1. The University of British Columbia, Vancouver, B.C. Canada February 2015 INTRODUCTION TO DESIGN AND ENGINEERING OF ELECTRICAL INSULATION SYSTEMS FOR POWER NETWORKS Professor K.D. Srivastava

2. At this Seminar on Insulation Technology, our principal motivation is to describe a design approach for large complex power apparatus, and in the process we use our accumulated previous experiences as best as we can. 2 Introduction

3. The dominant electrical parameters, for all gaseous insulation are: the local electric field, and the ambient gas pressure 3

4. These two parameters, jointly for any insulating gas, determine the initiation of the ionization process, when free electrons are present in the space where sufficiently high electrical field is present. The geometry of the metallic electrodes that create the high electric field “space” play a crucial role in determining the necessary local electrical field for the gaseous insulation. 4

5. The necessary initiatory electrons are present in the high electric field space either by cosmic radiation or by cold electron emission from a metallic surface that is subjected to high electric field or to high temperature, or both. The gaseous insulation medium may be open, such as open air atmosphere, or may be enclosed within a container. 5

6. The ionization characteristics of an insulating medium are determined by its intrinsic physio-chemical properties and its surrounding environmental context, that is, its specific usage for an electrical apparatus. If the electrical field is sufficiently high the insulating medium goes through a physical process of rapidly enhancing the ionization processes and creating a highly conducting gaseous channel, that is, an arc. 6

7. The final stages of insulation failure for all types of electrical insulation media (gaseous, liquid or solid) happens in a gaseous phase. Also, as mentioned earlier, in all electrical apparatus designs at least one solid insulating surface is subjected to the full design voltage of the specific apparatus. 7

8. The most common example of an open atmosphere gaseous application is the aerial electrical power transmission/distribution line. 8

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10. The electrical conductors are supported by post insulators, or suspension insulators or bushings at other high power apparatus such as transformers, circuit breakers or gas-insulated or air-insulated busbars at substations. The support insulators are a weak link in the insulation system. The surface flashover strength is significantly less than that of a gaseous gap between the electrical power conductors, or between the live conductors and the ground. 10

11. Other common failure modes for aerial power lines are: surface pollution on the support insulators from the ambient atmosphere icing of the insulator surfaces under low temperature weather and icicle formation ice formation also increases the conductor sag, thus reducing the air/gas clearance to ground. 11

12. also, under stormy conditions the live conductors also swing (often called “galloping”). This may cause mechanical failure or flashover to another phase of a three-phase 60Hz AC system. at some parts of the right-of-way there may be large trees and other tall vegetation. The right-of-way has to be regularly maintained and kept clear. 12

13. All insulation systems are also subject to power system generated high voltage transient overvoltages. The impact of such system disturbances is discussed later in this lecture. Lightning strikes will also generate high voltage transients on aerial power lines and the connected apparatus such as transformers, circuit breakers, and other equipment in a substation. 13

14. However, we recognize that technology very often depends upon a complex set of phenomena not fully understood and are not accessible for measurements and observation. Nonetheless, based on our accumulated experiences and continuing analyses and reinterpretations, we can still model them with reasonable accuracy. 14

15. This also informs us about the mutual interdependencies amongst the various critical "elements" that enable the device/apparatus to function and, in addition help the operators understand the ageing of the device and its various 'failure' modes. This methodology may be described as engineering modeling utilizing statistical methods. 15

16. In electrical power apparatus three types of insulation are used: gaseous insulating liquids solid insulating materials Interfaces between different insulation media present significant design challenges 16

17. always one solid insulator interface, which has significant tangential surface electric field, and is often subjected to the full design voltage of the specific apparatus. The final breakdown stages always occur in a gaseous phase 17

18. The breakdown voltage of an insulation system is a function of electric field non- uniformity. Local field enhancement can be a crucial factor. So is the applied voltage waveform, specifically, the rate-of- rise of voltage and the duration of application. 18

19. The density of the insulation medium also plays an important role. In liquids and solids local density variations occur, and molecular interactions would play a role in the failure processes. 19

