AC power transformers are one of the most sophisticated technologies in electrical engineering. However, they have some drawbacks facing volume-sensitive or weight-sensitive applications and energy conversion situations. The solid-state transformer is a promising power electronic technology that can solve the requirements of these applications. It utilizes power electronics and medium frequency (MF) transformers to achieve high power density and multiple functions in energy conversion. With the development of semiconductor switches, it is possible to handle medium voltage in the MF range (1kHz to 1MHz). Therefore, as an essential part of solid-state transformers, medium voltage high power medium frequency transformers require validated tools to model and optimize their design, which is the goal of this thesis. There are several challenges in designing such transformers. They include multiphysics models, Litz wire models, insulation design and design optimization, which are addressed in the thesis....@en

The integration of renewable energy resources has seen a marked increase in recent decades, driven by the need to address the growing severity of the global energy crisis. Distributed generators, which incorporate renewable energy sources alongside energy storage systems, present a viable pathway toward achieving substantial economic and environmental benefits.
In the context of microgrids, maintaining frequency and voltage amplitude within prescribed limits is essential for stable operation. Furthermore, the proper distribution of power—including active, reactive, and harmonic components—among interconnected converters is critical to achieving balanced performance under steady-state conditions. During dynamic transitions, it is equally important for voltage and power levels to evolve smoothly, ensuring a seamless adjustment to the steady-state operating point....@en

The future power grid must accommodate large-scale integration of variable renewable energy sources. Power electronic (PE)-based components, e.g. onverters, will play an essential role in the operation of the power system. However, the harmonics and transients generated by these PE-based components can significantly affect the lifetime and reliability of various power system components. These disturbances, particularly harmonics and transients, are challenging to eliminate. Thus, a more effective strategy is to assess the effects and enhance the durability and performance of the system components. To ensure the reliability of critical system components, these should meet stringent specifications and undergo rigorous testing prior to installation. Using a modular cascaded H-bridge (CHB) based high-voltage arbitrary waveform generator (HV-AWG), capable of replicating the dielectric stresses induced by PE components, enables a more accurate assessment of component resilience under operational conditions.
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge....@en

This thesis investigates the optimal sizing and siting of energy storage systems (ESS) within a distribution grid, focusing on the provision of ancillary services and the intelligent power control of flexible loads, including electric vehicles (EVs), heat pumps (HPs), and renewable energy sources (RES). The study aims to investigate the use of ESS in combination with EVs to participate in the automatic frequency restoration reserves (aFRR) market. The effect of different seasons and the sensitivity of electricity market prices to the optimal sizing are analysed.

The first level involves day-ahead scheduling of flexible loads (EVs, HPs) and ESS allocation. This stage utilises mixed-integer programming (MIP) to achieve optimal ESS sizing and siting based on day-ahead electricity prices, predicted data on EV arrivals and departures, and building occupancy. Additionally, the aFRR prices are known a day ahead, and the most lucrative time intervals for revenue maximisation are selected. The objective function minimises the total cost, which consists of penalties related to customer satisfaction, grid import/export cost, revenues from aFRR provision, and BESS CAPEX and O\&M cost. Two different MIP formulations were used to solve the optimisation in Gurobi using Python programming language.

The second level involves real-time control, where ESS operation is re-optimized using real-time data. This stage adjusts for forecast errors through rolling horizon optimisation. Within this level, the scheduled aFRR reserves are deployed.

The findings reveal that the optimal integration of ESS, considering additional revenue from aFRR provision, is achieved by using a centralised ESS closer to the substation. In contrast, optimal ESS integration for energy arbitrage alone involves sharing capacity between nodes but requires a lower CAPEX cost of BESS to be economical. The results indicate that optimal BESS allocation can differ based on specific conditions, such as imbalance price distribution, grid limits, peak load and EV flexibility. To maximise revenue and efficiency, the study suggests placing the BESS at nodes with the lowest resistance. Winter and summer results are similar in the optimal placement and sizing of BESS, although the operation of BESS and EVs is different. It was observed that during the winter, V2G utilisation was higher. The main difference for the seasons is in the case of only energy arbitrage. The cost savings from ancillary service provision of either BESS, EVs or both show greater potential for summer. Both of the proposed optimisations can find a solution, although the non-convex MIQCP formulation did not guarantee a global optimum for all cases. However, the iterative method proved to be more robust but had a longer simulation time.

