Transient overvoltages caused by a monopolar fault on a bipolar dc overhead line with dedicated metallic return

Abstract

This thesis deals with overvoltages caused due to a ground fault on pole conductor in an overhead bipolar Line Commutated Converter (LCC) HVdc system with a Dedicated Metallic Return (DMR). In HVdc systems with earth return, when a fault occurs, one pole may be lost causing the system to operate in monopolar mode with the ground return. During the fault, overvoltages are induced on the healthy pole resulting in its voltage to increase beyond its rated value. It is already known that these overvoltages reach up to 1.8 pu on the healthy conductor for an HVdc system with ground return due to a ground fault. Due to large ground currents flow during monopolar operation with ground return, nearby buried metallic structures are subjected to corrosion and these currents might interfere with buried metallic structures. To mitigate the effects caused by a large ground current flowing over a long period, a third Dedicated Metallic Return (DMR) conductor connecting both neutrals of converters, is laid along the line through which the pole current flows. The presence of a third conductor which is grounded only at one terminal and floating on other terminal completely changes the fault mechanics during a ground fault. In this thesis, the overvoltages on the healthy pole and neutral conductors of a dc overhead line with a DMR caused due to a monopolar ground fault are investigated. Ground faults are applied on a pole conductor at different locations on the line and overvoltages produced along the line are studied. Theoretic explanation of overvoltages is explained based on travelling waves theory and using lattice diagrams. Results showed that overvoltages of up to 2 pu on the healthy pole and 1.2 pu on the dedicated metallic return conductor are observed near the terminal close to ungrounded neutral. It is also observed that the highest overvoltage on the pole conductor occurs due to a fault closer to the ungrounded terminal unlike the case with ground return where maximum overvoltage occurs for a fault at the midpoint.