Basic Properties of Power Cable Insulations

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Basic Properties of Power Cable Insulations

• Bruce Bernstein
• Electric Power Research Institute
• (EPRI)

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Cable Components
Conductor Copper or Aluminum Stranded or Solid
Carries the electrical power from the generating
station to the customer
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Cable Components
Strand shield Black Semiconducting Crosslinked
Polyethylene
Used to fill in the interstices on a stranded
conductor and provide a smooth
cylindrical surface for the insulation to bond to
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Component RequirementsConductor Shield

• Surface smoothness
• Compatibility with interface materials
• Uniform conductivity
• Inseparable bond to insulation

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Cable Components
Insulation Shield Black Semiconducting Crosslinked
Provides a uniform cylindrical grounded surface
in intimate contact with the insulation
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Component RequirementsInsulation Shield

• Surface smoothness
• Compatibility with interface materials
• Uniform conductivity
• Controllable strippability

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Cable Components
Neutrals Always Copper Bare or Tinned
Provide a return path for the current and also
a continuous ground around the cable
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Cable Components
Jacket Black LLDPE Black Semiconducting Black
Track resistant High Density
Provides a moisture barrier Prevents corrosion
of neutrals Provides mechanical protection
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Fundamentals of Polymers Used in Extruded Cables

• Polyethylene
• Commonly used terminology
• Crosslinking
• Crystallinity
• Sol
• Gel
• Amorphous
• High Molecular Weight
• Fillers (in EPR)
• Crosslinking agent by-products
• Crosslinking agent
• What do these terms mean?

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Fundamentals of Polymers Used in Extruded Cables

• To answer these questions, it is first necessary
  to review a few fundamentals of polymer science
  and engineering
• After this review, the terms will be easy to
  understand
• All polymers can be depicted as wavy lines, like
  this

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The Addition Polymerization of Ethylene to
Polyethylene
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A Branched Polymeric Chain
A branched polymeric chain
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Chain Length (Molecular Weight)
Chain Length Molecular Weight
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Common Polymer Structures
These wavy lines
represent a simplified way to depict polymer
structure
For Polyethylene, the wavy line means
CH2- CH2 - CH2
CH3
CH3
CH2-C- CH2 - CH2-C
For Polypropylene, the wavy line means
CH3
CH2- CH2 - C - H2
For EPR, the wavy line means
H
For our purposes, the use of a wavy line is
adequate
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Molecular Weight
The length of the wavy line is significant -
it indicates molecular weight A short line
means Low Molecular Weight A long line means
High Molecular Weight A commonly used term is
chain or chain length
High molecular weight polyethylene was used as
the primary insulation for medium voltage cables
until the early 1980s
In general, the higher the molecular weight the
better the properties
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Polymeric Structure
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Polymer Chain Alignment
Conventional polyethylene has many such chains
The chains have a tendency to coil For
polyethylene, different chain segments also have
a tendency to align next to each other
The aligned portions cannot coil (the portions
that are not aligned will coil)
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Crystallinity

• The chain portions that are aligned are said to
  be crystalline
• The chain portions not aligned are said to be
  amorphous
• Crystallinity - these regions are what gives
  polyethylene its good properties
• Moisture resistance
• Gas permeation resistance
• Toughness (high modulus)
• Resistance to impurities

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Amorphous

• Amorphous - the non-aligned regions are what give
  polyethylene mixed properties
• Good
• makes the mix (crystalline and amorphous)
  malleable, extrudable, easy to process (a pure
  crystalline material would be brittle and
  unusable
• Bad
• places where impurities/contaminants locate
• high moisture/gas permeation
• Polyethylene is a blend of crystalline and
  amorphous regions
• That is why it is called a semi-crystalline
  polymer

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This Is How Polyethylene Cab Be Depicted This Is
Called Fringed Micelle Structure
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Additional Points

• The amorphous region is the location of
• Crosslinking agent
• Crosslinking agent by-products
• Antioxidant
• What are these?

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Polymer Additives

• Crosslinking agent
• additive placed into polyethylene to convert it
  into crosslinked polyethylene
• Crosslinking agent by-products
• organic chemical residues that sit in the
  amorphous regions after crosslinking has taken
  place
• Antioxidant
• additive placed into polyethylene to prevent it
  from decomposition in the extruder

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Why Add Antioxidants?

