energies

Article
Condition Assessment of HV Bushings with Solid
Insulation based on the SVM and the FDS Methods †
Jan Subocz, Andrzej Mrozik * , Patryk Bohatyrewicz and Marek Zenker
Faculty of Electrical Engineering, Department of Electrotechnology and Diagnostics, West Pomeranian
University of Technology, Sikorskiego 37, 70–313 Szczecin, Poland; jan.subocz@zut.edu.pl (J.S.);
bohatyrewicz@gmail.com (P.B.); marek.zenker@zut.edu.pl (M.Z.)
* Correspondence: andrzej.mrozik@zut.edu.pl; Tel.: +48-698-386-982
† This paper is an extended version of our paper published in 2018 IEEE Innovative Materials and Technologies
in Electrical Engineering (i-MITEL), 18–20 April 2018.




Received: 23 December 2019; Accepted: 14 February 2020; Published: 15 February 2020


Abstract: This paper presents the dielectric response of the insulation of bushings as an effect of
the simulated long-term aging process. The experiment was conducted under a condition of a
high temperature difference between the current circuit and the cover. The dielectric response was
measured with the FDS (Frequency Dielectric Spectroscopy) and the SVM (Step Voltage Measurement)
methods. The research has shown the correlation between the aging time and some parameters
obtained with the FDS and SVM analysis.

Keywords: power transformers; bushings; relaxation; step voltage measurement

1. Introduction
Transformer bushings belong to the basic equipment of each high voltage power transformer.
They serve as the elements providing the passage of power cables through the barrier which consists
of a transformer tank. Transformers, like bushings, are designed for around 30 years of operation.
However, as the operating practice shows, they are very often the cause of failure. The policy of the
distribution companies is aimed at lengthening the transformers operating time above the designed
lifetime i.e., 30 years what results in arising the situations in which there is big a group of transformers
with factory-installed transformer bushings [1,2]. In spite of positive results of the measurements of the
dielectric loss factor tgδ measured in mains frequency the bushings installed in such units have the signs
of beginnings of the aging processes which are noticeable in the low frequency range 0.01–0.001 Hz.
Such ageing processes are unnoticeable for the standard methods what may cause misdiagnosis in
the technical condition of the insulators and, as a result, it may lead to disastrous consequences such
as explosions. The statistics, which present that from 12 to 40% of the catastrophic failures of the
power transformers resulted from bushing malfunctions, can be found in the literature [3–6]. Due
to inefficiency of tg50Hz there is the need to search for new measurement solutions that increase
predictability of defects in HV (high voltage) bushings insulation [6,7]. According to the authors such
methods may be the FDS and the SVM method since they can contribute to detect accelerated aging in
the early stage of their development. The new use of the SVM method applied mainly in HV motor
insulation diagnostics, according to the authors, allows for diagnostics of the most exposed to aging
layers and such possibility results from a different distribution of the electric field constant in relation
to the alternating field [8]. Due to that issue, the constant field mainly breaks down at the stem, i.e.,
those most exposed to aging.
The main insulation of HV transformer bushings constructed using RBP (Resin-Bonded Paper)
or RIP (Resin Impregnated Paper) technology is a resinous composite in which the polymer matrix

Energies 2020, 13, 853; doi:10.3390/en13040853 www.mdpi.com/journal/energies

Energies 2020, 13, 853 2 of 13

consists of epoxy resin and the reinforcement rod is an electrotechnical paper foil and the aluminum
screen. The morphological construction of such structures can be classified according to the group
of layered composites with a mixed structure which are characterized by, among others, isotropic
electrical and mechanical properties [9,10].
The aging processes in such composites are caused by the superposition of the action of the electric
field, temperature and mechanical stress [11]. However, for a high concentration of factors, the synergy
effect accelerates the process of composite degradation.
The action of the electric field can be presented in the following equation:

LE = L0 (E/E0 )−n (1)

where
• LE —the “lifetime” for the electric field strength E,
• L0 —the “lifetime” for the electric field strength E0 ,
• n—the coefficient of the voltage durability (VEC).

The physical nature of the electric aging is hardly recognized. However, it is generally believed
that the process is dominated by partial discharges and injections of electrons from electrodes. The
presence of the electric field has no direct impact on the kinetics of the aging process. The value of the
coefficient “n” is usually determined based on long-term experimental processes.
The impact of the thermal exposure on the composite destruction process is shown in the following
equation:
1
LT = L0 exp(−BT T0−T ) (2)

where
• T0 —the reference temperature,
• LT —the lifetime for the temperature T,
• B—the coefficient associated with the activation energy of the major chemical reactions.

The impact of the mechanical stress can be described as in the following equation:

LM = L0 (M/M0 )−m (3)

where
• M—the coefficient of mechanical durability (MEC),
• L0 , M0 correspond to Equation (1).

The action of these factors in the initial stage of the aging processes primarily causes the occurrence
of microdefects in the composite structure. Considering the physical nature of the occurrence of defects,
Kimura put forward the phenomenological model of their formation (Figure 1) [12].
The practical experience of the authors has shown that, in most cases, the total destruction of
composite occurs as a result of the electrical breakdown but not as a result of mechanical damage.
This is probably due to the changes of the interface structure of the “polymer–filler matrix”. In the
initial stage of the aging process, this determines the kinetics of the physical and chemical processes.
Therefore, the good adhesion and mutual wettability of the resin and the filler determine the durability
of the insulation.
The strength of the adhesion between the polymer matrix and the reinforcement depends on
the type of the chemical bonds and the post-production mechanical stress as well as the existing
electrostatic and mechanical friction forces between the elements. The phenomenological model of the
matrix–filler interphase has been described by Theocaris I Varias (Figure 2) [13].
 • L0, M0 correspond to Equation (1).
The action of these factors in the initial stage of the aging processes primarily causes the
occurrence of microdefects in the composite structure. Considering the physical nature of the
occurrence of defects, Kimura put forward the phenomenological model of their formation (Figure
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1) [12].2020, 13, 853 3 of 13

Energies 2020, 13, x FOR PEER REVIEW 3 of 13

Figure 1. Schematic diagram of the generation and development of microdefects during the impact
of combined exposure on composite insulation.

