Method of manufacturing HVDC mass impregnated cable transition joint

Abstract

A method for creating a flexible transition joint between HVDC-MI cables having different diameters. The central wires of the conductors are thermally joined by a conical connection piece. The strands of the layers of stranded wires surrounding the central wires are rewound, cut and thermally joined along their respective lay lengths. The stranded are sanded/ground along the lay length of the strands to form a smooth uniform transition having the same slope as the conical connection piece. A paper lapping machine is used to form an insulation patch over the transition joint.

Claims

1. A method for creating a transition joint between two cables each having a conductor with a central wire surrounded by a plurality of layers of stranded wires wound about the central wire, said first and second cables being High Voltage Direct Current (HVDC) Mass Impregnated cables, wherein the method comprises the steps of: a. providing two cables wherein the conductor of a first cable of said two cables is of smaller diameter than the conductor of a second cable of said two cables, b. removing, from terminal portions of both of said first and second cables, any insulating layers or protective layers surrounding the conductors, c. unwinding and pulling back in an upstream direction the plurality of layers of stranded wires of said first and second cables, thereby exposing ends of the central wires of said first and second cables, d. thermally joining a connection piece between the ends of the central wires of said first and second cables, e. rewinding and thermally joining the stranded wires of the plurality of layers of the first cable to the stranded wires of corresponding layers of the second cable such that: each stranded wire of the first cable is abutted against, and joined end-to-end to, one stranded wire of said plurality of layers of stranded wires of the second cable, thus forming a flexible transition joint, and when a layer of one of the first and second cables comprises more strands than a corresponding layer from the other of the first and second cables, two strands of the one of the first and second cables are abutted against, and joined directly at their ends thereof, to an end of one strand of the other of the first and second cables, a sufficient number of times to account for a difference in total strands, and f. patching any insulating or protective layers about the flexible transition joint, wherein the plurality of layers of stranded wires are wound in tandem in a spiral about the respective central wires of said first and second cables, with a horizontal distance required for a strand to pass through a same circumferential position defining a lay length of a layer, said method further comprising the steps of: g. rewinding a first layer of strands of the first cable and cutting the strands such that ends of the first layer of strands of the first cable lay atop the connection piece in a row, extending linearly along an axial length of said flexible transition joint, corresponding to the lay length, h. rewinding a first layer of strands of the second cable and cutting the strands such that ends of the first layer of strands of the second layer lay adjacent corresponding ends of the first layer of strands from the first cable, i. thermally joining the ends of the first and second layer of strands, and j. performing steps g-i for remaining layers of the plurality of layers for each of the first and second cables.

2. A method for creating a transition joint according to claim 1, wherein the central wire of the conductor of the first cable is of smaller diameter than the central wire of the conductor of the second cable, and wherein the connection piece is conical.

3. A method for creating a transition joint according to claim 1, wherein the strands of the layers are of different height, and wherein the method further comprises the step of using a sanding or grinding device along the lay length in order to mechanically remove a portion of the higher of the strands to create a smooth transition between the strands, the removed portion defining a grind zone for the joined strands.

4. A method for creating a transition joint according to claim 3, wherein the connection piece is conical, and wherein the grind zone is ground to have essentially the same slope as the conical piece.

5. A method for creating a transition joint according to claim 1 wherein the strands of the layers of the first and second layers are held in their pulled-back positions by clamps.

6. A method for creating a transition joint according to claim 1 wherein the insulating layer are removed from the terminal portions of the first and second cables at a sloped angle, and wherein insulation is patched over the flexible transition joint by causing a paper lapping machine to oscillate back and forth between respective sloped surfaces, sequentially applying layers of impregnated paper lapping material to form an insulation patch.

7. A method for creating a transition joint according to claim 6, further comprising applying a supplement layer of paper lapping about the insulation patch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described in more detail with reference to the accompanying drawings, wherein:

(2) FIGS. 1A and 1B are perspective views of a first and second high voltage DC mass impregnated cable.

(3) FIGS. 2A and 2B are cross sectional and perspective views of the conductor of the cable from FIG. 1.

(4) FIG. 3 is a side elevational, exploded view of the conductors of the first and a second HVDC-MI cable, with stranded wires pulled back, and with a conical connection piece arranged between the center wires of the two conductors.

