Abstract: Vienna rectifier control systems require sensors to collect grid voltage and current information and the use of sensors increases system size and cost, and reduces system reliability. Aiming at the fact that the grid-free voltage sensor algorithm based on virtual flux cannot adapt to the grid voltage imbalance, a grid-free voltage sensor control strategy suitable for grid voltage balancing/unbalance is proposed. Firstly, the fourth-order quadrature signal generator is used to construct the grid voltage observer, which realizes the grid-side voltage estimation while eliminating the current differential calculation link, has good harmonic suppression ability, and combines the voltage observer with predictive power control. In addition, when the grid voltage is unbalanced, the traditional predictive power control causes serious grid-side current distortion, in order to improve the grid-side current quality of Vienna rectifier, the extended reactive power is applied to the predictive power control by introducing extended reactive power, so that the active power is constant and the grid-side current sine is good, so as to realize the normal operation of the Vienna rectifier under the condition of grid voltage balance/imbalance. Finally, the correctness and effectiveness of the proposed control strategy are verified by simulation.