Cause investigation of diode rectifier trouble by lightning in traction substation

On August 17^th, 2012, a diode rectifier at Nagano Traction Substation was broken. The broken rectifier was dismantled and investigated in detail and, as a result, it was estimated that the overvoltage caused by the thunder lightning caused the dielectric breakdown in the rectifier. To confirm the assumption of the broken process, the field measurement at Nagano Traction Substation and FDTD (finite-difference time-domain) calculation analysis at Nagoya Institute of Technology were carried out. As a result, it was indicated that the direct lightning to the elevated bridge over the substation caused the transient overvoltage of the grounding system of the substation and resulted in the breakdown within the rectifier. Because the concrete bridge pier is located adjacent to the substation, the surge voltage from the elevated bridge is able to influence on the potential of the substation mesh. In this paper, the investigation process to determine the cause of the trouble will be shown in detail.