Bernstein Danikas

7/30/2019 Bernstein Danikas

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ELECTRICAL PROPERTIES

OF

CABLE INSULATION MATERIALS

AND SOME FURTHER COMMENTS

BRUCE S. BERNSTEIN

MICHAEL G. DANIKAS


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Paper-Insulated Lead Covered Cables

PILC-Fundamentals


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INSULATION MATERIALS

MEDIUM VOLTAGE

Polyethylene[PE]

Crosslinked PE [XLPE]

Tree Retardant CrosslinkedPE [TR-XLPE]

Ethylene-Propylene

Elastomers [EPR]

PILC

HIGH VOLTAGE

Crosslinked Polyethylene

PAPER/OIL

Paper/Polypropylene [PPP]

SF6 Gas


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PILC

Cable is comprised of Paper strips wound over

conductor with construction impregnated with

dielectric fluid (oil)

Long Service History

Reliable/used since late 1800s

Gradually being replaced by Extruded Cables


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PILC

Paper derived form wood

Wood

Cellulose 40%

Hemicellulose 30% Poor Electrical Properties

Lignin 30% Serves as adhesive

Cellulose must be separated from others

Separation by bleaching

sulfate/sulfite process


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PILC

CELLULOSE-Insulation Material

HEMICELULOSE-Non-fibrous

more polar

losses higher vs. cellulose

LIGNIN-Amorphous

binds other components in the wood


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Paper/Oil

Cellulose chemical structure more complex vs. PE or

XLPE

Oil impregnates the cellulose/superior dielectric

properties

Different cable constructions for Medium vs. High

Voltage


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Paper/Oil

LEAD SHEATH over cable construction

Protects cable core

Benefit: Superior barrier to outside environment


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COMPARISON OF CABLE INSULATION

MATERIALS

PE / XLPE / TR-XLPE /EPR / PILC


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Polyethylene

Low Permittivity: limits capacitive currents

Low Tan Delta: Low Losses

Very High dielectric strength (prior to aging) Easy to process/extrude


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Crosslinked Polyethylene

All of the above PLUS

Improved mechanical properties at elevated

temperature

does not melt at 105C and above

thermal expansion

Improved water tree resistance vs. PE


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Tree-retardant XLPE

All of the above PLUS

Superior Water tree resistance to XLPE

TR-XLPE properties brought about by Additives to XLPE

Modifying the PE structure before crosslinking

Both


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Paper/Oil PILC

Long history of reliability

some cables installed 60 or more years !

More tolerant of some common diagnostic tests to

ascertain degree of aging

DC testing


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Advantages and disadvantages of paper/oil

insulation

The paper is the component most affected by temperature

The power factor increase is moderate and does not seem to beproportional to time or temperature

Cables examined after more than 20 years in service showedgood mechanical characteristics

BUT

Dielectric loss heating becomes significant compared withconductor losses for transmission voltages of 400 kV and higher

The increase of insulation thickness becomes excessive anduneconomical


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EPR

Compromise of extruded cable properties

Permittivity, Tan Delta > XLPEs

Dielectric strength slightly lower

High temperature properties :

Equal to or > XLPEs

Some types of EPR resistant to initiation and growth of water

and electrical trees even without additives

High thermal conductivity was also reported, higher thanXLPE at both 900 C and 1300 C (Brown 1983)


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Advantages of Extruded Cables

Reduced Weight vs. Paper/Oil

Accessories more easily applied

Easier to repair faults No hydraulic pressure/pumping requirements

Reduced risk of flammability/propagation

Economics

Initial and lifetime costs


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Extruded Cables at High Temperatures

PE/XLPE/TR-XLPE

At elevated temperatures, crystalline regions start to melt

Thermal expansion

Physical/mechanical strength reduced

At 105C crystallinity gone:

PE flows

XLPE and TR-XLPE crosslinking allows for

maintenance of FORM stability At high temperatures, crosslinks substitute for the

crystallinity at low temperature


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Extruded Cables at High Temperature

PE/XLPE/TR-XLPE

Although Crosslinks serve as Crystallinity substitute, they do

NOT provide same degree of

toughness

moisture resistance

Impact resistance

Crosslinking assists in maintaining form stability, but not

mechanical properties

Physical/electrical properties change as temperature

increases


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Extruded Cables at High Temperature

EPR

Little to no crystallinity initially

Form stability maintained due to presence of inorganic

mineral filler (clay)

Physical and electrical properties change to some extent as

temperature is increased

Present day issue: operating reliability at higher

temperatures vs. semicrystalline polymer insulation


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Some problems with polymeric materials in

cables

Oil composition may have an effect on polypropylene

since unlike cellulosic materials it may be soluble

in organic solvents (especially those with aromatic

molecules)

