Active DC fault management of a bipolar full-bridge MMC-HVDC scheme with metallic return

摘要

Besides offshore applications, MMC-HVDC is discussed for embedded onshore structures. In contrast to their maritime counterparts, these systems shall be carried out with overhead transmission lines to minimize investment costs. To avoid a full outage in case of a dc fault, bipolar topologies seem advantageous considering reliability aspects. While ground currents during asymmetric operation are unwanted, an additional return path needs to be introduced. This paper presents an active clearing sequence of a bipolar full-bridge MMC-HVDC system with metallic return. Subsequent to a pole to ground fault, detection, short term asymmetric operation and full system restoration are investigated. If blocking of the IGBT modules can be avoided to maintain the converters in a controllable mode, the midpoint voltage shift at the ungrounded terminal has to be explicitly considered in system controls. Transient simulations validate the developed methodology.