The practical experience of the authors has shown that, in most cases, the total destruction of
composite occurs as a result of the electrical breakdown but not as a result of mechanical damage.
This is probably due to the changes of the interface structure of the “polymer–filler matrix”. In the
initial stage of the aging process, this determines the kinetics of the physical and chemical processes.
Therefore, the good adhesion and mutual wettability of the resin and the filler determine the
durability of the insulation.
The strength of the adhesion between the polymer matrix and the reinforcement depends on the
type of the chemical bonds and the post-production mechanical stress as well as the existing
electrostatic and mechanical friction forces between the elements. The phenomenological model of
Figure 1. Schematic diagram of the generation and development of microdefects during the impact of
the matrix–filler interphase has been described by Theocaris I Varias (Figure 2). [13]
combined exposure on composite insulation.

Figure 2. Conception of the “polymer–filler” interphase.

Figure 2. Conception of the “polymer–filler” interphase.
The essential part of this is the transition zone which significantly influences such properties of
The essential
the entire composite part
asofitsthis is the transition
dielectric zone which
and mechanical significantly
relaxation mechanisms influences such properties
and electrical of
conductivity
the entire composite as its dielectric
in the field of low electric fields and temperature. and mechanical relaxation mechanisms and electrical
conductivity in thestage
In the initial field of
of the
lowaging
electric
offields andand
the RBP temperature.
the RIP composites, major changes in the phase
In the initial stage of the aging of
boundary morphology occur in the transition layer. the RBP and the RIP
Thiscomposites,
results in the major
losschanges in theand
of adhesion phase
the
boundary morphology occur in the transition layer. This results in the
occurrence of free spaces in the form of micro gas inclusions and composite delimitation. As a loss of adhesion and the
occurrence
result, there of is
free
anspaces in theat
occurrence form
the of micro
phase gas inclusions
boundary and composite
from which is formed, delimitation.
among other As factors,
a result,a
there is an occurrence at the phase boundary from which is formed, among
Maxwell–Wagner space charge which can significantly alter the local electric field strength and may other factors, a Maxwell–
Wagner
cause the space chargeof
generation which
partial can significantly
discharges evenalter
underthethelocal electric
nominal field strength
operating andof
conditions may
the cause the
insulation.
generation of partial discharges even under the nominal operating conditions
In off-line RIP insulation tests, the presence of Maxwell–Wagner charge as well as inclusion and of the insulation.
In off-line can
delamination RIP be
insulation
identified, tests, the presence
among others, of Maxwell–Wagner
using the spectroscopic charge as well as inclusion
measurements and
of dielectric
delamination
relaxation. Incan the be identified,
frequency among
domain, theothers, using the spectroscopic
most frequently used method is measurements
FDS (Frequency of dielectric
Dielectric
relaxation. In the frequency domain, the most frequently used method
Spectroscopy) method, in which changes in the dielectric loss factor tgδ in a wide frequency is FDS (Frequency Dielectric
range of
Spectroscopy)
−4 3 method, in
10 –10 Hz are noticed (Figure 3).which changes in the dielectric loss factor tgδ in a wide frequency range of
10−4–103 Hz are noticed (Figure 3).
 Energies 2020, 13, 853 4 of 13
Energies 2020, 13, x FOR PEER REVIEW 4 of 13

Figure 3. Typical
Figure relationships
3. Typical tgδ =tgδ
relationships f(f)=for
f(f)RIP
for bushings insulation
RIP bushings under
insulation different
under conditions.
different conditions.

