A power grid operation control system, device, and method for steadily operating a power grid while making the most of renewable energy. The power grid operation control system serves a power grid connected to a plurality of renewable energy and a plurality of thermal/hydroelectric power generators, and includes an automatic dispatching system, which controls the thermal/hydroelectric power generators, and a renewable energy generation control system, which controls the renewable energy. The automatic dispatching system controls the thermal/hydroelectric power generators by generating an automatic frequency control signal for correcting the power supply and demand imbalance of the power grid. The renewable energy generation control system determines an available connection capability and issues load instructions to ensure that the total power generation capacity of the renewable energy does not exceed the available connection capability.