Nonlinear Control Design and Stability Analysis of Power Inverters in Modern Smart Grids

Due to environmental concerns, the rise of renewable energy units is causing a paradigm change in the modern electric power system since bulky synchronous generators are being replaced by power electronic converters whose control algorithms are flexible but have low or no inertia. The inertia is a key concept for the power systems as it ensures frequency stability by balancing the generated and consumed power in case of system disturbances. To deal with the inertia problem in power electronic converters, research efforts have led to the formation of various control algorithms, such as droop control, virtual synchronous control, synchronverter, and virtual oscillator control. In addition, since the number of active players involved in power production significantly increases, system stability is another critical issue that should be considered for seamless power converter-based operations. Furthermore, power electronic converters are composed of semiconductor switches, which can be damaged if sudden changes, such as grid voltage sags and short-circuits, occur in the system. Therefore, advanced controllers are required to protect the power converter devices by limiting the key system states, i.e., currents and voltages, without increasing the total system cost.
In this thesis, the main aims are to propose novel nonlinear control algorithms that can ensure reliable operation of grid connected inverter-fed units and microgrids via system state limitation without additional protection schemes for both single and parallel-connected three-phase inverters, investigate the system stability, and provide the analytic stability conditions that can guide the prospective designers. The proposed controllers are tested both in grid-connected and stand-alone modes for power inverter and microgrid systems considering several system faults including voltage sags and short-circuits. Initially, for the three-phase grid-connected inverters, inverter current limitation is achieved by embedding droop control dynamics into both nonlinear bounded integral controller (BIC) and state-limiting PI (sl-PI) controllers, the closed-loop system stability is examined, and the analytic stability conditions are provided. Furthermore, an improved virtual synchronous control structure is proposed by coupling DC-link voltage and AC frequency dynamics and applied to three-phase grid-connected inverters. Finally, a nonlinear droop controller that can guarantee the current-limiting property and avoid the undesired circulating current issue in AC microgrids with parallel three-phase inverters is designed. The performances of proposed controllers are verified via simulation, experimental and hardware-in-the-loop studies considering both grid-connected and stand-alone modes. In all of the above cases, the proposed controllers are directly compared with the state-of-the-art control methods under both normal and abnormal (faulty) grid conditions to highlight the advantages of the proposed control frameworks in practice, in addition to the rigorous stability analysis.