Over Current Problems in Hydro Power Station

We are running one hYDRO plant machine in a certain area (11MW) ,Nowdays machine becomes more stable when excited by Load btn 4MW to 7.5 MW .We can not excited below 4MW due to CaVitation problem.,But when we start to excite machine above 8MW -10.5Mw which was formally adviced to be stable as High Load, machine trips by overcurrent fault but when we shunt down and wait for sometime before we start to excite with low Load(4MW -7.5MW) no probs happen......so could someone tell us technical what exactly hapen to our machine ???.Technical description will be highly appreciated