energies
Article
On-Line Partial Discharge Monitoring System for
Power Transformers Based on the Simultaneous
Detection of High Frequency, Ultra-High Frequency,
and Acoustic Emission Signals
Wojciech Sikorski 1, * , Krzysztof Walczak 1 , Wieslaw Gil 2
1
2
*
and Cyprian Szymczak 1
Institute of Electric Power Engineering, Poznan University of Technology, 60-965 Poznan, Poland;
krzysztof.walczak@put.poznan.pl (K.W.); cyprian.szymczak@put.poznan.pl (C.S.)
Mikronika, 60-001 Poznan, Poland; wieslaw.gil@mikronika.com.pl
Correspondence: wojciech.sikorski@put.poznan.pl
Received: 5 June 2020; Accepted: 22 June 2020; Published: 24 June 2020
Abstract: The article presents a novel on-line partial discharge (PD) monitoring system for power
transformers, whose functioning is based on the simultaneous use of three unconventional methods
of PD detection: high-frequency (HF), ultra-high frequency (UHF), and acoustic emission (AE). It is
the first monitoring system equipped in an active dielectric window (ADW), which is a combined
ultrasonic and electromagnetic PD sensor. The article discusses in detail the process of designing
and building individual modules of hardware and software layers of the system, wherein the
most attention was paid to the PD sensors, i.e., meandered planar inverted-F antenna (MPIFA),
high-frequency current transformer (HFCT), and active dielectric window with ultrasonic transducer,
which were optimized for detection of PDs occurring in oil-paper insulation. The prototype of
the hybrid monitoring system was first checked on a 330 MVA large power transformer during
the induced voltage test with partial discharge measurement (IVPD). Next, it was installed on a
31.5 MVA substation power transformer and integrated according to the standard IEC 61850 with
SCADA (Supervisory Control and Data Acquisition) system registering voltage, active power, and oil
temperature of the monitored unit. The obtained results showed high sensitivity of the manufactured
PD sensors as well as the advantages of the simultaneous use of three techniques of PD detection and
the possibility of discharge parameter correlation with other power transformer parameters.
Keywords: power transformer; partial discharge (PD); on-line PD monitoring system; active dielectric
window; acoustic emission; UHF antenna; PIFA; HFCT sensor
1. Introduction
Power transformers are very important devices of public and industry use, requiring high capital
investment. Their reliable operation ensures continuity of electrical energy supply and stability of
the power energy system. A serious global problem is a high share of power transformers, for which
the designed lifetime (usually estimated at 30–35 years) expired [1–3]. However, considering the
high cost of purchase, transportation, and insurance as well as very long delivery time (even over
a year), power transformers are not, at the moment of expiring their lifetime, immediately replaced
with new ones. The majority of distribution companies adopt a management strategy based on the
use of the old units as long as it is possible, which unfortunately increases the risk of a catastrophic
failure [4]. The report of the CIGRE working group A2.37 indicates that dielectric (partial discharge,
tracking, arcing, flashover) and electrical failures (open circuit, short circuit, poor joint/contact, ground
deterioration, floating potential) were nearly half (49.3%) the cause of the serious damages of power
Energies 2020, 13, 3271; doi:10.3390/en13123271
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Energies 2020, 13, 3271
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transformers in Europe [1]. The dynamics of the defect development caused by the activity of partial
discharges depends on a variety of factors, including the type of the discharge itself and its energy,
location of the defect, degree of aging and moisture content of the insulation system, varying load
affecting the oil temperature changes as well as moisture migration process. In some cases, the defect
may develop over many years of power transformer operation not leading it to damage. In other
cases, the defect may develop extremely rapidly, mostly as a consequence of a random event, such as
overvoltage or a sudden increase in oil temperature caused by overloading of a unit or malfunction of
the cooling system [1,5–7]. Very often the dynamics of the defect development grow in its final stage,
shortly before the damage itself [8]. For that reason, currently, the monitoring online systems have
gained popularity, since they have the advantage over periodic tests allowing for instant detection of
sudden growth of partial discharges activity [9–13]. Additionally, in combination with the SCADA
(supervisory control and data acquisition) system, which is installed on the majority of substations,
they allow to find the correlation between PD intensity and power transformer parameters, e.g., voltage
or oil temperature [14,15]. It increases the credibility of insulation system condition assessment and
enables the introduction of current limitations of power transformer exploitation, assuming that its
condition is not critical, and it does not require an immediate shutdown.
At present, the acoustic emission method (AE), electromagnetic methods (high-frequency (HF),
very-high frequency (VHF), and ultra-high frequency (UHF)), and dissolved gas analysis (DGA)
are some of the most important PD detection methods adjusted to operating in online monitoring
mode [16–25]. The last technique is indirect and because of its limitations, such as long time intervals
between successive measurements, the sensitivity depending on the dimensions of transformer tank and
distance of the gas sensor from the PD source as well as complex and non-standardized methods of DGA
results interpretation, it is not capable to detect very quickly developing defects. The acoustic emission
method and electromagnetic methods have both strengths and weaknesses, therefore none of them
may be recognized as more efficient or universal. For this reason, in the context of power transformers
diagnostics, the possibility of elaborating a hybrid system combining at least two measurement
methods is suggested more and more often. The main advantage of such a system application is
increase in PD detection reliability. This is because the individual methods are based on different
physical phenomena (detection of electromagnetic and acoustic waves) and different coupling methods
(inductive, capacitive, or acoustic), therefore each of them is sensitive to another type of disturbance.
For instance, in the AE method, an ultrasonic transducer may register core magnetostriction noise
(Barkhausen effect), pumped oil and fans noise, environmental noises (thunderstorms, rain, hail,
etc.), tap changer operation, and other random signals [26]. In the high-frequency method, sensor
HFCT is installed outside the transformer tank, therefore, it may register external electromagnetic
interferences (EMI) occurring on the substation, such as corona discharge from HV transmission
lines and signals accompanying switching operations. In turn, in the UHF method, the antenna may
receive a radio signal emitted by FM radio stations, aerial navigation systems, civilian and military
aviation, transmitters of civil services, DVB-T digital television, and GSM operators [27]. Therefore,
when a hybrid monitoring system, e.g., registers numerous UHF pulses and none of AE signals, it
may be assumed that the source of the signal is external EMI. In turn, when the system registers
pulses simultaneously by all methods and the fluctuations of their intensity are similar, then with high
probability one may assume that they are generated by partial discharges. As it was mentioned before,
the credibility of the diagnosis may be increased by finding a correlation between PD parameters and
power transformer parameters registered by the SCADA system. Those parameters that may affect the
partial discharge occurrence are voltage and oil temperature levels.
Other researchers also point to the benefits of combining two PD detection methods. Cavallini et
al. [28] believe that a combination of AE and UHF sensors can be successfully used as a tool to highlight
the presence of PD phenomena and can be used conveniently for monitoring and trending. Similar
conclusions were formulated by Witos and Gacek [29], who indicated that simultaneous registration of
PDs by two diagnostic techniques gives the possibility to elaborate new PD descriptors, which increase
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registration of PDs by two diagnostic techniques gives the possibility to elaborate new PD
descriptors,
increase
the efficiency
of defects recognition.
Fuangsoongnern
et al. [30]
the
efficiencywhich
of defects
recognition.
Fuangsoongnern
et al. [30] proved
that a combination
of proved
the AE
that HF
a combination
of eliminating
the AE and
methods
allows
eliminating theindicate
diagnostic
and
and
methods allows
theHF
diagnostic
errors
and simultaneously
if theerrors
problem
is
simultaneously
indicate
if
the
problem
is
caused
by
partial
discharge,
mechanical
faults,
arcing,
or
caused by partial discharge, mechanical faults, arcing, or loose parts inside the transformer. Combining
loose
parts inside
the transformer.
Combining
PD detection
methods
allows
obtaining of
additional
PD
detection
methods
allows obtaining
of additional
monitoring
system
functionalities.
For
instance,
monitoring system
functionalities.
simultaneous
registration
UHF and
AE improves
simultaneous
registration
of UHF For
andinstance,
AE improves
the accuracy
of PD of
sources
localization
with
the
accuracy
of
PD
sources
localization
with
trilateration
technique
[31‒33].
trilateration technique [31–33].
The presented
(PD)
monitoring
system
is based
on
The
presented in
in the
thearticle
articlenovel
novelon-line
on-linepartial
partialdischarge
discharge
(PD)
monitoring
system
is based
simultaneous
UHF,
HF,
and
AE
signal
registration.
It
is
a
world
first
system
equipped
in
active
on simultaneous UHF, HF, and AE signal registration. It is a world first system equipped in active
dielectric windows,
windows,which
whichare
are
a new
concept
of combined
acoustic
emission
and electromagnetic
dielectric
a new
concept
of combined
acoustic
emission
and electromagnetic
partial
partial discharge
fortransformers.
power transformers.
Each elaborated
prototype
PD
detector,
i.e.,
discharge
detectordetector
for power
Each elaborated
prototype PD
detector,
i.e.,
meandered
meandered
planar
inverted-F
antenna,
active
dielectric
window
with
built-in
ultrasonic
transducer
planar inverted-F antenna, active dielectric window with built-in ultrasonic transducer as well as
as well as high-frequency
current transformer
(HFCT)
wereoptimized
specially optimized
to theofdetection
of
high-frequency
current transformer
(HFCT) were
specially
to the detection
the partial
the partial occurring
discharges
in oil-paper
insulation.
to the implementation
of concepts,
new PD
discharges
in occurring
oil-paper insulation.
Thanks
to the Thanks
implementation
of new PD sensor
sensor
concepts,
a
significant
improvement
in
sensitivity
was
achieved
for
individual
partial
a significant improvement in sensitivity was achieved for individual partial discharge detection
discharge The
detection
The
shows that
the combination
of different
this
methods.
articlemethods.
shows that
thearticle
combination
of different
methods—in
this case,methods—in
electromagnetic
case,
electromagnetic
and
acoustic—increases
the
reliability
of
partial
discharge
detection.
and acoustic—increases the reliability of partial discharge detection.
The paper
paperisisstructured
structured
follows:
assumptions
and structure
the hybrid
monitoring
The
as as
follows:
thethe
assumptions
and structure
of the of
hybrid
monitoring
system
system
as the
wellprocess
as the process
of designing
PD detectors
testing
are presented
in Section
2. The
as
well as
of designing
and PD and
detectors
testing are
presented
in Section
2. The results
results
of
the
prototype
system
evaluation
conducted
on
a
new
330
MVA
large
power
transformer
of the prototype system evaluation conducted on a new 330 MVA large power transformer during
during
the induced
andexploited
on the exploited
31.5
MVAtransformer
power transformer
are discussed
in
the
induced
voltage voltage
test, andtest,
on the
31.5 MVA
power
are discussed
in detail in
detail in3.Section
3. General
conclusions
andplans
further
planstolinked
to the development
of the produced
Section
General
conclusions
and further
linked
the development
of the produced
system
system
are
included
in
Section
4.
are included in Section 4.
2. Design and Implementation of On-Line PD Monitoring System
2.1. General Structure of On-Line PD Monitoring System
In Figure 1, the schematic diagram of the general structure of the
the developed
developed on-line partial
discharges monitoring system is shown, whose unique feature is the possibility
possibility of simultaneous
simultaneous
registration of PD pulses with the use of three different diagnostic techniques,
techniques, i.e.,
i.e., high-frequency,
high-frequency,
ultra-high frequency,
frequency, and
and acoustic
acousticemission.
emission.
Figure
Schematic diagram
diagram of
of the
the general
general structure
structure of
Figure 1.
1. Schematic
of the
the developed
developed on-line
on-line partial
partial discharges
discharges
monitoring
system
for
power
transformers.
monitoring system for power transformers.
High-frequency current transformer (labeled in the system as sensor ISO-002-HF) and active
High-frequency current transformer (labeled in the system as sensor ISO-002-HF) and active
dielectric windows (ADW) with built-in acoustic emission sensor and mounted UHF antenna (the
dielectric windows (ADW) with built-in acoustic emission sensor and mounted UHF antenna (the set
set was labeled as sensor ISO-002-ADW/UHF) were employed as PD detectors. Each PD detector
was labeled as sensor ISO-002-ADW/UHF) were employed as PD detectors. Each PD detector has an
has an additional housing, in which there is a signal conditioning module and analog-to-digital
additional housing, in which there is a signal conditioning module and analog-to-digital (A/D) signal
(A/D) signal processing module. A module for fiber-optic signal transmission was included as well.
processing module. A module for fiber-optic signal transmission was included as well. This type of
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transmission
allows to minimize
influence
of external
EMI occurring
on substations.
The
This type of transmission
allows to the
minimize
the influence
of external
EMI occurring
on substations.
transmitted
digital
data data
is received
by the
data concentrator
labeled
as MPD-001.
It is
The transmitted
digital
is received
bymulti-channel
the multi-channel
data concentrator
labeled
as MPD-001.
aItspecialized
modular
device,
prepared
for
operation
in
a
substation
environment,
equipped
with
is a specialized modular device, prepared for operation in a substation environment, equipped with
multi-port
multi-port network
network communication
communication cards.
cards. ItItperforms
performsthe
theinitial
initialintegration
integrationof
ofdata
dataframes
framesreceived
received
from
PD sensors
sensorsinto
intoa aform
form
that
facilitates
their
transmission
to server
the server
and performing
from many PD
that
facilitates
their
transmission
to the
and performing
more
more
advanced
calculations
phased-resolved
partial
discharge
patterns
ortrend
data trend
analysis).
advanced
calculations
(e.g., (e.g.,
phased-resolved
partial
discharge
patterns
or data
analysis).
The
process of
ofdesigning
designingand
and
producing
sensors
elements
the monitoring
The process
producing
PDPD
sensors
and and
mainmain
elements
of the of
monitoring
system,
system,
i.e.,
module
for
conditioning
and
digital
processing
of
PD
pulses,
data
concentrator,
and
i.e., module for conditioning and digital processing of PD pulses, data concentrator, and modules
of
modules
of server
areindiscussed
in in
detail
later in the article.
server software
aresoftware
discussed
detail later
the article.
2.2. Partial
PartialDischarge
DischargeDetectors
Detectors
2.2.
2.2.1. Active
ActiveDielectric
DielectricWindow
Window
2.2.1.
The active
active dielectric
revision
hole
of the
power
transformer,
was
The
dielectric window
window(ADW),
(ADW),installed
installedininthe
the
revision
hole
of the
power
transformer,
elaborated
for the
of AE and
UHF
pulses
by partialby
discharges.
The previously
was
elaborated
fordetection
the detection
of AE
and
UHFgenerated
pulses generated
partial discharges.
The
conducted
research
demonstrated
that
ADW
assures
over
five-times
higher
AE
pulses
detection
previously conducted research demonstrated that ADW assures over five-times higher AE
pulses
sensitivitysensitivity
comparedcompared
to commonly
used contact
type PACtype
R15PAC
[33]. R15
It results
the
detection
to commonly
usedtransducer
contact transducer
[33]. Itfrom
results
fact
that
ADW
is
placed
inside
the
power
transformer
tank
and
registers
acoustic
signal
propagating
from the fact that ADW is placed inside the power transformer tank and registers acoustic signal
directly in oil,
whereas
a conventional
AE sensor is
the outer
of thewall
tank.
For
propagating
directly
in oil,
whereas a conventional
AEmounted
sensor ison
mounted
on wall
the outer
of the
that reason,
only registers
small part
of thepart
acoustic
energy
generated
by partial by
discharge.
tank.
For thatitreason,
it only aregisters
a small
of thewave
acoustic
wave
energy generated
partial
The
remaining
energy
is
lost
in
the
result
of
the
processes
of
the
acoustic
wave
reflection
and
absorption
discharge. The remaining energy is lost in the result of the processes of the acoustic wave reflection
by the
steel wallbyofthe
thesteel
transformer
tank.
and
absorption
wall of the
transformer tank.
The
active
dielectric
window
consists
of aa specially
specially formed
formed bowl-shaped
bowl-shaped ceramic
ceramic element,
element, in
in
The active dielectric window consists of
which
all
ultrasonic
transducer
components
were
built-in,
i.e.,
piezoelectric
crystal
with
electrodes
and
which all ultrasonic transducer components were built-in, i.e., piezoelectric crystal with electrodes
electric
wireswires
as well
matching
and backing
layer (Figure
2). The2).
bowl-shape
of the ADW
and
electric
as as
well
as matching
and backing
layer (Figure
The bowl-shape
of theenables
ADW
mounting-in
of
the
UHF
antenna
and
detection
of
radio
signals
generated
by
PDs.
enables mounting-in of the UHF antenna and detection of radio signals generated by PDs.
Figure2.2.Technical
Technicaldrawing
drawingof
ofthe
theactive
activedielectric
dielectricwindow.
window.
Figure
The dielectric
waswas
produced
from high-density
alumina ceramics,
is characterized
The
dielectricwindow
window
produced
from high-density
alumina which
ceramics,
which is
by
high
thermal,
mechanical,
and
chemical
resistance
(it
does
not
oxidize
and
does
characterized by high thermal, mechanical, and chemical resistance (it does not oxidizenot
andreact
doeswith
not
mineral
oils
and
esters).
The
main
parameters
of
the
used
ceramic
material
are
listed
in
Table
1.
react with mineral oils and esters). The main parameters of the used ceramic material are listed in
Table 1.
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Table 1. Properties of the high-strength aluminous porcelain used to construct the dielectric
Table 1. Properties of the high-strength aluminous
porcelain used to construct the dielectric window.
window.
Parameter
Parameter
Symbol Symbol
Apparent1 (bulk) density 1
ρb
ρb
Apparent (bulk) density
Flexural
strength 1
σ
1
σ
Flexural strength
K
Fracture
toughness 1 K
1
Fracture toughness
Young's
modulus 1 E
E
1
Young’s modulus
Abrasion
resistance
(volume
loss)
WV
Abrasion resistance (volume loss)
WV
1
VB
Dielectric
strength 1 VB
Dielectric strength
Resistivity 1
ρ
ρ
Resistivity 1
1
Tc
Thermal shock
resistance T1 c
Thermal shock resistance
11 Minimum values.
Minimum values.
Value
Value
2500
2500
140
140
2.0
2.0
100
100
172
172
20
20
13
10
1013
150
150
Unit
Unit
kg/m3
kg/m3
MPa
MPa
MPa·m1/2
MPa·m1/2
GPa
GPa
mm3
mm3
kV/mm kV/mm
Ω·cm
Ω·cm
K
K
Regarding the
the specific
specific environment
environment of
of the
the ultrasonic
ultrasonic transducer
transducer operation,
operation, which
which is
is mineral
mineral oil,
Regarding
oil,
the
matching
layer
is
a
very
important
element.
Its
proper
manufacturing
assures
high
transducer
the matching layer is a very important element. Its proper manufacturing assures high transducer
sensitivity. So
toto
piezoelectric
crystal
could
be
sensitivity.
So that
that the
theacoustic
acousticenergy
energytransmission
transmissionfrom
frommineral
mineraloiloil
piezoelectric
crystal
could
carried
outout
lossless
(without
reflections),
thethe
acoustic
impedance
of the
matching
layer
should
be the
be
carried
lossless
(without
reflections),
acoustic
impedance
of the
matching
layer
should
be
◦
geometric
mean
of
the
mineral
oil
impedance
(Z
oil
=
1.28
MRayl
at
25
°C)
and
used
piezoelectric
the geometric mean of the mineral oil impedance (Zoil = 1.28 MRayl at 25 C) and used piezoelectric
ceramics type
type PZT-5A
PZT-5A(Z
(ZPZT
PZT =
composite
ceramics
= 31
31 MRayl).
MRayl). The
Thematching
matching layer
layer was
was made
made of
of epoxy/alumina
epoxy/alumina composite
with
0.3825
volume
fraction
of
alumina,
which
allowed
for
obtaining
the
required
value
of
acoustic
with 0.3825 volume fraction of alumina, which allowed for obtaining the required value of acoustic
impedance
i.e.,
6.3
MRayl.
For
the
preparation
of
the
composite,
the
epoxy
resin
of
low
viscosity
(200
impedance i.e., 6.3 MRayl. For the preparation of the composite, the epoxy resin of low viscosity
◦ C) and
mPa·s
at 25at°C)
high high
purity
alumina
powder
with with
a grain
thickness
of 1 µm
used.used.
