Because of the high electric field intensity at the cut edge of the outer semi-conductive layer, a stress control element is always indispensible in the insulation of medium and high voltage cable accessories. In the case of most modern polymeric-insulated cables, two different design and material solutions have emerged and become widely accepted in the marketplace. These feature different advantages as well as application limits.
Figure 1 shows a classical geometrical stress control system, in this case for a high voltage application but suitable as well at all voltage levels from medium to extra high and for both AC and DC. The black element is a semi-conductive material with specific resistance of approximately 10-2 Ωm and shaped in the form of a cone in order to continuously increase distance between conductor and the outer semi-conductive layer.
Trends in Stress Control Design
Such geometrical stress control system offers a‘ loss free’ solution, considered extremely important for extra high voltage AC applications. However, it can prove disadvantageous for medium voltage accessories, where a very wide cross section range is required and where thickness of such a cone might constrain flexibility during installation. For such systems, fabrication of a cone embedded in an insulating material is more complex.
That’ s why there is now a clear trend toward refractive field control to manufacture more cost-effective medium voltage accessories. Here, the field control element is a slim tube – not only easier to manufacture but also applicable across the range of‘ cold shrink’ or‘ slip-on’ technologies. This black material( see Fig. 2) is not semi-conductive but rather made from an insulating material with specific resistance of approximately 10 11 Ωm and doped with carbon black to obtain a dielectric constant between 10 and 20.
Such a field control system can also compensate for errors during cable preparation and even for incorrect positioning of the accessory.
Two different design and material stress control solutions have emerged and become widely accepted in the cable accessory marketplace.
The principal negative consequences of refractive stress control materials are thermal losses at the point of highest field intensity. While this may prove a negligible problem for medium voltage applications, it becomes extremely important when talking about extra high voltage AC accessories. In the case of high voltage DC applications, however, given the phenomenon of space charges in the insulation, this can turn out advantageous in combination with resistive field control. Tests of such a stress control solution for DC are now on the way.
Figure 3 shows a geometric stress control system used for medium voltage applications where modifications have been made in the shape of the field control element so as to reduce wall thickness in the critical area. For medium voltage accessories, both refractive and geometric stress control are possible, each with certain advantages but also disadvantages.
Focus On Cable Accessories
54 20
YEARS
Q2 2013
Fig. 1: Typical stress control element in HV cable termination.
Fig. 2: MV termination with refractive stress control element included.
Fig. 3: MV termination with geometrical stress control element included.
Professor Klaus-Dieter Haim University of Applied Sciences Zittau / Görlitz, Gemany
KDHaim @ hs-zigr. de