Partial discharge spectral response to variations in the supply
voltage frequency
Cuthbert Nyamupangedengu and Ian R Jandrell
University of the Witwatersrand, Johannesburg
School of Electrical and Information Engineering,
Private Bag 3, Wits 2050,
Johannesburg, South Africa
ABSTRACT
Partial discharge (PD) spectral response to variations in the supply voltage frequency
was experimentally investigated through laboratory-based tests. The PD spectral
content of each defect type responded uniquely to variations in the sinusoidal supply
voltage frequency in the range 20 to 400 Hz. The findings are interpreted using the
theory of space charge dynamics in PD mechanisms. Prospective diagnostic applications
of the findings include PD recognition using supply voltage frequency sweeps.
Knowledge on supply voltage frequency dependency of partial discharges also helps in
comparing PD diagnostic test results obtained at different test voltage frequencies.
Index Terms — Partial discharges, supply voltage frequency, frequency spectra.
1 INTRODUCTION
IN partial discharge (PD) tests there is flexibility in the
choice of the supply voltage frequency (SVF) depending on
the type of equipment being tested. Questions arise concerning
the extent to which the changes in test voltage frequency affect
PD mechanisms. These questions have sustained efforts in
research into understanding the relationship between PD
characteristics and supply voltage frequency.
Frequencies other than power frequency (50/60 Hz) in PD
detection are used in order to reduce power ratings of the test
equipment. A test voltage frequency of 50/60 Hz may not be
technically and economically viable for testing largely
capacitive equipment such as power cables. In such cases
lower frequencies are desirable. Similarly, higher frequencies
are more suitable for inductive equipment such as transformers
and generators [1]. Consequently, technologies such as 0.1 Hz
and dumped alternating voltage (DAC) have been developed
and commercialised [2]. The ringing frequencies of DAC can
range from a few hundred to thousands of Hz depending on the
length of the cable under test. The flexibility in the choice of
frequency of PD test voltage poses a question on the
comparability or trending of PD results that are obtained using
test voltages of different frequencies [2-4].
In laboratory based research, higher voltages and
frequencies are usually used as accelerating agents in PD
ageing tests [5-7]. This is done in order to increase the rate at
which an insulation defect is exposed to PD induced
degradation. The rate of insulation degradation in service
under power frequency can then be inferred. The technique of
accelerated ageing of PD defects through increasing the test
Manuscript received on X Month 2010, in final form XX Month 2010.
voltage frequency is based on the assumption that the change
in the frequency only alters the rate of ageing mechanisms and
not the nature of the mechanisms [7]. There is however no
evidence in literature that supports this assumption.
Discharges can occur in insulation of equipment that is
exposed to power supplies polluted by harmonics. Whether
and how the harmonics aggravate the PD induced insulation
degradation is a question that is yet to be fully explored [3].
The effectiveness of PD diagnosis technologies in condition
based maintenance practice depends on how well the PD tests
results are interpreted. Correct interpretation of the results
requires sound understanding of the nature of PD mechanisms.
The relationships between test voltage characteristics and PD
mechanisms have been the subject of research by many
researchers as evident in the literature review presented in
Section 2. Aspects that are not yet fully explored and
understood are highlighted, and these motivated the research
work presented in this paper where the influence of the SVF
on PD frequency spectra was investigated.
The rest of the paper is structured as follows; Section 3
presents the experimental investigation procedure. The results
are presented, analysed and discussed in Section 4. The
prospective application and the conclusion are Sections 5 and
6 respectively.
2 INFLUENCE OF SUPPLY VOLTAGE
FREQUENCY ON PD CHARACTERISTICS: a
literature review
The frequency ranges considered by various researchers
working on PD characteristics dependency on SVF varies
significantly from case to case. A summary of some of the
important aspects of the literature on PD dependency on SVF
is presented in Table 1.
Other literature related to the effect of SVF on insulation
includes that by Mason who investigated the influence of SVF
on PD inception voltage (PDIV) and short time breakdown
voltage of insulation [8]. Mason concluded that in frequency
ranges up to a few kHz, the effect on PDIV was too small to
explain the observed reduction of short time breakdown
voltage of insulation. Localised heating due to repetitive PDs
was suggested as the main cause of the reduction in short time
breakdown voltage of insulation. Gockenbach & Hauschild [9]
also reported results on work involving investigation of SVF
effect on the insulation withstand voltage where they
concluded that in the frequency range 20 Hz to 300 Hz, the
insulation experienced the same electric stress effects.
