Insulation Medium

Terminations are generally filled with a dielectric fluid, usually a synthetic (polybutene or silicone based) insulating liquid, at or slightly above atmospheric pressure. The type and quantity of the fluid depends on the specific design of the termination. Poor quality of the liquid or contamination, due to external factors (humidity, water ingress, metallic or other polluting particles, etc), can reduce the electrical performance of the fluid and result in termination failure.One of the most common issues with the use of fluid is the risk of leakage through the sealing point areas, typically the weld/plumbing between the cable metallic screen and the bottom part of the termination or the mechanical seal onto the stress cone. A well-made seal depends mostly on the skill of the jointers.

There are also designs that use SF6 gas as the insulation medium, but this solution has to bear in mind the

environmental concerns of using SF6 gas.

2.2.4 Connectors

The connector electrically and mechanically joins the conductors of two cables or the cable and the top connector of a termination. Thus the connector must exhibit good electrical conductivity to avoid temperatures higher than that of the conductor in any operating condition and also present sufficiently high mechanical pull-out (tensile) strength to withstand thermo mechanical stresses during operation. It should be noted that TFB1.46 is currently working on Conductor Connectors (Mechanical and Electrical Testing). The following types of connectors are used for extruded cable connections:-

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2.2.4.1 Compression Connector

This connector includes a tube of the same material as the cable conductor into which the conductors to be joined are inserted. The tube is then compressed by a hydraulic press. The compression connector is the most commonly used type, because it is easy to install and does not require heat.

The cross section of the connector is at least equal to the cross section of the conductors to be joined. When the connector is exposed to an electric field, as in taped joints, it is necessary to provide suitable chamfers at both ends to minimize the effects of longitudinal electrical stresses.

Fig 7 Compression connector

A special bimetallic connector is used when it is necessary to join a copper conductor to an aluminium conductor. These connectors are half copper and half aluminium. The two connector halves are joined in the factory by friction welding.

Some companies use a copper alloy connector for both copper and aluminium conductors.

2.2.4.2 Cad Welding

Another way is to make a connection of copper and aluminium conductors by Cad-welding on site, though Cad welding is not used that often for aluminium. This is an exothermic welding process in which metal and metal oxide powders are placed in a special crucible mold around the parts to be welded. This mixture is ignited resulting in a short high temperature reaction,causing the flow of molten metals to form a localised solid connection.

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2.2.4.3 Soldered or Brazed Connector

Soldered connectors are used with small conductor cross sections (below 630mm2) and with cables having a short circuit current temperature below 160 °C, b ecause the solder can become soft during the cable system operation. Brazed connectors do not present this problem, but are more difficult to make.

2.2.4.4 MIG or TIG welded connection

The two conductors are fused together by the application of molten metal. A Metal Inert Gas (MIG) or Tungsten Inert Gas (TIG) welding process is applied in this case. Due to the high temperature developed during the process, air or water cooling clamps are required on both sides of the weld, in order not to damage the cable insulation The welding process is used for large aluminium conductors and for insulated wire copper conductors; in the latter the burning of the wire insulation, if necessary, ensures a good contact between strands. This technology requires an operator with a very high skill level and is time consuming. This weld provides a connection with an electrical conductivity, which is equivalent to that of the conductor itself. The connection is not subject to instability due to decrease of contact pressure as a result of load cycling. However the tensile strength of the welded connector is significantly (50 to 60 %) lower than the ultimate tensile strength of the conductor, due to the annealing of the conductor near the weld. If necessary, for submarine cables, the tensile strength can be improved by round compressing the conductor and the weld (hardening process).

Fig 9 Example of a MIG Weld

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2.2.4.5 Plug-in Connector

Two metal connectors, that terminate the conductor, are jointed through elastic or multi contact spring loaded contacts that are able to carry the current. Locking pins can be used to anchor the two parts together. Plug-in connectors can easily join conductors of different materials and cross section.

Fig 11 Plug-in connector (male contact) on prepared cable end. One of the advantages of a plug-in connection is the shorter length of the joint.

2.2.4.6 Mechanical bolted connector (shear bolts)

With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. The bolts shear off at a predetermined torque and are then finished flush with the surface of the connector. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface. Before using shear connectors consideration must be given to tensile strength during load cycling and pull out.

2.2.4.7 Mechanical bolted connector

With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface

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2.2.5 Non-buried Joints

Non-buried joints locations may be in tunnels, on bridges, in underground chambers or similar enclosures. Non-buried joints for XLPE cables usually have premolded joint bodies with additional covering for protection against moisture and mechanical damage. The additional covering could be heat shrink tubes or metal housings with additional insulating housings/coffins.

Transition joints for XLPE to oil filled cable are often installed as non-buried joints in underground chambers. They use metal-tubes combined with epoxy insulators as a barrier between the different insulating materials - XLPE and fluid impregnated paper. In the case of transition joints full quality control must take into account electrical and mechanical stresses for both sides of the joint and any interface locations.

Water can seep into a non buried joint, if any earth or bonding wire connections to the joint are not sealed properly.

Fig 13 Example of non-buried joints : 145kV single core cable joints installed in a cable jointing chamber/manhole

2.3 Assembly

TB 476 is a comprehensive document on assembly and quality control of XLPE accessories and the contents pages are attached as Appendix 3. It gives guidance on aspects of cable accessory workmanship that need to be carefully considered in evaluating the execution of the work, including the specific technical risks and the associated skills needed to mitigate them.

Where a termination is to be filled with compound, the manufacturers filling instruction should be followed. Filling compounds may be such items as polybutene, silicon oil or other dielectric fluid or gas.