Highlights: * Describes the type and composition of medium voltage electrical power cables. * Discusses various effects and processes that can cause ageing. * Agrees that assessment of cable condition is a major challenge. * Analyses the phenomenon of water tree ageing.

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Briefing Paper
Gérard Platbrood, Blandine Hennuy - Laborelec
Available online March 2009
Ageing of Medium Voltage Cables

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1. Introduction
Cables are important components in the transportation and distribution of
electricity. Due to the increased need for power, but also because
overhead power lines are often replaced by (underground) power cables
in densely populated areas, the importance of power cables has grown
over the years.
Successful asset management, in particular determining the residual
lifetime of installed cables, relies heavily on the use of information coming
from a variety of sources including the operating managers, the
manufacturers, the maintenance managers, the repairers and the
technical experts (who perform on-site electrical measurements and
material analyses). This often results in a very complex decision making
process, based on vast amounts of information that must be considered in
the context of a technical, economic and societal framework.
For the technical evaluation of the condition of a cable, a lot of expertise is
needed. Knowing the failure mechanisms and ageing processes is
mandatory in order to choose the right testing procedures. Also the
interpretation of the measurements and laboratory analysis is difficult,
especially for complex degradation mechanisms such as the growth of
water trees in polyethylene, which remains a subject for debate.
2. Technology
Medium voltage electrical power cables are composed of a low resistance
conductor (aluminium or copper) to carry the current, semiconductor and
insulation (impregnated paper or polymeric), screening and various layers

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(metal and/or polymeric sheath) to provide mechanical protection and
provide protection against moisture. Two different types of medium
voltage cables are mainly used: Paper Insulated Lead Covered and
Polyethylene cables. The cable can be a single-core cable or a three-core
belted cable.
2.1. PILC
Paper-insulated lead-covered (PILC) cables (widely used in urban
underground network systems) are made of copper or aluminium
conductors wrapped with paper, impregnated with dielectric fluid. The
cable is jacketed with a lead covering and may also have a plastic or jute
with tar outer jacket. They are no longer produced.
Mainly installed after World War II, they currently reach their end of life.
Nowadays there remains several thousands of km of PILC cables and it is
therefore important to assess the condition
of these cables. In order to maintain a
reasonable lifetime the temperature of the
paper should stay below 65°C. These
cables are progressively replaced by
polymeric insulated cables that can
withstand a higher temperature.

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2.2. XLPE
Extruded insulations such as cross-linked polyethylene (XLPE) were first
used as insulation of medium voltage distribution cables in the sixties in
place of the PILC cables. Although XLPE has a high electrical breakdown
strength, a low dielectric constant and low dielectric losses, it suffers from
the phenomenon of water treeing that reduces drastically its lifetime (see
3.2). In order to solve this problem, manufacturers of polyethylene have
developed different solutions. Copolymer has historically been used in
Europe. XLPE with water tree retardant (WTR) additive is mainly used in
the U.S. In order to maintain a reasonable lifetime the temperature of the
polyethylene should stay below 90°C.
2.3. EPR
Ethylene propylene rubber is another type of polymer. It presents more
important dielectric losses and therefore is not widely used. Rubber is
commonly used for appliance cords, portable cords, conductor leads, and
portable cables.
3. Ageing
It has been demonstrated by practical work that besides the theory of
electron generation and removal other many secondary processes, which
interact with and even supersede the electronic one, are the cause of
breakdown. The following effects must be considered: heating effect,
compression forces, treeing and electrochemical reactions with the
environment. [3]

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Electrical insulation is designed to withstand electrical, mechanical and
thermal stresses for at least 25 years. The power cable insulation is
mostly organic and susceptible to a deterioration in physical properties
when stressed. This is particularly the case with thermal stress. To assess
the lifetime of an insulation, accelerated thermal test are performed. Most
materials are complex in their chemical composition and their thermal
behaviour. Several physical processes like softening, melting or
crystallisation may affect properties. The most common deterioration on
routes involve oxygen. Oxidation will increase conductivity and dielectric
losses. It will dramatically alter mechanical properties. Thermal endurance
testing makes direct use of the empirical relationship commonly attributed
to Arrhenius which relates the rate of a thermally activated chemical
reaction (k) to the inverse of absolute temperature : k = A exp(-E/kT)
where E = activation energy and A = rate factor. In solids A is strictly
temperature, volume and entropy dependent. An empirical equation
relates life ζ to temperature in degrees, based in experience on thermal
degradation of transformer paper : ζ = A exp(-BT) where A,B = constant
characteristic of the material. This equation provides a platform for the “10
degree rule” i.e. that the thermal life of insulation is halved fo each
increase of 10°C or doubled for each 10°C decrease in temperature.[3]
As explained the cables are ageing but in the field the main observed
causes of degradations are mechanical aggression and the weakest
points are often the joints and terminations.

