Previously, oil-soaked paper was used as insulation for medium-voltage cables of 6-20 kV classes. At the same time, there were two core designs – round-shape or sector-shape. The advantages of cables with sector cores (Design 1) are shown on the slide.

Currently, cross-linked polyethylene (XLPE) is mainly used as cable insulation. Of course, the question arose again about which shape of the core is optimal – round or sector?

At first glance, cables with XLPE insulation and sector-shape cores (Design 2) are as good as old cables with oil-soaked paper (Design 1). However, in my opinion, this is not quite true, for two reasons.

✅ Applying paper insulation to the core of the sector shape was not a problem. The manufacturer has always been able to accurately control the number of layers of paper, ensuring an equal thickness of insulation on top of the core at all points on the surface of the core. However, for modern cables, insulation (XLPE) is extruded, and if the core has the shape of a sector, then the insulation thickness may be different. Most likely, the smallest insulation thickness will be at the “corners” of the sector – unfortunately, it is in this place that the highest electric field strength is present, which increases the risks of breakdown of thin XLPE insulation.

✅ When installing the cable joints and terminations, it is not always possible to carefully remove the semi-conductive layer over the XLPE insulation, and this is one of the reasons for the increased accident rate of the cable lines. It is even more difficult to remove such a layer for a cable with sector-shape cores than for a cable with a round-shape core.

For these two reasons, in my opinion, it is better to avoid using XLPE cables with cores of a sector shape. In combination with XLPE insulation it is preferable to have round-shape cores and not sector-shape.

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