Investigation of the Relay Protection Operation and the Electrical Safety at the Presence of Predominant Cable Part in Medium Voltage Power Lines

In the medium voltage power lines there is a tendency of enlargement of the cable part. This in turn leads to increase of the earth-fault currents. The practical experience has shown that it is important to consider that enlargement when designing the grounding of the neutral of mixed or cable power lines for medium voltage (normally through an active resistance). The value of the earth-fault current I_g has to be limited in view of limitation the level of the overvoltages and the damages of the equipment, the requirements for the selectiveness and the sensitiveness of the relay protection, the permissible touch U_t and step U_s voltages at the earth-fault place. The purpose of the paper is to investigate the operation of the relay protection and the electrical safety in the conditions of enlargement of the share of the cable part in the medium voltage power lines. The values of the possible touch and step voltages have been compared with the maximum permitted values, calculated according to the international IEEE 80-2000 standard Guide for Safety in AC Substation Grounding.