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3.1. PILC
The main causes of ageing of paper cables [1] are:
• thermal degradation related to an excessive conductor current
• partial discharges, i.e. electrical discharges that do not bridge the entire
insulation but nevertheless degrade the insulation material and finally
can lead to a breakdown between the phase and the earth of a power
cable
• aggression by the environment (humidity, vibrations, acidity of the soil,
...)
• transient over-voltages.
The first barrier to environmental aggression is the lead sheath. Four
categories of degradations can be found: fatigue cracking, extrusion
defects, fracture associated with internal pressure and corrosion. When
the lead sheath is cracked, it does not play its role anymore and therefore
moisture and impurities can impregnate the paper and reduce its
insulating properties.
The paper is mostly degraded by thermal effect. The temperature of the
paper is directly related to the electrical losses in the cable and to the
ability of the environment to drain the heat (thermal resistivity of the soil,
presence of other cables…).
3.2. Polyethylene
The main causes of ageing of polymeric cables [2] are:
• thermal degradation (leading namely to consumption of antioxidant)
• partial discharges due to manufacturing imperfections or to
mechanical damage
• water trees, i.e. tree-like micro-cracks that grow from defects when
the insulation is subjected to electrical stress and moisture (more
details are given in chapter 5)
• aggression by the environment.
• Losses

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As already mentioned the electrical losses are directly responsible for the
heating of the insulation and are therefore an important parameter when
considering the residual lifetime of a cable. It has been demonstrated that
the thermal resistivity of the soil is one of the important parameters when
determining the ampacity of a cable (i.e. the maximum admissible
current). Therefore an analysis of the soil is sometimes required in order
to assess its thermal properties. The losses can be divided in three
categories.
Conductor losses: they are produced by the transportation of the current
in a conductor. These losses are function of the type of conducting
material (copper or aluminium), the section of the conductors, the
temperature of the material (depending on the global losses and the
environmental conditions) and of course the transmitted current. Some
additional losses due to the use of alternating currents are to be taken into
account: skin and proximity effects.
Dielectric losses: they are related to the capacitance, the frequency, the
phase voltage and the dissipation factor of the insulating material. They
are also function of the temperature and the degradation level of the
insulating material (moisture content, presence of partial discharges…).
These losses are related to three different physical phenomena: leakage
current (independent of frequency), dielectric hysteresis (only present with
AC voltage) and partial discharges. For medium voltage cables the
dielectric losses are generally small in comparison with the conductor
losses.

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Sheath losses: they are due to the current induced in the sheath by the
current in the conductor of the
cable and possible other
cables in close proximity. A
part of these losses is related
to the sheath bonding.
Simulation softwares allow to
rapidly and accurately define
the relation between the load
profile of a cable and its temperature profile considering all the parameters
like soil resistivity, proximity of other cables or heating sources and
burying depth.
Additional losses due to harmonic content: in first approximation, we
can consider that losses inside the dielectric material is proportional to the
frequency and to the square of the voltage [5]. In medium voltage
distribution networks the voltage harmonics content is limited by the
standards EN 50160 [4]. With the limit of the standards the additional
losses inside the dielectric stay about 10% of the losses caused by the
fundamental, which are generally relatively low in regards of the other
losses. Non-linear loads induce current harmonics flowing through the
conductor. These harmonics are responsible for extra-losses and
therefore for an increase of the temperature. The underground system
must be dimensioned to carry the total rms current.
4. Cable assessment
A large part of Paper Insulated Cables, mainly installed 40 to 60 years
ago, are still in service. They have reached and even exceeded their
designed lifetime. On the other hand the first generation of polymeric
cables are degraded by unexpected water trees phenomenon. Therefore

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the assessment of the condition of the cable is an important and current
challenge. Many studies demonstrated that one diagnostic method is not
sufficient to assess the condition of cable insulation. The final decision
(complete or partial replacement, curing or further use) should be based
on the combination of different measurement results and analyses of the
failures. The correct interpretation of these results must be based on a
solid experience and knowledge of the degradation processes . This
assessment can be realized by means of various measurements on-site
(non-destructive electrical measurements) and in the laboratory
(destructive testing).
4.1. On-site
A wide range of methods are available to measure the degradation level
of the cable on-site. In order to apply the correct techniques, the maturity
of each technique (depending among others on the availability of
databases and interpretation models), its effectiveness and its applicability
must be evaluated. The most commonly applied on-site measurement
techniques are:
Dissipation factor (also called tan d):

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it is defined as the resistive current divided by the capacitive current
flowing through the insulation material. An aged cable will show higher
dissipation factor than a new one and so the value of the dissipation factor
is an indication of the global degradation of the insulation. To perform the
measurement, an AC high voltage is applied between the screen and the
conductor and the injected current is measured. The test is often
performed at Very Low Frequency (0.1Hz) in order to reduce the size of
the power source (the injected reactive power is proportional to the
frequency). The values of the dissipation factor at different voltage levels
provide an image of the global degradation of the insulation. A reference
for the interpretation is required and can be provided by a comparison with
adjacent phases or with a library built on previous measurements.
Electrical measurement of partial discharges: Some types of
degradation lead to the apparition of partial discharges. To measure the
discharges a high voltage is applied to the cable. The measurements can
be performed on-line or off-line. The advantages of the off-line
measurements are a lower noise level, the possibility to increase the
voltage and easier location whilst on-line measurements do not require a
separate power source to energize the cable. These discharges lead to
high-frequency signals that travel along the cable at a known speed
(depending on the type of the cable). At the end of the cable, the
discharges are reflected, which allows the location of the weak points.
Polarization index: it is a good additional source of information on the
degradation status of a cable. The evolution of the insulation resistance
over time is measured by applying a DC voltage to the insulation of the
cable.
External sheath test: the sheath is the first barrier for the environmental
contaminants (water, acidity…). Its integrity can be checked by measuring
the resistance between the screen and the earth.