Swelling of HDPE at 900 C because of oil

composition

Non-orientated LMWPE undergo dimensional

changes following prolonged immersion in hothydrocarbon oils (Ernst and Muller 1981)


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Paper/Oil at High Temperature

Cellulose: No significant thermal expansion

compare with extruded cables

Oil: Some thermal expansion

Degradation mechanisms differ at elevated

temperature


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Thermal Degradation

Paper/Oil

Cellulose degradation

consistent from batch to

batch

Starts to degrade

immediately under thermal

stress

Moisture evolves

Follows Arrhenius model

Oil may form wax over

time(Polymerization)

Extruded

Degradation is polymer

structure related

Degradation related toantioxidant efficiency

Does not start until

stabilization system

affected

No water evolution

No proven model exists


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Summary: Paper/Oil

Natural Polymer

Carbon/Hydrogen/Oxygen

More polar

Not Crosslinked

Linear Fibrils/no thermal

Expansion

Oil expands thermally

Thermal degradation of

cellulose at weak link (C-O)

DC : No harmful Effect on

Aged cable-does removeweak link


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Summary: Extruded Materials

Synthetic Polymer

Carbon/Hydrogen

Less Polar

Branched chains Non-fibril

Partly crystalline: much less

for EPR

Mineral fillers (EPR)

Thermal expansion on

heating

Crosslinked

Degrades at weakenedregions/crosslinks hold

together form stability

DC: Latent problem -effect

depends on age (XLPE)


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Electrical Properties

___________________

Determined By Physical and Chemical

Structure


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Electrical Properties of Polyolefins

The Electrical Properties of Polyolefins may be separated into

two categories:

Those observed at low electric field strengths

Those at very high field strengths LOW FIELD

Dielectric constant/dissipation factor

Conductivity

Determines how good a dielectric is the insulation

HIGH FIELD

-Partial discharges (corona)

Controls functioning and reliability


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How does Polymer Insulation Respond to

Voltage Stress

Polar Regions tend to migrate toward electrodes

Motion Limited

Insulation becomes slightly mechanically stressed Charge is stored

Properties change

Next few slides seek to picture events in idealized

terms


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Polymer Polymer+

No field DC field applied,

polymer becomes

polarized

POLARIZATION OF A POLYMER THAT CONTAINS

MOBILE CHARGE CARRIERS


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Electrode

ORIENTATION OF POLYMER

UPON APPLICATION OF VOLTAGE STRESS

Alignment of Charge Carriers

Electrode Electrode ElectrodePolymerPolymer


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IDEALIZED DESCRIPTIONorientation of polar functionality of polymer chains under

voltage stress

No Voltage Voltage Stress Applied


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POLARIZATION OF A POLYMER THAT CONTAINS

SIDE GROUPS WITH PERMANENT DIPOLES

No field field applied,

polymer becomes

polarized


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SCHEMATIC OF SOME NORMAL MODES

OF MOTION OF A POLYMER CHAIN

First Mode Second Mode Third Mode


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Application of Low Voltage Stress

Dielectric Constant:Ability to hold charge

Lower Polarity -> Lower K

Dissipation Factor: Losses that occur as a result of

energy dissipated as heat, rather than electrical

energy

> Polarity leads to > Losses


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DC vs. AC

Under DC- Polarization persists

Under AC-Constant motion of the

polymer segments due to changing

polarity


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Dielectric Constant

TECHNICAL DEFINITION

In a given medium (e.g. for our purposes, in a

specific polymer insulation)it is the Ratio of

(a) the quantity of energy that can be stored,

to

(b) the quantity that can be stored in a vacuum


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Dielectric Constant

Relatively small if no permanent dipoles are present

Approximately proportional to density

Influenced by presence of permanent dipoles:

Dipoles orient in the electric field

inversely proportional to temperature

Orientation requires a finite time to take place

is dependent upon frequency

Relaxation time for orientation of a dipole is also temperaturedependent


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Dielectric Constant

The dielectric constant of the electrical-insulating

materials ranges from:

a low of about 2 or less for materials with lowest electrical-

loss characteristics, up to 10 or so for materials with highest electrical losses


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Dielectric Constant of Common Polymers

Polyethylene 2.28

Polypropylene 2.25

Butyl Rubber 2.45

Poly MMA 2.7-3.2 Nylon 66 3.34

PPLP (Oil-Impregnated)

