I’m currently doing acceptance tests for new 4.16kV generators. I’m using a Megger S1-568 for the insulation resistance tests. Did polarization index tests, ramp voltage tests, etc. The S1-568 also has a “DD” or dielectric discharge option. I did some reading on the DD test, also known as the isothermic relaxation current (IRC) test, and ran it at 5kV. I have some understanding of how it works (the test measures the reabsorption current decay after the completion of an insulation resistance test, the more defects or “electron traps” in the insulation the longer it takes for the decay to occur, this helps to locate failed layers of insulation hidden between good layers of insulation in capacitively graded equipment, e.g., bushings).
What I’ve found online regarding DD/IRC analysis indicates that it’s very useful as a diagnostic test. However, the available literature is very academic and describes successful experiments with DD/IRC analysis, but not NETA-style general protocols. Luckily Megger does have a spiel on how to evaluate DD values (0 - 7, 0 is perfect, 7 is trash). Does anyone have experience with this test?