The
(200
mPa·s
25 and
purity
alumina
powder
a grain
thickness
of 1 was
µm was
backing
layer
was
made
of
composite
material
as
well.
In
this
case,
epoxy/tungsten
composite
with
The backing layer was made of composite material as well. In this case, epoxy/tungsten composite
0.08 volume
fraction
of tungsten
of acoustic
impedance
4.56 MRayl
used.
The
optimal
of
with
0.08 volume
fraction
of tungsten
of acoustic
impedance
4.56 was
MRayl
was
used.
Thevalues
optimal
acoustic
and geometric
dimensions
of all components
of the of
ultrasonic
transducer
(PZT
values
of impedance
acoustic impedance
and geometric
dimensions
of all components
the ultrasonic
transducer
crystal,
matching,
and
backing
layer),
which
affect
the
shape
of
the
frequency
response
curve
were
(PZT crystal, matching, and backing layer), which affect the shape of the frequency response curve were
designed based
based on
on the
the simulations
simulations performed
performed with
with the
the model
model of
of Krimholtz,
Krimholtz, Leedom,
Leedom, and
and Matthaei
Matthaei
designed
(KLM)
[34].
In
turn,
proper
proportions
of
epoxy,
alumina,
and
tungsten
for
the
elaborated
composite
(KLM) [34]. In turn, proper proportions of epoxy, alumina, and tungsten for the elaborated composite
materials were
were selected
selected using
using the
the Devaney
Devaney and
and Levine
Levine model,
model, which
which is
is based
based on
on aa self-consistent
self-consistent
materials
formulation of
of multiple-scattering
multiple-scattering theory
theory [35].
[35]. In
final effect,
effect, the
the transducer
transducer with
with relatively
relatively flat
flat
formulation
In the
the final
frequency
response
in
the
range
between
120
and
270
kHz
and
with
first
resonance
frequency
totaling
frequency response in the range between 120 and 270 kHz and with first resonance frequency totaling
134 kHz
kHz was
was elaborated.
elaborated. The
The transducer
transducer frequency
frequency parameters
parameters are
are aa compromise
compromise between
between high
high PD
PD
134
detection sensitivity
sensitivity and
remaining
parameters
of
detection
and the
the resistance
resistanceto
tocore
coremagnetostriction
magnetostrictionnoise.
noise.The
The
remaining
parameters
the
elaborated
ultrasonic
transducer
are
listed
in
Table
2,
whereas
detailed
information
on
the
design,
of the elaborated ultrasonic transducer are listed in Table 2, whereas detailed information on the
modeling,
and production
processes
were discussed
in [33].in
Figure
3 presents
a photograph
of the
design,
modeling,
and production
processes
were discussed
[33]. Figure
3 presents
a photograph
front
and
back
sides
of
the
active
dielectric
window
with
visible
components
of
the
built-in
ultrasonic
of the front and back sides of the active dielectric window with visible components of the built-in
transducer.
ultrasonic
transducer.
Figure
Figure 3.
3. Photographs
Photographs of
of the
the active
active dielectric
dielectric window
window produced.
produced.
Energies 2020, 13, 3271
6 of 37
Table 2. Parameters of piezoelectric transducer components.
Component
Parameter
Symbol
Value
Unit
Matching layer 1
Thickness
Density
Longitudinal velocity
Shear velocity
Bulk modulus
Shear modulus
Acoustic impedance
d
ρ
VL
VS
K
G
ZM
5.05
2079
3029
1640
11.62
5.59
6.3
mm
kg/m3
m/s
m/s
GPa
GPa
MRayl
Piezoelectric ceramics
(PZT-5A disc)
Thickness
Radius
Area
Acoustic impedance
d
r
A
Z0
6.2
6.35
126.61
31.0
mm
mm
mm2
MRayl
Backing layer 2
Thickness
Density
Longitudinal velocity
Shear velocity
Acoustic impedance
Bulk modulus
Shear modulus
d
ρ
VL
VS
ZB
K
G
8.8
2546
1790
845
4.56
5.74
1.82
mm
kg/m3
m/s
m/s
MRayl
GPa
GPa
1
epoxy/alumina composite with 0.3825 volume fraction of alumina; 2 epoxy/tungsten composite with 0.08 volume
fraction of tungsten.
2.2.2. Ultra-High Frequency Antenna
In the power transformers, the registration of the radio signals in the VHF/UHF band generated
by the partial discharges is carried out with the use of antennas mounted in special dielectric
windows [36,37] or antennas put in the tank interior through oil valves [38,39]. There are also
new solutions known, in which, antennas are placed outside the tank and detect the leakage of
electromagnetic waves through the gap (insulation gasket between the upper or lower ladle cover
and the main tank) [40] or bushing taps [41]. Antennas placed in the dielectric windows usually
allow to obtain the highest sensitivity in PD detection. The obstacle in the global popularization of
this solution is the fact that the new power transformers are still very rarely equipped with dielectric
windows. The factory installation of the interior UHF antennas takes place exclusively on demand of
the purchaser. Additionally, the exchange of the revision hatches in the transformer being exploited to
mount dielectric windows and antennas is an expensive, time-consuming, and complicated operation,
both regarding technical and logistic issues. Unfortunately, the remaining solutions, i.e., antennas
placed in the tank interior through oil valves and external antennas, regarding numerous limitations
are not a valuable alternative [39]. The first ones are simple monopole antennas of weak amplification
and a very narrow transmission band. Additionally, the shielding effect of the oil drain valve and
unfavorable placement inside the tank (usually it is a sidewall of the tank, near the bottom), have
a negative impact of their efficiency, because they may hamper the registration of the radio signals
generated by PDs [42]. In turn, the external antennas are vulnerable to the influence of external
electromagnetic disturbances, mainly corona from HV transmission lines. A significant problem is also
the fact that the signal registered by the antenna may have very low energy because of the EM wave
attenuation on the way from the PD source to the antenna [40]. To minimize the problem, it is necessary
to apply advanced algorithms of signals denoising, filtration, and recognition. It is worth mentioning
that only a small part of the useful signal gets out through gaps (if they are present in the given tank
construction at all) and bushing insulators—outside the transformer tank. Having considered all
advantages and disadvantages of the available methods of UHF signals registration, finally, it was
decided that for the needs of the monitoring system the antenna adjusted to installation in the dielectric
window would be elaborated. High EM signals detection sensitivity, which directly translates into
high effectiveness of the PD monitoring system operation, determined its choice.
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In the work of [21] the complex review of antennas applied for PD detection in the VHF/UHF
bandIn
was
Among
the commercial
solutions,applied
circularforplate
antenna (also
UHF band
disk
thepresented.
work of [21]
the complex
review of antennas
PD detection
in thecalled
VHF/UHF
sensor) prevails, since it has a simple construction and because it may be easily adjusted to the
was presented. Among the commercial solutions, circular plate antenna (also called UHF disk sensor)
dimensions of the dielectric window, which usually is round-shaped. A disadvantage of this
prevails, since it has a simple construction and because it may be easily adjusted to the dimensions
construction is a narrow bandwidth and the possibility to regulate the resonant frequency practically
of the dielectric window, which usually is round-shaped. A disadvantage of this construction is a
only by changing the diameter d of the circular plate (d = λ/4 where λ is the wavelength). Therefore,
narrow bandwidth and the possibility to regulate the resonant frequency practically only by changing
in many research centers, the works on the elaboration of the antenna not only of compact dimensions
the diameter d of the circular plate (d = λ/4 where λ is the wavelength). Therefore, in many research
but also of high gain and wide bandwidth, which covers the frequency band of the EM signals
centers, the works on the elaboration of the antenna not only of compact dimensions but also of high
generated by PD in oil-paper insulation system are carried out. Among the most common wideband
gain and wide bandwidth, which covers the frequency band of the EM signals generated by PD in
antenna constructions proposed by researchers, dedicated to PD detection are meander-line antenna
oil-paper insulation system are carried out. Among the most common wideband antenna constructions
[43], logarithmic spiral antenna [44], different types of Archimedean spiral antennas [45,46], Vivaldi
proposed by researchers, dedicated to PD detection are meander-line antenna [43], logarithmic spiral
antenna [47], microstrip patch antennas [48‒50], bio-inspired antennas [51,52], and fractal antennas
antenna [44], different types of Archimedean spiral antennas [45,46], Vivaldi antenna [47], microstrip
such as Hilbert curve fractal antenna [53,54], H-fractal antenna [55], Peano fractal antenna [56], or
patch antennas [48–50], bio-inspired antennas [51,52], and fractal antennas such as Hilbert curve fractal
Minkowski fractal antenna [57].
antenna [53,54], H-fractal antenna [55], Peano fractal antenna [56], or Minkowski fractal antenna [57].
From the viewpoint of efficient PD detection in a power transformer, the key antenna parameter
From the viewpoint of efficient PD detection in a power transformer, the key antenna parameter is
is properly adjusted bandwidth. The research results presented in [58] showed that PDs in oilproperly adjusted bandwidth. The research results presented in [58] showed that PDs in oil-immersed
immersed power transformers generate EM pulses of frequencies from 150 to 700 MHz, wherein most
power transformers generate EM pulses of frequencies from 150 to 700 MHz, wherein most of their
of their energy was transmitted in a band of lower frequencies (up to 400 MHz). In turn, in [59] the
energy was transmitted in a band of lower frequencies (up to 400 MHz). In turn, in [59] the authors
authors registered EM signals simultaneously with the use of three ultra-wideband antennas, which
registered EM signals simultaneously with the use of three ultra-wideband antennas, which jointly
jointly covered a wide measurement band from 20 MHz up to 18 GHz. The research results showed that
covered a wide measurement band from 20 MHz up to 18 GHz. The research results showed that
the main PD types in paper-oil insulation (PD in oil, PD in gas bubbles in oil, inter-turn discharges,
the main PD types in paper-oil insulation (PD in oil, PD in gas bubbles in oil, inter-turn discharges,
surface discharges and creeping sparks on pressboard) generate EM pulses, for which the majority
surface discharges and creeping sparks on pressboard) generate EM pulses, for which the majority of
of the energy is transmitted in the band from 250 to 390 MHz. For that reason, it was decided that the
the energy is transmitted in the band from 250 to 390 MHz. For that reason, it was decided that the
designed antenna would be optimized to operate in this band.
designed antenna would be optimized to operate in this band.
In search of the most efficient and scalable antenna design, the authors tested different
In search of the most efficient and scalable antenna design, the authors tested different constructions
constructions of wideband antennas, among others logarithmic spiral antenna, circular-cross
of wideband antennas, among others logarithmic spiral antenna, circular-cross antenna, H-fractal
antenna, H-fractal antenna, Hilbert curve fractal antenna, and broadband planar inverted-F antenna
antenna, Hilbert curve fractal antenna, and broadband planar inverted-F antenna (PIFA) [53,55].
(PIFA) [53,55]. The results of tests and simulations showed that the highest performance and
The results of tests and simulations showed that the highest performance and possibility of relatively
possibility of relatively easy bandwidth regulation is offered by the modified constructions of a
easy bandwidth regulation is offered by the modified constructions of a conventional PIFA. It is
conventional PIFA. It is currently the most popular antenna in the world applied in mobile devices
currently the most popular antenna in the world applied in mobile devices [60–62]. This is due to its
[60‒62]. This is due to its numerous advantages such as compact size compared to monopole antenna,
numerous advantages such as compact size compared to monopole antenna, high gain in both vertical
high gain in both vertical and horizontal states of polarization, close to omnidirectional radiation
and horizontal states of polarization, close to omnidirectional radiation pattern, and low manufacturing
pattern, and low manufacturing cost. Figure 4 presents a schematic diagram of the conventional
cost. Figure 4 presents a schematic diagram of the conventional PIFA, which consists of a ground
PIFA, which consists of a ground plane, a rectangular radiating element of length equal to the quarterplane, a rectangular radiating element of length equal to the quarter-wave, feeding pin and shorting
wave, feeding pin and shorting plate (or pin) to connect the ground plane with the top patch.
plate (or pin) to connect the ground plane with the top patch.
Figure 4. Schematic diagram of the conventional planar inverted-F antenna (PIFA).
Figure 4. Schematic diagram of the conventional planar inverted-F antenna (PIFA).
Resonant frequency fr of conventional PIFA may be approximately estimated with the use of
Resonant frequency fr of conventional PIFA may be approximately estimated with the use of
Equation (1), in which c is the speed of light in vacuum, εeff is the effective permittivity of the dielectric
Equation (1), in which c is the speed of light in vacuum, εeff is the effective permittivity of the dielectric
substrate, L1 and L2 are the width and length of the rectangular radiating element, H is the thickness of
substrate, L1 and L2 are the width and length of the rectangular radiating element, H is the thickness
Energies 2020, 13, x FOR PEER REVIEW
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of the substrate, and W is the width of shorting plate, assuming that L1 > W > 0 [63,64]. In order to
more accurately estimate the resonant frequency of PIFA, some authors propose more complex
the substrate, and W is the width of shorting plate, assuming that L1 > W > 0 [63,64]. In order to more
formulas
[65].
accurately estimate the resonant frequency of PIFA, some authors propose more complex formulas [65].
c
c
f=
(1)
(1)
fr = r 4√ εeff (L1 +L2 +H-W)
4 εeff (L 1 +L2 +H − W )
The
is the
the narrow
narrow bandwidth.
bandwidth. Simple
The major
major limitation
limitation of
of standard
standard PIFA
PIFA is
Simple methods
methods for
for widening
widening
the
bandwidth
of
PIFA
are
based
on
reducing
the
size
of
the
ground
plane
and
the
use
of
a
the bandwidth of PIFA are based on reducing the size of the ground plane and the use of a substrate
substrate
with
the low
low value
value of
of dielectric
dielectric constant
constant [64].
[64]. Another
with the
Another technique
technique is
is the
the employment
employment of
of specially
specially
designed
slits
in
front
patch
[66‒70],
ground
plane
[71],
or
in
both
these
structures
[72‒74].
designed slits in front patch [66–70], ground plane [71], or in both these structures [72–74]. A
A more
more
advanced
method
of
bandwidth
widening
is
based
on
the
use
of
stacked
patches,
often
also
equipped
advanced method of bandwidth widening is based on the use of stacked patches, often also equipped
in
changing
in slits
slits [75‒79].
[75–79]. Other
Other tuning
tuning techniques
techniques are
are based
based on
on varying
varying the
the position
position of
of feeding
feeding pin,
pin, changing
the
capacitance
value
of
the
shorting
plate,
or
changing
the
widths
of
feed
and
shorting
plates
the capacitance value of the shorting plate, or changing the widths of feed and shorting plates[80,81].
[80,81].
By
By applying
applying aa properly
properly selected
selected chip
chip resistor
resistor or
or chip
chip capacitor
capacitor load
load in
in place
place of
of the
the shorting
shorting plate
plate one
one
may
frequency of
of the
the antenna
antenna [82].
[82].
may reduce
reduce the
the resonant
resonant frequency
For
monitoring system,
construction with
For the
the needs
needs of
of the
the monitoring
system, it
it was
was decided
decided to
to apply
apply PIFA
PIFA construction
with the
the
meandered
loaded with
with chip
chip resistor
resistor [83].
[83]. Meandering
meandered radiating
radiating structure
structure loaded
Meandering the
the radiating
radiating patch
patch allows
allows
to
conventional PIFA
and obtain
obtain large
large gain
gain [84–88].
[84‒88]. In
turn, applying
applying
to reduce
reduce the
the dimensions
dimensions of
of the
the conventional
PIFA and
In turn,
the
chip
resistor
causes
not
only
lowering
of
the
resonant
frequency
but
also
widens
the
bandwidth
the chip resistor causes not only lowering of the resonant frequency but also widens the bandwidth
(even
(even several
several times
times compared
compared to
to standard
standard PIFA)
PIFA) [83].
[83]. A
A schematic
schematic diagram
diagram of
of aa meandered
meandered PIFA
PIFA is
is
presented
in
Figure
5.
presented in Figure 5.
Figure 5. Schematic diagram of the meandered PIFA.
Figure 5. Schematic diagram of the meandered PIFA.
In the initial stage of designing, the necessity to adjust the antenna to the dielectric window
In
the initial
stageinto
of designing,
the necessity
to adjust
the antenna
to 15
thecm
dielectric
dimensions
was taken
account (Figure
2) of internal
diameter
equal to
and thewindow
need to
dimensions
was taken
accounthousing.
(Figure 2)Since
of internal
diameter
to 15as
cmthe
and
the need
to
equip the antenna
in ainto
protective
the gain
of PIFAequal
increases
ground
plane
equip
the
antenna
in
a
protective
housing.
Since
the
gain
of
PIFA
increases
as
the
ground
plane
size
size increases [89], it was assumed that the largest possible diameter equaled 14 cm. The remaining
increases
[89],
it was
assumed
that thePIFA
largest
possible
diameter
equaled
14 iterative
cm. Thesimulations
remaining
dimensions
of the
prototype
meandered
(Table
3) were
optimized
through
dimensions
ofCST
the prototype
meandered
PIFA
(Table Systèmes
3) were optimized
through iterative
simulations
performed in
Microwave
Studio 2016
(Dassault
SE, Vélizy-Villacoublay,
France).
performed
in
CST
Microwave
Studio
2016
(Dassault
Systèmes
SE,
Vélizy-Villacoublay,
France).
The results of the simulation proved that the application of chip-resistor in place of the shorting
The results
of thethe
simulation
proved
that6),
the
application
of chip-resistor
in place
of thethe
shorting
pin allows
to increase
bandwidth
(Figure
whereas
by changing
the distance
between
patch
pin
to plane
increase
the
bandwidth
6), whereas
by changing
the distance
between
the patch
andallows
ground
one
may
regulate(Figure
the resonant
frequency
of the antenna
(Figure
7). Satisfying
and
ground
plane onefor
may
regulate the
resonant
frequency
the antenna
7). Satisfying
results
were obtained
chip-resistor
with
resistance
R = 5.4 of
Ω and
distance (Figure
H = 5 mm,
for which
results
were
obtained
for
chip-resistor
with
resistance
R
=
5.4
Ω
and
distance
H
=
5
mm,
for frequency
which the
the simulated voltage standing wave ratio was VSWR = 1.038 (S11 = -36.34 dB), resonant
simulated
voltage
wave
ratio waswas
VSWR
= 1.038The
(S11use
= -36.34
dB), resonant
fr = 347
fr = 347 MHz,
andstanding
the −6 dB
bandwidth
73 MHz.
of chip-resistor
of frequency
higher resistance
MHz,
and
the
−6
dB
bandwidth
was
73
MHz.
The
use
of
chip-resistor
of
higher
resistance
allows
to
allows to widen the bandwidth even more (e.g., for R = 7.2 Ω, the −6 dB bandwidth grew to 90 MHz).
widen
the bandwidth
more
R = 7.2
the −6
dB bandwidth
to 90fr ,MHz).
Unfortunately,
it is at theeven
expense
of (e.g.,
VSWRfor
growth
andΩ,
a slight
change
of resonant grew
frequency
which
Unfortunately,
it
is
at
the
expense
of
VSWR
growth
and
a
slight
change
of
resonant
frequency
fr,
is shifted to the lower frequencies. In turn, change in distance H between the patch and ground plane
Energies 2020, 13, x FOR PEER REVIEW
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which is shifted to the lower frequencies. In turn, change in distance H between the patch and ground
Energies 2020, 13, 3271
9 of 37
planeisinfluences
the lower
resonant
frequency
a greater
than the
bandwidth.
The results
of the
which
shifted to the
frequencies.
Into
turn,
changeextent
in distance
H between
the patch
and ground
simulation
proved
for Hfrequency
= 3 mm the
frequency
319 MHz, and
H = 6ofmm
plane
influences
the that
resonant
to aresonant
greater extent
thanwas
the bandwidth.
Thefor
results
the it
increased
to
357
MHz,
wherein
−6
dB
bandwidth
was,
respectively,
equal
to
47
and
56
MHz.
influences the
resonant
a greater
extent than
the bandwidth.