It is notable that there has been a considerable mix of
agreements and controversies in the conclusions on the various
research results. An example is that generally most of the
researchers except for Miller & Black [10] agree that the
minimal PD magnitude is independent of the SVF while the
maximal magnitude decreases with frequency. Forssen & Edin
Table 1:
presented a remarkable clarification on the question of PD
magnitude where they concluded that the dependency of PD
magnitude on supply voltage frequency was influenced by the
cavity size [11]. In small cavities (diameters equal to or less
than 1.5 mm) the PD magnitudes (both maximal and minimal)
were independent of the SVF. In larger cavities however
(diameters more than 1.5 mm) the PD magnitudes decreased
with increase in the SVF. This was attributed to the
possibilities of influence of multiple and simultaneous
occurrences of PDs in larger cavities. Another common
agreement among researchers is on how the PD inception
voltage increased and repetition rate decreased with increase in
SVF for cavity PDs.
An example of an area of controversy or disagreement is on
the extent of influence of SVF on PD phase-resolved patterns.
As an example while conclusions from Bodega et al [4] point
towards minimal influence, Cavallin et al [3] as well as
Forssen & Edin [11] reported significant changes, confirming
that work is still needed in this regard.
A scrutiny of most PD parameters studied, as summarised in
the fourth column of Table 1, shows that most of the workers
Summary of literature highlights on the subject of PD dependency on supply voltage characteristics.
Researchers
Frequency
range
considered
Type of defects
PD
characteristics
studied
Key findings
Miller & Black [10]
0.1 to 50 Hz
2-8 mm diameter
Cavities in epoxy as
well as polyethylene
insulation (also
surface discharges
in cable and stator
samples)
PD inception
voltage (PDIV)
and PD magnitude
- PD characteristics in the frequency range were generally
independent of the supply voltage frequency.
- Experimentation with polyethylene was more difficult than with
epoxy because of significant induced insulation condition changes in
the polyethylene.
- Surface PDs increased with increase in frequency of supply
voltage.
Radu et al [5]
1-30 kHz
0.5 mm gap between
dielectric in Helium
at atmospheric
pressure
Glow and Pseudo
– glow pulse
height, repetition
rate, width and
rise time
- Pulse height remained quasi constant.
- Repetition rate reduced with increase in supply voltage frequency.
- Pulse width and rise time reduced with increase in the supply
voltage frequency.
Wester et al [2];
Bodega et al [4];
Bodega et al [12]
50 Hz; 0.1
Hz; DAC
(260, 520 and
930 Hz)
2-3 mm cavities in
cast polyester resin
PD inception
delay,
PD magnitude and
phase patterns
- At frequencies below 50 Hz there were equal chances that the PD
characteristics could either be similar or different to those at 50 Hz.
- Above 200 Hz PD maximal magnitude decreased with increase in
supply voltage frequency.
- Overall shape of the phase resolved patterns (PRPDP) were
generally independent of the supply voltage frequency variations.
Cavallini &
Montanari [3];
Hauschild [13]
0.1; 20; 50 &
300 Hz
2 mm diameter
spherical cavities in
polyethylene,
surface and corona
discharges
PD maximal
magnitude
PDIV
PD repetition rate
PRPDP
- Maximal magnitude decreased with frequency.
- PDIV increased with increase in supply voltage frequency.
- PRPD patterns changed.
Forssen & Edin [11]
0.01 to 100
Hz
1.5 – 10 mm
diameter disc
shaped cavities in
polycarbonate
insulation
PD maximum and
minimum
magnitude, PD
repetition rate and
PDIV
- PDIV increased with voltage.
- Average & maximal PD magnitude depended on cavity diameter
such that no change occurred for 1.5 mm cavity but for bigger
cavities decreased with increase in supply voltage frequency.
- No change in minimal PD magnitude.
- Maximum PD magnitude decreased with increase in supply voltage
frequency.
- PRPD patterns changed.
have been focusing mainly on conventional PD characteristics
such as PDIV, extinction voltage (PDEV), magnitude and
phase-resolved PD patterns (PRPD).These parameters are
conventionally used to characterise partial discharge signals as
guided by popular standards such as the IEC60270 [14]. Of
late, however, there has been growing interest in the
unconventional characterisation of PDs such as in the high
frequency (HF) and ultra high frequency (UHF) techniques
[15]. In such cases interest is in the information contained in
PD pulse shape (in the time domain) or spectral characteristics
(in the frequency domain). The current knowledge on PD
characteristics dependency on supply voltage type should
therefore be extended to PD pulse shape and frequency
content. It is in this context that the work in this paper focused
on PD spectral characteristics.
