6.6kV Motor Earth Fault

Good evening all. I have a 6.6kV, 580kW, Westinghouse / Teco, single speed motor, tripping on earth fault. The windings are connected with an internal star point, not terminal box links. The protection relay is a GE Multilin Motor Manager 5. Earth Fault trip setting is 6A for 0.2 Seconds. At the motor, with the feed cables disconnected, PI results of 7 are achieved at 5kV DC. However, when attempting to commence an AC test, between the windings & earth, As soon as I try to apply the voltage, I have immediate leakage current which eventually trips the set where I preset the trip level to. 16 to 20 mA. I am using a 30kV capable tester. I cannot get any voltage applied with this set, & my target was initially 5kV for 1 minute. To fill in more detail, the motor has just been extremely expensively rewound, due to an earth fault. A 10kV Insulation Resistance test was carried out with a different Megger, by the repair contractor, & we experienced good results of several Gig Ohms after 1 minute. A second flash test was carried out with a different 5kV AC test set, by the repair contractor, & we experienced repeatable trips at 3.6kV. So, the protection relay agrees with 2 AC test sets, that there is an earth fault. And, the 2 IR / PI testers ( Meggers ) agree there's no fault. I know I am comparing AC to DC testing, but why are the Meggers showing nothing of concern. They should surely indicate a low reading in the presence of an earth fault ? When the motor was fitted yesterday, I did test run it for DOR purposes, uncoupled, & it accelerated up to speed, but i stopped it only a few seconds later. The relay was happy at this time. No fault evident. Operations ran the motor up last night & it immediately tripped on protection. This has "done my head in", & the motor repair contractors are equally bemused. The motor is heading back to the repair shop, where it will be Baker tested. Can anyone offer any advice ? Thanks in advance. CTB