22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Two modes of optical emission in partial discharges
S.A. Shcherbanev1, I.U. Nadinov1, P. Auvray1, S.M. Starikovskaia1, L.G. Herrmann2 and S. Pancheshnyi2
1
Laboratory of Plasma Physics (CNRS, Ecole Polytechnique, Sorbonne Universities, UPMC Univ. Paris 06, University
Paris-Sud), Palaiseau, France
2
ABB Corporate Research, Segelhofstrasse 1K, CH-5405 Baden-Dättwil, Switzerland
Abstract: Aging of polymer insulators is a complex physical and chemical process taking
typically tens of years in real conditions of high-voltage engineering devices. One of the
critical aging mechanisms can be modelled experimentally in laboratory conditions using
partial discharges. Emission optical spectroscopy was used to characterize the accelerated
aging of the voids in epoxy test samples including an air-filled cavity aged under 50 Hz AC
voltage at room temperature.
Keywords: partial discharge, emission spectroscopy
1. Introduction
One of the most serious technical problems arising in
high-voltage engineering is aging of dielectric materials, a
process which is typically characterized by a long time
scale – tens of years. According to the “standard”
scenario, the aging proceeds via creation of micro-cavities
(voids) in a dielectric material, with following electrical
discharge originating in the cavity and leading finally to
the breakdown of the dielectric. An artificial aging of
dielectric materials at the late stage before breakdown can
be achieved via applying high voltage to a sample,
containing already a small cavity (void). In this case, the
life time of the sample can be as small as a few weeks that
becomes suitable for laboratory studies. Typically, 50 Hz
sinusoidal AC signal is used in the experiments. The
discharge, igniting in the dielectric in this case, is called a
partial discharge (PD).
The physics of PDs is not well understood yet [1]. The
discharge is often considered as a barrier discharge, DBD
[2] as far as the electrodes and gas in the cavity are
separated by a dielectric layer. The difference between
the PDs and the DBDs is that (i) for PDs, the thickness of
the dielectric layer is much higher than the size of the
void, that is of the gas gap; (ii) the ratio “surface-tovolume” in partial discharge is much higher than in
DBDs, and (iii) encompassed in a bulk material, electric
current can flow around the void. These features cause
physical peculiarities of the PDs, as, for example, a strong
interaction between surface of the dielectric and gas in the
void, or limitations, at certain values of gas pressure, to
the electron multiplication on the typical size of the voids.
No study of the PDs spectra were reported at the
moment, and no systematic study of time-resolved
emission were made. The aim of the present work is to
study spectrum and time-resolved optical emission from
the partial discharge; to find, by means of spectroscopic
measurements, some specific parameter(s) which can be
used as indicator(s) for the aging process.
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2. Experimental setup ant methods
A general scheme of the experimental setup is
presented in Fig. 1. The sinusoidal high voltage (50 Hz,
up to 21 kV) is applied to the electrodes of the sample.
Two different methods to analyze the optical emission
from the discharge in the 300 - 800 nm spectral range
were used. Time integrated spectra were measured by
spectrometer Aston SP 2500 (Princeton Instruments) with
1200 l/mm grating connected to the PI-Max 4 1024i
ICCD camera (Princeton Instruments). The light was
collected by optical cable LG-455-020. Time-resolved
emission signal, selected by the narrow band wavelength
filter, was recorded by the photomultiplier H6610
(Hamamatsu), connected to the 50 Ohm input of the
LeCroy WaveRunner 600MHz oscilloscope. Triggering
generator was used to synchronize the time of the
collecting of the emission spectra with the voltage
waveform.
Fig. 1. Scheme of the experimental setup: 1) sample,
2) photomultiplier, 3) oscilloscope, 4) triggering
generator, 5) high-voltage generator, 6) optical cable,
7) adapter of optical cable, 8) spectrometer, 9) ICCD
camera, 10) computer.
