Experimental study on controlled unloaded transformer switching considering residual flux

Uncontrolled energization of unloaded transformers is usually associated with a large inrush current, several times larger than the rated transformer current. This transient can cause unfavourable consequences including reduction of transformer lifecycle, protective relay misoperations, and reduced power quality of the system. Controlled switching can efficiently eliminate these transients when energizing an unloaded transformer. The performances of controlled switching mainly depend on the optimal closing instants, and the magnitude and polarity of residual flux is the main parameters to determine the optimal instants. The theories of inrush transients and the principles of controlled switching are analyzed. A residual flux estimation method is presented to find the optimal closing instants. Experimental investigations are carried out to validate the accuracy of residual flux estimation method and the effectiveness of the controlled switching strategy. The results shows that the proposed switching strategy can reduce the excitation inrush current effectively when the unloaded transformers are energized.