Operating performance of a Petersen Earth Coil

This paper is a report on the Alabama Power Company's operating experience with a Petersen Earth Coil installed between the neutral and ground of a 120-mile, 44,000-volt, 3-phase, star-connected, 60-cycle system. A Petersen coil is essentially an inductive reactance of such value as to maintain resonance with the capacitance of the system to ground at the fundamental system frequency. With a ground on one wire the current through the fault is reduced to such a low value as to prevent maintenance of an arc. Therefore, on the assumption that the majority of phase-to-ground short circuits start as insulator flashovers, the installation of such a device as a Petersen coil which would snuff out flashover arcs should considerably reduce the number of interruptions to the line. By means of proper relaying, cases of trouble outside of the operating sphere of the Petersen coil, such as phase-to-phase short circuits and solid grounds have also been successfully taken care of. Previous to the installation of this coil numerous interruptions to service were experienced on this system during lightning storms, which are unusually severe in the territory covered by these lines. This system, therefore, offers an ideal location for a trial installation of the coil. Since the installation of the coil the number and duration of interruptions due to lightning flashovers have been reduced by 88.5 per cent and 94 per cent respectively. Several doubtful actions, however, have occurred during switching operations, indicating the presence of unusual phenomena. By proper relaying it is hoped to prevent the recurrence of such action. Further tests are also contemplated to investigate the unusual phenomena accompanying certain operating conditions.