The partial discharge was initiated in a nearly spherical
gas cavity encapsulated into a specially designed sample,
schematically presented in Fig. 2. The sample is made of
unfilled epoxy. Two cylindrical electrodes with rounded
1
edges are mounted into the sample. The electric field
between the electrodes is close to the field of a parallel‐
plate capacitor, the edge effects are minimized. The
distance between the end plates of the electrodes is equal
to 2.5 ‐ 3 mm. The cavity (void), 1 - 2 mm in diameter, is
situated in the gap between the electrodes at
approximately equal distance from each end plate.
Preliminary cured epoxy cube with a single air void is
immersed into the bulk epoxy between the electrodes.
Fig. 2. Scheme of the sample with a single void.
Four similar samples with a void diameter between 1.3
and 2.0 mm were used in the experiments. Typically, a
few hours of operating under the high voltage at
amplitude 21 kV were enough to ignite the discharge in
the void without any additional ionization source. After
the ignition, the samples were tested either up to their
breakdown or until PD activity stops.
The radiation intensity of partial discharges was low; all
the experiments were made in the dark room. Significant
number of accumulations of the signal was used to obtain
a proper signal-to-noise ratio when working with ICCD
camera. When working with PMT, a voltage close to the
limit value (2200 V at limit voltage of 2500 V) was used
for PMT power supply. The photo of the discharge is
presented in Fig. 3. A diffuse character of the optical
emission coming from the void is clearly seen.
emission is due to excitation of molecular nitrogen. The
emission system is known as the second positive emission
system of molecular nitrogen. Finally, a range of
400 - 440 nm was selected for day-by-day spectra
recording.
It was found that two distinctive time-resolved emission
signals co-exist in the system (see Fig. 4). We will
further designate them as signal A and signal B. The A
signals are small in amplitude, typically a few tens of mV,
and very short in amplitude, less than 5 ns. The
amplitude of the B signals is high, typically a few volts,
and their duration can be up to a few tens of nanoseconds.
Signals A do not change during the sample life time,
while the signals B modify significantly. Signals A are
repeated with a very high frequency, tens-hundreds of
spikes are observed during one high-voltage period.
Signals B is a rare event, one time per a few periods, with
pronounced decrease of the frequency of the appearance
on a long-scale period.
The spectra obtained
experimentally result of integrating of the signals A, the
input of the signals B is too low because of the low
frequency of the signals.
At the highest voltage
amplitude, signals A appear in the places of maximum
voltage derivative; as voltage amplitude decreases, the
signals A move to the maximum of the voltage signal.
Signals B are scattered and do not demonstrate a
correlation with the high voltage signal.
Fig. 4. Two types of optical signals detected in the
system (see the text for details).
Fig. 3. Photo of the emission from the partial
discharge.
To obtain the discharge spectra, the emission was
collected during 1 s by accumulating 1000 shots of ICCD
camera 1 ms each. Wavelength range of 300 - 800 nm
was tested; it was found that (i) the transmission of the
epoxy is close to zero below 350 nm, increasing gradually
to a constant value at about 550 nm; and (ii) the main
2
3. Emission spectra and aging
Integrated in time spectra in the wavelength range of
400 - 440 nm were taken nearly every day during the
discharge operation. Life time of the samples was
between 70 and 300 hours. Analysis of the emission
intensity vs cumulative work time allowed finding out the
regularities in behaviour of 4 different samples. Fig. 5
presents a summary plot of the emission intensity as a
function of time; the amplitude of the emission signal at
427 nm was taken as a reference value. The emission
intensity was firstly found monotonically decreasing with
a half-decay time of approximately 150 hours. This
probably was due to the decrease of frequency of PD
pulses of the type A with aging time. At later phases,
approximately after 150 - 190 hours, two different
scenarios of the discharge development were observed:
For two of the samples, the discharge has stopped
without breaking up the sample, no short-circuit was
observed. No evidences of discharge activity, including
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6
5
2+, 0-3
6
2+, 1-5
2+, 0-4
2+, 2-6
a)
2+, 3-8
2+, 2-7
4
2+, 1-6
2
40 hours
2+, 4-10
2+ system: N2(C3Πu-B3Πg)
0
375
6
b)
400
425
450
475
500
+
+
2 , 2-6
2 , 0-3
4
525
200 hours
2+, 1-5
2+, 0-4
2
0
375
6
2+ system: N2(C3Πu-B3Πg)
400
425
450
c)
4
475
500
525
224 hours
CO(A), 0-0
CO(A), 1-1
CO(A), 0-1
2
CO(A): CO Angstrom
0
375
400
425
450
475
500
525
Wavelength, nm
4
Fig. 6. Evolution of emission spectra: a) initial
spectrum, b) spectrum 2 days before, and c) 1 day
before breakdown. Molecular bands are marked in
the figure. Sample 1.
