  As we replace conventional synchronous generators with renewable energy, the
frequency security of power systems is at higher risk. This calls for a more
careful consideration of unit commitment (UC) and primary frequency response
(PFR) reserves. This paper studies frequency-secured UC under significant wind
power uncertainty. We coordinate the thermal units and wind farms to provide
frequency support, wherein we optimize the variable inverter droop factors of
the wind farms for higher economy. In addition, we adopt distributionally
robust chance constraints (DRCCs) to handle the wind power uncertainty. To
depict the frequency dynamics, we incorporate a differential-algebraic equation
(DAE) with the dead band into the UC model. Notably, we apply Bernstein
polynomials to derive tight inner approximation of the DAE and obtain
mixed-integer linear constraints, which can be computed in off-the-shelf
solvers. Case studies demonstrate the tightness and effectiveness of the
proposed method in guaranteeing frequency security.
