Propagation of Partial Discharge Signals in Stator Windings of Turbine Generators

Abstract

Abstract-We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.

A Review of Online Partial Discharge Measurement of Large Generators

“…A number of workers in this area, e.g., Wilson et al [33], Henriksen et al [41], Gear et al [34], Pemen et al [40], have approached this problem from a variety of experimental and analytical points of view. However, despite increasing the basic understanding of what happens to a PD pulse as it propagates through a stator winding there is, as yet, no definitive theory to quantitatively describe the process.…”

“…The experimental results obtained by Henriksen and Stone indicated that the pulse response was greatest in the phase where the PD occurs, when only the first peak of each pulse from three phase terminals was counted [41]. Further, results obtained by Pemen et al suggested that, if the complete response including two modes of all four terminals could be measured and integrated, the measured apparent charge may not depend very much on the origin of the PD, and a proper calibration can be achieved [40]. However, limited by the test generator with only one bar in each slot, Pemen et al only studied the coupling between end-winding of bars in adjacent slots, but not that between bars in the same slot.…”

“…Unfortunately, no thorough analysis was provided by the authors [39]. Along with the work of Cheng et al, Pemen et al [40] modified the experiment by drilling a small hole in the insulation to directly feed signals to conductors. They verified the two propagation modes of PD signal in stator windings, and showed experimentally that the absence of end-windings prevents the fast mode, and thus denied the result derived from Tarver's experiment on micro-rotating machine in [36].…”

Analysis and localization of abnormal partial discharge activity in generator units

“…Especially the cross-talk between the phases plays an important role. In Reference [17], we found that this cross-talk strongly depends on the location of a discharge. From Figure 2, it can be concluded that the cross-talk also depends on the measurement frequency.…”

Classification and separation of partial discharge signals by means of their auto-correlation function evaluation

“…Since the position of the PD source inside the apparatus is, in general unknown, PD signal shapes are distorted in an unpredictable way depending on the location and the design of the tested object. In particular, a signal shape can show two modes: a "pulse mode" that arrives at the terminal with a delayed risetime and mainly contains the higher frequency components; a "slow mode" composed of an oscillating tail due to partwinding resonances, mismatched impedances and traveling wave reflections that affect the damping coefficients, [55,56]. Consequently, pulses coming from different sites are often characterized by different shapes which can be very useful to separate contributions in PD patterns due to multiple sources.…”