Study on Inrush Current Restraining Technology when Energizing No-load Transformer

In this paper, an improved closing strategy is proposed and applied in restraining inrush currents when energizing no-load Y0 /Δ power transformer. It uses a combination of series resistor insertion and controlled closing technology. Considering the effects of residual flux to inrush transient, the grounding resistor connected to the transformer neutral point is used to restrain the inrush transient of the first closeing phase and then a "delayed closing-strategy" is employed to reduce the inrush current magnitude of the last two phases. The simulation by EMTP indicates that the strategy would restrain the inrush transient efficiently when the residual flux can't be measured.