3
2
1
PD stops
change of the spectra; then breakdown
0
0
24
48
72
96
120
144
168
192
216
240
Cumulative work time (hour)
Fig. 5. Evolution of emission in 400 - 440 nm
spectral range with aging time.
For one of the samples, the breakdown of the sample
occurs. The short-circuit event was registered at the end
of the operation of the sample. The taken photos clearly
demonstrate a presence of two channels of different
diameters, connecting the void with the electrodes. The
void changes the colour: instead of the transparent cavity,
the void and the channels of deep brown colour are
monitored after the breakdown.
The changes are
localized in the vicinity of the void. Significant (500%)
emission intensity increase in the considered spectral
range was detected two days before the failure with the
following intensity drop to a value of about 1/20 of the
initial emission.
This behaviour correlates with the behaviour of the
spectra.
For the principal descending part of the
amplitude of emission (0 - 180 hours in Fig. 5) the
spectrum corresponds mainly to a spectrum of the second
positive system of molecular nitrogen. This fact is
illustrated by Fig. 6a. If the discharge in void stops by a
breakdown of the void, the intensity of emission increases
dramatically two days before the failure but no change of
the spectrum is observed (Fig. 6b). The last day, severe
difference in emission composition is observed. Nitrogen
emission disappears completely. Simultaneously, three
transitions of CO (Ångstrom system) and one undefined
molecular band (apparently, the band of the molecule
containing some composition of C-H-O-N atoms) become
dominating over all the investigated, 440 - 520 nm,
spectral range (Fig. 6c).
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Intensity (a.u.)
λ = 405.9 nm
Sample 1 (breakdown)
Sample 2 (PD stop)
Sample 3 (PD stop)
4. Time-resolved emission
Time-resolved emission of the signal of type B was
used to evaluate gas pressure in the void during the
sample aging. The emission of the second positive
system of molecular nitrogen, in a semi-logarithmic scale,
is plotted in Fig. 7 as a function of time in nanoseconds;
the day of measurements is used as a parameter. It is seen
from the figure that (i) the decay is close to a monoexponential for majority of the data; (ii) the decay time
increases with aging time.
0.0
-0.5
ln(I)
Emission intensity (ab. un.)
7
Intensity (a.u.)
8
Intensity (a.u.)
light emission, were detected. No visible damage of
dielectric was observed. The last registered spectral
intensity was about 1/3 of the initial emission in the virgin
void. Likely, surface conductivity increases [3] that leads
to full or partial shielding of the electric field in the void
is the most probable explanation for the discharge stop. It
was not possible to re-ignite the sample by the constant
heating to 70oC during a few days or/and by keeping the
sample during a few days under the maximum voltage
21 kV.
-1.0
1st day
2nd day
3rd day
4th day
5th day
6th day
7th day
8th day
9th day
10th day
-1.5
-2.0
0.0
2.5x10-9 5.0x10-9 7.5x10-9 1.0x10-8 1.3x10-8 1.5x10-8
Time, s
Fig. 7. Decay of the PMT signal for different days of
the operation of the sample. Sample 4.
The following assumptions were made when treating
the emission data:
1) The main excitation process is direct excitation of the
N 2 (C3Π u ) state by electron impact, and the excitation
time scale is much shorter than the quenching decay;
2) The main depopulation processes are spontaneous
emission and collisional quenching;
3) The main quenchers are N 2 and O 2 molecules.