2.7

Sources: M.L.Miller Structure of Polymers,Dupont and Tervakoski Literature

Cellulose Acetate 3.2-7.0

PVC 2.79

Mylar (Polyester) 3.3

Kapton (Polyimide) 3.6 Nomex (Polyamide) 2.8

Cyanoethylcellulose 13.3


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DIELECTRIC LOSSES

From a materials perspective, losses result from

polymer chain motion

Leads to heat evolution

Chain motion influence on electrical properties are

depicted on next few slides


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AND AS A FUNCTION OF FREQUENCY

log

log

max


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Dispersion

Dipoles RIGIDLY attached - oriented by MAIN chain motion

Dipoles FLEXIBLY attached - orientation of pendant dipoles

and/or

orientation by chain segmental motion(shows TWO dispersion regions)

at different frequencies

PE that has been OXIDIZED

PE that is a copolymer with polar monomer, e.g.,

SOME TR-XLPE

Note: 60Hz is not necessarily where these phenomena show maxima


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Electrical Properties of Polyolefins

Poly Olefin Structure and Dielectric Behavior

The electrical behavior of insulating materials is

influenced by temperature, time, moisture and other

contaminants, geometric relationships, mechanical

stress and electrodes, and frequency and magnitudeof applied voltage. These factors interact in a

complex fashion.

Saturated hydrocarbons are non-polar

Dielectric constants are low Dielectric constants essentially frequency-independent (if

pure)

Dielectric constants change little with temperature

The change that occurs is related to density changes


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ELECTRIC BREAKDOWN

INABILITY OF INSULATION TO OPERATE or HOLD

CHARGE UNDER STRESS

Insulation inherent properties

Thermal

Stream of electrons released

Discharge-

Preceded by Partial Discharge


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Electric BreakdownTypes of Breakdown

Failure of a material due to the application of avoltage stress called the dielectric strength

expressed as kV/mm or V/mil Electric breakdown occurs when the applied voltage

can no longer be maintained across the material in astable fashion without excessive flow of current andphysical disruption

Theoretical understanding not clear even now What is clear is that there are several mechanisms of

failure


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Intrinsic Breakdown

Defined by the characteristics of the material itself in pure and

defect-free form under test conditions which produce breakdown

at the highest possible voltage. Never achieved experimentally.Thermal Breakdown

Occurs when the rate of heating exceeds the rate of cooling by

thermal transfer and thermal runaway occurs under voltage

stress


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Discharge-Induced Breakdown

Occurs when electrical discharges occur on the

surface or in voids of electrical insulation. Ionization

causes slow degradation. Corona, or partialdischarge, is characterized by small, local electrical

discharges

Treeing-Electrical

Results from partial discharge


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Two main types of trees

Electrical trees consisting of hollow channels whichare branched and have the general appearance ofbotanical trees

Water trees having fuzzy appearance andconsisting of water-filled microvoids (McMahon,1981)

Besides thosethere are electrochemical trees(similar to water trees) and chemical trees (short andvery dense)


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AC Breakdown Strength of 15 kV XLPE Cable

Vs. Position on Cable Run

For One 5000 Foot Reel of 50,000 Foot Run

200

400

600

8001000

1200

1400

0 80 160 240 320 400 480

Position on Cable Run (sample number)

ACBreakdown(volts/mil)


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WEIBULL

Commonly used to characterize time to failure

information

Defines

Characteristic Time to Failure

Scale Factor/ 63.2% probability of failure

Shape parameter/slope of failure times

Called two parameter Weibull distribution

Controversial: Most physical models do predict this type ofdistribution for failure as a function of time (but not necessarily of

voltage stress) Dissado and Fothergill, Page 323

Probability - Weibull


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10.00 100.00

1.00

5.00

10.00

50.00

90.00

99.000.9

1.0

1.2

1.4

1.6

2.0

3.0

4.0

6.0

Probability - Weibull

Time, (t)

Cumulative%o

fSamplesFailed

WeibullData 1

W2 RRX - SRM MEDF=8 / S=4CB[FM]@90.00%

2-Sided-B [T1]


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Short-Time Voltage Breakdown

of Polyethylene

0

10

20

30

40

50

60

70

0 50 100 150 200

Thickness

PeakVoltage(k

V)

AC and DC at -196C

DC at room temperature

AC (60 Hz) at room temperature


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Dielectric Strength

Important Points to Remember !

AC breakdown strength value NOT absolute

Related to rate of rise of the applied voltage stress

5 minute step rise

10 minute step rise

30 minute step rise

Ramp

Real world/stress is constant Voltage endurance


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Presence of micro-voids

Temperature

Pressure

Nature and morphology of material under

investigation

Parameters of electrical circuit

Damage path (surface of volume) Intensity, duration, waveform of applied voltage


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Treeing- Water

Water tree growth induced by water in presence of

voltage stress. Water trees generate at much lower

stresses than electrical trees. Not a direct cause ofbreakdown


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Water Treeing

First reported in 1968

Bahder, et al. 1972: First distinguished between

water and electrical trees

Water trees/electrochemical trees

Water and Voltage stress required

Lead to reduced dielectric strength

Cleanliness required

All known since mid-late 1970s

Progress (?)

