IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
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A Review on Characteristics and Assessment
Techniques of High Voltage Silicone
Rubber Insulator
Kaushik Sit , Student Member, IEEE, Arpan Kr. Pradhan , Senior Member, IEEE,
Biswendu Chatterjee , Senior Member, IEEE, and Sovan Dalai , Senior Member, IEEE
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Abstract — The usage of polymeric insulators in transmission and distribution overhead lines for the last few decades
has increased extensively. Silicone rubber (SiR) material
is used enormously to make high voltage (HV) polymeric
insulators. The SiR insulator provides insulation protection
for overhead HV lines. The life span of the SiR insulators
is shortened due to the aging effect. The progressive aging
ultimately leads to the failure of the insulation system as
well as the power supply. Therefore, a regular condition
assessment of SiR insulators can help prevent the power
system’s failures. A comprehensive study of the various
types of aging processes of SiR insulators is reported in
this article. Also, the comparative analysis of SiR insulators’
physical, chemical, and electrical properties under the influence of aging is reported here. The usefulness, limitation,
and difficulties confronted in the existing methodologies are
also reported in this article. This review article would be
helpful to implement in future research for applying the HV
overhead line insulator.
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Index Terms — Aging, analytical techniques, condition
assessment, high voltage (HV), over headline insulators,
physical, chemical and electrical properties of silicone
rubber (SiR) insulator, SiR insulator.
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I. I NTRODUCTION
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T
HE overhead line insulators are an essential part of
transmission and distribution systems. Insulators should
have a smooth function throughout their life span to provide
reliable electricity at the load end through the electric network.
Overhead line insulators are usually of two types based on
the materials used. These are ceramic, porcelain, and polymeric insulators [1]–[4]. Ceramic insulating materials have
good dielectric strength and broad temperature (i.e., −73.3 ◦ C
to 1260 ◦ C) withstand capabilities [5]. Despite having the
advantages mentioned above, ceramic insulators are not much
preferred on overhead lines due to their large size, heavyweight, and installation and maintenance difficulties [6], [7].
Manuscript received 24 March 2022; revised 5 July 2022; accepted
22 July 2022. Date of publication 27 July 2022; date of current version
28 September 2022. (Corresponding author: Kaushik Sit.)
The authors are with the Department of Electrical Engineering,
Jadavpur University, Kolkata 700032, India (e-mail: kaushik.sit@
gmail.com; arpan.pradhan85@gmail.com; biswenduc@gmail.com;
sovandalai@yahoo.co.in).
This article has supplementary material provided by the
authors and color versions of one or more figures available at
https://doi.org/10.1109/TDEI.2022.3194486.
Digital Object Identifier 10.1109/TDEI.2022.3194486
In contrast, polymeric silicone rubber (SiR) insulators are
preferred for power grid transmission and distribution line
applications. There are three main reasons why SiR composite
insulators are widely used in the power grid: first, they are
light, easy to install, and their mechanical strength is no less
than that of porcelain or glass insulators. Second, the manufacturing process is relatively simple; the shade configuration can
be changed according to the mold to meet the requirements
of different projects for creepage distance. Third and most
importantly, the anti-pollution flashover ability of SiR composite insulators is much stronger than that of porcelain or glass
insulator. Also, the SiR composite insulator has hydrophobicity migration properties [1]–[3], [6]–[8]. Some studies [2]
and [11] show that hydrophobicity is also found in the surface
contamination levels after a period of time because the transfer
of SiR components causes it. The performance advantages
of SiR composite insulators are further enhanced when they
apply power lines with voltage levels above 220 kV [5], [10]
and railway-yard substations [12], [39]–[41], [80]. The natural environment plays a decisive role in the aging of SiR
composite insulators. Climate factors include snow, rainfall,
sandstorms, fog, and mist and, so often, problems related to
the power grid operation department. As mentioned earlier,
climate factors directly influence the SiR insulators’ effectiveness [6]–[8], [10], [11].
