This thesis investigates the design, analysis and testing of medium-voltage planar transformers for modular Solid State Transformer applications, with a particular emphasis on insulation for medium-voltage level. The renewable energy and microgrid implementation in the generation and distribution segments of power grids is motivated by the global urgency of the energy transition, particularly in response to the challenges in the Netherlands related to the phase-out of natural gas and grid congestion. Central to this work is the development of modular Solid State Transformers (SSTs) tailored for microgrid applications which are one of key components in the transition to more sustainable energy, for which planar transformers in high-frequency operations have to be designed.

A thorough review of high-voltage planar transformer design is conducted, focusing on core and winding losses, parasitic elements, and the complexities of achieving sufficient insulation within compact designs. The thesis outlines the design of a high-frequency planar transformer optimized for modular SSTs, highlighting key aspects such as core selection, loss minimization, and insulation design. Simulations are performed to assess the impact of high-frequency effects on transformer performance, with particular attention to the optimization of winding and shield configurations for improved field distribution and reliable insulation.

Multiple critical aspects in the design of insulation are pointed out and the experimental work is dedicated to testing these aspects.The PCB-based insulation under high-voltage conditions, with a focus on partial discharges and breakdown voltage in scenarios involving parallel conductors within a PCB layer. The results demonstrate the robustness of PCB insulation while revealing challenges posed by surface discharges and the effects of external conditions such as oil and air. These findings provide critical insights into the dielectric performance of FR4 laminates, contributing to the enhancement of insulation system reliability in planar transformers.

The outcomes of this research offer valuable contributions to the design and optimization of high-voltage planar transformers, underscoring the importance of balancing compactness, efficiency, and reliability. These advancements are essential for the development of resilient and sustainable energy systems in the context of the global energy transition.

The global shift towards sustainable energy sources, together with rapid technological advancements, has driven a significant increase in the demand for electric vehicles (EVs) over recent years. This surge in EV adoption highlights the critical need for robust supporting infrastructure, particularly in the deployment of efficient and accessible EV chargers. Gallium Nitride (GaN) is a wide bandgap semiconductor material that offers better performance characteristics compared to traditional silicon-based components in medium-power and high-frequency application. Its higher electron mobility, saturation electron velocity and breakdown electric field make GaN an ideal candidate for high-frequency power conversion applications in EV chargers. To explore the potential of GaN technology in EV charging applications, an 11kW GaN based isolated bidirectional multi-port EV charger with a two-stage architecture has been designed to interconnect bipolar DC grid and electric vehicles. The two stages of the charger consist of a three-level triple active bridge converter and a three-level interleaved buck converter. The TAB converter is an extension of conventional dual active bridge (DAB) converter, offering capability to direct power transmission among three ports. However, the addition of a third port introduces complexity in managing power flow. The TAB converter can operate under various modulation schemes, including three-level and five-level modulation. An analysis of the switching modes associated with each modulation scheme has been
erformed. In order or derive the soft switching range of the converter, the current expression
of series inductors of each port are derived based on generalized-harmonic approximation method and equivalent circuit models. A detailed analysis of the commutation process during critical switching transitions has been conducted, leading to the derivation of the soft
witching criteria for both three-level and five-level operations of the TAB converter. To validate the design and verify the soft switching range, simulations have been carried out. Moreover, the prototype of the 11 kW multi-port EV charger is designed based on calculation and simulation results. The prototype employs a top-side cooled GaN high-electron-mobility transistor (HEMT) to enhance thermal dissipation. Testing of the prototype indicates that the converter can transmit 1.2 kW from the primary to the secondary side at the rated voltage
of 700 V.

This master's thesis presents the design and analysis of the inverter stage and controller architecture for a modular Solid-State Transformer (SST) to interconnect DC microgrids and a 10kV AC grid for bidirectional power flow. The research focuses on inverter topologies, modulation techniques, and modular controller architecture.

The inverter stage, a crucial stage of the modular SST, converts DC power from microgrids to AC power suitable for the 10kV AC grid. The most popular inverter topologies for modular converters are researched and investigated based on their compatibility with the specific application. Factors such as efficiency, power quality, harmonic distortion, number of components, control complexity, and scalability are considered for this process. Along with the topology, modulation techniques are important in producing high-quality output waveforms and efficient power transfer. Different modulation strategies, including pulse width modulation (PWM) techniques and space vector modulation schemes specifically for multilevel converter applications, are reviewed and discussed in terms of how they affect the system's performance.

