DC-Link Protection and Control in Modular Uninterruptible Power Supply

In this paper, a dc-link voltage protection (DCVP) control method is proposed to address the dc-link overvoltage issue due to power backfeeding in a parallel uninterruptible power supply (UPS) system. The proposed control method is able to protect the inverter against the excessive dc-link voltage, which increases the system reliability and robustness. Moreover, a current sharing control strategy is proposed by online regulating the virtual resistance of each UPS module. The proposed current sharing control strategy is able to address the circulating fundamental and harmonic currents caused by the line impedance mismatching or power backfeeding issue in the UPS system. In addition, an improved consensus-based distributed controller is proposed to alleviate the overshoot issue during the transient process in voltage amplitude and frequency restoration. Finally, the feasibility of the proposed methods is verified by experimental results from the parallel UPS prototypes.