Electricity + Control October 2018

CABLES + ACCESSORIES

investment is not equivalent to the benefit. Chart 1 compares use of different conductor types and shows the resistance increase with reference to the skin effect. The ratio of ac resistance to dc re- sistance is shown to be close to equivalent utilis- ing enamelled conductors, which are stranded and have every single wire insulated. Therefore, no cur- rent flows between the single strands, resulting in a great reduction of skin effect. This is a major advantage for cables but it chal- lenges design and therefore the function of cable accessories. Contact surface requires homogene- ous current distribution. Former solutions required removal of insulation layers by abrasives or heat, processes which are time consuming and are dependent on workmanship. Furthermore, addi- tional cable length is required due to the bending of strands. This therefore contradicts the goal of compact solutions. Some of the latest develop- ments make use of the front surface of the cable conductor as the connecting surface. Contact sur- face is offered with an incompressible yet flexible material. This solution is being used for joints, ter- minations and pluggable systems. Flexibility While increase in transmission capacity is a vital enabler for the grid of the future, network flexibility is another key aspect. Pluggable systems are be- coming more and more popular but any enclosed system, such as a cable system, needs additional thought when it comes to protection. Surge protection Surge arresters are usually either SF6 or air-insulat- ed and both have limitations. Use of SF6 is becom- ing more restricted and testing requires handling SF6 gas. Similarly, air insulation requires creepage and space and positioning is not optimal. Substa- tion grid layout when using cable usually does not allow an air-insulated arrester to be positioned as close to the transformer as possible. A pluggable, solid insulated arrester has been developed and tested up to Uc=144 kV (Ur=180 kV) with reference to the latest standards. The arrest- er’s main insulation is solid and there is no insula- tion liquid or gas such as SF6 included. The most important parts as far as arrester functionality is concerned are the metal oxide discs. These are used as a non-linear component with very low leakage current during operation.

Figure 4: Compensation device.

Figure 5: Cable clamp.

Decrease in skin factor De-centralised energy production, driven mostly by increasing generation of renewables, requires additional network capacity for energy transfer and distribution. Growing urbanisation further drives this by requiring additional distribution capacity. Increasing conductor cross-section is one op- tion. Nevertheless, due to the ‘skin effect’ the

Chart 1.

14 Electricity + Control

OCTOBER 2018