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

• 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.

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Introduction

• The dominant electrical parameters, for all gaseous insulation are:

• the local electric field, and• the ambient gas pressure

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• 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.

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• 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.

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• 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

• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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

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• 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

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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.

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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.

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

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• 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.

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• 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

• 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 αx

Collisional Ionization in Nitrogen-Uniform Electric Field

n0 = electrons initially at x = 0

n = electrons at x

α = ionization coefficient for the gas

• 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 N2Gas Spacer 0.1 MPa 0.2 MPa 0.3 MPa

SF6

With spacer 122 121 123

Without spacer 61 83 95

AirWith spacer 53 62 61

Without spacer 46 57 66

N2

With spacer 36 54 50

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 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

• 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%

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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 >15mm36

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]37

Energy and Industrial Culture

• Post World War II, energy (all forms)

usage was growing at the rate of ~3%

per year, in industrial nations

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• 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

• 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

• 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

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

• 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).

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• 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.

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• 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.

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• 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.

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• 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.

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• 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!

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• 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

• 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

• What are the alternatives?

• Three choices in technology:

Conventional underground power

cables

Compressed gas cables (SF6 - Sulphur

Hexa-fluoride)

Superconducting cables51

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• Land costs in urban areas

• Aesthetically “superior” to air insulated

substations

• Not affected by atmospheric pollution

Why GIS? Why GITL?

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• Completely sealed (metal-clad) permits

very low maintenance

• Demand for higher energy usage in urban

areas requires increased transmission

voltages; for example, 420 kV

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GITL• In addition to the advantages listed above

for GIS, there is a need for non-aerial

transmission lines near urban areas.

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• 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

• 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

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• 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.

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• 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.

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• 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.

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• 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 Section71

Growth of GIS72

Growth of GIS InstallationsBefore 1985 January After 1985 January

Voltage GIS CB-Bay-Yrs. GIS CB-Bay-Yrs.

1 230 28669 731 28215

2 227 21252 382 12808

3 123 10362 147 5678

4 45 3870 65 2904

5 26 3252 37 1273

6 - - 2 200

Total 751 67,405 51,078

Voltage Class

1 60 – 100 kV

2 100 – 200 kV

3 200 – 300 kV

4 300 – 500 kV

5 500 – 700 kV

6 >700 kV

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5. Current Transformer

6. Potential Transformer

7. Bus Section

8. Cable Termination

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Expansion joint

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• 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.

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SF6

• 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).

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• 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.

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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.

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• 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

• 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.

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• 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.

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• 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.

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Experimental Studies of Fast-Front Transients inOil Impregnated Paper

Insulation System

• 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

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• 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.

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

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Re-striking process at opening of vacuum circuit breaker

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.

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

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

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• 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

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• 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

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• 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

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Figure 1b

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Figure 2a

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Figure 2b

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

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Figure 3

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• 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

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Figure 4

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• 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.

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• 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.

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• 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 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.

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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

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Sample Layers FFILI

(base)SI

0.076 mmkraft

paper

one 0.85 1 1.15

two 0.93 1 1.16

0.254 mmkraft

paper

three 0.93 1 1.17

one 0.90 1 1.03

Results for Different Thicknesses of Kraft Paper

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SampleFFI

(kV/mm)

LI (kV/mm)

SI (kV/mm)

0.076mm 155 182 210

0.254 mm 142 156 160

Difference 8% 15% 24%

Effect of Number of Layers on V50 Value (0.076 mm thick kraft paper)

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SampleFFI

(kV/mm)

LI (kV/mm)

SI (kV/mm)

0.076mm 155 182 210

0.254 mm 142 156 160

Difference 8% 15% 24%

V50 Results for Different Impregnants (0.076 mm thick kraft paper)

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Oil TypeFFI

(kV/mm)LI (kV/mm) SI (kV/mm)

Transformer Oil 155 182 210

High Fire Point Oil

190 201 196

Difference -18% -10% 7%

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.

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• 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

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• 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.

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• 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.

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Thank you!

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