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The Vienna rectifier with unbalanced input voltage and load transient is analyzed. A nonlinear control strategy for Vienna rectifier under unbalanced input is proposed. From the view of positive and negative sequence components,the proposed nonlinear control strategy suppresses the twice frequency ripple and guarantees the dynamic response characteristic at the same time. Thanks to the proposed nonlinear control strategy,the DC bus capacitor can be reduced a lot since the voltage ripple and drop can be suppressed. A 10 kW Vienna rectifier is built to verify the proposed control strategy. After applying the proposed nonlinear control strategy,the voltage ripple is only 7 V and decreases over 75％ over the traditional PI control when the unbalanced degree is 20％. The voltage drop can be reduced about 80％ than former control strategy which is helpful to reduce the DC bus capacitor and achieve higher power density. The volume of the capacitor can be reduced by 83.3％ with the new control method.