ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
IMPACT OF DUTY CYCLE ON WIRE TO PLANE PARTIAL
DICHARGE UNDER CONDITIONS OF SQUARE WAVE VOLTAGE
ENERGISATION
F. Alrumayan1* and I. Cotton2
King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Saudi Arabia
2
The University of Manchester, PO Box 88, Manchester, M60 1QD, UK
*Email: rumayan@kfshrc.edu.sa
1
Abstract: This paper presents results from partial discharge testing that has been carried out using
square wave voltages of varying duty cycles. The voltage has been applied to wire whose
insulation thickness is not thick enough to prevent partial discharge, something of current concern
to the aviation industry where higher voltages are being used and wires can be subject to chafing.
The results show that there is a relationship between the duty cycle of the bipolar square waveform
and the PD inception voltage, the repetition rate of discharge and the average charge level.
Measurements taken at the 90% and 10% duty cycles present a particularly high PD inception
voltage value in comparison to those at lower duty cycles.
1.
increased, the partial discharge inception voltage fell.
In contrast, they reported little change in the inception
voltage of partial discharge within an air filled void for
a fundamental frequency between 6Hz and 1kHz. A
dramatic increase in the number of partial discharge
pulses per second did however occur. Cavallini et al.
[6] provide a theoretical analysis of the impact of
frequency on partial discharges taking place within
voids of solid insulation. These are then contrasted
with test results that show a significant effect of
frequency on the phase distribution of partial discharge
pulses. The main conclusion of the work was that
systems operating at lower frequencies may be subject
to lower numbers of partial discharges but that these
would be larger in magnitude.
INTRODUCTION
Insulation systems are one of the most important parts
of high voltage systems. However, if a defect is present
in the insulation, this can lead to a partial discharge
which can gradually degrade the insulation to failure.
The period before complete breakdown caused by
ageing will vary with the intensity of the PD, the
number of discharges and other environmental
parameters which can also accelerate breakdown [1, 2].
Partial discharges indicate weak points in electrical
insulation and are not usually tolerated.
Many studies have been published on the effect such
discharges have on insulation and measurement
techniques when sinusoidal and dc voltages are applied
to insulation. However, the effect of non-standard high
frequency voltages has not received this attention.
Given the increasing amounts of equipment that now
function at frequencies other than 50/60Hz, it is
important to be able to measure partial discharge when
the equipment is being tested using the supply
frequency for which it is intended.
Not all authors agree on the variation of inception
voltage and/or discharge magnitude as a function of
frequency (although generally test objects vary).
Plessow and Pfeiffer [7] have carried out work
examining the impact of frequency on the partial
discharge inception voltage of various materials
including a ceramic, a polyester and a phenolic resin.
They report increases in partial discharge inception
voltages as a function of frequency. They also report
higher discharge magnitudes as a function of frequency
[7]. Meanwhile, work by Wilder and Hebner states that
no variation in inception voltage is reported as a
function of frequency [8]. They also showed a different
in behaviour between samples that had been previously
exposed to high voltage and those tested as new.
The objective of this paper is to describe the response
of one insulation system energized by square wave
voltages with different duty cycles. The duty cycle is as
defined by IEC-61934 [3] as the ratio of the on portion
of the pulse width to the total time. According to [4],
changing the ratio between ON-time to zero-time (OFF
state) leads to changing the PDIV. This change is
associated with the effect of space charge deposition on
the insulators.
A number of authors have examined the impact of
squarewave voltages on partial discharge but such
work has usually been focused on enameled wire [9,
10]. One exception is [8] that looked at material
samples of PVC and PE. Discharge inception voltages
are shown to reduce with temperature in [9] but there is
no comparison with standard test frequencies within
this work. An analysis of the impact of space charge on
partial discharge in enameled wires is presented in
[10]. The work shows a difference in partial discharge
inception voltages depending on whether a bipolar or
Many pieces of literature exist that examine the impact
of voltage frequency on breakdown and partial
discharge inception in insulation systems. A number of
these are useful in further understanding of the topics
discussed within this paper. Kurihara et al. [5] discuss
the effect of a higher frequency voltage (created by
superimposing a higher frequency signal onto a lower
frequency fundamental) on an air filled void. As the
frequency of the higher frequency component
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
monopolar waveform is applied, the difference being a
result of space charge accumulation. The work
suggests that the partial discharge inception voltage is
largely unchanged as a function of frequency. An
increase in the partial discharge inception voltage of
PVC samples was attributed to a decline in permittivity
as frequency increases in the tests carried out in [11].
2.
cause damage to the cable insulation and could cause it
to fail in the longer term.