Drones -- small (or not so small) remotely controlled flying devices -- are seeing rapidly increasing use in many fields of application and human activities ranging from recreation, competitive sports, last-mile logistics, espionage, exploration, to media production, and even warfare. The abundance of these devices brings with it the risk of them becoming an audible nuisance due to the high pitched noise produced by their surface-mount (SM) PMSMs.
The acoustic noise produced in these motors is the product of multiple factors, but chief among them is the vibration of the motor's external shell, which is the stator for internal rotor type motors and the rotor for external rotor type. This shell vibrates in various oscillation modes as a result of the magnetic forces acting on it, which are an inevitable result of the motor's internal magnetic field and the armature currents that produce them.
This motivates research effort into reducing this noise through modulation or control strategies employed by the inverter powering the motor. In order to develop a control model, first the motor itself must be understood. In this thesis, the electromagnetic aspect of surface-mount-PMSMs will be developed, i.e. an analytical model will be established of the air gap magnetic field in SM-PMSMs.
First, the armature reaction field for arbitrary winding types will be derived, followed by a detailed derivation of equations typically used to model the rotor magnets in SM-PMSMs. A derivation of the effect of stator slotting on the air gap magnetc field will be provided, and concluded with a combination of the three previously mentioned aspects, and the dimension of time will be incorporated in the model.
Accompanying this thesis will be a set of MATLAB code that will be made publicly available for research and instruction in academia.

This thesis explores the development of mobile charging systems, which could significantly ease the challenges associated with current electric vehicle charging methods. The purpose of this thesis is to develop an autonomous system to fulfil estimated charging demand on a typical day under different conditions to exemplify how mobile charging systems can address these issues.

The research is mainly motivated by the limitations of conventional charging poles, such as their scarcity, lengthy charging times, unregulated demand, and urban space conflicts as EV usage grows. Future EV statistics and charging station projections are presented underscoring how these challenges can be amplified shortly. As an alternative solution, mobile systems are introduced highlighting products, prototypes, and studies in the market and literature. Through stakeholder analysis, these systems are shown to benefit investors, consumers, and the public, supporting the grid, facilitating convenient charging.

The thesis incorporates a charging demand estimation algorithm to simulate the charging tasks on a typical day. This demand estimation is represented as private, public, and workspace charging load, sampled by considering the probability of energy demand and connection times. Next, the study integrates an iterative optimization process to simulate how effectively this demand can be addressed by a robot-like mobile charging system.

The system is simulated with different price scenarios, grid capacity values of 50 and 100 kW, varying the number of units between 3 and 5, and battery sizes between 70 and 400 kWh. As a result, it is demonstrated that mobile charging systems can effectively reduce peak demand by decoupling charging load from the grid while offering more convenient charging experience. The profitability is assessed through energy arbitrage, operational revenues, and energy costs, noting improvements with seasonal effects and higher grid capacity.

The results show that the switchable battery configuration can effectively minimise the required investment costs because of the smaller number of necessary carrier units mobilising the battery units. A switchable battery setup with 3x270 kWh batteries and 2 carriers is identified as cost-effective for public and workplace demand, with a potential increase to 340 kWh for higher returns despite 20% more investment. The sizing process is reiterated for another demand scenario consisting of a private charging load and 260 kWh capacity is highlighted as a cost-effective choice, while the profits can be improved with 310 kWh capacity.

The thesis further discusses the mobility necessities of the system and the performance requirements of the powertrain. To maintain grounding, the study simulates the parking service area of P1 at the TU Delft campus. A driving cycle is developed by taking site measurements and also considering safety concerns and standards. Consequently, energy consumption and maximum power requirement are calculated by also integrating a weight estimation methodology regarding the main components of the system.

Lastly, the thesis introduces different power converter topologies that can act as a bridge between the system and EVs. As a consequence of a comprehensive analysis of different converters and the findings reported in the literature, various topologies are suggested to be used in different cases.

As society advances, the demand for sophisticated motor technologies grows. In particular, high-precision modular servo systems have become indispensable in addressing complex real-life challenges, ranging from automation in manufacturing to precision control in robotics. The GEMS project is dedicated to developing an online course that offers detailed tutorials alongside affordable educational hardware, making advanced learning opportunities more accessible to mechatronic students.

This thesis begins with a review of motor technology advancements and their suitability for servo systems. In line with the educational GEMS project’s goal of cost-effectiveness, a permanent magnet brushed direct current (DC) motor was chosen as the core device for electromechanical power conversion due to its simplicity, ease of control, and low cost. The drivetrain system incorporates 3D-printed gears, ensuring affordability and easy replication. For feedback control, two small magnets combined with magnetic Hall-effect sensors function as low-resolution quadrature encoders, offering essential functionality while minimizing costs.

