roop-controlled inverters small-signal impedance characterization for stability studies
Mira Gebauer, Nicolás Francisco
Microgrids are one of the key technologies to facilitate the integration of large amounts of renewable generation technologies to the main grid. The main power supplies inside the microgrid are power electronic devices which are responsible for energy conversion and provide the necessary control. Dynamical interactions between the microgrid and newly connected power electronic-based sources can lead to small-signal instability. Hence, several stability analysis approaches have been developed over the recent years, particularly methods to ensure stability by first dividing the system into source and load subsystems and then applying the Nyquist criterion to the respective source/load impedances ratio. Nevertheless, this aspect has been rarely studied considering droop-controlled inverters, as the active power droop control also impacts the output frequency of micro-sources and has a deep impact in the small-signal impedances of the inverters. The main objective of this thesis is to characterize the small-signal impedance of droop-controlled inverters typically used in microgrids through simplified models, in order to achieve a comprehensive understanding of their behavior. This work postulates as hypothesis that the general behavior of the small-signal impedance of droop-controlled inverters when operation conditions change can be characterized through the analysis of the transfer functions of linearized multi-input multi-output reduced-order inverter models, by contrasting them with the resulting small-signal impedances of more complex models. The obtained results show that the small-signal impedance of these inverters were effectively characterized, specially by one of the proposed models. Two indices were developed in order to quantify the graphically obtained results, which confirmed the performance of the developed models, specially with respect to DD, DQ and QD-Channels.The indices confirmed the identification of the operating variables that impact the small-signal impedance the most when perturbed. The results also indicate that the low-frequency range of the small-signal impedance is the most affected range when changing the operating conditions, as the high-frequency range tends to converge to the large-signal impedance. This work could lead to improved small-signal stability studies, in which one of the biggest problems nowadays is the dependence of the small-signal impedance on the changing operating point.