Common-mode currents in low-voltage DC underlay grids with AC-side grounding; 1. Auflage

RWTH Publications (RWTH Aachen)

Abstract

To counteract climate change, it is necessary to reduce the emissions of greenhouse gases, such as carbon dioxide. The electrification of heating and transportation systems, as well as the installation of renewable energy sources (RESs), plays a central role in achieving this goal. However, this puts a strain on the low-voltage and medium-voltage distribution grids, because RESs, heat pumps, and charging infrastructure are primarily connected to them. This can result in the local overloading of transformers or cable systems. DC underlay grids that support the existing AC grids are beneficial for integrating RESs and charging infrastructure because they are more efficient, require fewer components, and enable vertical grid coupling. Recently, a DC system with AC-side grounding was proposed that does not rely on galvanic isolation. However, this introduces two challenges, especially when multiple infeed converters are used: increased common-mode (CM) currents and electromagnetic interference. This work thoroughly analyzes these challenges and presents methods to mitigate their effects. First, a CM model is derived to determine potential reduction measures. The CM choke, the common-mode voltage (CMV) of the infeed converters and their switching alignment are identified as the primary influencing factors. These factors are addressed through a model-based CM choke design guide, optimized modulation strategies that reduce CMV for three-level inverters, and synchronization of locally distributed converters via the global navigation satellite system. Experiments on a demonstration setup resembling a DC underlay grid with AC-side grounding analyze all concepts.

Authors 1

1. Fabian Herzog corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen