Propagation of fast PD signals within stator bars, performance and limitations of a high frequency monitoring system

The propagation of PD current pulses along stator bars is studied by injecting fast nanosecond pulses, either in single bar specimens, or in a complete machine. PD pulse propagation along bars is rather different from a pure transmission line. This is partly due to the Roebel structure of bars, constituted of isolated conductors. Attenuation of very high frequencies, reflections and oscillations induce large modifications of amplitudes and shapes of signals. These investigations also point out the performance and limitations of a PD detection system based on the analysis of very high frequency signals. It offers an efficient way to reject noise, but on the other hand it provides only a local diagnostic of insulation (at a distance less than about 1m from sensors).