20. The breakdown voltage of an insulation system is a function of electric field non- uniformity. Local field enhancement can be a crucial factor. So is the applied voltage waveform, specifically, the rate-of- rise of voltage and the duration of application. 20

21. The breakdown voltage is also dependent upon the surface area of the electrodes, larger the area, the lower is the withstand voltage. 21

22. Local field enhancement would contribute to free electrons in the insulation material. The mean free path, in dense media is quite small (~0.5 to 2 nm), so the initial free electrons are likely to get “trapped” or “thermalized”. This process often leads to “regions” of lower density in the insulation and, is known to initiate the breakdown process. 22

23. In electrical insulation, subjected to repetitive fast-front transients, the initial measurable indication is Partial Discharges(PD) in the insulation, such as motors driven by pulse modulated power electronics. It is important to understand the precursor deterioration processes. For example, generation of localized pockets of space charges. These charge accumulations lead to PDs and eventually failure of the apparatus. 23

24. Oil-paper electrical insulation systems have been in use for over a century! Although electrical grade cellulose based paper quality has considerably improved, its aging in service is predominantly impacted by water, oxygen, oil acids, particulate impurities created by other materials present in the design, temperature of operation and the electrical and mechanical stresses it is subjected to in its working life. One such process of degradation is polymerization! 24

25. Polymerization Cellulose is a polymer, composed of repeating glucose molecule. The numbers of glucose monomer in a polymer's chain is called the degree of polymerization. The DP number of unused paper is around 1,000. As the paper ages the DP value drops, and at DP around 200, the paper is at the end of its useful life! 25

26. It would be useful to explain the basic physical process of how a gaseous gap between two conductors, which have a high electric field between them, sparkover. The following illustrate the basic processes: 26

27. 27 n = n 0 exp αx Collisional Ionization in Nitrogen-Uniform Electric Field n 0 = electrons initially at x = 0 n = electrons at x α = ionization coefficient for the gas

28. Ionization processes in a gas are: collisions thermal radiation, including photoionization and x- rays and nuclear radiations, including cosmic rays 28

29. De-ionization occurs through re- combination, thermal diffusion, loss of energy (cooling) at solid surfaces and at metal boundaries by conduction into the “external” electric circuit. 29

30. Some gases are electronegative, that is, affinity of molecules for “free” electrons in the ambient gas. Examples are oxygen and sulphur hexafluoride (SF 6). This capture of electrons by neutral molecules slows down the ionization process, since molecules are heavier and move slowly when an electric field is applied. 30

31. A single electron, in a uniform electric field, multiplies exponentially – it is often called an “avalanche” – and it has a positive and negative charge carrier separation. Negative at the “head” and positive at the “tail”. 31

32. Flashover voltage in SF6, air and N2 GasSpacer0.1 MPa0.2 MPa0.3 MPa SF 6 With spacer122121123 Without spacer618395 Air With spacer536261 Without spacer465766 N2N2 With spacer365450 Without spacer233145 32

33. Cable Technology - 21st Century 1960s-1980s:Fluid filled systems for HV and EHV 1980s-1990s:Low loss PPL systems to match the paper laminate’s performance for EHV 1970s-1990s:Parallel development of XLPE systems from MV up to EHV 275kV XLPE cables in service. In Japan 500 kV XLPE installed 33

34. 1970s-1990s:Gas-filled (SF 6 ) short lengths installed. Many lab models for higher voltages, including three phase designs in a single duct. Also, SF 6 /N 2 1990s:500 kV mass impregnated paper for submarine DC systems in the Baltic Sea 1970s-1990s:Low temp. cryogenic/supercon. designs tried. 1990s witnessed the phenomenal growth in HTS technology 34

35. Power Ratings: for conventional cable technologies 1.Paper fluid-filled100% 2.PPL fluid-filled~120% 3.XLPE~110% 35

36. Insulation Thickness: for conventional cables from 1990 to 1998 500kV - from ~35mm down to 25mm 220kV - from ~24mm down to >20mm 132kV - from ~22mm down to >15mm 36

