HT Motor with GRR Control Tripping on Over Current

How long has this system been in service, and what are your planned maintenance procedures...? ....because it sounds like you need a rebuild...and if it's a recent install, it sounds like something is undersized, or poorly installed...

https://cache.industry.siemens.com/dl/files/582/1109582/att_22063/v1/s7200_system_manual_en-US.pdf

Has this ever worked correctly & reliably?

When the rotor is short circuited, full speed, how does the actual current compare to the rated current?

Was your controller designed to operate at part speeds, such as continuous energization of the resistance elements? As they heat up, the resistance can change, shifting the breakdown torque of the motor.

Are the grids covered in cement dust, preventing the cooling designed into the original installation?

Can you verify that all of your steps are closed transition? Open circuit reduces your pullout torque significantly.

If the slip rings open circuit, the slip becomes very high, causing large stator currents.

Have you checked the resistance of each step? Are the resistances balanced for each phase?

Your fan air temperature has a very large effect on the load shown to the motor at any given speed. If the air cools off during the run, the rotor could start slipping in the air gap, drawing huge currents for apparently no reason.

What is the condition of your inlet vanes/damper, if you have leaves that are loose, unlinked, or plated over with caked in dust, these could shift position on your speed run-up, causing load upsets.

Keep in mind that the torque capability of your motor is directly linked to the secondary resistance. If the fan torque, changing at the square of the speed, exceeds the torque available from the motor, then you will get overcurrent.

This might help. It describes the same problem with the same equipment.

https://circuitdigest.com/case-study/ht-motor-tripping-with-overcurrent-fault-during-grr-control#:~:text=GRR%20stands%20for%20Grid%20Rotor,few%20combination%20of%20power%20contactors.

After I posted my message, I realized that the Step 7 was probably being used as a PI or PID controller for the air fan, so wanted to add one more question, did someone recently adjust the Integral on the controller?

The power drawn by any <...fan...> is proportional to something like the cube of the tip speed. So changing from <...600 RPM...> to <...990 RPM...> will involve a factor of (tap, tap tap, press equals) about 4.5 in the current, assuming constant supply voltage.

The electrical installation needs to be assessed as to whether or not it is up to the task, which includes the motor rating, the cables feeding it, the motor protection arrangements, and the upstream feeder as well as the <...GRR...> panel.

If in doubt, consult a qualified local Electrician.

The <...problem...> would seem to be a reluctance to do this, for reason(s) unknown.