Ferroresonance by open-phase on transformer with delta winding and grounded neutral

It was clarified that the ferroresonance overvoltage phenomenon is caused by an open phase in a power transformer with delta windings and a directly grounded neutral. An analysis of this phenomenon was performed using a 1050 kV transformer, and also a model test using three 3.3 kV transformers. The cause of magnetic saturation of the iron core, which is the cause of ferroresonance, was the appearance of a voltage in the transformer corresponding to the phase that was not connected to the power source, via the delta windings. Previously, the above ferroresonance phenomenon in a transformer was not very well known.