Energy storage system and control method thereof

The invention provides an energy storage system and a control method thereof. The energy storage system comprises a bus bar, M additional DC buses, M DC/DC converters, N battery clusters and a controller. One ends of the M DC/DC converters are connected to the M additional DC buses in a one-to-one correspondence manner, and the second ends of the M DC/DC converters are connected to the bus bar; wherein M is less than or equal to 0.5 N, and M and N are positive integers greater than 1; the controller divides the N battery clusters into K battery cluster groups based on the cluster-level electric quantity of the N battery clusters, and the absolute value of the difference value of the cluster-level electric quantity between the battery clusters in each battery cluster group is smaller than or equal to a first threshold value; wherein K is smaller than or equal to M, the first threshold value is larger than or equal to 0, and the cluster-level electric quantity is the sum of the battery charge quantities SOC or