Subsynchronous Resonance in Doubly Fed Induction Generator based Wind farms

The objective of this thesis is to investigate the risk for instabilities due to subsynchronous resonance (SSR) conditions in large wind farms connected to series-compensated transmission lines. In particular, the focus is on doubly-fed induction generator (DFIG) based wind farms. Analytical models of the system under investigation are derived in order to understand the root causes that can lead to instabilities. A frequency-dependent approach based on the generalized Nyquist criterion (GNC) has been applied to investigate the risk for SSR in DFIG based wind turbines. Through this approach, it is shown that the observed phenomenon is mainly due to an energy exchange between the power converter of the turbine and the series compensated grid. This phenomenon, here referred to as subsynchronous controller interaction (SSCI), is driven by the control system of the turbine’s converter, which results in a non-dissipative behavior of the DFIG system in the subsynchronous frequency range. The different factors that impact the frequency characteristic of the wind turbine, thereby making the system prone to SSCI interaction, are investigated. Through the analysis, it is shown that in a DFIG wind turbine, the current controller that regulates the rotor current plays a major role in the risk for SSCI, where an increased closed-loop bandwidth negatively impacts the system damping in the subsynchronous frequency range. The level of active power output from the wind farm also has an impact on the overall system stability; in particular, it is shown that the power-dissipation properties of the DFIG improves when the latter is operated in supersynchronous speed range (high-power output).Methods for proper aggregation of the wind turbines in the farm are investigated. Time-domain studies are performed on the aggregated model connected to a series compensated transmission line to verify the analytical results obtained through the frequency domain analysis. Based on the theoretical analysis, mitigation strategy is proposed in order to shape the frequency behavior of the wind turbine. The effectiveness of the proposed mitigation strategy is evaluated both theoretically through frequency-domain analysis and using detailed time-domain simulations