Static UPS failures-origin and possible prevention

Static inverters are used by various industrial and institutional facilities as uninterruptable power supplies (UPS), providing a reliable source of AC power to critical loads. In some applications, loads supplied by UPS inverters can tolerate occasional failures, but others, such as critical buses at nuclear power plants, are much more sensitive to a loss of these units. In many cases, a UPS failure at a nuclear power plant will result in a partial or total failure of reactor protection systems. In this paper we present our preliminary investigation of UPS failures at a nuclear power plant. An analysis of the failure along with the results of diagnostic testing led us to believe that the intermittent system transients resulted in an overlapping firing sequence of the thyristors in the inverter bridge. The overlapping firing sequence creates a condition in which all four thyristors in the bridge circuit conduct simultaneously resulting in a short circuit condition in the DC supply to the inverter bridge. In this presentation we detail our diagnostic approach in identifying the origin of the failure. In addition, we discuss the suggested corrective actions proposed to eliminate effects of transients on the UPS thyristor bridge. The discussion includes the design and evaluation of a proposed change in the circuit that will prevent inadvertent thyristor misfiring while leaving the proper operation of the bridge unaffected.