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SECTION 2
LITERATURE REVIEW
2.1
INTRODUCTION
Polymer insulated cables are widely used all over the world. As a result of the
rapid developments in solid dielectric extruded cable manufacturing techniques, the
polymeric insulated cables are very popular for HV applications due to their great
technical and economic advantage. The main polymer insulation material that is
dominating the market is XPLE, which has been used for low, medium and high voltage
cables for up to 500kV. Consequently, since the early 1960's, great quantities of XLPE
cables have been installed around the world leading to improved reliability, simplicity
and overall system economy.
Although XLPE has a very high intrinsic dielectric strength ( 1000 kV/mm),
the actual operating stress in practical cables are much lower than this value. This is
due to different defects which occur during extrusion as well as various factors which
influence aging and breakdown processes that are inherent in actual cables when used in
practical environment. Treeing phenomena is generally responsible for most of the
XLPE cable's insulation failures. In treeing, there are two types of processes, which are
termed as electrical treeing and water treeing. Both of these treeing processes have
distinct characteristics.
The electrical treeing produces clearly visible tree shaped
degradation paths which are caused mainly by high electrical stress in the polymer and
their growth can be observed experimentally. Thus, an electrical tree can be defined as
formation of a degradation path in a solid dielectric due to high electric stress. It
consists of tree-shaped hollow channels created through repeated partial dielectric
breakdowns occurring locally in a region that is small but has a very high electric stress.
Electrical treeing can also be defined as the growth process of hollow channels in the
solid dielectric caused by electrical pre-breakdown phenomena. Usually electrical
treeing is the dominant cause of cable failures in High and Extra High Voltage
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transmission class cables. At Medium Voltage primary distribution class cables, water
treeing is the predominant cause of cable insulation failures.
When the XLPE insulation was introduced in the early 1950s, an overall belief
existed that this material was immune to most of the chemicals and water available in
soil. Consequently, early construction of medium voltage cables in the United States
was without any protective covering. However, field experience showed the
shortcoming of this construction: many cable failures, which had occurred, were mainly
due to phenomenon called water treeing. Water trees are regions of microcavitation and
filamentary damage in a polymer such as XLPE caused by the penetration of aqueous
salts in the presence of an electric field. The determination of the extent of the damage
to polymeric cable insulation is a significant issue in the assessment of the cable’s
residual life. The various other failure reasons / mechanisms of XPLE cables related to
electrical, chemical and thermal stresses and as studied by various researchers [4, 20,
24, 37, 39, 51, 72, 73] are briefly reviewed in this section of this chapter. This review
mainly covers the work reported over the last decade and compliments the extensive
review reported earlier [304]. Moreover, the review mainly focuses on important issues
related to medium voltage XLPE cables which form the backbone of Saudi Arabia’s
distribution network and completes Task 1 of this applied research project.
2.2
WATER TREEING
During the last 15 years, significant amount of research work has been reported
in the literature on different aspects of water treeing. Table 2.1 presents a summary of
such publications regarding water treeing. It also classifies various sub topics and gives
pertinent references for each sub topic for an easy access to the earlier work.
Water and electrical treeing are complex phenomena involving several processes
with many synergistic effects. Water treeing is a well-known phenomenon responsible
for failures of the underground cables using polymeric insulation. The phenomenon of
water treeing is both phenomenal in its economic impact and fascinating from scientific
point of view. This is reflected in the vast number of papers that have appeared since
water trees were first discovered in Japan [24,39,51,72,73,81,136,147,173,304]. Still,
the subject requires attention in order to maintain and increase the cable’s insulation
reliability. Water trees are degradation structures in cables that are permanent, have
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grown due to at least humidity and electric field, have lower residual electric field
strength and are substantially more hydrophilic than the original insulation material.
Table 2.1: Recent Literature on Water Treeing.
Topic
General background, review type,
mechanisms
Mechanical, chemical and thermal aspects
Residual charge and space charge effects
Dielectric spectroscopy and analysis
Investigations using micro-PIXI, FTIR
and electron spectroscopy
Fractal behavior and analysis
Effect on electric-field
Dielectric properties
, r
Breakdown voltage
Diagnostics of water trees using:
Capacitance, tan 
Loss current waveform and its
nonlinearity with voltage
BD strength
Space charge
Broadband spectroscopy
Criterion of diagnostic
DC leakage, polarization /
depolarization currents
Current pulses during treeing
Time domain spectroscopy
Thermally stimulated currents
Non standard test voltages
Return / recovery voltage
measurements
Dielectric response
measurements
Ultrasound measurements
Time domain reflectrometery
Absorption current
measurements
Relaxation current
Reference Number
4, 20, 24, 39, 51, 72, 73, 81, 136, 147,
173, 304
2, 4, 12, 18, 39, 77, 90,97
1, 3, 8, 17, 22, 31, 53, 62, 93, 116, 135,
141, 146, 184, 185, 230, 246
5, 35, 47, 55, 56, 63, 67, 74, 87, 89, 121,
122, 124, 138, 153, 157, 251
6, 19, 32, 121
10, 28, 36
1, 11, 33, 53, 73
14, 44, 86, 92, 128, 162
45, 74, 75, 304
67
7, 54, 174, 249, 304
15, 17, 40, 74, 88, 89, 118
45, 74, 83, 95, 300, 304
17, 105, 135, 141, 144
103
152, 153
47, 82, 97, 99, 119, 151
26, 76
149
27
43, 66
60, 85, 110, 115, 154, 155
5, 55, 75, 104, 107, 108
139
140
101
145
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Table 2.1: Recent Literature on Water Treeing (Contd..)
Topic
Suppression methods using:
Tree-retardant materials
Cross linking
Additives
Barriers
Powders
Accelerated aging and assessment
methods
Benefits of TR – XLPE cables
Blue water trees
Factors affecting the growth of water
trees:
Bow-tie trees
Zero crossings of voltage
Frequency of voltage
Additives in insulation
Jacket and moisture barrier
Cross-linking methods
Irradiation
Ions and oxidation
Non standard voltages
Semi conducting screen
compounds
Low temperature
Effect of water without electrical stress
on ac breakdown voltage
Statistical analysis of water tree lengths
Estimation of water tree length
Restoration of water treed insulation
Effect of cable processing on:
Impurity content
Electrical properties
Water absorption
Cable performance
Reference Number
57, 68, 156, 168
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42, 71, 94
38, 123, 304
123
16, 30, 46, 57, 59, 68, 102, 114, 115,
120, 158, 158, 159, 294, 298, 304
98
23
70
21, 64
37, 41
13, 71, 94
38, 304
18, 49, 111
80
12, 41, 61, 96, 165, 292, 299
58, 65, 134
297, 301
9
143
50, 61, 69, 302, 304
52
45, 130
106, 150
111, 113, 160
112, 161
127, 245
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Table 2.1: Recent Literature on Water Treeing (Contd..)
Topic
Effect of conductor water on PD and
BDV
Water tree simulation and modeling
Conversion of water tree to electrical
tree
Propagation of electrical tree from water
tree
Economic diagnostic and life cycle cost
evaluation of cables
On site testing and diagnostics of cables
Characterization of un-aged cables
Life modeling of cables
Service experience of cables and
accessories and reliability evaluation and
failure modes
Reference Number
117, 143
34, 78, 100, 109, 164
25, 79
190
29, 84, 142
35, 40, 91, 129, 145, 110
148
163, 241
125, 137, 172, 176
Till now no investigation could give a clear and precise explanation of water
treeing mechanism. However, all the studies have in common the factors that contribute
to the initiation of water trees and their growth. Different mechanisms responsible for
water tree propagation [24] can be broadly classified as:
-
Processes related to electrical forces on water
-
Diffusion of hydrophilic species
-
Electrochemical processes like oxidation phenomenon
-
Condition dependent degradation
A brief discussion to some of these issues is provided next.