Ions influence

Jackets


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Original publications by Vahlstrom et al.

(1972)

They studied 15 kV and 22 kV PE cables in service of up to

eight years

Trees originated from contaminants and voids such defects

should be reduced as much as possible

Trees originated from insulating surface open to atmosphere

grow faster than trees wholly enclosed in the insulation

Tendency for tree initiation from a contaminant was probably

more affected by the contaminant material than by the size,

location or shape of the contaminant XLPE more resistant to electrochemical treeing than HMWPE

(probably because of the cross-linking byproduct acetophenone)


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Further comments

Influence of water and impurities are detrimental to

electrical characteristics in extruded cable insulation

Cables exposed to wet conditions are susceptible to

electrochemical treeing impurities make thesituation even worse

PE cables were more prone to trees than XLPE


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A Model of Water Trees in PE

cations

anionsPE outside

water tree

crystalline

amorphous

ionic end

groups

limiting

diameter

Void with

trapped saltlimiting

diameter

limiting

diameter


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General Procedure For Performing Electric

Breakdown Measurements

A number of specimens are tested in order that the

measurements may be a true reflection of the

statistical distribution of inhomogeneities in the

material under study The following variables must be strictly controlled and

identical in a series of tests:

Specimen thickness

Electrode shape and size Temperature

Frequency of applied field

Rate of increase of the field


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General Procedure For Performing Electric

Breakdown Measurements

In addition, care must be exercised in controlling the

moisture content and other pretreatment variables of

the specimen

In general, such breakdown tests are associated withsome particular conditions in which the insulation

material will be used, and these conditions therefore

become the dominant factors in designing the test

procedures

TYPICAL GEOMETRIC ARRANGEMENTS ILLUSTRATING


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TYPICAL GEOMETRIC ARRANGEMENTS ILLUSTRATING

THE USE OF PLASTICS AS AN INSULATOR

Uniform voltage gradient under electrode,nonuniform at edge of electrode and along

creepage path on the surface

An inset Rogowski (curved)electrode provides uniform

voltage gradient

Voltage gradient is not uniformacross the diameter; is highest

at the inner conductor

HV

Ground

HV

Ground

HV

Ground

X

t t

t

tX

R1

R2

Air, liquid, etc.Plastic

Metal electrode

Uniform voltage gradientalong surface of plastic


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Factors affecting the breakdown strength in

solids

Area effect

Effect of crystallinity (some materials showed a

decrease of breakdown strength with increasing

crystallinity, e.g. partially aromatic polymers)

Effect of impregnation XLPE showed improved

breakdown strength when impregnated with silicone

oil Impregnation of micro-porosities)

Temperature effect possibly the effect of thermalrunaway in localized electron system caused by the

interaction with conduction electrons)


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Factors affecting the breakdown strength in

solids

Interfaces very important a solid not well

impregnated is likely to breakdown at the interface

(However, carefully prepared paper-oil samples

indicated that breakdown does not necessarily takeplace at the interface, Kelly and Hebner, 1981)

Surface irregularities important particularly in thick

specimens


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Factors affecting liquid/solid insulation

Breakdown mechanisms

Failure mechanisms

ionic leakage current / chemical reactions of ions with

the rest of insulation

local thermal instability and possible increase of

acidity (Church and Garton - 1953, Gazzana-

Priaroggia et al. 1961, 1976)

ionizable impurities which may enhance space

charge losses (Bartnikas, Umemura et al.)

combination of moisture, temperature and high

operating electric stress (Wilkens)


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electochemical trees occur if adequate supply of polar liquid

(water) is exposed to a high electrical stress

swelling of polyolefins in oil indicated that swelling in partially

crystalline polymers is caused by molecules in the amorphous

regions incapable of going into true solution but enjoying a

quasi-soluble state (part of the molecule is locked into the

crystalline region and part diissolves in the immersion liquid)

contaminants are more injurious if they are near internal or

external screens than inside the insulation (Jocteur et al.

1977, Lyle and Kirkland - 1981)


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ELECTRICAL PROPERTIES

Survey of topics reviewed

Dielectric constant

Dissipation factor

Dielectric strength Water Treeing

Electrical Properties at low voltage stress are

dependent upon Chemical structure of Insulation

______________________________________ Reliability (Life) dependent upon other factors

Aging, local environment / beyond scope of this talk


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THANK YOU

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Bernstein Danikas