Basic Properties of Power Cable Insulations

1. Basic Properties of Power Cable Insulations Bruce Bernstein Electric Power Research Institute (EPRI)

2. Cable Components Conductor Copper or Aluminum Stranded or Solid Carries the electrical power from the generating station to the customer

3. 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

4. Component RequirementsConductor Shield • Surface smoothness • Compatibility with interface materials • Uniform conductivity • Inseparable bond to insulation

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

6. Component RequirementsInsulation Shield • Surface smoothness • Compatibility with interface materials • Uniform conductivity • Controllable strippability

7. Cable Components Neutrals Always Copper Bare or Tinned Provide a return path for the current and also a continuous ground around the cable

8. Cable Components Jacket Black LLDPE Black Semiconducting Black Track resistant High Density Provides a moisture barrier Prevents corrosion of neutrals Provides mechanical protection

9. 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?

10. 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:

11. The Addition Polymerization of Ethylene to Polyethylene

12. A Branched Polymeric Chain A branched polymeric chain

13. Chain Length (Molecular Weight) Chain Length Molecular Weight

14. 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 For Polypropylene, the wavy line means CH2-C- CH2 - CH2-C CH3 CH2- CH2 - C - H2 For EPR, the wavy line means H For our purposes, the use of a wavy line is adequate

15. 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 1980’s In general, the higher the molecular weight the better the properties

16. Polymeric Structure

17. 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)

18. 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

19. 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

20. This Is How Polyethylene Cab Be Depicted This Is Called “Fringed Micelle Structure”

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

22. 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

23. 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

24. 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

25. 1) R-O-O-R 2 R-O• 2) R-O• + H• + R-O-H Hydrogen Abstraction Polymer 3) 2 • Termination Polymer Radical Cross-linked Polymer The Peroxide Crosslinking Reaction

26. A Crosslinked Polymer

27. Thermal Decomposition of Dicumyl Peroxide

28. 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

29. 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

30. 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

31. 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

32. EPR

33. 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.

34. 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

35. Commercial EP Based Insulation Compounds

36. 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

37. 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

38. 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