37. Design Stresses: for conventional cables Paper - from 10kV/mm to ~15kV/mm PPL - from ~18kV/mm to ~20kV/mm XLPE - from ~5kV/mm to ~35kV/mm [Theoretical maxm. stress in 100% SF 6 is ~89kV/cm.bar] 37

38. Energy and Industrial Culture Post World War II, energy (all forms) usage was growing at the rate of ~3% per year, in industrial nations 38

39. But in industrial nations electricity usage was growing by more than 7% by displacing other forms of energy With oil crisis of 1970s and the growing environmental movement, the energy picture is very different now! 39

40. In Europe (Western) and North America the electricity usage is almost constant. In developing countries, however, the usage is growing between 7 and 10% per year. With declining “confrontation” between major world powers, the prospects for rapid world economic growth are pretty good. 40

41. The availability of useful forms of energy is not equal worldwide, and there are major geographical barriers to the movement of energy in the world. There is considerable world experience in transporting oil, natural gas and electricity over long distances (thousands of km) 41

42. Present Status of “Conventional” Cable Technology Both oil-paper and polymeric cables up to 500 kV system voltage are in service and commercially available. Experimental designs of oil-paper cable have been tested for both 750 kV and 1000 kV. 42

43. Cost differentials for such cable when compared to overhead lines are in excess of 25:1 (some estimates put this as high as 40:1). 43

44. Cellulose paper will have to be replaced by synthetic polypropylene paper or a composite. Impregnating mineral oil will have to be replaced by more acceptable (from environmental point-of-view) alkyl benzenes. 44

45. At such high operating voltages the margin to the high voltage “intrinsic” breakdown is lower. Hence very high oil pressures (~20 atmospheres) and very high quality control is needed. 45

46. Technology of making joints is still in an experimental/development stage. Conventional cable technology is very well established and over the past 100 years there have been many technological improvements. 46

47. Compressed gas cable technology has matured over the last 30 years, but its potential for bulk power transport is yet to be exploited and developed. 47

48. High temperature superconductor technology is developing rapidly but [is] not yet fully commercially viable for bulk power transport. None of the above three are free from technological areas of concern! 48

49. Geographically and technologically South Africa has the potential and opportunity to play an important strategic role. This prospect raises the technological and economic question of:  How does one move large amounts of electrical energy to major urban centres? 49

50. Over sparsely populated areas, overhead lines are, perhaps, the only proven and economic option for long distances. However, near urban centres overhead lines are no longer acceptable to the communities for environmental and aesthetic reasons. 50

51. What are the alternatives? Three choices in technology:  Conventional underground power cables  Compressed gas cables (SF 6 - Sulphur Hexa-fluoride)  Superconducting cables 51

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61. Land costs in urban areas Aesthetically “superior” to air insulated substations Not affected by atmospheric pollution Why GIS?Why GITL? 61

62. Completely sealed (metal-clad) permits very low maintenance Demand for higher energy usage in urban areas requires increased transmission voltages; for example, 420 kV 62

63. GITL In addition to the advantages listed above for GIS, there is a need for non-aerial transmission lines near urban areas. 63

64. There are currently only two alternatives:  Underground cables–conventional or superconducting, or  Gas Insulated Transmission Lines (GITL) GITL, compared to underground cables, have the additional advantage of reduced ground surface magnetic fields. 64

65. GIS/GITL installations have the usual components: Circuit breakers; disconnect, earthing/grounding switches Current and voltage measuring devices Busduct sections Variety of diagnostic/monitoring devices Design Features of GIS/GITL 65

66. Installations from distribution voltages right up to the highest transmission voltages (765 kV) have been in service for 30 years or more. Both isolated-phase and three-phase designs are in use. 66

67. SF 6 is the insulating medium at a pressure of 4 to 5 atmospheres. GITL units are factory-assembled in lengths of 40 to 50 feet. 67

68. The phase conductor is almost always of aluminium. The outer enclosure is also of aluminium, although earlier designs used mild steel. For lower voltages, stainless steel has also been used. 68

