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In response to the unbalanced DC-port fault of three-level neutral point clamped cascaded converter (3LNPC-CC), a sequence pulse modulation (SPM) voltage-balancing strategy is proposed in this paper to balance DC-link voltage, not only within the module but also among modules. With the steps of carrier cascaded calculation and sequence pulse generator, the voltage level of cascaded modules would take a smooth transition. Then the limitation of the SPM strategy is calculated according to the law of volt-second balance and the law of energy conservation. The proposed strategy has the advantage of simple calculation and control stability. Simulation and experimental results show the superiority of the proposed strategy.