In recent years, the rapid growth of deploying distributed energy resources (DERs) within the primary electrical grid and their widespread adoption in microgrid applications have significantly transformed the landscape of modern power systems. The increasing penetration of DERs, including photovoltaic systems, wind turbines, and energy storage units, necessitates the development of advanced control strategies for the power electronic converters that interface these resources with the grid. This shift is particularly critical as traditional control methods, designed for centralised generation, are no longer sufficient to address the dynamic and decentralised nature of DER-based systems. Inverters, one of the primary interfacing devices for controlling DERs, play a critical role in maintaining the stability and performance of modern power systems. The thesis aims to design and analyse novel control strategies for inverters, ensuring effective regulation of key parameters, including power, frequency, voltage, and current. The proposed control approaches aim not only to regulate these quantities, but also to keep them within prescribed operational limits to enhance the safety and reliability of the controlled system.
In particular, one of the primary contributions of this thesis is the development of state-limiting control strategies for all three main classifications of inverters in AC microgrids, as well as for inverters functioning as interlinking converters in hybrid AC/DC microgrid architectures. Initially, a novel control scheme is introduced for three-phase grid-following inverters, ensuring precise output power regulation while inherently constraining the magnitude of the inverter current. Subsequently, a dedicated current-limiting controller is proposed for interlinking converters, enabling power balance between interconnected AC and DC subgrids. Additionally, a new control method has been developed for grid-forming inverters in isolated microgrids, which can limit both frequency and current, even under transient conditions. An equivalent grid-supporting variant of this control approach has also been formulated for applications requiring grid connectivity. The last contribution lies in the systematic design of an optimal resonant control strategy for single-phase grid-connected inverters, which reduces current overshoots during grid voltage phase jumps. Finally, the theoretical validation of system stability under the proposed control frameworks is provided, demonstrating robust performance across a broad spectrum of dynamic operating conditions. The effectiveness of the developed controllers is further verified through detailed simulations and hardware-in-the-loop testing to assess the stability and performance of modern power systems.