69. Usually busducts are of rigid design although flexible and semi-flexible designs have been proposed. None are in use. 69

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71. Typical Cable Section 71

72. Growth of GIS 72

73. Growth of GIS Installations 73

74. 5.Current Transformer 6.Potential Transformer 7.Bus Section 8.Cable Termination 74

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85. Expansion joint 85

86. Sulphur hexafluoride is a man-made gas, and it is an electronegative electron attaching gas. It has been in industrial use for almost a century. Since its introduction in major equipment for electrical power industry, it has raised some serious environmental concerns. 86 SF 6

87. Before WWII it was mainly used as a tracer gas. With the advent of nuclear power generation, its use increased for refining uranium ore. It is included in the Kyoto Protocol. SF 6 is an excellent electrical insulating gas and has been used in power circuit breakers, gas insulated substations (GIS) and more recently in Gas Insulated Transmission Lines (GIL). 87

88. Its main physical properties are: basic electric breakdown strength 89 kV/cm normal condensation temperature 63° Its thermal properties are also very favourable for application in GIS/GIL. However, under arcing conditions its byproducts are both corrosive and toxic. The operative standards require it to be reclaimed and recycled. 88

89. Oil-Paper Composite Insulation There have been investigations for assessing the usefulness of vegetable oils in electric power apparatus as an impregnating insulation medium. The main components of such oils are triacylglycerols, the fatty acid components vary quite a lot. These are more prone to oxidation. 89

90. The three main chemical processes of insulation degradation are called hydrolysis, oxidation and pyrolysis. The degradation by-products are carbon monoxide, carbon dioxide, various organic acids, water, and free glucose molecules. Free glucose molecules can decompose further into a class of compounds called furans. These different compounds can be monitored and analyzed in the context of insulation deterioration and the applied external stresses. 90

91. In the longer term the higher fatty acid content protects the paper surface from further degradation, and also lowers the water content in the treated paper insulation. For similar reasons the Furanic content, at the same level of DP, is also less in paper treated with vegetable oils- this is also an advantage. 91

92. Published good reviews of the current methods for measuring space charges in dielectrics. The resolution capability is as high as 2 micrometer, but the sample thickness for such high resolution is also low, (<200 micrometer). Deconvolution of the measurements in some cases is necessary to get the actual distribution of the space charges within the sample. 92

93. Accumulated pockets of charge in composite and solid insulation systems have a very significant impact on the electric breakdown, aging and dielectric losses. For the oil-paper composites the energy loss is very important. However, for the longer-term failure modes the frequency of partial discharges (PDs) accelerates the failures. 93

94. 94 Experimental Studies of Fast- Front Transients in Oil Impregnated Paper Insulation System

95. In modern electric power systems there is a significant increase of power electronics devices such as : inverters/converters for HVDC numerous applications for renewable energy apparatus Such equipment generates repetitive fast- front-transients, and those transients are known to cause failure of oil-paper insulation systems in motors and transformers 95

96. The impact of high power power- electronics devices adds to the impact of vacuum interrupters and compressed gas insulated substations equipment which are known to generate fast-front transient overvoltages. In Europe and Canada aging studies of oil- paper insulation systems, subject to fast- front transient overvoltages, have been undertaken for over 25 years. 96

97. In this paper, two such studies are described and the findings are discussed. 97

98. 98 Re-striking process at opening of vacuum circuit breaker

99. Applied voltage: 300kV, 0.4 MPa (SF6) (81kV/div, 20 ns/div FTO waveform measured by 1-GHz surge sensor Source: M.M. Rao & M.S. Naidu, III Workshop on EHE Technology, Bangalore, India, 1995.