Furthermore from the third column in Table 1 it is evident
that most of the studies on the influence of SVF on PD
characteristics have been conducted using cavity defects. This
trend could be attributed to an assumption that cavities are
among the most common defects found in solid insulation as
stated by Gutfleisch & Niemeyer [16] as well as Cavallini &
Montanari [3]. It is however known that surface discharges and
corona are also a concern in insulation systems although to a
lesser extent. An extension of knowledge on the relationship
between SVF and PDs to other types of discharges such as
surface discharges and corona could reveal valuable
knowledge that enriches the effectiveness of PD diagnosis
technology. An experiment was therefore conceived in which
equal attention was given to cavity, surface and corona
discharges as explained in the following section.
3 THE EXPERIMENTAL SETUP
The experimental setup used for investigating the PD
spectral content dependency on supply voltage frequency is
depicted in Figure 1. The test specimens were designed to give
undistorted frequency response up to 1 GHz and this was
verified through measurement using a network analyser. The
PD signal detection was through a ring guard electrode and
measuring electrode setup. The measuring electrode disc was
connected to the grounded guard ring electrode using 3 x 150
Ω resistors in a star formation. This ensured impedance
matching with the 50 Ω signal cable.
The test cells showing the type and dimensions (not drawn
to scale) of the artificial PD defects are schematically shown in
Figure 2. The defect dimensions were chosen such that in all
cases the PDIV was the same, approximately 6 kV at 50 Hz.
All the test samples were preconditioned by continuous
stressing at 7 kV for an hour. This was to avoid taking
measurements in the first hour of voltage application where
PD behavior, particularly in voids, could be more influenced
by rapid physiochemical changes in the defect than the
controlled variables [6]. Each test cell was then tested
separately at 7 kV.
The PD signals were captured as frequency spectra using a
Rohde & Swarz model FS300, 9 kHz - 3 GHz sweep tuned
spectrum analyser (SA). The SA was set in full span and
maximum amplitude hold mode. The SA settings were kept at
constant optimal values throughout all the tests. Each spectral
record at every SVF was a resultant trace of maximum signal
magnitude registered at each spectral frequency component
over a period of 2 minutes of continuous repetitive sweeps.
The SVF was varied from 20 to 400 Hz and incremented in
steps of 20 Hz. Five independent measurements were taken at
every step. The 2-minute test duration and number of
measurements per every test step for each sample were
optimally minimal in order to limit PD induced ageing effects
that would otherwise adversely influence the test results. In
order to check results repeatability, a number of independent
but similar tests were conducted.
The experimental tests were performed in a screened
laboratory environment at ambient atmospheric conditions of
20 to 25oC and 50% humidity. Each set of measurements did
not take more than 3 hours including the preconditioning time.
Related tests, but in the time domain, were conducted on
similar test samples. The same experimental setup and
procedures were used but with a suitably fast digital
oscilloscope as the detection instrument instead of the
spectrum analyser. Details of this work are described
elsewhere in [17].
4 RESULTS
Variations of the PD spectral bandwidth in response to
changes in the SVF were plotted for each defect type. Best
fitting trend lines were generated for each scatter plot. A
qualitative comparison of the trends showed close similarity
for each defect type in all three sets of independent but
identical measurements.
This section presents the measurement results, analysis and
Figure 1. The experimental setup for investigating the influence of supply discussion for respectively: void, surface and corona
voltage frequency on partial discharge characteristics.
discharges.
Figure 2. Test samples showing the various PD defect models and dimensions used in the experimental investigation of PD dependency on supply voltage frequency.
The tip radius of the corona discharging copper needle was about 50 m.
800
4.1 VOID DISCHARGES
and is given by equation 1.
600
500
400
300
200
100
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Supply voltage frequency (Hz)
(a)
-30
-35
-40
-45
Magnitude (dBm)
4.1.2 CAVITY DISCHARGE MECHANISM ANALYSIS
Figure 4 illustrates the sequence of events in a cavity
discharge process used to interpret the experimental
observations on cavity PD dependency on SVF. All equations
and analytical expressions used in explaining the cavity
discharge process are adopted from Niemeyer’s generalised
PD model [18].
The process begins with establishment of the resultant field
( E i ) that is responsible for initiating a discharge in the cavity
PD spectral bandwidth (MHz)
700
4.1.1 MEASUREMENTS AND OBSERVATIONS
The spectral bandwidth of cavity PD was generally not
responsive to changes in the SVF from 20 to 400 Hz, and this
was irrespective of the cavity position between the electrodes.
The cases investigated were: HV electrode bound, earth
electrode bound and fully embedded cavity in insulation. The
scatter plot of the spectral bandwidths as a function of the SVF
for an HV bounded cavity is shown in Figure 3a. Examples of
the spectra recorded at SVF of 20 Hz and 400 Hz are shown in
Figure 3b. In Appendix A results of the embedded and earth
electrode bound cavities are given. It is notable that the
degrees of stochastic scatter, as indicated by the confidence
range at each measuring point in the plots, were more
pronounced than in surface discharges and corona. A closer
look at the scatter plots also suggests some quasi-modulation
in the trends.