(5) FIG. 4 is a perspective view of the first layer of stranded wires of the first conductor cut and arranged upon the conical connection piece.

(6) FIG. 5 is a top view showing stranded wires of the first layer from the first and second conductors with their respective ends aligned prior to being thermally joined.

(7) FIG. 6 is a top view corresponding to FIG. 5, with the ends of the strands joined together and sanded.

(8) FIG. 7 is a perspective view showing an operator sanding or grinding the first layer of connected strands.

(9) FIG. 8 is a transverse cross sectional view illustrating the difference in height between the strands of the two conductors at the joint area.

(10) FIG. 9 is a longitudinal cross sectional view showing the grinding area of material removed by the operator from FIG. 7.

(11) FIG. 9 is a side elevational view of the completed first layer

(12) FIG. 10 is a top view of the completed outer layer of joined strands, also showing two strands from one conductor joined to single strand from the other conductor.

(13) FIG. 11 is a side cross sectional view showing a completed with the dimensions of Table 1.

(14) FIG. 12 is a cross sectional view illustrating the reapplying of paper insulation to the joint area.

DETAILED DESCRIPTION

(15) The invention will be now be described with reference to a specific example of joining two HVDC-MI cables. It should be understood however that the invention is suitable for the joining of other types of cables than HVDC-MI cables so long as the cable is of the type having a conductor with a central wire surrounded by stranded wires.

Example

(16) A first cable 20 of the type as illustrated in FIG. 1A is to be joined with a second cable 40 of similar type as illustrated in FIG. 1B (albeit with cable 40 having a conductor with different diameter and/or configuration than that of cable 20).

(17) As shown in FIGS. 1A and 2A/B, first cable 20 comprises a conductor 22 surrounded by a plurality of insulating/protective layers. Conductor 22 comprises a central wire 24 surrounded by layers of stranded wires 26. Such a configuration with a central wires and stranded wires, known in the art, improves the flexibility of the cable.

(18) As shown in FIG. 2A/B, central wire 24 is a round wire, and stranded wires 26 are keystone shaped in order to be tightly packed about the central wire 24. In the example used herein, there are five layers 25a-e of stranded wires 26. Stranded wires 26 are wrapped in a spiral about central wire 24, with alternating layers being wrapped in alternating directions, as shown in FIG. 2B.

(19) Surrounding conductor 22 are a plurality of insulating and/or protective layers. Immediately adjacent conductor 22 is an insulation layer 28. In this example, the insulation layer is a mass-impregnated paper insulation known in the art, comprising a plurality of wrapped layers of oil-impregnated paper.

(20) Outside the insulation layer 28 is a water tight lead barrier layer 30. About lead layer 30 is arranged polyethylene layer 32. A strengthening layer 34 of galvanized steel is arranged about polyethylene layer 32. An armor layer 36 comprising galvanized steel bands protects the cable from abrasion and other forces. Finally, the cable comprises an outer protective layer 38 of bitumen/polypropylene yarn.

(21) Second cable 40 as shown in FIG. 1B is, in this example, also a HVDC-MI cable of similar configuration as first cable 20, albeit with a conductor 42 having a central wire 44 with a larger diameter and different total number of stranded wires 46 than conductor 22 of cable 20. For the purposes of the example, it is assumed that second cable 40 has the same arrangement of insulating/protective layers as first cable 20, namely: an insulation layer 48, a water tight lead barrier layer 50, a polyethylene layer 52, a galvanized strengthening layer 54, an armor layer 56 and an outer protective layer 58.

(22) Table 1 below lists the parameters and dimensions of the center wire 24/44 and stranded wires 26/46 of the conductors of the two cables joined in the example. It should be understood, however, that the dimensions discussed are for illustrative purposes, and are not meant to necessarily be limiting for the invention. Cables having other dimensions and configurations can be connected by the method of the invention.

