1.9 MEDIUM VOLTAGE CABLE DEVELOPMENT
In the mid-1960s,conventional polyethylene became the material of choice for the rapidlyexpanding URD systems in the United States . It was known to be superior tobutyl rubber for moisture resistance, and could be readily extruded. It wasused with cloth taped conductor and insulation shields, which achieved theirsemiconducting properties because of carbon black. By 1968, virtually all ofthe URD installations consisted of polyethylene-insulated medium voltagecables. The polyethylene was referred to as “high molecular weight” (HMWPE);this simply meant that the insulation used had a high “average” molecularweight. The higher the molecular weight, the better the electrical properties.The highest molecular weight polyethylene that could be readily extruded wasadopted. Jacketed construction was seldom employed at that time.
Extruded thermoplastic shields were introduced between 1965 and1975, leading to both easier processing and better reliability of the cable.
XLPE was first patented in 1959 for a carbon filled compound andin 1963 as unfilled by Dr. Frank Percopio. It was not widely used because ofthe tremendous pressure to keep the cost of URD down near the cost of anoverhead system. This higher cost was caused by the need for additives(cross-linking agents) and the cost of manufacturing based on the need formassive, continuous vulcanizing (CV) tubes. EPR was introduced at about thesame time. The significantly higher initial cost of these cables slowed theiracceptance for utility purposes until the 1980s.
The superior operating and allowable emergency temperatures ofXLPE and EPR made them the choice for feeder cables in commercial andindustrial applications. These materials did not melt and flow as did the HMWPEmaterial.
In order to facilitate removal for splicing and terminating, thoseearly 1970-era XLPE cables were manufactured with thermoplastic insulationshields as had been used over the HMWPE cables. A reduction in ampacity wasrequired until deformation resistant and then cross-linkable insulationshields became available during the later part of the 1970s.
A two-pass extrusion process was also used where the conductorshield and the insulation were extruded in one pass. The unfinished cable wastaken up on a reel and then sent through another extruder to install theinsulation shield layer. This resulted in possible contamination in a verycritical zone. When cross-linked insulation shield materials became available,cables could be made in one pass utilizing “triple” extrusion of those threelayers. “True triple” soon followed, where all layers were extruded in a singlehead fed by three extruders.
In the mid-1970s, a grade of tree-retardant polyethylene(TR-HMWPE) was introduced. This had limited commercial application and neverbecame a major factor in the market.
Around 1976, another option became available—suppliers provided agrade of “deformation resistant” thermoplastic insulation shield material. Thiswas an attempt to provide a material with “thermoset properties” and thuselevate the allowable temperature rating of the cable. This approach wasabandoned when a true thermosetting shield material became available.
By1976, the market consisted of approximately 45% XLPE, 30% HMWPE, 20% TR-HMWPE,and 5% EPR.
In the late 1970s, a strippable thermosetting insulation shieldmaterial was introduced. This allowed the user to install a “high temperature”XLPE that could be stripped for splicing with less effort than the earlier,inconsistent materials.
Jackets became increasingly popular by 1980. Since 1972–1973,there had been increasing recognition of the fact that water presence undervoltage stress was causing premature loss of cable life due to “watertreeing.” Having a jacket reduced the amount of water penetration. This led tothe understanding that water treeing could be “finessed” or delayed byutilizing a jacket. By 1980, 40% of the cables sold had a jacket.
EPR cables became more popular in the 1980s. A breakthrough hadoccurred in the mid-1970s with the introduction of a grade of EPR that could beextruded on the same type of equipment as XLPE insulation. The higher cost ofEPR cables, when compared with XLPE, was a deterrent to early acceptance evenwith this new capability.
In 1981, another significant change took place: the introductionof “dry cure” cables. Until this time, the curing, or cross-linking, processwas performed by using high-pressure steam. Because water was a problem forlong cable life, the ability to virtually eliminate water became imperative. Itwas eventually recognized that the “dry cure” process enabled faster processingas well as elimination of the steam process for XLPE production.
Anothermajor turning point occurred in 1982 with the introduction of tree-resistantcross-linked polyethylene (TR-XLPE). This product, which has supplanted conventionalXLPE in market volume today, shows superior water tree resistance when comparedwith conventional XLPE. HMWPE and TR-HMWPE were virtually off the market by1983.
By 1984, the market was approximately 65% XLPE, 25% TR-XLPE, and10% EPR. Half the cables, sold had a jacket by that time.
During the second half of the 1980s, a major change in the use offilled strands took place. Although the process had been known for about 10years, the control of the extruded “jelly-like” material was better understoodby a large group of manufacturers. This material prevents water movementbetween the strands along the cable length and eliminates most of theconductor’s air space, which can be a water reservoir.
In the late 1980s, another significant improvement in thematerials used in these cables resulted in smoother and cleaner conductorshields. Vast improvements in the materials and processing of extruded, mediumvoltage power cables in the 1980s have led to cables that can be expected tofunction for 30, 40, or perhaps even 60 years when all of the proper choicesare utilized. In 1995, the market was approximately 45% TR-XLPE, 35% XLPE, and20% EPR.
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