Softstarter Fail To Start Motor

Hi all,

Recently, I came across a 350kW 415V Low Speed Submissible Pump (high inertia load) with FLC 788A but failed to start using a softstarter
set at 70% Voltage and 3.5 times starting current.
The supply is from a 22/0.415kV 750kVA (ie. 1000A) Transformer with 6% impedance.

The scenerio is like this:

The supply is only 1000A from the authority. The external overcurrent relay was defeated so that it cannot trip the main 1600A ACB during starting. But the ACB electronic relay was set much higher but trip at about 4100A. Parties involved claimed that the supply is insufficient, pump is jammed or the problem is with the softstarter. Now one party wanted to verify the status of the pump by shorting the softstarter and on testing on no-load, failed (pump didn't run) with the current exceeding 14000A and the ACB flashover.

Can anyone say what's the problem? I estimated the starting current for full-load to be about 4 x 788A = 3152A which gives about 3152/1000A x 6% = 18.9% volt drop which I believe is way above manufacturer's voltage tolerance of 10% drop, meaning starting torque from the motor is too low to start the motor. I think a 1500kVA transformer at 6% impedance will work as it gives about 9.46% drop. But I reckon that if the 750kVA transformer was used to start the pump without the softstarter on no-load it should work though the final requirement is still on load. But why did the no-load test also failed?

If a VSD is used, what will be its rating and what will be the minimum rating for the transformer size at 6% impedance? Can anyone explain with calculations. Thanks!