The drive system consists of four key modules: power, sensing, communication, and motion control. Each module's printed circuit board (PCB) functions are detailed based on schematic analysis. The thesis focuses on the motion control module, which is responsible for managing the motor's performance, including speed, position, and torque control. It deals with the low-resolution encoder feedback and implements the control algorithms.

A Simulink model was developed to simulate the DC motor with a 3D-printed drivetrain gearbox. Conventional cascaded PI control is applied, with a current loop as the inner loop and a speed/position loop as the outer loop. To address speed measurement challenges posed by low-resolution quadrature encoders, an encoder tracking algorithm was implemented in the Simulink model, enabling closed-loop control. Finally, the control firmware, based on the Simulink model, was developed in C for the ESP32 microcontroller. Prospective future work is also outlined.

The integration of wind and solar energy through power electronic converters has introduced new challenges to High Voltage (HV) equipment in the electrical power system. Switchgear, cables, and transformers are now subject to higher dV/dt stress and complex wave shapes due to solid-state switching. This poses a threat to the reliability of the grid by weakening the dielectric material of these assets. Existing HV test sources face limitations in generating complex wave shapes and have restricted current capabilities. Building a customized test setup is time-consuming when combining multiple HV test sources for complex waveforms.

To overcome these challenges, an Arbitrary Wave shape Generator (AWG) for dielectric testing of HV grid assets is proposed. The Modular Multilevel Converter (MMC) topology is chosen for its modular structure, low harmonic content, and scalability to higher voltage levels. The initial focus is on dielectric testing of Medium Voltage (MV) class equipment, with the ultimate goal being the development of a modular prototype as part of a PhD project.

HV test requirements and procedures for conventional tests of MV class equipment are compiled, along with specifications for non-standard wave shapes in consideration of the hybrid grid. Two main HV test requirements are addressed in the PhD thesis: the output voltage range of 10 kV to 100 kV with a load capacitance range of 50 pF to 10 nF and a large-signal bandwidth up to 2.5 kHz. The second requirement involves generating steep pulses with a rise time of a few microseconds for a voltage magnitude of 250 kV across a capacitive load of 10 nF.

Despite the maturity of MMC technology for HVDC transmission, adapting it for HV AWG applications presents unique challenges. The thesis explores design trade-offs related to MMC parameters such as the number of Submodules (SMs) per arm, arm inductance, arm resistance, modulation technique, SM capacitance, and control system. Design criteria are developed and demonstrated through simulation models and a scaled-down prototype.

The control hardware of the HV AWG is addressed using a commercially available Real Time Simulator (RTS) named Typhoon-HIL. This choice is based on its flexibility to program arbitrary waveforms in the FPGA without coding in any special hardware description language. The performance is demonstrated in the scaled-down prototype, achieving sinusoidal waveforms up to 5 kHz reference frequency with THD less than 5%.

The second HV test requirement, steep pulse generation, is investigated with the MMC topology. It is found that the series-connected SMs of MMC make it challenging to obtain a short rise time across a large capacitive load. To address this, an integrated hybrid circuit of MMC and Marx generator circuit is proposed for complex waveforms with a rise time faster than 100 μs. Proper guidelines for choosing circuit parameters are provided and experimentally validated with a scaled-down prototype.@en

Electric vehicle (EV) charging plays a crucial role in paving the way for zero-emission vehicle markets, which require massive installations of EV chargers. However, that causes degraded power quality of the power grid, which may lead to non-compliance issues. To prepare for massive installations, a comprehensive understanding of EV chargers’ potential power quality impacts is essential. Furthermore, mitigation measures for potential power quality issues should be prepared beforehand to prevent catastrophic power quality issues from happening. Therefore, the thesis aims to clarify the potential power quality issues caused by EV chargers, prepare modelling methods to understand the root cause of the power quality issues and develop mitigation measures to alleviate chargers’ power quality impacts.@en

This thesis aims to construct a systematic framework for integrating Electric Vehicles (EVs) into Low Voltage (LV) distribution grids. The ultimate goal is to develop a multi-functional, flexible and reliable smart charging (SC) algorithm enabling EV mass deployment in LV distribution grids. The framework for achieving the main research objective is segmented into several key parts:

