CHAPTER
INSULATING
5
MATERIALS FOR CABLES
Bruce S. Bernstein
1.0 INTRODUCTION
Electrical insulation materials are employed over the metallic conductors of
underground cables at all voltage ratings. Polymeric materials are employed as
the insulation, but the nature of the polymer may vary with the voltage class.
Transmission cables, which are defined as cables operating above 46 kV, have
traditionally used paper / oil systems as the insulation. The paper is applied as a
thin film wound over the cable core. Some years back, a variation of this paper
insulation was developed, the material being a laminate of paper with
polypropylene (PPP or PPLP). Since the advent of synthetic polymer
development, polyethylene (PE) has been used as an insulation material, and in
most countries (France being the exception) the use of polyethylene was limited
to the crosslinked version (XLPE). XLPE is considered to be the material of
choice due to its ease of processing and handling, although paper / oil systems
have a much longer history of usage and much more information on reliability
exists.
For distribution voltage classes (mostly 15 to 35 kV), the prime material used in
the past was conventional PE; however, this was replaced by XLPE as the
material of choice in the 1980s. The installed PEinsulated cables are gradually
being replaced. In recent years, ethylene propylene co- or ter-polymers have
been used (EPR or EPDM, respectively). The use of EPR, which is an
elastomer, (XLPE is semi-crystalline) requires the incorporation of inorganic
mineral fillers. The term EPR has been used to generically describe both EPR
and EPDM cables and that terminology will be employed here.
At even lower voltages, the possible choices of polymeric materials widens.
Here it is possible to use polyvinyl chloride (PVC), silicone rubber (SIR), or
other polymers that are readily available and processable. PVC was used for a
time in Europe for medium voltage cables in the 10 kV class, but that practice
has been discontinue