Impact of Fast-Front Overvoltage Transients on Electrical Oil-Paper Insulation Systems

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INTRODUCTION TO DESIGN AND ENGINEERING OF
ELECTRICAL INSULATION SYSTEMS FOR POWER NETWORKS
Professor K.D. Srivastava
The University of British Columbia, Vancouver,
B.C. Canada February 2015
2
Introduction

• 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.

3

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

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.

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.

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.

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.

8

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

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• 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.

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.

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.

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.

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.

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.

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

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

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.

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• 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.

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• 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.

21

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

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.

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.

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!

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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!

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• 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

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• n n0 exp ax
• Collisional Ionization in Nitrogen-Uniform
  Electric Field
• n0 electrons initially at x 0
• n electrons at x
• a ionization coefficient for the gas

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

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• 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.

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• Some gases are electronegative, that is, affinity
  of molecules for free electrons in the ambient
  gas. Examples are oxygen and sulphur hexafluoride
  (SF6). 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.

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• 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.

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Flashover voltage in SF6, air and N2
Gas Spacer 0.1 MPa 0.2 MPa 0.3 MPa
SF6 With spacer 122 121 123
SF6 Without spacer 61 83 95
Air With spacer 53 62 61
Air Without spacer 46 57 66
N2 With spacer 36 54 50
N2 Without spacer 23 31 45
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Cable Technology - 21st Century

• 1960s-1980s Fluid filled systems for HV
  and EHV
• 1980s-1990s Low loss PPL systems to match
  the paper laminates 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

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• 1970s-1990s Gas-filled (SF6) short lengths
  installed. Many lab models for higher
  voltages, including three phase designs in a
  single duct. Also, SF6 /N2
• 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

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Power Ratings for conventional cable
technologies

1. Paper fluid-filled 100
2. PPL fluid-filled 120
3. XLPE 110

36
Insulation Thickness for conventional cables
from 1990 to 1998

• 500kV - from 35mm down to 25mm
• 220kV - from 24mm down to gt20mm
• 132kV - from 22mm down to gt15mm

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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 SF6 is
  89kV/cm.bar

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Energy and Industrial Culture

• Post World War II, energy (all forms) usage was
  growing at the rate of 3 per year, in
  industrial nations

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!

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.

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)

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.

43

• Cost differentials for such cable when compared
  to overhead lines are in excess of 251 (some
  estimates put this as high as 401).

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.

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.

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.

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.

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!

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?

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.

51

• What are the alternatives?
• Three choices in technology
• Conventional underground power cables
• Compressed gas cables (SF6 - Sulphur
  Hexa-fluoride)
• Superconducting cables

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Why GIS? Why GITL?

• Land costs in urban areas
• Aesthetically superior to air insulated
  substations
• Not affected by atmospheric pollution

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

63

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

64

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

65
Design Features of GIS/GITL

• 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

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.

67

• SF6 is the insulating medium at a pressure of 4
  to 5 atmospheres. GITL units are
  factory-assembled in lengths of 40 to 50 feet.

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.

69

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

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Typical Cable Section
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• Growth of GIS

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• Growth of GIS Installations

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1. Current Transformer
2. Potential Transformer
3. Bus Section
4. Cable Termination

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Expansion joint
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SF6

• 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.

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.
• SF6 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).

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.

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.

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.

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.

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, (lt200 micrometer). Deconvolution of the
  measurements in some cases is necessary to get
  the actual distribution of the space charges
  within the sample.

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.

94
Experimental Studies of Fast-Front Transients
inOil 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

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.

97

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

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

101

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

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

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

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

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Figure 1a
106
Figure 1b
107
Figure 2a
108
Figure 2b
109

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

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Figure 3
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

112
Figure 4
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.

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.

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.

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• 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

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• 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.

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• 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.

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• The power loss measurements (Tan d) 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.

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• 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.

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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

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• 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.

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• 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

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

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

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A special pulse generator and a special electrode
system were designed for this purpose
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A special pulse generator and a special electrode
system were designed for this purpose
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A typical fast-front impulse waveform
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• 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.

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• 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).

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• 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.

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Effect of Risetime
Sample Layers FFI LI (base) SI
0.076 mm kraft paper one 0.85 1 1.15
0.076 mm kraft paper two 0.93 1 1.16
0.254 mm kraft paper three 0.93 1 1.17
0.254 mm kraft paper one 0.90 1 1.03
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Results for Different Thicknesses of Kraft Paper
Sample FFI (kV/mm) LI (kV/mm) SI (kV/mm)
0.076mm 155 182 210
0.254 mm 142 156 160
Difference 8 15 24
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Effect of Number of Layers on V50 Value (0.076 mm
thick kraft paper)
Sample FFI (kV/mm) LI (kV/mm) SI (kV/mm)
0.076mm 155 182 210
0.254 mm 142 156 160
Difference 8 15 24
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V50 Results for Different Impregnants (0.076 mm
thick kraft paper)
Oil Type FFI (kV/mm) LI (kV/mm) SI (kV/mm)
Transformer Oil 155 182 210
High Fire Point Oil 190 201 196
Difference -18 -10 7
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Discussion and Conclusions

• In both case-studies described above, the
  laboratory investigations were triggered by a
  large number of equipment failures in the
  industry

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• 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.

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• 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.

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• 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.

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.

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• 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

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.

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

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

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• 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.

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

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• 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.

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• 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.

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• 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.

150

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

151
Thank you!