The Thepresence
presenceof inclusions
of inclusionsor delamination
or delamination results in the
results in creation
the creation of a of spatial charge
a spatial in which
charge in which
the relaxation
the relaxation timetimeconstant is usually
constant in the
is usually in therange of 1–100
range of 1–100s, ons, tg = f(f),
on=tgf(f), which characteristically
which characteristically
corresponds
corresponds to the frequency
to the frequency range of 10
range of–310 –3 –10Hz.
–10Hz. In this range,
In this range,a large increase
a large increase in in
thethecoefficient
coefficient tgδ
tgδ is
is observed,
observed, andand thethefrequency
frequency in in
which
whichits its
value is maximum
value is maximum is defined
is defined as the resonance
as the resonancefrequency
frequency
corresponding
corresponding to thetovalue of theofrelaxation
the value time constant
the relaxation (Figure
time constant 3, curve
(Figure 3, 2). Similar
curve changeschanges
2). Similar in the in
agedtheRIP insulation
aged can be observed
RIP insulation in the rage
can be observed in above
the rage 100above
Hz. These
100 Hz.are associated with the dipole
These are associated with the
relaxation of low-molecular-weight
dipole relaxation productsproducts
of low-molecular-weight of the resinof theand resinfiller. Similarly,
and filler. in the
Similarly, in Cole–Cole
the Cole–Cole
diagram
diagramε” =ε” = f(ε’),
f(ε’), the frequency
the frequencyat which
at whichthe value ε” isε”
the value atismaximum
at maximum corresponds
corresponds to the
to relaxation
the relaxation
timetime
constant. In the
constant. Inoperating
the operatingmeasurements,
measurements, oftenoften
instead of a of
instead thorough
a thorough analysis of the
analysis ofrelationship
the relationship
tgδ =tgδ = f(f),
f(f), a faster determination
a faster determination of the
of range
the range of changes
of changes in thein value of the
the value of time constant
the time constantis applied
is applied
using the relationship
using the relationship ε” =ε” = f(ε’).
f(ε’).
In this domain,
In this domain, however,
however, the the
PDCPDC (Polarization–Depolarization
(Polarization–Depolarization Current)
Current)methodmethodor the SVM
or the SVM
(Step Voltage
(Step VoltageMeasurement)
Measurement) method
methodcan can
be used.
be used.TheThelatter consists
latter of measuring
consists of measuring the change
the changeof the
of the
charging
chargingcurrent overover
current timetimeunder the conditions
under the conditions of a of
step increase
a step in the
increase measuring
in the measuring voltage to the
voltage to the
value Umeas
value U=meas = 1.5·U
1.5∙U 0. It is0.mainly used used
It is mainly to testtoHV testmotor
HV motor insulation, and and
insulation, the measurement
the measurement conditions
conditions
and and
the the
interpretation
interpretation of the results
of the are are
results included
included in the IEEE
in the number
IEEE number 4, 1978 standard
4, 1978 standard(“Standard
(“Standard
Techniques
Techniques forfor High-Voltage
High-VoltageTesting”), standard number
Testing”), standard number43,43, November
November 1974 1974 (“Recommended
(“Recommended Practice
Practice for Testing Insulation Resistance of Rotating Machinery”),
for Testing Insulation Resistance of Rotating Machinery”), and in [14]. The method of calculating and in [14]. The method of the
calculating
absorption the absorption
coefficient Ka coefficient
defined inKathese
defined in theseisstandards is
standards
𝑖30𝑀
KaKa= = i30M (4)
𝑖30𝐶 (4)
i30C
where i30M and i30C are, respectively, the measured and calculated leakage currents after 30 min of the
SVM test. i30M and i30C are, respectively, the measured and calculated leakage currents after 30 min of the
where
SVM test.
The absorption coefficient Ka for the new insulation based on epoxy resins should be less than
The
2, while in the absorption coefficient
case of degraded Ka for the
insulation, itsnew
valueinsulation
should bebased
5–7. on
In epoxy resins should
this manuscript, the be less than
authors
2, while in the case of degraded insulation, its value should be 5–7. In this manuscript,
assumed that the morphology of the RIP composite is very similar to the structure of the vacuum the authors
assumed
pressure that the morphology
impregnation of the RIPof
used in the insulation composite
HV engines.is very similar to the structure of the vacuum
pressure impregnation used in the insulation of HV engines.
Therefore, the manuscript adopts the methodology for measuring SVM and assessing the
conditionTherefore, the manuscript
of the insulation adoptswith
in accordance the methodology for measuring
the above-mentioned SVM and
standards. assessing
A detailed the condition
description
of the insulation in accordance with the above-mentioned
of the FDS and the SVM methods is provided later in the manuscript. standards. A detailed description of the FDS
and the SVM methods is provided later in the manuscript.
2. Test Subject and Research Methodology
2. Test Subject and Research Methodology
The subjects of the laboratory tests were two transformer bushings: an RIP type, Micafil CTKF
The subjects of the laboratory tests were two transformer bushings: an RIP type, Micafil CTKF
145 kV, and an RBP type, Haefely CRPT 52 kV (Figure 4), which were retired from service after
145 kV, and an RBP type, Haefely CRPT 52 kV (Figure 4), which were retired from service after
around 10 years of operation. The heat sources were installed inside the bushings to simulate the
most unfavorable conditions during the actual operation (Figure 4c). The temperature inside the
current path and the adjacent layers was T = 130 °C, which is likely during the summer period for
Energies 2020, 13, 853 5 of 13

around 10 years of operation. The heat sources were installed inside the bushings to simulate the
most unfavorable conditions during the actual operation (Figure 4c). The temperature inside the
Energies
current 2020,and
path 13, x the
FOR adjacent layers was T = 130 ◦ C, which is likely during the summer period
PEER REVIEW 5 of 13
for
tgδ50Hz ≈ 1.5% due to the thermal model analysis [15]. The temperature was controlled by the use of a
tgδ50Hz ≈ 1.5% due to the thermal model analysis [15]. The temperature was controlled by the use of a
Czaki-type temperature controller.
Czaki-type temperature controller.

(a) (b)

(c)

Figure 4. The experimental setup for: (a) CRPT bushing; (b) CTKF bushing, (c) diagnostic scheme of
the Figure
Frequency Dielectric
4. The Spectroscopy
experimental setup for:(FDS) method.
(a) CRPT bushing; (b) CTKF bushing, (c) diagnostic scheme of
the Frequency Dielectric Spectroscopy (FDS) method.

The SVM tests were performed directly after the deinstallation of bushings and after 140, 340
and 740 h of aging. The instrument used for measurements was an MI 3200 Metrel Theraohm at a
voltage of 10 kV and measurement accuracy current of 5% ± 0.5 nA in the full measurement range.
Energies 2020, 13, 853 6 of 13

The SVM tests were performed directly after the deinstallation of bushings and after 140, 340 and
740 h of2020,
Energies aging.
13, xThe
FORinstrument
PEER REVIEW used for measurements was an MI 3200 Metrel Theraohm at a voltage 6 of 13
of 10 kV and measurement accuracy current of 5% ± 0.5 nA in the full measurement range. The voltage
The voltage
levels and thelevels and the
respective respective
time sequencestime sequences
used used in this
in this procedure are procedure
presented inare presented
Table 1. The in Table 1.
combined
Thetime
test combined
was 30test time
min, andwas 30its
after min, and afterthe
conclusion, its conclusion, thewas
Ka coefficient Ka coefficient
calculated. was calculated.

Table 1. The
Table1. The voltage
voltage levels
levels and
and time
time sequence
sequence used
used in
inthe
the Step
Step Voltage
VoltageMeasurement
Measurement(SVM)
(SVM)tests.
tests.

