When it comes to medium voltage cables, the past few decades years have seen great diversification in design and composition. Indeed, the number of different products on the market in Europe alone now exceeds 100. This extreme diversification within the MV range is not due solely to choices between single-core versus three-core cables. Rather, it is the result of differences in conductor materials, structures and shapes, differences in insulation materials as well as differences in design of outer semi-conductive layer, metallic screen and outer sheath. Given the broad array of different MV cable designs, attempts by different players in the industry to harmonize construction have so far not met with much success – in spite of discussions toward this goal at international conferences.
Recently, however, there seems to have been a slow but fundamental change underway in cable design. This change has been driven by the common customer requirement of finding the most cost-effective solution for the entire system and not simply the cable itself. That means users are increasingly looking for the optimal combination of cable, cable accessories and cable laying characteristics. The main requirements in this regard include:
• mostly single core cables
• aluminium solid conductor
• high performance thermoplastic elastomer (HPTE) insulation instead of XLPE
• fully bonded outer semi-conductive layer
• laminated aluminium foil screen instead of copper wire screen, and
• extra thick high density polyethylene (HDPE) outer sheath
Based on these requirements, it appears that a standard cost-optimized medium voltage cable of the future might look like what is shown below. In such a design, the screen is a laminated aluminum foil and the cable is radial watertight. However, in contrast to copper wire-screened cable, such an aluminum screen cable needs a special earthing device for joints and terminations, also shown below. For example, such a cable design having an aluminium screen but with XLPE insulation has been used in France without major problems for over 30 years. Similarly, more and more cables with laminated aluminium screen are already in service in countries such as Switzerland, Finland and Spain. In Germany, the first lot of this type of cable was recently in production for qualification tests.
There now exist two design variants for the aluminium screen in such cables and these require different screen connection devices. The first solution is a fully bonded outer sheath on the aluminium screen. In this case the position of the earthing device is beneath the aluminium screen. The second option is a spacer tape between aluminium screen and outer cable sheath. Here, the earthing device can be located above the aluminium screen.
Of course, the common problem of all kinds of screen connection devices relates to transmission capability of fault currents from the screen to ground potential in a substation. Deterioration of the contact can also affect the reliability of accessories. Finally, it is important not only to have good contact but also to ensure adequate heat emission from the screen to the outer sheath of the cable.
Professor Klaus Dieter Haim