Unnormal Voltage Drop during Start Up Motor with 450 kW and 770 A current

hi all dears. 

we have a  AF compressor with a 450 kw electromotor with soft starter WEG. when we start the motor till to ramp up  catches 2200 A for 20 second from netwok. we have atransformer with 20Kv and 2000KVA specification in our network .we connected this transformer with 6*1*240mm^2 cables (distance is less than 20 m) per each phase to a disjunctore and in asame way to main distribution panel (MDB) . we connect main distribution panel to compressor with 3*1*150 mm^2 per each phase . in the ramp up we have 40 volt drop till to MDB and  40 volt drop in our motor input .its strange !  i chechked my cable design and every connections .there was ok what do you think about  this too high voltage drop ?? .

best regards 

Minimum ampacity for the motor conductors = 1.25x770=963A. The ampacity of 3*1*150 mm^2 per each phase = 3x335 =1005 A which is fine since exceeds the minimum. For a 450 kW (380~400V) motor, starting current is likely around 5850A or lower since it uses a soft starter. This causes a voltage drop of around 11.7 V for the said conductors with a resulting voltage at the terminals around 368.3V. This accounts for a 3.1% voltage drop, the NEC recommends that the branch circuit VD should not exceed 3%. The resulting 3.1% voltage drop does not even account for other motors or loads that may simultaneously start with the 450kW motor and the normal load current at the MDB. At full load, the 2000 kVA transformer can have as much as 3038A at the mains this entails a .5~.7% voltage drop at the MDB mains (depends on the power factor). For 6*1*240mm^2 per phase the ampacity is = 6×375 = 2250A which is even less than the rated transformer current at the 380V transformer secondary. Because the transformer secondary cables are undersized (minimum feeder ampacity = 1.25x3038 =3799A) and this may cause significant voltage drop at the mains due to combined factors such as high load current, simultaneous motor starting, and conductor heating (not to mention harmonics present in the system).