jωD ( ω )
If the displacement field
( D(ω ))
Eq. 2-14
is expressed in terms of χ ' ( ω ) and χ ''( ω ) , then
Eq. 2-14 can be expressed as:
J ( ω ) = ⎡⎣σ 0 /ωε 0 +χ ''( ω ) +j (1+χ ' ( ω) ) ⎤⎦ ωε 0 E ( ω )
Where
( σ /ωε 0
0
Eq. 2-15
+χ ''( ω ) ) is in phase with the driving field and therefore
generates power loss. However (1+χ ' ( ω ) ) is in quadrature with driving field and does not contribute to the loss. Since χ '' ( ω ) refers to the polarization loss, χ '' ( ω ) is normally named the dielectric loss. Normally the material has several polarization processes coexisting but not significantly interacting. Therefore the susceptibility could be divided up, with one susceptibility representing each process:
χ ( ω) =
∑ χ ( ω) i
Eq. 2-
i
16
By definition:
1+χ ' = ε 'r
and
Eq. 2-17
17
χ '' = ε ''r
Eq. 2-18
Where ε 'r is the real part of relative permittivity, and ε ''r is the imaginary part of relative permittivity. Together they provide the complex permittivity ( ε ( ω ) ) of the material in the following expression:
⎡ ⎣
ε ( ω ) = ε' ( ω) -jε''( ω) = ε 0 ( ε 'r ( ω) -jε''r ( ω) ) = ε 0 ⎢1+
⎤
∑ χ ( ω) -j∑ χ ( ω) ⎥⎦ ' i
i
'' i
Eq. 2-19
i
The entity dissipation factor Tanδ , can be expressed as:
σ tan δ ( ω ) =
ωε 0
+ε ''r ( ω )
ε 'r ( ω )
2.1.4 Dielectric loss in cable [15] Consider the cables as a parallel plate capacitor as shown in Figure 2-1.
FIGURE 2- 1 The cable equivalent circuit
The capacitor can be calculated as:
Eq. 2-20
18
c=
2πε 0ε r
Eq. 2-21
ln ( R/r )
or
χ =
ε0ε ρ Α
Eq. 2-22
δ
where ε 0 = permittivity of free space ε r = relative permittivity Capacitors are used for a wide variety of purposes and are made of many different materials in many different styles. For purposes of discussion we will consider three broad types that is, capacitor made for ac, dc, and pulse applications. The ac case is the most general since ac capacitors will work in dc and pulse application, where the reverse may not be true. It is important to consider the losses in ac capacitors. All dielectrics (except vacuum) have two types of losses. One is a conduction loss, representing the flow of actual charge though the dielectric. The other is a dielectric loss due to movement or rotation of the atoms or molecules in an alternating electric field. One way of describing dielectric loss is to consider the permittivity as a complex number, defined as:
δ
ε = ε'-jε'' = /ε/ε -j
Eq. 2-23
where
ε' = ac capacitivity ε'' = dielectric loss factor
= dielectric loss angle
Capacitor is a complex number
c
∗ in this definition, becoming the expected
real number C as the losses go to zero. That is, we define
19 Eq. 2-24
c* = c-jc''
One reason for defining a complex capacitance is that we can use the complex value in any equation derived for a real capacitance in a sinusoidal application, and get the correct phase shifts and power losses by applying the usual rules of circuit theory. This means that most of our analyses are already done, and we do not need to start over just because we now have a lossy capacitor. Equation 2-23 expresses the complex permittivity in two way, as real and imaginary or as magnitude and phase. The magnitude and phase notation is rarely used. Instead, people usually express the complex permittivity be
ε '
and tan
Where
tanδ =
1 ωR pC p
=
ε''
Eq. 2-25
ε'
Where tan is called either the loss tangent or the dissipation factor (DF). The real part of permittivity is defined as: ε' = ε r ε 0
Where ε r is the dielectric constant and
Eq. 2-26
ε 0
is the permittivity of free space. If
we consider the term power factor (PF) may also be defined for ac capacitors. It is given by the expression.
PF = cosθ
Eq. 2-27
Where θ is the angle between the current flowing through the capacitor and the voltage across it. The capacitive reactance for the sinusoidal case can be defined as:
Xc =
1 ωC
Eq. 2-28
20 where ω = 2πf rad/sec, and f is in Hz. In a lossless capacitor, ε'' = 0 and the current leads the voltage by exactly 90°C. If ε'' is greater than zero, then the current has a component is phase with the voltage.
ε''
cos θ =
( ε'') For a good dielectric, ε'
2
+ ( ε')
Eq. 2-29
2
ε'' , so:
cos θ
≈
ε'' ε'
= tan δ
Eq. 2-30
Therefore, the term power factor is often used interchangeably with the terms loss tangent or dissipation factor, even though they are only approximately equal to each other. We can define the apparent power flow into a parallel plate capacitor as:
S =UI =
U2 -jXc
= jU 2 ωC* = jU 2 = U2
ωA d ωA d
( ε'-jε'') ε r ε 0 (j+DF)
Eq. 2-31
By analogy, the apparent power flow into any arbitrary capacitor is:
S = P+jQ = U 2ωC ( j+DF )
Eq. 2-32
The power dissipated in the capacitor is:
P = U2 ωC'' = U 2 ωC ( DF ) = U 2 ωC ( tan δ )
Eq. 2-33
21 And is called loss angle. In case low dielectric loss as ε''
ε' or tan δ
1,
so Eq. 2-26 can be calculated as:
ε r = ε'
The relative dielectric constant ε r depend on temperature. Generally, if dielectric temperature is increased, ε r will be increased.
2.2 Dielectric Loss Measurement [15]
Power loss in dielectric can be calculated by Eq. 2-33 by measuring tan and capacitance.
In general case tan
and capacitor can be measured by watt meter
method and bridge method. One of the most commonly used methods for measuring loss tangent and capacitor is the Schering bridge. It is suitable for medium frequency as 50 Hz to 100 kHz. The bridge measures the capacitance and loss angle of a capacitor by comparing it with a gas-filled standard capacitor which has negligible loss over a wide frequency range. The arrangement shows in Figure 2-2.