Voltage
Voltage Level Level Time Interval
Time Interval
2 kV 2 kV 10 min
10 min
4 kV 4 kV 5 min
5 min
6 kV 6 kV 5 min
5 min
8 kV 5 min
8 kV 5 min
10 kV 5 min
10 kV 5 min
The absorption coefficient was also appointed for the other RIP type CTF 245 kV bushings, which
The absorption coefficient was also appointed for the other RIP type CTF 245 kV bushings, which
were retired from service after the long-term operation period. In order to verify the SVM method
were retired from service after the long-term operation period. In order to verify the SVM method
results, the dielectric dissipation factor tgδ was measured in a wide frequency spectrum (FDS
results, the dielectric dissipation factor tgδ was measured in a wide frequency spectrum (FDS method).
method). The FDS method was performed using the meter Dirana, from the company Omicron, in
The FDS method was performed using the meter Dirana, from the company Omicron, in the frequency
the frequency range 10−3 to 5 kHz at a voltage of 200 V; the measurement accuracy tgδ for 1 mHz < f
range 10−3 to 5 kHz at a voltage of 200 V; the measurement accuracy tgδ for 1 mHz < f < 100 Hz was
< 100 Hz was 1% + 3 × 10−4, while for 1 mHz < f 100 Hz, this value was 2% + 5 × 10–4. In measurements,
1% + 3 × 10−4 , while for 1 mHz < f 100 Hz, this value was 2% + 5 × 10–4 . In measurements, the tree
the tree electrode method was used. An internal inspection of the inner insulation of the selected
electrode method was used. An internal inspection of the inner insulation of the selected bushing was
bushing was also performed by creating a few cross-sections of the core. Advanced insulation aging
also performed by creating a few cross-sections of the core. Advanced insulation aging in the form of
in the form of visible discoloration was observed in the layers located close to the conduction, and
visible discoloration was observed in the layers located close to the conduction, and compounds of
compounds of carbon insulation degradation were detected.
carbon insulation degradation were detected.
3. Results
3. Results
3.1. Impact
3.1. Impact of
of Thermal
Thermal Aging
Aging on
on SVM
SVM Test
TestResults
Resultsofofthe
theInsulation
InsulationofofBushings
Bushings
Figure5 5shows
Figure shows thethe effects
effects of thermal
of thermal agingaging
on theon
SVMthemeasurement
SVM measurement results
results for the RBPforinsulated
the RBP
insulated
type CRPTtype CRPT
52 kV 52 kV [16].
bushing bushing [16].
After 10After
years10ofyears of operation,
operation, the absorption
the absorption coefficient
coefficient valuevalue
was
was K a = 1.37 (Figure 5a), which suggests a good overall condition without advanced aging processes.
Ka = 1.37 (Figure 5a), which suggests a good overall condition without advanced aging processes. After
After
340 and340
740 and
h of740 h of aaging,
aging, a significant
significant increaseincrease in the current
in the leakage leakagewascurrent was observed
observed forvoltages
for the step the step
voltages
of 8 and 10 ofkV.
8 and
As10 kV. Asthe
a result, a result, the calculated
calculated absorptionabsorption
coefficientcoefficient
values werevalues
Ka =were
3.79 K a = 3.79 (Figure
(Figure 5b) and
5b) and K a = 4.9 (Figure 5c), respectively. These results may be considered to be a result of the ongoing
Ka = 4.9 (Figure 5c), respectively. These results may be considered to be a result of the ongoing aging
aging process,
process, whichtoleads
which leads to the presence
the presence of the products
of the products of the thermal
of the thermal decomposition.
decomposition.

(a)
(b)

Figure 5. Cont.
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(c)
(c)

Figure 5.
Figure The SVM
5. The SVM test
test chart
chart of
of CRPT
CRPT 52
52 kV
kV bushing
bushing insulation
insulation after
after (a)
(a) 10
10 years
years ofof operation;
operation; (b)
(b) an
an
additional 340
Figure 5.340
additional The h of
h SVM aging after
testafter
of aging deinstallation;
chartdeinstallation;
of CRPT 52 kV (c) an additional
(c)bushing 740
insulation
an additional h of
740 hafteraging after
(a) 10after
of aging yearsdeinstallation [16].
of operation;[16].
deinstallation (b) an
additional 340 h of aging after deinstallation; (c) an additional 740 h of aging after deinstallation [16].
Different results were obtained for the RIP type CTKF 145 kV bushing (Figure 6). It was observed
Different results were obtained for the RIP type CTKF 145 kV bushing (Figure 6). It was observed
that, during an results
Different additional
were thermal
obtained aging
forof ofRIP
the 740 h, the value of the leakage current
6). Itdecreased in
that, during an additional thermal aging 740 h,type CTKF
the value 145
of kVleakage
the bushing (Figure
current decreasedwas in
observed
each
each of the voltage steps. The authors believe that this is a consequence of the so-called additive
of that, during steps.
the voltage an additional thermal
The authors aging
believe of this
that 740 h,
is athe value of theofleakage
consequence currentadditive
the so-called decreased in each
process
process
of the and physical
voltage relaxation
steps. The of thebelieve
authors composite,
that which
this is has
a been validated
consequence of with
the the dielectric
so-called response
additive process
and physical relaxation of the composite, which has been validated with the dielectric response
measurements
and physicalin in the frequency
relaxation domain
of thedomain
composite,(FDS) [15].has Simultaneously, nowith
changes in the value of
measurements the frequency (FDS)which been validated
[15]. Simultaneously, no changes theindielectric
the valueresponse
of the
themeasurements
absorption coefficient
in the were
frequency noted (Figure
domain 6), suggesting no significant aging changes in of
thethe
absorption coefficient were noted (Figure 6),(FDS) [15]. Simultaneously,
suggesting no significant agingno changes
changes in the composite
in the value
composite structure.
absorption coefficient were noted (Figure 6), suggesting no significant aging changes in the composite
structure.
structure.