2.2.1 Mechanical, Thermal and Chemical Aspects
In the mechanical process, some molecular bonds are broken by mechanical
stresses, which eventually result in micro cracks that are filled with water and finally
result in a water tree [2,4,18,39]. It is known that water droplets at high ac field could
generate local pressures that are well above the local mechanical strength of the
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molecular chains. Amorphous polymers being more flexible and thus resisting better to
fatigue should then grow smaller water trees [39]. Temperature is neither really a
chemical nor a mechanical parameter. However, even then experiments show that water
tree length increases when samples are wetted on both sides whereas it decreases when
samples are wetted only on one side. The chemical theories rely on the interaction
between water and contaminants, additives, and O2. Since many cables are directly
buried in the ground which may contain almost all the chemical species, hence such
chemicals can easily contaminate the cables. In this case, the water treeing is a two steps
process: first, penetration of water or ions assisted by Maxwell stresses and second,
chemical deterioration leading to permanent micro voids in the insulation [77, 90, 97].
2.2.2 Investigation Using Micro-PIXI, FTIR and Electron Spectroscopy
The water tree consists of micro voids which are connected by micro channels or
tracks. The density of micro voids (number per unit volume) decreases from the root to
the front of the water tree. The micro voids contain the water and ions which have
penetrated the polymer under the action of the electric field [6,19]. Reference [32]
illustrates the range of water tree structures observed in medium voltage cable insulation
using the scanning electron microscope (SEM) and also high resolution imaging system
based on the transmission electron microscope (TEM).
Recently, the nonlinear
dielectric response of water treed XLPE cables has been explained by voltage assisted
water ingress into water treed regions [121]. Water content of up to 2% was found in
vented water tree structures. Some sections of the tree branches were, however, found to
be hydrophilic, indicating water being bound to the polymer. The water content was
highest in the tree branches and lowest at the tip of the trees as well as in the root region
close to the insulation screen. The length of these regions was typically 200-300 μm.
Scanning with the micro-PIXE technique was employed to analyze water trees
in the XLPE insulation of a field-aged underground HV cable [6]. X-ray spectra of bow
tie and vented water trees, the inner and outer semi conductive compounds, and an
insulation spot free from any water tree were acquired. Simultaneously, twodimensional elemental distribution profiles across the water trees were also measured.
Various trace element impurities were identified in the analyzed spots and their possible
sources were suggested. Differences in elemental distribution profiles in the scanned
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areas were observed. This study demonstrated the effectiveness of the micro-PIXE
facility in analyzing water trees in underground power cables.
Traditionally, water and ions in the water tree had been examined separately, by
different methods, such as infrared spectroscopy, neutron activation analysis or X-ray
analysis. In reference [20] , the authors presented a study of water and ions distribution
in the same water tree by FTIR micro spectroscopy and tried to correlate their results
with the proposed mechanisms for the growth of water trees.
2.2.3 Fractal Behavior and Analysis
Water trees arise from the penetration of water into the XLPE dielectric of
medium and HV power cables when operating in a humid or wet environment. They
appear in the insulation of service aged power cables and can lead to electrical failure.
The visual appearance of water trees suggests underlying fractal characteristics. The
calculation of the fractal dimension can help to confirm the fractal character and to
understand the physical mechanisms that lead to water tree formation and growth [10].
The growth process and shape of water tree in XLPE dielectric under ac electric field is
random and branched and cannot be described using Euclidean geometry. However, the
fractal dimension and the branching tendency of water tree in XLPE dielectric can be
estimated based on partial electric breakdown and the fractal theory [28,36]. The shapes
and structures of water trees observed by an optical microscope proved to have fractal
characteristics, and it is concluded in these studies that water trees are fractal objects.
2.2.4 Effects of Water Tree on Electric Field
It is reported that ac breakdown strength of the aged XLPE is reduced as the
growth length of water tree is increased and the trend of the reduction has been similar
to that of water tree growth rate. It has been found that the homo-charge is introduced
due to treeing which is heavily concentrated at the tip of the water tree path while a little
hetero-charge is gathered in front of electrode contacted to the water tree in virgin and
aged cables. By the electric field calculation using the space charge distribution, the
local field enhancement at the tip of water tree path has also been verified. Based on the
experimental results and analysis, it has been shown that because of the increase in
length and permittivity of the water tree, the electric field in front of the degraded area
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is amplified because of the homo-charge concentration at the tip of water tree path
causing the reduction of the breakdown strength of XLPE [1,11,33].