(23) TABLE-US-00001 TABLE 1 Direction of spiral Number Dimension Lay (Right or of strands (mm) length Layer height Left) per layer First Cable Center wire 12.0 6.0 (radius) 1.sup.st layer of 18.9 207.8 3.5 R 8 stranded wires 2.sup.nd layer 27.3 225.0 4.2 L 12 3.sup.rd layer 32.7 328.0 2.7 R 18 4.sup.th layer 40.5 372.0 3.9 L 18 5.sup.th layer 46.3 472.0 2.9 R 26 Second cable Center wire 15.0 7.5 (radius) 1.sup.st layer 23.4 213.00 4.2 R 9 2.sup.nd layer 31.6 296.0 4.1 L 15 3.sup.rd layer 39.6 376.0 4.0 R 18 4.sup.th layer 47.8 458.0 4.1 L 20 5.sup.th layer 54.7 530.0 3.5 R 24

(24) According to the method of the invention, the outer protective and insulation layers are removed from terminal portions of the first and second cables, thus exposing their respective conductors 22 and 42 as shown in FIG. 3.

(25) Stranded wires 26/46 from outer layer 25e/45e are unwound and pulled back in the upstream direction, and clamped off with a clamp 60. This operation is repeated for layers 25/45 d, c, b and a, thus exposing central wires 24 and 44. For simplicity sake, FIG. 3 shows strands 26/46 shorter than in a real word scenario. A more realistic representation of the pulled back strands 26/46 may be seen in FIG. 7.

(26) A conical connection piece 62 as shown in FIG. 3 is thereafter thermally joined between central wires 24 and 44 and sanded to a smooth surface using a sanding device 64 as shown in FIG. 7.

(27) In the next step of the method, the stranded wires 26 from first layer 25a of first cable 20 are rewound about the central wire 24 and up upon conical connection piece 62 and cut, such that the ends of stranded wires 26 lay upon conical connection piece 62, as shown in FIG. 4. First layer 25a comprises 8 strands as shown in Table 1, with the horizontal distance between the ends of the outermost strands corresponding to the lay length from table 1. Likewise, strands 46 from layer 45a of the second cable 40 are rewound and cut so that the ends are adjacent the ends of strands 26 as shown in FIG. 5. The ends of strands 26 and 46 are welded or soldered together at a joint area 66 as shown in FIG. 6. According to one aspect, the ends are arranged to rest at the top of the conical piece for ease of the welding operation. As shown in Table 1, the first layer 45a comprises one more strand than corresponding layer 25a. Therefore, two strands are welded to a single strand at an appropriate location. In the event that there is more than one extra strand, the double welds should preferably be spaced apart to the extent possible.

(28) When joining cables of different dimensions according to the method of the invention, it is necessary to account for the stranded wires having a different thickness where this is the case. As shown in Table 1, in the current example the stands 26 of first layer 25a of the first cable are 3.5 mm in height whereas the strands 46 of first layer 45a of the second cable are 4.2 mm in height. This difference in height is illustrated in FIGS. 8 and 9, transverse and longitudinal cross sections respectively. An operator therefore utilizes sanding device 64 as shown in FIG. 7 to remove material from strands 46 in a grind zone 68 to a achieve an smooth surface having a slope corresponding to conical connection piece 62. The result is a smooth conical transition from the first cable to the second cable over the connection piece, as shown in FIG. 6.

(29) The above procedure is repeated for remaining layers 25/45b, c, d and e, resulting a completed transition joint as shown in FIG. 10.

(30) FIG. 11 is a graphical representation of a cross section of the completed transition joint, according to the dimensions from Table 1. The numbers enclosed in circles represent the lay length of the layer having the longest lay length for that particular layer. The number enclosed in boxes represent the grinding zone 68. where material is removed from the thicker of strands 26 or 46, as the case may be. (in this example, the strands from cable 40 are always the thickest, with the exception of layer 25b/45b, where in that instance strand 26 is the thickest.)

(31) After the transition joint is completed, the insulation layer is reapplied over the transition joint. As shown in FIG. 12, the original insulation layers 28 and 48 are preferably removed such that a sloped surface 70 is achieved. A paper lapping machine (not illustrated) is caused to oscillate back and forth between sloped surfaces 70, sequentially applying layers of impregnated paper lapping. The lapping is applied to achieve a uniform transition from the first cable to the second cable. As can be seen in FIG. 12, the result is a generally trapezoidal cross section insulation patch 72. A surplus insulation patch 74 made of impregnated paper lapping is then applied about the insulation patch 72.

(32) The remaining protective/armor layers from the two cables are thereafter patched about the transition joint using know techniques in the art.