In addition to the inverter stage, the thesis focuses on designing a modular controller architecture for the SST. The controller design prioritizes maintaining flawless data flow and synchronization between all controllers for converter and AC grid coordination. The central controller and distributed control architectures are studied and evaluated for scalability, modularity, and flexibility. The distributed architecture enhances the system's overall flexibility but is subject to synchronization issues and limitations in communication bandwidth. These issues are addressed in the developed design.

To evaluate the performance of the proposed inverter stage and controller architecture, extensive simulations and experimental validations are carried out. The simulations consider various operating conditions, such as islanded and grid-connected modes, to assess the system's stability, control, harmonic content, and power quality in the output waveforms. The simulation results indicate that the chosen inverter design and modulation strategies successfully attain high efficiency and minimal harmonic distortion in the operation of the converter. The modular, distributed controller design demonstrates its ability to provide seamless operation and effective coordination between DC microgrids and the AC grid.

Overall, this master's thesis contributes to the advancement of solid-state transformer technology by delving into the design of the inverter stage and controller architecture for interconnecting DC microgrids and a 10kV AC grid and providing useful insights. The findings and recommendations can be a valuable framework for future research and development of modular SSTs for grid-connected applications.

As renewable energy sources and DC energy storage continue to increase in modern power systems, the power electronic converters required to interface them with existing power grids are also rapidly increasing. This is resulting in increasing amounts of the total power exchanged on the grids being converted between DC and AC. Additionally, the majority of today's grid loads ultimately convert their input power to some form of DC whether they are connected to an AC or DC grid. Therefore, the concept of DC grids is becoming more attractive for applications such as DC distribution systems and electric vehicle charging stations. However, the existing AC grids will continue to play a fundamental role in the power infrastructure. Therefore, it is, important to consider how LVDC distribution grids will be connected to, and exhange power with, the AC grid. A transformerless interface between LVDC distribution grids and the LV AC grid via a power electronic converter is an attractive option in terms of size and cost reduction. However, the challenge of ground leakage current is a major challenge for such an interface.

This thesis assesses the different possible three-phase DC-AC converter topologies for a transformerless interface between a bipolar DC microgrid and the LV AC grid. The DC-AC converter modulation is explored to understand the challenges of ground leakage current and determine the feasibility of a transformerless interface. A detailed analysis of the common-mode voltage, common-mode impedance and resulting circulating currents in a grid-tied DC-AC converter is performed. The effect of this circulating current on AC grid protection equipment is considered.

A survey of converter modulation methods for common-mode reduction and DC bus voltage balancing is conducted. Simulations are used to verify the performance of the different modulation methods in terms of common-mode reduction and DC balancing capabilities in the bipolar DC grid. A new modulation method is proposed for simultaneously mitigating ground leakage current and providing DC voltage balancing. The limitations of this method are analyzed and simulations are performed to evaluate its effectiveness. The developed modulation method is also shown to eliminate the low-frequency voltage ripple in the DC bus caused by the conventional modulation methods. An experimental hardware prototype DC-AC converter is designed and built for testing the different modulation methods. Finally, recommendations for future work are made.