In the time-domain, except for negative PD pulse height that
decreased with increase in SVF, all the other PD pulse
parameters were not responsive to the changes in the SVF in
range 20 to 400 Hz as published earlier by the authors [17].
At 400 Hz supply
voltage frequency
-50
-55
-60
-65
-70
-75
0
At 20 Hz
supply voltage
frequency
100
200
300
400
500
600
700
800
900
1000
Frequency (MHz)
(b)
Figure 3. (a) Cavity PDs spectral bandwidth as a function of the supply
voltage frequency. (b) Typical spectra at lowest and highest frequency of the
supply voltage.
E i = fE 0 + E res
(1)
Where:
E 0 = electric field in the insulation due to the
externally applied voltage [V/m].
f = stress enhancement factor, which is a function
of the cavity dimensions. In spherical cavities
3ε r
and ε r is the dielectric constant
f =
2ε r + 1
of the insulation [F/m].
Eres = stress created by the space charge deployed on
the cavity walls after a discharge event [V/m].
The stress in the cavity increases until the stress conditions
become conducive to PD inception, that is, the resultant stress
( E i ) becomes at least equal to the streamer initiation
involves detrapping of electrons from residual charge that
would have been deployed on the cavity surfaces by the
previous discharge events. In small cavities not exposed to
radioactive radiation and where the cavity surfaces have a high
work function, the dominant seed electron source can be
assumed to be surface emission process [3]. Some of the
residual charge is lost through conduction across the cavity
walls and this process is governed by a decay time constant
( λcond ) derived from the resistance and capacitance (RC)
model of the cavity and is given by:
λ cond =
Where:
threshold stress ( E str ). In an air filled spherical cavity the
threshold stress is given by:
E str = p (
Ei
p
) cr [1 + B
2 rp
]
Ei
p
) cr =
B =
p =
r =
εr
D cavity
= the insulation dielectric constant [F/m].
= the cavity diameter [m].
σs
= the cavity surface conductivity and this
(3)
parameter increases with ageing of the cavity
under continuous exposure to PD [S].
(2)
Where for air in the cavity:
(
ε r D cavity
4σ s
25 [V/Pa.m].
8.6 [Pa.m].
gas pressure in the cavity [Pa].
cavity radius [m].
Even if the stress is sufficiently high for streamer discharge
initiation a discharge only occurs when an initiatory or seed
electron becomes available in the cavity at strategic positions
such as near the anode. The source of such an electron can be
either volume or surface emission processes. Volume
generation of the seed electron involves release of free
electrons by gas molecules or negative ions due to ionisation
by cosmic/radioactive energy. The surface emission process
The remaining electrons, that would have survived loss
through conduction, further suffer decay as some migrate deep
into the insulation. This loss is accounted for through a decay
term given by:
(−
e
t
λtr
)
(4)
Where:
λtr
t
= the time constant of loss through the
migration [s].
= the time elapsed since the last PD event [s]
Figure 4. An illustration of the sequence of events in a cavity discharge process showing the role of the residual space charge
The rate of detrapping electrons from the cavity walls
avalanches that can be characterised through the discharge
current pulse shape or spectral content. Assuming the residual
charge decay parameters remain constant, λ cond (due to cavity
.
( N e ) is given by:
.
N e = N e v 0 exp( −
Φ−
eE i
( 4 πε 0 )
kT
)
(5)
Where:
(Φ −
eE i
( 4 πε 0 )
is the Schottky term.
v0 = fundamental phonon frequency for the insulation [s-1].
e
= the elementary charge [C].
= the effective detrapping work function [eV].
= permittivity of vacuum [F/m].
Φ
ε0
k
T
Ne
= the Boltzmann constant [JK-1].
= the temperature [K].
= total number of electrons available for migration.
E = the electric field on the emitting cavity surface
temperature [V/m].
At instances when the cavity surfaces are negatively
charged, the total number of electrons available for detrapping
( N e ) is scaled down by a factor ( ξ ) that accounts for the
more difficult process of detrapping electrons from negatively
charged insulation surfaces [16].
At any given instant the production of an initiatory electron
from the cavity surface is therefore a stochastic process that
can be expressed as a probability function and denoted by;
P ( dt ) = 1 − e − ( N e dt )
(6)
When initiated the discharge event causes the field in the
cavity to change from E i to a residual value E res . The
difference in the electric field is associated with a charge
transfer given by:
∆ q ( t ) = ε 0 π r 2 ( E i ( t ) − E res )
(7)
Where:
r
E i (t )
= the cavity radius [m].
= the resultant field in the cavity [V/m].
