Improving the power conversion process in UPS systems

Typical specifications for uninterruptible power supply systems require constant regulated output ac voltage under variable or temporarily defaulted ac utility bus conditions. Such an operating environment results in a variable dc link operation with serious derating of the UPS inverter, the output transformer and the output filter components. Two link voltage control methods that overcome this problem are discussed in this paper. With the first method, a boost chopper is activated to maintain the dc link voltage constant when the battery voltage dips below its rated value. In this mode, the chopper operates only during small time intervals, thus not contributing to system losses. With the second method, the battery voltage is stepped-up under feed-back control through a boost chopper. This allows the use of fixed inverter switching patterns with further improvement in the utilization of system components. Specifically, it is found that power handling capability is improved by 65% without compromise in the system complexity and/or system performance.