The input terminal of three-level neutral point clamped（3L-NPC）topology is a structure of series capacitors.Owing to non-ideal factors in the circuit，there exists the problem of capacitor voltage unbalance.Based on the equivalent circuit of the 3L-NPC derived，the relation between the capacitor voltage difference and the capacitor current is analyzed in detail.Theoretical analysis indicates that 3L-NPC possesses the feature of capacitor voltage natural balance，whose natural balance current mainly depends on the direct current（DC）component of the output current.Two types of the voltage balance control strategy are proposed based on the analysis:type one，that capacitor voltage difference feed-forward or DC component of modulation wave feedback is utilized with the proportion-integration（PI）regulator or proportion-resonant（PR）regulator to ensure the natural balance;type two，particular regulators are employed without damaging the natural balance.Based on type two，the quasi-resonant regulator is proposed which can balance the capacitor voltage and simplify the system control at low cost.Moreover，in order to improve the dynamic response performance，a double-loop control scheme is proposed.The theoretical analysis is verified by both simulation and experimental results.