Transformer inrush current mitigation concept for hybrid transformers

Large inrush currents can occur when power transformers are connected to the grid. In the past, a wide range of countermeasures has been developed whereof the application of power electronic converters recently received more attention. With hybrid transformers combining a conventional transformer with a power electronic converter, the elimination of inrush currents can be realized without considerable additional hardware effort. For such hybrid transformers, a novel mitigation procedure is proposed, which is based on the injection of a synchronous core flux before the transformer is connected to the grid. In addition, dedicated demagnetization strategies applicable for grid connected converters with filter elements are presented. The effectiveness of the proposed procedure is verified by comprehensive simulations and a comparison to a conventional method.