You asked about the reliability of 345-kilovolt (kv) solid dielectric underground transmission cables. Such cables have been proposed as an alternative to Northeast Utilities' (NU) plans to build an overhead power line from Bethel to Norwalk.

SUMMARY

There seems to be a general consensus among experts that solid dielectric transmission cables, insulated with cross-linked polyethylene (XLPE), will soon become the technology of choice for high voltage transmission lines. However, there is some disagreement about how far the technology has progressed. Some feel the technology has not yet reached the point at which they can unreservedly endorse its use, particularly when, as in the case of the Bethel to Norwalk line, the XLPE line would be by far the longest such high voltage cable in the U. S.

BACKGROUND

NU has applied to the Connecticut Siting Council for permission to build a 345-kilovolt transmission line from Bethel to Norwalk. Because of local opposition to construction of an overhead line, NU has alternatively proposed placing the transmission cable underground along public streets, a route of about 21. 6 miles. According to NU, placing the line underground would be substantially more expensive than routing it overhead.

CABLE TECHNOLOGY

NU considered several types of cable technologies. These include fluid-insulated cables, such as high pressure fluid filled (HPFF), or self contained fluid filled (SCFF), and solid dielectric cables. Solid dielectric cables, insulated with cross-linked polyethylene (XLPE), do not require liquid insulation. The choice of cable technology depends on a variety of factors, including cable length, the amount of voltage transmitted, and environmental concerns.

Although fluid filled cables are a proven technology that has been used extensively in the U. S. , they pose an environmental risk because of the possibility that insulating fluid may leak. NU says a 21. 6-mile fluid filled cable would require about 500,000 gallons of insulating fluid. While the fluid is considered non-toxic, a leak would still pose hazards to nearby wetlands and water sources, and NU estimates that at least 5,000 gallons would be released before a leak detection system alarm would alert the company.

There are no such environmental concerns with XLPE. NU, in fact has said that if the Siting Council directs it to bury the line, it would "probably prefer the technical and reliability risks of an XLPE installation to the environmental risk of an HPFF cable" (Testimony of NU vice-president Roger Zaklukiewicz before the Siting Council, March 12, 2002). However, NU has stated it would prefer to defer such use until the industry had gained more experience with XLPE, and to reserve its use for cases in which there is no overhead alternative (NU's October 15, 2001 Siting Council Application).

XLPE DEVELOPMENT

Uncertainty about XLPE seems to stem mainly from the lack of experience with this relatively new and developing technology, particularly at the lengths and high voltage levels NU would require.

Commercial XLPE cables up to 230-kv first came into widespread use in the 1980s. Although some high voltage XLPE lines were installed in Japan in the late 1980s and early 1990s, these differ from the prospective NU line in not having any splices or joints. These joints, which link two lengths of cable together, are often the cause of cable failure (see below). The first 400-kv system with a significant number of joints (72) was installed in Copenhagen in 1997. A 500-kv line was installed in a tunnel in Tokyo in 2000. There is now about 100 miles of 345-kv XLPE cable worldwide, mostly in 5 and 10-mile lengths.

In the U. S. , XLPE is most commonly used for 138-kv and lower voltage levels. The first 230-kv XLPE line in the U. S. was installed in 1992, and there are now about 20 miles of XLPE at that voltage. The first 345-kv XLPE line in the U. S. was installed in Everett, Mass. in 2001. Those lines, however, are each only 400 to 500 feet long.

Cable Splice and Joints

Long cables with joints are inherently more failure prone that shorter cables because of the way cables are assembled. Cables are built in sections and spliced together - in a way, like linking together a string of extension cords. Splicing is a delicate operation, and, failures often occur at these cable junctions. Because XLPE cables are thicker than equivalent fluid-filled cables of the same voltage capacity, less cable can be wrapped on each cable spool. Less cable per spool means more splices, increasing the chance of failure. A 21. 6-mile XLPE cable would require about 60 splices.

Cable Consulting International (CCI), which reviewed cable technology for NU, notes that the proposed NU cable will require more splices than now exist worldwide.

XLPE INSTALLATIONS

There is some discrepancy about the reported failure rates of existing 345-kv XLPE cables. Different installation methods and the limited experience with 345-kv lines make it hard to extrapolate from existing cable systems.

Three prominent systems now exist in Tokyo (a 25-mile, 500-kv line installed in a tunnel in 2000); Copenhagen (about 21. 2 miles of 400-kv); and Berlin (about 4 miles in a 400-kv system installed in 1998). (The mileage represents the mileage of cable from point A to point B. Because each line may consist of several circuits, the actual amount of cable may be greater. The Copenhagen line, for example, has more than 62. 2 miles of cable. )

Gregory Lawson, a consultant with Power Delivery Consultants, (PDC) reports no failures in the Berlin line at least through September 2001 and no failures in the Copenhagen line through September 2002. He does not mention the Tokyo line, but Harry Orton, of Orton Consulting Engineers, International, reports that Japan has had two years of trouble free service since October 2000.

CCI reports one failure in the Berlin line during testing and one in the Tokyo line while in service. CCI also notes, in a report prepared for NU, that in the past 18 months four of six major projects calling for high voltage lines have opted for SCFF lines.

XLPE SUITABILITY

But while SCFF lines are now a viable alternative to XLPE lines, many experts, including CCI, see a worldwide trend towards XLPE. Among the reasons for this, says Orton, are XLPE's "environmental compatibility, better electrical properties, stronger fire resistance, easier handling and simpler maintenance. "

But while XLPE may be the obvious choice ten to 15 years from now, experts disagree about how suitable a choice it is right now. CCI cautions that XLPE technology has not yet matured. Not enough time has elapsed since the installation of the XLPE lines in Copenhagen and Tokyo, CCI says, "to have demonstrated the absence of teething problems and the long term serviceability of such systems. " XLPE technology, CCI concludes in its review, "has not yet been demonstrated to have reached a sufficiently high level for long length circuits of XLPE cable to be considered for reliable and safe commercial application of 300-kv and above. "

Reviewing the same history, PDC finds that XLPE "has an excellent, but limited operating history overseas" and "is considered suitable for 345-kv operations. " PDC's Lawson cites XLPE's "perceived advantage" in operation and maintenance, and the elimination of environmental problems associated with the leakage of fluid from fluid-filled systems.

Lawson notes that most members of a subcommittee of CIGRE, the Paris-based International Council on Large Electric Systems, meeting in August 2002, found that extruded high voltage and extra high voltage cables, such as XLPE, have reached "a high technical and functional level and therefore are generally considered as reliable and mature components in transmission networks. " Nevertheless, the subcommittee, as have all other experts, cautions that the technology is still evolving.

Lawson said the International Electrotechnical Commission's issuance of a pre-qualification test for XLPE cables also indicates growing acceptance of the technology. "Traditionally conservative electric utilities [in Berlin and London] have accepted the...pre-qualification test as a good indicator of the reliability for 400-kv XLPE cable systems, so that if (NU) eventually opts for a 345-kv XLPE system they would be in good company," he said.

All observers agree on at least one point: the key to the successful installation and operation of an XLPE cable would be NU's intimate and intense involvement in the design, production, testing and installation of the cable. "The risk is directly related to the expertise, process capabilities and quality assurance procedures of the manufacturers and the knowledge and intense participation of the users," said Walter Zenger of the Electric Power Research Institute.

PF: ro