The type of material used in SiR is an elastomer. This elastomer is made of mainly silicon and oxygen atoms. According to the chemical structure, methyl groups are in the side
chain [shown in Fig. 1(a)]. SiR is colorless and they look
like oil or rubber type of substance. SiR is used in electrical insulation, heat insulators, and medical science applications [2], [8]. Global surveys have shown that SiR is used
as insulators in the highest percentage in the Middle East
(94.7%), Latin America (93.4%), and Asian (100%) continents [7], [8]. The survey has also shown that 91.4% of
SiR components are used to manufacture insulators worldwide [8]. SiR insulators have long-term reliable performance,
but they are affected by electrical discharge, natural or artificial aging, and environmental stresses [3], [6]–[8], [10], [11].
Therefore, various research works have reviewed the longterm reliability of polymeric insulators [3]. It has been
found that healthy SiR insulators can work satisfactorily near
about 8–20 years [7], [8], [10].
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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
TABLE I
T YPES OF F ILLER E LEMENTS AND T HEIR C HARACTERISTICS [8]
Fig. 1. Chemical construction of PDMS. (a) PDMS solo unit monomer.
(b) Cyclic PDMS molecule [13].
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This article briefly discusses the impact of aging in overhead SiR insulators, which are used for outdoor applications.
This article discusses the chemical structure, adoption of filler
elements, types of SiR insulators, and physical properties of
SiR insulators. Finally, the degradation of the physical and
chemical properties of SiR insulators due to aging and their
consequent effects on electrical performance are presented in
this article.
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II. C HEMICAL S TRUCTURE
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The chemical composition of silicone elastomer or SiR
is explained in detail in this section. Organic fillers (i.e.,
organically modified montmorillonite (OMMT), nanofibrillated cellulose (NCF), etc.) and inorganic fillers (i.e.,
calcium carbonate (CaCO3 ), silica, zinc oxide (ZnO), etc.) are
mainly used in SiR polymers [13]. These fillers provide the
more rigid structure of SiR polymer through the vulcanization
process [14], [15]. A fundamental form of SiR is made up of
polydimethylsiloxane (PDMS) with organic methyl groups of
silicon-oxygen structure. The PDMS is expressed as CH3 [Si
(CH3 )2 O]n Si(CH3 )3 , where the letter n indicates the quantity
of monomers [16]. The chemical structure of the single
monomer unit (linear) and general ring structure of PDMS is
shown in Fig. 1.
Due to the Si–O bond in the SiR chemical structure, SiR
insulators exhibit good thermal conductivity, hydrophobicity,
and anti-oxidant properties. Jiang et al. [5] reported that the
base material in any SiR insulator is PDMS, where the primary
molecular components are composed of carbon (50%), oxygen
(25%), and silicon (25%). The mass of the cyclic molecule in
SiR is 341. Methyl groups of the chemical structure of PDMS
have the inherent quality of water repellence [16]. SiR can
maintain their qualities over wide temperatures (i.e., +180 ◦ C
to −50 ◦ C) due to its chemical structure [7], [8]. Two fundamental characteristics are viscosity and volatility, determined
by the molecule chain’s length [14], [16]. Due to the effect of
crosslinking events on the chemical structures of the material, the outer surface of the aged SiR insulator becomes
hard [17]. The study [18] said that the surface of the SiR
insulator becomes rougher after continuous aging.
III. F ILLER E LEMENTS
Fillers are the primary agents of the SiR insulator
to improve electrical, mechanical, and thermal properties.
The Fundamental component of SiR is pure PDMS that has
low mechanical strength and less intermolecular energy [2],
[7], [8]. The main inorganic agents used as filler in SiR insulators are silica (SiO2 nH2 O) [9], [19] or Alumina Trihydrate
(ATH) (Al2 O3 0.3H2 O) [19]–[22], feldspar, and Kaolin [23].
