Hi subhpoto,
looking at the figures i think you will find you have a low resistance on phase c,which using ohms law (for simplicity ),is whats causing a higher current on that phase ,either that or you have a high resistance on the other 2 phases,the current on the neutral is the indicator ,it wasnt there when the thing was built ,and i think im correct in saying its only there because of the inballance between phases,
I would turn it off ,and check all connections at the motor end,if you find 2 loose ones you could be very lucky if there all nice and tight ,it might be a long night,have a check the control end as well but i think its at the motor ,or rather "in" the motor,
so if all connections are nice and tight ,dissconect them all at the motor end and carry out an insulation resistance test on each winding ,if one is lower than the rest ,you have a break down in the insulation to earth ,as idicated by the control relay
problem is im not sure if even at 1000volts you will find it ,when the normal operating voltage is 11000,maybee you nead special kit to detect it,but i bet ya a pound to a pinch of **** fella ,thats what it is.
to rectify it if it is you might need a rewind,as it wont get better,it will get worse ,i would monitor it running and i think it will trip more ,often ,could **** up the relay as well but thats a risk you will need to weigh up in your own head ,but dont worry about that just yet,if your in charge you might be pumped by then.
Shakey might have a different approch mind,i know hes got good experiance of motors and gennys,but i think by the time you have checked the connections he might reply.and dont panic ,and remember Electricians dont have problems ,we have solutions,
Please let me know what it turns out to be ,
all the very best in the whole wide and wonderfull world