3
In this case, if the decay of emission is long comparing to
excitation in the short-time discharge, the reverse efficient
radiative time can be written as
𝟏
𝟏
𝑵
𝑶
=
+ 𝒌𝒒 𝟐 [𝐍𝟐 ] + 𝒌𝒒 𝟐 [𝐎𝟐 ]
𝝉𝟎
𝝉
In this case, at known radiative lifetime τ, quenching rate
constants k O2 , k N2 and known gas composition, a gas
pressure can be calculated. It was further assumed that
gas temperature in the void is equal to the ambient
temperature. Gas pressure was calculated in two different
assumptions: (a) the gas inside the void is molecular
nitrogen; (b) the gas inside the void is a dry air
(N 2 :O 2 =4:1 mixture). The results are given by Fig. 8.
Although the absolute values of pressure are different, the
decay by a factor of 1.6 is observed during 250 hours.
This result is in a good correlation with the direct
measurements of pressure in partial discharges [4, 5].
Still, additional measurements are necessary to make a
conclusion about percentage of molecular oxygen in the
void during a stable phase with gradual decrease of
emission intensity.
0.7
N2
dry air (N2-O2 80:20)
0.6
Pressure (bar)
0.5
0.4
0.3
0.2
the second positive system of molecular nitrogen was
observed in near UV and visible region (350 - 500 nm).
In the case of dielectric breakdown, a sharp (500%)
increase of N 2 emission was observed 2 days before
breakdown; the last day a spectrum of CO and some other
C-N-O-H containing molecules was observed instead of
the spectrum of nitrogen. This fact suggests that it is
possible, analyzing the broad emission spectra, to predict
a breakdown at least a few hours before it happens.
Measurements of the emission of the 0 - 4 and 1 - 5
vibrational transitions of the 2+ system of N 2 at
(430 ± 10) nm with nanosecond resolution demonstrated
the coexisting of two well distinguished signals with
different duration and amplitude:
A. Fairly small optical signals of less than 2 - 3 ns in
duration and a few tens mV in amplitude were
observed continuously in the vicinity of the voltage
maxima and minima. The observed emission spectra
are mainly determined by these small optical pulses.
Based on their short excitation and decay times, these
pulses can be short surface discharges inside the void.
B. The optical signals of about 40 ns in duration and up to
a few volts in amplitude were observed stochastically
during the period with a frequency, small comparing to
the signals of the type A. The amplitude of these
signals was found to be decreasing with the aging time.
These pulses are probably originated in the bulk of
gaseous voids.
It was shown that the modification of the emission
decay time during the operation period can be used for
analysis of the gas pressure in the void. Pressure decay
by a factor of 1.6 was observed during 250 hours.
0.1
0.0
0
50
100
150
200
250
Cumulative work time (hour)
Fig. 8. Evaluated gas pressure (at room temperature)
variation with aging time. Sample 4.
5. Сonclusions
Emission optical spectroscopy was used as a technique
to study the partial discharges in a dielectric. Four
different samples were subjected for aging process which
has taken between 70 and 300 hours before the stop of the
discharge operation at voltage amplitude about 20 kV.
Two scenarios of the discharge failure were observed:
stop of the discharge activity without break of the
dielectric and without a short circuit event, or the sharp
local disruption of the dielectric with formation of two
conductive channels connecting the void with the
electrodes and producing a short circuit.
It was shown that, with the help of emission
spectroscopy, it is possible to follow, in non-intrusive
manner, the changes of gas mixture composition during
the aging process. For both scenarios, a stable initial
phase with gradual decrease of emission intensity from
the discharge was typical for a few hundreds of hours of
non-stop discharge operation. At this stage, a spectrum of
4
6. References
[1] L. Niemeyer. IEEE Trans. Dielectr. Electr. Insul.,
2, 510-528 (1995)
[2] U. Kogelschatz. Plasma Chem. Plasma Process.,
23, 1-46 (2003)
[3] C. Hudon, R. Bartnikas and M.R. Wertheimer. in:
IEEE Int. Symp. on Electrical Insulation. 153-155
(1990).
[4] A.C. Gjaerde. IEEE Trans. Dielectr. Electr. Insul.,
4, 674-680 (1997)
[5] A.C. Gjaerde. IEEE Trans. Dielectr. Electr. Lnsul.,
5, 463-465 (1998)
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