There appears to be some misunderstanding regarding the methods and reasoning in curing XLPE insulated power cables. Perhaps the following information may be of some help in explaining those methods used.

Steam curing has always been the standard method of processing thermosetting insulation on power conductors, whether it be elastomeric or polymeric material. However, in the early 1980's micro-treeing was identified in some XLPE insulated cables, which developed within typical micro voids in the insulation and possibly resulted from voltage impulsing such as lightning surges. Since most of the tree forming micro voids contain some water which was thought to be a partial cause, a company in Finland, Nokia, had the notion that by curing the extruded layers of cable in the catenary pipe using heated nitrogen gas instead of steam, the water could be extracted from the insulation and therefore produce a relatively dry cable without water in the micro voids. There was hope that the hot dry gas cure would also reduce the number and size of micro voids. However, this was not effectively achieved because in a catenary cure process the insulation is allowed to expand in the cure pipe due to the high temperature, which creates the voids.

Although most of the micro voids become partially or completely dehydrated, an important factor was overlooked. When curing XLPE cables in such a short interval of time in the catenary cure pipe, the XLPE continues to cure or vulcanize into a thermoset material for months later. During this delayed curing process, the oxidant mixed with the polyethylene absorbs the hydrogen molecules from the polyethylene to form water (H2O), which migrates into the insulation micro voids. This results in the same outcome as the steam cure process.

Most power cable producers who purchased these dry gas machines continue to install and use them because they are easier and more convenient to operate in the process than creating and piping steam under pressure.

In later years, a prominent XLPE compound producer, Union Carbide, realized that the curing process was not the solution to eliminating potential high stress micro-treeing and as a result developed a new tree retarding XLPE insulating compound, identified as "TR-XLPE". By adding special ingredients along with the peroxide into the XLPE, the insulation dielectrics were slightly reduced resulting in a higher SIC value, but this compound appears to have alleviated the potential micro-treeing issue.

The final consensus is that there is no significant difference between a steam and a nitrogen gas cured cable, except for preference by the manufacturer in processing. Although, the TR-XLPE compound is not really required for most medium voltage levels and is premium priced compared to the standard XLPE, it is still considered to be a worthwhile investment.

For extra high voltage (EHV) XLPE insulated cables such as for 230kV and higher voltage applications, the TR-XLPE is not used because of its added impurities that results in higher power losses. The standard XLPE is used but is generally produced on "state of the art" machinery called a "long land die continuous vulcanizer" (LLDCV). This machine produces the triple extruded layers of strand shield, insulation and insulation shield under extremely high pressure, in a restrictive special sized tube which disallows the insulation to expand prior to setting. This process virtually eliminates any possible micro voids from forming. Unfortunately, there are no LLDCV machines available in North America, but if available, they would be cost prohibitive for medium voltage cable designs due to their high set up costs and slow speed of operation.

I hope this information will assist your technical team in better understanding the relevancy of these process methods used in the wire and cable industry today.

If you should have any further questions regarding this information, please do not hesitate to contact us.

Bob Pawluk, P.Eng.
United Wire & Cable Inc.

Copyright © 2003, United Wire & Cable Inc.