- Conducting a thorough study on the EV mass deployment in distribution grids through grid load flow analysis.
- Performing a comparative investigation of representative heuristic EV charging tactics to establish a foundation for a smart charging algorithm.
- Developing a Power Transfer Distribution Factors (PTDF) based grid congestion prevention mechanism from the Distribution System Operator (DSO) perspective in anticipation of widespread EV connections.
- Designing, refining and validating a flexible, efficient and reliable hierarchical mixed integer programming (MIP) EV smart charging algorithm.
a. The developed algorithm is equipped with a passive stochasticity processing function and considers practical constraints in protocols such as IEC/ISO 15118 and IEC 61851-1. It is verified and assessed in a Power Hardware-In-the-Loop (PHIL) testbed.
b. Based on the experimental results, the algorithm's effectiveness is further enhanced in: charging current command levelling for a steadier charging process, upgrading grid balancing services, and acquiring a higher level of proximity to optimality. The stochasticity managing function is also upgraded for ad hoc admittance of (future) erratic charging events and self-correction of charging parameters.
c. The advanced EV smart charging algorithm is then assessed by comparing with uncontrolled and one heuristic charging method presented in part 2 above.@en

In the last decade, the market for Electric Vehicles (EV) has expanded at an unprecedented rate. Concurrently, there has been a surge in the demand for public Direct Current (DC) EV charging stations. To accommodate a diverse array of EVs, these charging stations are required to support a broad operational voltage and power spectrum. However, this necessity introduces a trade-off between the operational range and the efficiency and utilization of the installed power within the charging infrastructure. The core of the issue lies in the fact that the charging stations, designed to cater to a wide range of operational conditions, often do not operate at their peak efficiency outside their designated optimal performance region.
To address this challenge, this thesis investigates the advantageous DC/DC power electronics solutions that can maintain high efficiency across a wider operational range while optimizing the utilization of installed power. The research topics include the suitable DC/DC converters for the back-end power modules of the charging system, the multi-objective design process of the power modules, and the advanced operation of the power modules.@en

One of the main technical challenges in the energy transition is the seamless integration of renewable energies into the grid ensuring, among others, system stability and acceptable power quality levels. This thesis explores the effect that voltage imbalance has on the small-signal stability and harmonic performance of the grid-side converter in grid-following type 4 wind turbine generators (full converter). Within the scope of this thesis is voltage imbalance that appears in steady-state (typically, a low value limited by grid codes and international power quality standards) and also voltage imbalance that appears during transient situations (typically, a higher value due to temporary conditions like faults).@en

The electric vehicle (EV) market has experienced significant expansion in recent years, underscoring the pressing need for advanced EV charging infrastructures. In addition to conductive charging, wireless EV charging has proven to be a promising charging solution as it provides safe, convenient, and automated charging for EVs. The most commonly adopted technology for wireless EV charging is inductive power transfer (IPT). Moreover, in wireless EV charging systems, achieving a wide operating range is imperative due to several contributing factors. Firstly, EV battery loads vary significantly during the constant current-constant voltage (CC-CV) charging process. Secondly, coil misalignment and capacitance drift lead to notable deviations in the parameters of resonant circuits. Thirdly, accommodating diverse EV models to enhance interoperability introduces significant variations in air gaps, receiver coil configurations, and nominal EV battery voltages. These factors collectively expand the operating range requirements for wireless EV charging systems. Nevertheless, ensuring highly-efficient and wide-range operation under these varying conditions is challenging. This thesis aims to address this challenge by implementing advanced control and modulation techniques for power converters and compensation networks in wireless EV charging systems.@en

Aviation is a growing industry responsible for over 2% of the energy-related CO2 emissions in 2021. To achieve the 'Net Zero Emission by 2050 scenario', the aviation industry is turning towards modern propulsion technologies that reduce carbon and NOx emissions. Research is taking place on many prospective aircraft designs, such as hybrid/turbo-electric powertrains, fuel cell/liquid hydrogen-powered aircraft and fully electric aircraft. Although fully electric aircraft offer the cleanest possible air travel, these aircraft are considered the solution for short-haul flights due to the range extension issue caused by the deficient specific energy of batteries compared to currently used aviation fuel. The electric aircraft concept designers rely on the potential development in battery technology while proposing their designs, and sporadic state-of-the-art battery designs are present concerning electric aviation.

