Dynamic frequency-constrained unit commitment in isolated grids with DFIGs for frequency regulation

The use of doubly fed induction generators (DFIGs) in isolated grids (IGs) deteriorates system inertial and primary frequency regulation ability. In this paper, integrating DFIGs in the inertial response is used to maintain the frequency dynamic security. The rate of change of frequency (RoCoF) and frequency nadir expression after contingence are derived by encapsulating the DFIGs' inertial response. Due to the time variation of the inertial response of DFIGs, a method based on curve fitting is derived to calculate the frequency nadir. A Unit Commitment (UC) model with dynamic frequency constraints is established accordingly. The model represents a complex nonlinear mixed integer optimization problem. Then, the model is decomposed into a master problem for conventional UC with the RoCoF constraint which is a mixed-integer linear programming problem and a sub-problem for frequency verification. The so-called optimization cuts are generated to eliminate the frequency limitation in the iteration method. An isolated grid (IG) test system is utilized to demonstrate that the proposed method ensures system frequency security while retaining favorable economic performance. Published by AIP Publishing.