A frequency control method for use in a frequency control system including: a server that receives, from a power system operator, a power command for controlling a frequency of a power grid within a predetermined frequency; at least one distributed energy resource; and a local controller connected to the server through a communication network and to the at least one distributed energy resource, the frequency control method includes: receiving the power command from the power system operator; obtaining a frequency measurement of the power grid; predicting a next power command using the frequency measurement, before the next power command is received from the power system operator; and controlling an input or output of the at least one distributed power energy resource using the predicted next power command, before the next power command is received from the power system operator.