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.

The use of DC microgrids in combination with photovoltaic (PV) energy generation sources results in higher efficiencies by avoiding AC to DC conversion. Moreover, bipolar DC grids allow for more system flexibility and increased reliability at the cost of an additional conductor when compared to unipolar DC. However, maximum power point tracking (MPPT) converters are still required in order to optimize the power delivered to the microgrid by PV systems. The development of more efficient and cost-effective DC-DC MPPT converters will be proven vital considering the expected growth in solar PV installation as a consequence of the relentlessly increasing global energy demand.

The objective of this thesis is to design a DC-DC MPPT converter for bipolar DC microgrids based on gallium nitride (GaN) high-electron-mobility transistors (HEMTs). GaN transistors are wide-bandgap semiconductor devices that can operate at very high switching frequencies due to their superior electron mobility compared to other semiconductor materials used in electronic switching. Increasing the operational frequency of an electronic converter allows for smaller passive components, which in turn has the potential to result in system size and cost reductions. A complete electrical and thermal analysis of the performance of these components is simulated and presented in this report, and the implications of employing these devices for the selected application are evaluated. The design overview of the DC-DC MPPT converter using these GaN HEMTs is also showcased in this work, as well as the results obtained from testing the assembled DC-DC MPPT converter for bipolar DC microgrids. The main conclusions of this work aim to clarify the advantages of GaN devices in MPPT applications, as well as demonstrate a design solution to the integration of MPPT converters to DC microgrids. The project was developed in collaboration with the company DC Opportunities, and the designed converter was assembled and tested in their laboratory.

To tackle the effect of climate change while meeting the ever-growing demand of energy, a high scale of integration of renewable energy resources has been performed in the past decade. Solar Photovoltaics have played a monumental role in this process, thus contributing in the fulfillment of the energy demand. The scalability and the flexibility of use has led to a significant development in the technologies implemented for assimilation of Solar PV. In addition, the development in power electronics has made the process of integration seamless. Due to Solar PV's dependency on irradiation there is an ever growing need for establishment of an improved control on power converters.
The aim of this thesis is to implement Maximum Power Point Tracking and Droop control algorithms on PV powered DC-DC Converters while considering the dynamics of Solar PV. Mathematical modelling is used to model the PV generator to consider its dynamics along with the dynamics of the converters to establish control. The output of these converters is connected to a DC Microgrid. After completion of the modelling process, the internal control design was performed followed by the implementation of both the algorithms. The project was performed at the lab of the company DC-Opportunities.
The findings of this research are presented in the current document. Its goal is to outline the procedure taken from the start of the project and the literature review all the way through the design and testing of the power electronic converters. Finally, suggestions for the future are offered, which might serve as a roadmap for more advancements.

The increasing penetration of the distributed energy sources and the increasing load on the AC grid demands for integration of DC grids in AC distribution network. This has resulted in the requirement of power electronic converters such as DC/AC converters. For different operating conditions of the DC/AC converter it is not recommended for the converter to operate in open loop. Hence, while designing the control for the converter there are challenges such as the presence of unbalance on the grid leading to higher losses, offsetting of AC voltages at the output and grounding of the converter leading to higher common mode voltage and currents. This thesis explores the control strategy that can be implemented on the converter to mitigate the unbalance in different operating conditions and compares different modulation techniques to reduce the common mode voltage and currents.
To begin with the islanded operation is considered for the converter and the voltage control is implemented on this type of converter. Further a modification on this type of control is proposed for the operation of converter under overcurrent scenario. Subsequently the control strategy for grid connected operation is analyzed such that the issue of unbalanced currents on the grid is mitigated. Three different operating modes are considered under grid connected operation for validating the control strategy proposed. Following the controlled operation in islanded and grid connected scenario, the active methods for reducing the common mode voltage are reviewed by comparing different modulation techniques.
On the discussed control strategies for different operating conditions of the DC/AC converters, an optimized implementation of the control on the microcontroller is presented. Other functionalities such as protection of the converter against unwanted actions is investigated through implementation of state machine. The results of the controlled DC/AC converter operating in islanded operation is presented and modifications in the existing PCB are discussed to make it compatible to operate under grid connected condition. The operation of control strategies implemented is validated for the real time application of the converter with unbalanced load conditions.