Uninterruptible power supply voltage fluctuation 2

Introduction An uninterruptible power supply (UPS) is an electrical apparatus that provides a continuous, stable, and uninterrupted supply of power to critical loads. UPSs can supply power to an important load for a certain amount of time when faults occur with utility power, so the user has ample time to deal with it.

Diagram of output voltage regulation for paralleled uninterruptible power supply system. When the control system detects the active circulating current and reactive circulating current in the parallel system, the increase in the inverter output voltage amplitude is calculated according to Eq. (15.40).

For UPS applications, unbalanced load current is a normal situation whereas output voltages need to be balanced all of time to keep a higher-power quality to the load. There is a zero-sequence current in the three-phase four-wire system owing to the unbalanced load.

Soft-switching uninterruptible power supplies The traditional back-to-back converter based on silicon devices uses a hard switching mode. Its switching frequency is limited, resulting in larger filters and higher audio noise. Soft switching technology can reduce the switching loss of power semiconductor devices and improve conversion efficiency.

Conceptual diagram of unified control scheme for uninterruptible power supply system. Because of the three-phase four-wire configuration, the control for each phase in both the PWM rectifier and inverter can be decoupled. Therefore, a single-phase independent control approach can be adopted.

Simplified single-phase circuit models of uninterruptible power supply system: (A) Pulse-width modulation rectifier; (B) Inverter. By comparing the mathematical models in (15.24), (15.25), a unified control plant for a single-phase PWM rectifier and inverter in the stationary coordinate system can be obtained, as shown in Fig. 15.32.