Provided are: a grid controller for use in a smart grid system, the grid controller being capable of effectively smoothing transmission power transmitted to the outside without requiring a high-capacity power storage unit to be additionally introduced; a smart grid system including the grid controller; and a method of controlling the smart grid system. A grid controller (2) is communicably connected to controllers (51, ...) of a plurality of power storage units (31, ...). The grid controller (2): obtains transmission power (Pall) transmitted from the smart grid system to an external power system, the transmission power (Pall) being a sum of electric power generated by power generating units (21, ...), electric power consumed by loads in the smart grid system, and electric power charged into and discharged from the power storage units (31, ...); calculates differential power (”P) between the transmission power (Pall) and a smoothing operation output (Pall'), the smoothing operation output (Pall') being obtained by performing smoothing operation on the transmission power (Pall) by using a smoothing filter; and performs control of smoothing the transmission power (Pall) by performing allocation of the differential power (”P) of the transmission power (Pall) in accordance with a charge-discharge state of each of power storage parts (31, ...) of the plurality of power storage units (31, ...).
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