Influence of zero sequence impedances of station auxiliary transformers on equipment performance under open-phase faults

Abstract

Primary open-phase faults on station auxiliary transformers (SATs) are characterized by the presence of voltage unbalance at the auxiliary equipment level that could lead to protective device tripping, increased motor acceleration times, overheating or failure to start critical safety loads, particularly in the nuclear power industry. The transformer primary to ground zero sequence impedance has a profound impact on the voltage balance on the secondary (equipment) level but its effects not been fully analyzed in the context of nuclear power plant operation. This study investigates the influence of zero sequence impedance to ground as seen from the transformer primary terminals during an open-phase condition on the performance of nuclear plant auxiliary equipment, in particular the effect on large motor starting and running performance. Dynamic models for the motors were employed and adapted to the sequence network representing the overall system. The resulting system was simulated for a number of open-phase conditions using transformer characteristics representative of the SATs encountered in the nuclear power industry. It was revealed that a lower value of this impedance has a beneficial effect on motor acceleration time and voltage balance for both starting and steady state conditions. The influence of neighboring transformers with a path to ground was also investigated and found to serve towards further improvement of performance.