2.2.5 Dielectric Properties
The dielectric properties of the XLPE insulation are affected by water treeing as
reported in the literature. A brief review of these aspects is given next.
2.2.5.1 Dielectric constant and loss tangent
The good electrical and dielectric properties of PE remain largely unchanged
during cross-linking process. XLPE cables therefore, like PE cables have a very small
and insignificant temperature dependent dielectric constant (εr) and loss tangent (Tan δ).
As a result, the dielectric loss of XLPE cables is small in comparison with those of PVC
and paper insulated cables. Hence XLPE cables are specially suitable for long cable
routes and high voltages since in both cases the dielectric loss is of great significance
[14,44]. Due to the dependence of ion mobility on the presence of water, the wet water
trees have much higher conductivities than the dry water trees and than the PE [44]. It
has also been reported that the conductivity of water-treed polyethylene is more than
106 times higher than that of non-treed polyethylene, while the permittivity increase is
only a few times [86,92,128]. The Maxwell-Wagner-Sillars (MWS) model proved a
reasonable fit to the experimental data providing that the conductivity in water treed
region lies between 10-3 S/m and 10-2 S/m.
2.2.5.2 Breakdown voltage
The breakdown strength of XLPE insulation is degraded due to water treeing.
Due to this reason, the breakdown voltage of cable insulation reduces as the length of
water tree increases [304]. However, if an insulation fluid such as silicone oil is injected
in the water-treed region of the insulation, the dielectric strength can be restored to
some extent. The effects of silicone restoration liquid on water treed XLPE power cable
insulation materials under ac and impulse breakdown strengths showed that the shortterm effect of restoration is to strongly increase the ac and impulse breakdown strength.
It also increases low-frequency dielectric loss [45,74]. This effect is a non linear
function of time and voltage.
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2.2.5.3 Conductivity
The conductivity of water trees is high at power-frequency ac voltages because
charges were observed around the tip of water trees and on the electrode at the nondegraded side, but not on the electrode at the water-treed side. Whereas at high
frequency of applied voltage such as 2 kHz, the conductivity of water trees is slightly
low due to charge deposition on the electrode at the water-treed side [1,11,33,53,73].
2.3
DIAGNOSTIC OF WATER TREEING
Various techniques have been reported in the literature for diagnostics of water
trees in the cable insulation. Hvidsten et. al. [67] show that the degree of water treeing
in XLPE cables rated at 36 kV and above is strongly dependent upon the cable design
and the system voltage. The water trees are typically relatively short, but their density is
similar to that observed in medium voltage cables with similar design. The results from
nondestructive diagnostic testing indicated that it can be possible to detect water treeing
also in old high voltage cables extending diagnostic techniques developed for medium
voltage cables. These diagnostic techniques are based on measurements of various
parameters as outlined next.
2.3.1 Capacitance and Loss Tangent
The temperature dependence of loss tangent and capacitance is a very significant
parameter. Hence, the differential variation of loss tangent and capacitance and its
dependence on the temperature can be used as a measure of water tree degradation of
polymer insulated medium voltage cables as suggested in several recent reports
[7,54,174,249].
2.3.2 Loss Current Waveform and its non-linearity with Voltage
It has been reported that water trees introduce higher harmonics into the
insulation loss current waveform and that the harmonic distortion of the loss current can
be correlated with the length of the water trees. Current waveforms (current
comparative technique [118]) for insulation free of water trees or insulation in which the
water trees were completely dried out showed negligible harmonic distortion. Insulation
with partially dried water trees maintained significant harmonic distortion of the loss
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current. Harmonic components arise as a result of the nonlinear voltage-current
characteristic of the water treed insulation [89]. The results confirm that water treeing
causes reduced residual ac breakdown strength and high and nonlinearly increasing lowfrequency dielectric loss [74].
2.3.3 Space Charge
Space charge behavior is also affected by the degradation of insulating materials
by water treeing. The charge distribution at any phase angle of the applied voltage
inside water-treed polyethylene was examined using pulsed electro acoustic method
(PEA) in [144]. Thus several studies have attempted to understand the space charge
behavior of water treed insulation and the measurement of space charge as a diagnostic
tool [17,105,135,141,144].
.