Large capacity battery energy storage system is one of the effective means to solve the photovoltaic and other renewable energy power station grid-connected issues. In view of the problems of SOC imbalance in the proposed star hybrid cascade energy storage system, the balancing control of SOC among clusters, namely the bridge arms of each phase in this system, is implemented by means of injecting zero-sequence voltage. And the SOC balance of units within the cluster is realized through adding corresponding modulation wave with SOC as feedback components. Its reasonableness and feasibility are verified by an example of hybrid cascade energy storage system established in PSCAD/EMTDC.