| Popis: |
The wide diffusion of distributed energy resources (DERs) has led to a scenario where the penetration of renewables is very high and can significantly affect the grid stability. The increasing complexity of these systems requires a suitable stability approach: the impedance-based analysis has one of its main advantages in the possibility to characterize the components separately, e.g. source and load, and to estimate the stability at a certain interface applying the Nyquist criterion to the impedance ratio. This method has been widely used in DC systems, to investigate the converters interactions and anticipating the stability of the final scenario also in case of multiple paralleled converters, often using criteria to limit the interactions and guarantee a stable configuration. Then, the method has been extended to three-phase system, where the multi-input multi-output configuration needs the generalized Nyquist criterion (GNC) for the stability assessment. The first case presented in this work is a grid-connected large photovoltaic (PV) farm, where the inverter control is provided in abc-frame, and considering a balanced and symmetrical system the equivalent single-phase inverter is used in this analysis. The stability is addressed according to the aforementioned impedance-based approach, including also the equivalent generator contributions. The impedance multiplication effect is here formalized also for the case of different parallel inverters. The influence of the line impedance and of the power rating of the inverter are considered. The outcome of the study is an approach featuring both accurate stability analysis, as in multi-input multi-output based approaches, and modularity, as in impedance-based approaches. Moreover, the grid sensitivity is investigated for the case of multiple paralleled inverters, in order to analyze how it changes with an increasing number of connections. Recently, the interest on the hybrid-grids with diesel generators and battery energy storage systems (BESSs) are gaining higher attention because nearly one in five people in the world live without access to electricity. This off-grid solution is then able to provide a continuous generation and also integrate the renewables in the same system. The second part focuses on the modeling of a three-phase hybrid-grid, where the diesel generator is controlled in isochronous mode, and the inverters interfacing the BESSs are droop-controlled with an additional external loop to provide the exact tracking of the power references when the generator is connected. The experimental results of a system with a 400kVA diesel generator and up to 300kVA coming from the BESSs are included. The analysis has led to the full reproduction of the interaction between the diesel generator and an increasing number of connected inverters, where the total inertia of the system changes. However, in literature there is no stability analysis accurate enough to analyze such a complex system and predict instabilities. The modularity of the impedance-based stability analysis can then provide a subdivision of this complexity, and so represents a suitable approach. In this work, the output impedance of a droop-controlled inverter is determined, in order to characterize this element widely used in off-grid applications. After determining the operating point, the analytical model of the output impedance is derived in both controller and system frame, including the effect of the decoupling impedance and the inverter inner dynamics. Finally, this work presents a mathematical tool to convert impedance between different dq-frames. The application of this conversion tool to the aforementioned droop-controlled inverter case will be provided, in order to prove the correctness of the transformation. |
