The fast and slow signal components of partial discharges in SF6 measurements of the electron and ion contributions to PD-signal

For many years the IEC 270 partial discharge (PD) measurement technique has been the international standard. To improve the ratio of PD signal to thermal amplifier noise and to reduce sensitivity to external noise, “ultra”-wideband techniques have been proposed as an alternative to IEC 270. This paper will show that in the absence of external noise the assumption of improved S/N is false and based on a general misconception of the PD signal characteristics. Partial discharge signals in gas-insulated systems (GIS) are usually said to be very fast signals in the nanosecond range, created by fast electrons. We measured not only a fast but also a slow signal component for a typical discharge source under normal GIS operating conditions. These signal components can be attributed to fast moving electrons and slow moving positive and negative ions. We found that the slow part of the PD pulse is 2-14 times larger than the electronic part (measured at 100 kHz, in IEC 270), and also that the ratio of these two parts depends strongly on pressure and geometry. Since the IEC 270 method integrates fast and slow signal components as well as the smaller consecutive pulses we find that the disadvantage of thermal noise is compensated for by the resulting increase in PD signal. Therefore the conventional technique can still be recommended in quality insurance testing of GIS