PD monitoring is generally present in all stator windings on the rotating machines and is one of the common failure problems. There is no red line on the actual limit of a new machine that was set by IEC (standard says this should be decided between the end user and manufacturer) however rule of the thumb is – if PD doubles in 6 months is a clear sign that the machine is failing.
Measuring PD offline has no big significance on Generators (no mechanical forces which tend to move the components and no thermal expansion).