Motor Failure due to high transient voltage

I have come across one case where motors experienced damages during commissioning. motors are 11kV rated 650kW. motors are switched with vacuum CB. It is speculated that failure is due to high transient voltages while switching on and off.

Any comments

Auto Transformer Motor Starting

I am carrying out motor starting study for one of the existing plant. The existing motors rated 3.3kV 770kW are started with the auto transformer at 67% tap. Auto Transformer is in ckt for 2.5 sec and then bypassed.

According to my calculations the voltage drop at motor terminals is about 87% during motor starting if started direct on line with worst system condition. And the simulation shows motor can be succesfully accelarted.

Hence I am unable to understand why they would have gone for Auto Transformer Starters.

My simulation with Auto transformer starting shows that the drop during first 2.5 sec is reduced compared to direct on line, however after bypassing, motor again draws the same current as Direct on line for some time leading to same voltage drop. Also the accelation time is increased. Then what is the gain in having auto transformer start.

Please shed some light!