Change of Dielectric Parameters of XLPE Cable due to
Thermal Aging
Martin GERMAN-SOBEK, Roman CIMBALA, Jozef KIRÁLY∗
Abstract
The change of dielectric parameters points to the aging or defect of HV equipment insulation. Cross-linked
polyethylene (XLPE) is the globally preferred modern insulation for power cables, both for distribution and
transmission system applications. Many studies and experiments show an influence of aging to degradation of
XLPE insulation. It is subsequently reflected by changing of dielectric parameters of insulation. The
measurement of dielectric parameters of XLPE cable sample was carried out by the method of dielectric
relaxation spectroscopy (DRS) in frequency domain. DRS represents method suitable for measuring the
dielectric parameters of polymeric composites during aging. The results are compared and it was observed a
change of parameters in consequence of additional aging.
Keywords: insulation, XLPE cable, aging of XLPE, capacitance, dissipation factor, thermal aging, dielectric
relaxation spectroscopy
1. Introduction
The insulation system is an important
and sensitive part of any high-voltage
electric equipment. Power cables are very
important and sensitive devices in the
power system, and they play an important
role in the safety of the power load and
reliable transmission of electricity.
Nowadays, the majority of power cables
are insulated with polymeric materials.
Cross-linked polyethylene (XLPE), as the
main polymeric insulation, is widely used as
electric insulation material for high-voltage
distribution power cables. This insulation
material provides excellent physical,
chemical and electric properties.
During operation, the power cables may
be exposed to high currents and voltages
and they are critical parts of the
transmission infrastructure. Therefore, is
∗
Martin, GERMAN-SOBEK: Eng., Technical University of
Košice, Faculty of Electrical Engineering and Informatics,
Department of Electric Power Engineering, Mäsiarska 74,
04120 Košice, Slovak Republic; martin.germansobek@tuke.sk
Roman CIMBALA: Univ.Prof., Technical University of Košice,
Faculty of Electrical Engineering and Informatics,
Department of Electric Power Engineering, Mäsiarska 74,
04120 Košice, Slovak Republic; roman.cimbala@tuke.sk
Jozef KIRÁLY: Eng., Technical University of Košice, Faculty
of Electrical Engineering and Informatics, Department of
Electric Power Engineering, Mäsiarska 74, 04120 Košice,
Slovak Republic; jozef.kiraly@tuke.sk
expected their high resistance against
possible failures [1]. Damage of insulation
can lead to equipment failure and other
disorders.
The insulation degradation is inevitable
during the operation and the failure rate of
XLPE cable increases with the service time
[2]. The cables are permanently exposed to
thermal ageing during operation. It may
cause change in dielectric parameters of
cables and also irreversible damage of cable
insulation. The primary initiators for the
degradation of cable insulation are high
currents, voltage, mechanical, chemical
and thermal stress and pollution of
environment. Interaction various factors
together may significantly speed up the
degradation processes. Process of aging of
insulation is the most acting on the
parameters and quality of insulation and it
is a phenomenon that is essentially cannot
be affected.
In most cases, failures in the insulation
are related to integral degradation of the
insulation like water treeing in XLPE cables.
It is also known, that the insulation failures
may be caused by lower dielectric strength
due to aging processes or by internal
defects in the insulation. [1]
It is necessary to assess degradation and
insulating state of cables.
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ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ,vol. 62 (2014), Nr. 3
2. About aging of XLPE insulation
Polyethylene (PE) is a long chain semi
crystalline polymer manufactured through
the polymerization of ethylene gas. PE was
very popular, compared to paper insulation,
as insulation for cables because of its low
cost, good electric properties (low
dielectric constant, low dielectric loss, and
high breakdown strength), processability,
mechanical toughness and flexibility, good
resistance to chemicals and moisture. [3]
XLPE is a material produced by the
compounding of low density polyethylene
(LDPE) with a crosslinking agent such as
dicumyl peroxide. XLPE has good dielectric
properties for high voltage applications.
However, aging of XLPE material cannot
avoidable after long time in operation
under various stress conditions. XLPE
insulated
cables
for
high
voltage
applications have been studied and
investigated in order to evaluate a function
of service stresses and aging time. Many
researchers are studied improved XLPE
properties in order to improve dielectric
performance of XLPE material. [4, 5]
During the production of XLPE, this is
molten and cooled serveral times. The
material properties are influenced by the
manufacturing process, heating and cooling
temperatures and time. During the cable
production the material is also heated and
cooled several times during the different
steps. The cooling process influences
significantly the morphology and thus the
electric properties of XLPE cable. The
electric breakdown strength is depending
on the density and it is also depending on
the increase of the amorphous structures
inside the polyethylene. [6]
XLPE is composed of crystalline phase
and amorphous phase. Defects such as
submicrovoids and microvoids in XLPE may
be formed and developed at the interface
of crystalline and amorphous area, which
can be regarded as weak point of
insulation. [7].
