Achieving More Cost Effective & Reliable Cable Accessories through Improved Layout, Production & TestingDecember 1, 2018 • ARTICLE ARCHIVE, Cable Accessories, Testing
Among the different possible constructions, slip-on technology has grown to become the world standard to connect extruded high voltage cables. One of the reasons for this is that use of pre-fabricated, routine-tested components for the main insulation minimizes risk of faults during installation. At the same time, demand for higher voltage levels, combined with advances in XLPE materials, is resulting in cable designs with higher field stresses placed on their accessories. To allow progress to continue, there will be a need to optimize electrical design and further improve both material and production technologies.
Mechanical properties of cable systems will also need on-going development. With many installations now taking place under demanding site and service conditions, robust and suitable designs will be required to ensure trouble-free service. Moreover, since the quality of an installation can differ from one country to the next, a certain fault tolerance will be yet another characteristic needed in the next generation of accessories.
Apart from the expertise required to design cable accessories for high and extra high voltages, suppliers in this business must also maintain large investments in production and testing facilities. Moreover, since pre-fabricated insulating parts are only suitable for a defined range of cable dimensions, different sizes have to be offered depending on voltage and cable cross-section. That means additional investment, this time in moulds for each size. Ideally, some standardization will be desirable to limit the number of possible different parts.
Modular layouts for rubber components can help in this regard by allowing more cost effective production as well as stock keeping. Moreover, since changes in design are costly, it will also be important to develop layouts that can be used on all the different types of construction offered by manufacturers of cables.
Finally, international standards for cables and their accessories require factory-tested components. In this regard, routine testing is an additional cost driver in this business. While opportunities to optimize already well-defined test procedures may be limited, more sophisticated equipment and handling of samples could still help reduce testing costs.
This article from 2017, contributed by Thomas Klein, Eckhard Wendt and Stefan Zierhut of STRESCON – a German firm of experts, specializing in consulting on cable accessories – reviewed these challenges and proposed recommendations to deal with them.
The cost effectiveness and reliability of insulating components used in accessories such as stress cones and joint bodies depend greatly on the sophistication of the layout where they are produced. Fortunately, the insulation of modern XLPE cables is now more or less standardized and that means there is no longer a need to customize stress cones in each case. Indeed, modern accessories can meet the requirements of all XLPE cables being supplied by different manufacturers. However, international standards still ask for a common type test to guarantee this – a reasonable requirement given that there can be significant differences in insulation dimensions when it comes to HV cables.
For example, 245 kV cables from some manufacturers have insulation thickness of 22 mm or more while this is only 16 mm in the case of other suppliers. Combined with allowed tolerances, eccentricities and further reduction in thickness by peeling during installation, accessories may have to manage the enhanced field stress of only 13 mm of XLPE insulation. Fig. 1 illustrates the tangential component of field strength along the interface between cable insulation and an accessory’s stress cone rubber. It is apparent that the accessory could not be installed on XLPE cables with insulation thickness below 19 mm.
However, given optimized layout of stress cones and deflectors (i.e. the embedded semi-conductive elements), maximum field stresses can be reduced and make these products suitable even for thinner cable insulation. Such an optimization process can be achieved using numeric simulation tools. In this regard, it is essential to also include the deformation of the expanded stress cone or joint body after the process of slipping it onto a cable – an effect that is especially relevant when simulating joints and fluid-filled terminations. This is because, for these types of accessories, there are no additional mechanical forces available apart from expansion to create defined pressure between cable insulation and stress cone or joint body. Such pressure is needed to ensure sufficient electrical strength along the interface, given high tangential stress.
All cable accessories of this type use geometric-capacitive field control, which is highly dependent on shape of the integrated conductive parts, i.e. the deflectors. Fig. 2 demonstrates deformation caused by expansion.
Deformations of deflectors and stress cones can cause increased field stresses in the accessory. Because of the wide range of possible cable diameters, a certain accessory design calls for a number of stress cone sizes. To keep this number as small as possible requires optimized deflector designs. It is also important to take into account both minimum and maximum expansion for any given size of stress cone or joint body. The objective in both cases is that all parts remain within the safe limits of electrical stress for that given design. The best field distribution will be reached at some intermediate expansion. Only in very specific cases will deformation phenomena limit the expansion range of field control elements. Usually, the minimum expansion is given by the required pressure, while the maximum by the mechanical properties of the elastomeric material such that no ripping occurs due to overstretching.
But reliability is not the only important characteristic of cable accessories these days. With the huge diversity in cable dimensions (e.g. voltage levels up to 550 kV and conductor cross-sections up to 3000 mm2), many sizes of insulating components are needed. Since pre-moulded parts are involved, that translates into large investments in moulds for all the different types and sizes of insulating components. Cost efficient layouts are therefore essential and can be achieved by ‘modular designs’ of these parts, where this term means that identical insulating components can be used for different types of accessories, provided cable dimensions are the same. For example, the same earth deflectors can be used for stress cones of fluid-filled and dry-type terminations and for joint bodies as well. This helps reduce both mould and stock keeping costs.