Bipolaire VSC-HVDC-Verbindingen met Netvormende Omvormers – Studie van het Dynamisch Gedrag en de Klein-Signaal Stabiliteit

The ongoing energy transition, aimed at mitigating global warming, is transforming electric power systems. Environmentally friendly converter-interfaced Renewable Energy Sources (RES) are replacing polluting Synchronous Generator (SG)-based units. However, this transition raises concerns as SGs naturally stabilize the electric system after disturbances through their intrinsic inertia provision and their voltage control. This inherent stabilizing behavior is lost when SGs are replaced with power electronic converters operating in traditional Grid Following (GFL) mode. To address this issue, a different converter control concept, known as Grid Forming (GFM) control, is being introduced to recreate the stabilizing behavior historically provided by SGs. Simultaneously, the increasing transmission power demand and the need for grid reinforcements are leading to a large-scale deployment of Modular Multilevel Converter (MMC)-based High Voltage Direct Current (HVDC) connections, which are moving from a symmetrical monopolar to a bipolar layout. The large ratings of these connections and the potential of GFM control strategies make their combined application an attractive solution to alleviate power system stability issues. Despite these promising prospects, the implementation of GFM control strategies in MMC-based bipolar HVDC systems has not yet been addressed, leaving several research questions unanswered. Therefore, a detailed examination of transient response and small-signal stability is essential to evaluate performance and identify any potential unwanted interactions between GFM converters and Alternating Current (AC) power systems. This analysis is crucial to mitigate risks associated with integrating bipolar HVDC connections with GFM-MMCs in power systems, as the behavior of GFM-MMCs fundamentally differs from that of the state-of-the-art GFL-MMCs. This thesis evaluates the impact of inertia support from an HVDC bipolar connection with GFM-MMCs between two asynchronous power systems and the presence of interactions between converters and AC systems. The inertia support evaluation highlights the influence of GFM control parameters combined with different AC system properties and the available power on the frequency dynamics of the connected AC system. Based on the obtained insights, an improved design for inertia support is introduced. Time-domain simulations illustrate how this control modification can improve the dynamics of the connected AC system without affecting the steady-state power transmission in the HVDC connection. The small-signal stability analysis includes a detailed assessment of the sub-synchronous dominant dynamics of bipolar HVDC connections with GFM converters. The influence of the location of GFM converters on observed sub-synchronous interactions is thoroughly analyzed, showing how the location of GFM converters is a key influencing factor in causing interactions between asynchronous AC systems. The influence of the Direct Current (DC) side layout on these interactions is assessed by comparing the sub-synchronous dynamics of bipolar and parallel monopolar HVDC connections. It is also revealed that when GFM converters are controlled with the same control strategy and with similar parameters, the sub-synchronous interactions can be divided into in-phase and anti-phase oscillations for the two converters connected at the same side of a bipolar connection. The in-phase interactions relate to the combined response of the GFM converters with the AC system. In contrast, the anti-phase interactions represent unwanted local power oscillations between the two GFM converters, and do not contribute to stabilizing the AC system they are connected to. To better analyze these interactions and their impact, a coordinate transformation is introduced. Building on this transformation, a novel centralized control strategy is proposed to individually shape the in-phase converter interactions and dampen the unwanted anti-phase interactions.