Hi
The megger should be set to 10k and the reading to earth should be 0.ie no leakage.If you are testing between windings then you should have equal readings between windings.Iam not sure that you should be doing this test as you do not sound competent to do this,as the motor is a HV class system.No disrespect intended.
Owing to the nature of the insulation systems used in HT machines such as this, a simple megger test will prove inconclusive. What is required is a Polarization (Or PI) test. The issue is covered by IEEE standard No43 and is a lengthy and involved subject. As Laundry says its best left to the experts. The link will provide you with a lot more information than I can.
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As far as I know, megger readings are more of a reference as to motor condition than an absolute failure indicator. I have seen some motors (low voltage) that have read off the scale and still ran for years. I've learned some new (high voltage) stuff here myself.
Life is what happens while we're making other plans.
Get IEEE STD 43 and study it in detail. That is the safe way to go. Do not follow past experiences not based on scientific bases. I work on a repair shop and the bad experiences of negative results due to mistakes made by technicians with 20+ years of practical experience are countless.