How do you measure insulation resistance between windings for 120KVA alternator?

Use a megger or a insulation tester. Place one lead on any of the phases nd other lead the alternator boy. Observe the indicator. In megger it s good with infinity or it must be minimum 100M OHM, IF IT SHOWs 50 it could be coz of moisture in winding by heating the alternator the insulation resistance value may improve