Abstract: The grid-connected renewable energy with a high penetration rate puts forward higher requirements for the power system frequency regulation. In view of the problem that the traditional primary and secondary frequency regulation capabilities are insufficient when the system suffers from large power disturbances, it is necessary to use the operational reliability to release the system ability to resist large power disturbances in advance. Therefore, a proportional-integral（PI） control link based on the operational reliability index of expected energy not supplied（EENS） is designed to realize the preventive regulation of the active power output of conventional power generators during normal operation. The proposed PI control link takes the minimum EENS of the system as the control reference value and the real-time predicted value of EENS as the feedback, while uses the deviation between the two values as the basis to redistribute power outputs of units. A method for determining feasible parameter ranges of the PI controller based on the Jury criterion is proposed. The effectiveness and superiority of the proposed control link are verified by simulation.