Flexible Joint for 275kV XLPE Cable Tatsuya Nagata*, Akitoshi Watanabe, Seiichi Okuyama, Shigekazu Yokoyama, Kazuaki Yoshida and Satoshi Kaneko Recently, XLPE cable has been used for main underground power lines in metropolitan areas, due to its many superior features. However, enhancing reliability and a reduction in the construction cost of cable lines are expected. Using very long cable is one of the answers and now about 1,800 meter length can be used. If we can make the joint drawable in underground tunnel all through the way as well as cable, this may be a best way. We make a prototype of unswelling and flexible EMJ(Extrusion Type Molded Joint)which is assembled in the factory and evaluated its performance level. This is progress report about development of flexible EMJ for 275kV XLPE cable 1. Introduction Extra-high-voltage XLPE cables are commonly used for major power lines in metropolitan areas. However, they pose some construction-related problems, such as the need to improve reliability by using a smaller number of joint boxes and the need to reduce construction time. One feasible measure is the use of longer cables, since 1,800-meter-long cables are now available. We have studied extrusion-type molded joints (EMJs) that can be used to connect long cables in a plant or a connection base station before being transported and laid.1)2) This report describes our flexible EMJs, which are under development. 2. Design mm, which is the value required for laying of such long cables when they are transported using a suspended tray. Considering other conditions required for laying these cables, the tensile strength was set at 171.5 kN (68.6MPa)for a linear portion. For a bent portion, it was set at 4 ton, taking into account the allowable lateral pressure (14.7kN/m) of a bend. 2.2 Design Table 3 shows the designed values, which were based on the performance targets. Conductors of the same diameter were welded, taking into account the finished diameter and bend characteristics. The reTable 1. Construction of 275kV 1 2,500mm2 XLPE Cable Item Unit Value Sectional area mm2 2,500 Diameter mm 61.2 Conductor shielding thickness mm Approx. 2.5 Insulation thickness mm 27.0 Thickness mm Approx. 1.0 Diameter mm Approx. 122 Cushion layer thickness mm 3.0 Diameter and number of shield wire mm No. Stainless steel sheath thickness mm 0.8 PVC jacket thickness mm 5.0 Outer diameter mm Approx. 159 Nominal weight kg/m Approx. 42.5 2.1 Performance Targets In this development, we used 275kV, 2,500mm2 CSZV cables (insulated-strands-conductor), which have been used for Nagoya route. Table 1 shows their structure. Table 2 shows the performance targets for a flexible EMJ. The life exponent n=15 was applied for the required electrical performance. We also planned to apply n=9, which corresponds to the withstand voltage of a conventional 275kV EMJ. In determining the performance targets, we considered the transportation and laying of cables connected with EMJs. The allowable bending radius R was set at 2,500 * : Chubu Electric Power Co., Inc. 42 Conductor Insulation shielding 2.0 80 quired thickness of reinforcing insulation over the sleeve was set at 27 mm, corresponding to that of existing cables, following a design for 500kV EMJs. As a result, the insulation had a finished diameter close to that of the cable. The sheath over the joint was designed to be made of corrugated flexible metal that could withstand the force applied when laying the cable. The cut edge of the shield layer was designed to be as small as possible. Fig. 1 is an illustration of the trial design of a flexible joint. 3. Method of Construction and Evaluation of Trial Construction 3.1 Connection of Conductors Two methods of connecting conductors were studied : TIG welding of each segment (Type A connection) and batch welding (Type B connection). Table 4 and Fig. 2 show the characteristics of the two methods. Type A connection required more working time. Table 2. Performance Targets Table 4. Conductor Jointing Methods Item Requirement Conductor jointing Connect by same diameter within 110% resistance(at 2m length) Method Reinforcing insulation Connect by similar diameter (including shield break) Weld by each segment (Type A) Allowable bending radius Tensile stregth 400mm TIG (silver solder) Weld by each segment (for a conventional submarine cable) Working time Approx. 10h Approx. 4h Approx. 700mm TIG (phosphor copper solder) Approx. 8h R=2,500mm Linear portion 171.5 kN(68.6MPa) Bent portion 39.2 kN(41.7kN/m) Others Soldering Batch welding (Type B) AC withstand voltage 525kV 1h (610kV 12h) Lightning Imp. withstand voltage 1,445kV 3shots ( 1,590kV 3shots) Electrical performance Connection length same with 275kV conventional EMJ Table 3. Designed Value Type A : Welding of each segment Designed value Method Connection of conductor Approx. 