This paper discusses a grid-connected energy storage system based on two three-level (3L) converters: a DC-DC one and a neutral-point-clamped inverter. When compared to a system using standard two-level converters, the main advantages of this system are: higher efficiency, smaller reactive components allowing for a system cost reduction and its capability to handle higher voltage and power. A modulation strategy for the DC-DC converter which includes the neutral-point voltage balance control is proposed. The capability of the DC-DC converter to cancel the inverter's neutral-point voltage oscillations is also analyzed. Results are presented to illustrate the extended operation area without imbalances. Design guidelines to size the DC-link capacitors are given for a practical implementation. Experimental results taken from a 10 kW 3L DC-DC converter prototype are provided to show the capability of this particular converter to keep neutral-point voltage balance.