(a) (b)
(a) (b)
Figure 6. The SVM test chart of CTKF 145 kV bushing insulation after (a) 10 years of operation; (b) an
additional
Figure 740SVM
6. The h of test
aging after
chart ofdeinstallation
CTKF 145 kV [16].
bushing insulation after (a) 10 years of operation; (b) an
Figure 6. The SVM test chart of CTKF 145 kV
additional 740 h of aging after deinstallation bushing insulation after (a) 10 years of operation; (b) an
[16].
Considering
additional 740 h of aging after deinstallation [16]. of RIP type composites over the RBP type, which
the much higher thermal endurance
are classified respectively as class F (155 ◦ C) and E (120 ◦ C), the obtained data of coefficient K changes
Considering the much higher thermal endurance of RIP type composites over the RBP a type,
which are classified respectively as class F (155 °C) and E (120 °C), the obtained data of coefficient type,
during Considering
the aging the
process much
were higher
as thermal
expected in endurance
both cases. of RIP type composites over the RBP Ka
which
In the
changes areauthors’
classified
during respectively
opinion,
the aging even
process asthough
were class F a(155 °C)inand
bothE cases.
non-standard
as expected (120
SVM °C), the obtained
method data [8],
was used of coefficient
there is aKa
changes
In theduring
dependency authors’theopinion,
between aging process
the absorption were as aexpected
coefficient
even though Ka andin both
non-standard cases.method
the SVM
level of the thermal wear
was used [8],ofthere
RBP and
is a
RIP In the
insulation. authors’
This opinion,
dependency even
was though a
validated non-standard
in Sections SVM
3.2 and method
3.3 forwas used
selected
dependency between the absorption coefficient Ka and the level of the thermal wear of RBP and RIP [8], there
examples is a
of
dependency
RIP-insulated between
bushings the
afterabsorption
long-term coefficient
operation K
underand the level
various of the thermal
conditions.
insulation. This dependency was validated in Sections 3.2 and 3.3 for selected examples of RIP-
a wear of RBP and RIP
insulation.
insulated This dependency
bushings after long-termwas validated
operation in Sections
under various 3.2 and 3.3 for selected examples of RIP-
conditions.
insulated bushings after long-term operation under various conditions.
Energies 2020, 13, 853 8 of 13
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3.2.
3.2.SVM
SVMTest
TestofofRIP
RIPType
TypeBushings
Bushingsafter
afterLong-Term
Long-TermOperation
Operation
Figure
Figure7a7ashows
shows thethe
SVM test test
SVM results of RIP-insulated
results CTF 245
of RIP-insulated CTFkV 245
(serial
kVnumber
(serial 75D76)
numberbushing,
75D76)
which was in operation for around 30 years. Similar to the laboratory aging experiments,
bushing, which was in operation for around 30 years. Similar to the laboratory aging experiments, a a significant
non-linearity in the dielectric
significant non-linearity response
in the (leakage
dielectric current)
response was observed,
(leakage current) wasespecially for the
observed, voltagefor
especially steps
the
of 8 and 10
voltage kV.of
steps The calculated
8 and absorption
10 kV. The coefficient
calculated Ka =coefficient
absorption 8.86 gradesKathe insulation
= 8.86 grades asthebeing heavily
insulation as
aged. This diagnosis was validated with the measurements of the dielectric loss factor,
being heavily aged. This diagnosis was validated with the measurements of the dielectric loss factor, which was
measured tgδ50Hz = at
which wasatmeasured 1.68%, significantly
tgδ50Hz exceeding the
= 1.68%, significantly permissible
exceeding value for this
the permissible class
value forofthis
bushings
class of
(tgδ
bushings
50Hz ≤ 0.7%)
(tgδ [17,18].
50Hz ≤ 0.7%) Therefore,
[17,18]. the bushing
Therefore, the was withdrawn
bushing was from
withdrawnfurther
from operation.
further operation.

(a) (b)

Figure 7. The analysis of the CTF 245 kV (serial number 75D76) bushing: (a) SVM test chart;
(b) cross-section
Figure of the core
7. The analysis [16].
of the CTF 245 kV (serial number 75D76) bushing: (a) SVM test chart; (b) cross-
section of the core [16].
Figure 7b presents the cross-section of the analyzed bushing, which was performed slightly
aboveFigure
the mounting flangethe
7b presents at around one-third
cross-section of the
of the insulation
analyzed height.which
bushing, According to the insulation’s
was performed slightly
morphology, two fundamental areas can be distinguished. The first, which includes
above the mounting flange at around one-third of the insulation height. According to the insulation’s over a dozen
layers near the current rod, shows clear symptoms of thermal aging, especially
morphology, two fundamental areas can be distinguished. The first, which includes over a dozen on the boundary with
the aluminum
layers near thescreen.
current Furthermore,
rod, shows clearthe presence
symptoms of carbon compounds
of thermal was foundon
aging, especially inthe
thisboundary
part. It needs
with
to be emphasized that the performed simulations of the temperature distribution
the aluminum screen. Furthermore, the presence of carbon compounds was found in this part. It have shown that,
inneeds
this area,
to bethe distribution
emphasized thatis the
much higher than
performed in the other
simulations parts.
of the The second
temperature area, which
distribution included
have shown
the outer
that, layers
in this of the
area, the distribution
composite, showed is muchahigher
morphology
than intypical for insulation
the other parts. The in a good
second technical
area, which
condition [15].
included the outer layers of the composite, showed a morphology typical for insulation in a good
Takingcondition
technical into consideration
[15]. the distribution of the electrical field intensity in the SVM test (Figure 2),
an assumption can be made that
Taking into consideration the thedistribution
first few layers were
of the diagnosed.
electrical field Therefore,
intensity inthe
theobtained
SVM testvalue of
(Figure
the absorption coefficient
2), an assumption can be made K a = 8.86 seems reasonable.
that the first few layers were diagnosed. Therefore, the obtained value
Onabsorption
of the the other hand, Figure
coefficient Ka8=presents
8.86 seems thereasonable.
SVM test results of Micafil type CTF 245/630 bushings,
whichOn were deinstalled after 25 years of
the other hand, Figure 8 presents the SVM service. Thetest
absorption
results ofcoefficient
Micafil typeKaCTF
of the bushings
245/630 with
bushings,
serial
whichnumbers 73D56979after
were deinstalled and 73D56981
25 years ofwere, respectively,
service. 4.17 and
The absorption 6.8, which
coefficient Kainofline
the with standard
bushings with
practice marks them
serial numbers as excessively
73D56979 and 73D56981 aged. were,
In contrast, the serial
respectively, number
4.17 73D56980
and 6.8, which inbushing
line with a = 2.34)
(Kstandard
shows
practice symptoms
marks them of the early process
as excessively of insulation
aged. decomposition.
In contrast, These
the serial number conclusions
73D56980 were(Kverified
bushing a = 2.34)
with
shows thesymptoms
Dielectric Frequency
of the earlyResponse
process of tests (FDS), which
insulation will be discussed
decomposition. later in this were
These conclusions paper.verified
with the Dielectric Frequency Response tests (FDS), which will be discussed later in this paper.
Energies 2020, 13, x FOR PEER REVIEW 9 of 13
Energies 2020, 13, 853 9 of 13
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(a) (b)
(a) (b)

