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.

Disadvantages to PILC include the fact that lead is usually used as an outer sheath and the motivation not to use lead for new installations is very high. Paper is also highly susceptible to deterioration from moisture.

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