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.

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.