ELECTRICAL PROPERTIESOFCABLE INSULATION MATERIALS



BRUCE S. BERNSTEIN
Paper-Insulated Lead Covered Cables
PILC-Fundamentals
INSULATION MATERIALS
MEDIUM VOLTAGE
Polyethylene[PE]
Crosslinked PE [XLPE]
Tree Retardant Crosslinked PE [TR-XLPE]
Ethylene-Propylene Elastomers [EPR]
PILC

HIGH VOLTAGE
Crosslinked Polyethylene
PAPER/OIL
Paper/Polypropylene [PPP]
SF6 Gas
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
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

PILC
CELLULOSE-Insulation Material
HEMICELULOSE-Non-fibrous
more polar
losses higher vs. cellulose
LIGNIN-Amorphous
binds other components in the wood
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
Paper/Oil
LEAD SHEATH over cable construction
Protects cable core
Benefit: Superior barrier to outside environment
COMPARISON OF CABLE INSULATION MATERIALS
PE / XLPE / TR-XLPE /EPR / PILC
Polyethylene
Low Permittivity: limits capacitive currents
Low Tan Delta: Low Losses
Very High dielectric strength (prior to aging)
Easy to process/extrude
Crosslinked Polyethylene
All of the above PLUS
Improved mechanical properties at elevated temperature
does not melt at 105°C and above
thermal expansion
Improved water tree resistance vs. PE
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

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
EPR
Compromise of extruded cable properties
Permittivity, Tan Delta > XLPE’s
Dielectric strength slightly lower
High temperature properties :
Equal to or > XLPE’s
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
Extruded Cables at High Temperatures
PE/XLPE/TR-XLPE
At elevated temperatures, crystalline regions start to melt
Thermal expansion
Physical/mechanical strength reduced
At 105°C 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
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
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
Paper/Oil at High Temperature
Cellulose: No significant thermal expansion
compare with extruded cables
Oil: Some thermal expansion
Degradation mechanisms differ at elevated temperature
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 to antioxidant efficiency
Does not start until stabilization system affected
No water evolution
No proven model exists
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 remove weak link
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 weakened regions/crosslinks ‘hold together’ form stability
DC: Latent problem -effect depends on age (XLPE)
Electrical Properties ___________________Determined By Physical and Chemical Structure

Polyolefin Properties
Electrical properties/General
Dielectric constant
Dissipation factor
Volume resistivity
Dielectric strength
Polyolefin properties
Structure/property relationships
Dielectric strength
Partial Discharge
Measurement Methods
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
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
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

DC vs. AC
Under DC- Polarization persists

Under AC-Constant motion of the polymer segments due to changing polarity
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


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 temperature dependent
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

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
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
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
Dispersion
At frequencies where dispersion occurs
some energy stored
some energy dissipated a

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