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.