A Novel Phase-Selective Switching Method for Suppressing Magnetizing Inrush Current in Dyn11 Distribution Transformers

The magnetizing inrush current in distribution networks can lead to malfunctions in upstream line protection systems. Traditional strategies are only applicable to the transformer with primary side grounding, leaving the Dyn11-type distribution transformer less studied. This paper proposes an inrush current suppression strategy for Dyn11-type distribution transformers based on residual flux. First, a magnetic circuit model of the Dyn11-type distribution transformer considering hysteresis and saturation characteristics is established, and an analytical expression for the core magnetic flux during closing is derived. On this basis, the influence of residual flux on inrush current is analyzed in depth, leading to the development of an optimal phase sequence and timing for closing operations. Secondly, the proposed strategy is compared with the traditional three-phase simultaneous closing method, and 17 sets of simulations are conducted under five typical residual flux modes. The results demonstrate that the proposed strategy can significantly suppress inrush current, reducing it to below 2% of the rated current under various residual flux conditions. This effectively prevents malfunctions and equipment damage caused by inrush current, thereby improving system reliability.