Detection of insulation defects on generator bars and coils using an instrumented impact test

Abstract

The methodology used by the industry to identify defects in the insulation system of generator bars and coils is based on a subjective acoustic detection test, commonly known as tap test. These defects are in the form of delamination between the insulation layers, decohesion between the insulation and copper or cavities. However, the tap test can be adapted using well-known scientific principles in modal analysis of structures and in high frequency acoustic emission detection. It is therefore possible to propose an instrumented methodology currently followed to carry out this test more objectively. This paper presents a rapid review of modal analysis principles useful for defects detection on structure. Also, based on this knowledge, an impact test method is proposed for generator stator bars and coils. This methodology offers a stricter and more defined procedure than the usual tap test as well as more objective criteria for localizing the defects. It required the use of an instrumented impact hammer and a microphone. In parallel, another test is proposed using an acoustic camera to obtain a visual localisation of potential defect location based on high frequency emission. This paper presents the results of both tests perform on a single hydrogenerator stator coil and correlate to tap test and microscope examination. The acoustic camera test results are well correlated with delamination detected with the tap test and confirmed with microscope examination. More investigation is in progress to correlate insulation defects with the instrumented impact test results.