E res
= the field remaining in the cavity after a
discharge event [V/m].
The resultant field in the cavity therefore varies with time as
a function of the sinusoidal external stress. Using equations 6
and 7, the pulse phase positions and corresponding charge
magnitudes can be determined.
4.1.3 CAVITY DISCHARGE TEST RESULTS
ANALYSIS AND DISCUSSION
It is evident from the discharge model outlined above that
the residual space charge in the cavity plays a critical role in
determining the point at which discharge occurs along the test
voltage cycle. This in turn affects the nature of the discharge
surface conductivity) and λ tr (due to charge migration to
deeper insulation traps), the amount of residual charge
generated at the instant of a new PD depends on the length of
the time ∆ t PD between consecutive pulses. With reference to
Figure 4, ∆ t PD is proportional to the rate of increase of the
resultant field E i (t ) which in turn is a function of the rate of
change of the supply voltage. If the rate of change of E 0 ( t ) is
much slower relative to λ cond and λ tr then the discharge
characteristics are not influenced by the SVF. This is
considered as a possible explanation for the observation in this
experimental work where the positive PD pulse parameters
were not influenced by the variations of the SVF in the range
20 to 400 Hz.
An increase in the SVF causes reduction of the time slot
( ∆ t PD ) that is available for PD pulses to occur and therefore
causes reduction in the number of PD pulses. This relationship
was noted by some researchers while investigating the
influence of SVF on PD characteristics [3,5].
In the zero crossing regions, particularly on polarity reversal
from positive to negative, different conditions exist. The
amount of residual charge remaining after time ∆ t 1 PD , (and
assuming no significant changes in λ cond and λ tr ) is
proportional to the number of PD pulses in the period
preceding this point in time. Since the latter is a function of the
SVF, the residual charge in the zero crossing region can
therefore be assumed to be dependent on the SVF. It follows
that the PDs associated with this residual charge are expected
to respond to changes in the SVF.
In the zero crossing region discharges occur at bigger gap
overvoltages
because
E 0 ( t ) and E res are in phase.
Furthermore where polarity changes from positive to negative,
it is more difficult to extract seed electrons from negatively
charged surfaces. These factors mean that maximal PD
magnitudes occur in this region and the PDs are more sensitive
to changes in the SVF. This was confirmed by the findings in
the time-domain measurements where the negative PD pulse
magnitude decreased with increase in SVF as presented in
[17]. Other researchers have attributed the appearance of
‘rabbit ear’ like portion of PRPD patterns to these type of PDs.
The ‘ears’ were observed to disappear with increase in SVF
[3] thus further confirming the model.
When observed in the frequency domain in this work, the
cavity PD frequency spectral characteristics were generally not
responsive to changes in the SVF as shown in Figure 3. This is
explained as follows: Since the frequency spectra were
recorded using a spectrum analyser in maximum hold mode,
the frequency components of both positive and negative PDs
were overlayed. Although the negative PDs reduced in
magnitude with increase in SVF, the resultant overall spectral
profile did not change as it was dominated by the positive PDs
that did not change with changes in the SVF.
4.1.4 KEY FINDING ON CAVITY DISCHARGES
In small cavities the cavity PD spectral content is immune
to variations in the supply voltage frequency. The life span
of residual space charge in the cavity (after each PD event)
relative to the rate of change of the supply voltage,
determines how the cavity discharge characteristics respond
to the supply voltage frequency.
PD spectral bandwidth (MHz)
300
250
200
150
4.2 SURFACE DISCHARGES
The experimental observations and results discussion of
surface discharge tests are presented in this section.
100
50
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
Supply voltage frequency (Hz)
(a)
-30
-35
-40
-45
Magnitude (dBm)
At 400 Hz supply
voltage frequency
-50
-55
4.2.1 MEASUREMENTS AND OBSERVATIONS
Surface discharge spectral bandwidth increased with
increase in the supply voltage frequency from 20 to 400 Hz as
shown in the scatter plot of Figure 5a. The same trend was
observed for the other two cases of surface discharges, that is,
on the earth electrode and those on the high voltage (HV)
electrode. As the supply voltage frequency was increased from
20 Hz to 400 Hz the average spectral bandwidth increased by
about 170% for HV electrode surface discharges and about
40% in the case of the earth electrode surface discharges.
Other forms of energy such as optical and sound increased in
intensity as the SVF increased. Related measurements
conducted in the time domain showed that negative surface
discharge PD pulse shapes changed in response to changes in
the supply voltage frequency in the range 20 to 400 Hz. More
details of this work are described elsewhere in [17].
