| Περίληψη: | This dissertation is placed in the general context of power systems modeling, control, and analysis. Specifically, modern power systems of varying topology featuring large scale integration of renewable energy sources (RES) are considered for the modeling of which, a generalized method is proposed. By taking into account all units located within a modern electricity grid infrastructure, including locally controlled power converter interfaces and electromechanical components, the introduced modeling approach effectively captures the entirety of the system dynamics. Since the proposed formulation retains its structure under changes in grid layout, it can be deployed so as to model varying topology systems, whereas the form of the model itself is nonlinear in order to match and accurately describe the inherently nonlinear nature of modern grid topologies. One of the remarkable properties this approach features is the capability of directly establishing strong stability and state convergence properties for any kind of system that is compatible with this formulation, regardless of its scale or structure. This fact is rigorously proven by adopting a theoretical framework based on advanced nonlinear analysis tools. In this sense, the introduced modern power system representation has a twofold use; it stands both as an accurate and universal power system model and as a direct stability analysis tool.
An important part of this dissertation is also devoted on developing suitable control schemes for various types of power converter interfaces, mainly focusing on the one type dominating the power grids nowadays, the three-phase voltage source converter (VSC). The proposed control designs are of simple PI-type and they feature several innovations in their structure that effectively withdraw many adversities this kind of control schemes usually have. In this frame, several VSC-based configurations are considered, including the cases of a VSC connected to a stiff or a weak grid. For the latter case, a novel PLL mechanism is also proposed that effectively copes with the challenging issue of maintaining synchronism. In addition, VSC-based dc-link configurations are considered and appropriate control designs are proposed for such topologies as well. The stabilizing effect of the developed control designs on the closed-loop performance of these systems can be proven by adopting the theoretical framework used in the unified CIAT model analysis. As it becomes apparent, all of the considered topologies are absolutely compatible with a simpler variation of the CIAT model and thus, strong stability and state convergence properties can be directly extracted.
The case of constant power loads (CPLs) is also investigated in relation to the developed CIAT representation. Several aspects regarding the need of characterizing some loads as constant power ones in the modern power system paradigm is also discussed, whereas the basic properties of such loads are examined by considering a typical dc/dc boost converter/CPL configuration. Finally, the complete CIAT formulation is employed in order to accurately model a realistic modern DG-based power system example featuring both controlled power converter interfaces and electromechanical components. In this process, the remarkable properties accompanying this modeling approach are being fully showcased.
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