FIGURE 2-2 Schering Bridge
22 where Cx = dielectric capacitance R x = dielectric resistance C N = standard capacitor R 3 = pure resistance R 4 = pure resistance C4 = pure capacitance G = null indicator The balance condition can be occurred by adjust R 3 and R 4 until null indicator indicated as zero then we can be written equation as:
Z1 Z2
=
Z3
Eq. 2-34
Z4
Where Z1, Z2, Z3, and Z4 are an impedance of part I, II,III and IV and
Z1 =
Z2 =
R x 1+jωC x R x -j ωC N
Z3 = R 3 Z4 =
R 4 1+jωC4 R 4
then
R x R 3 (1+jωCx R x )
=
-J ωC N R 4
(1+jωC4 R 4 )
23 by compare with real value we obtain
R x 2
2
1+ω C x R x
2
=
C4 R 3
Eq. 2-35
CN
Figure 2-3 shows vector diagram parallel circuit of Cx and R x
FIGURE 2-3 Vector diagram parallel circuit of Cx and R x
and then cos δ =
ωC x R x 2
2
2
1+ω C x R x cos δ =
ω2C 2 x R 2 x 1+ω2 C2 x R 2 x
replace above equation into Eq. 2-35 which can be written as:
Cx =
and from Figure 2-4
C N cos 2 δ ω2 C x C 4 R x R 3
Eq. 2-36
24
tan δ =
ωC4 1/R 4
and
tan δ =
ωC4 R 4 = 1 Cx R x
1/R x ωC x
=
1 ωC x R x
1 ωC x R x 2
= ω C4 R 4
replace 1/C x R x into Eq. 2-36, so that:
Cx =
C N R 4Cos 2 δ R 3
and
Cx =
C N R 4 R 3
tan δ = ωC4 R 4
Eq. 2-37
Now a day, the instrument which to accept popularly in a field test is very low frequency (VLF). It can be direct measurement tan
and it consists of a high voltage
divider and a fiber optically linked measurement box. The high voltage divider measures the voltage input to the cable, sends this information to the controller, which analyzes the voltage and current wave forms and calculates the tan connected laptop computer displays and stores the results.
number-A
25 A VLF is shown in Figure 2-4 and the example of VLF tested result, Figure 2-5. The VLF are also widely used for testing newly installed and/or repaired cable before reenergizing to insure and for besting critical cable runs.
(A)
(B)
FIGURE 2-4 Very low frequency
FIGURE 2-5 The result of a VLF test
26 The cable to be tested must be de-energized and each end isolated. Using a VLF, the test voltage is applied to the cable while the tan delta controller takes measurements. Typically, the applied test voltage is raised in step, with measurements first taken up to 1U 0 , or normal line to ground operating voltage. If the tan delta numbers indicate good cable insulation, the test voltage is raised up to 1.5-2 U 0. The tan
value at the higher voltages are compared to those at lower voltages and an
analysis is made. Two reasons that VLF used instead of a regular power frequency (50 Hz or 60 Hz). First, to test a cable with 50 Hz or 60 Hz power requires a very large high voltage supply. It is not practical, nearly impossible, to test a cable of several thousand feet with a power frequency supply. At a typical VLF frequency of 0.1Hz, it takes 600 times less power to test the same cable compared to power frequency, secondly, the magnitude of the tan
value increase as the frequency decreases,
making measurement easier. As the below equation shows, the lower the frequency (f), the higher the tan value.
tanδ = I R /IC =
1
Eq. 2-38
2ωCR
Benefits of voltage testing with VLF sinusoidal wave forms in the field [16]. 1. Low power requirement for on site testing 2. Compact portable design of test equipment 3. Easier
diagnosis of water tree deteriorated insulation at low frequencies
compared to higher frequencies such as 50 Hz using tan
(TD)
4. Easier to integrate cable diagnostic interfaces such as TD and partial discharge (PD) etc
2.3 The Test Result Interpreted
If a cable’s insulation is perfect, the loss factor (tan δ) will not change as the applied voltage is increased. The capacitance and loss will be similar with 1 kV or 10 kV applied to the cable. If the cable has water tree contamination, thus changing the capacitive nature of the insulation, then the tan
value will be higher at higher
27 voltages. Rather than a flat curve for the loss value versus voltage, the curve will be non linear see the Figure 2-6.
0.06
New and Aged 15 kV XLPE Cable
) a t l 0.05 e d n 0.04 a t ( e 0.03 l g n A0.02 s s o 0.01 L
New cable Aged cable
0
2.5
5
7.5
10
Voltage (kV rms)
FIGURE 2-6 The tan loss of new and age 15kV XLPE cable
2.4 The standard for XLPE dissipation factor testing [16, 17, 18]
2.4.1 IEC 60502-2 The cables are still in good condition if
tan
at maximum conductor
temperature in normal operator plus 5°C up to 10°C less than 0.008. 2.4.2 IEC 141 The cables are still in good condition, if:
[tan (2 U0) - tan (0.5U0)] < 0.001
2.4.3 IEEE Std 4002 TM-2004 This standard is suitable for field testing of shielded power cable systems using very low frequency (VLF). The cables are still in good condition, if:
tan (2U0) < 0.012 and [tan (2 U0) - tan (U0)] < 0.006
28 The cables are in bad condition (or to be replaced immediately) if:
tan (2U0) > 0.012 and [tan (2 U0) - tan (U0)] > 0.006
CHAPTER 3 THE METHODOLOGY OF THE CABLE TESTING 3.1 Introduction
The tan
measurement was performed on various cable samples using the
Schering bridge to measure the cables dielectric loss. In-order to perform tests on the acquired cable samples, modifications had to be made to the cables themselves. The following chapter outlines the setup for performing the dielectric loss measurement. 3.2 Cable Setup
Figure 3-1 shows the cable setup, the cable was divided in two sections, measuring electrode and guard electrode and the two ends of the cable was connected with the terminator, the copper tape was connected to terminator and ground. The terminator is used to protect the sliding discharge at the end of the cable. The guard electrode is used to separate the part of the cable that does not need to be measure [15].
FIGURE 3-1 Cable setup
30 3.3 Stress Control [19]
The sliding discharge much occurred to be specific on skill of insulator that have two different types such as air and solid insulator and it occurred in form of sliding brush as in Figure 3-2.
FIGURE 3-2 Sliding discharge
If no stress control were applied, discharge could occur and the life of the termination would be limited depending on the stress at the end of the shield and the discharge resistance of the primary dielectric. Figure 3-3 shows the stress concentration at the end of the screen of medium voltage cables when no stress control is used.
FIGURE 3-3 Uncontrolled cable end-potential distribution
31 The tradition method of reducing the electrical stress and ensuring long cable services is to install a cone shaped insulating material on the outer conductive electrode over the cable shield end as shown in Figure 3-4.
FIGURE 3-4 Stress control
3.4 Partial Discharge Protection [15]
The external partial discharge may occurred in non-uniform field, e.g. in point plane gaps or coaxial cylinders, the partial discharge are in form of corona can grading in three type, glow discharge, branch discharge and brush discharge. The glow and brush discharge is shown in Figure 3-5.
FIGURE 3-5 External partial discharge
32 At the joint of the terminal external partial discharge may be occurred, it is protected by guard ring, Figure 3-6.
FIGURE 3-6 Partial discharge protection 3.5 Testing Transformer
The testing transformer setup is shown in Figure 3-7, and the output voltage is measured by the capacitance divider.
