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Plans for future power systems indicate expansion of DC systems including application of HVDC transmission. This may lead to new hybrid AC-DC systems where two or more HVDC links of different technologies are connected to the same AC grid, and this raises concerns of new stability issues. Such scenario is expected in the North French topology which is used as the test system. There is an old 2 GW HVDC link (IFA2000) based on the traditional LCC, and experimental studies showed that it may destabilize the dominant 6.3 Hz torsional mode of a nearby nuclear generator (Gravelines plant). This is a well-documented phenomenon known as the subsynchronous torsional interactions (SSTI). It is studied in more depth, using analytical models and eigenvalue methods, to understand the influence of various key parameters and HVDC controller gains. Then, a study is conducted on the HVDC link based on the emerging MMC. It represents the new 1 GW French system Eleclink, and detailed representation of MMC and frequency-dependent DC cable in state-space enable analysis of the new systems. The results show that it may also introduce a significant risk of SSTI. This is contrary to previous studies which mostly consider simplified models representing early two-level VSC topologies. The Eleclink system is being installed in close proximity of the IFA2000 system (2020). A systematic comparison of dynamic stability limits is conducted between the point-to-point systems and the dual-infeed topology (both MMC and LCC) analyzing adverse interactions between the fast HVDC controls and the weak AC grid. The study shows that MMC HVDC can operate at the weakest AC grids and that it may improve stability of LCC HVDC. However, it may also deteriorate the risk of SSTI. Lastly, a subsynchronous damping controller is developed on MMC which can successfully mitigate all the identified SSTI.

Vulnerability of voltage source converters to DC faults is recognized as the main obstacle towards widespread implementation of meshed DC networks. Because of low surge current capability of IGBTs, the converters are blocked under high currents in order to prevent permanent damage to the modules. Converter blocking abruptly interrupts the power flow and causes a major power imbalance in both the AC and DC grid. This thesis aims to improve reliability and security of DC grids by preventing loss of terminals under DC faults. Fault response of a modular-multilevel converter is studied analytically and through simulations. It is identified that the MMC goes through several stages under a DC fault and each stage has different dynamics. The impact of converter modelling is also taken into account. DC grid topology has a major impact on protection system design and performance. If multiple converters are connected to the same bus, fault current is shared between the converters and fault current slope of each individual converter is reduced. Converter blocking can consequently be avoided and protection system requirements can be relaxed. A novel DC grid protection device based on capacitive energy storage is proposed. It is demonstrated that the device reduces the magnitude and frequency of MMC fault current, as well as DC voltage drop. In symmetrical monopole grids, pole voltage deviations under pole-to-ground faults are greatly reduced and converter blocking is avoided. Temporary MMC blocking is studied as another method for avoiding loss of terminals under DC faults. The benefit of this approach is low protection system cost and reduced impact on normal grid operation. A thermal valve model is developed to assess peak thermal stress on antiparallel diodes. Coordination between MMC self-protection and fault detection relays is proposed, as well as the use of converter DCCBs for backup protection.