A Direct Modulation Index Control Strategy for Three-Phase Vienna Rectifier at Light Load

The three-phase VIENNA rectifier suffers from the inherent problem of increased or unstable output voltage on the DC side at light loads. In this paper, a light load state judgment criterion is derived based on the VIENNA equivalent model. A direct modulation index control strategy is further proposed to optimize the light load performance, which is implemented by proportional control and voltage feedforward. The proposed method significantly reduces the duty cycle by directly modifying the modulation index, ensuring that at any given time, only one phase switch is in PWM mode at most, which reduces the energy transfer path and simplifies the operating modes. To verify the validity of the proposed strategy, a VIENNA rectifier prototype is designed and built. The proposed strategy improves the DC voltage stability and dynamic characteristics under light load at the cost of input current harmonics. The experimental results show that the DC voltage ripple rate is less than 0.25% in steady-state and the voltage overshoot is less than 1.7% in transient.