The results
of the
simulation
proved
that frequency
for H = 3 to
mm
the resonant
frequency
was 319 MHz,
and for
H =simulation
6 mm it
proved that
for MHz,
H = 3 wherein
mm the −6
resonant
frequency
was
319 MHz, and
fortoH47=and
6 mm
it increased to
increased
to 357
dB bandwidth
was,
respectively,
equal
56 MHz.
Table
3.
Dimensions
of
the
prototype
meandered
PIFA.
357 MHz, wherein −6 dB bandwidth was, respectively, equal to 47 and 56 MHz.
Table 3. Dimensions of the prototype meandered PIFA. Value
Parameter
Indication
Table 3. Dimensions
of the prototype meandered
PIFA. (mm)
Value
Parameter
Indication
Width of the patch
L1
62.5
(mm)
Parameter
Indication
Value (mm)
Length
of
the
patch
L
2
100
Width of the patch
L1
62.5
Width of the Length
patch of the slit
L1
1
50 62.5
S
Length
of the patch
L2
100
Length of the
patch
L
100
2
Widthofofthe
theslit
slit
502 50
Length
S1S2
Length of the
slit
S1
Distance between
patch and ground plane
Width
S2H
25 2
Width of the
slit of the slit
S2
Diameter
of
the
circular
ground
plane
D
Distancepatch
between
and
ground plane H
H
514 5
Distance between
and patch
ground
plane
Distance
between
feeding
point
and
resistor
d
26.3 14
Diameter
of the
circular
ground plane
D
14
Diameter of
the circular
ground
plane
D
Distance between
point
and resistor
Distancefeeding
between
feeding
point and resistor d
d
26.3 26.3
Figure 6. Simulated VSWR for meandered PIFA with various loading chip resistors and constant H =
Figure
6. Simulated VSWR for meandered PIFA with various loading chip resistors and constant
5.0
mm.
Figure
6. Simulated VSWR for meandered PIFA with various loading chip resistors and constant H =
H = 5.0 mm.
5.0 mm.
Figure 7. Simulated VSWR for prototype meandered PIFA with various distances H between patch
Figure 7. Simulated VSWR for prototype meandered PIFA with various distances H between patch
and ground plane and with the same loading chip resistor with resistance R = 5.4 Ω.
and
plane and
with
same loading
chip resistor
with various
resistance
R = 5.4 Ω.
Figureground
7. Simulated
VSWR
forthe
prototype
meandered
PIFA with
distances
H between patch
and
ground
plane
and with the
same loading
chip resistor
with resistance
R = 5.4 Ω. PIFA of dimensions
Based
on the
simulation
results,
the prototype
construction
of meandered
Based on the simulation results, the prototype construction of meandered PIFA of dimensions
presented in Table 3 was manufactured. The antenna was equipped in cylindrical protective housing
presented
in Table
3 was manufactured.
The antenna
was equipped
in cylindrical
protective
housing
Based
the simulation
results,(PTFE)
the prototype
construction
of meandered
PIFA
of dimensions
made
fromon
polytetrafluoroethylene
and matched
to the cavity
of the active
dielectric
window.
made from
polytetrafluoroethylene
(PTFE)
matched
to the cavity cylindrical
of the activeprotective
dielectrichousing
window.
presented
in Table
3 was
manufactured.
The and
antenna
was equipped
The antenna
with the
active
dielectric window,
inspection
hole cover,inand
the module for conditioning
The
antenna
with
the
active
dielectric
window,
inspection
hole
cover,
and
the
module
for
made
from polytetrafluoroethylene
and matched
to the
the active
dielectric window.
and digital
processing of PD pulses(PTFE)
constitutes
a complete
PDcavity
sensoroflabeled
as ISO-002-ADW/UHF
The
antenna
with the active dielectric window, inspection hole cover, and the module for
(Figure
8).
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conditioning and digital processing of PD pulses constitutes a complete PD sensor labeled as ISOconditioning
and(Figure
digital8).
processing of PD pulses constitutes a complete PD sensor labeled as ISO002-ADW/UHF
Energies
2020, 13, 3271
10 of 37
002-ADW/UHF (Figure 8).
Figure 8. Components of
N-type
connector,
3
Figure
of the
the ISO-002-ADW/UHF
ISO-002-ADW/UHF sensor:
sensor:11==meandered
meanderedPIFA,
PIFA,2 2= =
N-type
connector,
Figure
8. Components
of the ISO-002-ADW/UHF
sensor:
1 = meandered
PIFA,
= N-type
connector,
3
PTFE-housing,
4 4= =active
window,
55==enclosure
for
discharge
3= =cylindrical
cylindrical
PTFE-housing,
activedielectric
dielectric
window,
enclosure
for2partial
partial
(PD)
=pulses
cylindrical
PTFE-housing,
4and
= active
dielectric window,
5 =module,
enclosure
partial
discharge
(PD)
conditioning
module and
analog-to-digital
processing
module,6 =6for
= fiber
optic
cable
gland,
pulses
module
analog-to-digital
processing
fiber
optic
cable
gland,
7=
pulses
conditioning
module
and
analog-to-digital
processing
module,
6
=
fiber
optic
cable
gland,
7
=
7cable
= cable
gland
for power
supply,
= shielding
for signal
cables,
9 = inspection
cover.
gland
for power
supply,
8 = 8shielding
tubetube
for signal
cables,
9 = inspection
holehole
cover.
cable gland for power supply, 8 = shielding tube for signal cables, 9 = inspection hole cover.
Having
antenna
in the
dielectric
window,
the VSWR
was measured
Havinginstalled
installedthe
the
antenna
in active
the active
dielectric
window,
thecharacteristic
VSWR characteristic
was
Having
installed
the
antenna
in
the
active
dielectric
window,
the
VSWR
characteristic
using
a vector
network
analyzer
typeanalyzer
KC901S+type
(Deepace,
Guangdong,
The measurements
measured
using
a vector
network
KC901S+
(Deepace,China).
Guangdong,
China). was
The
measured
using
a
vector
network
analyzer
type
KC901S+
(Deepace,
Guangdong,
China).
The
showed
that
compared
to
the
simulation
results,
the
VSWR
curve
is
slightly
shifted
towards
lower
measurements showed that compared to the simulation results, the VSWR curve is slightly shifted
measurements
showed
compared
simulation
results,
theantenna
VSWR curve
is slightly
shifted
frequencies
(Figure
9). that
It is(Figure
probably
an
effect
of the
antenna
surroundings
influence,
i.e.,
metal
towards lower
frequencies
9). Itto
isthe
probably
an effect
of the
surroundings
influence,
towards
lower
frequencies
(Figure
9).
It
is
probably
an
effect
of
the
antenna
surroundings
influence,
transformer
tank and inspection
cover, hole
which
werewhich
not taken
in the
simulations.
i.e., metal transformer
tank and hole
inspection
cover,
wereinto
notaccount
taken into
account
in the
i.e.,
metal
transformer
tank resonant
and inspection
holeof
cover,
which
were
not
taken
into Saccount
in
the
The
measured
resonant
frequency
of the
antenna
was
equal
to 329
MHz
(VSWR
= 1.13;
dB),
11 = −24.3
simulations.
The
measured
frequency
the
antenna
was
equal
to
329
MHz
(VSWR
= 1.13;
simulations.
The
measured
resonant
frequency
of
the
antenna
was
equal
to
329
MHz
(VSWR
=
1.13;
and
dB dB),
bandwidth
to 73 MHz
(from
to 353(from
MHz).
S11 =−6
−24.3
and −6 equal
dB bandwidth
equal
to 280
73 MHz
280 to 353 MHz).
S11 = −24.3 dB), and −6 dB bandwidth equal to 73 MHz (from 280 to 353 MHz).
Figure 9.
9. Simulated and measured VSWR of the prototype meandered PIFA.
PIFA.
Figure
Figure 9. Simulated and measured VSWR of the prototype meandered PIFA.
In order
order to check the performance of meandered PIFA,
PIFA, comparative
comparative research
research was
was conducted,
conducted, in
In aorder
to checkUHF
the performance
of meandered
PIFA,
comparative
research
was conducted,
infr
which
commercial
disk sensor
sensor with
with
diameter
15
cmand
andmeasured
measured
resonant
frequency
which
disk
aadiameter
ofof15
cm
resonant
frequency
which
a MHz
commercial
UHF
sensor
with a diameter
of 15were
cm and
measured
fr
f=r =
475
wasused
used
reference
antenna.
Bothsensors
sensors
were
installed
in an
anresonant
transformer
475
MHz
was
asasa adisk
reference
antenna.
Both
installed
in
oil-filled frequency
=tank
475
MHz
was
used
as
a
reference
antenna.
Both
sensors
were
installed
in
an
oil-filled
transformer
1200
× 800
× 750
mm,mm,
equipped
in twoinceramic
dielectric
windowswindows
and an electrode
tank ofofdimensions
dimensions
1200
× 800
× 750
equipped
two ceramic
dielectric
and an
tank
of
dimensions
1200
×
800
×
750
mm,
equipped
in
two
ceramic
dielectric
windows
and
an
system
for
generating
surface
discharges
on
the
pressboard
sample.
The
distance
between
the PD
electrode system for generating surface discharges on the pressboard sample. The distance between
electrode
system
for
generating
surface
discharges
on
the
pressboard
sample.
The
distance
between
source
the central
point
of the
antenna,
the case of
sensors,
was the
same was
and equal
to about
the PDand
source
and the
central
point
of theinantenna,
inboth
the case
of both
sensors,
the same
and
the
PDto
source
and
the The
central
point
of athe
antenna,with
inoscilloscope
the
case of type
both Tektronix
sensors, was
the3104
same
anda
45
cm.
The
pulses
registered
with
four-channel
MDO
with
equal
about
45 were
cm.
pulses
were
registered
a four-channel
oscilloscope
type
Tektronix
equal
about
pulses
wereof
registered
withlevel
a four-channel
oscilloscope
sampling
frequency
of 5The
GS/s.
The level
the
apparent
charge
theapparent
generated
partialoftype
discharges
was
MDO to
3104
with45a cm.
sampling
frequency
of
5 GS/s.
The
ofofthe
charge
the Tektronix
generated
MDO
3104
with
a
sampling
frequency
of
5
GS/s.
The
level
of
the
apparent
charge
of
the
generated
monitored
by the use
the standard
measurement
set compliant
with the
60270 standard
partial discharges
wasof
monitored
by the
use of the standard
measurement
setIEC
compliant
with
the [90]
IEC
partial
discharges
was
monitored
(Figure
10).
60270 standard
[90]
(Figure
10). by the use of the standard measurement set compliant with the IEC
60270 standard [90] (Figure 10).
of the surface discharges of the average apparent charge equal to qavg = 895 pC, the growth of the
amplitude of the registered UHF pulses compared to the disk antenna by 7.80 ± 0.9 times on average
was found. In turn, in case of high energy creeping sparks with an average apparent charge equal to
qavg = 5990 pC, lower growth of the amplitude, equal to 1.75 ± 0.7 times on average was observed. The
comparison
of3271
the exemplary time waveforms of the PD pulses registered by the prototype antenna
Energies 2020, 13,
11 of 37
Energies
2020,
13,
x
PEER
REVIEW
11 of 38
and commercialFOR
disk
antenna
is demonstrated in Figure 11.
The research results confirmed the high sensitivity of the prototype antenna. For the initial form
of the surface discharges of the average apparent charge equal to qavg = 895 pC, the growth of the
amplitude of the registered UHF pulses compared to the disk antenna by 7.80 ± 0.9 times on average
was found. In turn, in case of high energy creeping sparks with an average apparent charge equal to
qavg = 5990 pC, lower growth of the amplitude, equal to 1.75 ± 0.7 times on average was observed. The
comparison of the exemplary time waveforms of the PD pulses registered by the prototype antenna
and commercial disk antenna is demonstrated in Figure 11.
Schematic diagram
diagram of
of the
the measurement
measurement setup
setup in
in which
which PD detection sensitivity
sensitivity of the
Figure 10. Schematic
prototype
UHF
antenna
was
tested:
U
=
high-voltage
supply,
Z
=
short-circuit
current
limitinglimiting
resistor,
prototype UHF antenna was tested: U = high-voltage supply, Z = short-circuit current
OSC = four-channel
oscilloscope,
TT = oil-filled
transformer
tank, PDtank,
= electrode
system for
surface
resistor,
OSC = four-channel
oscilloscope,
TT = oil-filled
transformer
PD = electrode
system
for
partial
generation,
PA = prototype
meandered
PIFA, CA PIFA,
= commercial
UHF disk antenna
as
surfacedischarge
partial discharge
generation,
PA = prototype
meandered
CA = commercial
UHF disk
reference,asCreference,
= coupling
(measuring
impedance),impedance),
CC = connecting
couplingCD
capacitor,
CD device
= coupling
device (measuring
CC =
antenna
CK =capacitor,
K = coupling
cable, M = conventional
partial discharge
PCdetector,
= computer.
connecting
cable, M = conventional
partialdetector,
discharge
PC = computer.
The research results confirmed the high sensitivity of the prototype antenna. For the initial form
of theFigure
surface
of the average
apparent charge
equal
to PD
qavgdetection
= 895 pC,
the growth
of the
10. discharges
Schematic diagram
of the measurement
setup in
which
sensitivity
of the
amplitude
of the
registered
pulses U
compared
to the disk
antenna
by 7.80 ± 0.9
timeslimiting
on average
prototype
UHF
antenna UHF
was tested:
= high-voltage
supply,
Z = short-circuit
current
was found.
In
turn,
in
case
of
high
energy
creeping
sparks
with
an
average
apparent
charge
resistor, OSC = four-channel oscilloscope, TT = oil-filled transformer tank, PD = electrode systemequal
for to
qavg =surface
5990 pC,
lower
growth
of
the
amplitude,
equal
to
1.75
±
0.7
times
on
average
was
observed.
partial discharge generation, PA = prototype meandered PIFA, CA = commercial UHF disk
The comparison
of the exemplary
time waveforms
of =the
PD pulses
registered
by the
prototypeCC
antenna
capacitor, CD
coupling
device
(measuring
impedance),
=
antenna as reference,
CK = coupling
and commercial
disk
antenna
is
demonstrated
in
Figure
11.
connecting cable, M = conventional partial discharge detector, PC = computer.
(b)
(a)
Figure 11. The comparison of the exemplary UHF pulses registered by the commercial UHF disk
antenna and prototype meandered PIFA for (a) surface PD of an apparent charge of q = 625 pC, (b)
creeping spark of apparent charge of q = 4670 pC.
2.2.3. High-Frequency Current Transformer
The high-frequency current transformer is an inductive PD detector, (b)
which is usually installed
(a) investigated high voltage device. That wire is simultaneously a primary
on the wire grounding the
winding of the current transformer. A typical sensor HFCT consists of toroidal ferrite core, on which
Figure 11.
11. The comparison of the exemplary UHF pulses registered by the commercial
commercial UHF
UHF disk
disk
Figure
from several to over a dozen turns (secondary winding) are wound-up. This winding is terminated
apparent
charge
of of
q =q625
pC,pC,
(b)
antenna and prototype
prototype meandered
meanderedPIFA
PIFAfor
for(a)
(a)surface
surfacePD
PDofofanan
apparent
charge
= 625
with creeping
a low-inductance
sense
resistor.
A
current
pulse
of
high
frequency
flowing
through
the
spark
of apparent
charge
of qof= q4670
pC.pC.
(b) creeping
spark
of apparent
charge
= 4670
grounding wire (primary winding) causes the occurrence of a time-varying magnetic field, which
2.2.3.
High-Frequency
Current
passes
through the secondary
winding and induces a voltage across the sense resistor. Figure 12
2.2.3. High-Frequency
Current Transformer
Transformer
presents a model and equivalent circuit of the HFCT sensor, where i is the current flowing through
The
current transformer
is an inductive
PD
which is
installed
The high-frequency
high-frequency
transformer
PD detector,
detector,
is usually
usually
installed
the primary
winding, R iscurrent
the sense
resistor, VisR an
an inductive
induced voltage
in the which
secondary
winding,
Lm is
on
the
wire
grounding
the
investigated
high
voltage
device.
That
wire
is
simultaneously
a
on the wire grounding the investigated high voltage device. That wire is simultaneously a primary
primary
winding
winding of
of the
the current
current transformer.
transformer. A
A typical
typical sensor
sensor HFCT
HFCT consists
consists of
of toroidal
toroidal ferrite
ferrite core,
core, on
on which
which
from
several
to
over
a
dozen
turns
(secondary
winding)
are
wound-up.
This
winding
is
terminated
from several to over a dozen turns (secondary winding) are wound-up. This winding is terminated
with
sense
resistor.
A current
pulse of
highof
frequency
flowing through
grounding
with aalow-inductance
low-inductance
sense
resistor.
A current
pulse
high frequency
flowingthe
through
the
wire
(primary
winding)
causes
the
occurrence
of
a
time-varying
magnetic
field,
which
passes
through
grounding wire (primary winding) causes the occurrence of a time-varying magnetic field, which
the
secondary
winding
and induces
a voltage
theasense
resistor.
Figure
12 presents
a model
passes
through
the secondary
winding
and across
induces
voltage
across
the sense
resistor.
Figureand
12
presents a model and equivalent circuit of the HFCT sensor, where i is the current flowing through
the primary winding, R is the sense resistor, VR an induced voltage in the secondary winding, Lm is
Energies 2020, 13, 3271
12 of 37
Energies 2020, 13, x FOR PEER REVIEW
12 of 38
equivalent circuit of the HFCT sensor, where i is the current flowing through the primary winding, R is
the sense
magnetizing
of the
currentintransformer,
Rc winding,
is the coreL parallel
equivalent resistance, Ll
the
resistor,inductance
VR an induced
voltage
the secondary
m is the magnetizing inductance
and
Rscurrent
are the leakage
inductance
andcore
series
resistance
of the secondary
The
of
the
transformer,
Rc is the
parallel
equivalent
resistance, winding,
Ll and Rsrespectively.
are the leakage
remaining
parasitic
components
included
in
the
model
are
stray
capacitance
C
and
inductance
L
R of
inductance and series resistance of the secondary winding, respectively. The remaining parasitic
the sense resistor
[91].in the model are stray capacitance C and inductance L of the sense resistor [91].
components
included
R
Figure
High-frequency current
current transformer
Figure 12.
12. High-frequency
transformer model
model with
with parasitic
parasitic components.
components.
The HFCT sensors are applied in the diagnostics of high-voltage power cables, power generators,
The HFCT sensors are applied in the diagnostics of high-voltage power cables, power
metal-enclosed switchgears, and gas-insulated systems (GIS) [92–94]. In the case of power transformers,
generators, metal-enclosed switchgears, and gas-insulated systems (GIS) [92‒94]. In the case of power
the HFCT sensor may be installed on a grounding wire of the tank, on a grounded measuring tap
transformers, the HFCT sensor may be installed on a grounding wire of the tank, on a grounded
of the bushing insulator, or a grounding of the neutral point of the transformer winding. The last
measuring tap of the bushing insulator, or a grounding of the neutral point of the transformer
option assures the highest detection sensitivity of the PDs occurring in the power transformer main
winding. The last option assures the highest detection sensitivity of the PDs occurring in the power
insulation system.
transformer main insulation system.
The toroidal HFCT sensor core is most commonly made from soft ferrite. This material is
The toroidal HFCT sensor core is most commonly made from soft ferrite. This material is
ferromagnetic and consists of iron oxides mixed with oxides of manganese and zinc (MnZnFe2 O4 ),
ferromagnetic and consists of iron oxides mixed with oxides of manganese and zinc (MnZnFe2O4), or
or nickel and zinc (NiZnFe2 O4 ). Manganese-zinc soft ferrites have high relative permeability
nickel and zinc (NiZnFe2O4). Manganese-zinc soft ferrites have high relative permeability (µr ≈ 350–20,000
(µr ≈ 350–20,000 at 10 kHz), medium saturation flux density (Bs ≈ 320−545 mT at 1200 A/m),
at 10 kHz), medium saturation flux density (Bs ≈ 320−545 mT at 1200 A/m), operating frequency range
operating frequency range from a few kilohertz to 4 MHz, and low resistivity ρ (from 10 Ωm at 10 kHz
from a few kilohertz to 4 MHz, and low resistivity ρ (from 10 Ωm at 10 kHz to 0.01 Ωm at 100 MHz).
to 0.01 Ωm at 100 MHz). In turn, nickel-zinc soft ferrites are characterized by low relative permeability
In turn, nickel-zinc soft ferrites are characterized by low relative permeability (µr ≈ 15−2000 at 10 kHz),
(µr ≈ 15−2000 at 10 kHz), slightly lower saturation flux density (Bs ≈ 220−380 mT at 1200 A/m), much
slightly lower saturation flux density (Bs ≈ 220−380 mT at 1200 A/m), much higher maximum
higher maximum operating frequency (up to about 200 MHz), and much higher resistivity
ρ (from
operating
frequency (up
to about 200 MHz), and much higher resistivity ρ (from 105 Ωm at 10 kHz to
5
3
10 3 Ωm at 10 kHz to 10 Ωm at 100 MHz) [95].
