This paper proposes a new type of interphase voltage sag/swell compensator based on voltage-fed quasi-Z-source ac-ac converter to implement a four quadrant voltage injection. By PWM duty-ratio control, the converter performs as a "solid-state transformers" to provide a larger range of output ac voltage with buck-boost, reversing or maintaining phase angle, reducing in-rush and harmonic current, reducing passive component ratings, and improving reliability. The injected voltage amplitude can vary in a large range and its phase angle can vary in the whole 2π range due to the circuit features. Compared to conventional parallel series back to back dynamic compensator, the quasi-Z-source converter has smaller capacitor and inductor requirement, and also smaller switch voltage and current stress. The theory analysis can be divided into three parts: basic circuit analysis; passive component stress and switch stress comparison to conventional topology; voltage sag/swell compensation strategy. Simulation and laboratory test are also carried out on a prototype to validate the theory analysis.