AU2012216603A1 - Insulating cradle for a cross bonding link box - Google Patents

Abstract

An insulating cradle for a cross bonding link box, said link box having an upper link arm and a lower link arm for connecting power transmission cable sections, the insulating cradle including a body for installation between said upper and lower link arms, said body having an upper surface including an upper guide channel and a lower surface including a lower guide channel, wherein said upper guide channel is adapted to receive said upper link arm and said lower guide channel is adapted to receive said lower link arm.

Description

1 Insulating cradle for a cross bonding link box Field of the Invention [0001] The present invention relates to an insulating cradle for a cross bonding link box. The invention is particularly useful in relation to an insulating cradle for 3-phase cross bonding link boxes, and it will therefore be convenient to describe the invention in that environment. However, it should be understood that the invention is intended for broader application and use. Background of the Invention [0002] In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of the common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned. [0003] Underground power cable systems often present unique design and installation challenges not found in the overhead power cable environment. Poor thermal dissipation, confined spaces, lack of visibility, more difficult fault diagnosis, circulating earth currents, and frequent cable joints are common problems experienced when dealing with underground power cable systems, and problems that cable engineering must overcome to deliver a reliable long term installation. A well designed cable system delivers maximum power and minimum losses within a small footprint. [0004] The current rating of high voltage (HV) underground cable circuits is predominantly influenced by environmental factors and cable losses. Specialised techniques are often used to improve cable heat dissipation. The heat generated in such cables is caused by electrical losses in insulation and metallic components, and can be classified as either current dependent or voltage dependent losses. [0005] Current dependent losses are commonly generated in cable conductors, metallic sheaths, armouring and piping designed to carry fault currents. In a simplified cable system the three main loss generators are main conductors, insulation and metallic sheaths / wire screens. [0006] By contrast, voltage dependent losses are caused by capacitive effects such as, for example, the cable itself which acts as a large capacitor. Charging current and dielectric losses 2 are dependent on dielectric constant (e), electrical resistance plus magnitude and frequency of applied voltage. As such, voltage losses are consistently being generated while the cable is connected to the power grid. [0007] Current dependent losses can also be viewed as ohmic losses manifested as heat generated by current flowing in cable conductors. Alternating current (AC) electrical resistance (Q) is dependent on a number of factors including skin effect, proximity effect and temperature variation. Skin effect is caused by conductor self-inductance being greater at the centre than the periphery, which leads to current flow concentration at the surface. By contrast, the proximity effect is generated by cable conductor current induced magnetic field (and other parallel conductor currents). [0008] If the current in parallel conductors flows in the same direction, the magnetic flux is higher on the inner parallel sides of these conductors, thus increasing AC resistance in these parts of the conductors. As a result, current density is greater in the external (remote) halves of these conductors. [0009] Proximity effect is influenced by cable direct current (DC) resistance, system frequency, cable spacing and cable diameter, whereas skin effect is influenced only by resistance and frequency. While such proximity effects may be ignored for small conductors (e.g. up to 800mm2) carrying low currents, the effects are significant for high rating cables with large conductors. As a result it is generally necessary to compensate for such proximity effects by using "Milliken" or "Segmental" conductors of several alternated, insulated sector shaped strands. [0010] Sheath losses are commonly generated in cable metallic sheaths as a result of induced currents in these sheaths when load current flows in cable conductors. Sheath currents in single-core cables are induced by "transformer" principles; by the magnetic field of current flowing in the cable conductor which induces voltages in the cable sheath and other parallel conductors. Sheath induced electromagnetic forces generate two types of losses: circulating current and eddy current losses. Eddy currents are generated by similar principles to skin and proximity effects. Eddy currents are generally smaller than circuit (circulating) currents of solidly bonded cable sheaths and may be neglected except for very large segmental conductors. A circulating current is induced in the sheath by the conductor current's electromagnetic forces if the sheath is double bonded to form a closed loop via earth or a return conductor.

