Numerical calculation of inrush current in single-phase transformers

Digital techniques for modelling the inrush current associated with the energisation of single-phase transformers are described. The digital results are compared to site measurements on a 6 MVA transformer. The techniques developed were used to investigate the factors such as remanent flux, point on wave switching and system and transformer resistance. The technique developed has particular application where the level of inrush is important, not only for the first cycle but sometimes even a few seconds later. The variation of the harmonic content of the current wave over a long period is also described.