Mitigating System Impacts of Energization of Large Power Transformers of a Nuclear Plant

Direct energization of large power transformers from a utility grid can be expected to result in high magnitude inrush currents to be drawn from the grid that may take several seconds to decay. The phenomenon was verified and characterized by evaluating measurements that captured energization of a large Generator Step-up transformer (GSU) in an existing nuclear facility. The highly reactive and unbalanced nature of these inrush currents means that they have the potential to cause several undesirable system and facility impacts including protective relay maloperations, significant system voltage changes and electromagnetic torques on nearby operating generating units. A comprehensive time-domain modeling and simulation study was carried out to evaluate these concerns for new nuclear units being developed by Southern Company at Vogtle Nuclear Plant (NP) in Augusta, Georgia. The AP1000 nuclear reactors to be used have a design based on a generator breaker for unit synchronization which allows large GSU and Unit Auxiliary Transformers (UATs) to be energized from the transmission grid instead of the conventional practice of energization with the generator as it builds voltage prior to synchronization with the transmission grid. To mitigate the potential system impacts, several solutions were researched and evaluated and the most optimal solution for this application in the form of Pre-Insertion Resistors (PIR) switching was identified.