Understanding of the Abnormalities in the Measurements of Insulation Dielectric Losses for High Voltage Rotating Machines

Employing dielectric losses measurement for the insulation of high voltage rotating machines (HVRM) is widespread for most manufacturers of the winding for high voltage rotating machines. Typically, trending the dielectric losses versus test voltage allows us to qualify the extent of the insulation cure and insulation integrity. Losses in the end turns grading system creates significant impact on the measurements and does not allow us to use this test without guarding out losses created by the current through the end turns voltage grading surfaces. The North American standard IEEE 286 [1] and the European standard IEC 60034-27-3 [2] recommend different guarding techniques for accurate dielectric losses measurement. Unfortunately, these techniques have some deficiencies discussed in this article. The influence of higher harmonics, both in the test voltage and test set supplied voltage, creates additional problems in the testing. Employing inner corona protection (ICP) for single-turn coils, such as Roebel bars, also distorts the dielectric losses measurements. All abovementioned factors contribute to distortion in dielectric losses measurements related to the phenomenon of negative tip-up of dielectric losses. Negative dielectric losses tip-up caused by vast delaminations observed at the higher test voltages are sometimes mistaken with negative dielectric losses tip-up related to distorted measured values. The present work attempts to summarize the potential problems with measurements of dielectric losses using different guarding techniques and different testing devices.