Abstract
Condition assessment of high voltage equipment based on
partial discharge measurements is often performed after a
voltage pre-conditioning period. The aim of this paper is to
present results from experimental examinations of time variance
of partial discharge activity and to propose physical explanations
of the phenomena observed. Experiments were performed on
laboratory made 3 mm thick discs of generator bar insulation,
consisting of mica and glass fiber reinforced epoxy with a
0.5 mm thick cylindrical void surfaces of 10 mm in diameter.
The effect of conducting and insulating void surfaces was
examined using copper tape as upper and lower electrodes of the
voids. All objects were tested by 12 one-minute long AC voltage
frequency sweeps at frequencies from 50 Hz to 0.1 Hz
distributed in time from start of the experiment, after initial onehour
constant 50 Hz voltage application and during object shortcircuiting
for 20 h. The main result shows that in case of
insulating voids the apparent charges vanished after the onehour
constant 50 Hz voltage application. After a grounding
period of 5 minutes, the charge magnitudes slowly increased
with time until reaching steady state after about 4-8 hours. Test
objects with conductive void surfaces showed such reduction in
case of PD testing at 0.1 Hz only. At voltage frequencies above
10 Hz the measured PD magnitudes were found to be nearly
constant, close to the expected high theoretical value. This
indicate that PD by-products strongly affected the void
resistivity and thereby affect the PD activity of insulating voids.
These by-products are temporary and disappear with time.
partial discharge measurements is often performed after a
voltage pre-conditioning period. The aim of this paper is to
present results from experimental examinations of time variance
of partial discharge activity and to propose physical explanations
of the phenomena observed. Experiments were performed on
laboratory made 3 mm thick discs of generator bar insulation,
consisting of mica and glass fiber reinforced epoxy with a
0.5 mm thick cylindrical void surfaces of 10 mm in diameter.
The effect of conducting and insulating void surfaces was
examined using copper tape as upper and lower electrodes of the
voids. All objects were tested by 12 one-minute long AC voltage
frequency sweeps at frequencies from 50 Hz to 0.1 Hz
distributed in time from start of the experiment, after initial onehour
constant 50 Hz voltage application and during object shortcircuiting
for 20 h. The main result shows that in case of
insulating voids the apparent charges vanished after the onehour
constant 50 Hz voltage application. After a grounding
period of 5 minutes, the charge magnitudes slowly increased
with time until reaching steady state after about 4-8 hours. Test
objects with conductive void surfaces showed such reduction in
case of PD testing at 0.1 Hz only. At voltage frequencies above
10 Hz the measured PD magnitudes were found to be nearly
constant, close to the expected high theoretical value. This
indicate that PD by-products strongly affected the void
resistivity and thereby affect the PD activity of insulating voids.
These by-products are temporary and disappear with time.