FIGURE 3-7 Testing transformer setup
33 3.6 Heating Current Setup
To make the temperature in the cable, the cylinder shape transformer is used, and the temperature control is used to maintain the temperature constant, Figure 3-8.
FIGURE 3-8 Heating current setup 3.7 Schering Bridge Setup
The Schering Bridge involves a measurement carried out in a bridge arrangement, Figure 3-9, and Figure 3-10 comparing the cable sample with thoroughly loss free standard capacitor, with the capacity and loss factor, tan
,
known. The bridge is balanced by setting the resistance, R 4 and R 3 as well as the capacitance C4 until the indicator on the screen of null indicator is horizontal. The capacitance is then calculated using:
CX = C N *R 4 /R 3
The loss factor is calculated using:
tan δ = R 4 *ω*C4
34
FIGURE 3-9 Schering Bridge
FIGURE 3-10 Null indicator
35 For the experiments completed using the Schering bridge the standard capacitor used, C N, was rated at 100 pF .
3.8 Experimental Detail
In order to investigate the influence of voltages and temperatures of underground XLPE cables, the new cables, the 5-year used cables and the 10-year used cables were selected. The dissipation factor measurements were all performed on 6 meter long, 12/20 (24 kV) underground XLPE cable sample with 50 mm 2 stranded copper conductors, all produced by the same cable manufacture. The sample of completed cable was heated by cylinder core transformer. The tan
measured with an alternating voltage of 0.5 U 0, U0, 1.5U0 and 2U0 with the
temperature of 20°C to 100°C. Figure 3-11 shows the dissipation factor measuring.
FIGURE 3-11 The dissipation factor measuring setup
In order to investigate the influence of moisture, age and time, also studies by others, two cables were selected the new cable and the 10-year used cable. The cables were placed in the cylindrical insulating tank. These test were not done by 100°C because of the cable was melted at this temperature. Then the water temperature was
36 maintained at 90°C. The voltage of U 0 (13.86 kV AC) was applied conductor to ground continuously during aging. The samples for the evaluation were measured every 100 hours. It is planned to continue the aging time for 1000 hours. In Figure 3-12, the new cable and the 10-year used cable under wet condition were tested.
FIGURE 3-12 Cable under wet condition test
CHAPTER 4 EXPERIMENTATION RESULTS 4.1 The Dissipation Factor (tan ) of New Cable and Used Cable Test Results
The objectives of the testing are to investigate the changing of dissipation factor (tan ) of 12/20 (24 kV) copper conductor with tape shield XLPE new cable, 5-year used cable at 20°C to 100°C temperature with 0.5 time of phase voltage (U 0) to 2 time of phase voltage (0.5U 0 to 2U0). The changing of the tan
in dielectric of cable
produced in Thailand is an index to know that the new cable is good or bad by compare with IEC 60502-2. If the tan
of new cable at maximum conductor
temperature (90°C) in normal operation plus 5°C up to 10°C is less than 80 x 10 -4, the cable will be good. In order to investigate the changing of dissipation factor of the 5year used cable and the 10-year used cable, the IEC 141 standard is used. If the tan
of used cable at phase voltage (U 0) and 0.5U0 to 2U0 is less than 5x10-3 and 1x10-3 respectively, the cable will be still in a good condition. On the other hand, it is a bad and the water tree may occur in the cable insulator. The test result of the new cables, the 5-year used cable and the 10-year used cables are as follows: 4.1.1 The dissipation factor (tan ) of new cable test result The test result of new cable is shown in Table 4-1.
Figure 4-1 shows the
dissipation factor versus voltage at 0.5U0 to 2U0 with nine different temperature (20°C to 100°C). The dissipation factor at 20°C to 90°C do not increase follow with voltage raise that can be observed with Figure 4-1, at 0.5U0 to 2U0 each graph is a straight line. However at 100°C the dissipation factor was increased with voltage raise ( 0.5U0 to 2U0). The consideration of dissipation factor at 100°C with IEC 141 standard, the tan
(2U0 to 0.5U0) is less than 1x10-3, so this cable is good. However, the increase
of the dissipation factor with the voltage raise at 100°C cause from the increase of the cable insulation’s conductance when the cable is used at high temperature.
38 TABLE 4-1 The dissipation factor of 12/20 (24 kV) XLPE new cable Temperatures °C
Voltages (kVrms) U0 1.5U0 1.3e-4 1.3e-4 2e-4 2e-4 3e-4 3e-4 10e-4 10e-4 20e-4 20e-4 32e-4 32e-4 37e-4 37e-4 40.8e-4 40.8e-4 43.9e-4 44.6e-4
0.5U0 1.3e-4 2e-4 3e-4 10e-4 20e-4 32e-4 37e-4 40.8e-4 43.2e-4
20 30 40 50 60 70 80 90 100
2U0 1.3e-4 2e-4 3e-4 10e-4 20e-4 32e-4 37e-4 40.8e-4 45.3e-4
50 45 20°C
40
30°C
35 ) 0 1 x ( δ
n a t
40C°
30
4 -
50°C
25
60°C
20
70°C 80°C
15
90°C
10
100°C 5 0 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-1 The dissipation factor of 12/20 (24 kV) XLPE cable versus voltage
with nine different temperature (20°C to 100°C) The relationship between tan
and temperature with four different voltage
(0.5U0 to 2U0) is shown in Figure 4-2. The dissipation factor at 20°C to 40°C was slow increased, however at 40°C to 100°C it was fast increasing of tan with Eq.2-23,
'
ε ε
" or tan
1 that
ε
r
=
ε
was follow
' , so the dielectric loss of tan
changed with temperature. Consider Figure 4-2, clearly the tan
was
was depended on
applied temperature level but do not increased follow with voltage raise.
39
50 45
)
4 -
0 1 x ( δ
n a t
40
0.5Uo
35
1Uo
30
1.5Uo
25
2Uo
20 15 10 5 0 20
30
40
50
60
70
80
90
100
0
temperature ( C)
FIGURE 4-2 The dissipation factor of 12/20 (24 kV) XLPE new cable versus
temperature with four different voltage (0.5U0 to 2U0) 4.1.2 The dissipation factor (tan ) of 5-year used cable test result The test result of 5-year used cable is shown in Table 4-2. The relationship between tan
and electric field (voltage) on conductor screen at four different voltage
(0.5U0 to 2U0) with nine different temperature (20°C to 100°C), Figure 4-3. Consider Figure 4-3 the dissipation factor at 20°C to 90°C, it do not increase with voltage raise. For example, at 20°C, the dissipation factor with 0.5U0 to 2U0 is 1.5x10-4, at 30°C the dissipation factor with 0.5U0 to 2U0 is 2.3x10-4 and at 40°C the dissipation factor is 3.2x10 -4. However the dissipation factor at 100°C was slowly increased with voltage raise. Table 4-2 shows that the dissipation factor at 100°C is less than 0.8% (80x10-4) which accords with the IEC 60502-2 specifications on dissipation factor for conventional XLPE cable (tan <80x10-4), so this cable is still in good.
