It is expected that DC grids will evolve into large meshed networks, which will have multiple DC voltage levels. Two DC grids operating at different voltage levels could be interconnected using a DC-DC converter. The role of the DC-DC converter in meshed DC networks is similar to the role of a transformer in traditional AC systems. The DC-DC converter could take a range of functions in DC grids. The DC-DC converter should prevent the propagation of DC faults and should also have the ability to isolate DC faults. Because of its high cost, the DC-DC converter cannot be used as DC breaker in meshed DC grids, but it can be strategically placed in the DC grid to provide protection zone separation. In case multiple HVDC (High Voltage Direct Current) lines need to be interconnected, a multiport DC-DC converter should be used. The DC hub is a multiport DC/DC converter which consists of an inner AC circuit and AC/DC converters interfacing each HVDC line. The main goal of the DC hub is the interconnection of multiple HVDC lines operating at different voltage levels and with different DC grid configurations.

In this thesis, the operation of a DC hub based on the MMC (Modular Multilevel Converter) converter was investigated. A three-port MMC-based DC hub was simulated using Matlab/Simulink. An important contribution of this thesis is a comparison of different DC hub topologies which could be used for the interconnection of multiple HVDC lines. Several designs for the inner AC circuit are suggested and the advantages and disadvantages of each recommended design are described in detail. Furthermore, a methodology is presented for the clearing of AC faults within the DC hub and the protection of the system during the fault. An AC fault within the DC hub should be cleared by opening the AC circuit breaker of the faulted port. A methodology is also presented for the safe connection or disconnection of additional ports from the DC hub. Any port should be easily connected or disconnected from the DC hub without affecting the operation of the other ports. The DC hub should also offer connection points to allow the interconnection of more ports. Finally, the operation of the DC hub is investigated when generation or consumption units are connected to its inner AC circuit. In case the DC hub is used for the interconnection of multiple offshore HVDC cables, the loads of the offshore platform or a nearby offshore wind farm could be directly connected to its inner AC circuit.

With the concepts analysed in this thesis, HVDC network system designers will be able to select the most suitable DC hub topology for the interconnection of multiple HVDC lines, use a methodology for the clearing of AC faults within the DC hub, use a methodology for the safe connection or disconnection of ports and understand the design limitations of the DC hub when generation or consumption units are connected to its AC side.

As the transformation of the electricity generation progresses, the number of power electronic-based devices connected to the electricity grid is expected to grow. To ensure the stability of the power system in a scenario with high penetration of power electronic converters, the dynamics and stability characteristics of these devices have to be comprehensively studied. This is especially important for bulk converters — with capacities in the hundreds of megawatts range — such as high-voltage direct current (HVDC) systems.

Small-signal analysis is extensively used in power system stability studies. The calculation of the system eigenvalues provides insights regarding the dynamic behavior and the nature of the interactions within the system. However, generating a small-signal representation of power electronic converters is known to be problematic, due to the switched operation of such converters and their complex control structures. Nevertheless, usage of modular multilevel converters (MMC) in HVDC systems is rapidly increasing. This creates the need for extensive studies regarding its dynamic behavior. Due to the MMC unique submodule-based topology, special actions have to be taken if the stability of the converter is to be analyzed.

In this thesis, a small-signal model of a generic MMC was developed by linearizing all the converter plant and control equations, to analyze the converter stability. Averaging was used to eliminate nonlinearities brought forth by the converter switching actions.

The main contributions of this thesis are the application of classic linear-time invariant theory to a time-varying system by linearizing the full converter model at different points in time, and the application of modal analysis methods to the MMC.

The small-signal model was verified against a large-signal one, both developed in Matlab/Simulink. Additionally, the system eigenvalues obtained analytically and numerically —using Matlab/MuPaD and Simulink Control Design Toolbox, respectively — were benchmarked. After obtaining the system state-space matrix, its transfer functions were used to estimate the input impedance of the MMC in the frequency domain. Using the linearized system matrix and an optimization algorithm, namely MVMO, the locations of the converter eigenvalues were optimized by finding a new set of controller gains. Finally, calculating the converter eigenvalues, understanding the effects of these eigenvalues on the system variables and applying advanced tools to tune the controllers of the MMC, and possibly also its main circuit parameters, may aid system designers in controller design and parametrization studies.