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The combination of these four on-site measurements gives a good idea of
the degradation of the cable. Some other techniques are available, such
as acoustic measurements of partial discharges, dielectric spectroscopy,
recovery voltage techniques… but they are not so widely used on-site.
4.2. In the laboratory
4.2.1. Visual inspection and material analyses
The examination of the different layers of the cables, in particular the main
insulation layer (paper or polyethylene), is useful to determine the
condition of the cable. The first analyse to be performed is visual
inspection. It requires a good deal of
experience to detect all the ageing signs
(traces of water, corrosion, mechanical
damages, presence of gas bubbles or wax
in paper insulated cables…). In addition
some material analyses can assess the
degree of degradation of the insulation.
The cables or joints are often analysed
after a breakdown in order to evaluate the
global degradation and help in the decision
process (replacement or further use).
Moisture content. Water affects the electrical properties of the insulation.
An increase of moisture normally enhances the dissipation factor and
decreases the dielectric strength of the insulation. Karl-Fisher titration
enables to measure the quantity of water in the paper or in the
polyethylene.
Total acidity. It is used to assess the degradation of the oil for paper
insulated cables.
Degree of polymerisation of the paper. It gives a good indication of the
thermal degradation of the paper.

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Differential Scanning Calorimetry, Infra-Red Spectroscopy, X-Ray
Diffraction. These are some of the useful techniques to assess the
quality and the degradation of polymeric materials.
Water trees visualization. In order to visualize the water trees in the
insulation, thin slices are taken using a microtome, dyed with methylene
blue and observed with a microscope.
4.2.2. Electrical measurements
If the received sample is long enough, some electrical measurements can
be performed in laboratory: partial discharges measurements, dielectric
spectroscopy, dielectric strength, DC resistance. The combination of
electrical measurements and material analyses allows a complete
understanding of the ageing process and is useful for further investigation
on site.
5. Water tree ageing
Among the already mentioned degradation mechanisms, water treeing is
certainly the most studied but nevertheless also the less understood
mechanism. Water trees grow from defects – contaminants (ions),
protrusions or voids - when the insulation is subjected to electrical stress
and moisture.
Water trees lower the dielectric strength and have caused a large number
of failures of cables in service, particularly in older vintage cables that
have higher levels of defects than more recent cables. But also cables
produced in more recent years, namely cables with copolymer insulation
or with addition of water tree retardant, may be susceptible to water tree
degradation.

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The phenomenon was first published in the late sixties. The discovery was
made in the USA. The literature [2] shows that these develop through
changes to the physical and chemical factors leading to charge
transportation and storage effects, resulting in an increase of the local
electric field. This electric field induces electro-dynamic forces at
interfacial boundary layers. Impurities at the inner semiconductor layer are
often starting points of “vented” water trees which may eventually convert
into an electrical tree. Changes in the bulk properties of the material
resulting from cracking of the amorphous–crystalline regions can lead to
micro-voids. Electrical failure usually occurs when an electrical tree
initiates from a water tree and bridges the insulation or by thermal
runaway when a water tree that bridges the insulation reaches a
sufficiently high conductivity.
Therefore, the estimation of water trees resistance and lifetime of cable
insulations represents an important issue for cable owners all over the
world. Unfortunately there exists no theoretical or practical model
integrating all the parameters to give a cable lifetime determination.
Accelerated tests exist on flat samples with defects created artificially by
abrasive paper or on a bulk sample using needles, but they cannot
account for all the influencing parameters on real cables. This incited
Laborelec to develop a new method of accelerated aging at high
frequency and high voltage on samples of real cables. The very short time
necessary to let water trees grow with this technique allows its use for
decision making not only for maintenance but also for purchase.

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References
[1 ] Members of WG 21.05 ; “Diagnostic methods for HV paper cables and
accessories” ELECTRA N°.176 February 1998
[2] L.A. Dissado and J.C. Fothergill; ”Electrical degradation and
breakdown in polymers”, Published by Peter Peregrinus for the
IEE,1992.
[3] A Bradwell; “Electrical Insulation”, Published by Peter Peregrinus for
the IEE,1983.
[4] EN 50160 – Voltage characteristics of electricity supplied by public
distribution systems; European Standard, version 1999.
[5] Wildi and Sybille; “Electrotechnique –Published by de boek 2005.
[6] S. Y. King and N.A. Halfter; “Underground Power Cables” – Published
by Longman 1982.
[7] G.F. Moore; “Electric Cables Handbook” – Published by Blackwell
1997