Existing DC tram traction networks (DCTTN) provide intermittent regenerative energy and quasi-persistent surplus capacity as they are dimensioned for peak power demand. The deployment of DC microgrids is increasingly on the rise largely due to the compatibility with DC-native loads and avoiding AC grid congestion issues. In particular, the bipolar DC microgrid (BDCMG) is considered, as it enables different voltage levels, requires less copper for cabling and enhances reliability compared to unipolar architectures. Interlinking these two energy networks through isolated bidirectional DC-DC converters (IBDCs) offers a favorable opportunity to enhance the operation of the DC microgrid, particularly during periods of local demand fluctuations or utility disturbances.
This thesis considers three IBDC topologies for the interface: the CLLC resonant converter, single- and three-phase dual active bridges (DABs). Simulation-based comparisons of the 1-phase and 3-phase DAB demonstrate that the 3-phase DAB achieves higher efficiencies across operating points considered based on industry standards, requires smaller output capacitance, which enables a higher power density. This is advantageous for facilitating a modular architecture with high-frequency SiC MOSFETs, which not only allows passive components to be smaller in size and thus lower the overall system cost, but also ensures redundancy and scalability towards different energy demands of systems (DCTTN and BDCMG) at different locations or cities. To ensure robustness against the electrically demanding tram traction environment, it is necessary to consider a dedicated input circuit to protect the downstream components against damage from surge overvoltages as well as ensuring that the input circuit does not compromise the stability of the IBDCS. Simulation-based comparisons of two different passive damping methods (RC and RL) used on multi-stage LC filter topologies (part of the input circuit) were investigated. Results indicate that the RL damped filters require smaller footprint size, achieve lower output impedance and improved transient voltage attenuation. Based on Middlebrook’s impedance-based stability criterion, the inclusion of the input circuit and the DCTTN’s overhead line impedance did not compromise the stability of the system (input circuit + IBDC). Adopting a modular approach of stacking IBDCs, two distinct configurations of connecting the input circuit with the IBDCs were taken into consideration. A single input circuit for a 150 kW load showed a smaller size and weight and lower cost per kW compared to using three smaller input circuits for 50 kW loads each.

With the energy transition, electrical energy consumption is increasing heavily. Electric vehicles replace cars with internal combustion engines, and electric heating replaces gas-heated homes. The AC electricity grid becomes congested with the increase in load. Local electricity generation through PV panels or windmills also increases, but there is generally a mismatch in generation and demand. Besides, practically all electrical equipment uses DC directly or in the conversion process. This makes using DC grids much more efficient than AC grids. The distribution grid can not convert directly to AC. DC microgrids could prove an attractive solution to support the distribution grid and to move gradually to a DC grid. These locally operated grids combine load, generation, and energy storage. A bipolar DC microgrid further increases efficiency by providing an additional voltage level and doubling the power compared to a unipolar DC grid with the same pole voltage. A DC/AC converter will interface the LV AC grid and the DC grid to enable energy exchange between the grids.

Research is conducted to the design and modulation techniques for a transformerless bipolar 700 V DC grid interfacing converter. Mitigating the isolation transformer increases efficiency and reduces material usage, but will also provide a path for ground leakage current to flow. An NPC converter topology is selected and an 11-segment modulation scheme is implemented using sub-references and carriers. This scheme mitigates the oscillations in the DC neutral point, while simultaneously reducing the ground leakage current to acceptable levels. Furthermore, a novel dead time compensation method is implemented to reduce the ground leakage current further. Finally, a new concept to provide unbalanced power to the poles based on the pole voltages is introduced. Simulations and experimental tests show that the developed modulation technique can enable a transformerless connection.

Over the past decade, global efforts to combat climate change have focused on transitioning from fossil fuels to renewable energy sources and battery-based energy storage. This shift is essential to meet the rising global energy demand and integrate renewable power into the grid. However, the current AC based grid infrastructure poses challenges for incorporating DC based renewable sources like photovoltaic (PV) systems and battery storage. A rapid transition from AC to DC systems is impractical, necessitating a grid system that leverages the existing AC infrastructure while integrating DC systems. Hybrid microgrids (HMGs) have emerged as a solution to this challenge.

Our research focuses on a crucial component of HMGs: the Interlink Converter (ILC). Ideally, the two subgrids in a hybrid microgrid generate power to meet their respective demands. When one subgrid cannot meet its demand, the other supports it through the ILC. This system works effectively most of the time. However, during simultaneous power deficiencies in both subgrids, current standards prioritize AC loads, transferring power from the DC to the AC subgrid. This can lead to voltage fluctuations or blackouts in the DC subgrid, which is increasingly problematic as critical DC loads like EV charging stations and data centres grow.
The thesis titled ‘AC-DC Priority Control of an Interlink Converter’ aims to develop an optimal control strategy for ILCs, enhancing their ability to prioritize power transfer between AC and DC subgrids based on priority modes. This research is crucial for future grid stability and the integration of renewable energy sources.

This study explores the effects and severity of microcracks within Solarge’s innovative lightweight polymer-based photovoltaic modules, aiming to recommend a grading criterion specific to the company’s product range. Unlike the traditional PV modules which predominantly employ a glass and aluminum frame, Solarge utilizes lightweight, fully recyclable polymers. The uniqueness of Solarge’s composition presents a novel landscape in terms of durability and degradation trajectories. A pivotal concern arises from the introduction of microcracks, which inherently diminish a PV module’s power output. Leveraging Solarge’s extensive database of current-voltage measurements and electroluminescence images, an initial analysis discerned the emergence of small diagonal and v-type cracks post-production, predominantly after the lamination process. Subsequent experimental methodologies, encompassing mechanical loading and temperature cycling on these modules, demonstrated the growth of these existing cracks into diverse and more complex formations. This evolution of stressors significantly compromised module performance. A categorization system classifying cracks into noncritical (Category 1), critical (Category 2), and very critical (Category 3) was developed, with associated power losses identified for each. Using linear regression, predictive models for crack progression in each category were established. The study culminated in the formulation of a grading criterion proposed to Solarge, aiming to refine their quality assurance.