40 TABLE 4-2 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable Temperatures °C 20 30 40 50 60 70 80 90 100
Voltages (kVrms) U0 1.5U0 1.5e-4 1.5e-4 2.3e-4 2.3e-4 3.2e-4 3.2e-4 13e-4 13e-4 22e-4 22e-4 36e-4 36e-4 38e-4 38e-4 42e-4 42e-4 43.5e-4 45e-4
0.5U0 1.5e-4 2.3e-4 3.2e-4 13e-4 22e-4 36e-4 38e-4 42e-4 43.5e-4
2U0 1.5e-4 2.3e-4 3.2e-4 13e-4 22e-4 36e-4 38e-4 42e-4 46e-4
50 45 30°C
40
40°C
35 )
4 -
0 1 x (
50°C
30
60°C
25
70°C
δ
n a t
20
80°C
15
90°C
10
100°C
5 0 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-3 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable versus
voltage with nine different temperature (20°C to 100°C) The relationship between tan
and temperature with four different voltage
(0.5U0 to 2U0) is shown in Figure 4-4. The dissipation factor at 20°C to 40°C was not increase, however at 40°C to 70°C it was fast increase. After 70°C to 100°C the dissipation factor begins slow increased because of, the cable dissipation factor (tan ) consist of 3 part, conductor ohmic loss (in insulator), dielectric polarization loss and dielectric partial discharge or ionization loss (tan
= tan
σ
+ tan
p +
tan
i).
The
41 ionization loss (tan ) or partial discharge loss is a loss that occur in the voids of cable insulator. After temperature increased from 70°C to 100°C, the void filling occurs more rapidly with temperature increase that relates with the melting temperature rank for the crystallites of XLPE [20]. This lead to the decrease of ionization loss (tan
i)
so tan at 70°C to 100°C is decreased.
50 45 0.5Uo 40
1Uo
35 )
4 -
0 1 x (
1.5Uo
30
2Uo
25
δ
n a t
20 15 10 5 0 20
30
40
50
60
70
80
90
100
temperature ( 0 C)
FIGURE 4-4 The dissipation factor of 12/20 (24 kV) XLPE 5-year used cable
versus temperature with four different voltage (0.5U0 to 2U0) 4.1.3 The dissipation factor (tan ) of 10-year used cable test results Table 4-3 shows the test result of 10-year used cable and the relationship between tan
and electric field (voltage) on conductor screen at four different voltage
(0.5U0 to 2U0) with nine different temperatures (20°C to 100°C) is shown in Figure 45. Table 4-3 at 20°C, tan
is 1.5x10-4 and remains constant with voltage raise from
0.5U0 to 2U0. At other temperatures, the tan remain still constant with voltage raise, except at 100°C, the voltage of 0.5U0 to U0, 1.5U0 and 2U0 are 44x10-4, 45x10-4 and 46x10-4 respectively. The cause is from the increase of insulator’s conductance at high temperature.
42 TABLE 4-3 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable Temperatures °C
Voltages (kVrms) U0 1.5U0 1.5e-4 1.5e-4 2.3e-4 2.3e-4 4e-4 4e-4 15e-4 15e-4 22e-4 22e-4 36e-4 36e-4 40e-4 40e-4 42e-4 42e-4 44e-4 45e-4
0.5U0 1.5e-4 2.3e-4 4e-4 15e-4 22e-4 36e-4 40e-4 42e-4 44e-4
20 30 40 50 60 70 80 90 100
2U0 1.5e-4 2.3e-4 4e-4 15e-4 22e-4 36e-4 40e-4 42e-4 46e-4
50 45 20°C
40
30°C
35 )
4 -
0 1 x (
40°C
30
50°C
25
60°C
δ
n a t
20
70°C
15
80°C
10
90°C
5
100°C
0 0.5
1
1.5
2
U0 (kV rm s)
FIGURE 4-5 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable
versus voltage with nine different temperature (20°C to 100°C) The dissipation factor (tan
)
versus temperature (20°C to 100°C) with four
different voltage (0.5U0 to 2U0) is shown in Figure 4-6. The dissipation factor at 20°C to 40°C was slow increased, however at 40°C to 100°C it was fast increased that can observe in Figure 4-6. Consider Figure 4-6, it shows a similar pattern with the dissipation factor between 0.44 to 0.46%. Power cable rarely operate at above 90°C. The most important parameters are the cable life and the long term stability of dissipation factor as the cable ages.
43 50 45
)
4 -
0 1 x (
0.5Uo
40
1Uo
35
1.5Uo
30
2Uo
25
δ
n a t
20 15 10 5 0 20
30
40
50
60
70
80
90
100
temperature ( 0 C)
FIGURE 4-6 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable
versus temperature with four different voltage (0.5U0 to 2U0) The cable are durable even those in 10 year used, by observing, the tan does not increase follow with voltage raise. 4.2 The Dissipation Factor (tan ) of New Cable and Used Cable Under Wet Condition Test Results
The objective of testing is to investigate the changing of dissipation factor (tan ) of 12/20 (24 kV) XLPE new cable and 10-year used cable where the cables was immersed in water tank, and the water temperature was maintained at 90°C. The voltage of U0 (13.86 kV) was applied across conductor and ground continuously during aging. The samples for the evaluation were measured every 100 hours. It is planned to continue the aging time for 1000 hours. After 1000 hours the temperature was decreased 10°C by step and tan
was measured at the same time. This testing is
to investigate the influence of moisture, age and time by observing with changing of the dissipation factor (tan ) and the test results also, shows the performance of XLPE cable produced in Thailand by comparing with IEC 60502-2 standard and IEC 141 standard too. The test result of the new cable under wet condition test and the 10-year used cable under wet condition test are as followed:
44 4.2.1
The dissipation factor of 12/20 (24 kV) XLPE new cable under wet
condition The test result of new cable under wet condition is shown in Table 4-4 and the relationship between dissipation factor and time at 100 hours to 1000 hours with four different voltages (0.5U0 to 2U0) is shown in Figure 4-7. Consider the dissipation factor at 100 hours to 400 hours, it is 43x10 -4 and it remains constant with voltage raise (0.5U0 to 2U0), at time 500 hours to 1000 hours the dissipation factor was increased with voltage raise (0.5U0 to 2U0) such as at 500 hours the dissipation factor was increased from 43x10-4 (at 0.5U0) to 47.6x10 -4 (at 2U0) and at 1000 hours it was increased from 43x10-4 (at 0.5U0) to 48.4x10 -4. The consideration of dissipation factor at 500 hours to 1000 hours, the cause of increasing of tan
with voltage raise is the
decreasing of the cable insulation’s resistance where the cable is used at high temperature with long time in wet condition, the cable insulator may be to get worse in quality then the water can be thoroughly absorbed to the cable insulation and i t is to make decreased the cable insulation’s resistance, this is the cause of increasing of cable dissipation factor. The comparison of the dissipation factor with IEC 60502-2 standard, the dissipation factor at 1000 hours with voltage of 2U 0 is less than 80x10-4, and the comparison with IEC 141 standard tan
(2U0 to 0.5U0) less than 1x10-3 then,
the cable is durable even when long time tested with wet condition.
