CHAPTER 4: RESULTS AND DISCUSSION
4.5 USE OF MODERN ACCESSORIES
Any user of a cable system will have had to make decisions regarding termination and jointing of the cables used in that system. Historically, bitumen compound was used when terminating
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paper cables in compound filled cable enclosures. The modern trend of switchgear is toward more compact, air-filled enclosures designed for screened, single-core polymeric cables.
Modern accessories cater for a variety of installation constructions and conditions, and it is possible to terminate paper insulated cables into modern switchgear, within reason. The compact nature of modern switchgear places a limit on the maximum size of cable (especially three-core cable) that can be practically terminated within a compact enclosure [148].
Correct preparation is essential to the reliability of any accessory and details in this respect are provided throughout Goulsbra’s book [149].
Of more interest with reference to this dissertation, is the subject of joints as this is where the difficulties and challenges associated with network integration are most obvious. For instance, accessories traditionally used for the jointing of belted paper cables cannot be used to join a belted paper cable to an XLPE cable. Furthermore, paper cables usually have sector-shaped cores, while XLPE has round conductors. To complicate matters more, conductors may be solid or stranded, and made from aluminium or copper, and be of different cross-sectional area. The physical construction of the two cable types is, of course, also very different.
The functions of a joint have already been listed in Chapter 2. The first function is the joining of conductors, and this may be achieved by means of crimped or mechanical (torque shear) connectors.
When crimping ferrules onto conductors, the ferrule must be chosen to correspond to the type and size of conductor being joined. Usually, copper connectors are crimped using the hexagonal method, whereas aluminium conductors are crimped with the deep indent method [150]. Both of these methods and constructions are shown in Figure 4-7 overleaf. The correct preparation of aluminium is particularly important to give good electrical contact due to the rapid formation of an oxide film on exposed aluminium. The disadvantages of crimping are that a range of ferrules must be held in stock, corresponding to the different sizes and types of conductor that may need to be joined. In the case of joining copper to aluminium conductors, special bi-metallic conductors need to be used, although SANS 10198 does not recommend their use [151]. Each crimp ferrule will require the use of a tool to provide the necessary pressure, as well as a unique set of dies designed to be used with a specific ferrule.
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Figure 4-7: Crimping methods [152]
Mechanical connectors have the advantage that they are suitable for joining copper and aluminium conductors (they are inherently bi-metallic), sector-shaped or round conductors, solid or stranded. They are also range-taking. They can be water-blocked i.e. provided with a solid centre so that any water or oil present in a section of cable cannot continue to migrate down the cable. They are more expensive than crimp ferrules but have the advantage that no special tools and dies are required to install them. Their installation is simple in that the jointer tightens the hexagonal heads until they shear off at a pre-determined (by the manufacturer) torque, thus making sure correct installation is largely independent of the skill set of the jointer.
In South Africa, NRS 075 [153] details the preferred requirements of the major users for these connectors. The preferred ranges are given [154] as 16 to 35mm2, 25 to 70 mm2, 50 to 95 mm2, 120 to 240 mm2 and 185 to 300 mm2. The ranges have been chosen to overlap and thus provide maximum flexibility. Larger sizes are designed for circular conductors and are not range-taking.
An example of a mechanical connector is shown below in Figure 4-8.
Figure 4-8: Mechanical Torque Shear Connector (© Tyco Electronics)
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Having dealt with the physical connection of the actual conductors, the other joint functions need to be taken into account. The elimination of any air or voids from the joint, as well as stress control, are dealt with by stress relieving tape, or the Faraday cage principle [155]. With the former, layers of semi-conductive tape are wrapped around the ferrule and conductor interface in order to provide a smooth profile. With the Faraday cage principle, a conductive sleeve or accessory (either separate or integral to other parts of the joint) is installed over the interface. Care must be taken to ensure that the joint is dimensionally compatible and suitable and to this end it is advisable to purchase the joint as a system, rather than as discrete parts. The construction and stress distribution of a typical single-core polymeric joint are shown in Figure 4-9 below.
Figure 4-9: Joint with stress control [156]
Modern accessories may be cold applied (push on or cold shrink), heat shrink, or hybrid whereby more than one technology is employed [157]. In all cases they need to be designed to fulfil the functions required of them, with critical areas being sealing and mechanical strength.
For users considering the migration from a PILC to an XLPE cable platform, the accessory of most interest may be the transition joint. This is already employed by some large users who have experienced difficulties in terminating three-core PILC cable into modern compact cable termination enclosures. The solution has been to install a short length of single-core polymeric cable to facilitate easier termination within the switchgear, and then jointing these polymeric tails to the standard three-core PILC cable just outside the substation, or within the cable trench.
The use of transition joints is covered in SANS 10198-11 [158], and the recommended method is the resin-filled system. However, this document and part 10 date back to 1988 and are overdue for revision, as the reference to asbestos string [159], for instance, is out of step with the modern focus on health and safety.
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