Power distribution networks are changing rapidly as integration of renewables as well as creeping urbanization require updated concepts in power infrastructure. Compactness is no longer only desirable but now even a crucial performance requirement. Similarly, downtime has to be minimized while safety aspects are more complex. This edited recent contribution by Ruben Grund and Michael Zerrer of Pfisterer, discusses these challenges in the context of development of ‘next generation’ cable accessories.
Increasing Transmission Capacity
Urbanization requires higher transmission capacity that could either be triggered by growth in number of customers or due to fluctuating power generation and consumption due to integration of renewables. Moreover, energy has to be transported over long distance since points of generation and demand in most cases do not overlap. This requires detailed consideration of power losses and one approach to reduce these has been increased system voltage, as has occurred over past decades. While the first three-phase AC line in 1891 had a system voltage of 25 kV and carried power over 176 km, these days system voltage of 550 kV is used for extra high voltage cable lines, allowing power transmission up to 1.5 GVA. The main reason for higher system voltages is power loss reduction. For example, increasing system voltage by 10 leads to a reduction in power losses by a factor of 100, using same diameter cable. If cable diameter is reduced by 10, power losses are still reduced by a factor of 10.
Cable Accessories up to UM=550 kV
Accessories are NOW increasingly available for high voltage cables up to 550 kV. This includes dry, pluggable cable connections for transformers and gas insulated substations (GIS), cable terminations, cable joints and pluggable cable joints as well as blind dummy plugs. These components are type tested and can be used for all types of XLPE cable, independent of core diameter, thickness of insulation or manufacturer. All field control units and electrically stressed materials are pre-fabricated and routine-tested, which ensures highest long-term operational safety.
One type of dry pluggable system can handle cable diameters of 3000 mm² CU or AL conductor, carrying up to 4000A nominal current. Maximum diameter over insulation is 144 mm and the principal set up is shown in Fig. 1. An additional benefit is the housing, which is touch-proof, waterproof and salt-water resistant, meaning it can be used in coastal areas as well as in offshore surroundings.
The extensively tested latest technology of cable joints ensures simple installation of the waterproof external housing as well as maximum operational safety. Using a bolted connector allows the conductor to be connected with optimal contact force, without special tools. Treatment of the cable shielding can be adapted individually for cable type and customer needs (see Fig. 2 as example).
The qualification requirement is performing a type and a pre-qualification test, according to IEC. A sample set up as per IEC 62067 is shown in Fig. 3.
A major hurdle is the so-called Annex G test at which the joint is submersed in water at 1 bar pressure and cycled, with number of cycles depending typically on regional specifications. Chart 1 shows heat cycle measurements during such a test, while temperature at a dummy is used for calibration.