I am seeking innovative solutions to improve the growth and maintenance of its electrical infrastructure while reducing environmental impact.
Do you have any new processes, tools, or applications that can improve the traditional approach of digging, trenching, and tunneling for underground Medium Voltage cable installation?
Dear doctor
"Underground Medium and Low Voltage Cable Systems up to 36 kV Medium voltage (MV) cables up to 36 kV are deployed for the connection of the LV network to the primary distribution network. Low voltage (LV) cable systems up to 1 kV serve to connect buildings and carry power over short distances of not more than several hundred meters. A transformer connects the LV cable with the medium voltage system. Together MV and LV cables represent the distribution system.
Design Since the 1960s, MV and LV voltage cable technology moved from paper mass impregnated to polymeric insulations. While still today many mass impregnated cables are in service in large part of the European distribution network, they are no longer produced in Europe except for replacements and repairs. Any new MV and LV cable uses polymeric insulation technology. The standard design of MV and LV cables consists of a metallic conductor surrounded by an insulation system. In case of higher voltages (>1 kV) a metallic screen shall be added. The screen is necessary to provide a homogeneous electric field to carry fault currents and for touch protection of the high voltage part. The cable, containing one or more conductors, is surrounded by an outer sheath which serves as mechanical protection as well as a protection against water. LV cables normally have a low electric field around the conductor, therefore screening is not mandatory, but it can be helpful to carry fault currents and to give touch protection to the cable. A screening function can also be provided by metallic armour.
MV cables need a special insulation system because of the higher electric field. As the electric field is dependent on the geometry of the electrodes (in our case the conductor forms one and the screen the other) the surface of these electrodes needs to be as smooth as possible. This can be reached by introduction of semi conductive layers between the conductor and the insulation as well as between the insulation and the metallic screen. Medium voltage cables are available in single-core, triplex single core and three-core designs.
Reliability of electricity supply is fundamental to the functioning of today´s society. Given the continuous increase in electrification, the relevance of security of supply can only further increase. Distribution networks are just as critical as transmission networks as they cover “the last mile” to the end users. MV and LV overhead lines are exposed to environmental impacts hence supply may be compromised by adverse weather conditions (storms, lightning, ice, etc.). MV and LV voltage underground cables, on the other hand, are well protected against such conditions. MV and LV voltage cables systems are proven to offer long-term reliability. CENELEC harmonised documents HD603 or HD620 define the design and the requirements that MV and LV cables need to comply with to ensure reliable power distribution. In addition, MV and LV cables have been tested before authorization for use on the distribution network. Installation of MV and LV underground cables shall only be performed by qualified personnel to ensure correct handling and installation. Once installed into the ground, MV and LV underground cables systems are well protected against all external weather influence. There is now an important volume of MV and LV underground cables installed on the European Distribution Network
over 35,000 km above 100kV;
over 1,3 million km between 1 – 100 kV;
over 3 million km of 1 kV underground cables.
According to Eurelectric statistics, already today in Europe 55% of all 1kV distribution lines and 41% of all distribution lines above 1 up to 100kV are underground. Looking into the future, more and more EU member states are moving towards full undergrounding of their electricity distribution networks.
The reliability of distribution systems not only depends on the reliability of the distribution technology but is also dependent on the network configuration. Two major models are to be distinguished. In Fishbone network configuration, customers may lose energy only in case of a breakdown on a circuit below the HV/MV transformer. Most of the overhead MV networks are based on this principle and may be subject to failures specifically in rural areas with forests. This has been the configuration in France during the 1999 storm. Contrary to that, underground MV network configurations are well interconnected in loops. In case of a breakdown on one branch, low voltage customers can be supplied through another branch from another transformer with very little interruption of the service. Most utilities are working in this direction thus increasing the lengths of underground lines. Well interconnected network configurations are particularly important to secure electricity supplies in the case of individual failure on either overhead or underground cable transmission systems. While single faults on MV or LV overhead lines are likely to entail shorter repair times than single faults on MV or LV underground cables, a failure resulting from a series of overhead poles being broken will relate to similar repair times. In order to shorten repair times on MV and LV underground cable systems, it is advised that DSO keep spare parts in stock, notably spare accessories."
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