When such a system is placed under test, the partial
discharge patterns shown in Figure 2 result. These
show a typical example of the discharge distribution
along the voltage waveforms for both sine wave and
square wave energisation [13]. In the case of the square
wave, discharges can be seen at the transition between
half cycles. The absence of any change in voltage
between these transitions prevents further discharges
from taking place in that half cycle. This is because the
constant voltage between transitions allows no
charging through the capacitance of the surrounding
insulation, but forces recharging to take place through
the large insulation resistance, a process that takes too
long in comparison to the cycle time [14]. In contrast,
multiple pulses can be seen on the rising edges of the
positive and negative half cycles of the sine wave.
WIRE TO PLANE DISCHARGE
PD Amplitude (V)
In a system using unscreened cables to carry power
from an aircraft generator to the loads, discharge could
occur in the air gaps between the conductors or
between an individual conductor and the duct. To carry
out a theoretical analysis of the safe operating voltage
of the cable system as limited by the presence of the air
gap, a simple uniform field system can be modelled
where the two electrodes are separated by insulation of
a set thickness and an air gap.
5
5
4
4
3
3
2
2
PD Amplitude (V)
ISBN 978-0-620-44584-9
1
0
-1
1
0
-1
-2
-2
-3
-3
-4
-4
-5
50
100
150
200
Phase
250
300
350
-5
50
100
150
200
Phase
250
300
350
Figure 2: Wire to plane discharge patterns for square
wave and sine wave voltage energisation [13]
The rest of this paper will be devoted to examining the
change in the partial discharge performance under
squarewave energisation.
Figure 1: Example of discharge between an aerospace
cable and a ground plane
3.
EXPERIMENTAL ARRANGEMENT
In the experiments reported in this paper, a square
wave voltage was applied to a test object left in a low
pressure environment of 100 mbar ± 3 mbar (this
correlates to an altitude of around 50,000ft). The test
object consisted of a wire with an insulation thickness
of 0.3 mm and a conductor radius of 0.3 mm that was
placed above a plane electrode in a manner similar to
Figure 1. The wire is a type that is designed for use in
modern aircraft power systems. The wire was in
contact with the plane electrode for an approximate
distance of 5 cm, the ends of the wires then rose off the
ground plane. Discharge was expected in the region
where the wire lifted off the plane. In an aircraft this
form of discharge could take place between two wires,
where a wire moves against a grounded object (a
ground plane or a duct) or when wires move into a
connector.
In the dielectric system made up of solid insulation and
an airgap, the system voltage is divided between the
two components according to their relative dimensions
and the relative permittivity of the solid insulation. The
larger the gap, the higher the proportion of the system
voltage that will exist across it. As the air gap is made
smaller, when a cable moves towards the duct for
example, the voltage across that gap will reduce while
the voltage across the solid insulation will increase.
The higher the relative permittivity of the insulation,
the higher the proportion of the system voltage dropped
across the airgap. In most aerospace systems, air gaps
will always vary in size along a cable run and
conductors move closer together in the proximity to
connectors or lie flat on ducts at mid-span. Therefore,
the exact separations cannot be guaranteed
A choice of insulation thickness must therefore be
made to ensure that such discharge does not occur. If
this is not the case, when a discharge takes place in the
air gap, the entire system voltage will be placed across
the cable insulation. As long as that can withstand the
voltage, a disruptive discharge will not occur.
However, the partial discharges in the air gap will
The circuit diagram of the test arrangement is shown in
Figure 3. A power amplifier rated at 2.5 kW and 20 kV
peak supplied the test voltage. It was controlled using a
20 V pk-pk signal generator. Experimental data was
collected using a fast oscilloscope with results then
being processed by Matlab. A commercial PD detector
was not used as while they are available from various
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
manufacturers, they only work for sinewave and
generally not for frequencies above 400 Hz. The
configuration shown in the figure represents a balanced
circuit technique where two test objects (Cto1 and
Cto2) are tested, each one acting as the coupling
capacitor for the other. If the test object impedances are
perfectly matched, the equal flow of power supply
current through each test object can be removed using
an LCR impedance made up of differential windings.
This will significantly reduce noise levels if operated
properly. However, the test objects need to be clearly
matched or additional tuning impedances need to be
provided in the circuit and this is not always
straightforward. This type of experimental arrangement
was used in the testing described to minimise noise
levels. It would not normally be required if tests were
to be carried out with 50 Hz/60 Hz only. The
maximum noise level in the testing carried out was 12
pC. This was achieved even under squarewave voltage
energisation.
inception and extinction voltages were measured over a
frequency range of 50 Hz to 1 kHz. The inception
voltage measured was the voltage at which continuous
PD activity was first recorded, the extinction voltage
was the voltage at which it then stopped. At each
frequency, partial discharge data was then collected by
applying a voltage 10 % above the inception voltage.