Defects in XLPE may be introduced in the
process of transporting or laying, although
cable manufacturing is improved. In
practical service condition, electric field
deformations around these defects are very
sharp and exert negative effects on the
XLPE insulation breakdown. The breakdown
of XLPE is closely related to electric trees
and partial discharges and it was found that
the breakdown voltage decreased with an
increase in frequency. [8]
In general, during the operation an
insulation system is subjected to one or
more stress that causes irreversible changes
of insulating material properties with time.
This process is called aging and ends when
the insulation is no more able to withstand
the applied stress. The relevant time is the
time-to-failure
or
time-to-breakdown,
alternatively called insulation life time. In
the case of electric insulation, the stresses
most commonly applied in operation are
electric field (due to voltage) and
temperature (due to loss), but also other
stresses, such as mechanical stresses
(vibration) and environmental stresses
(pollution, humidity) can be present. [5]
Stability
of
microstructure
and
composition of the insulating material is
changing due to degradation processes.
These changes result to changing behaviour
of insulation material from view of
polarization processes. Aging in a polymer
changes the electric, physical, mechanical
and morphological properties of the
insulation [9]. All these properties are
influencing the dielectric parameters and
characteristics of insulation [10]. During
thermal aging, several structural changes
occur such as variation in crystalline, chain
scission and variation in heat of fusion and
in melting point. [7, 11]
Integral ageing of XLPE cables changes
the morphological properties of the
insulation. It is well known that the aged
XLPE
cable
insulations
have
many
microvoids whose number increases with
the distance from the cable conductor.
Their dimensions and number depend on
the technology and the kind of cable
insulation. It is generally assumed that
during production, microvoids, impurities,
water
and
residual
products
from
crosslinking will be collected in amorphous
regions of the insulation. Increasing of aging
temperature, the microvoids are of larger
size. [12]
XLPE insulation is a low-mobility material
in which the mean free path length is quite
small. However, the injected high energy
electrons
may
easily
cause
bond
dissociation in insulation. With the
development of bond dissociation, some low
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), Nr. 3
density regions, low cohesive energy
density regions and thus free volume can be
generated. In these regions, impact
ionization is more likely, and thus leading
to avalanches, partial discharge and
eventually breakdown of the XLPE
insulation. Such a process can be
accelerated when there are weak points on
the interface of void and insulation. [8]
Aging of XLPE cables is related to the
temperature of the insulation. All XLPE
cables contain antioxidants which protect
the XLPE from oxidation during the
extrusion and cross-linking process, and also
during the service life of the cable. The
rate at which the antioxidant is used up is
dependent on temperature. The normal
maximum operating temperature of XLPE
cables is 90 °C. Tests have shown that XLPE
cables can operate at a temperature of
105 °C for a limited time without
significantly reducing the service life of
cables. At temperatures in excess of 105°C
deformation of XLPE readily occurs,
particularly at positions where the
insulation is under mechanical stress. The
maximum overload temperature of XLPE is
limited to 105 °C. [3, 13]
The rate of consumption of anti-oxidant
has been calculated to provide a cable life
of a minimum of 30 years at normal
maximum
operating
temperature.
Increasing the operating temperature of the
cables will increase the rate at which the
anti-oxidant is used up and hence reduce
the service life. Small increase in
temperature has a significant impact on the
ageing of the XLPE. The XLPE will start to
oxidize and become brittle, once the antioxidant in the cable is used up. This then
leads to the cable will be subject to stress
cracking and electric failure at positions of
mechanical stress. [13]
Quantitative assessment of ageing or
damage of the XLPE insulations can be
separated assessment of the relaxation
mechanisms. XLPE cable ageing has been
studied for nearly 40 years and many
methods have been proposed to evaluate
the properties of XLPE. [7].
3. Measurement Method
One of the methods for detection of
insulating state of XLPE insulation is
measurements of dielectric parameters of
49
insulation using the dielectric relaxation
spectroscopy (DRS). DRS is a one of the nondestructive measurement methods and uses
polarization as the response of the sample
on a time-dependent electric field. For
electrolyte solutions, polarization essentially originates from the oriented
fluctuations of permanent dipoles (solvent
molecules, ion pairs), from intramolecular
polarizability and from ion motion. It can be
investigated either in the time domain or as
a function of the frequency of a harmonic
field. Thus, principle of this method is
based on examination of molecular
dynamics of polarized and polar materials.