The diameter of the filler particles should be less than or
equal to 100 nm [8]. The thermal endurance of SiR insulators can be improved by adding the filler particles of the
macro range [9], [19], [23]. Large filler particles may cause
problems in SiR insulators such as surface roughness, poor
heat circulation, and less durability. It has been reported that
low-density ATH fillers in SiR insulators absorb fewer water
molecules on their surface at high temperatures [24]. As a
result, the dielectric strength of the insulator material decreases
and the flow of leakage current (LC) through its outer surface
increases. In [25], the article states that the mass of 50%
microsilica filler is equal to that of 10% nanosilica filler, which
helps to improve the physical properties of SiR insulators.
The study said that boron nitride (BN) fillers show better
results than Aluminum Nitride (AlN) fillers in SiR insulators’
performance [26]. In [27], another study revealed that hybrid
fillers effectively show much better results for SiR insulators
during aging than independently used fillers. Different types of
filler elements and their advantages are summarized in Table I.
IV. T YPES OF S I R I NSULATORS
The SiR insulators are classified based on the temperaturedependent vulcanization process. According to the temperature
variation, SiR insulators are classified into three categories.
These are room-temperature vulcanized (RTV-SiR) [30]–[33],
low-temperature vulcanized (LTV-SiR) [2], [7], [8], and
high-temperature vulcanized (HTV-SiR) [35]–[37] SiR insulators. The polymeric insulators commonly used on transmission
and distribution lines are HTV and LTV SiR insulators [33],
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SIT et al.: REVIEW ON CHARACTERISTICS AND ASSESSMENT TECHNIQUES OF HIGH VOLTAGE SiR INSULATOR
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TABLE II
D IFFERENT T YPES OF S I R S W ITH T HEIR C HARACTERISTICS AND A PPLICATIONS [8]
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whereas RTV-SiR is used to coat the porcelain insulator’s
surface [8]. RTV-SiR is commercially available in two forms
such as: 1) RTV-1 and 2) RTV-2. These two RTV-SiR insulators are made of the same base material and filler elements.
The molecular weight of RTV SiR is low. Molecules of RTV
SiR are arranged in cyclic patterns [8], [13]. Wang et al. [34]
reported that insulators made of RTV coating are better surface
resistance than porcelain insulators under the same aging and
wet conditions. Generally, the linear and cyclic molecular
structures of HTV-SiR are found in practice. The commercial
grades of HTV-SiR are Elastosil R401/40, Elastosil R401/50,
Elastosil R4001/50, Elastosil R401/60, and in other grades [8].
HTV-SiR insulators offer better flashover performance in a
polluted environment than other SiRs. The HTV-SiR insulators
include high crosslink degrees and low [-OH] groups. In addition, they are well resistant to UV-A due to their uniform
filler configuration among them [38]. LTV-SiR is made with
the same base material as HTV-SiR and RTV-SiR. LTV-SiR
is cured at high temperatures and fillers are added as per
the requirements. The viscosity of LTV-SiR is low compared
with HTV-SiR [33], [36]. Aging increases the surface hardness
resulting in a change in the ratio of base material and filler
elements [37]. Table II shows the characteristics of the three
categories of SiR insulators mentioned above in detail [8].
V. D ESCRIPTION OF C ONSTRUCTION AND P HYSICAL
D IMENSION OF S I R I NSULATORS
The SiR insulators contain four parts in their physical
structure [28], [33], [39], [40]. These are: 1) two end fittings
(metal); 2) FRP rod; 3) SiR made polymeric housing; and
4) edge between the FRP rod and SiR housing [shown in
Fig. 2(a)] [6]. The interface condition between the FRP rod
and SiR polymer housing is the most vital factor in determining the insulator’s longevity. The steep forward impulse
voltage and water diffusion test help to determine the volume
and surface resistance at the interface of SiR polymers and
FRP rods by the IEC-60093 standard [29].
In Fig. 2, pictures of overhead line SiR insulators of two
different ratings (e.g., 11 and 33 kV) are shown. In addition,
the pure and artificially contaminated SiR insulators are shown
in that Fig. 2. The thermal stability range of the SiR insulators
is from +180 ◦ C to −50 ◦ C. The reason for having such
a wide thermal range is due to the stable Si–O bond [8].
