Modeling and analysis of three-phase four-leg PWM boost-type rectifier for double conversion transformerless UPS

A dual-loop control scheme in a-b-c coordinate for three-phase four-leg pulse width modulation (PWM) boost-type rectifier for double conversion transformerless uninterruptible power supplies (UPS) is presented in this paper. Analysis of the power flow from front-end rectifier to back-end inverter is performed according to instantaneous power theory. The inverter will absorb additional oscillating power when the load is unbalanced. The additional oscillating power can be provided by DC-bus capacitor, but the DC-bus voltage will have the same oscillating component which has a negative impact on the controller for rectifier. The DC-bus voltage feedback in outer control loop can be delayed by quarter fundamental period to filter out the oscillating component. The performance of proposed control scheme is verified by experimental results.