Comparison Of Cable Insulating Materials

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.

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.

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.


Major Differences Between Paper and Polyolefinic Insulations

Paper / Cellulose                                                   Polyethylene

Natural                                                                   Synthetic
Carbon / hydrogen/oxygen                                   Carbon / hydrogen/oxygen
More polar / medium losses                                   Less polar, low losses
Chains linear                                                           Chains branched
Fibrils                                                                   Non-fibrils
Partially crystalline / Relatively constant                   Partially crystalline / Varies with grade employed
No thermal expansion on heating                           Significant thermal expansion
Not crosslinked                                                   Not crosslinked
Thermal degradation via cleavage at weak link   Degrades at weak links

Crosslinked Polyethylene                              Ethylene Propylene Rubber

Synthetic                                                              Synthetic
Carbon / hydrogen                                              Carbon / hydrogen
Less polar, low losses                                      Losses due to additives
Chains branched, crosslinked                              Chains branched, crosslinked
Non-fibril                                                              Non-fibril
Slightly less crystalinevs PE                              Least crystaline of all
Same thermal expansion as PE                      Slight thermal expansion
Crosslinked                                                      Crosslinked
Degrades at weak links                                      Same as XLPE

This table provides a comparison of the properties of paper, polyethylene, crosslinked polyethylene, and ethylene propylene rubber insulations. Only the paper is a natural polymer and is therefore processed differently. Paper is obtained fi-om 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. The latter is present as fuctional hydroxyl or ether groups. The contribute a measure of polarity that is absent in the synthetic polymers. (Polarity means increased dielectric losses.)

Of special note is the concept of thermal expansion during heating. While all of the synthetic polymers undergo thermal expansion during heating, this does not occur with cellulose-although the oil will do so. How these insulations respond on aging is a well studied subject since it is directly related to reliability of the cable after installation and energization. When cellulose degrades, it does so at a "weak link," the region of the oxygen linkage between the rings. When this happens, the DP is reduced.

On the other hand, polyolefins degrade by a completely different mechanism–oxidative degradation at specific sites.
Protection against degradation is imparted to  polyolefins by adding an antioxidant to the pellets prior to extrusion. Note that adding antioxidants to oil to prevent it from degradingis rather common. One further point should be noted on the chart: the different response of the insulation types to dc testing. DC testing of cables has traditionally been performed to ascertain the state of the cable at specific times during their use, such as before peak load season. This is a technique that was adopted for PILC cables many years ago.

This was later carried over to extruded dielectric cables. 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 XLP
    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.

Author: Bruce S. Bernstein & electrical-engineering-portal