On the use of the HMCPL shielding system in renewing the underground HV power lines in big cities

Using underground cables in big cities, one of the most important acceptability issues is the magnetic field mitigation. This paper focuses on the joint bays, which are a critical part of the buried power lines regarding the magnetic field generation. For technical reasons, within a joint bay cables have to be arranged in flat configuration with an increment of the cable distances which cause a higher magnetic field generation compared to the trefoil arrangement. For a standard buried power line the magnetic field above a joint bay can be 10 times higher than the one of the trefoil. Joint bays are usually mitigated by means of passive shields. The literature covers comparisons of flat metallic shields, passive loops and passive loops with improved magnetic coupling (called HMCPL). In the renewal of the HV grid of Torino, from 2010 to 2014 sixteen new 220 kV lines were laid. The total length is about 50 km. The cables were provided from three different manufacturers. A total of 70 joint bays were installed. 20 of 70 joint bays required, for their position, a mitigation action. In 14 of them, HMCPL system has been used; in the others 6, the classical passive loops system has been installed. In this paper the results of measurement data concerning a sample of the joint bays are presented. The measurement results serve to compare the magnetic field with joint bays without a mitigation system and also to compare the HMCPL with the classical passive loops. Regarding the innovative HMCPL shielding system, in Torino it has been made the more extensive application; furthermore, the results are very satisfactory: the average value measured at the reference point is reduced of about ten times. We also experienced that the site assembly of the system is fast and flexible.