Generally speaking, the behavior of the
dielectric material is characterized DC
conductivity, dielectric response function
and high-frequency component of the
relative permittivity. DRS is widely applied
in the characterization of ion-conducting
solids and polymers. [14, 15]
In
the
frequency
domain,
the
polarization
phenomena
follow
the
alternating electric field. DRS is focused on
the measurement of the frequency
dependence of real and imaginary
components of the impedance of the
investigated samples. DRS evaluates the
dielectric response function in the
frequency domain using the dielectric
dissipation loss factor tan δ and complex
capacitance C(ω). Using DRS in frequency
domain can be evaluated the effect of
temperature and thermal aging on
dielectric parameters and change of
properties
of
investigated
insulation
material after aging and degradation
processes. [15]
The total current flowing across the
material when exposed to voltage U(ω) can
be expressed as [15]:

 σ

I (ω) = iωC0 ε ∞ + χ′(ω) − i 0 + χ′′(ω) U (ω)
ε
ω
 0
 

(1)
Complex electric induction D(ω) is
proportional to the complex dielectric
permittivity ε(ω) according to the relation
[15]:
D (ω ) = ε 0 ε (ω )E (ω ) = ε 0 [1 + χ′(ω ) − iχ′′(ω )]E (ω )
(2)
where
ε(ω) = ε′(ω) − iε′′(ω) = (1 + χ′(ω)) − iχ′′(ω)
(3)
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ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ,vol. 62 (2014), Nr. 3
Actual measurements of this dielectric
response in the frequency domain are
difficult to perform, if the frequency range
becomes very large. The measured relative
dielectric permittivity is defined from the
following relation [15]:
j (ω ) = iωε 0 ~ε r (ω )E (ω )
(4)
Therefore
~ε (ω ) = 1 + χ ′(ω ) − i  χ ′′(ω ) + σ 0 
r


ε 0ω 

(5)
The dielectric dissipation factor:
ε ′r′ (ω ) +
tan δ (ω ) =
σ0
ε 0ω
ε ′r (ω )
(6)
The real part of (5) represents the
capacitance of a test object, whereas the
imaginary part represents the losses. Both
quantities depend on frequency. It should
be noted that all dielectric quantities are
more or less dependent on temperature and
dissipation
factor
tanδ
is
strongly
dependent on temperature. Any comparison
or measurement of these quantities must
take this into account. Increased interfacial
polarization also produces the increase in
dissipation factor, mainly in the low and
very low frequency range. The value of
dissipation factor is related to the dielectric
loss and its value can be regarded as a
measure of the quality of the insulation
system. Measurements in the frequency
domain need voltage sources of variable
frequencies and, for applications related to
HV power equipment, output voltages up to
at least some hundreds of volts. Such
measurements become quite lengthy if very
low frequencies are considered. [15, 16]
Figure 1. Frequency dependence of capacitance
before and after aging process
4. Experimental measurement
The experimental measurement was
performed on two cable sample by the DRS
method in frequency domain:
• sample A - XLPE cable of operationally
unknown technical condition,
• sample B – operationally unaged,
undamaged XLPE cable.
The cable samples were a power cables
with an aluminium core and XLPE
insulation. Sample A of cable was
approximately 25 cm long and protective
cable jacket and semiconducting layer have
been removed (2 cm at both ends) and
shielding taken out. Sample B of cable was
approximately 1 m long and protective
cable jacket and semiconducting layer have
been removed (10 cm at both ends) and
shielding taken out.
On sample A were recorded changes of
capacitance and dielectric dissipation loss
factor depending on frequency and the
measurement was repeated after an
accelerated thermal aging of sample at
90 °C for 360 hours in an air oven. On
sample B were recorded changes of
capacitance and dielectric dissipation loss
factor depending on frequency at various
temperatures. The frequency range of
measurements was 20 Hz to 2 MHz and the
measurements were performed using the
precision LCR meter Agilent E4980A. The
increase of frequency was decimal. Aim
measuring has been comparison of
measured frequency dependencies of
capacitance and dissipation factor.
In Figure 1 and Figure 2 are shown
comparison of frequency dependencies of
capacitance and dissipation factor before
and after aging process, respectively.