TABLE 4-4 The dissipation factor of 12/20 (24 kV) XLPE new cable under wet
Condition Times (Hours) 100 200 300 400 500 600 700 800 900 1000
0.5U0 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4
Voltages (kVrms) U0 1.5U0 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 43e-4 47.3e-4 47e-4 47.6e-4 47.7e-4 47.7e-4 47.7e-4 47.7e-4 47.7e-4 47.7e-4 47.7e-4 47.7e-4
2U0 43e-4 43e-4 43e-4 43e-4 47.6e-4 47.9e-4 48e-4 48.4e-4 48.4e-4 48.4e-4
45 49 100h
48
200h 47
300h
46
400h
0 1 x (
45
500h
δ
44
600h
)
4 -
n a t
700h
43
800h
42
900h
41
1000h
40 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-7 The relationship of dissipation factor versus voltage of new cable
under wet condition with ten different time (100 hours to 1000 hours)
The relationship between tan
and time is shown in Figure 4-8, consider the
dissipation factor at 100 hours to 400 hours with four different voltage (0.5U0 to 2U0), the straight line is shown the tan do not immediately increased follow with time raise, but it had increased about after 400 hours.
49
)
4 -
0 1 x ( δ
n a t
48
0.5Uo
47
1Uo
46
1.5Uo
45
2Uo
44 43 42 41 40 100
200
300
400
500
600
700
800
900 1000
time (hours)
FIGURE 4-8 The relationship of the dissipation factor versus the time of the new
cable under wet condition with four different voltages (0.5U0 to 2U0)
46 After1000 hours test, the temperature was decreased 10°C by step, and the tan
was measured at the same time, it is shown in Table 4-5. The relationship between
the dissipation factor versus the temperature (20°C to 90°C) with four different voltage (0.5U0 to 2U0) is shown in Figure 4-9, at 20°C to 50°C the dissipation factor do not change with voltage raise but at 60°C to 90°C the dissipation factor was increased with voltage raise that it shows the beginning of the worse in quality of the cable insulation. TABLE 4-5 The dissipation factor of new cable under wet condition
Temperatures °C 20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C
Voltages (kV rms) U0 1.5U0 4.56e-4 4.56e-4 4.56e-4 4.56e-4 9e-4 9e-4 18e-4 18e-4 26.7e-4 31.2e-4 34.9e-4 39e-4 40e-4 40e-4 47.7e-4 48.4e-4
0.5U0 4.56e-4 4.56e-4 9e-4 18e-4 22.3e-4 34.9e-4 40e-4 43e-4
2U0 4.56e-4 4.56e-4 9e-4 18e-4 31.2e-4 39.5e-4 40e-4 48.4e-4
60 50 20°C )
30°C
40
4 -
0 1 x ( δ
n a t
40°C 30
50°C 60°C
20
70°C 80°C
10
90°C
0 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-9 The relationship of dissipation factor versus voltage of new cable
under wet condition with eight different temperature (20°C to 90°C)
47 The dissipation factor versus temperature (20°C to 90°C) of new cable under wet condition test with four different voltage (0.5U0 to 2U0) is shown in Figure 4-10. At 20°C to 30°C the dissipation factor does not increase but begins increase at 30°C to 90°C. Consider the dissipation factor at 50°C to 90°C, it was increased with voltage raise. So, it indicate the beginning damage of the cable insulator. 60
50 0.5Uo 1Uo
40 )
4 -
0 1 x (
1.5Uo 30
2Uo
δ
n a t
20
10
0 20
30
40
50
60
70
80
90
temperature ( 0 C)
FIGURE 4-10 The dissipation factor versus temperature of new cable under wet
condition
4.2.2 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable under wet condition The test result of 10-year used cable under wet condition is shown in Table 4-6. The relationship between dissipation factor and time at 100 hours to 1000 hours with four different voltage (0.5U0 to 2U0) is shown in Figure 4-11. The dissipation factor at 100 hours to 1000 hours was increased with voltage raise. For examples, at 100 hours of the testing, the voltages were 0.5U0, U0, 1.5U0 and 2U0 that the dissipation factors were 42.7x10-4, 42.7x10 -4, 44.2x10 -4 and 45.7x10 -4 respectively and at 1000 hours the dissipation factors were 52.5x10 -4, 52.5x10 -4, 56.7x10 -4 and 60.8x10 -4 respectively. The increasing of the dissipation factor follow with voltage and time raise was indicated the starting of the cable worse in quality.
However, the
48 comparison with IEC 60502-2 standard and IEC 141 standard, after 1000 hours test, the 10-year used cable is still in good condition.
TABLE 4-6 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable under
wet condition Times (Hours)
Voltages (kV rms) U0 1.5U0 42.7e-4 44.2e-4 44.3e-4 44.8e-4 48.4e-4 50e-4 48.4e-4 50e-4 48.4e-4 50e-4 48.7e-4 50.5e-4 49e-4 54e-4 49e-4 55.3e-4 52.2e-4 58.3e-4 52.5e-4 58.7e-4
0.5U0 42.7e-4 44.3e-4 48.4e-4 48.4e-4 48.4e-4 48.7e-4 49e-4 49e-4 52.2e-4 52.5e-4
100 200 300 400 500 600 700 800 900 1000
2U0 45.7e-4 45.9e-4 50e-4 50e-4 50e-4 50.5e-4 57e-4 57e-4 60.5e-4 60.8e-4
70 100h 60
200h 300h
50
400h
)
4 -
0 1 x ( δ
n a t
500h
40
600h 30
700h 800h
20
900h 1000h
10 0 0.5
1
1.5
2
U0 (kV rm s)
FIGURE 4-11 The relationship of dissipation factor versus voltage of new cable
under wet condition with ten different time (100 hours to 1000 hours)
49 The relationship of the dissipation factor versus time is shown in Figure 4-12, at
0.5U0, U0, 1.5U0, and 2U0 the dissipation factor was increased follow continuous with time raise. The increasing of the dissipation factor indicated the decreasing of the resistance of the cable insulation. The cause of the decreasing of the resistance of the cable insulation are age and moisture, where aged cable is tested in water tank with high temperature the cable insulator may be cracked and the moisture may be thoroughly absorbed in to cable insulator. 70 60 )
50
4 -
0 1 x ( δ
n a t
40 0.5Uo
30
1Uo
20
1.5Uo
10
2Uo
0 100
200
300
400
500
600
700
800
900
1000
time (hours)
FIGURE 4-12 The relationship of the dissipation factor versus the test time of the
10-year used cable under wet condition with four different voltage (0.5U0 to 2U0)
After 1000 hours test, the temperature was decreased 10°C by step and the dissipation factor was measured at the same time, it is shown in Table 4-7. The relationship between the dissipation factor versus the temperature (20°C to 90°C) with four different voltage (0.5U0 to 2U0) is shown in Figure 4-13. The dissipation factor was increased with voltage raise that shows the worse in quality of cable insulation after 1000 hours wet condition test. At 20°C with 0.5U0 the dissipation factor is 7.3x10 -4 and at 2U0 the dissipation factor is 8x10-4.