3 [0011] These losses are determined by the magnitude of current in the cable conductor, frequency, mean diameter and resistance of the cable sheath and the distance between single core cables (i.e. the mutual inductance). Sheath losses can often be significantly reduced or eliminated by applying special sheath bonding systems in conjunction with cable transposition techniques at joint bay positions. [0012] A common approach is to use a cross bonding link box at every splice point or joint bay to transpose cable sheaths and thus balance cable sheath impedances. For cross bonding, the cable length is commonly divided into three approximately equal sections. Each of the three alternating magnetic fields induces a voltage with a phase shift of 1200 in the cable shields. The cross bonding generally takes place in the link box. Ideally, the vectorial addition of the induced voltages results in a zero overall induced voltage. However, in practice, the cable length and the laying conditions will vary, resulting in a small residual voltage and a negligible current. Since there is no current flow, and the total of the three voltages is zero, the ends of the three sections can be grounded. In addition to cross bonding the shield, the induced voltage can be further reduced by cyclically transposing the main conductors of the 3-phase system. [0013] Surge voltage limiters or arrestors are routinely used between sheaths and earth at each link node to limit induced voltage spikes that may occur during transient and fault conditions. However, there remains a significant risk of arcing between the links of these link boxes, particularly during assembly, maintenance or repair of the link boxes. [0014] In view of these limitations, there is a need for a insulating cradle for a cross bonding link box that provides an improved level of safety to an operator. There is also a need for a insulating cradle for a cross bonding link box that can withstand the current impulse between links of a link box, and thus minimize any unnecessary damage to the components due to arcing. Summary of the Invention [0015] The present invention relates to an insulating cradle for a cross bonding link box that provides impulse resistance between the link arms of the link box, and effectively insulates two or more phases of power transmission at a connection point between cable sections. [0016] According to one aspect of the present invention, there is provided an insulating cradle for a cross bonding link box, said link box having an upper link arm and a lower link arm for connecting power transmission cable sections, the insulating cradle including: 4 (a) a body for installation between said upper and lower link arms, said body having an upper surface including an upper guide channel and a lower surface including a lower guide channel, wherein said upper guide channel is adapted to receive said upper link arm and said lower guide channel is adapted to receive said lower link arm. [0017] Preferably, the lower surface also includes a further lower guide channel adapted to receive a corresponding further lower link arm. The further lower guide channel may be formed by parallel ridges that extend below the lower surface. [0018] It is also preferable that the lower guide channel is formed by parallel ridges that extend below the lower surface. In a representative embodiment of the present invention, the parallel ridges forming the lower guide channel and the parallel ridges forming the further lower guide channel are adjacent and share at least one common central ridge. [0019] Preferably, the upper guide channel is formed by parallel ridges that extend above the upper surface. [0020] A vertical plane extending through the upper guide channel may intersect with a vertical plane extending through the lower guide channel. However, it should be understood that the angle of intersection of these vertical planes may vary. [0021] According to a further aspect of the present invention, there is provided a cross bonding link box for connecting power transmission cable sections, the link box including: (a) an upper link arm and a lower link arm; (b) an insulating cradle having a body for installation between said upper and lower link arms, said body having an upper surface including an upper guide channel and a lower surface including a lower guide channel, wherein said upper guide channel is adapted to receive said upper link arm and said lower guide channel is adapted to receive said lower link arm. [0022] The upper and lower link arms are preferably fixable between first and second connection terminal pairs respectively so as to permit power transmission through the cable sections.

