Core saturation failures while motor starting

Runaway power oscillations and core saturation: A motor starting with soft starter of a Tyre Industry's Banbury Mixture Motor of rating 400kw, 660v, 3ph, 50hz. The recorded RMS graph view, lasting totally about 6secs, shows high oscillations at the starting instant with the 3phae system voltage and motor currents, which lasted for a longer time than the usual time of a few milliseconds. Thereafter the system voltage and motor currents did not droop back to their rated values, which happens with normal motor starting. But remained constant in the next 5 to 6secs and then abruptly dropped down to their rated values, which depicts saturation with the motor core.

This Squirrel Case Induction Motor SQIM is always loaded with its charge of rubber mixtures from its Banbury.  Due to its high starting current, and even after providing with a soft starter; it's in-addition provided with a standalone 1250kva, 11kv/660v isolation transformer at the immediate upstream of the motor. Still, while switching it ON, the motor has had a history of switch on failures - stator winding insulation fails- burn/flash/blast. In-additions even the sturdy rotor bar twists and/or dislodges. Its root cause is diagnosed as the motor core saturation while starting, as shown in the recorded motor starting inrush graphs.