-60
4.2.2
-65
-70
At 20 Hz supply
voltage frequency
-75
0
50
100
150
200
250
300
350
400
450
500
Frequency (MHz)
(b)
Figure 5. (a) Surface discharge spectral bandwidth as a function of the
supply voltage frequency. (b) Typical spectra at lowest and highest frequency
of the supply voltage.
SURFACE DISCHARGE MECHANISM
ANALYSIS AND RESULTS DISCUSSION
The response of surface discharges to variations in the
supply voltage frequency can be analytically explained in
terms of space charge dynamics along the insulation interface
with the metallic electrode. Murooka et al (as summarised in
[19]) studied surface discharge mechanisms using dust and
photographic figure methods, and this gives useful basis for
interpreting surface PD mechanisms under varying SVF.
Figure 6 gives an illustration of the surface discharge
mechanism.
Figure 6. An illustration of surface discharge mechanisms showing the influence of the slower positive ion cloud.
The space charge dynamics’ response to increase in SVF
can be deduced from the illustration of the mechanisms. In the
positive half cycle at the tip of the positive ion cloud the stress
due to this space charge superimposes on the background
stress. The maximum electric field therefore occurs at the
advancing plasma tip [19,20]. Assuming constant
space charge decay, an increase in the supply voltage
frequency gives less time for the positive ion space charge to
disperse during the full cycle of the supply voltage. By
increasing SVF, the amount of positive ion space charge
available in each discharge event increases. An increase in the
amount of the positive ion space charge results in more stress
enhancement at the tip of the positive ion space cloud causing
faster avalanches and further extension of the discharge
streamer. This manifests as increased discharge frequency
bandwidth as well as optical and audible emissions as
observed in the experimental tests conducted for this work.
In the negative half cycle the positive ion cloud near the
cathode causes enhanced stress region between the ion cloud
and the cathode. This region is known as the cathode fall [19].
More avalanches are consequently initiated producing more
positive ions as electrons (being much lighter) are quickly
swept away to the opposite electrode. Since increased SVF
results in increased positive ion space charge, it follows
therefore that the size of the cathode fall region increases with
increase in the SVF. The shielding effect of the positive ion
cloud increases resulting in further limitation of the negative
discharge streamer growth. This is the reason why in the timedomain negative discharge pulse magnitudes were observed to
decrease with increase in the SVF.
In the frequency domain, where both positive and negative
discharges were detected simultaneously using a spectrum
analyser in a maximum hold mode, the frequency components
of the positive discharges prevailed over those of the negative
discharges. The overall surface discharge spectral bandwidth
therefore increased as the SVF increased although that of the
negative discharges would have decreased due to the increased
cathode fall.
Increasing pulse rise-time is expected to be associated with
decreasing spectral bandwidth. The time-domain pulse
parameter measurements taken as a function of SVF compared
with the corresponding frequency domain spectral
measurements, showed non-conformance to this principle. In
the time-domain generally the positive pulse parameters
remained unchanged as the SVF increased and yet in the
frequency domain the bandwidth increased. A possible
explanation for this could be the effect of pulse superpositions.
Incidences of pulse superposition have a high probability of
occurrence in surface discharges particularly with the type of
samples used in this work, where the test sample was made up
of a disc electrode pressed against a layer of dielectric
insulation. The phenomenon of PD pulse parameters distortion
due to superposition was also experimentally and analytically
explored by Reid et al [21] as well as Brosche et al [22]. They
reported that current pulses could occur in bursts of up to ten
individual pulses in as short time as 1 ns. Such pulses are
difficult to resolve using ordinary signal measuring
instruments.
4.2.3 KEY FINDING ON SURFACE DISCHARGES
Surface discharge spectral bandwidth increases with
increase in supply voltage frequency. Positive ion clouds
effectively determine how surface discharges respond to
variations in the SVF through either influencing streamer
mechanisms in the positive half cycle or the cathode fall
magnitude in the negative half cycle.
4.3 CORONA DISCHARGES
The point to plane corona experimental test results as well
as observations are presented and discussed in this section.
4.3.1 MEASUREMENTS AND OBSERVATIONS
The variation of spectral bandwidth of point to plane corona
on HV electrode in air as a function of SVF (in the range 20 to
400 Hz) is shown in the scatter plot in Figure 7a. Examples of
the corona spectral signatures recorded at the lowest (20 Hz)
and at highest (400 Hz) SVF frequencies are shown in
Figure 7b. Corona on the earth electrode behaved generally the
same as that on the HV electrode as shown in the plots in
Appendix A. The corona discharges spectral bandwidth
decreased with increase in the SVF. The trends for both HV
electrode and earth electrode corona had a step-wise profile.
