**7. Conclusion**

The Slovenian power system designers tend to reduce the number of voltage levels. In the future, only four levels will probably exist: 0.4; 20; 110 and 400 kV. The 10 kV levels are in the middle of the range and are present only in large towns (Ljubljana, Maribor). One of the major problems is abandonment of the 220 kV voltage level in the transmission network. The designers are thinking about preservation of the 220 kV power line platforms and the transition to 400 kV conductors. The simplest solution seems to be the erection of new overhead power lines, yet this would involve substantial funds and new permissions. The proposition is the use of covered conductors. The purpose of this chapter was to determine whether it is possible to use the existing platforms and transmission towers of the 220 kV power lines with the new 400 kV conductors. We proposed a covered conductor with a carbon fibber core and a conductive layer made from aluminium, surrounded by in insulation made from polyurethane. The insulation thickness was calculated as the double insulation of the conductor was made from two layers, the one being polyurethane and the other air. We determined a radius at which the electric field intensity at the edge of the insulation is not high enough to cause breakdown of the surrounding air (the electric field intensity has to be lower than the dielectric strength of air). For the reduction of weight of the conductor we assume that we can replace the steel core with a core made from carbon fibbers.

We attempt to calculate the electric field intensity in the air (at the edge of the insulation) with the following values. For the radius of the conductor we take the radius of the current 220 kV conductor, which is 15.3mm. For the thickness of the insulation we use 15mm and its dielectric strength εr = 3.4. For the voltage we use 400 kV. The result we get with these values is 2.39 MV/m, which is less than the dielectric strength of air.

The proposed conductor will have a core made from carbon, a conductive layer made from aluminium and the insulation made from polyurethane. According to the usual labelling of conductors we named the suggested conductor PUAC 2150/490/65 mm2. Here 2150 mm2 stands for the cross section of the polyurethane mantel, 490 mm2 for the aluminium and 65 mm2 for the core made form carbon fibber. The electrical resistance of the covered conductor doesn't change in comparison with a normal conductor and is R = 0.0592 Ω/km. Likewise the dielectric strength of the insulation mantle does not affect the electrical reactance of the conductor, which is = 0,414 Ω/km. For the proposed conductor PUAC 2150/490/65 mm2 with a 15 mm thick insulation layer made from polyurethane (εr= 3.4) the capacitance is *C* = 12.6 nF/km.

The over ground conductors are used to transfer electricity between two points and they lead through various parts of the area. We calculated the mechanical properties of the proposed cable and the sag in the middle of the imaginary span and over obstacles. With this data we calculated the impact of non-ionizing radiation that over ground lines exert on the environment.

The installation of an over ground power line is disruptive to the environment. The frequency that we use for the transfer of electricity in the distribution network is 50 Hz, and it causes a magnetic field with the same frequency. This electromagnetic field falls in to the category of low frequency fields. As negotiated at an international level it actually belongs to electromagnetic fields with very low frequencies (ELFF), with frequencies ranging from 30-300 Hz. This is the range at which we talk about electric and magnetic fields separately, instead of electromagnetic fields. The electric field is the result of electric charge on the conductor and in the ground. It is also indirectly linked to the voltage between conductor and ground, the higher the voltage the higher the electric field is. If we look at the limit values that are determined in the Slovenian legislation the electric field is more problematic than the magnetic field. We calculated the electric field intensity in the critical points, and found that it is smaller than the value that is allowed under the regulation about non-ionizing radiation. We also calculated the electric field intensity perpendicular to the axis of the over ground conductor in the point of the greatest sag. It fall on the specified value determined by the regulation for new buildings in a distance of 75 m from the axis of the over ground conductor. We checked the results with a calculation using the finite element method.

We found that the proposed covered conductor does not need a wider corridor as it is already set for the 220 kV overhead power line with bare conductors and allows the transfer of energy at 400 kV.

As future work we propose the construction of a prototype of such a conductor, laboratory experiment of these theoretical calculations and an economic analysis: cost of new conductors and the replacement of these on the existing transmission towers - the price of building the new above ground power line with all the necessary permits.
