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To enhance the lifespan of Vienna rectifiers, the electrolytic capacitors in the dc link can be substituted with slim-film capacitors. However, the decrease in capacitance leads to an increase in neutral point (NP) voltage fluctuation and input current harmonics. To solve this issue, a model predictive control strategy based on three-vector duty cycle optimization is proposed. The negative influence of reduced capacitance and high modulation index on the NP fluctuation and the input current harmonics is analyzed. In order to suppress the NP fluctuation and eliminate weighting factors, the subsectors division and the NP voltage hysteresis are introduced to select effective voltage vectors, reducing 27 candidate voltage vectors to three sets of switch sequences. Then the Karush–Kuhn–Tucker condition is used to derive the duty cycle of the convex quadratic optimization issue within the boundary and constraint interval. With the proposed strategy, both the NP voltage fluctuation and the input current harmonics can be reduced simultaneously. The effectiveness is verified on an electrolytic capacitorless Vienna rectifier platform.

When grid voltage is unbalanced and distorted, the input current harmonics in Vienna rectifier will increase. To solve the issue, a grid voltage sensorless control strategy based on direct-axis current orientation is proposed for Vienna rectifier. In the reference frame, by ensuring that the reference voltage and the input current are in phase, the proposed strategy can avoid the separate operation of positive and negative sequence while reducing the zero-crossing distortion of the input current. The virtual reference voltage is constructed according to the current path and conducting state in the diode rectifier mode, which can reduce the overcurrent during the start-up process. To reduce the harmonics of input current caused by unbalanced grid voltage, the direct axis current control strategy with low-frequency fluctuation is developed in the fundamental plane. In the harmonic plane, the harmonics of the grid voltage are estimated in real time by combining single phase second-order generalized integral, which can suppress the harmonics of the input current under distorted grid voltage. The effectiveness of the proposed strategy is verified on a Vienna rectifier platform.

In Vienna rectifier, amplitude of the common mode voltage (CMV), fluctuation of the neutral point (NP) voltage, and harmonics of the input current are mutually coupled, making it difficult to balance these performances using the conventional strategy. A novel CMV-reduction modulation strategy based on the nearest four voltage vectors optimization is proposed, which can simultaneously decrease the NP voltage fluctuation and the input current harmonics. First, the issue of large switch-order harmonics in input current using the conventional strategy is analyzed according to the characteristics of the current ripple vector trajectory. The proposed strategy adopts four voltage vectors with low CMV amplitudes, which are nearest to the reference voltage vector. The duty cycles are distributed by the zero-sequence component to ensure that the average NP current is zero, which can reduce the NP voltage fluctuation and CMV simultaneously. Furthermore, to reduce switch-order harmonics and zero-crossing distortion of the input current, the optimal switching sequence is obtained by permutation and combination to minimize the vector synthesis error of the reference voltage. The duty cycles are optimized by limiting the range of zero-sequence component to eliminate the large voltage vector. Finally, the effectiveness of the proposed strategy is verified on a Vienna rectifier platform.