Equivalent measurements were conducted on corona in the
time-domain. The corona pulse parameters (rise-time, width,
fall-time and height) exhibited similar trends as those of
frequency domain measurements as discussed in detail
elsewhere [17]. As in the frequency domain the pulse rise-time
and height decreased and in distinct steps as the SVF
increased. The corresponding audio and optical emissions
from the corona PDs also decreased with increase in SVF.
4.3.2
CORONA MECHANISM ANALYSIS AND TEST
RESULTS DISCUSSION
The behaviour of corona discharges under varying SVF is
remarkable as it contrasts that of surface and cavity discharges.
With reference to Figure 8, showing schematic illustrations of
corona discharge mechanisms derived from literature [23-26],
the tests results and observations are discussed.
350
PD spectral bandwidth (MHz)
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Supply voltage frequency (Hz)
(a)
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Figure 8. An illustration of positive corona mechanism [23].
Magnitude (dBm)
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At 20Hz supply
voltage frequency
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0
At 400 Hz supply
voltage frequency
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Frequency (MHz)
(b)
Figure 7. (a) Corona discharge spectral bandwidth as a function of the
supply voltage frequency. (b) Typical spectra at lowest and highest frequency
of the supply voltage
Corona in air gives distinct phase-resolved patterns [26].
Discharges that occur while the needle tip is at negative
potential are typically very small and uniform and this is
negative corona. With the needle tip at positive potential,
much bigger pulses occur on the voltage cycle crest (positive
corona). When an oscilloscope was used to detect the corona
pulses for pulse shape analysis as a function of frequency, the
negative corona (also known as Trichel pulses) were below the
detection level of the system. Only positive corona pulses
could therefore be detected [17]. Similarly in the frequency
domain when both negative and positive corona were detected
simultaneously using a spectrum analyser in the maximum
hold mode, the resultant spectrum was that of the positive
corona pulses. Analysis and discussion of experimental
findings in this work are therefore only about positive corona
as they were the only corona discharges detected under the
given experimental conditions. The corona discharge models
in literature [23-26] are used to analyse and discuss the
experimental results obtained in this work where the behaviour
of corona under varying SVF was investigated.
According to literature, distinctly different modes of corona
discharges occur depending on the stress conditions at the
anode. At an electric stress that is significantly above the
discharge inception level the discharges are dominated by the
onset corona streamer [23,24]. As illustrated in Figure 8, after
initial avalanches, a space charge cloud enhances the field in
the gap and this initiates secondary avalanches that extend
radially into the lower field regions of the gap. Unlike in the
negative corona where negative ions influence the process
[25], in positive corona, electrons are always under the
influence of strong electric field and therefore are quickly
swept away by the anode without a chance to form negative
ion molecules. An increased size of the positive ion space
charge forms around the anode thereby effectively shielding
the anode from the opposite electrode. The local electric field
at the anode drops and the discharge extinguishes. The positive
ion cloud then drifts away under the influence of the electric
field thus clearing up the shielding effect [23,27]. The reestablishment of the high stress conditions at the anode
initiates another discharge event, and the process then repeats.
Under alternating voltage conditions the stress in the gap
changes in magnitude and polarity as a function of time. After
the peak, the stress decreases until it gets to zero and then
increases again in the opposite direction. The effect on the
positive ion cloud (still present in the gap after the last positive
corona discharge) is a reduction in the rate of drift. On stress
polarity reversal the space charge cloud drifts in the reverse
direction. An increase in the rate of change in stress polarity
results in increased retention of the residual positive ion cloud
and therefore causes more effect on the nature of the
subsequent discharges. This is the reason why the corona
spectral bandwidth and magnitude decreased as the SVF
increased from 20 to 400 Hz as shown in Figure 7.
APPENDIX A
4.3.3 KEY FINDING ON CORONA DISCHARGES
Point to plane corona spectral bandwidth in air decreases
with increase in the supply voltage frequency. Positive corona
discharge mechanisms depend on the dynamics of the positive
ion cloud in the discharge gap. Factors such as increase in the
SVF that cause prolonged presence of the space charge cloud
in the discharge gap inherently alter the positive corona
discharge characteristics as observed in the experiments.
The influence of the supply voltage frequency on the PD
spectral content was unique for each defect type. The response
was however independent of the relative positions of the
defects between the electrodes. Figure A1 and A2 show the
embedded cavity and earth electrode cavity respectively.
Figure A2 and A3 are scatter plots of the spectral bandwidth
variations and examples of spectra recorded at highest and
lowest supply voltage frequency for surface discharges on
earth electrode/insulation interface and corona on earth
electrode respectively.
In the case of surface discharges and corona, the changes in
the spectral content as a function of the supply voltage
frequency also manifested as changes in the audio and optical
energy emissions from the discharges.