10 Ωm at 100 MHz) [95].
The clamp-type HFCT sensor devoted to PD monitoring in power transformers should have high
The clamp-type HFCT sensor devoted to PD monitoring in power transformers should have
sensitivity, wide bandwidth, closer to the constant line frequency response characteristics, and be
high sensitivity, wide bandwidth, closer to the constant line frequency response characteristics, and
equipped in enclosure shielding from external electromagnetic interferences. The sensitivity S of the
be equipped in enclosure shielding from external electromagnetic interferences. The sensitivity S of
HFCT sensor depends on resistance value R and the number of windings N, as shown in Equation (2).
the HFCT sensor depends on resistance value R and the number of windings N, as shown in Equation
(2).
R
S=
(2)
N
R
(2)
S=
The lower −3 dB cutoff frequency of HFCT sensor
N is given by
Rc ||(Rsensor
+ Rs ) is given
R by
The lower −3 dB cutoff frequency of HFCT
fL =
≈
(3)
2πLm
2πLm
Rc ||(R+Rs )
R
(3)
≈
fL = The HFCT sensors
where Lm is the magnetizing inductance.
2πLmare mostly loaded with non-inductive
2πLm
resistor R = 50 Ω, in order to assure a good impedance matching with the transmission line (usually it
is
a standard
50-ohm
coaxial cable)
and with
analog
inputare
of mostly
signal acquisition
Therefore,
where
Lm is the
magnetizing
inductance.
Thethe
HFCT
sensors
loaded withunit.
non-inductive
to
achieve
low
of the
fL , the magnetizing
inductance
should be line
possibly
the
resistor
R =a 50
Ω,value
in order
to cutoff
assurefrequency
a good impedance
matching with
the transmission
(usually
highest.
The
magnetizing
inductance
L
on
the
secondary
side
is
given
by
m
it is a standard 50-ohm coaxial cable) and with the analog input of signal acquisition unit. Therefore,
to achieve a low value of the cutoff frequency fL, the magnetizing
inductance should be possibly the
µr µ0 N2 Ac
highest. The magnetizing inductance Lm on
the
secondary
side
is
given
by
Lm =
(4)
lc
Lm =
µr µ0 N2 Ac
lc
(4)
Energies 2020, 13, 3271
13 of 37
where µ0 is the vacuum permeability (magnetic constant), Ac is the core cross-sectional area, and lc
is the toroidal mean length. Equation (4) shows that the high value of the magnetizing inductance
Lm may be obtained the easiest using a large number of turns N. Unfortunately, with increasing the
number of turns, the sensor sensitivity is reduced. The other way is a choice of a core made of a
material with high relative permeability µr (e.g., MnZn soft ferrites) and possibly the smallest toroidal
mean length lc [95,96].
In turn, the higher –3 dB cutoff frequency of the HFCT sensor mainly depends on stray capacitance
C and the leakage inductance Ll (Equation (5)). To obtain a high value of fH , both parameters should
be reduced.
1
(5)
fH =
r
L 2
2
l
2π (RC) + Rc
The stray capacitance C consists of several capacitive couplings, which are present in high-frequency
current transformer, i.e., turn-to-turn capacitance, winding-to-core capacitance, winding-to-shielded
enclosure capacitance, and primary-to-secondary winding capacitance. The stray capacitance can be
minimized by reducing the number of turns N, increasing the insulation layer of core and secondary
winding (using a material with low dielectric constant εr ), increasing the distance between turns, and
using a Faraday electrostatic shield [95].
The reduction of the number of turns N is also favorable from the viewpoint of leakage inductance
Ll minimizing. Additionally, the secondary winding should be made wide and have a small thickness.
As one may notice, the factors which have the greatest influence on the sensitivity and width of
the HFCT sensor band, are the number of windings N and the type of material, which the toroidal
core is made from. The cores made from MnZn and NiZn soft ferrites were selected for the tests.
The physical parameters of the materials are listed in Table 4.
Table 4. Specifications of soft ferrite materials used in the study.
Ferrite Type
Parameter
Symbol
Value
Unit
Conditions
MnZn
Initial permeability
Amplitude permeability
Magnetic flux density
Specific power loss
Loss factor
Resistivity
Curie temperature
Density
µi
µa
B
Pv
tan
ρ
Tc
D
900 ± 20%
≈ 1700
≈ 410
≈ 130
n/a
≈ 10
≥ 220
≈ 4700
−
−
mT
kW/m3
−
Ωm
◦C
kg/m3
25
≤ 10 kHz; 0.25 mT
100 ◦ C; 25 kHz; 200 mT
25 ◦ C; 10 kHz; 1200 A/m
100 ◦ C; 1 MHz; 30 mT
−
DC; 25 ◦ C
−
−
NiZn
Initial permeability
Amplitude permeability
Magnetic flux density
Specific power loss
Loss factor
Resistivity
Curie temperature
Density
µi
µa
B
Pv
tan
ρ
Tc
D
125 ± 20%
n/a
≈ 380
n/a
≤ 80 × 10-6
≈ 105
≥ 350
≈ 4500
−
−
mT
−
−
Ωm
◦C
kg/m3
25 ◦ C; ≤ 10 kHz; 0.25 mT
−
25 ◦ C; 10 kHz; 3000 A/m
−
25 ◦ C; 3 MHz; 0.25 mT
DC; 25 ◦ C
−
−
◦ C;
The dimensions of the applied ferrite cores were selected so that, after equipping them in shielding
enclosure, they could be easily installed on the wire grounding the transformer tank or the neutral
point of the star windings. Since the width of the grounding wire/rail of power transformer usually
does not exceed 40 mm, it was decided to choose toroidal cores with an internal diameter equal to
65 mm.
The external diameter MnZn ferrite core was 107 mm, and its height was 18 mm. For NiZn
ferrite core they were, respectively, 103 and 16 mm. To reduce the skin effect, for manufacturing of the
secondary winding, the high-frequency Litz-wire with a diameter of 0.48 mm and external insulation
with a thickness of 0.02 mm made from natural silk of dielectric constant εr = 2.6 was used.
Energies 2020, 13, x FOR PEER REVIEW
14 of 38
Energies 2020, 13, 3271
14 of 37
secondary winding, the high-frequency Litz-wire with a diameter of 0.48 mm and external insulation
with a thickness of 0.02 mm made from natural silk of dielectric constant εr = 2.6 was used.
Litz-wire consists
consists of
of 920
920 strands
strands insulated
Litz-wire
insulated electrically
electrically from
fromeach
eachother,
other,wherein
whereinaadiameter
diameterofofa
(AWG54).
TheThe
cores
were
splitsplit
in half
using
a diamond
cutting
disc.disc.
The
asingle
singlewire
wireis is0.016
0.016mm
mm
(AWG54).
cores
were
in half
using
a diamond
cutting
secondary
winding
with
a
counter-wound
compensation
turn
was
applied
to
one
of
the
core
halves.
The secondary winding with a counter-wound compensation turn was applied to one of the core
To reduce
the stray
capacitance
(or more
precisely,
the
distance
halves.
To reduce
the stray
capacitance
(or more
precisely,
theturn-to-turn
turn-to-turncapacitances),
capacitances), the
the distance
betweenindividual
individualturns
turnswas
wasmaximized.
maximized. The
The enclosure
enclosure of
of HFCT
HFCT sensor
sensorwas
was made
madefrom
fromaluminum
aluminum
between
withaathickness
thicknessof
of22mm
mmtotoattenuate
attenuateradiofrequency
radiofrequencyelectromagnetic
electromagneticinterferences
interferences and
and improved
improved
with
performance in
in an
anelectrically
electrically noisy
noisy substation
substation environment.
environment. It
It was
was equipped
equipped with
with aa 11 mm
mm slit
slit for
for
performance
better
magnetic
coupling
between
primary
and
secondary
winding
[97].
The
sensor
HFCT
housing
better magnetic coupling between primary and secondary winding [97]. The sensor HFCT housing
and the
the housing
housing of
of the
the conditioning
conditioning and
and A/D
A/D processing
processing module
module were
were integrated
integrated and
and jointly
jointly are
are aa
and
completesensor
sensorISO-002-HF
ISO-002-HFwith
withthe
thefunction
functionof
offiber
fiberoptic
opticsignal
signaltransmission
transmission(Figure
(Figure13).
13).
complete
Energies 2020, 13, x FOR PEER REVIEW
15 of 38
visible for sensors with MnZn soft ferrite core, whose magnetizing inductance is high regarding the
high value of permeability µ at low frequencies. For comparison, the value fL for current transformer
with Figure
MnZn 13.
ferrite
core and 10 turns is lower than 100 kHz, in turn, for a current transformer with
The components of sensor ISO-002-HF: 1 = MnZn ferrite core with secondary winding,
13.
The
ofnumber
sensor4ISO-002-HF:
= MnZn
ferrite
core
secondary
winding,
2 = the
NiZn2Figure
ferrite
core
with
the same
ofhigh-frequency
turns, fL 1equals
450
kHz.
Forwith
a lower
number
of
turns,
= insulation, 3components
= N-type
connector,
=
current
transformer
(HFCT)
sensor
housing,
3 = N-type
connector,
4 = high-frequency
(HFCT)
sensor
housing,
differences
more
clear.
For
instance,
fLlatch,
for HFCT
sensortransformer
with
MnZn
ferrite
core and
3 5is=also
5insulation,
= slitare
in even
metallic
housing,
6 = adjustable
7 = current
waterproof
enclosure
for electronics,
8N
= =cable
slit
in
metallic
housing,
6
=
adjustable
latch,
7
=
waterproof
enclosure
for
electronics,
8
=
cable
gland
very low
and
equals
250
kHz.
The
current
transformer
of
the
same
construction,
but
with
the
ferrite
gland for power supply, 9 = fiber optic cable gland.
for
power
supply,
9
=
fiber
optic
cable
gland.
core NiZn, has fL equal to even 3 MHz. It should be emphasized that for the HFCT sensors devoted
investigation
on important
the influence
of the number
N and
material
the usedfHcore
on
to PDThe
detection,
the more
parameter
than theoffL turns
is higher
−3 dB
cutoff of
frequency
, since
The investigation
on
theinsulation
influence emit
of
thehigh-frequency
number
turns
N and mainly
material
the used
on
the
the frequency
response
of the
prototype
HFCT
sensorsof
was
performed
using the
measurement
setup
the
discharges
in oil-paper
waves
inofthe
rangecore
between
frequency
response
of [98].
the
HFCT
was performed
using
theTektronix
measurement
setup
presented
in
Figure
14.
Theprototype
measurement
consisted
of the
oscilloscope
MDO
3104
approx.
2 and
20 MHz
In
this case,
it setup
is sensors
more
favorable
to apply
a lower
number
of
turns.
It
presented
in
Figure
14.
The
measurement
setup
consisted
of
the
oscilloscope
Tektronix
MDO
3104
with the
frequency
of 5 GS/s,
1 GHz
bandwidth,
and high
built-in
signal
The sinusoidal
allows
tosampling
obtain high
sensitivity
S of the
HFCT
sensor and
value
of generator.
the parameter
fH, which
with the
sampling
frequency
5 range
GS/s,
1 range
GHz
bandwidth,
andwith
built-in
signal
generator.
The
signal
wasthe
generated
in
theof
frequency
from
100of
kHz
to 30frequencies
MHz
a(>step
frequency
of 100
kHz.
improves
performance
the of
sensor
in the
higher
1 MHz).
One
may
notice
sinusoidal
signal
was
generated
in
the
frequency
range
from
100
kHz
to
30
MHz
with
a
step
It allowed
to cover
the positive
full rangeeffect
of high-frequency
(HF)
band, i.e.,
MHz. The
in for
the
in
Figure 14,
that the
resulting from
reduction
of 3–30
the number
of current
turns isflow
more
frequency
ofthe
100core
kHz.made
It allowed
to
cover
the
full range
high-frequency
band,
i.e.,
3‒30 MHz.
primary
winding
was
forced
by using
a 50
Ω
noninductive
resistor
RL inµ series.
same
resistor
was
sensors
with
from
NiZn
soft
ferrite
since of
permeability
of (HF)
the The
applied
material
is
a
The
current
flow
in
the
primary
winding
was
forced
by
using
a
50
Ω
noninductive
resistor
R
L in
connected
in
parallel
with
the
secondary
winding
of
the
HFCT
sensor.
practically constant value in the wide range between 1 MHz and approx. 50 MHz.
series. The same resistor was connected in parallel with the secondary winding of the HFCT sensor.
The measurement of the frequency response was automatically realized by the application
implemented in LabVIEW environment and launched on a computer connected to the oscilloscope.
The individual points of the frequency response curve FRHFCT(f) were calculated as
𝐹𝑅
(𝑓)=20∙log
VR (𝑓)
,
VGEN (𝑓)
(6)
where VR(f) is the RMS value of voltage measured on the output of the HFCT sensor at frequency f,
VGEN(f) is the RMS value of voltage measured on the output of signal generator at frequency f.
Figure 15 presents the frequency response curves of the prototype HFCT sensors. As one may
notice, simultaneously with the increase in the number of turns, the sensor sensitivity decreases. It
results from the fact that the values of that parameter depend directly proportional to the load
Figure14.
14.Schematic
Schematicdiagram
diagram of
measurement setup
for
measuring
the
frequency
response
of
Figure
thethe
measurement
forto
measuring
the inversely
frequency
response
of the
resistance
R (which
in this caseofwas
constant andsetup
equal
50 Ω) and
proportional
to the
the
HFCT
sensor;
OSC
=
oscilloscope
type
Tektronix
MDO3104,
GEN
=
built-in
arbitrary
function
HFCTofsensor;
= oscilloscope
type Tektronix
MDO3104,
GEN
= built-in arbitrary
function
number
turns OSC
N. Increasing
the number
of turns
raises the
magnetizing
inductance
Lm, which
generator, PC
PC ==computer,
computer,HFCT
HFCT==high-frequency
high-frequencycurrent
current transformerunder
undertest.
test.
generator,
causes
the reduction
of lower −3 dB
cutoff frequency fL oftransformer
the HFCT sensor.
This effect is even more
Energies 2020,
2020, 13,
13, 3271
x FOR PEER REVIEW
Energies
15
15 of
of 38
37
visible for sensors with MnZn soft ferrite core, whose magnetizing inductance is high regarding the
of µ
the
response
automatically
realized
by the transformer
application
high The
valuemeasurement
of permeability
at frequency
low frequencies.
For was
comparison,
the value
fL for current
implemented
in
LabVIEW
environment
and
launched
on
a
computer
connected
to
the
oscilloscope.
with MnZn ferrite core and 10 turns is lower than 100 kHz, in turn, for a current transformer with
The
points
frequency
FRHFCT
were
calculated
as
NiZnindividual
ferrite core
with of
thethe
same
numberresponse
of turns,curve
fL equals
450(f)
kHz.
For
a lower number
of turns, the
differences are even more clear. For instance, fL for HFCT sensor with MnZn ferrite core and N = 3 is also
VR ( f )
very low and equals 250 kHz. The current
FRHFCT (transformer
f )= 20· log of the same
, construction, but with the ferrite
(6)
VGEN ( f ) that for the HFCT sensors devoted
core NiZn, has fL equal to even 3 MHz. It should be emphasized
to PD detection,
the
more
important
parameter
than
is higher
−3 dB
cutoff
frequency
fH, sincef,
where
VR (f ) is the
RMS
value
of voltage
measured
onthe
thefLoutput
of the
HFCT
sensor
at frequency
the
discharges
in
oil-paper
insulation
emit
high-frequency
waves
mainly
in
the
range
between
VGEN (f ) is the RMS value of voltage measured on the output of signal generator at frequency f.
approx.
2 and
20 MHz the
[98].frequency
In this case,
it is more
favorable
to apply aHFCT
lowersensors.
numberAs
of one
turns.
It
Figure
15 presents
response
curves
of the prototype
may
allows to
obtain high sensitivity
S of the in
HFCT
sensor and
high value
of thesensitivity
parameter decreases.
fH, which
notice,
simultaneously
with the increase
the number
of turns,
the sensor
improves
the
performance
of
the
sensor
in
the
range
of
higher
frequencies
(>
1
MHz).
One
may
It results from the fact that the values of that parameter depend directly proportional to thenotice
load
in
Figure
14,
that
the
positive
effect
resulting
from
reduction
of
the
number
of
turns
is
more
for
resistance R (which in this case was constant and equal to 50 Ω) and inversely proportional to the
sensors
with
the
core
made
from
NiZn
soft
ferrite
since
permeability
µ
of
the
applied
material
is
number of turns N. Increasing the number of turns raises the magnetizing inductance Lm , which causesa
practically
constant
value
in cutoff
the wide
range between
1 MHzsensor.
and approx.
50 MHz.
the
reduction
of lower
−3 dB
frequency
f of the HFCT
This effect
is even more visible for
L
sensors with MnZn soft ferrite core, whose magnetizing inductance is high regarding the high value of
permeability µ at low frequencies. For comparison, the value fL for current transformer with MnZn
ferrite core and 10 turns is lower than 100 kHz, in turn, for a current transformer with NiZn ferrite
core with the same number of turns, fL equals 450 kHz. For a lower number of turns, the differences
are even more clear. For instance, fL for HFCT sensor with MnZn ferrite core and N = 3 is also very
low and equals 250 kHz. The current transformer of the same construction, but with the ferrite core
NiZn, has fL equal to even 3 MHz. It should be emphasized that for the HFCT sensors devoted to PD
detection, the more important parameter than the fL is higher −3 dB cutoff frequency fH , since the
discharges in oil-paper insulation emit high-frequency waves mainly in the range between approx.
2 and 20 MHz [98]. In this case, it is more favorable to apply a lower number of turns. It allows to
obtain high sensitivity S of the HFCT sensor and high value of the parameter fH , which improves the
performance of the sensor in the range of higher frequencies (> 1 MHz). One may notice in Figure 14,
Figure
14. Schematic
diagram of
the measurement
for measuring
the is
frequency
response
the the
that the
positive
effect resulting
from
reduction ofsetup
the number
of turns
more for
sensorsofwith
HFCT
sensor;
OSC
=
oscilloscope
type
Tektronix
MDO3104,
GEN
=
built-in
arbitrary
function
core made from NiZn soft ferrite since permeability µ of the applied material is a practically constant
PC =range
computer,
HFCT
= high-frequency
current
transformer under test.
valuegenerator,
in the wide
between
1 MHz
and approx.
50 MHz.
(a)
(b)
Figure 15.
onon
the
number
of of
turns
N
Figure
15. Frequency
Frequencyresponse
responseofofthe
theprototype
prototypeHFCT
HFCTsensors
sensorsdepending
depending
the
number
turns
and
the
core
material
used:
(a)
MnZn
soft
ferrite,
(b)
NiZn
soft
ferrite,
compared
with
frequency
N and the core material used: (a) MnZn soft ferrite, (b) NiZn soft ferrite, compared with frequency
response of
of commercial
commercial HFCT
HFCT sensors
sensors (RFCT-4
(RFCT-4 and
and HFCT
HFCT 140/100).
