The prediction of fault currents in a large multiwinding reactor transformer

The fault currents occurring in power transformers are determined by the leakage reactances within the windings. Where the transformers are used in a phase-shifting mode, there is additional coupling between phases which influences the fault current. This work describes the modelling of the self and mutual inductances in a 90 MVA autotransformer with a tertiary winding, on the assumption that the airgap formed by the transformer window dictates the reluctance of the leakage flux paths. Recordings made during a short-circuit between two phases of the tertiary winding show a remarkably close comparison with the predicted waveforms.