Scanning Individual Stator Bars and Coils with an Antenna to Detect Localized Partial Discharges

Partial discharge (PD) measurements have been carried out for years on generators, individual bars and form-wound coils. Even if the determination of quantitative criteria for what is considered normal and abnormal PD still represents a challenge in the industry, phase resolved partial discharge (PRPD) pattern recognition has been successfully used to recognize the nature of PD activity [1-3]. In the present work, conventional PD measurements using a capacitive coupler were compared with measurements made using an antenna to detect localized PD sites on individual bars and coils. The antenna was used to map localized PD sites along the entire length of the straight portion. The measurements were carried out on bars and coils of four different designs. The results revealed that the two measurement techniques give complementary information. It was found that the antenna had a limited sensitivity to PD occurring in micro-voids occluded within the groundwall insulation. For most of the bars and coils tested, the antenna did not detect any signal, even if there was always PD activity measured with the standard capacitive coupler. However, some of the specimens tested with the antenna revealed localized PD sites with distinctive PRPD patterns, suggesting that some anomalies were present at these sites. This information can thereafter be used to determine where to perform dissection of bars and coils.