Uninterruptible Power Supply for Critical AC Loads -- A New Approach

An uninterruptible ac supply system consisting of two power sources (static inverter and ac line) and a thyristor switch is presented. The thyristor switch is operated in such a way that the equalization current flow between the sources is prevented. Consequently continuous parallel operation of the sources may be applied. Faults in one of the sources do not affect the voltage supplied to the load. In order to obtain these features the trigger mode of the switch must meet certain requirements. The analysis of these requirements is given and an implementation of the system is presented. Test results are illustrated by oscillograms. The advantages of the system are that 1) load transfer is performed in an instantaneous and smooth way, 2) the system is able to deliver large inrush currents required by the load and to blow fuses in branch circuits of the load, and 3) no voltage monitors are required.