IEEE 1434-2014 pdf free download – IEEE Guide for the Measurement of Partial Discharges in AC Electric Machinery.
4.2 Forms of PD pulses The form of PD intrinsic to a given discharge site will depend on the gap length or diameter of the cavity, the gas and pressure within, the nature of the surface at the site between where the discharge takes place, and the statistical time lag. The product of the gap separation and the gas pressure establishes the voltage at which the PD occurs, and if the resulting discharge is of the pulse (spark) type, it also determines, in conjunction with the portion of the overall capacitance discharged, the magnitude of the PD pulse. The statistical time lag represents the time necessary for a free electron to appear within the gap (e.g., because of cosmic or natural radiation), which is required to initiate the electron avalanche and breakdown of the gap once the applied voltage across the gap attains a value equal to the breakdown value. If the appearance of the free electron is delayed, the potential across the gap continues to rise sinusoidally until a free electron finally does appear and the breakdown event is initiated at some value of voltage in excess of the nominal breakdown voltage. The difference between the actual breakdown voltage and the nominal breakdown voltage of the gap is a statistically variant quantity and is commonly referred to as the overvoltage across the gap. The larger the overvoltage, the more intense becomes the space charge field developed adjacent to the cathode; this leads to pronounced secondary electron emission at the cathode caused by photon impact, resulting in rapid (fast rise time) pulses having high amplitudes. The larger the overvoltage, the larger the pulse amplitude and the shorter its rise time (see Bartnikas and Novak [B4]). Assuming that all other parameters controlling the discharge process remain invariant, statistical changes in the time lag are reflected in a train of pulses of varying amplitude and separation in time. At large overvoltages, the detected discharge pulse current consists almost entirely of the electron current component, whereas, at zero or low overvoltage, the more protracted discharge current pulses, evince an ion current component tail due to the slower moving ions. VHF and UHF detectors are particularly suited for the measurement of rapid rise-time pulses, though with the slower rise pulses they omit the ionic tail contribution of the pulse. In order to record the total current of a discharge pulse, much lower bandwidth detectors need to be used, which essentially integrate the pulse charge, thereby yielding a measure of the total apparent charge transfer per pulse.
4.3 Glow and pseudoglow discharges Under certain conditions, the discharge process within the cavities or air gaps may assume a pseudoglow or even a pulseless glow character (see Bartnikas and McMahon [B2]). Pseudoglow discharges exhibit features common to both pulseless glow and pulse-type discharges in that, although they exhibit a visually apparent glow, they, in fact, consist of very minute discharge pulses of long rise time, which evade detection by conventional PD detectors as do true pulseless glow-type discharges. Bridge techniques need to be employed to properly measure and assess the extent of pulseless and pseudoglow discharge activity. Because these discharges are determined by their dissipated energy, the sensitivity of such measurements is low compared with conventional pulse detectors.IEEE 1434 pdf download.