• Polyethylene is provided as a pellet by the
  compound supplier to the cable manufacturer who
  heats the pellets in an extruder (which can be
  visualized as a giant meat grinder) and forces
  the melted pellets out of an orifice (die), over
  the center conductor (copper or aluminum)
• The manufacturer converts the pellets into cable
  insulation
• The antioxidant prevents the heat from thermally
  decomposing the polyethylene
• If decomposition occurred, the length of the wavy
  line would be shortened, causing it to be a
  poorer insulation

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Crosslinking Agent

• Crosslinking Agent -This chemical called a
  peroxide
• The most commonly used is dicumyl peroxide
• The peroxide sits in the amorphous regions of
  the pellet when the PE is extruded, it remains
  there quietly
• After the extrusion is complete, and the pellets
  have been converted into cable insulation, the
  newly formed cable now passes into a long heated
  tube (CV tube)
• The new cable is now subjected to even higher
  temperature and pressure
• This causes the peroxide to decompose and causes
  crosslinking of the chains

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The Peroxide Crosslinking Reaction
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A Crosslinked Polymer
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Thermal Decomposition of Dicumyl Peroxide
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Crosslinking

• Crosslinking can only take place in the amorphous
  regions. But, it is important to note that when
  the PE is crosslinked at the high
  temperature/pressure, the material is completely
  amorphous.
• After the crosslinking process is over, and the
  cable is cooled down, alignment (crystallinity)
  reforms
• The cooling process/crystallization assures that
  the residual crosslinking agent (if any is left),
  residual antioxidant, and crosslinking agent
  by-product are pushed into the amorphous region

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Gel Sol

• The cable now consists of crosslinked and
  uncrosslinked regions
• A typical insulation is 70-80 crosslinked
• The crosslinked region is called the gel fraction
• The uncrosslinked region is called the sol region

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Crosslinked Polyethylene

• Polyethylene XLPE
• Residual amounts of dicumyl peroxide
• Crosslinking agent by-products
• Acetophenone
• Cumyl alcohol
• Alpha methyl styrene
• Antioxidant plus some antioxidant by-products

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Tree-Retardant Crosslinked Polyethylene (TR-XLPE)

• XLPE
• Tree-retardant additives
• Residual amounts of dicumyl peroxide
• Crosslinking agent by-products
• Antioxidant plus some antioxidant by-products

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EPR
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EPR Insulation Formulation Basics

• XLPE is semi-crystalline. This imparts a
  stiffness to the insulation at ambient
  temperatures.
• EP is rubbery. This imparts softness to the
  insulation at ambient temperatures.
• EP must be compounded with mineral fillers, due
  to its inherent softness
• XLPE can be compounded with mineral fillers. This
  is not normally done for medium voltage cables
  for utility applications.

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EPR Insulation Components

• Rubber Compound
• semi-crystalline
• amorphous
• variable ethylene content
• variable molecular weight/MWD
• Inorganic fillers
• Filler surface treatment agent(s)
• Crosslinking agent(s)
• Processing aids (oils, stearates, others)
• Antioxidant
• Ion scavenger(s)
• Zinc Oxide

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Commercial EP Based Insulation Compounds
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EP Compounding Considerations

• Semicrystalline EP - continuous mixing
• Low density PE improves processing
• Zinc oxide and antioxidant improve heat aging
• Red lead maximizes wet electrical stability
• Silane treated Kaolin (clay) optimizes physical
  properties and wet electrical stability
• Vinyl Silane improves Kaolin/EP interface
• Process oil - processing aid
• Paraffin wax - release agent
• Dicumyl peroxide - crosslinking agent

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EPR Crosslinking

• The crosslinking principles apply to EPR as well
  as XLPE
• The major difference is that EPR has little or no
  tendency to crystallize
• So how does EPR get toughness (high modulus)?
• Fillers are added to EPR to achieve this
• Fillers are inorganic ...Clay
• The filler can make up a majority of an EPR
  formulation, which may vary from one manufacturer
  to another

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Semiconducting Shields

• All cables contain semiconducting shields
• Semiconducting shields based on ethylene
  copolymers or EPR and contain high carbon black
  contents.
• The carbon black particles are in very close
  proximity to each other as they are dispersed
  through the polymer matrix
• Carbon black particles provide the semiconducting
  properties