140/100).
response
Based on the analysis of the measured frequency response curves, for further research, the two
prototype HFCT
HFCT sensors
sensors were
were selected.
selected. The first of the chosen sensors was manufactured from ferrite
prototype
core MnZn with the three-turn secondary winding. The main advantages of that construction are a
Energies 2020, 13, 3271
16 of 37
Energies 2020, 13, x FOR PEER REVIEW
Energies 2020, 13, x FOR PEER REVIEW
16 of 38
16 of 38
core MnZn with the three-turn secondary winding. The main advantages of that construction are a
very
veryhigh
highsensitivity
sensitivity(higher
(higherthan
thanpopular
popularcommercial
commercialsensors),
sensors),wide
widebandwidth
bandwidth(250
(250kHz–25.5
kHz–25.5MHz),
MHz),
very
high
sensitivity
(higher
than
popular
commercial
sensors),
wide
bandwidth
(250
kHz–25.5
MHz),
and
relatively
flat
frequency
response
curve.
The
second
from
the
selected
constructions
was
built
from
and
relatively
flat
frequency
response
curve.
The
second
from
the
selected
constructions
was
built
from
and relatively flat frequency response curve. The second from the selected constructions was built
ferrite
core
NiZn
with
four
turns.
It
is
characterized
by
slightly
lower
sensitivity
(≤
2.67
dB)
than
the
ferrite
core core
NiZnNiZn
withwith
four four
turns.
It is characterized
by slightly
lower
sensitivity
(≤ 2.67
dB)dB)
than
the
from
ferrite
turns.
It is characterized
by slightly
lower
sensitivity
(≤2.67
than
transformer
with
MnZn
core,
but
its
frequency
response
curve
is
almost
ideally
flat
in
a
wide
band
transformer
withwith
MnZn
core,core,
but but
its frequency
response
curve
is almost
ideally
flat inflat
a wide
band
the
transformer
MnZn
its frequency
response
curve
is almost
ideally
in a wide
between
1.8
30
In
first
stage
of
the
laboratory tests,
measurements of
the impulse
between
1.8 and
and
30 MHz.
MHz.
In the
the
first
stage
of of
the
impulse
band
between
1.8 and
30 MHz.
In the
first
stage
thelaboratory
laboratorytests,
tests,measurements
measurements of the impulse
response
prototype
HFCT
sensors
were
conducted.
For
this
purpose,
standard
LDCresponseof
ofthe
theselected
selectedprototype
prototypeHFCT
HFCTsensors
sensorswere
wereconducted.
conducted.For
Forthis
thispurpose,
purpose,aaastandard
standardLDC-5
LDCresponse
of
the
selected
55 PD
calibrator
(dDoble
Engineering
Company,
Marlborough,
USA)
was
used,
which
allowed
to
PD
calibrator
(dDoble
Engineering
Company,
Marlborough,
USA)
was
used,
which
allowed
to
PD calibrator (dDoble Engineering Company, Marlborough, USA) was used, which allowed to inject
inject
pulses
with
an
apparent
charge
of
500
pC
and
duration
of
50
ns
into
the
primary
circuit.
The
inject pulses
an apparent
500duration
pC and of
duration
of the
50 ns
into the
primary
circuit. The
pulses
with anwith
apparent
charge ofcharge
500 pCofand
50 ns into
primary
circuit.
The waveforms
waveforms
of
calibration
pulses induced
the
circuit
of
the
sensor
were
waveforms
of the
the
calibration
induced in
incircuit
the secondary
secondary
circuit
ofwere
the HFCT
HFCT
sensor
were
of
the calibration
pulses
inducedpulses
in the secondary
of the HFCT
sensor
registered
using
an
registered
using
an
oscilloscope
(Figure
16).
registered
using
an
oscilloscope
(Figure
16).
oscilloscope (Figure 16).
(a)
(a)
(b)
(b)
Figure 16. Examination of HFCT sensors: (a) Schematic diagram of the measurement setup for
Figure 16.
16. Examination
Examination of
of HFCT
HFCT sensors:
sensors: (a)
Schematic diagram
diagram of
of the
the measurement
measurement setup
setup for
for
Figure
(a) Schematic
measuring the impulse response of the HFCT sensor using standard PD calibrator: OSC = oscilloscope
measuring
the
impulse
response
of
the
HFCT
sensor
using
standard
PD
calibrator:
OSC
=
oscilloscope
measuring the impulse response of the HFCT sensor using standard PD calibrator: OSC = oscilloscope
Tektronix MDO3104, HFCT = high-frequency current transformer under test, CAL = partial discharge
TektronixMDO3104,
MDO3104,HFCT
HFCT==high-frequency
high-frequencycurrent
currenttransformer
transformerunder
undertest,
test,CAL
CAL==partial
partialdischarge
discharge
Tektronix
calibrator type Doble LDC-5; (b) time waveform of PD calibration pulse with an apparent charge of
calibrator type
type Doble
Doble LDC-5;
LDC-5; (b)
(b)time
timewaveform
waveform of
of PD
PDcalibration
calibration pulse
pulse with
with an
anapparent
apparentcharge
chargeof
of
calibrator
500 pC.
500pC.
pC.
500
Figure
Figure
17
shows
the
recorded
waveforms
of
the
impulse
response
of
the
tested
HFCT
sensors.
Figure17
17shows
showsthe
therecorded
recordedwaveforms
waveformsof
ofthe
theimpulse
impulseresponse
responseof
ofthe
thetested
testedHFCT
HFCTsensors.
sensors.
The
highest
detection
sensitivity
of
the
pulse
of
apparent
charge
500
pC
was
shown
by
the
prototype
The
highest
detection
sensitivity
of
the
pulse
of
apparent
charge
500
pC
was
shown
by
the
prototype
The highest detection sensitivity of the pulse of apparent charge 500 pC was shown by the prototype
current
three
turns.
current
transformer
made
from
MnZn
ferrite
core
with
At
the
output
of
this
current
current transformer
transformer made
madefrom
from MnZn
MnZnferrite
ferrite core
corewith
withthree
threeturns.
turns. At
At the
the output
output of
of this
thiscurrent
current
transformer,
the
pulses
of
amplitude
A
max = 146 mV were registered, which means that its sensitivity
transformer,
transformer,the
thepulses
pulsesofofamplitude
amplitudeAAmax
max =
= 146 mV were registered, which
which means
means that
that its
its sensitivity
sensitivity
was
higher
than
the
prototype
sensor
made
from
NiZn
ferrite
core
(A
max = 86 mV) and
was
by
about
70%
wasby
byabout
about70%
70%higher
higherthan
thanthe
theprototype
prototypesensor
sensormade
madefrom
fromNiZn
NiZnferrite
ferritecore
core(A
(A
max=
= 86
86 mV)
mV) and
and
max
commercial
HFCT
sensor
type
RFCT-4
(A
max = 88 mV).
commercial
88 mV).
commercialHFCT
HFCTsensor
sensortype
typeRFCT-4
RFCT-4(A
(Amax =
= 88
max
Figure 17. Impulse response of the prototype (MnZn ferrite core with three turns, NiZn ferrite core with
Figure 17. Impulse response of the prototype (MnZn ferrite core with three turns, NiZn ferrite core
Figure
17. Impulse
responseHFCT
of thesensors
prototype
(MnZn
ferrite core
with three
turns,with
NiZnanferrite
core
four
turns)
and commercial
to partial
discharge
calibration
impulse
apparent
with four turns) and commercial HFCT sensors to partial discharge calibration impulse with an
with four
turns)
charge
of 500
pC. and commercial HFCT sensors to partial discharge calibration impulse with an
apparent charge of 500 pC.
apparent charge of 500 pC.
Time
Time parameters
parameters of
of the
the pulses
pulses generated
generated by
by standard
standard PD
PD calibrator
calibrator are
are optimized
optimized for
for the
the
conventional
method
IEC
60270,
in
which
case
the
registration
of
PD
pulses
is
performed
in
conventional method IEC 60270, in which case the registration of PD pulses is performed in aa
Energies 2020, 13, 3271
17 of 37
Time parameters of the pulses generated by standard PD calibrator are optimized for the
Energies 2020, 13, x FOR PEER REVIEW
17 of 38
conventional method IEC 60270, in which case the registration of PD pulses is performed in a relatively
low
frequency
(100–500
For that
reason,
the reason,
tests with
usewith
of thethe
PDuse
calibrator
do
relatively
low band
frequency
bandkHz).
(100‒500
kHz).
For that
thethe
tests
of the PD
not
allow
unambiguous
statement
that
the
prototype
HFCT
sensor
with
core
MnZn
is
an
optimal
calibrator do not allow unambiguous statement that the prototype HFCT sensor with core MnZn is
construction
of the detector
PD pulses. Nevertheless,
the use of
an optimal construction
of of
thehigh-frequency
detector of high-frequency
PD pulses. measurements
Nevertheless, with
measurements
calibrator
showed
that dueshowed
to the lowest
lower
−3 dB
cutoff
frequency
kHz) among
L = 250
with the use
of calibrator
that due
to the
lowest
lower
−3 dB(fcutoff
frequency
(fL =the
250tested
kHz)
HFCT
sensors,
it
may
be
successfully
employed
both
in
the
standardized
frequency
range
according
to
among the tested HFCT sensors, it may be successfully employed both in the standardized frequency
IEC
60270
as welltoasIEC
at higher
frequencies
to 25.5
MHz.
range
according
60270 as
well as at up
higher
frequencies
up to 25.5 MHz.
In
the
second
stage
of
the
laboratory
research,
the
detection
sensitivity
of of
PDPD
pulses
generated
in
In the second stage of the laboratory research, the detection
sensitivity
pulses
generated
oil-paper
insulation
was
tested,
which
were
simultaneously
registered
by
four
HFCT
sensors
(two
in oil-paper insulation was tested, which were simultaneously registered by four HFCT sensors (two
prototype
and two
twocommercial).
commercial).The
The
research
was
conducted
with
the of
use
of a transformer
tank
prototype and
research
was
conducted
with
the use
a transformer
tank filled
filled
with
mineral
oil.
The
electrode
systems
devoted
to
generating
PD
in
oil
and
surface
PD
on
with mineral oil. The electrode systems devoted to generating PD in oil and surface PD on the
the
pressboard
pressboard sample
sample were
were put
put inside
inside the
the tank.
tank. Detailed
Detailed information
information concerning
concerning the
the geometry
geometry of
of the
the
applied
The HFCT
HFCT sensors
sensors were
were installed
installed on
grounding wire
applied electrode
electrode systems
systems was
was presented
presented in
in [24].
[24]. The
on grounding
wire
going
out
of
the
tank
through
the
ceramic
bushing
insulator.
The
PD
pulses
were
registered
with
going out of the tank through the ceramic bushing insulator. The PD pulses were registered with
the
the
a four-channel
oscilloscope
Tektronix
MDO3104
withthe
thesampling
samplingfrequency
frequencyofof 55 GS/s.
GS/s.
use use
of aoffour-channel
oscilloscope
Tektronix
MDO3104
with
Additionally,
Additionally, the
the standard
standard IEC
IEC 60270
60270 measurement
measurement setup
setup was
was applied
applied to
to control
control the
the level
level of
of the
the PD
PD
apparent
charge
(Figure
18).
Discharges
in
oil
of
apparent
charge
in
the
range
between
about
200
apparent charge (Figure 18). Discharges in oil of apparent charge in the range between about 200 and
and
850
InIn
turn,
thethe
surface
discharges
hadhad
a significantly
higher
intensity
and
850 pC
pC were
weregenerated
generatedatat2929kV.
kV.
turn,
surface
discharges
a significantly
higher
intensity
apparent
charge
(q
=
450–5100
pC).
This
type
of
discharge
was
generated
by
a
voltage
of
15.4
kV.
and apparent charge (q = 450–5100 pC). This type of discharge was generated by a voltage of 15.4 kV.
Figure 18. Schematic diagram of the measurement
measurement setup for generating partial discharges in oil-paper
insulation and assessing the sensitivity of high-frequency PD pulses detection by prototype and
commercial HFCT
HFCT sensors:
sensors:U U
= high-voltage
supply,
Z = short-circuit
current
limiting
= high-voltage
supply,
Z = short-circuit
current
limiting
resistor,resistor,
OSC =
OSC
=
four-channel
oscilloscope,
HFCT
=
high-frequency
current
transformers
under
test,
TT == oil-filled
oil-filled
four-channel oscilloscope, HFCT = high-frequency current transformers under test, TT
transformer
tank, PD
PD==electrode
electrodesystem
systemforfor
partial
discharge
generation,
= coupling
capacitor,
transformer tank,
partial
discharge
generation,
C =Ccoupling
capacitor,
CD
CD
= coupling
device
(measuring
impedance),
CC = connecting
M = conventional
partial
= coupling
device
(measuring
impedance),
CC = connecting
cable, Mcable,
= conventional
partial discharge
discharge
detector,
PC = computer.
detector, PC
= computer.
Figure
Figure 19
19 presents
presents typical
typical partial
partial discharge
discharge time
time waveforms
waveforms and
and power
power spectral
spectral density
density (PSD)
(PSD)
characteristics
registered
with
the
use
of
HFCT
sensors.
The
results
of
frequency
analysis
showed
that
characteristics registered with the use of HFCT sensors. The results of frequency analysis
showed
PDs
oilin
generated
pulsespulses
of high
in the band
12.4 and
18.3
MHz,
surface
that in
PDs
oil generated
offrequency
high frequency
in thebetween
band between
12.4
and
18.3whereas
MHz, whereas
PD
generated
pulses
in
a
narrow
band
between
10.8
and
11.6
MHz.
The
PD
detection
sensitivity
of
surface PD generated pulses in a narrow band between 10.8 and 11.6 MHz. The PD detection
both
prototypes
was
higher
than
commercial
sensors.
In
the
case
of
discharges
in
oil,
the
highest
sensitivity of both prototypes was higher than commercial sensors. In the case of discharges in oil,
sensitivity
observed
forobserved
the prototype
sensor
with sensor
four turns
the highestwas
sensitivity
was
for theHFCT
prototype
HFCT
withand
fourNiZn
turnssoft
andferrite
NiZncore.
soft
Its
sensitivity
was
36%
higher
on
average
than
the
sensors
from
MnZn
soft
ferrite
with
three
ferrite core. Its sensitivity was 36% higher on average than the sensors from MnZn soft ferriteturns.
with
On
theturns.
other On
hand,
construction
on the MnZn
showed
higher
sensitivity
in thesensitivity
detection
three
thethe
other
hand, thebased
construction
basedcore
on the
MnZn
core showed
higher
of
surface
discharges
on
the
pressboard
sample.
The
average
growth
in
sensitivity
compared
to the
in the detection of surface discharges on the pressboard sample. The average growth in sensitivity
prototype
with
NiZn
core
was
46%.
The
detection
possibility
of
surface
discharges
is
very
important
compared to the prototype with NiZn core was 46%. The detection possibility of surface discharges
from
the
viewpoint
of the
andperformance
effectivenessand
of the
monitoring
operation.
It is
is very
important
from
the performance
viewpoint of the
effectiveness
ofsystem
the monitoring
system
operation. It is one of the most energetic and destructive types of partial discharges. Therefore, for
the designed monitoring system, finally, the HFCT sensor with MnZn core was selected.
Energies 2020, 13, 3271
18 of 37
one of the most energetic and destructive types of partial discharges. Therefore, for the designed
Energies 2020, 13,
x FOR PEER
REVIEW
18 of 38
monitoring
system,
finally,
the HFCT sensor with MnZn core was selected.
(a)
(b)
Figure 19.
19. Comparison of typical partial discharge time waveforms and power spectral density (PSD)
Figure
characteristics registered for (a) PD
PD in
in oil,
oil, and
and (b)
(b) surface
surface PD
PD with
with the
the use
use of
of selected
selected prototype
prototype and
and
characteristics
commercial (RFCT-4,
(RFCT-4,HFCT
HFCT140/100)
140/100)HFCT
HFCTsensors.
sensors.
commercial
2.3. Module for Conditioning and Analog-to-Digital Signal Processing of PD Pulses
2.3. Module for Conditioning and Analog-to-Digital Signal Processing of PD Pulses
The schematic diagram of electronic modules for conditioning and analog-to-digital processing of
The schematic diagram of electronic modules for conditioning and analog-to-digital processing
PD pulses is presented in Figure 20. Conditioning of the UHF signals is realized in an electronic circuit
of PD pulses is presented in Figure 20. Conditioning of the UHF signals is realized in an electronic
consisting of two low noise RF amplifiers basing on a chipset SPF5043Z of maximal amplification of
circuit consisting of two low noise RF amplifiers basing on a chipset SPF5043Z of maximal
G = 18 dB at 900 MHz, tunable bandpass filter, and a system of peak detector based on multistage
amplification of G = 18 dB at 900 MHz, tunable bandpass filter, and a system of peak detector based
demodulating logarithmic amplifier type AD8313. Application of a peak detector circuit (also called
on multistage demodulating logarithmic amplifier type AD8313. Application of a peak detector
frequency converter or envelope detector) is a common practice in the case of the registration of UHF
circuit (also called frequency converter or envelope detector) is a common practice in the case of the
pulses because it allows to significantly reduce the sampling frequency [99,100]. Due to that, it is
registration of UHF pulses because it allows to significantly reduce the sampling frequency [99,100].
possible to process the PD pulses and determine their parameters in real-time. Unfortunately, this
Due to that, it is possible to process the PD pulses and determine their parameters in real-time.
is done at the expense of losing information on time-domain parameters (e.g., rise and fall time) of
Unfortunately, this is done at the expense of losing information on time-domain parameters (e.g., rise
individual pulses. The AE signals conditioning module consists of a low-noise preamplifier (with 40 dB
and fall time) of individual pulses. The AE signals conditioning module consists of a low-noise
gain) based on high-speed instrumentation amplifier AD8421BRZ, active bandpass filter (20–500 kHz)
preamplifier (with 40 dB gain) based on high-speed instrumentation amplifier AD8421BRZ, active
based on Sallen–Key architecture and voltage follower (based on AD813ARZ op-amp) to buffer the
bandpass filter (20‒500 kHz) based on Sallen–Key architecture and voltage follower (based on
output. Since the PD pulses registered with the use of sensitive HFCT sensor may reach the amplitude
AD813ARZ op-amp) to buffer the output. Since the PD pulses registered with the use of sensitive
of even a few volts, it was decided to give up on the dedicated conditioning system and to process the
HFCT sensor may reach the amplitude of even a few volts, it was decided to give up on the dedicated
conditioning system and to process the raw HF signals. At the input of the analog-to-digital signal
processing module, there is A/D converter type AD9266BCPZ-40 with 40 MS/s sampling rate and 16bit resolution. To each PD sensor, a separate A/D converter was allocated.
Energies 2020, 13, x FOR PEER REVIEW
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Energies 2020, 13, 3271
19 of 37
raw HF signals. At the input of the analog-to-digital signal processing module, there is A/D converter
type AD9266BCPZ-40 with 40 MS/s sampling rate and 16-bit resolution. To each PD sensor, a separate
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2020, 13, xwas
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PEER REVIEW
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A/D
converter
Figure 20. Schematic diagram of PD signal conditioning modules and A/D signal processing module
and 3D drawing showing the mechanics of these modules: A = signal conditioning boards, B = analogto-digital signal processing board, C = internal power supply module.
Figure 21 shows exemplary time waveforms of conditioned AE and UHF pulses and raw HF
pulses registered during laboratory tests of the signal conditioning modules and A/D signal
processing module. During the tests, in the oil-filled transformer tank, the surface discharges on a
Figure sample
20. Schematic
diagram of PD
signal conditioning
modules
and A/D signal
processing
pressboard
were generated.
apparent
charge was
in
the
range
400 and
5300
pC. As
Figure 20. Schematic
diagram of Their
PD signal
conditioning
modules
and
A/Dbetween
signal processing
module
module
and
3D
drawing
showing
the
mechanics
of
these
modules:
A
=
signal
conditioning
boards,
one can
in Figure
21a, the
themodules:
peak detector
circuit
at a relatively
sampling
andsee
3D drawing
showing
theapplication
mechanics ofof
these
A = signal
conditioning
boards, Blow
= analogB = analog-to-digital signal processing board, C = internal power supply module.
frequency
equal
to 40
MS/s allows
to Ceffectively
detect supply
the UHF
pulses. A comparative analysis of the
to-digital
signal
processing
board,
= internal power
module.
results
obtained
with
the
HF
method
demonstrated
that
the
UHF
all PDand
pulses
Figure 21 shows exemplary time waveforms of conditionedmodule
AE anddetected
UHF pulses
rawand
HF
Figure 21
shows
exemplaryoftime
waveformssensor.
of conditioned
AEresignation
and UHF pulses
and
raw
HF
it
confirmed
the
high
sensitivity
the
ISO-002-HF
Despite
the
from
the
HF
signal
pulses registered during laboratory tests of the signal conditioning modules and A/D signal processing
pulses
registered
during
laboratory
tests
of
the
signal
conditioning
modules
and
A/D
signal
conditioning
module,
pulses
with
the high-frequency
had
on average
module. During
the tests,
in registered
the oil-filled
transformer
tank, the current
surface transformer
discharges on
a pressboard
processing
module.
