The effects of thermal, electrical, mechanical and environmental stresses acting upon the insulation windings will eventually result in Partial Discharges (PD). Certainly you should consider that, the higher the voltage the higher risk for PD on motor stator insulations. However PD has been found on machines operating as low as 2 kV. The lowest voltage for application of a PD detection system ($$) will be depending upon the importance of a particular machine and the cost implied for an unexpected winding failure.
At 3kv, you won't detect pd until the winding is very near failure (if at all) because pd doesn't occur until the insulation is almost totally compromised. The diagnostic value of the test is very small.
At 13.2kv, you will often detect pd long long before any failure might occur (in some but not all cases the pd could go for decades without causing problems). There is a lot more diagnostic value. It helps us to prioritize our refurbishments and rewinds.
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