Parallel uninterruptible power supplies based on Z-source inverters

One limitation of the conventional parallel uninterruptible power supplies (UPSs) is that the battery and rectifier output DC voltages must be almost equal to each other and higher than the AC output peak voltage. To remove this constraint, a new configuration of parallel UPSs based on Z-source inverters (ZSIs) is proposed. This system supplies a common AC load with DC sources of different voltages and power ratings. Control of the proposed system consists of the ZSI and the current sharing control. To control the ZSI output voltage, dual-loop control technique is implemented. For ZSI DC link voltage control, direct and indirect methods are discussed and their effects on circulating current and system characteristic equation are studied. Moreover, two strategies are proposed for current sharing: equal current sharing (ECS) and proportional DC source nominal power rating current sharing (PCS). For implementing these strategies, a new current sharing algorithm based on 3C method is developed. The system operation with these methods is compared from the viewpoint of Z-source network response and maximum power supply duration. The performance of the proposed system is verified by simulation and is validated via experimental results.