100. Case Study A: High Power Electronic Switching Systems Industrial applications high capacity power electronics devices are now ubiquitous Many components of such energy conversion systems make extensive use of composite oil-paper insulation in motors and transformers 100

101. Numerous equipment failures have been reported. A comprehensive laboratory study of such electrical insulation failures has been reported in Europe 101

102. The technology of power inverters and converters is very well established in power systems and numerous industrial applications. It utilizes fast solid-state switching devices, such as rectifiers, thyristors, inverted gate bipolar transistors (IGBT) and MOSFETS. Pulse width modulation methodology is commonly used 102

103. A high quality AC waveform switching device operates at higher frequencies up to 10 kHz In one study the rise time of a “square wave” was changed and the impact on the time to failure was measured; the insulation tested was for an adjustable speed motor 103

104. The circuit diagrams for these test voltages are shown in Figures 1a & 1b, and Figures 2a & 2b show the actual applied voltages to the insulation test samples 104

105. Figure 1a 105

106. Figure 1b 106

107. Figure 2a 107

108. Figure 2b 108

109. Figure 3 shows the test samples with noted visual differences. 109

110. Figure 3 110

111. Figure 4 shows that the time to failure decreases as the rate of rise of the applied voltage pulse is increased. In the European study, a spark generator has been designed to observe the impact of fast- front overvoltage on electrical insulation systems 111

112. Figure 4 112

113. Two different types of test voltages used in the experimental work: a combination of power frequency (50Hz) with a superimposed high frequency modulating signal a double-exponential fast-front impulse The modulated power frequency waveform is 50Hz with a peak magnitude of 5 kV. The modulating frequency is 10 kHz with a peak magnitude of 1 kV. 113

114. A typical single fast-front pulse is also shown. Its peak magnitude and the rate- of-rise of fast-front can be varied. The modulated power frequency waveform is used for insulation aging studies, and the single double exponential pulses are used to explore the physical processes for insulation deterioration. Single pulse polarities can also be reversed. 114

115. In the European studies kraft paper 0.06mm in thickness was used and was impregnated with Shell Diala B. the AC breakdown of the paper sample is 3.2 kV rms. The IEC standard IEC 60243 was followed for these studies. Both positive and negative voltage polarities were used. 115

116. When oil-paper insulation samples were subjected to a modulated power frequency voltage and fast repeating higher frequency components, ranging between 0.5 kHz and 10 kHz of bipolar pattern, rate of rise 1 kV/µs and average magnitude of 1 kV: 116

117. 1.The average value of breakdown was 3.2 kV superimposed 2.At 5 kHz, 8 kHz and 10 kHz, with a peak transient magnitude of 1 kV and power frequency magnitude of 2.91 kV, the average time to insulation failures were 22, 12.5 and 10.1 hours respectively. 117

118. In the absence of high frequency superimposed transients, the power frequency breakdown delay was about 168 hours. Clearly the observed failures in service may be attributed to the high frequency transients generated by power electronics high speed switching. 118

119. The power loss measurements (Tan δ) also show very significant increases. The insulation paper samples were also inspected visually The samples subjected to high frequency transient show signs of carbon deposits, perhaps due to local partial discharges. 119

120. It should be noted that in these European studies, the fast-front transient is superimposed on top of the power frequency (50 Hz) applied voltage. This is very different from the case study B, described below, where the magnitude of the fast-front transient is significantly higher than the power frequency applied voltage. 120

121. Case Study B: GIS Fast-front Transient Impact on Oil Paper Insulation Numerous failures of transformers and their bushings connected to compressed gas insulated substations have been reported, for system voltages from 220 kV to 765 kV 121

122. Field tests showed that fast-front transients, up to 1.2 per unit peak voltages with a risetime of 25 ns could be attributed to GIS disconnect switch operation. The principal gaseous insulation in GIS is sulphur- hexafluoride gas, which is an electronegative gas. 122

123. In 1980 the Canadian Electrical Association sponsored a laboratory study to explore the possible behavior of oil-paper insulation when repetitive fast-front impulses were applied to samples of oil-paper insulation 123

124. The experimental work was done at the BC Hydro research laboratory, Powertech Labs Inc., in Surrey, BC, Canada 124

125. A special pulse generator and a special electrode system were designed for this purpose 125

126. A special pulse generator and a special electrode system were designed for this purpose 126

127. A special pulse generator and a special electrode system were designed for this purpose 127

128. A typical fast-front impulse waveform 128

129. The fast-front high voltage pulse generator, for peak voltages up to 100 kV, could be synchronized with a half wave 60 Hz power source and is capable of generating impulses at the rate of 1500 pulses per minute, that is up to 2.5 million per day. 129