The concept of reconfigurable battery packs involves using power switches to modify the arrangement of connected battery cells based on specific requirements. This innovative technique can potentially significantly reduce the weight of battery packs. The primary objective of this thesis was to conduct a comprehensive analysis and comparison between fixed configuration and reconfigurable battery packs in the context of electric aviation. It was imperative first to design these battery packs to facilitate this comparison. Given the limited availability of open data on electric aircraft designs, the power profile was estimated using available reference aircraft specifications and reasonable assumptions. The literature review on power systems in aircraft revealed a significant correlation between system-level voltage and the weight of power cables. This discovery led to estimating an optimal system-level voltage, a critical constraint in battery sizing. For the fixed configuration battery pack, sizing was conducted using both a high-specific energy cell and a high-specific power cell. The design of a reconfigurable battery pack involved strategically leveraging both cell types. This innovative approach created a reconfigurable battery pack capable of dynamically connecting and disconnecting an internal high-specific energy battery pack called the 'primary battery pack' and a high-specific power battery pack known as the 'secondary battery pack' through power switches, allowing them to complement each other during high-power demand phases of flight, such as take-off and climbing.

Software simulations were conducted for the validation of this technique. These simulations revealed that the reconfigurable battery pack experienced higher C-rates than the fixed configuration battery pack. Given that higher C-rates can impact battery health by inducing capacity loss over multiple cycles, a preliminary ageing analysis was performed to quantitatively assess the adverse effects of higher C-rates on the reconfigurable battery pack.

The results quantified that around 400 kg of potential weight savings is possible by employing reconfigurable battery packs over fixed configuration battery packs at only 0.4% more capacity loss over 500 charging-discharging cycles. The weight savings can be translated into three different scenarios. Firstly, payload weight capacity can be enhanced. Secondly, flying with lesser weight will offset the power profile, saving energy. Lastly, an additional number of cells equivalent to the mass saved can realise the range extension of the electric aircraft.

To utilize electronic power converters for water electrolysis in industrial electrical networks, reliable testing methods are required to ensure safe control operation as malfunctions can instigate hydrogen explosion and fierce electrical hazards. In this thesis, Digital Twin modeling via OPAL-RT software and hardware is compared to Simulink and PLECS simulation methods by creating the same power-to-power hydrogen network for all three with a Dual Active Bridge, a Medium Voltage DC grid, and an Alkaline electrolyzer whilst utilizing PI control via single phase shift modulation.

The Digital Twin model showed great control tuneability, while the PLECS model showed superior control performance, modeling complexity, error occurrence, and time-based performance for the designed specifications. It was concluded that the Digital Twin model needs to be developed further or reassessed
to outperform the other modeling methods. The recommendations made included re-evaluating the OPAL-RT Digital Twin results for different signal measurement methods and validating the OPAL-RT Digital Twin’s performance by comparing the results with other Digital Twin brands. Future work suggestions included creating a hardware-in-the-loop system and expanding on the current network design to include all components of a power-to-power hydrogen network.

Global average temperatures are setting new records compared to the previous decade, primarily due to the accumulation of greenhouse gases such as CO₂. The growth potential in emerging economies worsens the current global situation. These CO₂ emissions are needed to be curtained and efforts are underway. The emissions target set by the International Maritime Organization seems far-fetched given the forecast for the growth of the shipping sector. Although electrification has proven to reduce emissions, the energy efficiency metrics (EEDI, EEXI) are incapable of providing the quantitative benefits that can be realized by switching to a bipolar DC (BiDC) grid when compared with the AC grid.
This thesis aims to evaluate the feasibility of BiDC grids over the complete operating profile of the ship by comparing ten arrangements, five for AC and five for DC, with the Key Performance Indicators (KPIs). The arrangements are two diesel generators (AC1 & DC1), two diesel generators and battery (AC2 & DC2), two diesel generators, battery, and shore power (AC3 & DC3), one diesel generator and battery (AC4 & DC4), one diesel generator, battery, and shore power (AC5 & DC5). The comparison is made for key performance indicators like CO₂ emissions, fuel consumption, electrical power requirement, propulsion power cable losses, capital costs, operating costs, propulsion system weight, and load carrying capacity of the ship. In this study, a ferry is chosen to compare the two grid architectures.
To establish the required background knowledge for the reader of this thesis, the literature on the current emission regulations, the different propulsion systems, and the differences in AC and DC grids are presented. The ship’s propulsion and auxiliary power demand are not freely available and are in the possession of the ship operator. Therefore, first, a preliminary methodology was developed for estimating the ship’s mechanical power requirements. Second, the ship’s electrical power requirements are estimated from the understanding of the literature. The cable power losses are also calculated for the AC and BiDC grid topology. Third, the diesel generator model was developed from the data points of a marine diesel generator for the AC and BiDC grid operation, i.e., the fixed speed and variable speed operation. Finally, an optimization problem was formulated for all the ten arrangements for minimising the CO₂ emissions over the complete operational profile of the ship.
Overall, it was observed that the minimum emissions achieved by BiDC grids, when compared with AC grids for all the arrangements, the emissions from the former were lower. Furthermore, these results depend on the operating profile of the ship, the shore infrastructure, the diesel generator and battery configuration and their technical parameters. However, these findings demonstrate the possible potential of BiDC grids in emission reduction from the shipping sector.