Fig. 2. (a) Cross-sectional image of physical structure of SiR insulator
[6], [28], [91]. (b) Image of 33- and 11-kV pure and contaminated SiR
insulators [42].
TABLE III
T ECHNICAL D ESCRIPTION OF 11- AND
33- K V S I R I NSULATOR [41], [42]
Si–O bonds are much stronger than conventional C–O and
C–H bonds [7], [40]. Also, Si–O’s bond energy is 8.3 eV.
The technical specification of the 11- and 33-kV outdoor SiR
insulators is shown in Table III [41], [42].
VI. AGING A SSESSMENT OF S I R I NSULATORS
Any defects or surface deterioration may appear in the
composite insulators after an extended period of service and
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TABLE IV
S UMMARY OF VARIOUS E XTERNAL FACTORS AND T HEIR E FFECTS ON S I R I NSULATORS [43]
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ultimately threaten the operation of the stable power system.
The faults and degradation events in SiR insulators are classified based on where the defects occurred. The elaborate study
of aforesaid events has been given in [44].
Three different analysis processes are used to review the
degradation of SiR insulators. These are physical analysis,
chemical analysis, and electrical analysis [2], [8]. Khattak
and Amin [43] reported that two types of external causes
(such as environmental and electrical factors) are responsible
for the overall deterioration of SiR insulators. The different
types of stresses and their overall effect on SiR insulators are
summarized in Table IV.
A. Analysis of Physical Properties
The physical property of SiR insulators after aging can be
analyzed after thorough investigation by the following methodologies such as: 1) hydrophobicity and 2) accumulation of
pollutants on SiR insulator surface.
1) Hydrophobicity: Hydrophobicity is a physical characteristic of SiR’s chemical structure. SiR molecules inhibit water
film formation on its surface because of their hydrophobic
property. As a result, the SiR insulators create water droplets
on the surface instead of forming a continuous water film.
This signature characteristic is called water repellence [2],
[8], [10]. On the contrary, water molecules attract hydrophilic
molecules [2]. The SiR insulators are made of low hydrophilic
property-based materials; that is why they show superior performance. SiR insulators can recover their lost hydrophobicity even after the aging event due to their durable material
structure. The following factors such as hydrophobicity, carbonization, effects of arcing on insulator sheds, insulator
design, penetration of the water molecule in the FRP rod,
tracking, and erosion resistance of SiR insulators assist to
predict the longevity of performance. It is noteworthy that the
most dangerous agent is water molecules that initially enter
the SiR housing and then the FRP rod. After the molecules
of water penetrate in FRP rod, the temperature rise, localized
discharge, and then arcing phenomena are started. Ultimately,
this degrades the interface resistance between the FRP rod
and the SiR housing and cause punctures of the insulators.
It was reported in [44] that the brittle and decay-like fractures
have all occurred due to interface failure and poor adhesive
application between the FRP rod and SiR housing. In [50],
it is reported that in 500-kV transmission line insulators, after
Fig. 3.
Schematic
(b) Dynamic CA [49].
representation
of
CA. (a)
Static
CA.
the water enters the core of the composite insulators, either
nitric acid is formed due to the surface discharge phenomenon
or sulfuric acid is generated due to acid rain. This acidic
reaction ultimately leads to stress corrosion cracking (SCC)
in the FRP rod. It is also reported that the SCC is formed
due to three main factors: humidity, the impact of the electric field, and surface contamination of the SiR insulator’s
housing.
Hydrophobicity is determined by measuring the contact
angle (CA) of deposited water droplets on the surface of the
insulator. Four methods are used to calculate hydrophobicity.