During than
the tests,
in the oil-filled
transformer
tank,The
theplacement
surface discharges
on a
1.8
times
higher
amplitude
the
strongly
amplified
UHF
pulses.
of
theone
HFCT
sample were generated. Their apparent charge was in the range between 400 and 5300 pC. As
can
pressboard
sample
werewire
generated.
Their
apparent
charge
was inthe
the range
between
400 and had
5300apC.
As
sensor
on
the
ground
of
the
electrode
system,
nearby
PD
source,
certainly
high
see in Figure 21a, the application of the peak detector circuit at a relatively low sampling frequency
one
can
see
in
Figure
21a,
the
application
of
the
peak
detector
circuit
at
a
relatively
low
sampling
influence
onMS/s
obtaining
a good result.
equal to 40
allowssuch
to effectively
detect the UHF pulses. A comparative analysis of the results
frequency
equal
tothe
40 MS/s
allows to
effectively
detect
the UHF pulses.
A amplitude
comparative
analysis
of the
In
the
case
of
AE
method,
the
pulses ofthat
comparable
the UHF
were
registered.
obtained with the HF method demonstrated
the UHF to
module
detected
all PD
pulses
and it
results
obtained
with
the
HF
method
demonstrated
that
the
UHF
module
detected
all
PD
pulses
and
In
Figure 21b
visible
time differences
(T = 272sensor.
µs) in UHF
andthe
AEresignation
signal arrival
result
from
the
confirmed
thethe
high
sensitivity
of the ISO-002-HF
Despite
from
the HF
signal
it confirmed
the high
sensitivity
of the ISO-002-HF
sensor.
Despite
the in
resignation
fromacoustic
the HF signal
different
velocity
of
EM
wave
propagation
(approx.
2/3
speed
of
light
vacuum)
and
wave
conditioning module, pulses registered with the high-frequency current transformer had on average
conditioning
module,
registered with
the high-frequency
current
transformer
had oncm
average
in
(1400‒1500
m/s). pulses
The than
piezoelectric
transducer
built
in pulses.
the ADW
placed about
from
1.8oil
times
higher amplitude
the strongly
amplified
UHF
Thewas
placement
of the 39.5
HFCT sensor
1.8
times
higher
amplitude
than
the
strongly
amplified
UHF
pulses.
The
placement
of
the
HFCT
the
PDground
source, wire
which
that system,
acoustic
wavethe
velocity
in thecertainly
transformer
with oil
on the
of means
the electrode
nearby
PD source,
had atank
highfilled
influence
on
sensor
onm/s.
the ground wire of the electrode system, nearby the PD source, certainly had a high
was
1452
obtaining such a good result.
influence on obtaining such a good result.
In the case of the AE method, the pulses of comparable to the UHF amplitude were registered.
In Figure 21b the visible time differences (T = 272 µs) in UHF and AE signal arrival result from the
different velocity of EM wave propagation (approx. 2/3 speed of light in vacuum) and acoustic wave
in oil (1400‒1500 m/s). The piezoelectric transducer built in the ADW was placed about 39.5 cm from
the PD source, which means that the acoustic wave velocity in the transformer tank filled with oil
was 1452 m/s.
(a)
(b)
Figure
Figure 21.
21. Exemplary
Exemplarytime
timewaveforms
waveforms registered
registered during
during laboratory
laboratory tests
tests of
of the
the PD
PD signal
signal conditioning
conditioning
modules:
(a) conditioned
conditionedUHF
UHFPD
PDpulses
pulses
and
raw
pulses,
(b) conditioned
PD
modules: (a)
and
raw
HFHF
PD PD
pulses,
(b) conditioned
singlesingle
UHF UHF
PD pulse
pulse
and
conditioned
acoustic
emission
(AE)
PD
pulses.
and conditioned acoustic emission (AE) PD pulses.
In the case of the AE method, the pulses of comparable to the UHF amplitude were registered.
In Figure 21b the visible time differences (T = 272 µs) in UHF and AE signal arrival result from the
(a)
different velocity of EM wave propagation
(approx. 2/3 speed of light(b)
in vacuum) and acoustic wave in
Figure 21. Exemplary time waveforms registered during laboratory tests of the PD signal conditioning
modules: (a) conditioned UHF PD pulses and raw HF PD pulses, (b) conditioned single UHF PD
pulse and conditioned acoustic emission (AE) PD pulses.
Energies 2020, 13, 3271
20 of 37
oil (1400–1500 m/s). The piezoelectric transducer built in the ADW was placed about 39.5 cm from the
PD source, which means that the acoustic wave velocity in the transformer tank filled with oil was
1452 m/s.
Digital signals from the A/D converter then go to the FPGA (Field-Programmable Gate Array) type
Intel 10M50DAF256I7G with 256 MB cache memory type DDR2. In that logic circuit, two operating
modes were implemented: basic (monitoring mode) and extended (diagnostic mode).
In the monitoring mode, for HF and UHF method, separate data frames of duration time 55.5 µs
were analyzed. Such data frame length was selected for two reasons. First, it is the time corresponding
to the angle 1◦ of voltage period (20 ms per 360◦ which is approximately 55.5 µs per 1◦ ), which enables
determination of phase-resolved partial discharge patterns (PRPD). Second, the duration time of a
single UHF PD pulse usually amounts to less than 2 µs. It allows to minimize the risk that the UHF PD
pulse would not be detected by the monitoring system. In the case of the AE method, the data frames
are aggregated in one-second records. It is caused by the fact that the AE pulses of the high-energy
partial discharges (e.g., creeping sparks or interturn discharges), which are registered with resonant
piezoelectric transducer may have a time duration of even over a dozen milliseconds [24].
The algorithm implemented in the FPGA for each data record (55 µs for the HF and UHF method,
and 1 s for the AE method) determines the following parameters:
•
•
•
•
•
the number of pulses detected (n),
the maximal amplitude of the pulses (Amax ),
the maximal energy of the pulses (Emax ),
the average amplitude of the pulses (Amean ),
the average energy of the pulses (Emean ).
In the diagnostic mode, the FPGA does not do the calculations. Data frames are sent via a
fiber-optic Ethernet cable to the multi-channel data concentrator, and afterward to the monitoring
system server. The 1-Gigabit Ethernet physical layer (PHY) was made using the Marvell Alaska
88E1512 chip. That system supports Synchronous Ethernet (SyncE) and Precise timing control protocol
(PTP) time stamping, which is based on IEEE1588 [101] and IEEE802.1AS [102] standards. The 88E1512
system on the first side is connected using the RGMII interface to the FPGA chip and on the second it
is connected using SERDES interface to the small form-factor pluggable (SFP) optical transceiver.
2.4. Multi-Channel Data Concentrator Module
Measurement data is sent to the multi-channel data concentrator (labeled as MPD-001 module)
using protocol elaborated based on the so-called Process Bus (PB) according to the guidelines of the
international substation automation standard IEC 61850-9-2 [103]. As it was mentioned before, the
fiber-optic cables allow to effectively reduce the influence of external electromagnetic interferences
occurring on the substation. The schematic diagram and photograph of the MPD-001 module are
demonstrated in Figure 22.
The data into the MPD-001 module are entered through an integrated, specialized MSO-001
network card containing up to eight fiber-optic interfaces (ISO). The MSO-001 network card transmits
the measurement data via the PCX/1 bus to the PJC-900 module based on the E660T ATOM processor.
Individual data streams are decoded by the FPGA type CYCLONE V implemented in the PJC-900
module. The FPGA also supports the following external network transmission channels:
•
•
•
the basic channel of the Ethernet network in standard FX1000 used for data transmission, warning
signals, download the parameterization from the server,
configuration channel in TX100 standard dedicated to programming, local parameterization,
and service,
synchronizing channel in 1PPS or IRIGB standard.
Measurement data is sent to the multi-channel data concentrator (labeled as MPD-001 module)
using protocol elaborated based on the so-called Process Bus (PB) according to the guidelines of the
international substation automation standard IEC 61850-9-2 [103]. As it was mentioned before, the
fiber-optic cables allow to effectively reduce the influence of external electromagnetic interferences
occurring on the substation. The schematic diagram and photograph of the MPD-001 module are
Energies 2020, 13, 3271
21 of 37
demonstrated in Figure 22.
Figure
22.Schematic
Schematic
diagram
photograph
of developed
multi-channel
data concentrator
Figure 22.
diagram
andand
photograph
of developed
multi-channel
data concentrator
module
module
MPD-001.
MPD-001.
The PCX/2 bus is used to transfer data streams from FPGA to the embedded PC (EPC) computer
module. As with the signal processing module, MPD-001 can work in the monitoring or diagnostic
mode. In the monitoring mode, based on the data sent from the analog-to-digital signal processing
module, the embedded PC calculates for three different time intervals (one minute, fifteen minutes, and
one hour) parameters such as the number of PD pulses, maximum and average pulse amplitude, as
well as maximum and average pulse energy. In diagnostic mode, the computer prepares data for PRPD
patterns. For this purpose, it aggregates 1-s data frames with a duration of 55.5 µs. The data prepared
in this way is sent to the server. If necessary, data from the MPD-001 module can be transferred to
other local or remote SCADA systems using the DNP-3.0 or IEC 61850 protocol.
The module MPD-001 is powered with the use of the 220 V AC/DC power supply type MZA-502,
in which the voltage level is supervised by the PJC-900 module.
After manufacturing the complete prototype of MPD-001 module, the EMC properties of the
housing ports and power ports were tested in accordance with IEC 60255-26 standard [104].
2.5. Modules of Server Software
In the firmware of the monitoring system server, the following software modules were
implemented: virtual data concentrator (VDC), analytic module, the module of graphic data
presentation, and reporting module. In the monitoring mode, the VDC module receives from
the MPD-001 concentrator notifications of the PD pulse occurrence. This causes that information about
the event with calculated partial discharge parameters (number of pulses, their energy, and amplitude)
is saved to the database on the server. In turn, in the diagnostic mode, VDC enables direct access to the
transmitted data frames, including the preview in real-time the PD waveforms of the registered in
particular channels. This functionality also enables gathering the samples to perform further, more
advanced analysis, including the elaboration of PRPD patterns.
Based on the data gathered in the database parameters, the analytical module regularly checks
if the threshold values of the PD parameters have not been exceeded. Additionally, using the least
square method, a linear trend estimation is conducted for the data from the last week and the last
month. All information is shared with the user through the module of the graphic data presentation,
which displays in the application window, i.e.,
•
•
•
•
the number of detected PD pulses and the average value of their energies and amplitudes,
the result of self-diagnosis of measurement channels,
warning or alarm state after exceeding the threshold values for the sum of registered pulses, their
average energy, and amplitude for various time intervals: 15 min, 60 min, 24 h, and 30 days,
the warning or alarm state when the values of PD parameters collected during last week and last
month show a rising trend.
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images are created based on the 20 ms aggregation of data frames from HFCT and UHF sensors and
voltage sine wave registered by the current/voltage transformer (CVT) on the substation.
The functions of the reporting module enable elaborating summary reports or details for the PD
The system also provides the possibility to wirelessly measure the high voltage in a power
parameters registered in the database and events description.
transformer, which is realized with the use of a special capacity probe. The structure of the probe and
As it was mentioned before, the system in the diagnostic mode functions as a multichannel partial
assumptions of the measuring method is discussed in detail in [105]. The signal from the probe is
discharges detector allowing to observe the PD waveforms or PRPD pattern images. PRPD images are
registered with an additional, A/D converter of sampling frequency equal to 4 MS/s dedicated
created based on the 20 ms aggregation of data frames from HFCT and UHF sensors and voltage sine
exclusively for this task. This converter is placed in the circuit of every analog-to-digital signal
wave registered by the current/voltage transformer (CVT) on the substation.
processing module.
The system also provides the possibility to wirelessly measure the high voltage in a power
The angular resolution of the created PRPD patterns is 1°, whereas the amplitude resolution
transformer, which is realized with the use of a special capacity probe. The structure of the probe
equals 16 bits for the output voltage range equal to 0–3.6 V. The software function responsible for
and assumptions of the measuring method is discussed in detail in [105]. The signal from the probe
preparing PRPD patterns was tested in the laboratory conditions. For this purpose, the comparative
is registered with an additional, A/D converter of sampling frequency equal to 4 MS/s dedicated
research, during which PRPD patterns were simultaneously registered with a standard partial
exclusively for this task. This converter is placed in the circuit of every analog-to-digital signal
discharges detector type Doble PD-Smart and monitoring system under test. A schematic diagram of
processing module.
the measurement setup was presented in Figure 23.
The angular resolution of the created PRPD patterns is 1◦ , whereas the amplitude resolution equals
The PRPD images obtained during the research were the effect of about 60–90 min registrations
16 bits for the output voltage range equal to 0–3.6 V. The software function responsible for preparing
of the partial discharges pulses. In this case, instead of pressboard samples, glass samples with a
PRPD patterns was tested in the laboratory conditions. For this purpose, the comparative research,
thickness of 6 mm were used. This change was caused by the fact that the pressboard samples quickly
during which PRPD patterns were simultaneously registered with a standard partial discharges detector
degraded under the influence of the action of high-intensity surface discharges with the apparent
type Doble PD-Smart and monitoring system under test. A schematic diagram of the measurement
charge of a few nC. This, in turn, led to PD extinction or breakdown.
setup was presented in Figure 23.
Figure
of the
Figure 23.
23. Schematic
Schematic diagram
diagram of
the measurement
measurement setup
setup in
in which
which the
the monitoring
monitoring system
system software
software
module
for
performing
PRPD
patterns
was
tested:
MS
=
partial
discharge
monitoring
module for performing PRPD patterns was tested: MS = partial discharge monitoring system,system,
MPD =
MPD
= multichannel
signal concentrator,
S == server,
PC U
==computer,
U supply,
= high-voltage
supply,
multichannel
signal concentrator,
S = server, PC
computer,
high-voltage
Z = short-circuit
Z = short-circuit current limiting resistor, TT = oil-filled transformer tank; PD = electrode system for
current limiting resistor, TT = oil-filled transformer tank; PD = electrode system for partial discharge
partial discharge generation, C = coupling capacitor, CD = coupling device (measuring impedance),
generation, C = coupling capacitor, CD = coupling device (measuring impedance), CC = connecting
CC = connecting cable, M = conventional partial discharge measuring instrument.
cable, M = conventional partial discharge measuring instrument.
The PRPD images obtained during the research were the effect of about 60–90 min registrations
In Figure 24 the exemplary results of the 80-min registration of the surface discharges of average
of the partial discharges pulses. In this case, instead of pressboard samples, glass samples with a
apparent charge qmean = 283 pC and maximal qmax = 5702 pC are presented. A typical PRPD distribution
thickness of 6 mm were used. This change was caused by the fact that the pressboard samples quickly
for surface discharges was obtained. The most PD pulses were registered in the first (0–90°) and third
degraded under the influence of the action of high-intensity surface discharges with the apparent
quarter (180–270°) of a sine wave of test voltage.
charge of a few nC. This, in turn, led to PD extinction or breakdown.
In Figure 24 the exemplary results of the 80-min registration of the surface discharges of average
apparent charge qmean = 283 pC and maximal qmax = 5702 pC are presented. A typical PRPD distribution
for surface discharges was obtained. The most PD pulses were registered in the first (0–90◦ ) and third
quarter (180–270◦ ) of a sine wave of test voltage.
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(a)
(a)
(b)
(b)
Figure 24. Measurement results obtained with conventional
electric method: (a) instantaneous value
of apparent charge q, (b) PRPD pattern.
Figure 24.
24. Measurement results obtained with
with conventional
conventional electric
electric method:
method: (a) instantaneous value
apparent
charge
q, (b)
(b) PRPD
PRPD
pattern.
of apparent
charge
q,
pattern.
The results
obtained
by the
prototype monitoring system (Figure 25) confirmed the properly
implemented functions of PRPD pattern creation. In this case, also most of the pulses were registered
The
obtained
by the
prototype
monitoring
systemcompared
(Figure 25)
confirmed
properly
prototype
in
theresults
first and
third quarters
of the
voltage sine
wave. However,
to the
IEC 60270the
method
implemented
functions
of
PRPD
pattern
creation.
In
this
case,
implemented
functions
of
PRPD
pattern
creation.
also
most
of
the
pulses
were
registered
(where the number of pulses did not depend clearly on the voltage polarization), both
in theelectromagnetic
first and thirdmethods
quartersregistered
of the voltage
sine wave.
However,
compared to the IEC 60270 method
more pulses
of positive
polarization.
Currently,
there
equipment
of voltage
the analytical
the electromagnetic
function of both
(where
number
of pulses
did conducted
notdid
depend
on clearly
the
(where the
the
number
ofis work
pulses
notonclearly
depend
on polarization),
the module
voltageinboth
polarization),
automatic
recognition
PD type
based
the analysis
of the polarization.
PRPD patterns.
methods
registered
more of
pulses
of positive
polarization.
electromagnetic
methods
registered
moreonpulses
of positive
Currently, there is work conducted on equipment of the analytical module in the function of
automatic recognition of PD type based on the analysis of the PRPD patterns.
(a)
(b)
Figure
25. PRPD
patterns
obtained
monitoring system
system for
pulses
registered
in in a
Figure
25. PRPD
patterns
obtained
inina amonitoring
forpartial
partialdischarges
discharges
pulses
registered
a measurement channel: (a) of high frequency, (b) of ultra-high frequency.
measurement channel: (a) of high frequency, (b) of ultra-high frequency.
(a)
(b)
3. Tests of the Developed PD Monitoring System on Power Transformers
Currently, there is work conducted on equipment of the analytical module in the function of
Figure 25. PRPD patterns obtained in a monitoring system for partial discharges pulses registered in
automatic
recognition
of PDSystem
type based
on
the analysis
of the
PRPD
3.1.
Test
of the Monitoring
During
Induced
Voltage
Test
of 330
MVApatterns.
Power Transformer
a measurement
channel: (a)
of high
frequency,
(b)
of ultra-high
frequency.
After
laboratory
tests, iton
was
decided
to check the effectiveness of the
3. Tests of
the successfully
Developed finished
PD Monitoring
System
Power
Transformers
3. Tests
of the
Developed
PDpower
Monitoring
System
Power
Transformers
system
operation
on a real
transformer.
Firston
such
measurement
was carried out during the
induced
test with
partial
discharge
measurement
(IVPD)
on aMV
power
transformer
of 330 MVA.
3.1. Test
of thevoltage
Monitoring
System
During
Induced
Voltage Test
of 330
A Power
Transformer
3.1. Test
the of
Monitoring
Systemtransformer,
During Induced
Voltage
Test
of 330 MVA
Power
The of
tank
the investigated
already
at the
production
stage,
wasTransformer
equipped in two
After successfully
finished
laboratory
tests,ofitthe
was
decided
tothe
check
the effectiveness
ceramic
dielectric windows
placed
in side-walls
tank,
i.e., near
L1 phase
(Figure 26) andof the
After
successfully
finished
laboratory
tests,
it was
decided
to check
the
effectiveness
of the
the
system
operation
on
a
real
power
transformer.
First
such
measurement
was
carried
out
during
the L3 phase. This unfortunately prevented the active dielectric window with the built-in
acoustic
system
operation
on(ISO-002-AWD/UHF)
a real
power
transformer.