At 90°C with 0.5U0 the
dissipation factor is 52.5x10-4 and at 2U0 the dissipation factor is 60.8x10-8. However, the dissipation factor less than the standard, so this cable is still in good condition.
50 TABLE 4-7 The dissipation factor of 12/20 (24 kV) XLPE 10-year used cable under wet condition Temperatures °C 20 30 40 50 60 70 80 90
Voltages (kV rms) U0 1.5U0 7.3e-4 8e-4 12.2e-4 15e-4 18e-4 20.3e-4 25e-4 27.4e-4 33.2e-4 33.2e-4 42.4e-4 45e-4 45.3e-4 47e-4 52.5e-4 58.7e-4
0.5U0 7.3e-4 12.2e-4 18e-4 25e-4 30.5e-4 42.4e-4 45.1e-4 52.5e-4
2U0 8e-4 15e-4 20.3e-4 27.4e-4 35e-4 46.3e-4 47e-4 60.8e-4
70 60 20°C 30°C
50
40°C
)
4 -
0 1 x ( δ
n a t
40
50°C 60°C
30
70°C 20
80°C 90°C
10 0 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-13 The relationship of dissipation factor versus voltage of 10-year used
cable under wet condition with eight different temperature (20°C to 90°C) The relationship of the dissipation factor versus temperature with four different voltage is shown in Figure 4-14. The dissipation factor at 0.5U0 to 2U0 was fast increased with the temperature raise, consider at 0.5U0 at 20°C the dissipation factor is 7.3x10-4 and at 90°C the dissipation factor is 52.5x10 -4. If consider the dissipation
51 factor at 2U0, it is the highest. Clearly, the influence of age, moisture, temperature, and electrical field (voltage) have affected to increasing of the dissipation factor in the cable. 70
) 4 0 1 x ( δ
n a t
60
0.5Uo
50
1Uo 1.5Uo
40
2Uo
30 20 10 0 20
30
40
50
60
70
80
90
temperature (0C)
FIGURE 4-14 The dissipation factor versus temperature of 10-year used cable under
wet condition 4.3 The Comparison of The Dissipation Factor Test Result
The comparison of the dissipation factor test result is used to compare the test result of all test. The comparison indicates the influence of voltage, temperature, and service time to the new and used cables by considering the dissipation factor and the influence of moisture, observe from the dissipation factor of the new cable and the 10year used cable under wet condition test. The comparisons of the dissipation factor of the all test results are as follows: 4.3.1 The comparison of new cable and used cable The relationship of the dissipation factor versus the electrical field (voltage) of the comparison of the new cable and the used cable with three different temperature (20°C, 60°C and 100°C) is shown in Figure 4-15. The dissipation factors of the new cable, the 5-year used cable and the 10-year used cable have a little bit different, clearly the cables which produced in Thailand are durable even those in long time
52 used. However, the service time is the major of the increasing of the dissipation factor, by observe with Figure 4-15, the highest dissipation factor occurred on the 10year used cable.
50 45
)
4 -
0 1 x (
δ
n a t
40
20°C N
35
20°C 5Y 20°C 10Y
30
60°C N
25
60°C 5Y
20
60°C 10Y
15
100°C N 100°C 5Y
10
100°C 10Y 5 0 0.5
1
1.5
2
U0 (kVrms )
FIGURE 4-15 The comparison of the dissipation factor of the new cable and the
used cable The comparison of the new and the used cable with the relationship of the dissipation factor versus the temperature with U0 is shown in Figure 4-16.
The
dissipation factor at 20°C to 40°C was slow increased and after 40°C it was fast increased with temperature raise. Figure 4-16 shows, the dissipation factor of the 10year used is the highest, the second is the 5-year used, and the new cable is the lowest. Clearly, the influence of the service time increased the cable tan . From Eq.2-33 the power dissipation in the cable is P=U2ωC (tan capacitance.
).
It depended on tan
and
53 50 45 40 35 )
4 -
0 1 x ( δ
n a t
30
at U0
25 20
new
15
5 year
10
10 year
5 0 20
30
40
50
60
70
80
90
100
temperatures ( 0C)
FIGURE 4-16 The comparison of the dissipation factor of the new cable and used
cable
4.3.2 The comparison of the new cable and the new cable under wet condition The relationship of the dissipation factor versus the electrical field (voltage) of the comparison of the new cable and the new cable under wet condition test with three different temperature (20°C, 60°C and 90°C) is shown in Figure 4-17.
The
dissipation factor of new cable under wet condition test obviously more than the dissipation factor of the new cable and it is increased follow with voltage raise but the dissipation factor of new cable do not increased with voltage raise. The cause of increasing of the dissipation factor of the cable under wet condition test is the decreasing of cable insulator resistance with influence of the temperature and the moisture when it was tested in wet condition.
54
60 at U0
50
)
20°C
40
4 -
0 1 x (
20°C wet condition 30
60°C
δ
n a t
60°C wet condition 20
90°C 90°C wet condition
10 0 0.5
1
1.5
2
U0 (kV rms )
FIGURE 4-17 The comparison of the dissipation factor of the new cable and the new
cable under wet condition The relationship of the dissipation factor versus the temperature of the new cable and the new cable under wet condition test at 90°C and U0 is shown in Figure 4-18. It is very clearly, the tan of new cable under wet condition test is higher than the tan
of new cable.
