Increased penetration of large-scale renewable sources such as wind and solar power creates additional constraints for power system maintenance

reliability and security. The intermittent and fluctuation characteristics of renewable energy require robust operation methodologies. Co-optimized scheduling of multiple energy resources is regarded as an efficient way to accommodate renewable energy. As the second largest source of electricity in the world

hydropower is clean and has the merits of ease of regulation

low cost and high flexibility. In this regard

it can play an important role in mitigating the uncertainty of wind power. This paper proposes a robust wind-hydro-thermal unit commitment (UC) model that provides reliable day-ahead unit commitment decisions. The hydro units are operated in longer time scales than the thermal units

so as to capture reservoir operations. The hydro production capability curve is linearized by using variable separation and the piecewise linear (PWL) function

leading to a mixed integer linear programming (MILP) problem that can be solved using mature software such as CPLEX. Numerical tests on an IEEE 39-bus system demonstrate the effectiveness of the proposed model to increase system flexibility in accommodation of uncertain wind power.