Earth fault with Meduim voltage motor

[emadshaaban]

dear all
The mentioned motor 2600kw 6.6kv was tripped by earth fault (50N/51N),after disassembling the motor found the insulation resistance 4MΩ ,no sign of burn or moisture inside winding ,all winding all the physically checks shows no sign of damage .we tried to clean winding by air but we get the same results PI=0.65.
What is your advices for diagnostic and solving?????

 

[gattie]

Agree with rhatcher.
This type of earth fault is typical where a coil has blown out at the end or middle of
a stator slot.
The earth fault blows itself open & clear so subsequent tests do not reveal any poor
megger readings.
Look for hole in winding isulation, copper spatter & carbon footprint.