Hybrid winding configuration in high-current injection transformers based on EMC issues

In designing high-current injection transformers (CITs), many factors need to be considered for its compatibility with the internal and external propagation and generation of disturbances within electromagnetic environment, i.e. the magnetic leakage field must be reduced to a level that will make the operation of CIT system safe. On the other hand, inrush current (transient current) may inadvertently affect the operation of the protective relay system and necessitate strengthening of the mechanical structure of the CIT. The inrush current and leakage field do not favour each other, but they are considered as two aspects of EMC considerations. The prior art has only addressed asymmetric winding configuration (S-P-S-P) for inrush current reduction. For comparative purposes, the authors propose a novel hybrid approach that is a composition of auxiliary windings and S-P-S-P structures based on an active shielding concept for EMC improvement. To design an optimal choice of the distributive ratios of S-P-S-P of the main transformer windings, the formulae of the leakage inductance are calculated from the structural parameters of the transformer. Hence, as a case study, there comes the need for advanced numerical techniques for correctly calculating the leakage flux distributions and EMC considerations within a 25 kA CIT enclosure.