DC Corrosive Challenges in Radial Multi-Terminal HVDC Systems: Understanding the Role of HVDC Electrodes Under Emergency Operations

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) systems are increasingly adopted for their controllability and suitability in modern transmission grid applications. Depending on technology, in both meshed and radial multi-terminal topologies, current return to earth via electrodes may become essential during maintenance or fault operating conditions. However, estimating the duration and impact of such electrode usage–especially in the presence of nearby metallic infrastructure-remains a challenge due to the probabilistic nature of component failures and operational interdependencies. To this end, this paper presents a probabilistic methodology to evaluate the expected full current electrode usage in a radial three-terminal VSC-HVDC system. A simplified approach is used to determine current redistribution and electrode polarity under emergency operation, providing potentially useful insights for corrosion risks. Specifically, the methodology is presented through numerical examples, that combine graph-based analysis with a probabilistic evaluation, comparing systems with and without Metallic Return Transfer Breakers (MRTBs).