The way in which the PD frequency spectra responded to
variations in the supply voltage frequency depended on the
defect type. Void defect PDs were immune to changes in
frequency of the supply voltage. Spectral bandwidth of surface
discharges increased with increase in supply voltage
frequency. The opposite was observed in corona discharges
where frequency components diminished as the supply voltage
frequency increased.
The observed supply frequency dependent behaviour of PDs
suggests the following possible diagnostic application-:
• By sweeping the test voltage frequency through a range
such as 10 Hz to 400 Hz, unknown PD source can be
recognised from how the PD spectra change during the
sweep. This technique can be used as a quick PD defect
pre-classification or elimination of corona interference by
elevating the frequency of the test voltage.
ACKNOWLEDGMENT
The authors would like to acknowledge with gratitude
Eskom for their support of the High Voltage Engineering
Research Group through TESP. They would also like to
express gratitude to the Department of Trade and Industry
(DTI) for THRIP funding and to thank the National Research
Foundation (NRF) for direct funding of the research group.
250
PD spectral bandwidth (MHz)
5 PROSPECTIVE APPLICATION
6 CONCLUSION
The supply voltage frequency (in the 20 to 400 Hz range)
influences the frequency content of partial discharges in a
manner that depends on defect type causing the discharges.
Implications of this knowledge include the following:
200
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0
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Supply voltage frequency (Hz)
(a)
-35
ii) Where frequency spectra are used as characteristic
signatures for each defect type in the frequency domain
PD diagnosis, the influence of the SVF can pose a
challenge in the reliability of the diagnostic results. As an
example, spectral patterns of surface discharges in a power
cable termination that is energised at 0.01 Hz test voltage
can be remarkably different form that obtained at 50 Hz
test voltage. It is imperative therefore that in classifying
PD sources using spectral characteristics, the influence of
SVF on the spectral features is appropriately taken into
account.
-40
-45
Magnitude (dBm)
i) Comparative analyses of PDs obtained through test
voltages of different frequencies should be done in
cognisance of the different behaviour of PD types under
different test voltage frequencies.
-50
At 400 Hz supply
voltage frequency
-55
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-75
0
At 20 Hz
supply voltage
frequency
100
200
300
400
500
600
700
800
900
1000
Frequency (MHz)
(b)
Figure A1. (a) Imbedded cavity PDs spectral bandwidth dependency on
supply voltage frequency. (b) Typical spectra at lowest and highest frequency
of the supply voltage.
300
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PD Spectral bandwidth (MHz)
PD spectral bandwidth (MHz)
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0
(a)
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(a)
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At 400 Hz supply
voltage frequency
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Magnitude (dBm)
Magnitude (dBm)
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Supply voltage frequency (Hz)
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At 20 Hz
supply voltage
frequency
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At 20 Hz supply
voltage frequency
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0
100
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300
400
500
600
700
800
900
1000
Frequency (MHz)
-70
(b)
Figure A2. (a) Earth electrode bounded cavity PDs spectral bandwidth
as a function of the supply voltage frequency. (b) Typical spectra at lowest
and highest frequency of the supply voltage.
250
At 400 Hz supply
voltage frequency
-80
0
100
200
300
400
500
600
700
800
900
1000
Frequency (MHz)
(b)
Figure A4. (a) Earth electrode corona PDs spectral bandwidth as a
function of the supply voltage frequency. (b) Typical spectra at lowest and
highest frequency of the supply volt.
PD spectral bandwidth (MHz)
200
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150
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100
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20
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60
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Supply voltage frequency (Hz)
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At 400 Hz supply
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Cuthbert Nyamupangedengu was born in
Zimbabwe. He received a Bachelor of
Technology honours degree in electrical
engineering from the University of Zimbabwe in
1994. At the University of the Witwatersrand,
Johannesburg, in 2004, he was awarded the
M.Sc. Engineering (with distinction) and in
2011, the PhD. He is a lecturer at the University.
Before moving to the University of the
Witwatersrand, Cuthbert was an engineer in the
Zimbabwe Electricity Supply Authority from
1995 to 2005 in various capacities mainly in power system planning and
development. His passion is research on diagnosis of high voltage
dielectric insulation.
Ian R. Jandrell (M’85) was born in East
London, South Africa. He received a B.Sc.
degree in electrical engineering, GDE (Elect.)
and Ph.D. degree from the University of the
Witwatersrand, Johannesburg, RSA in the
period 1981 to 1990. Ian Jandrell is the “CBI
Electric Professor of Lightning” and holds the
rank of “Personal Professor” in the School of
Electrical and Information Engineering at the
University of the Witwatersrand, Johannesburg.
Since June 2001 he has been the Head of the
School of Electrical and Information
Engineering. He is a registered professional engineer in South Africa and is
also a director of a number of companies.