Medium voltage captive transformer specification

[Ants]

Good day gentlemen! I am currently investigation the possibilty of upgrading a 5 MW 6600V 3phase motor to about 6.2MW (not yet standard size). My problem is that it feeds from a captive transformer (is this term used worldwide?) rated 8MVA and 33kV / 6.6 KV , 4.07% impedance at 33kV. What is the biggest motor I can put on this transformer? Currently the transformer saturates during a 40% load start-up, and the start-up current is restricted to 4.7 times the transformer FLC of 700A at 6.6kV. Will I be able to succesfully operate the system if the motor is bigger, but the start-up load is only 10%? What is the killer concerns?
Assume for the moment that cable and feeder sizes are adequate.

 

[bigamp]

Two main things with captive transformers is the short term current rating and the reactive voltage drop caused by the transformer impedance.

Ideally, you would do a motor starting study and work out the extent and the duration of the motor starting current. Then, consult the transformer manufacturer and seek his opinion as to the suitibility of the transformer for its proposed new starting duty. The suitability of your existing transformer will depend on the characteristics of your proposed new motor. Low starting current/high torque motor designs help and you are doing yourself a big favour by being able to restrict your start-up load to 10%. The lower the starting load, the more rapid the run-up to full speed. It may be that the transformer will be OK for a time but it may not last for too many starts, only the manufacturer can confirm this (assuming he still has records and design data for the transformer).

During motor starting you will get a voltage drop across the transformer (mostly reactive because motor starting current is at a very low power factor) and also in the cables to the motor. If the voltage drop is excessive and the driven equipment is too highly loaded the voltage at the motor terminals will not be sufficient to allow the motor to develop enough torque to run up to full speed and you will get a "rotating stall" (reduction in torque is proportional to the square of the reduction in voltage).

Regards