Figure 2. Frequency dependence of dissipation
factor before and after aging process
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 62 (2014), Nr. 3
Due to the large fluctuations in the
measured values at low frequencies from 20
Hz to 100 Hz, the measured values are not
within the zone of termination capable and
therefore are not shown. Figure 1 shows a
decrease of the values of capacitance after
aging process. The change of capacitance is
very small, order of 10-11 Farad. From
frequency circa 200 kHz occurs a noticeable
decrease of capacitance in measurement
after aging process. Such measurement is
sensitive to disturbances and parasitic
capacitance. Figure 2 shows a change of the
values of dissipation factor after aging
Figure 3. Frequency dependence of dissipation
factor at various temperature
The measured values at low frequencies
from 20 Hz to 100 Hz are not shown due to
the large fluctuations. Figure 3 shows a
change of the values of dissipation factor at
higher temperatures. A significant change
of dissipation factor is at high frequency
from circa 700 kHz where there was an
increase of measured values. In frequency
range between 100 Hz and 20 kHz were
measured smaller values of dissipation
factor at higher temperatures compared to
measurement at 20 °C. Thus, similar to the
measurement of capacity, there is also here
a change of values of dissipation factor with
a change of temperature. The trend of
frequency dependencies was similar. Figure
4 shows a decrease of the values of
capacitance at higher temperatures. The
change of capacitance is very small, order
of 10-10 farad. With increasing temperature
there is also a significant and steeper
increase of capacitance from frequency
circa 400 kHz. At a temperature 20 °C,
there is a slight increase of capacitance
from circa 1 MHz. Thus, with a change of
51
process. The change of dissipation factor is
significant at high frequency from 10 kHz
where there was a significant and steeper
increase of measured values. The values of
dissipation factor are greater after aging
process. The accelerated thermal aging
process caused a change of frequency
dependence of dissipation factor.
In Figure 3 and Figure 4 are shown
comparison of frequency dependencies of
dissipation factor and capacitance at
temperature 20 °C, 60 °C, 70 °C and 80 °C,
respectively.
Figure 4. Frequency dependence of capacitance at
various temperature
temperature occurs a change of values of
capacitance and also a change the curve of
the frequency dependencies, but the trend
of dependencies is similar.
5. Conclusions
It is well known that temperature and
thermal aging of insulation influences on
the parameters and quality of insulation.
Using DRS in frequency domain can be
evaluated the effect of temperature and
thermal aging on dielectric parameters and
change of properties of investigated
insulation material after aging and
degradation processes.
Aim of this paper was to observe a
change of dielectric parameters of XLPE
cable due to thermal aging. The comparison
and evaluation of measured frequency
dependencies
of
capacitance
and
dissipation factor of two samples of XLPE
cable show that temperature and aging
process change dielectric properties of
samples of XLPE cable. Such changes can be
related to structural changes in the
52
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ,vol. 62 (2014), Nr. 3
polyethylene morphology due to thermal
aging. It is possible to confirm that
degradation led to changes in the
polarization
processes
occurring
in
insulation. For an overall assessment of the
effect of aging is needed to know changes
of measured values and their development
of previous measurements.
The
measurement
confirmed
the
influence of thermal ageing to dielectric
parameters of XLPE insulation. The
measurement confirmed the suitability of
DRS for investigate cable insulation system.
It is not possible exclude the surrounding of
a disturbance during the measurement,
which could affect the measured results. It
is important to continue to investigate the
process of ageing and their impact on the
insulation system with use modern and
suitable diagnostic methods and measuring
equipment.
6. Acknowledgment
We support research activities in Slovakia
/Project is cofinanced from EU funds. This
paper was developed with support of operating
program Research and development for the
project: "Univerzitný vedecký park Technicom
pre inovačné aplikácie s podporou znalostných
technológií" (University Science Park Technicom
for innovative applications with support of
knowledge
technologies),
code
ITMS:
26220220182, co-financed from European funds.
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8. Biography
Martin GERMAN-SOBEK was born
in Košice (Slovakia), on July 26,
1987.
He graduated the Technical
University of Košice, Faculty of
Electrical Engineering and
Informatics in Košice (Slovakia), in 2011.
53
He is PhD student at the Technical University of
Košice, Faculty of Electrical Engineering and
Informatics in Košice (Slovakia).
His research interests concern: diagnostic of
high-voltage solid insulation system, highvoltage equipment, power cables.
Roman CIMBALA was born in
Košice (Slovakia), on April 28th,
1962.
He graduated the Technical
University of Košice, Faculty of
Electrical Engineering and
Informatics in Košice (Slovakia), in 1989.
He received the PhD degree in electrical
engineering from the Slovak University of
Technology in Bratislava (Slovakia), in 1994.
He is Professor at the Technical University of
Košice (Slovakia).
His research interests concern: dielectric
spectroscopy, diagnostics of HV systems.
Jozef KIRÁLY was born in Košice
(Slovakia), on May 3th, 1987.
He graduated the Technical
University of Košice, Faculty of
Electrical
Engineering
and
Informatics in Košice (Slovakia),
in 2011.
He is PhD. student at the Technical University
of Košice (Slovakia).
His research interests concern: dielectric
spectroscopy, diagnostics of HV systems,
ferrofluids.