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Explanatory note

Slide №2. It is natural to choose an underground solution only when OHL is not possible.

Slide №3. The standard installation location of cables is trenches placed aside of the roads in order to minimize the impact on the traffic. In most cases, more than one circuit of cables will be installed for the transmission of the the requested load. In presence of limited length and obstacles along the selected route (e.g. road crossings), the configuration of the cables may be changed and other installation techniques may be adopted in order to overcome the obstacle. The document Cigre TB 194 gives the guidance for the construction, laying and installation techniques for extruded and self-contained fluid-filled cable systems. Where the direct burial installation in trenches is not feasible for various reasons such as a very crowded underground by other existing services, it is possible to install cables in dedicated tunnels of shared infrastructures.

Slide №4. The service experience of HV extruded insulation cables dates back more than 40 years for 245 kV and more than 30 years for 420 kV, with excellent performances. This technology can now be considered as consolidated. The reliability characteristics of the individual equipment items play a decisive role in the security of the electrical supply. Nowadays there are specific and stringent IEC standards and Cigre recommendations that are applicable to the design and requirements of the transmission cable systems in order to assure a high quality and reliability of the components for the designed or expected life time. The current rating of the cable system is depending on the cable laying conditions; typically the maximum power that can be transmitted for a cable system is in the range of 1 000-1 500 MVA per circuit (three single core cables) that is working at voltages in the range of 420-550 kV. By adopting forced cooling a maximum power in the range of 1 500-2 000 MVA per circuit can be transmitted. GIL or cable installations are designed for specific current ratings. These ratings are based on network requirements and are given for a specific ambient temperature. The GIL or cables are consequently designed to meet the requirements of their corresponding standards for the maximum allowed temperature rises. This can also be required when a circuit is out of order and the load needs to be transported through the remaining circuits. In such case, we talk about temporary overload. In both cases, it is possible to go beyond the rated values without jeopardizing the integrity of the equipment. As a result, if the temperature is lower than the 40 °C, the GIL or cable equipment can be operated continuously at a current higher than the rated continuous current. If the current is lower than the rated continuous current, the maximum ambient temperature of operation of the equipment can be higher than 40 °C.