130. In addition to the custom built pulse generator tests were also done with DC and 60 Hz voltage and standard lighting impulse 1.2/50µs and 5.7/130µs and switching impulses and a fast-front impulse (10 ns/2500 µs). 130

131. Test samples were one, two and three layers of 0.076 mm thick kraft paper, one layer of 0.254mm thick kraft paper, one layer of 0.76 mm thick Nomex paper and one layer of 0.254mm thick polyester sheet. Two kinds of impregnants were used: standard transformer oil and a high fire-point oil. 131

132. Effect of Risetime 132 Sample LayersFFI LI (base) SI 0.076 mm kraft paper one0.8511.15 two0.9311.16 0.254 mm kraft paper three0.9311.17 one0.9011.03

133. Results for Different Thicknesses of Kraft Paper 133 Sample FFI (kV/mm ) LI (kV/mm) SI (kV/mm) 0.076mm155182210 0.254 mm142156160 Difference8%15%24%

134. Effect of Number of Layers on V50 Value (0.076 mm thick kraft paper) 134 Sample FFI (kV/mm ) LI (kV/mm) SI (kV/mm) 0.076mm155182210 0.254 mm142156160 Difference8%15%24%

135. V50 Results for Different Impregnants (0.076 mm thick kraft paper) 135 Oil Type FFI (kV/mm) LI (kV/mm)SI (kV/mm) Transformer Oil155182210 High Fire Point Oil 190201196 Difference-18%-10%7%

136. Discussion and Conclusions In both case-studies described above, the laboratory investigations were triggered by a large number of equipment failures in the industry 136

137. In both case studies, the focus of the laboratory investigations has been the oil- paper insulations system, since it is very extensively used in a wide range of voltage classes from HV to UHV. 137

138. As the equipment use continues, in time, pockets of space charges develop in the insulation systems. These space charge discontinuities play a very major role in the aging and failure modes of the apparatus and equipment. 138

139. Several factors, including the quality of materials, the sample preparation, the applied voltage magnitude and waveform and the number of fast-front pulses and the intervals between the impulses are just a few factors that would impact the aging phenomena is the field. 139

140. There are some overall impacts of fast-front repetitive applications on power apparatus and devices. For example, the breakdown electrical strength reduces as the risetime gets shorter. 140

141. Both the European results and the Canadian results confirm this, albeit that the voltage magnitudes and the context of the operation of the specific equipment are quite different 141

142. In another aspect these are different since the power system operating voltages are vastly different, in the case of European investigation and the Canadian one. 142

143. Increasing the number of impulse applications at higher system operating voltage does reduce the safe impulse electric field magnitudes 143

144. The results for the power electronics application may also indicate: longer term aging is present in lower system voltage applications. 144

145. The breakdown phenomena have to be studied in order to understand the deeper physical/chemical processes that may be determining the deleterious impact on insulation ageing and useful lifetime of composite oil-impregnated paper/cellulose insulation systems. 145

146. Current work may provide a better understanding of the failure modes of the complex composite insulation systems. 146

147. It is evident from the accumulated results of laboratory studies and the industry's manufacturing, testing and field experience that the impregnated oil-paper insulation system is a very complex combination of material, processing technologies and very poorly understood physical and chemical processes during manufacturing and contexts under which the equipment is used in the field. 147

148. The equipment designers, manufacturers and industrial users, for almost over a century, have oversimplified the physical/chemical framework under which in practice the insulation must operate in apparatus and equipment. The focus has been on adopting a phenomenological approach. 148

149. A very useful set of design criteria, material selection, manufacturing practices and development testing protocols have served the industry well. The good news is that, in the last several decades very useful research and development has taken place in industrial countries. 149

150. Major long-term basic research and R&D is currently underway. References are two such examples of research, development, fundamental measurement techniques and testing protocols. 150

151. Thank you! 151