A large portion of the global population, mostly in the Sub-Saharan African region, still has no access to electricity. Due to the remoteness and limited accessibility of some communities, off-grid solutions such as mini-grids are required to electrify these regions. An essential part of such an isolated mini-grid is the energy storage system. DC Opportunities aims to establish these mini-grids as a third step in the electrification process. As a means of energy storage, the small-scale employment of the mature technology of pumped hydro storage (PHS) should be explored, making use of height differences in terrain often present in these areas. While PHS is the most common energy storage technology, it has thus far only be employed on a large scale and only recently research on its small scale applicability has been gaining ground.
In order to conduct a feasibility study on the use of PHS as a means of energy storage for isolated mini-grids in low-resource settings, such as those in Sub-Saharan Africa, local challenges are identified. These include a suitable operating range, quality of supply and power issues, limited component and material availability, potential arrival of the main grid, funding, low tariffs and revenue, a low-trust society, limited technical skills, theft and vandalism, political vertical networks and corruption, regulatory issues, secondary water use and environmental impact. Based on these challenges, a program of requirements is presented, which consider the technical, economic and socio-cultural requirements.
Subsequently, the available design options are discussed, and based on the program of requirements a technical design synthesis leads to a provisional technical design. This exploration has led to the conclusion that earth dam reservoirs should be used for the water storage system, Glass Reinforced Plastic or Mild Steel pipes for the water conveyance system and a binary unit utilizing a Pump-as-Turbine or a ternary unit employing a Pelton turbine for the powerhouse configuration; this configuration can be either designed for an one-time installation or scalable installation. Based on the provisional technical design and the local challenges, an organizational structure, using the existing local power structure, ensures the PHS plant embeds itself in the community through community involvement and participation, as well as local capacity building by the project facilitator. The capacity requirements of the reservoir, as well as the diameter and material of the penstock, and the powerhouse configuration appear to be highly dependent on site-specific conditions. Therefore an energy flow model was developed, which in combination with a penstock selection tool and a generated demand profile is able to determine the requirements of a Hybrid Energy Storage System (HESS) utilising PHS as its main component for an isolated mini-grid. This model has been deployed for a case study in Adi Araha, Ethiopia, yielding positive results. Further exploration into the economic implications of the modeled system, and socio-cultural factors to be considered for the case study, led to the conclusion that for the case study a PHS plant could prove to be a technically, economically and socially feasible solution. Further research is however warranted, specifically on site.
The developed model was further employed to attain a conclusion on the general economic feasibility. While the socio-cultural factors are very site specific and can not easily be generalized, the results exhibit that for a situation where the gross head is at least 250 meter, and preferably around 300 meter, and the maximum power output requirements of the PHS plant is at least 150 kilowatt, technical and economic feasibility is attainable. Isolated mini-grids requiring a storage solutions, adhering to these conditions, should thus be further investigated to more accurately determine the feasibility of a small pumped hydro storage plant.

The modern world demands more and more electric energy. At the same time, almost a billion more people are expected to gain access to electricity over the next decade. All of the above compose a very challenging future, where a continuously increasing amount of electric energy will be needed. However, the planet is not able to provide us anymore with the necessary resources for that, jeopardizing the environmental stability. The aim of this thesis is the design and testing of power electronics converters that will contribute positively to the production and distribution of sustainable energy. In particular, a Maximum Power Point Tracking Converter is designed and tested. The 6.4kW converter is able to connect to two separate PV strings. The output of the converter is connected to a bipolar DC distribution grid. Afterwards, a 3.68kW Voltage Balancing Converter was designed for the stabilization of the voltage between the two phases of a bipolar DC grid. Particular interest is given to the challenges that arouse for the power supply of a DC Micro-Grid from a Photo-Voltaic plant. The project took place in the DC lab of the company DC Opportunities.

Roughly 840 million people, predominately from rural communities in sub-Sahara Africa, still lack access to electricity. The direct current (DC) microgrid is an emerging grid infrastructure that meshes efficiently with DC based technology such as photovoltaics, batteries, consumer electronics, and electric vehicles. This characteristic of DC microgrids designates them as a preferred solution for new grid infrastructures in rural electrification applications. In order to establish a DC microgrid, power electronic interfaces (PEI) are required for regulating power flow and interconnecting different grid components of the microgrid. In this thesis, the PEI (operating at 200-900 W) connecting a 350 V DC-microgrid to a solar home system (compatible with USB-C) is investigated. A unidirectional half-bridge LLC converter and a bidirectional dual active half-bridge (DAHB) converter (both utilizing gallium-nitride (GaN) transistors and planar transformers (PT)) are designed and tested. A working prototype of the half-bridge LLC converter with a center-tapped secondary is presented and the waveforms at maximum load and no-load conditions are analyzed. A complete design and efficiency approximation for a DAHB converter with a center-tapped secondary and active snubber circuits is included. In addition, simulations (in PLECS) of the designed DAHB converter provide waveform results for both the forward and reverse power flow direction. The results of the work discuss how high frequency operated, half-bridge isolated DC/DC topologies with GaN transistors and planar transformers are an excellent composition of technology for these rural electrification applications. The GaN transistor is most effective in a low voltage (up to 650 V), high performance scheme, and offers inherent benefits which allow for high frequency operation and thus, smaller passive components. Moreover, the effect of current collapse (an adverse effect in GaN transistors) is discussed and analyzed from a design standpoint. The benefits of planar transformers in low-medium power (up to 900 W) rural electrification applications and an in-depth PT design process are presented. Additionally, rural electrification safe extra-low voltage (SELV) standards require that PT designs must have reinforced (or double) isolation between primary and secondary windings. Taking this into consideration, two proposed PT configurations using a U-core and planar E-core, respectively, are compared. The main conclusions of this work aim to bridge the gap for the design and implementation of efficient DC microgrid variable power output converters for use in rural electrification applications.