An average of 100 cycles of data was collected for
each frequency measurement.
4.
EXPERIMENTAL TEST RESULTS AND
DISCUSSION
4.1.
PDIV and PDEV
Figure 5 (a and b) show, respectively, the level of
inception and extinction voltage for the wire to plane
sample as a function of squarewave frequency and duty
cycle. The graph illustrates a reasonable level of
variation of both voltages regardless of the frequency.
It should be noted that the unit of voltage chosen is
peak to peak. Fabiani et al. [14] point out that PDIV
mainly depends on the peak-to-peak value of the
applied voltage.
PDIV (kVpk-pk)
2.2
2
1.8
1.6
1.4
1.2
Figure 3: Circuit diagram as used in experiments
0
The duty cycle of the voltage applied to the test object
was changed by varying the ratio of the ON-time to the
total period. The values used were 10%, 30%, 50%,
75%, and 90%. Equation 1 represents the ratio of the
ON-time to the total period of a bipolar square wave.
An example of a 10% duty cycle of bipolar waveform
is shown in Figure 4.
400
600
800
Frequency (Hz)
1000
(a) PDIV
1.7
PDEV (kVpk-pk)
t+ ve
DCbipolar =
t− ve + t + ve
200
(1)
1.6
1.5
1.4
1.3
1.2
0
200
400
600
800
1000
Frequency (Hz)
(b)PDEV
Bipolar_90%
Bipolar_30%
Bipolar_75%
Bipolar_10%
Bipolar_50%
Figure 5: The effect of PDIV and PDEV.
Of importance in this paper is the level of change in the
measured parameters with respect to duty cycle values.
Accordingly, the graph was redrawn as function of
duty cycle, as shown in Figure 6. The graph
demonstrates that the inception voltage increased when
the duty cycle changed toward the extremes (at 10%
and 90%), leading to a U-shape as displayed in the
figure. The increase in inception voltage is higher with
a 90% duty cycle.
Figure 4: Bipolar voltage waveform
The rise time of the voltage was not fixed and tended
to vary from 80 µs at low frequencies (around 50Hz)
and reached 110 µs at higher frequencies (around
1kHz). In the experiments, the partial discharge
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
more significant and consistent as shown in Figure 8.
Fabiani et al. [14] state that as frequency increases,
space charge accumulation in insulation defects
decreases. The figure agrees with this finding.
PDIV (kVpk-pk)
2. 2
2. 1
2
1. 9
1. 8
1. 7
1380
1. 6
1180
1. 5
20
30
40
50
60
70
80
90
q (pC)
10
Duty Cycle (%)
980
780
580
380
180
Figure 6: PDIV of bipolar waveform as function of
duty cycles
0
10
20
30
40
50
60
70
80
90
100
Duty Cycle (%)
(a) Mean charge, q
This finding is useful when it comes to the
consideration of how to test equipment that operates
under square waves with a duty cycle other than 50%.
As the 50% duty cycle gives the lowest PD inception
voltage, this is the worst case test and would therefore
be suitable for use in equipment qualification. the
previous results show that there is a relationship
between the duty cycle of the square waveform and
PDIV. The measurements taken at 90% and 10% duty
cycle lead to increased PDIV values.
4010
3510
N (1/s)
3010
2510
2010
1510
1010
510
10
0
20
40
60
80
100
Duty Cycle (%)
(b) Repetition rate, N
4.2.
Impact of duty cycle on charge magnitude
The behaviour of PD charge magnitude is now
discussed as a function of frequency and duty cycle.
The experimental results are illustrated in Figure 7.
The graph clearly shows a dramatic reduction of charge
magnitude as frequency increases. It is thought that the
lower time for charge deposition which exists may
contribute in reducing the charge magnitude at high
frequency.
Figure 8: Mean amplitude of charge as a function of
frequency
In terms of the application of these results, an
important factor can be concluded, namely that testing
of equipment intended for operation at different duty
cycle can be achieved by using 50 % duty cycle. This
will provide a safe testing level.
1400
q (pC)
1200
5.
1000
CONCLUSIONS
800
Partial discharge activity has been investigated using
square wave applied voltages of different duty cycle
levels. Results show that there is a relationship
between the duty cycle of the square waveform and the
PDIV, the repetition rate of discharge and the average
charge level. Those measurements taken at 90% and
10% present a high PDIV value no matter what the
type of defect.
