Let me pay attention to one very common mistake that can be very expensive. In the construction of a single-core cables, the main elements are the core and XLPE-insulation. The metal screen (Cu, Al) and the outer sheath of the cable are usually perceived as auxiliary elements of the cable. As a result, people carry out the thermal calculation of the cable line, taking into account only the processes in the core, and forgetting about the processes in the screen.

At the same time, with two-side grounding of screens, currents are induced in the screens, which causes the screens to heat themselves and heat the XLPE-insulation. Thus, in a single-core cable there are two sources of active power losses and insulation heating at once:
1. Power losses in the core.
2. Power losses in the screen.

The selection of the core is made during the so-called thermal calculation. Within the framework of this calculation, all sources of heating of the cable and all conditions of its cooling should be taken into account. How then can you make a thermal calculation of the cable line if you took into account the losses in the core and did not take into account the losses in the screen?

Someone may say – “We remember about the losses in the screens, but they are small compared to the losses in the core, and therefore we did not take them into account”. This is a big misconception.

The losses in the screen depend on the parameters of the screen contours:
1. The screen cross-section (16, 25, 35, 50,…. 240, 265, 300 mm2).
2. The screen material (Cu, Al).
3. The distance between single-core cables of phases ABC.

For example, I can give an examples of ordinary 6-35 kV cable lines, where the losses in the screens were 1.5-2 times (!!!) greater than the losses in the core. That is, the screen in these examples is the main (!!!) source of XLPE-insulation heating. How is it possible not to take into account the processes in the screens when performing the thermal calculation of the cable line and selecting the cable core section???

A cable (6-35 kV or 110-500 kV) selected without taking into account power losses in the screen can be overheated to such a temperature that its insulation degrades, and immediately along the entire route of the cable line. The cost of replacing such a cable can be up to 1 million euros for each km of the route.

If someone provides you with the results of the thermal calculation of the cable line and the selection of the cross-section of the core, but did not say a word about the screen parameters, then such a person does not understand what he is doing and cannot be trusted.

The voltage class does not matter. Anyone who thinks that the problem of screens has nothing to do with 6-35 kV networks is making a mistake. The problem of currents and losses in screens is common to all networks where single-core cables are used – this means 6-35 kV and 110-500 kV.