Standard or Typical Settings for Undervoltage Relays 3

Maybe you need to add some PT ratio information.

PT ratios are
33KV / 100V
6.6KV / 100V
0.4KV / 100V

The undervoltage relays are to protect the motors, so at what voltage level will the motors run into problems?

As the voltage decreases, the motors will draw more current, so at what voltage level will the motors either stall, or be in over current?

That is the level around where you want to base your voltage setting.

The setting of 0.8Vn for 100ms seems quite sensitive. If these relays are intended as loss of phase protection, the ideal settings might be faster and lower than settings intended for 3 phase undervoltage protection.

Do you have generators connected at any of the busbars - 6.6kV or 400V?
UV relays at the incomer level are for supply changeover purpose at 6.6kV and 400V levels. At 33kV, it is for disconnecting from the grid, presuming the power supply from the grid authority is taken at 33kV level.
Can you check that / clarify!

@RRaghunath
Thanks for the reply. Yes, at 400V we have a generator connected to the bus in case of power outage or transformer failures.
in fact, there are two voltage monitoring relays at 400V level, one with (27, 47 & 59) functions sensing busbar voltage which kills the control circuit voltage of all MCC feeders when activated. the other with (27) function sensing secondary voltage of 6.6/0.4KV transformer which first trips the incoming breaker from transformer supply and with a short delay closes the breaker from generator supply. And here is the problem, as you can see when these relays trip, the entire plant shuts down and it takes about 1 hour to restart it. so in my opinion a very sensitive setting is not optimal.
Both of these voltage monitoring relays have a time range of (0.1s ~ 10s) and voltage range of (0.7Vn ~ 1.3Vn).
I think (0.8Vn, 2s) would be a good setting for both. what is your opinion? and is there any standard or document that I can refer to and read more?
And for the second part, also yes. the power supply from grid is taken at 33KV level.

The undervoltage element which trips the 400V incomer is also giving start command to the backup generator. So, in my opinion any delay is not good as starting of backup generator gets delayed in case of genuine loss of power supplies.
Voltage threshold setting of 70% should help as the voltage is never expected to touch that low level during motor starting etc. However, the voltage can go down to 70% or lower in case of grid faults or faults in your power system.
To avoid unnecessary tripping of incomer for a fault elsewhere, 2s delay seems appropriate in order to allow fault to be cleared by protection operation in the affected feeder.