Multiple solutions for solving optimal power flow (OPF) have been employed, most of them using a centralised approach. However, there is another approach where every node is responsible for the local problem in order to reach a solution: the decentralised optimal power flow (D-OPF). In this thesis, the aim was to improve the speed and flexibility of a D-OPF algorithm based on the Consensus and Innovation (C+I) method using physical measurements from a direct current (DC) system. While in previous implementations, the system would work towards finding the solution for one point in time, the suggested implementation works by performing online optimisation, meaning that there is less time required in propagating information around the system and faster solutions are reached. A possible interaction between the physical and optimisation layers was suggested, to make online control feasible. Using current droop control for a DC system, it was possible to react to sudden changes the system and, in the long term, optimise the electrical resources. The improvements in speed were then demonstrated by the simulations results, where the time to reach the optimal solution was reduced, when compared to previous implementations. In order to increase the flexibility of the system, adaptive behaviour for the critical optimisation variables was suggested. To reduce the oscillatory behaviour of the system, some gains were made proportional to rate of change of said variables, meaning that the system didn't have to rely on user determine values in order to converge. It was also implemented a solution to calculate the line resistance between two nodes, further reducing the need for external inputs. These implementations were tested and it was concluded that it improved convergence speeds, while increasing the flexibility pf the system. Finally, a test case, based on a real existing lighting grid, was designed in order to test the algorithm under larger networks. The results showed that for a 25% increase in the size of the network there was no significant increase in the time required to reach a solution, indicating that the system can be scaled further, and might be dependent mainly on the network structure, and not its size.

Access to electricity is a key actuator in order to tackle poverty in areas of limited resources. While the population without access to electricity has a decreasing tendency, still almost 1 billion people continue to live without this basic amenity. This thesis has been carried out at the company DC Opportunities as part of the rural electrification project. On this field, the main activity focuses on developing cost-effective DC solutions in the form of a solar home system which consists of a total of 4 devices including low-power LED lights, a solar charging station, a high-power rate power bank and a power hub. The report analyses various existing electrification projects identifying the characteristics of their approaches in order to recognize the shortcomings of market available products and to build a design criterion. Given that the main target application is aimed at rural areas, a modular approach has been the base of the design criterion with the purpose of tailoring the SHS to developing countries with limited resources. The project analyses each step within this modular process together with the power electronics involved in every stage. The main objective of the project is to perform direct charging between the charging station and the power bank, which consists in feeding power directly from the bus voltage of the charging station into the battery cells with only one active converter. This feature is implemented as a result of the introduction of the power delivery protocol released together with the USB type C connector . This offers a wide variety of possibilities which includes negotiable voltage levels ranging from 5 to 20V. The first element taking part in the direct charging is the charging station .This thesis addresses the challenge of designing and building a circuit with dynamic adjustment of voltage and current in a cost-effective and reliable manner. By adding a digital to analogue converter and 2 operational amplifiers, it is possible to add this feature to a wide range of standard voltage converters available in the market. The report analyses this circuit in 3 steps: first with circuit theory, secondly by simulation and eventually by including the results obtained in an empirical test implemented in one of the prototypes. The power delivery protocol is also beneficial for a power-bank application, which is the second element of the direct charging. It is one of the purposes of the thesis to evaluate the performance of direct charging. Eventually, the report presents the main conclusions derived from the analysis of the aforementioned elements and also covers future lines which include improvements to be implemented within the components of the solar home systems.