Service experience with numerical relays in distribution systems

The utility Energieversorgung Oberfranken operates its 20 kV distribution systems in predominantly rural areas with Peterson Coil earthed neutrals and in urban areas with a high percentage of power cables solidly grounded. The individual load areas are fed by some 60, 110/20 kV substations, initially each with only one 40 MVA power transformer, which will not be duplicated until system growth demands it. Therefore, for system availability, in case of loss of one system component, the 110 kV system, as well as the 20 kV distribution system, is meshed. This system configuration requires fast and selective fault clearance, as well as efficient communication facilities for system restoration. Furthermore, for enhancement of system security, comprehensive fault reporting was required for preventive maintenance. It is the utility´s policy to minimize the total costs of the distribution systems i.e. initial equipment costs, as well as operation and maintenance over their entire life time. These considerations lead to a fully numerical relay and control concept