(c)
(c)

Figure
Figure 8. 8.
TheTheSVMSVMtesttest results
results of of
CTFCTF 245
245 kVkV bushings
bushings after
after 2525 years
years ofof operation:
operation: (a)(a) serial
serial number
number
73D56979;
73D56979; 8.(b)
Figure(b) Theserial
SVM
serial number
number 73D56980;
test results (c)(c)
of CTF
73D56980; 245serial number
kVnumber
serial bushings 73D56981
after
73D56981 [16].
25 years
[16]. of operation: (a) serial number
73D56979; (b) serial number 73D56980; (c) serial number 73D56981 [16].
Figure
Figure 9 presents
9 presents thethe changes
changes in the
in the coefficient
coefficient Ka values
Ka values of the of selected
the selected bushings
bushings depending
depending on
on the aging
Figure time.
9 The
presents assumption
the changes was
in the made
coefficient that Kin case
a values
the aging time. The assumption was made that in case of bushings which were in service for ofof bushings
the selected which were
bushings in service
depending for
on
20–30
the
20–30 aging
years, time.
there The
were assumption
usually 12 was
days made
annually that in
with case
an of bushings
ambient which
temperature were
T ≈in
28–35 ◦ C, for
service 20–30
which led
years, there were usually 12 days annually with an ambient temperature T ≈ 28–35 °C, which led to a
to ayears, there ◦ C.
temperature risewere
temperature ofrise
theusually
ofporcelain12 days
the porcelaincoverannually
cover
to 50–60to with °C.an
50–60 ambient temperature
Considering
Considering the the T ≈model
thermal
thermal 28–35
model°C, which
of
of the the led to a
bushings,
bushings,
thistemperature
indicated rise for
that of the
tgδ porcelain
≈ cover to 50–60 °C. Considering the thermal model of the bushings,
this indicated that for tgδ50Hz 50Hz≈ 0.006–0.008, the insulation surrounding the current rod exceeded a a
0.006–0.008, the insulation surrounding the current rod exceeded
this indicated
temperature of that◦ C
100 for[14,18].
tgδ50Hz ≈These
0.006–0.008,
conditions the insulation
may result surrounding the current rod exceededand a
temperature of 100 °C [14,18]. These conditions may result inin accelerated
accelerated paperdegradation
paper degradation and
sometemperature
other types of 100 °C [14,18]. These conditions may result in accelerated paper degradation and
some other types ofof resins
resins (e.g.,
(e.g., epoxy
epoxy oror phenol-formaldehyde)
phenol-formaldehyde) within
within RBP RBPandand RIPRIP composites.
composites.
some other the
Additionally, types of resins (e.g.,
assumption was epoxy
made or
thatphenol-formaldehyde)
the duration of such within RBP and
insulation duringRIP these
composites.
days
Additionally, the assumption was made that the duration of such insulation during these days is is
Additionally,
around 4–5 the assumption was made that the duration of such insulation during these days is
around 4–5 h.h. Assuming
Assuming these
these conditions,
conditions, thethe timespan
timespan has has been
been evaluated
evaluated inin which
which thethe accelerated
accelerated
around
processesare 4–5 h.
arelikely Assuming
likelyto to occur.
occur. Inthese
In the conditions, the timespan has been evaluated in which the accelerated
processes the case
caseof of2525yearsyearsofof service
service time,
time,thethe
insulation
insulationlastslasts
circacirca
1200–1500
1200– h,
processes
whereas for areyears
20 likelyof tooperation,
occur. In the caseisofa 25
there years ofofservice
duration circa time, the insulation
1440–1800 h [19,20]. lasts
Figure circa
9 1200–
reveals
1500 1500
h, whereas
h, whereas for 20foryears
20 years of operation,
of operation, therethereis aisduration
a duration of of
circa
circa1440–1800
1440–1800h h[19,20].
[19,20].Figure
Figure99
that the
reveals experiment
that the the performed
experiment in the operational
performed in inthethe conditionsconditions
operational confirmed the laboratory observations.
reveals that experiment performed operational conditionsconfirmed confirmedthe thelaboratory
laboratory
The insulation
observations. subjected
The Theinsulation to long-term
subjected aging
to long-term factors, regardless
aging factors, of the bushing
regardless and composite type,
observations. insulation subjected to long-term aging factors, regardlessofofthe thebushing
bushingand and
is generally
composite distinguished
type,type,
is generally by higher
distinguished K coefficient values.
a by higher Ka coefficient values.
composite is generally distinguished by higher Ka coefficient values.

Figure
Figure 9. 9.
The9. impact
TheThe impact
of of
impact of
the the aging
aging
the time
aging time
time onthe
onon
the the absorption
absorption coefficient
coefficient
absorption KaK
coefficient Kvalue
a value calculated for different
calculated forfor
different
Figure a value calculated different
RBP
RBPRBP
andandand RIP-insulated
RIP-insulated
RIP-insulated bushings
bushings
bushings[16].[16].
[16].
 Energies 2020, 13, 853 10 of 13
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Energies 2020,
2020, 13,13, x FOR
x FOR PEER
PEER REVIEW
REVIEW 10 13
10 of of 13

