Partial discharge propagation distortion and implications for feature extraction methods in on-line monitoring

Partial Discharge (PD) pulses occurring in power transformers are influenced by distortion when the pulses propagate through the transformer, away from the site of the discharge activity. Attenuation and dispersion of PD pulse waveforms are potentially significant for some modern on-line partial discharge measurement instruments. These modern instruments use fast sampling digital oscilloscopes to capture PD waveforms and analyze the data using feature extraction algorithms that identify characteristics of the PD pulse shape. Feature extraction methods have shown promise in their ability to separate internal PDs, from interfering noise and external air-corona. In this paper, an experiment is performed which investigates distortion of PD pulses propagated in a transformer winding. A salvaged power transformer, without oil, was used for simulation of an on-line PD monitoring measurement. A pulse generator simulated PDs at different locations within the transformer winding. A digital oscilloscope measured the response of the injected pulses from sensors located at the transformer bushing capacitance taps. The injected pulses were identical in shape and magnitude. Only the location of injection along the transformer winding changed throughout tests. The intent of this experiment was to observe the effects of propagation distortion on PD pulse waveforms, and examine what influence this has for the outcome of a Feature Extraction Algorithm, when the pulses are propagated different distances through the transformer winding.