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.
Click here to access the full article