400mmL 61.2 1.0mm Joint by soldering Reinforcing insulation Minimum thickness 27mm Outer diameter Approx. 130mm Extrusion type molded joint(EMJ) Protective tube for EMJ Approx. 230mm Corrugated metal with insulator Approx. 2,200mm Equal to conventional 275kV class EMJ 195 Reinforcing insulation (EMJ) Fig. 2. Conductor Jointing Methods. Insulator (Epoxy resin) Protective tube (Corrugated metal) 230 Metal sheath joint (Welding) 130 Length of joint Type B : Batch welding Shield wire 159 Item Approx. 2,200 Fig. 1. Structure of Flexible Joint. Fujikura Technical Review, 2000 43 Welding of five segments at seven points caused the connection length to be the same as that obtained by batch welding. Type B connection required a smaller number of welds and the connection length was much shorter than that of welding method for a conventional submarine cable. Thus, the number of work hours required was significantly reduced. The designed weld in this study had a larger volume than a conventional weld. Therefore, we used a welding material of high conductivity to reduce the resistance of the weld. Table 5 shows the performance of conductor jointing methods. The DC resistance of the conductor in all the methods met the performance target. The tensile strength was satisfactory : 152MPa for Type A connection and 153MPa for Type B connection. These values are close to the value obtained using sleeve compression. The bending characteristics for the two types of connection met the performance target. 3.2 Reinforcing Insulation We used an extrusion-type molded joint(EMJ)to prepare a reinforcing insulation. This type of joint was most effective in reducing the insulation thickness. According to the trial design described in the previous section, the connection length of conductors was as small as 400 mm. Consequently, the internal insulation had a longitudinal dimension similar to that of a conventional EMJ. Therefore, it was possible to apply the existing method of constructing a conventional EMJ. Both the welding of conductors and the reduction of insulation thickness were considered to lead to an uneven distribution of thickness. We made a trial joint, in which the distribution value of insulation thickness was about 92% in the center. This was equivalent to the thickness distribution for a conventional EMJ. Table 5. Performance of Conductor Jointing Methods 44 Method Conductivity ratio (to orignal cable) Tensile strength (MPa) Bending test Weld by each segment (Type A) 101% 152 Good Batch welding (Type B) 101% 153 Good Weld by each segment (for a conventional submarine cable) 105% 161 Good 3.3 Protective Tube To ensure the flexibility of joint while transporting and laying the cables, each joint was covered with a protective tube made of corrugated metal. This metal was the same type of stainless steel as that used for the cable sheath. The protective tube was welded to the cable sheath, and the weld strength was satisfactory because the welded materials were of the same type. Water tightness of the weld was ensured by plumbing, as is the case for conventional EMJ. The cut edge of the shield layer was assembled together with an insulation tube in a plant to reduce the amount of work required after laying cables in the field. The insulator was set made small as the electrical and mechanical characteristics allowed with a finished outside diameter of about 220 mm(230 mm when cables were laid).This size had no significant influence on the flexibility of the joint. Table 6 shows the results of a bending test. This test revealed that the tube had satisfactory resistance to repeated bending. The measured flexural rigidity(EI)of the joint, which is representative of its mechanical behavior, was similar to that of a cable. 4. Evaluation of Determined Properties 4.1 Electrical Test Table 7 shows the results of an electrical test of flexible joint. EMJ specimens were prepared by connecting conductors and attaching a reinforcing insulation, as described in previous sections. Initial AC withstand voltage and resistance to lightning impulses were determined at room temperature. The results of this test showed that the joint had the required properties, equivalent to those of conventional 275kV EMJs. This indicates that the design was acceptable. 