White paper on DC technologies for Switzerland’s electricity transmission and distribution

Existing AC power systems, established more than a century ago, are increasingly challenged by DC technologies, enabled by significant advancements in the power electronics and related scientific fields. Three major application areas for DC transmission are established: transferring bulk power over long distances, interconnecting grids and connecting offshore wind. Medium and low voltage DC application become more appealing based on the improved controllability, more effective integration of renewable energy sources, higher power density and better compatibility with underground cables. Such technologies will be attractive for the Swiss energy transition as they might provide more effective solutions for the densification of power systems, the integration of converter-based renewable energy sources and pumped-storage plants. In order to achieve this, several research challenges, however, need to be overcome and standardization must further advance. Several academic partners from Switzerland contribute to these research problems in WP3: “Multi-Terminal AC-DC Grids and Power Electronics” within the SCCER FURIES. In this paper six major topics are presented: -“General overview of DC options” where present and future applications of DC technologies as wells as MVDC grids development issues are discussed. -“MMC-based MVDC converters” where a selection of modular multilevel converter as a platform in order to provide flexibility in addressing a multitude of applications and conversion needs is shown. Several topological adaptations are proposed, leading to novel converter topologies. -“AC/DC resonance analysis” where analysis allowing to find the resonance location and to analyzeres-onance nodes contribution to critical mode. These frequency analysis methods permit to foresee net-work frequency behavior that is becoming an important issue due to the growing number of power electronics converters in the network. -“Overview of HVDC breaker technologies” where basic requirements for fast and reliable HVDC circuit breakers as well as the differences to HVAC technology are introduced. -“HVDC circuit breakers: testing methods and challenges” where the limits of HVDC circuit breakers are explored. In this section, a flexible, modular high current source is presented. The source is intended to act as a hardware-in-the-loop test benchfor future HVDC circuit breakers, by driving highly dynamic and arbitrary current waveforms through dynamic loads (e.g. DC arc). -“Fault location principles” where the significant influence of fault location on the network security of supply and quality is drawn. A newly-developed technique which is based on the electromagnetic time reversal (EMTR) theory that can be applied to radial/meshed AC/DC power transmission or distribution networks is presented and compared to other Travelling Wave –based methods. The outputs and outlooks are drawn in order to conclude the paper.