5 [0023] During assembly of the link box, it is preferable that the lower link arm be fixed between the second connection terminal pair prior to the upper link arm being fixed between the first connection terminal pair. Once the lower link arm is fixed between the second connection terminal pair, the insulating cradle is preferably positioned on top of the lower link arm such that the lower link arm is located in the lower guide channel. It is then possible to fix the upper link arm between the first connection terminal pair, by locating the upper link arm in the upper guide channel. Advantageously, the alignment of the upper guide channel relative to the first connection terminal pair prevents the upper link arm from being fixed between the first connection terminal pair unless the upper link arm is located within the upper guide channel. [0024] Also during assembly of the link box, the upper link arm may not be fixable between the first connection terminal pair until the lower link arm is located within the lower guide channel of the insulating cradle. [0025] When the upper link arm is fixed between the first connection terminal pair and the lower link arm is fixed between the second connection terminal pair, a portion of the upper link arm may overhangs the lower link arm. [0026] It is preferable that the lower surface of the body also includes a further lower guide channel adapted to receive a corresponding further lower link arm. [0027] The further lower link arm is preferably fixable between a third connection terminal pair so as to permit power transmission through the cable sections. [0028] During assembly of the link box, the further lower link arm is fixed between the third connection terminal pair prior to the upper link arm being fixed between the first connection terminal pair. In a particularly preferred embodiment of the present invention, once the lower link arm is fixed between the second connection terminal pair and the further lower link arm is fixed between the third connection terminal pair, the insulating cradle is positioned on top of the lower link arm and further lower link arm such that the lower link arm is located in the lower guide channel and the further lower link arm is located in the further lower guide channel. In addition, when the lower link arm is fixed between the second connection terminal pair, and the further lower link arm is fixed between the third connection terminal pair, the lower link arm and further lower link arm may be parallel to one another. It is then possible to fix the upper link arm between the first connection terminal pair, by locating the upper link arm in the upper guide channel. Advantageously, the alignment of the upper guide channel relative to the first 6 connection terminal pair prevents the upper link arm from being fixed between the first connection terminal pair unless the upper link arm is located within the upper guide channel. [0029] Also during assembly of the link box, the upper link arm may not be fixable between the first connection terminal pair until the further lower link arm is located within the further lower guide channel of the insulating cradle. [0030] When the upper link arm is fixed between the first connection terminal pair and the further lower link arm is fixed between the third connection terminal pair, a portion of the upper link arm overhangs the further lower link arm. In a particularly preferred embodiment of the present invention where the lower link arm is fixed between the second connection terminal pair, a different portion of the upper link arm overhangs the lower link arm. Brief Description of the Drawings [0031] Embodiments of the present invention will now be described with reference to the accompanying drawings. These embodiments are given by way of illustration only and other embodiments of the invention are also possible. Consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description. In the drawings: Figure 1A is a top view perspective drawing of an insulating cradle according to an embodiment of the present invention; Figure 1B is a bottom view perspective drawing of the insulating cradle of Figure 1A; Figure 1 C is a top view perspective drawing of an insulting cradle according an alternative embodiment of the present invention; Figure ID is a bottom view perspective drawing of the insulating cradle of Figure IC; Figure 2A is a perspective drawing of a cross bonding link box including the insulating cradle of Figure 1A; Figure 2B is a schematic drawing of the cross bonding link box of Figure 2A; and Figure 2C is a perspective drawing of a cross bonding link box including the insulating cradle of Figure IC.

7 Description of Preferred Embodiments [0032] Embodiments of the insulating cradle for a cross bonding link box will now be described with reference to the accompanying drawings. The invention is particularly useful in relation to an insulating cradle for 3-phase cross bonding link boxes, and it will therefore be convenient to describe the invention in that environment. However, it should be understood that the invention is intended for broader application and use. [0033] Referring to the drawings, Figures 1A and 1B illustrate an insulating cradle for a cross bonding link box according to a representative embodiment of the present invention. In addition, Figures IC and ID illustrate an insulating cradle for a cross bonding link box according to an alternative embodiment of the present invention, although the majority of the features of this insulating cradle are the same as those shown in Figures 1A and 1B. It will therefore be convenient to describe these two embodiments together. [0034] An insulating cradle 100, as shown in Figures 1A and 1B, includes a body 110 having an upper surface 102 and a lower surface 112. The upper surface 102 of the body 110 includes an upper guide channel 104 formed by parallel ridges 106 that extend along and above the upper surface 102. The upper guide channel 104 is adapted to receive an upper link arm 222 (shown in Figures 2A and 2B). Similarly, the upper guide channel 104 shown in Figures IC and ID is adapted to receive an upper link arm 222 (as shown in Figure 2C). [0035] The lower surface 112 of the body 110 includes two lower guide channels 114 formed by parallel ridges 116 that extend along and below the lower surface 112. The two lower guide channels 114 are adapted to receive corresponding lower link arms 224 and 226 (as shown in Figures 2A and 2B). In addition, the two lower guide channels 114 are also separated by a void channel 120 which also assists to insulate the two lower link arms 224 and 226 from one another during use. By contrast, in the embodiment of the invention shown in Figures IC and ID, the lower surface 112 of the body 110 includes two lower guide channels 114 formed by depressions that extend along the lower surface 112. The two lower guide channels 114 are adapted to receive corresponding lower link arms 224 and 226 (as shown in Figure 2C). [0036] The insulating cradle 100 is preferably manufactured from a high density polyethylene (HDPE) material of high electrical insulation characteristics. In a particularly preferred embodiment of the invention, the internal structure of the insulating cradle 100 may have a lattice configuration as shown in Figure ID of the drawings.