Batteries play a crucial role in powering contemporary devices and systems, ranging from smartphones and electric vehicles to renewable energy storage. With the increasing demand for more efficient and reliable battery technologies, the need for accurate monitoring and assessment of battery condition and performance has also grown. Online estimators, that continuously analyse battery conditions in real-time, have emerged as valuable tools to meet these goals. This thesis focuses on constructing model-based online state estimators for real-time estimation of a battery's State of Health (SOH).

These estimators, necessitate a state space model for a lithium-ion cell. In the initial part of the thesis, a physics-based reduced-order model (ROM) of a lithium-ion cell is developed. This model accounts for major aging mechanisms such as Solid Electrolyte Interphase (SEI) layer formation, Loss of Active Material (LAM), and Lithium Plating (LIP). To incorporate temperature effects on cell parameters, a simplified lumped thermal model is integrated into the battery model. The model is subsequently transformed into a state space model using the Discrete Realization Algorithm (DRA) process.

Building upon the noisy outputs from the ROM models, a set of five estimators is formulated: State of Charge (SOC), Voltage, SEI loss, LAM loss, and LIP loss estimators. These estimators are constructed based on Kalman filters and collectively contribute to real-time prediction of a battery's SOH. Given their reliance on the model, the ROM model's output is employed as a reference to gauge the precision of the estimators.

To address the real life scenarios and to check the robustness of the estimators, a series of sub-questions were analysed:
Sub question1: How to estimate the SOH of a cell in real-time using adaptive control techniques ?
Sub question2: How fast can the estimator react to changing initial states of the cell?
Sub question3: Can the estimators be adapted to concurrently estimate both the states and the time varying cell parameters in case of an old cell? How fast can this be
achieved in real-time ?

Overall, this research contributes to the development of model-based online estimators, which are poised to have a significant impact on enhancing battery performance, prolonging lifespan, and facilitating the transition towards a more sustainable energy future.

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.

Recently, there have been numerous DC microgrids coming up in every sector. Some of these microgrids operate as independent entities with their own renewable energy sources, active loads, distribution networks and energy storage units. However, they are not highly reliable as the renewable energy sources are location and climate dependent. Having a large energy storage unit can be one possible solution, but that involves high expenditure and occupies a large amount of space. Thus, an alternative is to connect the independent DC microgrid to an established AC grid. With the advancements in power electronic technologies, such a connection can be established by a multi-stage converter known as Solid state transformer (SST).

Numerous studies have been conducted on SSTs for low-voltage levels, and there is a wide scope for research in the area of medium and high-voltage converters. Also, the scope of using Planar transformer (PT) as high-frequency transformers in DC-DC converters for medium and high-voltage applications is yet to be extensively explored. Hence, the main focus of this thesis would be to develop an isolated bidirectional DC-DC converter with a planar transformer that can be used in an SST which can potentially connect a low-voltage DC microgrid comprising a solar-based car parking system, to a medium-voltage AC grid.

Firstly, various DC-DC converter topologies have been investigated, and the single phase Dual active full bridge (DAFB) is chosen as the best suitable topology for this application. Secondly, various control strategies have been investigated, and single-phase shift control has been implemented due to its simple control techniques and low data transfer requirement across high isolation. Thereafter, a script is developed to identify the optimal parameters for the Dual active bridge (DAB) and PT design. Further, the planar transformer with high isolation is realized by considering various factors like shielding and termination into account. Finally, a single-cell prototype is developed and tested for DAB operation and isolation requirement of the designed planar transformer.

The result of the presented work is a single-cell isolated bidirectional DC-DC converter rated for 12 kW designed and developed with a PT rated for 16.3 kV isolation.