Generator Insulation Testing

[jplinn]

I am interested in three tests used on the insulation systems; both stator and rotor windings. These test are the Poalrization Index, Dielectric Discharge and Step Voltage. I have used PI testing on my generators forever and am considering adding both Dielectric Discharge and Step Voltage to our maintenance program. The few times I have perfromed these 2 test they have produced wildly different results for sister units and non-related units. Especially, between different rotor windings.

I am looking for a detailed discussion of how the results are to be evaluated and how results differ over time and between stator/rotor windings. In addition, are the results affected (and how) by surface contamination, temperature or humidity? Are the reults trendable?

Any information or references would be greatly appreciated.

 

[Addy71]

I have been using these tests for some years now. I find the DD test to be more or less in line with PI. In some slow speed motors, I have been getting high DD values for years & the winding seems to be fine. Tan delta values are good, took the motor apart, did not see anything. Found contaminated motors with DD too, where DD improved later.

Which step voltage are you referring to? Some companies test the winding at five different voltages & plot IR vs. voltage. I never found this useful.

Others apply DC voltages in steps to cross the operating voltage, measuring the leakage current at each point. If the knee point is above the operating voltage, the machine gets accepted. This does give some trendable results.

Regards,

Aditya