Page 52 - Basic PD Theory

PD Investigations

7.2.1 Phase-to-phase PD
Phase to Phase discharges are described in Sections 1.4 and 2.4.7.4. Electrical field distribution controls partial discharge.
Several PD sources may be found that are dependent upon phase-to-phase electrical stress. Often the presence of these
discharge sources can be related to Contamination, Phase to phase discharges, poor contact between the semicon and the field-
graded area or, as in the case of PD in the groundwall insulation, poorly impregnated windings. Although most of the
mentioned phenomena give rise to surface discharges, as opposed to discharges internal to the insulation, these phenomena may

C

30º phase- Yes Phase-to-
shifted? phase PD

eventually lead to a phase-to-phase or phase-to-ground failure. It should be mentioned that discharge activity on the ring bus or
between phase leads is also phase-to-phase dependent discharge activity.
Since the fields in the endwinding area are phase-to-phase voltage
dependent and the electrical fields in the slot area are phase-to-ground
voltage, it is often possible to distinguish between discharge activities
occurring in the slot part versus the endwinding part of the winding
using Phase Resolved Plots [see Section 4.3]. By creating an LPD for
each phase of a given machine, with the reference phase-to-ground
voltage shifted 120° between phases based on phase rotation, the
discharge activity will be presented in the time domain (Figure 36).
Phase-to-phase voltage dependent PD pulses will be ± 30° phase shifted
from the positions of phase-to-ground PD, and centred at 15°, 75°, 195°
and 255° positions along the reference sine wave, based on the phase
rotation of the machine. Phase-to-phase activity is only confirmed by
these pulse positions if the corresponding pulse patterns in the two
phases at the same time are approximately equal in magnitudes and also
have opposite polarities, as shown in Figure 47. For example, PD in one
phase will be centred on 75° while the PD in the other phase must be
centred on 195° of the AC cycle.
Although these pulses tend to be more erratic, it is sometimes possible
to distinguish these pulses from others by observing their location with
reference to the phase-to-ground voltage. Since this activity is frequently
isolated to a specific area of the machine, the PD pattern may reveal
which phases are involved. This is only possible if the two affected Note the opposite polarity of the clusters in the two
phases both have detectable endwinding activity and their respective 30° phases.
phase shifts correspond to what would be expected according to the
machine rotation (Figure 47). Sometimes this PD may be occurring Figure 47: Phase-to-phase Discharges
between a top and bottom coil in a slot, and visual verification of the
activity may be difficult.

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