System modeling for an electric power system and the discrete-time data for learning control policy. (A) Load frequency control of an electric power system is modeled by considering its nominal continuous-time system around an operating point specified by a constant load value. Only partial system state Δf(t) is available for measurement, requiring the policy design to follow the output-feedback principle. The state-space equation of the power system is highlighted in the figure, but the system dynamics are unknown to the control policy designer. (B) The continuous-time control policy is inferred from the discrete-time reward. The time for the power system’s evolution is sampled non-uniformly. (C) Sampled input-output data are collected for generating the discrete-time reward.