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Improving Controllability and Plug-and-Play Operation of Wind Farms Using B2B Converters

We show that a critical factor deciding the controllability of a doubly fed induction generator (DFIG) in a wind power system is the ratio of its leakage reactance to resistance. If this ratio is high then the DFIG has two uncontrollable slow resonant modes. In that situation any type of control action for attenuating disturbances inside the wind farm becomes impossible. In order to prevent this uncontrollability, we propose to add a back-to-back (B2B) converter in the stator line of the DFIG. This new converter has two benefits: 1) it improves the controllability of the wind farm significantly and 2) its signal-flow diagram reveals a cascade structure where the grid is shown to impact the DFIG dynamics, but not vice versa. The cascade structure also enables one to design controllers for regulating the DFIG currents and its dc-link voltages in a completely modular and plug-and-play fashion. We illustrate various implementation aspects of this control mechanisms via numerical simulations of the IEEE 68-bus power system model with one wind farm.