8 [0037] In Figures 2A and 2B of the drawings, the insulating cradle 100 of Figures 1A and lB is shown installed in a cross bonding link box 200. Similarly, in Figure 2C, the insulating cradle 100 of Figures IC and ID is shown installed in a similar cross bonding link box 200. [0038] The link box 200 includes a housing 240 and three cable openings 250 which facilitate the insertion into the link box 200 of the three cable sections (not shown) required for 3-phase power transmission. Within the housing 240 are three terminal connection pairs, which consist of three concentric connector terminals 202, 206, and 210, and three corresponding and opposing stalk terminals 204, 208, and 212. The three concentric connector terminals 202, 206, and 210 are connected and supported by an insulation brace 230. In a particularly preferred embodiment of the invention, the insulation brace 230 is an epoxy glass insulated support strip that is bolted between the concentric connector terminals 202, 206, and 210, to restrict movement under the clamping effect of the 50kA fault current. Similarly, the three stalk terminals 204, 208, and 212 are also connected and supported by an insulation brace 230. In order to facilitate the connection of the three concentric connector terminals 202, 206, and 210 to the corresponding three stalk terminals 204, 208, and 212, in a cross bonding arrangement, the link box also includes two lower link arms 224 and 226, and an upper link arm 222. In Figure 2C of the drawings, the two lower link arms 224 and 226, and the upper link arm 222 are predominantly covered with an insulating material 270 such as, for example, heat-shrink tubing in order to achieve the required lightning impulse withstand values. [0039] The link box 200 shown in Figures 2A, 2B and 2C is illustrated in its assembled condition with the insulating cradle 100 already in place. However, in order to assemble the link box 200 with the insulating cradle 100, either for the first time (e.g. new installation) or in the course of a maintenance operation, a user will be required to perform the following actions (in the order that they appear): (i) fixing lower link arm 226 between a first connection terminal pair, namely concentric connector terminal 202 and connector strut 204, and fixing lower link arm 224 between a second connection terminal pair, namely concentric connector terminal 206 and connector strut 208 (using convention fixing means such as, for example, a nut and bolt combination); (ii) positioning the insulating cradle 100 on the lower link arms 224 and 226 such that lower link arms 224 and 226 are received in the corresponding lower guide channels 114 of the insulating cradle 100; 9 (iii) positioning upper link arm 222 in the upper guide channel 104 of the insulating cradle 100; and (iv) fixing upper link arm 222 between a third connection terminal pair, namely concentric connector terminal 210 and connector strut 212. [0040] In representative embodiments of the present invention, shown in Figures 2A, 2B and 2C, the alignment of the upper guide channel 104 relative to concentric connector terminal 210 and connector strut 212, prevents the upper link arm 222 from being fixed between concentric connector terminal 210 and connector strut 212 unless the upper link arm 222 is located within the upper guide channel 104. [0041] In addition, it is clear from Figures 2A, 2B and 2C that the upper link arm 222 can not be fixed between concentric connector terminal 210 and connector strut 212 until the lower link arms 224 and 226 have been positioned within the lower guide channels 114 of the insulating cradle 100. The height of the insulation cradle 100 somewhat regulates the space between the upper link arm 222 and the lower link arms 224 and 226. This arrangement of the lower link arms 224 and 226 within the lower guide channels 114, and the arrangement of the upper link arm 222 within the upper guide channel 104, ensures that only one possible configuration of the link box 200 and insulating cradle 200 can be obtained during assembly. This minimises the risk of incorrect assembly of the link box 200 which may otherwise lead to damage (or destruction) of components and unnecessary downtime of the power transmission line. More importantly, the insulating cradle 100 acts to insulate the phases by providing an impasse to withstand current between the link arms 222, 224 and 226. [0042] The embodiment of the invention shown in Figures 2A, 2B and 2C illustrates the link arms 222, 224, and 226 in a configuration such at (in the assembled form) upper link arm 222 overlaps lower link arms 224 and 226. However, it should be understood that alternative configurations may also be possible. [0043] The word 'comprising', and forms of the word 'comprising', when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

10 [0044] As the present invention may be embodied in several forms without departing from the essential characteristics of the invention, it should be understood that the above described embodiments should not be considered to limit the present invention but rather should be construed broadly. Various modifications, improvements and equivalent arrangements will be readily apparent to those skilled in the art, and are intended to be included within the spirit and scope of the invention.