It is well known that when the cables are aged in wet
environment, the deterioration under effect of the water is occurred. Among many hypotheses about the mechanism of the water influence on service life of the cable it can be outlined that the water penetrates into solid cable insulation from the outside and condenses at defects such as voids or impurities [1]. This is a cause of the increasing of the dissipation factor of the new cable under wet condition test. 60
)
4 -
0 1 x ( δ
n a t
at U0
50
new
40
wet condition
30 20 10 0 20
30
40
50
60
70
80
90
temperatures ( 0C)
FIGURE 4-18 The comparison of the dissipation factor of the new cable and the new
cable under wet condition
55 4.3.3
The comparison of the new cable under wet condition and the 10-year
used cable under wet condition The comparison of the dissipation factor versus voltage with three different temperature is shown in Figure 4-19. 4-19. The dissipation factor of the new cable under wet condition test and the dissipation factor of the 10-year used cable under wet condition test was increased follow with voltage and temperature raise and the dissipation factor of the 10-year used cable was higher than that of the new cable under the same test. Clearly, the moisture and service time is the cause of the increase of the dissipation factor follow the voltage and the time raise. 70 60 50
20°C new wet wet condition
)
4 -
0 1 x ( δ
n a t
20°C 10Y wet condition
40
60°C new wet wet condition 30
60°C 10Y wet condition 90°C new wet wet condition
20
90°C 10Y wet condition 10 0 0 .5
1
1 .5
2
U0 (kV rms ) rms
FIGTURE 4-19 The comparison of the dissipation factor of the new cable under wet
condition and the 10-year used cable under wet condition
The relationship of the dissipation factor versus the temperature of the new cable under wet condition test and the 10-year used cable under wet condition test is shown in Figure 4-20. The dissipation factor remain increase with the temperature raise and the dissipation factor of the 10-year used cable under wet condition test is higher than that of the new cable under wet condition test, clearly service time is one of the cause of the increasing of of the dissipation factor. However, the XLPE cables which produced in Thailand are durable even those in long times used and long time test, the dissipation factor is less than the standard.
56
60 50
)
40
0 1 x (
30
4 -
δ
n a t
at U0
20
new wet wet condition
10
10Y wet condition
0 20
30
40
50
60
70
80
90
temperature ( 0C)
FIGURE 4-20 The comparison of the dissipation factor of the new cable under wet
condition and the 10-year used cable under wet condition
4.3.4 The comparison of the new cable, 5-year used cable, 10-year used cable, new cable under wet condition and 10-year used cable under wet condition The comparison of all test with four different voltage (0.5U 0 to 2U0) at 90°C is shown in Figure 4-21. The highest tan
is the 10-year used cable under wet condition
test and the second is the new cable under wet condition test, the lowest is the new cable. It is well know that the aged XLPE XLPE cable insulations have many many micro voids whose number increases with the distance from the cable conductor. The dimensions and number depend on the technology and the kind of cable insulation. When the aging process at the temperature is higher than the prolong room temperature to aging time, the micro voids are of larger size.
Generally, it is assumed that during
production, micro void impurities, water and re sidual products from cross linking l inking will be collected in amorphous regions of the insulation and a nd on service life or during test the water may penetrates into solid cable insulation from the outside and condense at defects such as voids or impurities [1]. So the moisture and and the service time are the cause of the increased of the dissipation factor of the XLPE cable.
57 70 60 50 )
4 -
0 1 x (
40
δ
30
n a t
90°C new 90°C 5Y
20
90°C 10Y
10
90°C new wet wet condition 90°C 10Y wet condition
0
0 .5
1
1 .5
2
U0 (kV rms ) rms
FIGURE 4-21 The comparison of the dissipation factor of the new cable, 5-year
used cable, 10-year used cable, new cable under wet condition and 10-year used cable under wet condition
The relationship of the dissipation factor versus the temperature of the new cable, the 5-year used cable, the 10-year used cable, the new cable under wet condition test and the 10-year used cable under wet condition test is shown in Figure 4-22. At 20°C to 40°C the dissipation factor of of the 5-year used cable, 10-year used cable, and new cable under wet condition test was slow increased, after 40°C it was fast increased, exempt the 10-year used cable under wet condition test, it was continuous increased with 20°C 20°C to 90°C. Clearly, the moisture and the service time is the main cause of the increasing of the dissipation factor of the 10-year used cable. at U0
60
new 50 )
4 -
0 1 x ( δ
n a t
5 year
40
10 year new wet wet condition
30
10Y wet condition 20 10 0 20
30
40
50
60
70
80
90
temperature (0C)
FIGURE 4-22 The comparison of the dissipation factor of new cable, 5-year used
cable, 10-year used cable, new cable under wet condition and 10-year used cable under wet condition
58 4.4 The Conclusions of The Test Result
The test result of the new cable, the 5-year used cable and the 10-year used cable show the influence of the temperature, the service time and electrical fields (voltage), the dissipation factor of the new cable, the 5-year used cable and the 10-year used cable was increased follow with temperature. The dissipation factor of the 10-year used cable is the highest that it was indicated of the influence of the service time. At 20°C to 90°C the dissipation factor of all cable do not increase with voltage raise but at 100°C dissipation factor of all cable was increased follow with voltage raise. It shows that at 100°C the cable insulator begin get worse in quality. By comparing with IEC 60502-2 and IEC 141, all cable are still in good condition because of the dissipation factor was lower than that of the standard. The test result of the new cable under wet condition and the 10-year used cable show the influence of the moisture when the cable was immersed in the water tank. The water temperature was maintained at 90°C and the voltage of U 0 was applied conductor to ground continuously during aging. The dissipation factor of the new cable under wet condition test and the 10-year used cable under wet condition test are higher than the dissipation factor of the new cable, the 5-year used cable and the 10year used cable.
We can conclude that influence of the moisture increased the
dissipation factor of the XLPE cable.
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
This chapter was concluded the all of test result then the matter was divide as the new cable and the used cables test result, the new cable under wet condition test and the 10-year used cable under wet condition test. The others were described the advantage of this thesis and the improvement of this thesis involves the recommendations for the future study as follow.
5.1 The Conclusions of The New Cable and Used Cables Test
To consider the test result, in the case of good cable the increasing of the dissipation factor only increase follow with the temperature raise but for the bad cable the dissipation factor increase follow with temperature and the voltage raise. The increasing of the dissipation factor lead to the thermal breakdown in the future.
5.2 The Conclusions of The New Cable Under Wet Condition Test and The 10year Used Cable Under Wet Condition Test
To consider the test result, the dissipation factor do not immediately increased follow with the time raise. After 400 hours the dissipation factor begin with voltage and time raise that show the worse in quality of the cable insulator. The highest dissipation factor is the 10-year used cable because it used for a long time, so its insulator is easy cracked and water was thoroughly absorbed into the cable insulator lead to increasing of the insulator conductance.
5.3 Conclusions
In order for an appropriate conclusion to be made, it is important to keep in mind the specific reasons for undertaking and therefore this places stress on the current high voltage equipment.