600
400
200
50
250
450
650
850
1050
Frequency (Hz)
Bipolar_90%
Bipolar_75%
Bipolar_50%
Bipolar_30%
Bipolar_10%
Figure 7: Mean amplitude of charge as a function of
frequency
A reduction in the repetition rate and the average
charge magnitude has been noticed in data collected at
the 10 and 90% duty cycles for the wire to plane
samples.
The previous graph was further analyzed by studying
the behaviour of charge magnitude and repetition rate
at different duty cycle levels. This is illustrated
graphically in Figure 8a and 8b. It was noticed that at a
low duty cycle (particularly 10%) and a high duty
cycle (particularly 90%), the charge magnitudes are
slightly lower than those at 50% duty cycles for some
frequency measurements and act independently at other
values. The reductions in repetition rate however, are
It has also been concluded that, any acceptance tests
for equipment that will operate under PWM with
varying duty cycle can be done with a 50% duty cycle
testing voltage as a worst case for PDIV.
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
of Dielectric Materials, 2003. Proceedings of the
7th International Conference on, 2003.
REFERENCES
[1] J. P. Steiner, "Partial discharge. IV. Commercial
PD testing," IEEE Electrical Insulation Magazine,
vol. 7, pp. 20, 1991.
[12] A. Nelms, "Electrical Discharge in The More
Electric Aircraft Power System", PhD Thesis, The
University of Manchester, 2007.
[2] S. A. Boggs, "Partial discharge. II. Detection
sensitivity," IEEE Electrical Insulation Magazine,
vol. 6, pp. 35, 1990.
[13] F. Alrumayan, I. Cotton, and A. Nelms “Partial
Discharge Testing of Equipment for Aerospace
Electrical Systems with Variable Frequency
Sinewaves and Squarewaves’ IEEE Transaction on
Aerospace and Electronic systems, 2008.
(Accepted and waiting for publication).
[3] I.E.C 61934, "Electrical measurement of partial
discharge under short rise time and repetitive
voltage impulses," British Standard, 2006.
[14] D. Fabiani, G. C. Montanari, A. Cavallini, and G.
Mazzanti, "Relation between space charge
accumulation and partial discharge activity in
enameled wires under PWM-like voltage
waveforms," Dielectrics and Electrical Insulation,
IEEE Transactions on, vol. 11, pp. 393-405, June
2004.
[4] N. Foulon, J. P. Lucas, G. Barre, R. A. M. R.
Mailfert, and J. A. E. J. Enon, "Investigation of the
failure mechanism of insulation subjected to
repetitive fast voltage surges", Conference on
Electrical Manufacturing & Coil Winding, pp.
401-406, 1997.
[5] T. Kurihara, S. Tsuru, K. Imasaka, J. Suehiro, and
M. Hara, "PD characteristics in an air-filled void at
room temperature under superimposed sinusoidal
voltages," Dielectrics and Electrical Insulation,
IEEE Transactions on, vol. 8, pp. 269, 2001.
[15] P. H. F. Morshuis, R. Bodega, M. Lazzaroni, and
F. J. Wester, "Partial Discharge detection using
oscillating voltage at different frequencies",
Proceedings of the 19th IEEE Instrumentation and
Measurement Technology Conference on, vol. 1,
pp.431-435, 2002.
[6] A. Cavallini and G. C. Montanari, "Effect of
supply voltage frequency on testing of insulation
system," Dielectrics and Electrical Insulation,
IEEE Transactions on, vol. 13, pp. 111, 2006.
[7] R. Plessow and W. Pfeiffer, "Influence of the
frequency on the partial discharge inception
voltage," presented at Electrical Insulation and
Dielectric Phenomena, 1994. IEEE 1994 Annual
Report., Conference on, 1994.
[8] A. T. Wilder and R. Hebner, "Frequency
dependence of partial discharge initiation voltages
with embedded electrodes," presented at Power
Modulator Symposium, 2004 and 2004 HighVoltage Workshop. Conference Record of the
Twenty-Sixth International, 2004.
[9] M. Kaufhold, G. Borner, M. Eberhardt, and J.
Speck, "Failure mechanism of the interturn
insulation of low voltage electric machines fed by
pulse-controlled inverters," Electrical Insulation
Magazine, IEEE, vol. 12, pp. 9-16, 1996.
[10] D. Fabiani, G. C. Montanari, A. Cavallini, and G.
Mazzanti, "Relation between space charge
accumulation and partial discharge activity in
enameled wires under PWM-like voltage
waveforms," Dielectrics and Electrical Insulation,
IEEE Transactions on, vol. 11, pp. 393, 2004.
[11] K. Wada, K. Tsuji, and H. Muto, "Partial
discharge inception voltage for two insulating
materials (PVC and PE) under inverter surge
voltage," presented at Properties and Applications
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