Since
Since temperature
temperatureis aisacrucial
Sincetemperature acrucial factor
crucial influencing
factor
factor the dielectric
influencing
influencing response
thethedielectric
dielectric characteristic
response
response of insulation,
characteristic
characteristic of of
it needs
insulation,
insulation, to be
it needs taken
it needs into
to to
bebe consideration
taken
taken intointo during
consideration
consideration analysis.
during
during This issue
analysis.
analysis. also
This Thisconcerns
issue
issue also the
also SVM
concerns
concerns tests,
thetheas
SVMSVMthe
temperature
tests,
tests, as as
thethe of the bushings
temperature
temperature of of may
the the influence
bushings
bushings the
maymaycalculated
influence
influence value
thethe of the absorption
calculated
calculated value
value ofcoefficient
of
thethe Ka . This
absorption
absorption
coefficient Ka. This especially applies when measurements were taken in conditions of a within
especially
coefficient applies
K a. when
This measurements
especially applies were
whentaken in conditions
measurements of
werea big temperature
taken in difference
conditions of abigbig
the bushings’
temperature
temperature core.
difference
difference For this
within
within reason,
thethe an
bushings’ experiment
bushings’ core.
core. ForFor was
this
this conducted
reason,
reason, anan to determine
experiment
experiment was wasthe dependence
conducted
conducted to to
between
determine
determine temperature
the the dependence
dependence andbetween
Kbetween
a coefficient values.and
temperature
temperature Measurements
and
KaKcoefficient
a coefficient were taken
values.
values. for the temperature
Measurements
Measurements were
were range
taken
taken
◦ C. Figure 10 presents the K values as a function of temperature for the PTK 52 kV bushing.
forof
for15–110
thethe temperature
temperature range
range of of 15–110
15–110 °C.°C. Figure
aFigure 1010 presents
presents thethe KaKvalues
a values as as a function
a function of of temperature
temperature
forThe
for absorption
thethe
PTKPTK 5252 kVkV coefficient
bushing.
bushing. ranged
The The in value
absorption
absorption from 0.7 ranged
coefficient
coefficient toranged
0.98,in which
in value
value means
fromfrom that,
0.70.7 for0.98,
to to
0.98, a which
temperature
which means
means of
∆T
that,
that, =for
for 95 a◦ C, the relative
temperature
a temperature ∆Tchanges
of of ∆T
= 95 = 95 of the
°C,°C,
the Krelative
amounted
a relative to around
changes
changes of of
KaK 40%. Considering
a amounted
amounted to to around
around that
40%. 40%. the impact of
Considering
Considering
the
that
that temperature
the the impact
impact onthe
of of
the the dissipation
temperature
temperature onfactor
on
thethe tgδ 50Hz is much
dissipation
dissipation factor
factor higher,
tgδtgδ
50Hz the
is is
50Hz absorption
muchmuch higher,
higher, coefficient
thethe Ka as a
absorption
absorption
descriptor
coefficient
coefficient KaofKas a the
as acondition
descriptor
a descriptor ofof
of insulation
thethecondition is much
condition of of more temperature-independent
insulation
insulation is is much
much more
more compared to the
temperature-independent
temperature-independent
dielectric
comparedto
compared loss
tothefactor. Furthermore,
thedielectric
dielectric loss Ka changes
lossfactor.
factor. in a wide
Furthermore,
Furthermore, atemperature
KaKchangeschangesin ina range,
awide classifying
widetemperature
temperature it within
range,
range, a
certain
classifying range
classifying it withinof
it withincriteria values
a certain
a certain range for
range insulation
of of criteria
criteria assessment.
values
values forfor insulation
insulation assessment.
assessment.

Figure
Figure
Figure 10.
10.10.
TheThe
The effect
effect of
of of
effect temperature
temperature on
onon
temperature KaKKa values
values for
forfor
a values PTKPTK 52
52 52
PTK kV
kVkV bushing
bushing [16].
[16].
bushing [16].

3.3.
3.3.
3.3. FDSFDS
FDS Analysis
Analysis
Analysis of Bushings
of Bushings
of Bushings Insulation
Insulation
Insulation
The
The spectroscopic
spectroscopic measurements
measurements of of
thethe dielectric
dielectric response
response of of
thethe insulation
insulation CTF
CTF 245
245 kVkV (serial
(serial
(serial
number
number 75D76)
75D76) bushing
bushing is is presented
inin
presented
presented in Figure
Figure
Figure 11.11.
11.

(a)(a) (b)(b)

Figure 11. The dependencies measured in the CTF 245 kV bushing: (a) tgδ = f(f); (b) ε” = f(ε’).
Figure
Figure 11.11.
TheThe dependencies
dependencies measured
measured in in
thethe
CTFCTF
245245
kVkV bushing:
bushing: (a)(a)
tgδtgδ = f(f);
= f(f); (b)(b)
ε’’ ε’’ = f(ε’).
= f(ε’).
Primarily, a clear relaxation process was observed which occurred within a frequency range
of Primarily,
10–4000 Hza (Figure
Primarily, a clear
clear 11a). Based
relaxation
relaxation on the
process
process was dielectric
was observed
observed response
which
which representation
occurred
occurred aon
within
within a the Cole–Cole
frequency
frequency range
range plot
of of
(Figure
10–4000 11b),
Hz the
(Figurevalue of
11a). the
Basedresonant
on the frequency
dielectric was estimated
response to be
representation
10–4000 Hz (Figure 11a). Based on the dielectric response representation on the Cole–Cole plot in the
on range
the of 90–110
Cole–Cole Hz
plot
which
(Figure
(Figure corresponds
11b),
11b), thethevalue to
valueofthe
of relaxation
thethe resonant
resonant time
frequency
frequency ≈ was
of τ was
1.4–1.8 × 10–3
estimated
estimated tos.
beThe
to be
in in presence
thethe ofofof
range
range this process
90–110
90–110 HzHz is the
which
which
reason for to
corresponds
corresponds theto excessive
thethe
relaxationincrease
relaxation time ofinof
time τthe ≈value
≈τ 1.4–1.8∙10of–3tgδ
1.4–1.8∙10 –3 which
s. The
s. 50Hz
The leads
presence
presence of oftothis
this aprocess
withdrawal
process from
is the
is the further
reason
reason forfor
operation.
the excessiveIn terms
increaseof medium
in the (MF)
value of and
tgδ low (LF) frequencies, the changes of
the excessive increase in the value of tgδ50Hz which leads to a withdrawal from further operation. In In
50Hz which leads to a withdrawal from tgδ
furtherand the capacity
operation.
terms
terms of of medium
medium (MF)
(MF) andand low
low (LF)
(LF) frequencies,
frequencies, thethe changes
changes of of
tgδtgδ and
and thethe capacity
capacity values
values were
were
typicalforforthermosetting
typical thermosettingcomposites
compositesat atananearly
earlystage
stageof ofthethethermal
thermaldegradation,
degradation,which
whichis is
 Energies 2020, 13, 853 11 of 13
Energies 2020, 13, x FOR PEER REVIEW 11 of 13

