Bipolar VSC HVDC connection with cables –an in-depth analysis of dynamic system performance of grid-forming converters

High Voltage Direct Current (HVDC) technology, and its Voltage Source Converter (VSC) variant in particular has become a popular solution for grid expansion projects. The technology has shown its potential to interconnect several unsynchronised control areas, provide grid reinforcements within one synchronised area, or to connect remote offshore wind resources. The current-day increased power ratings achievable with VSC HVDC make that the bipolar configuration becomes an interesting alternative to be considered over its symmetrical monopolar counterpart. Although significant practical experience has been gained with Line Commutated Converter (LCC) HVDC technology in bipolar set-ups, experience with VSC HVDC is at this moment still lacking, and consequently, open questions remain on the interactions between the different converters in the bipolar set-up. At the same moment, concerns are rising regarding the low inertia levels of our future power systems as a result of the phase-out of traditional synchronous units and the increased penetration of power electronic devices. One of the proposed ways forward that has received significant attention in recent years, are converter control concepts that rely on ‘grid-forming’, rather than ‘grid-following’ principles. Grid-forming control concepts cannot only be applied to power electronic interfaces for renewables-based generation, such as wind turbines, but also to converters of VSC HVDC links. In this way, one of the sides of the HVDC link can actively contribute to the system inertia. When it comes to a bipolar VSC HVDC link, however, including grid-forming principles to the picture raises a number of outstanding research questions, which will be addressed in this PhD project.