Analysis of Induced Electromotive Force in Phase Conductors of 35 kV Line Caused by Phase-to-Ground Fault in 400 kV Overhead Power Line

摘要

The paper presents the results of the analyses performed to estimate possible values of the electromotive force induced in phase conductors of a medium voltage (MV) overhead power line during a phase-to-ground fault occurring on one of two parallel high voltage (HV) overhead power lines. The MV line lies partially parallel to the HV lines and then intersects them. The analyses were necessary to ensure safe conditions for the employees during maintenance of the MV line. It is assumed that during the fault on one HV line, the other one is out of operation. In order to determine the worst possible situation, both cases were analyzed, i.e. the first HV line was considered to be under phase-to-ground fault while the second one was out of operation, and vice versa. During the maintenance work the phase conductors of the MV line are grounded in the substations at the ends of the line, forming a closed loop that consists of the MV line phase conductor impedance, grounding resistances at the ends of the MV line and the impedance of the ground return path. Since the impedance of the ground return path and the resistances of substations' groundings in relation to the line impedance can be assumed to be small, the current that flows through the loop mainly depends on the line impedance. For the analysis of the employees' safety conditions, the most critical case scenario is when the MV line is out of operation, grounded at the ends and the work is carried out on any MV line tower outside the segment where the lines intersect and run approximately parallel, since in these circumstances the electromotive forces induced in the MV line phase conductors are of maximum value. The analyzed MV line does not have a ground wire, so this situation is not considered. The danger to the personnel arises after grounding of all phase conductors at the tower mentioned, because of the current flow through the tower grounding electrode, which appears as a result of the induced electromotive force. Since the grounding resistances of the MV line towers are relatively high, the main voltage drop in the circuit occurs at the tower grounding resistances. As a consequence, hazardous values of touch voltages can appear in the vicinity of the tower where work is performed, which was confirmed by the analyses carried out.