values were typical for thermosetting composites at an early stage of the thermal degradation, which is
characterized by the presence of relaxation with a large time constant and small capacity increases
characterized by the presence of relaxation with a large time constant and small capacity increases [21].
[21].
The frequency characteristics of the dissipation factor tgδ of the selected CTF 245 kV bushings
The frequency characteristics of the dissipation factor tgδ of the selected CTF 245 kV bushings
are presented in Figure 12. In all cases, the shape of the response characteristics was typical for
are presented in Figure 12. In all cases, the shape of the response characteristics was typical for
insulation in the advanced degradation process. Additional relaxation processes were observed both
insulation in the advanced degradation process. Additional relaxation processes were observed both
in high (HF) and low (LF) frequencies, which suggest the presence of the products of the composite
in high (HF) and low (LF) frequencies, which suggest the presence of the products of the composite
decomposition or the potential delamination of the layers. The relaxation in the high-frequency range
decomposition or the potential delamination of the layers. The relaxation in the high-frequency range
in all bushings shows that a thermal decomposition of resin or paper had already begun on the
in all bushings shows that a thermal decomposition of resin or paper had already begun on the
screen–insulation boundary.
screen–insulation boundary.

Figure
Figure12.12.
Frequency characteristics
Frequency of the
characteristics tgδ coefficient
of the of CTF
tgδ coefficient 245 kV
of CTF 245bushings after after
kV bushings 25 years of
25 years
service.
of service.

TheThecharacteristics
characteristics of of
thethe
73D56980
73D56980 bushing
bushing attract attention
attract attentionas as
they
theyshow
showa fairly acceptable
a fairly acceptable
absorption coefficientKKaa == 2.34
absorptioncoefficient withno
2.34 with noclear
clear relaxation
relaxation process
process in MF
in the the range,
MF range,
whichwhich
occurs occurs
typically
typically for advanced
for advanced insulation
insulation degradation
degradation and theand the presence
presence of Maxwell–Wagner
of Maxwell–Wagner charge. charge.
In In
thethe
case of of
case thethe
bushing
bushing with serial
with number
serial number73D56981,
73D56981,with a very
with highhigh
a very coefficient a = 6.83,
Ka =K6.83,
coefficient it
was observed
it was thatthat
observed twotworelaxation processes
relaxation in the
processes LF range
in the LF rangewere present,
were present,andand
thethe
insulation
insulationcapacity
capacity
increased
increased bybyalmost
almost twotwo orders
ordersofofmagnitude.
magnitude.These Thesedata prove
data provethethepoor
poorcondition
condition of of
thethecore
core
insulation,
insulation, which
whichrequires
requires immediate
immediate withdrawal
withdrawal from
fromoperation.
operation.

4. 4. Conclusions
Conclusions
In In bushings
bushings with
with solid
solid insulation,
insulation, thermal
thermal aging
aging in in operating
operating conditions
conditions mainly
mainly causes
causes thethe
decomposition of the insulation layers close to the current rod. A
decomposition of the insulation layers close to the current rod. A similar morphology of layered similar morphology of layered
insulation
insulation in in high-voltage
high-voltage electrical
electrical machines
machines andand high-voltage
high-voltage bushings
bushings makes
makes thethe
SVM SVM method
method
suitable
suitable forfor diagnostics
diagnostics in in
RIP RIPandandRBP RBP bushings.
bushings. This
This procedure
procedure cancan
bebe used
used forfor both
both RIP RIP and
and RBP
RBP
insulation assessment due to the specific static electrical field distribution in the layered insulation of of
insulation assessment due to the specific static electrical field distribution in the layered insulation
thethe bushings’
bushings’ cores.The
cores. Theelectrical
electricalstresses
stresseswhich
whichoccurred
occurred during
during the SVM SVM test
test in
inthe
thefirst
firstlayers
layersofofthe
insulation are comparable to the stresses occurring in high-voltage electrical
the insulation are comparable to the stresses occurring in high-voltage electrical machine insulation. machine insulation.
TheThe experimentshave
experiments have shown that thatthe
thedevelopment
development of the
of aging processes
the aging in RIPin
processes andRIPRBPandinsulation
RBP
leads to the
insulation leadsincrease
to theofincrease
the absorption coefficient calculated
of the absorption coefficient from the SVM
calculated from test.
theThe
SVM assessment
test. The of
insulation of
assessment based on Ka values
insulation basedwas on positively
Ka values verified with measurements
was positively verified with of the dissipation factor
measurements of thetgδ
and the capacity
dissipation factor tgδin aand
widethespectrum
capacity of in frequency (FDS method).
a wide spectrum of frequencyFurthermore, the great
(FDS method). advantage of
Furthermore,
thethe SVM
great test is theofpractically
advantage the SVM test stable and
is the temperature-independent
practically coefficient Ka value.coefficient
stable and temperature-independent As a result,
Ka value. As a result, insulation evaluation is somewhat dependent on temperature values within
insulation evaluation is somewhat dependent on temperature values and the distribution and thethe
core, which
distribution is generally
within the core, hard to define.
which is generally hard to define.
The measurements carried out—especially
The measurements carried out—especially using usingthethe
SVMSVM method—determine
method—determine thethe condition
condition of of
aging of the crucial area of the bushing insulation located
aging of the crucial area of the bushing insulation located close to the current close to the current channel; i.e., the place
place in
in which
which the
the processes
processesof ofdamage
damageand andelectrical
electricalbreakdown
breakdownare areinitiated
initiatedmost
mostoften.
often.
Author Contributions: All authors have read and agreed to the published version of the manuscript.
Conceptualization, J.S and A.M.; methodology A.M. and M.Z.; resources, J.S.; investigation, A.M., P.B. and M.Z.;
Energies 2020, 13, 853 12 of 13

Author Contributions: Conceptualization, J.S and A.M.; methodology A.M. and M.Z.; resources, J.S.; investigation,
A.M., P.B. and M.Z.; supervision, J.S.; visualization, A.M., P.B. and M.Z.; writing—original draft preparation,
J.S., A.M., P.B. and M.Z.; writing—review and editing, A.M. and P.B. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

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