Doable, but you've got some limitations. The drive will accept a 0-10V signal. You can scale that to CT's 0-5V signal.
Your setpoint will be full speed at 175A. (You say 0-150A, then you finished with 175A, I'll go with 175A). Any current above 175A on the grinder and you want the motor to slow down. If the motor needs to ramp to 0Hz, that's ok, it'll set there until the grinder amps come down. Be sure to adjust your acceleration to not slam to the grinder as my guess is the grinder current will come down dramatically.
Your feedback, the CT, is 0-400A for the 0-5V. That's not going to give you much resolution since you're somewhat defining your top acceptable limit as 200A. With your setpoint at 175A, that only really gives you .31V (6%or of control range. The closer your feedback sensor matches your actual process, in this case 0-200A, the better. I think you can still achieve better results with the CT and PID than stopping and starting with the relay, but it'll be highly dependent on setting up your acceleration and decal to match your process feedrate.
I'd use trim control, set P458 to 5 (0-10V). That way you can set your speed reference for full speed, then set your PID setpoint for the V level that corresponds to 175A. Anything above that and the PID will 'trim' down the speed. Whereever you're setting your running speed (ie, keypad, preselected freq, etc.) set that to A471. For the feedback, to use just your 0-5V CT, set A472 to 0. Set t091(low ref) to 175A in the 0-5V scale: 43% and set t092 (high ref) to 400A in the 0-5V scale: 100%.
