ABB Review Special Report - ABB - ABB Group

CR13025201510501986 1988 1990 1992 1994 1996 1998 2000installation, and reduced thermal contraction/expansionof the insulating material.Experience accumulated duringEHV XLPE cable system development,improvements made in materials andprocesses, and the excellent servicerecord of XLPE, have reduced the thicknessof cable insulation to 12–15 mmfor 132-kV cable systems. This placesthe XLPE cable systems versus overheadline transmission scenario in anew light, where the cable solutionoften can be an attractive alternative.Underground cables versusoverhead linesThere are, of course, many operational,security, environmental, reliability and2Comparison of cost ratio (CR) for XLPE cable systems and overheadtransmission lines400 kV130 kVeconomic parameters that distinguishXLPE cable systems from overheadlines [1]. For modern XLPE cable systems,the reduced cost ratio and environmentaland reliability benefits arethe most obvious and important considerations.Due to their larger crosssectionalareas, cables usually exhibitfewer losses per MVA than comparableoverhead lines. A summary of the benefitsof XLPE cable systems is given inthe table on page 52.The ratings of the overhead lines aresometimes dictated by high winterloads, which include a lot of electricalheating equipment. During hot summerdays the overhead line carries someRating of overhead line (OHTL) versus underground XLPE cable. The dashed lineindicates that higher powers could be transmitted if the daily load cycle profile istaken into account.50% less electricity than in winter, makingthem less attractive if load profileshave to be smoothed out in the future.In areas where there are many air-conditioningunits, for example, the benefitsof XLPE underground cables makethem a genuine alternative 2 .Underground transmission lines alsohave a better overload capacity forperiods of time shorter than 90 minutesdue to the high thermal mass of thesurrounding soil.Qualification of 400–500 kV cablesystemsThe IEC emphasizes the importance ofreliability and coordination of the cablesand accessories by recommending thatthe performance of the total system,consisting of cable, joints and terminations,be demonstrated. The comprehensivetest program, including a ‘prequalification’test, is described in detailin IEC 62067.ABB qualified as a supplier of cablesystems for the 400-kV voltage level in1995.Quality, materials and manufacturingOnly certified suppliers are contractedto deliver essential materials. All ABBmanufacturing sites for HV cables andaccessories are ISO 9001 and 14001 certified.The XLPE cable core is producedon a dry curing manufacturing line. Thecable insulation system, including theconducting layers, is extruded in a singleprocess using a triplex extrusioncross head located, together with thethree extruders for the insulating andconducting materials, in a clean-room 3.AmpacitySoilAir/soil temperatureXLPE cablesOHTLCable design4 shows a 400-kV XLPE cable. Thecable’s copper conductor, which has across-sectional area of 2500 mm 2 , isdivided into five segments to reduceskin effect losses. ABB uses segmented(Milliken) conductors made of strandedwires for cross-sections greater than1000 mm 2 . For cross-sections smallerthan 1000 mm 2 , the conductors arehighly compacted to obtain a rounder,smoother surface.48Special ReportABB Review