First such
measurement
wastransformer
carried
outofduring
the
induced
voltage
test
with
partial
discharge
measurement
(IVPD)
on aThe
power
330
emission
sensor
from
installation and
testing.
induced
voltage test
wasMVA.
induced
voltage
test
with
partial
discharge
measurement
(IVPD)
on
a
power
transformer
of
330
MVA.
carried
out investigated
strictly according
to the scheme
described
in the IEC 60076-3
[106] and
taking
The tank
of the
transformer,
already
at the production
stage, standard
was equipped
in two
ceramic
The tank
of the investigated
transformer,
at near
the high
production
was
equipped
two
into account
the
strict in
internal
safety
in
the
voltage
laboratory.
dielectric
windows
placed
side-walls
of regulations
thealready
tank, i.e.,
the L1
phasestage,
(Figure
26)Due
and to
thethese
L3in
phase.
ceramic
dielectric windows
in side-walls
the tank,
i.e.,
near
the L1
phase emission
(Figure 26)
and
This
unfortunately
preventedplaced
the active
dielectric of
window
with
the
built-in
acoustic
sensor
the
L3
phase.
This
unfortunately
prevented
the
active
dielectric
window
with
the
built-in
acoustic
(ISO-002-AWD/UHF) from installation and testing. The induced voltage test was carried out strictly
emission sensor
(ISO-002-AWD/UHF)
installation
and testing.
The
induced
voltage the
teststrict
was
according
to the scheme
described in thefrom
IEC 60076-3
standard
[106] and
taking
into account
carried
out
strictly
according
to
the
scheme
described
in
the
IEC
60076-3
standard
[106]
and
taking
internal safety regulations in the high voltage laboratory. Due to these restrictions, the installation of the
into account the strict internal safety regulations in the high voltage laboratory. Due to these
Energies 2020, 13, x FOR PEER REVIEW
Energies 2020, 13, 3271
24 of 38
24 of 37
restrictions, the installation of the high-frequency current transformer (ISO-002-HF sensor) on the
grounding
of the
point
was(ISO-002-HF
not allowed.
Therefore,
registration
the PD
pulses
Energies 2020,
13,
xneutral
FOR PEER
REVIEW
24
of was
38 was
high-frequency
current
transformer
sensor)
on the the
grounding
of theofneutral
point
not
performed
using
two
UHF
antennas,
whereas
the
employees
of
the
HV
test
station
conducted
the
allowed. Therefore, the registration of the PD pulses was performed using two UHF antennas, whereas
restrictions,ofthe
installation
of the
high-frequency
current
(ISO-002-HF
sensor) on
the PD
measurement
charge
(according
IEC
60270transformer
standard)
using
multichannel
digital
the employees
of apparent
the HV test
station
conductedtothe
measurement
of apparent
charge (according
to IEC
grounding of the neutral point was not allowed. Therefore, the registration of the PD pulses was
detector
type DDX
9121b
(Haefely, Basel,
60270 standard)
using
multichannel
digitalSwitzerland).
PD detector type DDX 9121b (Haefely, Basel, Switzerland).
performed using two UHF antennas, whereas the employees of the HV test station conducted the
measurement of apparent charge (according to IEC 60270 standard) using multichannel digital PD
detector type DDX 9121b (Haefely, Basel, Switzerland).
Figure 26.
26. The
The installation
installation of
of the
the UHF
UHF antenna
antenna (ISO-002-UHF
(ISO-002-UHF sensor)
sensor) in
in the
the dielectric
dielectric window
window of
of the
the
Figure
330
MVA
power
transformer.
330 MVA
Figurepower
26. Thetransformer.
installation of the UHF antenna (ISO-002-UHF sensor) in the dielectric window of the
330 MVA power transformer.
The time sequence for the application of test voltage for induced voltage test with partial discharge
The time sequence for the application of test voltage for induced voltage test with partial
measurement
(IVPD)
is shown
Figure
27. Inof
thetest
first
stage, theinduced
voltage voltage
was increased
to partial
the level of
time
sequence
for in
the
application
voltage
test was
with
dischargeThe
measurement
(IVPD)
is shown
in Figure
27.
In thefor
first stage, the
voltage
increased to
√
(0.4 ×discharge
U )/ 3, measurement
which for a transformer
with
offirst
a winding
Uvoltage
= 410 was
kV corresponded
to
(IVPD) is shown
inrated
Figurevoltage
27. In the
stage, the
increased to
the levelr of (0.4 × Ur)/√3, which for a transformer with rated voltage rof a winding Ur = 410 kV
the level
of 94.7
(0.4 kV.
× UrAt
)/√3,
which
for alevel,
transformer
with ratednoise
voltage
a pC)
winding
r = 410 kVThen,
the value
Um =
that
voltage
the background
(q =of3.5
was U
registered.
corresponded to the value Um = 94.7 kV. At that voltage level,√the background noise (q = 3.5 pC) was
corresponded
to the value
= 94.7tokV.
that of
voltage
(q =partial
3.5 pC)discharge
was
the voltage
was increased
forU1m min
theAtlevel
(1.2 ×level,
Ur )/the3 background
= 284 kV, atnoise
which
registered.
Then,
thethe
voltage
was
increased
for
11 min
minto
tothe
thelevel
level
of
(1.2
×r)/√3
Ur)/√3
= 284
kV,
at which
registered.
Then,
voltage
was
increased
for
of
(1.2
×
U
=
284
kV,
at
which
pulses of the apparent charge from a several dozen to 150 pC were registered occasionally. According
partial
discharge
pulses
of
the
apparent
charge
from
a
several
dozen
to
150
pC
were
registered
partial discharge pulses of the apparent charge from a several dozen to 150 pC were registered
to the
scheme of the IVPD test, the voltage was increased to the 1-h PD measurement voltage level
occasionally.
According
to to
the
test,the
thevoltage
voltage
was
increased
to 1-h
thePD
1-h PD
occasionally.
According
thescheme
schemeof
of the
the IVPD
IVPD test,
was
increased
to the
(Um = 374 kV). This voltage level was held for 5 min. During that time stable PD pulses of apparent
measurement
voltage
level
(U(U
m m
= =374
voltagelevel
levelwas
was
held
5 min.
During
that time
measurement
voltage
level
374kV).
kV). This
This voltage
held
forfor
5 min.
During
that time
charge q = PD
600–700
pC
were registered.
Then
the voltage
was raisedThen
to thethe
enhancement
voltage
level
pulses
of apparent
chargeqq==600‒700
600‒700 pC
voltage
was was
raised
to
stablestable
PD pulses
of apparent
charge
pC were
wereregistered.
registered. Then
the
voltage
raised
to
(Um =
473.4
kV)
and
held
there
for
1
min.
At
this
test
stage,
the
apparent
charge
grew
to
approx.
the
enhancement
voltage
level
(U
m
=
473.4
kV)
and
held
there
for
1
min.
At
this
test
stage,
the
apparent
the enhancement voltage level (Um = 473.4 kV) and held there for 1 min. At this test stage, the apparent
1500–1800
pC. to approx. 1500‒1800 pC.
charge
charge
grewgrew
to approx. 1500‒1800 pC.
Figure
Time
sequencefor
forthe
theapplication
application of
induced
voltage
test test
withwith
partial
Figure
27. 27.
Time
sequence
of test
testvoltage
voltageforfor
induced
voltage
partial
discharge
measurement
(IVPD).
discharge
(IVPD).
Figure
27.measurement
Time sequence
for the application of test voltage for induced voltage test with partial
discharge measurement (IVPD).
For IVPD
the IVPD
test,
monitoringsystem
system was
toto
thethe
operation
in a in
diagnostic
mode,mode,
For the
test,
thethe
monitoring
wasconfigured
configured
operation
a diagnostic
which
enabled
the
observation
andregistration
registration of
of UHF
time
waveforms
in real-time.
In Figure
28 the28 the
which
enabled
the
observation
and
UHF
time
waveforms
in
real-time.
In
Figure
For the IVPD
the monitoring
system
to thewere
operation
in afor
diagnostic
mode,
exemplary
timetest,
waveforms
of duration
20 ms was
wereconfigured
presented, which
registered
three main
exemplary
timethe
waveforms
of duration
20 ms were
presented,
which were
registeredInfor
three28
main
which
enabled
observation
and
registration
of
UHF
time
waveforms
in
real-time.
Figure
IVPD test voltage levels, i.e., 284, 374, and 473 kV. PD pulses were detected by the UHF probe the
IVPD
test
voltage
levels,
i.e.,
284,
374,
and
473
kV.
PD
pulses
were
detected
by
the
UHF
probe
installed
exemplary
time
of duration
ms were
which
three
installed
nearwaveforms
the phase L3.
The probe20installed
onpresented,
the opposite
side were
of theregistered
tank, near for
phase
L1, main
near
the
phase
L3.
The
probe
installed
on
the
opposite
side
of
the
tank,
near
phase
L1,
independently
the i.e.,
level284,
of the374,
test voltage,
background
noise (about
20‒30
mV).probe
IVPDindependently
test voltage from
levels,
and 473registered
kV. PD only
pulses
were detected
by the
UHF
fromThis
the near
level
ofthat
the
testPD
voltage,
registered
onlyinon
background
(about
20–30
mV).
means
meansthe
the
source
was probably
phase
L3 or noise
its neighborhood.
For
the This
partial
installed
phase
L3.
The probe
installed
the opposite
side
of the
tank,
near
phase
L1,
that
the
PD
source
was
probably
in
phase
L3
or
its
neighborhood.
For
the
partial
discharges
of
low
discharges
of
low
intensity
and
low
apparent
charge
(max.
150
pC),
the
UHF
probe
registered
pulses,
independently from the level of the test voltage, registered only background noise (about 20‒30 mV).
intensity
andthat
lowthe
apparent
charge
(max.
150
pC),
the
UHF
pulses,
however,
their
however,
their
amplitude
did not
exceed
45 mV
1.5‒2
times
higher than
the background
This
means
PD source
was
probably
in(approx.
phase
L3probe
or
itsregistered
neighborhood.
For
the partial
noise). did
In turn,
by Um = 45
374mV
kV,(approx.
for partial
discharges
of apparent
equal to 600‒700
amplitude
not exceed
1.5–2
times higher
than charge
the background
noise).pC,
In the
turn, by
discharges of low intensity and low apparent charge (max. 150 pC), the UHF probe registered pulses,
Um = 374 kV,
foramplitude
partial discharges
apparent
equal to1.5‒2
600–700
pC,higher
the UHF
pulses
of definitely
however,
their
did notofexceed
45 charge
mV (approx.
times
than
the background
higher In
amplitude
were
registered.
For the
enhancement
voltage
(473.4 kV),
PD
noise).
turn, by (120–135
Um = 374mV)
kV, for
partial
discharges
of apparent
charge
equallevel
to 600‒700
pC, the
Energies 2020, 13, 3271
25 of 37
Energies 2020, 13, x FOR PEER REVIEW
25 of 38
UHF
pulses of charge
definitely
higher amplitude
(120‒135 mV)
were
registered.
the enhancement
pulses
of apparent
1500–1800
pC corresponded
to the
UHF
pulses ofFor
maximal
amplitude 175
voltage
level (473.4
kV), PD pulses
of apparent
charge
1500‒1800
corresponded
to the
UHF
pulses
mV. The
obtained
measurement
results
during the
IVPD
test of pC
power
transformer
330
MVA
showed
of
maximal
amplitude
175
mV.
The
obtained
measurement
results
during
the
IVPD
test
of
power
the correct design and implementation of the sensor ISO-002-UHF mechanics and high sensitivity of
transformer 330 MVA showed the correct design and implementation of the sensor ISO-002-UHF
the UHF antenna, which enabled detection of partial discharges with relatively low apparent charge
mechanics and high sensitivity of the UHF antenna, which enabled detection of partial discharges
(100–150 pC).
with relatively low apparent charge (100‒150 pC).
(a)
(b)
(c)
Figure
28. Exemplary
UHFPD
PD pulses
pulses registered
forfor
different
voltage
levelslevels
duringduring
the IVPD
(a) test:
Figure
28. Exemplary
UHF
registered
different
voltage
thetest:
IVPD
284 kV, (b) 374 kV, (c) 473 kV.
(a) 284 kV, (b) 374 kV, (c) 473 kV.
3.2. of
Test
the System
on 31.5
MVA
PowerTransformer
Transformer
3.2. Test
theofSystem
on 31.5
MV
A Power
The next
object,
which
thePD
PDmonitoring
monitoring system
was
a transformer
withwith
a rated
The next
object,
on on
which
the
systemwas
wastested
tested
was
a transformer
a rated
power of 31.5 MVA and voltage of 115/33 kV. Before the prototype system installation, the
power of 31.5 MVA and voltage of 115/33 kV. Before the prototype system installation, the localization
localization of PD sources was carried out by the trilateration technique. For this purpose, eight
of PD sources was carried out by the trilateration technique. For this purpose, eight piezoelectric AE
piezoelectric AE sensors type PAC R15α and 8-channel AE system type DiSP (Physical Acoustic
sensors
type PAC Princeton,
R15α and NJ,
8-channel
AE system
type
DiSP (Physical
Corporation,
Princeton,
Corporation,
USA) were
used. The
coordinates
of theAcoustic
AE sensors
mounted on
the
NJ, USA)
were
used.
The
coordinates
of
the
AE
sensors
mounted
on
the
transformer
tank
of
dimensions
transformer tank of dimensions 4 × 1.3 × 2.6 m are listed in Table 5.
4 × 1.3 × 2.6 m are listed in Table 5.
Table 5. Coordinates of AE sensors.
Table 5. Coordinates of AE sensors.
Sensor Index
Sensor Index 1
1
2
3
4
5
6
7
8
2
3
4
5
6
7
8
X (m)
2.05
1.20
3.05
2.05
4.00
3.05
3.00
4.00
2.00
3.00
1.06
2.00
0.00
Y (m) Z (m)
1.40Y (m)
1.30
0.45
1.30
1.40 1.401.30
1.10 0.450.65
0.75 1.400.00
0.45 1.100.00
1.05 0.750.00
0.95 0.450.65
1.06
0.00
1.05
0.95
X 1.20
(m)
Z (m)
1.30
1.30
1.30
0.65
0.00
0.00
0.00
0.65
Energies 2020, 13, 3271
Energies 2020, 13, x FOR PEER REVIEW
26 of 37
26 of 38
The amplitude
amplitude of
of the
the registered
registered signals
signals was
was relatively
relatively low
low and
and equal
equal to
to 30–45
30‒45 dB
dB ref
ref 100
100 µV
µV
The
(peakatataround
around
dB).
results
the localization
obtained
with
the trilateration
(peak
66 66
dB).
TheThe
results
of theoflocalization
obtained
with the
trilateration
techniquetechnique
showed
showed
in the investigated
power transformer
there
are two
sources
of partial The
discharges.
The
that
in thethat
investigated
power transformer
there are two
sources
of partial
discharges.
first source
firstdetected
source was
on the high
side
(115 kV),
between
phases
L1 and L2 (coordinates:
was
on detected
the high voltage
sidevoltage
(115 kV),
between
phases
L1 and
L2 (coordinates:
x = 1.9 m,
1.9 m, zy =
= 1.0
z =In1.25
m).the
In second
turn, the
second
source
was on
detected
onvoltage
the lowside
voltage
side
yx== 1.0
1.25m,
m).
turn,
PD
sourcePD
was
detected
the low
(33 kV),
(33
kV),
at the
point
of LV connections
(coordinates:
= 1.3,
= 1.5graphic
m). The
graphic
results
at
the
point
of LV
connections
(coordinates:
x = 1.3, y =x0.1
m, y
z = 0.1
1.5 m,
m).zThe
results
of the
PD
of the PD
source localization
are in
presented
in Figure 29.
source
localization
are presented
Figure 29.
Figure 29. Graphic presentation of the localization results of two partial discharges sources by the
Figure 29. Graphic
presentation
of the localization
results
of two(blue
partial
discharges sources by the
trilateration
technique
and the distribution
of the eight
AE sensors
dots).
trilateration technique and the distribution of the eight AE sensors (blue dots).
After performing the measurements which showed the existence of partial discharge sources,
Afterpreceding
performing
measurements
which showed
existence
of partial
discharge
sources,
the works
thethe
prototype
system installation
werethe
carried
out. First,
the power
transformer
the
works
preceding
the
prototype
system
installation
were
carried
out.
First,
the
power
transformer
was turned off and disconnected from the HV transmission lines. Then, the oil excess was drained to
was turned off and disconnected from the HV transmission lines. Then, the oil excess was drained to
the level enabling opening the revision hole in the upper tank cover. After opening the revision hole,
Energies 2020, 13, 3271
Energies 2020, 13, x FOR PEER REVIEW
Energies 2020, 13, x FOR PEER REVIEW
27 of 37
27 of 38
27 of 38
the level enabling opening the revision hole in the upper tank cover. After opening the revision hole,
the
dielectric windows and the remaining elements of the ISO-002-ADW/UHF sensors were
theactive
active dielectric windows and the remaining elements of the ISO-002-ADW/UHF sensors were
immediately
in order to minimize the time of contact
immediatelyinstalled,
contactof
ofthe
theoil
oilwith
withhumid
humidair
air(Figure
(Figure30).
30).
installed, in order to minimize the time of contact
of
the
oil
with
humid
air
(Figure
30).
Figure
windows with
UHF antennas
(ISO-002-ADW/UHFFigure30.
30.The
Theinstallation
installationofof
ofthe
theactive
activedielectric
dielectric
Figure
30.
The
installation
the
active
dielectric windows
windows with
with UHF
UHF antennas
antennas(ISO-002-ADW/UHF-1
(ISO-002-ADW/UHF1and
and ISO-002-ADW/UHF-2)
ISO-002-ADW/UHF-2) in
in the
the revision
revision holes
holes of
the
upper
cover
of
the
31.5 MVA
of
the
upper
cover
of
the
MVA power
power
1 and ISO-002-ADW/UHF-2) in the revision holes of the upper cover of the 31.5
31.5 MVA
power
transformer
tank.
transformer tank.
tank.
transformer
The investigatedpower
power transformerwas
was equippedinintwo
two sensorsISO-002-ADW/UHF
ISO-002-ADW/UHF andone
one
The
Theinvestigated
investigated powertransformer
transformer wasequipped
equipped in twosensors
sensors ISO-002-ADW/UHFand
and one
sensor ISO-002-HF installed
installed
the grounding wire
of the neutral
point ofof
the HV
winding (Figure
31).
sensor
sensorISO-002-HF
ISO-002-HF installedon
on thegrounding
groundingwire
wireofofthe
theneutral
neutralpoint
point ofthe
theHV
HVwinding
winding(Figure
(Figure
TheThe
optic
fiber
andand
power
supply
cables
werewere
protected
by putting
themthem
into into
the corrugated
PCV PCV
pipes
31).
optic
fiber
power
supply
cables
protected
by
putting
the
corrugated
31). The optic fiber and power supply cables were protected by putting them into the corrugated PCV
with awith
diameter of 25 mm.
A waterproof
enclosure of the monitoring
system with data concentrator
pipes
pipes with aa diameter
diameter ofof 25
25 mm.
mm. AA waterproof
waterproof enclosure
enclosure ofof the
the monitoring
monitoring system
system with
with data
data
module MPD-001,
server,
and power
supply
was
placed
opposite
toplaced
the power
transformer.
The last
concentrator
module
MPD-001,
server,
and
power
supply
was
opposite
to
the
concentrator module MPD-001, server, and power supply was placed opposite to the power
power
stage of the installation
works was
configuring thewas
data transmission
between
the PD monitoring
transformer.
transformer.The
Thelast
laststage
stageofofthe
theinstallation
installationworks
works wasconfiguring
configuringthe
thedata
datatransmission
transmissionbetween
between
system monitoring
and SCADA substation
system type
SYNDIS ES (Mikronika,
Poznan, Poland),
which registered
the
thePD
PD monitoringsystem
systemand
andSCADA
SCADAsubstation
substationsystem
systemtype
typeSYNDIS
SYNDISES
ES(Mikronika,
(Mikronika,Poznan,
Poznan,
basic operationregistered
parameters, such
as voltage, load, andsuch
oil temperature.load, and oil temperature.
Poland),
Poland),which
which registeredbasic
basicoperation
operationparameters,
parameters, suchasasvoltage,
voltage, load, and oil temperature.
Figure31.
31.Photograph
Photograph
of PD
the on-line
PD on-line
monitoring
system
installed
31.5substation
MVA substation
Figure
of the
monitoring
system
installed
on 31.5on
MVA
power
Figure 31. Photograph of the PD on-line monitoring system installed on 31.5 MVA substation power
power transformer.
transformer.
transformer.