It is then important for the condition of this
equipment to be monitored thus emphasizing the point that reliable condition monitoring techniques need to be employed. For this reason, studies have been under
60 taken with the goal of determining a valid and reliable technique for monitoring the condition of equipment. This thesis focuses on underground power cables because of their growing demand in the power industry. Also the type of insulation in the cables experimented on were different, where as previous studies focused more on XLPE insulation. This method was looked at as a method for determining the insulation condition. The following conclusions are draw from the research and implementation of this study: 1. Applying the tan in quite easy
when
measurement method for diagnostic testing of insulation
compared
to
the other methods. Also the
tan
measurement can be applied to varying types of insulation not just XLPE cable. 2. The experimental results from the tan
curve indicated that there is indeed a
correlation between the voltage (electrical field), aging, moisture and time. The tan
curve is directly related to the thermal aging processes and this reiterated by the results obtained. 3. Comparing the results obtained with theory it was found that 10-year used cable was determined to be in the worst condition, new cable and 5-year used cables were in better condition. This shows the increase of cable’s conductance due to it was used in long time. 4. The dissipation factor of the new cable under wet condition was not immediately increased but it was increased after 400 hours. It
was different
with 10- year used cable under wet condition that the dissipation factor was increased after 100 hours. This shows the increase of the cable’s conductance due to it was used in long time. 5. The XLPE cables which produced in Thailand were durable even those in long time use and long time test. The tan also less than 80x10
-4
which
accord
was not increased follow voltage raise and with
the IEC 60502-2 specifications on -4
dissipation factor for conventional XLPE (tan <80x10 ). Overall, the tan
measurement method is an effective tool for determining
the condition of insulation in high voltage underground power cables. The results obtained support the previous studies in this area. Therefore this thesis has obtained
61 the main objectives, which are studying the techniques of the tan
measurement
method and determining the validity of its use on high voltage underground cables.
5.4 Recommendations
From the research completed during this thesis, some recommendations for future studies in this area are listed as follows: 1. Experimenting on an industry cable still in use and comparing the results against those obtained in the lab would be beneficial for overall conclusions. Future studies could incorporate a site visit involving experimentation on cable in the field. 2. The cables experimented on was XLPE. Hence future the testing in this area could be done on cables with different types.
REFERENCES
1. Nikolajevic, S.V. “Investigation of Water Effects on Degradation of Crosslinked Polyethylene (XLPE) Insulation.” IEEE Transactions on Power Delivery. 8 (October 1993) : 1682-1688. 2. Kuschel, M. and KalknerW. “Dielectric Response Measurment in Time and Frequency domain of Different XLPE Homo-and Copolymer Insulated Medium Voltage Cables.” IEE Proc.-Sci. Means. Technol. 146 (September 1999) : 243-248. 3. Bolarin Oyegoke, Petri Hyvonen, Martti Aro and Ning Gao “Application of Dielectric Response Measurment on Power Cable Systems.” IEEE Transactions on Dielectrics and Electrical Insulation. 5 (October 2003) : 862-873. 4. Williams, J.A. Underground Transmission Systems. New York : Electrical Power Research Institute, Inc., c1992. 5. Haddad, A. and Warne, D. Advances in High Voltage Engineering. London : MPG Books Limited, c2004. 6. Phelps Dodge Thailand Limited. High voltage Power Cables and Their Applications. New York : Pleasant Hill, c1985. 7. Chan, J.C., Hartley, M.D. and Hiivala, L.J. “Performance characteristics of XLPE Versus EPR as Insulation for High Voltage Cables.” IEEE Electrical Insulation Magazine. 9 (May/June 1993) : 8-12. 8. Chan, J.C., Cometa, E.T., Hiivala, L.J. and Hartley, M.D. “Electrical Aging Performance of Tree-Retardant XLPE Versus Standard XLPE as Insulation For distribution Cables.” IEEE Transactions on Power Delivery. 7 (April 1992) : 642-648. 9. Katz, C., Fryszcyn, B. and Walker, M. “ Comparative
Laboratory Evaluation
of TR-XLPE and XLPE Cables With Super-Smooth Conductor Shields.” IEEE Transactions on Power Delivery. 19 (October 2004) : 1532-1537.
63 10. Faremo, H. and Lldstd, E. “Water treeing and dielectric loss of WTR-XLPE Cable Insulation.” IEE Proceedings-A. 140 (September 1993) : 393-396. 11. Nikolajevic, S.V. “The Behavior of Water in XLPE and EPR Cable and its Influence on The Electric Characteristics of Insulation.” IEEE Transactions on Power Delivery. 14 (January 1999) : 39-45. 12. R.Bartnikas, R. “Performance Characteristics of Dielectric in the Presence of Space Charge.” IEEE Transactions on Dielectrics and Electrical Insulation. 4 (October 1997) : 544-557. 13. Nikolajevic, S.V. “Accelerated aging of XLPE and EPR Cable Insulation in wet Conditions and its Influence on electrical Characteristics.” IEEE International Conference on Conduction and Breakdown in Solid Dielectrics. (June 1998) : 337-340. 14. Jeong Park, Jong, S.Lee., and Chin, C. “Microwave Measurements on Dielectric Constants and Dissipation Factors of Dielectric Materials.” 2003 Electronic Components and Technology Conference. 1800-1803. 15. Kuffel, E. and Zaengl, W.s. High-Voltage Engineering. New York : Pergamon Press, c1984. 16. IEEE Power Engineering Society : 400.2, IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequnecy (VLF). 17. IEC : 141-1, International Standard. 18. IEC : 6502-2, International Standard. 19. 3M Thailand Limited, 3M Termination. 20. Namiki, Y., Shimanuki, H., Aida, F. and Morita, M. “A Study On Microvoids and Their Filling in Crosslinked Polyethylene Insulated Cables.” IEEE Transactions on Electrical Insulation. 15 (December 1980) : 473-480.
APPENDIX A
Water Tree
65 Water tree are small tree shaped channels found within the insulation of a cable, caused by the presence of moisture. They are very prevalent in service aged XLPE and other solid dielectric cables, like PE and EPR cables. These tree shaped moisture channels, in the presence of an electrical field, eventually lead to the inception of partial discharge (PD), which eventually leads to the formation of electrical trees, which grow to a point where insulation failure occurs. The tan of water tree damage in a cable, Fig.A-1, A-2 and A-3.
FIGURE A-1
The water tree
FIGURE A-2
The water tree
test shown the extent
66
FIGURE A-3
The extent of water tree damage in the cable
APPENDIX B
Dielectric Losses
68 The amount of power losses in a dielectric under the action of the voltage applied to t is commonly known as dielectric losses [2]. When metal conductor is
connected to a DC source the loss of Power P in watts is given by:
P
=
U
2
/ R
Where U is the voltage applied and R is he insulation resistance. Figure B-1 depicts a phasor diagram of currents and voltages in a capacitor energized by voltage. The phase angle φ is slightly less than 90° and the total current I through the capacitor is resolved into two components; active I c and reactive I r currents.
FIGURE B-1
Diagram of currents in a lossy dielectric
The phase angle describes a capacitor from the viewpoint of losses in a dielectric. The angle is called the dielectric loss angle. Since the phase angle is very close to 90° in a capacitor with the tangent of this angle is equal to the ratio between the active and reactive currents:
tan δ
=
I c
/ I r
Above equation is known as the loss factor.
