Improvements in the Identification of Defects on the Semiconducting Slot Coating of Stator Bars Using Partial Discharges

The present work describes how the detection of surface defects located in the slot portion of high-voltage bars (or coils) can be improved with the use of partial discharge (PD) tests. In spite of all care, surface defects capable of damaging the semiconducting slot coating of bars can eventually be created during their manufacturing process or later on, during their installation at site. Although the most commonly used methods to search for surface defects are based upon visual inspections, this work will explore a possible improvement in such methodologies by using electrical methods, such as the acquisition and discrimination of PD pulses. The results shown here represent a first progress report of a larger ongoing project that aims to develop a test method suitable for the quality control of bars and coils mass production. To address the topic in laboratory scale, a series of artificially created defects were produced and systematically tested with different parameters. A number of test conditions was employed to determine under which circumstances the test can be sensitive enough to permit a clear detection and discrimination of the surface damages. It was found that although the mere recording of PD amplitudes is a simpler and straightforward method of detection, it is the analysis of the complete PD patterns that provides the most useful information to identify the defects.