After fillingthe
the power transformer
transformer tank with
oil and
checking the
tightness of of
revision holes,
the
After
Afterfilling
filling thepower
power transformertank
tankwith
withoil
oiland
andchecking
checkingthe
thetightness
tightness ofrevision
revisionholes,
holes,
power
transformer
was
turned
on
and
the
partial
discharges
monitoring
system
was
started.
Figure
32
the
thepower
powertransformer
transformerwas
wasturned
turnedon
onand
andthe
thepartial
partialdischarges
dischargesmonitoring
monitoringsystem
systemwas
wasstarted.
started.
presents
the
characteristics
of
PD
intensity
(pulse
rate
per
15
s),
active
power,
voltage,
and
top-oil
Figure
Figure32
32presents
presentsthe
thecharacteristics
characteristicsofofPD
PDintensity
intensity(pulse
(pulserate
rateper
per15
15s),
s),active
activepower,
power,voltage,
voltage,and
and
temperature
obtained
within
8-days-operation
of
the
PD
monitoring
system.
top-oil
top-oiltemperature
temperatureobtained
obtainedwithin
within8-days-operation
8-days-operationofofthe
thePD
PDmonitoring
monitoringsystem.
system.
Energies 2020, 13, 3271
Energies 2020, 13, x FOR PEER REVIEW
28 of 37
28 of 38
.
Figure 32.
Partial discharges intensity characteristics, voltage, load, and oil temperature
Figure 32. Partial
discharges
intensity characteristics,
voltage,
load,
and
temperature
characteristics
registered
during 8-days-operation
of the monitoring
system
on the
31.5oil
MVA
substation
characteristics
registered during 8-days-operation of the monitoring system on the 31.5 MVA
power
transformer.
substation power transformer.
•
•
The conducted data analysis proves that in the monitored power transformer:
The conducted data analysis proves that in the monitored power transformer:
Partial discharges were registered every day, and their activity was the highest mainly for hours
Partial
discharges
were
registered
every
day, and
activity
was the
from 8:45
a.m. to 5:45
p.m.
(these time
intervals
aretheir
indicated
in Figure
32highest
as grey mainly
bands). for
Thehours
sum
from
a.m.from
to 5:45
time
intervals 68.9%
are indicated
in Figurepulses.
32 as grey bands). The
of PD8:45
pulses
thisp.m.
time (these
intervals
constitutes
of all registered
sum of PD pulses from this time intervals constitutes 68.9% of all registered pulses.
Energies 2020, 13, 3271
Energies 2020, 13, x FOR PEER REVIEW
••
••
29 of 37
29 of 38
The
Thepeak
peakintensity
intensityof
ofpartial
partialdischarges
dischargesoccurred
occurredbetween
between 1:00
1:00 p.m.
p.m. and
and 3:00
3:00 p.m.
p.m. In
In this
this short
short
period,
almost
one-quarter
(24.4
%)
of
all
PD
pulses
were
detected.
period, almost one-quarter (24.4 %) of all PD pulses were detected.
The
ThePearson
Pearsoncorrelation
correlationcoefficient
coefficientbetween
betweenthe
thehourly
hourlydistribution
distributionof
ofthe
the UHF
UHF and
and AE
AE pulses
pulses
for
sensor
ISO-002-ADW/UHF-1
was
r
=
0.87
with
significance
p
=
0.001
(very
strong
for sensor ISO-002-ADW/UHF-1 was r = 0.87 with significance p = 0.001 (very strong positive
positive
correlation),
and for sensor ISO-002-ADW/UHF-2 it was r = 0.45 with significance p = 0.027
correlation), and for sensor ISO-002-ADW/UHF-2 it was r = 0.45 with significance p = 0.027
(moderate
positive correlation). Both hourly distributions of the UHF and AE pulses are
(moderate positive correlation). Both hourly distributions of the UHF and AE pulses are presented
presented
in
Figure 33.
in Figure 33.
(a)
(b)
Figure
UHF
and
AEAE
pulses
forfor
sensor:
(a) ISO-002-ADW/UHF-1
andand
(b)
Figure 33.
33.Hourly
Hourlydistribution
distributionofofthe
the
UHF
and
pulses
sensor:
(a) ISO-002-ADW/UHF-1
ISO-002-ADW/UHF-2.
(b) ISO-002-ADW/UHF-2.
••
••
••
•
•
The
Thehourly
hourlydistribution
distributionofofthe
thePD
PDpulses
pulsesregistered
registeredwith
withthe
thehigh-frequency
high-frequencycurrent
current transformer
transformer
ISO-002-HF
themost
mostwith
withthe
thedistribution
distribution
obtained
transducer
(r =
ISO-002-HFwas
was correlated
correlated the
obtained
forfor
the the
AE AE
transducer
(r = 0.68,
0.68,
p < 0.001)
the UHF
antenna
= 0.62;
p = 0.001)
of the
sensor
ISO-002-ADW/UHF-1.
In
p < 0.001)
and and
the UHF
antenna
(r = (r
0.62;
p = 0.001)
of the
sensor
ISO-002-ADW/UHF-1.
In the
the
case
of
the
sensor
ISO-002-ADW/UHF-2
the
correlation
was
lower
at
the
level
of
r
=
0.44
with
case of the sensor ISO-002-ADW/UHF-2 the correlation was lower at the level of r = 0.44 with
significance
significancepp==0.029
0.029for
forthe
thedistribution
distributionobtained
obtained for
for the
the AE
AE transducer
transducer and
and rr == 0.22
0.22 with
with
significance
p
=
0.031
for
the
distribution
obtained
for
the
UHF
antenna.
significance p = 0.031 for the distribution obtained for the UHF antenna.
The
Thevoltage
voltagevalue
valuewas
waschanging
changingininaanarrow
narrowrange
rangefrom
from113.3
113.3toto117.6
117.6kV,
kV,wherein
whereinthe
theaverage,
average,
2
2
variance,
and
standard
deviation
were,
respectively,
equal
to
M
=
115.3
kV,
s
=
0.47,
and
=
variance, and standard deviation were, respectively, equal to M = 115.3 kV, s = 0.47, and SD =SD
0.69.
0.69.
Due
to
the
small
voltage
variation,
it
was
decided
not
to
investigate
its
influence
on
partial
Due to the small voltage variation, it was decided not to investigate its influence on partial
discharges
dischargesintensity.
intensity.
During
the
8-days-operation
During the 8-days-operationofofthe
thesystem,
system,the
thepower
power transformer
transformerwas
was loaded
loaded with
with relatively
relatively
low
power
from
only
0.68
to
11.34
MVA,
which
constituted,
respectively,
2.15%
and
low power from only 0.68 to 11.34 MVA, which constituted, respectively, 2.15% and 36.00%
36.00% of
of its
its
2
rated
power.
The
average
value
of
the
load
was
equal
to
M
=
5.31
MVA,
variance
s
=
6.16,
and
2
rated power. The average value of the load was equal to M = 5.31 MVA, variance s = 6.16, and
standard
standarddeviation
deviationSD
SD==2.48.
2.48.
The oil temperature was changing in a wide band from 14.0 to 36.1 °C.
It was observed that daily
The oil temperature was changing in a wide band from 14.0 to 36.1 ◦ C. It was observed that daily
“hills” and local peaks of the oil temperature curve overlap the period of higher activity of partial
“hills” and local peaks of the oil temperature curve overlap the period of higher activity of partial
discharges. (from 8:45 a.m. to 5:45 p.m.) (see Figure 32). In turn, for daily “valleys” of the oil
discharges. (from 8:45 a.m. to 5:45 p.m.) (see Figure 32). In turn, for daily “valleys” of the oil
temperature curve, a clear reduction or complete disappearance of the partial discharges activity
temperature curve, a clear reduction or complete disappearance of the partial discharges activity
was observed. In order to test the hypothesis relating to the influence of the level of oil
was observed. In order to test the hypothesis relating to the influence of the level of oil temperature
temperature on partial discharges intensity in the monitored power transformer, it was decided
on partial discharges intensity in the monitored power transformer, it was decided to conduct a
to conduct a one-way analysis of variance (ANOVA) for independent samples. One of the main
one-way analysis of variance (ANOVA) for independent samples. One of the main assumptions
assumptions of the ANOVA is that the independent variable consists of two or more categorical,
of the ANOVA is that the independent variable consists of two or more categorical, independent,
independent, and equinumerous groups. The categorization of the daily oil temperature values
and equinumerous groups. The categorization of the daily oil temperature values (independent
(independent variable) was carried out using tertiles, in order to obtain three equinumerous
variable) was carried out using tertiles, in order to obtain three equinumerous groups. The “low”
groups. The “low” temperature category was ascribed to lower tertile, whereas “moderate” and
temperature category was ascribed to lower tertile, whereas “moderate” and “high” temperature
“high” temperature categories to middle and higher tertiles, respectively. Since the assumption
categories to middle and higher tertiles, respectively. Since the assumption of homogeneity of
of homogeneity of variance was violated (Levene's test for equality of variances was statistically
variance was violated (Levene’s test for equality of variances was statistically significant), therefore
significant), therefore the Brown–Forsythe and Welch F-ratios are reported. The results of both
the Brown–Forsythe and Welch F-ratios are reported. The results of both robust tests of equality
robust tests of equality of means are listed in Table 6. The results of the analysis showed that the
of means are listed in Table 6. The results of the analysis showed that the compared groups differ
compared groups differ from one another statistically, which means that the oil temperature level
from one another statistically, which means that the oil temperature level differentiates the number
differentiates the number of PD pulses. Figure 34 illustrates the value of the average number of
the AE, UHF, and HF pulses between particular oil temperature levels (low, moderate, and high),
Energies 2020, 13, 3271
30 of 37
of PD pulses. Figure 34 illustrates the value of the average number of the AE, UHF, and HF pulses
Energies
2020, 13,particular
x FOR PEERoil
REVIEW
of 38
between
temperature levels (low, moderate, and high), which indicate a strong30causal
relationship between these variables. Figures obtained for the sensors ISO-002-ADW/UHF1 and
which
indicate a strong
causal
relationship
these variables.
Figures
for the
ISO-002-ADW/UHF2
show
that the
higher the between
oil temperature
level was, the
higherobtained
was the number
sensors
ISO-002-ADW/UHF1
and
ISO-002-ADW/UHF2
that the highercurrent
the oil temperature
of the registered
AE and UHF
pulses.
The exception isshow
the high-frequency
transformer
level
was, thesensor,
higher for
waswhich
the number
of theaverage
registered
AE and
pulses.
The
exceptionfor
is the
ISO-002-HF
the highest
number
of UHF
HF pulses
was
registered
the
high-frequency
current transformer
ISO-002-HF
sensor,
for which
the are
highest
averagesignificantly
number of
average oil temperature
level. To investigate
which
compared
groups
statistically
HF
pulsesfrom
was registered
for the
the Gamesa–Howell
average oil temperature
level.
investigate
which compared
different
one another,
post hoc
test,To
which
is recommended
in the
groups
are
statistically
significantly
different
from
one
another,
the
Gamesa–Howell
hoc
situation when the assumption of homogeneity of variance in the compared groups waspost
violated.
test,
is recommended
in the situation
whensignificant
the assumption
of homogeneity
ofbetween
variance all
in
Thewhich
post hoc
comparisons revealed
statistically
differences
(p < 0.0005)
the
compared
groups
was
violated.
The
post
hoc
comparisons
revealed
statistically
significant
groups (oil temperature levels) except the groups “moderate” and “high” temperature for the
differences
< 0.0005) between
groups
(oil temperature levels)
thehighest
groupsdifferences
“moderate”
UHF pulses(pregistered
with the all
sensor
ISO-002-ADW/UHF-2
(Tableexcept
7). The
in
and
“high” temperature
for the
UHF pulses
registered
with and
the sensor
ISO-002-ADW/UHF-2
the number
of PD pulses were
observed
between
groups “low”
“high” temperature.
As it was
(Table
7). The
highest
differences
the number
PDTherefore,
pulses were
observed
between
groups
mentioned
before,
the voltage
valueinvariability
wasof
low.
while
interpreting
the obtained
“low”
“high”
temperature.
Asthe
it monitored
was mentioned
the voltage
value variability
was
resultsand
it may
be concluded
that in
powerbefore,
transformer,
the oil temperature
growth
is
low.
Therefore,
while
interpreting
the
obtained
results
it
may
be
concluded
that
in
the
monitored
a significant factor inducing partial discharges. This hypothesis is in line with the research results,
power
temperature
growth
is a significant
factor
inducingmoisture
partial discharges.
which transformer,
showed that the
the oil
temperature
growth
in combination
with
an increased
content of
This
hypothesis
is in linesystem
with the
research
whichgrowth
showed
the discharges
temperature
growth
the oil-paper
insulation
may
induceresults,
the activity
ofthat
partial
[107–110].
in
combination
with
an
increased
moisture
content
of
the
oil-paper
insulation
system
may
induce
Additionally, the presence of water molecules in oil as a result of the moisture migration process,
the
activity
growth
of
partial
discharges
[107‒110].
Additionally,
the
presence
of
water
molecules
as well as the effect of gas bubbles exuding from cellulose insulation, may significantly influence
in
oilintensification
as a result of of
thethe
moisture
migration [15,111–116].
process, as well as the effect of gas bubbles exuding
the
PD phenomenon
from cellulose insulation, may significantly influence the intensification of the PD phenomenon
[15,111‒116].
Table 6. Results of robust tests of equality of means.
PD Sensor Label
Table
6. Results ofTest
robust tests of Statistics
equality of
1 means.
Pulses
df1
PD Sensor Label
Pulses
Test
Welch
UHF
Brown–Forsythe
Welch
ISO-002-ADW/UHF-1
UHF
Brown–Forsythe
Welch
ISO-002-ADW/UHF-1 AE
Welch
Brown–Forsythe
AE
Brown–Forsythe
Welch
UHF
Welch
Brown–Forsythe
UHF
ISO-002-ADW/UHF-2
Brown–Forsythe
Welch
ISO-002-ADW/UHF-2
AE
Welch
AE Brown–Forsythe
Brown–Forsythe
Welch
Welch
ISO-002-HF
HFCT
ISO-002-HF
HFCT Brown–Forsythe
Brown–Forsythe
1
1
Statistics
289.93
371.11
289.93
371.11
149.51
149.51
176.34
176.34
60.18
60.18
39.05
39.05
1772.95
1772.95
977.32
977.32
1486.10
1486.10
914.83
914.83
1
df12
2 2
2 2
2 2
2 2
2 2
2
2
2 2
2
2
2
2
2
Asymptotically F distributed.
Asymptotically F distributed.
Figure 34. Estimated marginal means of PD pulses.
Figure 34. Estimated marginal means of PD pulses.
df2
p
df2
28,969.85 p < 0.0005
32,492.03< 0.0005
< 0.0005
28,969.85
32,492.03
<
0.0005
24,644.93
< 0.0005
24,644.93
20,351.35< 0.0005
< 0.0005
20,351.35
<
0.0005
25,693.85
< 0.0005
25,693.85
35,227.18< 0.0005
< 0.0005
35,227.18 < 0.0005
23,518.60
< 0.0005
23,518.60
< 0.0005
32,563.54
< 0.0005
32,563.54 < 0.0005
25,652.90
< 0.0005
25,652.90 < 0.0005
26,939.20
< 0.0005
26,939.20 < 0.0005
Energies 2020, 13, 3271
31 of 37
Table 7. Results of multiple comparisons of mean PD pulses within groups using post hoc
Games–Howell test.
PD Sensor Label
Pulses
Temperature Groups
Mean Difference
p
UHF
Low vs. Moderate
Low vs. High
Moderate vs. High
−18.60 *
−79.08 *
−60.49 *
< 0.0005
< 0.0005
< 0.0005
AE
Low vs. Moderate
Low vs. High
Moderate vs. High
−4.95 *
−22.10 *
−17.15 *
< 0.0005
< 0.0005
< 0.0005
UHF
Low vs. Moderate
Low vs. High
Moderate vs. High
−0.74 *
−1.00 *
−0.26
< 0.0005
< 0.0005
0.154
AE
Low vs. Moderate
Low vs. High
Moderate vs. High
−386.96 *
−534.64 *
−147.68 *
< 0.0005
< 0.0005
< 0.0005
HFCT
Low vs. Moderate
Low vs. High
Moderate vs. High
−39.46 *
−25.98 *
13.48 *
< 0.0005
< 0.0005
< 0.0005
ISO-002-ADW/UHF-1
ISO-002-ADW/UHF-2
ISO-002-HF
*
The mean difference is significant at the 0.05 level.
4. Conclusions
The article presents a prototype, hybrid on-line PD monitoring system. Its unique feature is a
combination of the functionality of three methods of PD detection (AE, HF, and UHF) and integrating
them into one complementary system. Another important function of the system is the possibility
of its integration using IEC 61850 protocol with remaining substation automation devices, but first
of all with SCADA system. Due to that, the monitoring system can analyze in real-time the PD
parameters registered using various detectors as well as the basic transformer parameters that affect
the phenomenon of partial discharges, i.e., voltage and oil-temperature. Tests conducted on 31.5 MVA
power transformer showed that in its case partial discharges ignited in particular times of day (from
approx. 8:45 a.m. to 5:45 p.m.), and their increased activity was observed during daily “hills” and
local peaks of the oil temperature curve. The results of the statistical analysis of the HF, UHF, and
AE signals presented in the paper allow to conclude with a high probability that in these periods, the
partial discharge phenomenon was the source of the majority of pulses registered by detectors.
Laboratory tests of partial discharge detectors developed for the monitoring system have confirmed
their high performance. The active dielectric window with built-in ultrasound transducer assures on
average 5.80 times higher sensitivity than the popular contact AE sensor type PAC R15D. Such high
sensitivity was obtained because the ultrasonic transducer is located inside the oil-filled transformer
tank and is equipped with a specially designed matching layer that facilitates the transfer of acoustic
energy from mineral oil to a piezoelectric crystal. In turn, the prototype HFCT sensor with MnZn
ferrite core showed—in comparison to the commonly used RFCT-4 sensor—on average 1.48 times
higher sensitivity of PD in oil detection and on average 1.50 times higher detection sensitivity of surface
PD. Additionally, very good results were obtained for the prototype construction of the meandered
planar inverted-F antenna. The antenna bandwidth (280–353 MHz) was optimized for the detection of
PDs typical for oil-paper insulation. Tests performed in a shielded laboratory and with the use of a
power transformer tank equipped in the dielectric windows showed that the prototype meandered
PIFA compared to standard disk antenna is on average 7.80 times more sensitive in the detection of
low-radiating PD (an initial form of surface PD with apparent charge below 1 nC) and on average 1.75
more sensitive in the detection of high energy creeping sparks with apparent charge about 6 nC.
Future research will focus primarily on the development of the system’s software layer, including
development and implementation of an algorithm for identifying the type of partial discharge pulses
based on PRPD patterns (for HF and UHF method) and implementation of the FFT (Fast Fourier
Energies 2020, 13, 3271
32 of 37
Transform) algorithm in the FPGA for the acoustic emission channel, which will create the ability to
identify the type of PDs based on selected frequency domain features and artificial neural network
(ANN) based classification. In the case of the hardware layer, research works are underway to adapt the
system to diagnose power equipment other than transformers. The first successful implementations of
the prototype monitoring system on 110 kV gas-insulated switchgear are described in [117].
Author Contributions: Conceptualization, W.S., K.W. and W.G.; methodology, W.S, K.W. and W.G.; software,
W.G., W.S. and K.W.; validation, W.S., W.G. and K.W.; formal analysis, W.S.; investigation, W.S, W.G., C.S.,
K.W.; resources, W.S and W.G; data curation, W.S.; writing—original draft preparation, W.S.; writing—review
and editing, W.S., K.W., W.G. and C.S.; visualization, W.S.; supervision, W.S.; project administration, W.S. and
W.G.; funding acquisition, W.S., W.G and K.W. All authors have read and agreed to the published version of
the manuscript.
Funding: This work was supported by the Polish National Centre for Research and Development, within the
Applied Research Programme, grant No. PBS3/A4/12/2015.
Conflicts of Interest: The authors declare no conflict of interest.
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