Comparison of PD amplitudes of stator bars taken with different instruments

PD measurement is a commonly used tool for quality control. For major rewinding project of hydro generator, PD amplitude is a pass/fail criterion for new stator bars. A stator bar is small enough to be considered as a lumped object in the frequency range of IEC 60270 standard. Stator bars usually have a high-PD activity that leads to the superposition of the time response of PD pulses in this frequency range. This paper reports the results of comparative tests performed on two stator bars and one coil with simultaneous measurement using four commercial PD measuring systems. The results show a scattering up to a factor of two. The paper also compares scalar quantities extracted from the PD pattern and suggests an improved definition.