XLPE: DECADES OF IMPROVEMENT
The XLPE insulations you buy today aren't made the way they used to be.
- Published in Southwire's T&D Update newsletter in April 1998
- Reprint permission granted
Insulations made of cross-linked polyethylene (XLPE) and tree-retardant cross-linked polyethylene (TRXLPE) have become the standard for direct buried underground distribution cables. They combine toughness, good installed cost, low electrical losses, and proven field performance to provide a low total cost of operation. Modern XLPE and TRXLPF insulated cables are not the same as they were 20 years ago.
"Today's cables are the result of more than 20 years of continuous improvements in cable construction, materials and processing technology," says Thomas Arnold, manager of power cable technology. "These improvements all contribute to the current long-life cable performance."
Eliminate a Contaminant, Stop a Tree
One of the principal hazards to underground medium-voltage cable is water treeing. In wet environments, both XLPE and ethylene propylene rubber (EPR) insulations may develop trees. Reducing contaminants that provide initiation points for trees can be expected to deliver better cable performance.
Reducing tree initiation points and incorporating the new TRXLPE material has, in fact, resulted in improved performance of cables both in laboratory testing and in the field.
Today's medium-voltage insulation materials are much cleaner than those of 20 years ago. Laser scanner technology can detect very small contaminants in the insulation. Identification of contaminants has led to greatly reduced contamination levels, and specifications on contaminants have become much tighter since the 1970s.
Current specifications for XLPE limit contaminant size to 100-124 microns (4-5 mils). Current laser scanning equipment can detect contaminants in XLPE down to 25 microns.
Laser scanners can also detect non-uniform material. If a situation in the reactor where the compound is manufactured causes a non-uniform condition or scorching of the compounds, these defects can be detected and process corrections made.
Once It's Clean, Keep It Clean
The entire insulation manufacturing environment has also seen significant improvements.
The clean insulation compounds have to be kept clean during packing, shipping, movement in the cable plant and during extrusion. At the compound manufacturing end, that calls for measures such as cleanroom conditions for packaging, packaging that eliminates fiber contaminants, and dedicated clean hopper cars. At the cable plant, there are clean-room conditions for unloading, and carefully designed non-contaminating transport systems to the extruder.
"There have been many improvements in the XLPE and TRXLPE compounds used in today's cable," says Arnold. "There have also been many improvements in the actual cable manufacturing over the past 20 years." |
