This paper presents a control structure of Solid-State Transformers for three-phase AC/AC, to reduce the required size of the capacitors with load unbalanced compensation capability. The structure discussed in this paper consists of an AC/DC converter based on cascaded H-bridge (CHB) converters, isolated DC/DC converters, and a DC/AC inverter. A high capacitance for the smoothing capacitors is usually required due to oscillating instantaneous power caused by an unbalanced load current. This study proposes a method which can compensate for the unbalanced currents with a reduced capacitance of the capacitors. The proposed method can be achieved by the use of smoothing capacitors connected to the CHB converters as an energy buffer for the oscillating instantaneous power. The design case study on a 300-kVA system shown in this paper calculated that the overall capacitor volume can be reduced by 83 % compared to the conventional control. This control method was demonstrated in time-domain simulation and experiments with a 6-kVA laboratory prototype. By applying the proposed control technique, the voltage of the smoothing capacitor was confirmed to be almost constant while the load currents were unbalanced.