4.2 Mechanical Test We tested cables connected with flexible joint to determine whether they had the mechanical properties required for transporting and laying the cables. Table 6. Performance of EMJ Flexible Metal Sheath Test item Test condition Results Bending test R=2,500mm + tension 39.2kN 10times Good EI Ratio to original cable 1.50 5m sheave Table 7. Electrical Properties of Flexible Joint Tension wire Test item Test condition Results AC (RT) 525kV 1h 610kV 12h consequently 50kV 1h step up 525kV 1h good 610kV 12h good 1,010kV 1h good (Emean=33.7kV/mm *) 1,445kV 3shots 1,590kV 3shots consequently 50kV 3shots step up Lightning Imp. (RT) Roller Tension 39.2kN 2 Flexible joint 1,445kV 3shots good 1,590kV 3shots good 2,290kV 1shot B.D. (Emean=76.3kV/mm*) * : Evaluate by measured insulation thickness(=30mm) Fig. 3. Schematic of Tensile and Bending Test. Table 8. Tensile and Bending Test Test item Test condition Results Bending radius 2,500mm Tensile strength 39.2kN (lateral pressure 14.7kN/m) Number of bending 30times Good Table 9. Electric Test after Bending Test Test item A C(RT) Test condition 525kV 1h 610kV 12h 50kV 1h step up Results 525kV 1h 610kV 12h 1,010kV 1h good good good Fig. 4. View of Tensile and Bending Test. 4.2.1 Tensile and Bending Test A tensile and bending test was conducted by simulating the tension and bending force applied to the joint during cable laying. Table 8 shows the test conditions. Fig. 3 illustrates the test apparatus. Fig.4 shows a cable going round a wheel. A visual inspection revealed no abnormality. In addition, when the tensile stress was relieved(reduced below 4.9kN) after the above test was conducted, the joint went through the bent support satisfactorily and behaved well on the straight portion of the cable. This observation indicates that the joint would bend as required and that the bend would more or less disappear when the joint went along a straight portion after passing a bent portion. After the tensile and bending test, electrical test was carried out. Table 9 shows the results. The joint had good properties, similar to that of other sample that were not subjected to a mechanical test. Fujikura Technical Review, 2000 Fig. 5. View of Long Distance Laying Test. 4.2.2 Long Distance Laying Test A circular loop was used to simulate cable laying over a long distance curved tunnel. Table 10 shows the test conditions. Fig. 5 shows a part of the test loop. 45 Table 10. Long Distance Laying Test Condition Test item Test condition Distance 20km(use 16m circular loop) Speed 7 10m/min Others Stop and go repeated actions 5times (for vehicle tunnel, et al.) Table 11. Result for Long Distance Laying Tests Test item Test condition Results Long distance laying test 7 10m/min 20km Good AC withstand voltage test 525kV 1h 610kV 12h Good Shield break Lightning Imp. voltage test 50kV 3shots (between outer insulation tube) 100kV 3shots (between inner shield break) Good Water protection test 98kPa Good Disassemble research protection tube,insulation tube, reinforcing insulator, et al. No damage Visual inspection revealed that some dirt stuck to the rollers on the protective covering sheath used for laying a cable but there was no damage to the cable. Following the laying test, the cable and joint were subjected to an AC voltage. In addition, tests for lightning impulse withstand voltage between shield layers and water protection test were conducted. The results were all acceptable, as shown in Table 11. 5. Conclusion We developed flexible extrusion-type molded joints having an outside diameter similar to the cable diameter. We used them to connect longer cables at a connection base station. Several tests were conducted to determine whether cables connected using EMJs could be transported and laid in the field. The conclusions we obtained are as follows : (1)The conductor connection procedure allowed the EMJ length to be made similar to the length of a conventional EMJ. There was no large increase in the electrical resistance of joint. (2)The outside diameter of developed EMJ was close to that of cables. The reinforcing insulation of developed joint met the electrical requirements for a 275kV EMJ. (3)The joint satisfied the required mechanical conditions for transporting and laying. We feel that further studies will be necessary with regard to the following aspects : (1)Long-term reliability(carrying out a long-term loading test). (2)Application to actual power lines(considering transportation, laying procedures and modes, and thermal behavior). We are grateful to all the persons concerned for their guidance and cooperation in the course of this study. References 1) T. Nagata, H. Ishihara, et al. : National Meeting of IEE Japan, No. 1849, 1997 2) T. Nagata, H. Ishihara, et al. : National Meeting of IEE Japan, No. 1749, 1998 46