Large transient neutral currents during energisation of 3-phase transformers

A magnetising inrush phenomenon characterised by a large exponentially decaying direct component of current in the neutral has been recorded in a number of 275 kV transformers. Laboratory investigations have shown that the effect can be produced in 3-limb transformers with certain combinations of nonsimultaneous energisation of the three phases. In particular, for the first two limbs to have their windings energised the sum of the fluxes in those limbs must be greater than the flux in the third limb when the third winding is energised. The initial magnitude of the d.c. component is proportional to the difference between these fluxes.