Since
paper insulation was used first in the power industry, and was later replaced
in low and medium voltage applications, any comparison of properties usually
employs the paper-fluid system as the standard.
Paper is obtained from a wood or cotton source. The
synthetic polymers are produced by polymerization of monomers derived from
petroleum. All consist of carbon and hydrogen, but paper also contains oxygen.
Research and development in the past few years has shown
that PE and XLPE may be harmed by the use of a dc test, but this does not occur
with paper-oil systems. EPR cables have not been studied to the same extent and
no conclusions can be drawn at this time about the effect of dc testing on the
insulation.
Advantages of polyethylene:
- Low permittivity (low dielectric constant)
- Low tan delta (low dielectric loss)
-
High initial dielectric strength
-
Advantages of crosslinked polyethylene (in addition to
the ones above):
- Improved mechanical properties at elevated temperature
- No melting above 105 “C but thermal expansion occurs
- Reduced susceptibility to water treeing
Advantages of EPR:
- Reduced thermal expansion relative to XLPE
- Reduced sensitivity to water treeing
- Increased flexibility
Advantages of PILC:
- Lack of sensitivity to dc testing
- Known history of reliability
Particular advantages of synthetic polymer insulations
over PILC:
- Reduced weight
- Accessories more easily applied
- Easier to repair faults
- No hydraulic pressure / pumping requirements
- Reduced risk of flame propagation
- Reduced initial cost
Some of these advantages are electrical and some are not.
Care must be taken in seeking to compare EPR to XLPE to
TR-XLPE. There are many different EPR formulations. The nature of the
non-polymeric additives, including fillers, plays a major role in influencing
properties as well as the nature of the mixing process.
What is clear is that any EPR formulation will have higher
losses than a non-mineral filled PE or XLPE system. Some EPR systems may have
very high losses. This may influence resistance to water treeing.
However, EPR systems are generally ‘‘softer” due to their
lack of crystallinity and therefore easier to handle in the field-especially at
very low temperatures.
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