These are the CA measurements [49], sliding angle measurement [50], Swedish Transmission Research Institute Index
(STRI-I) [50], and water-soaked test [46], [47]. The static
and dynamic CA measurement [8], [49] is the most popular
technique for estimating hydrophobicity. Ethylene iodide or
distilled water is applied to the surface of the SiR insulator
to measure the CA [49], [50]. The CA (θC ) depends on the
three interfacial force factors. These interfacial forces exist
between water and air (Fwa ), solid and water (Fsw ), and air
and solid (Fas ) [shown in Fig. 3(a) and (b)]. These three interfacial forces are represented by Young Dupre’s mathematical
expression [8]
Fas = Fsw + Fwa cos θC .
(1)
The CA (static) is measured with the assistance of the goniometer, magnifying instrument, or
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computerized photography [2], [8]. The static-CA
measurement schematic representation is shown in Fig. 3(a).
Hydrophobicity is the dynamic property of any material.
Therefore, the dynamic CA measurement is essential, but
the procedure is more complex than the measurement of
static-CA. The dynamic CA is measured either by advancing
the slope (i.e., increasing the water droplet mass) or using a
receding slope (i.e., decreasing the water drop volume). The
schematic of the dynamic CA is shown in Fig. 3(b).
If a water drop is placed on the slope of the SiR material, then the advancing angle (θadv ) is formed at its bottom.
Similarly, a receding angle (θrec ) is formed at the upper side.
These two angles are essential for quantifying dynamic CAs.
A logarithmic relationship is found between the recovery time
and the receding angle [48]. In [45], it has been reported
that SiR insulators can be considered an excellent dielectric
strength with good aging resistance if the value of the CA is
close to 110◦ after hydrophobicity testing. The slope creates a
sliding angle (θS ) with the horizontal axis [shown in Fig. 3(b)]
and its measurement is essential to determine hydrophobicity.
As reported in [49], the sliding angle is directly proportional
to the surface wetness. The measurement of CA is affected
by various factors. These are droplet size, water droplets
addition or subtraction rate, and the interval between the
tests and slope angle range [49]. The authors suggest that
measurements should be made frequently and average results
should be considered to achieve good precision. For testing,
the droplet volume range should be from 5 to 150 μL [2].
The hydrophobicity of SiR insulators is inversely proportional
to the soluble electrolytes of the contaminant surface. The
CA (static or dynamic) measurement is more effective than
other conventional methods such as electron spectroscopy for
chemical analysis (ESCA) and the examination of crossover
voltage (COV) [45]. It has been reported in [48] that prolonged
artificial UV aging reduces the hydrophobicity of SiR insulators and can lead to quick flashover. The surface accumulated
water molecules get blocked by resistance at the interface
between the FRP rod and the SiR insulator housing. The
impact of prolonged aging can degrade the interfacial resistance between the SiR insulator housing and the FRP rod.
Consequently, the hydrophilic [-OH] group appears on the
surface of HTV-SiR insulators and reduces the hydrophobic
condition [50], [51].
This article reports a relationship among the four parameters: angle of contact, surface condition of the insulator,
surface contamination, and pollutants accumulations degree
[equivalent salt deposit density (ESDD)]. The relationship
among the above parameters is presented in Table V.
2) Accumulation of Pollutants on SiR Insulator Surface: Environmental pollutants are the leading cause of corrosion of SiR
insulator surfaces. These contaminants come from a variety
of sources. For example, salt is a polluting factor found in
coastal and industrial areas. Similarly, manure, coal, and smog
come from the fertilizer industry, harvesting fields, coal mills,
and automobile emissions. In addition, high relative humidity, rising temperatures, accumulation of hydroxide ions on
the surface, acid rain, and thermal aging reduce SiR insulators’ life [24], [56]–[58]. According to the reported works
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TABLE V
R ELATIONSHIP A MONG THE CA, S URFACE C ONDITION OF THE
I NSULATOR , P HYSICAL C ONDITION OF THE I NSULATOR , AND
C ONTAMINATION L EVEL (ESDD)
TABLE VI
D IFFERENT C ATEGORIES OF C ONTAMINATION ’ S
C LASSES [8], [59], [62]
[59], [60], aforesaid contaminants introduced the dry band
arcing on the surface of SiR insulators. Subsequently, they
can cause a complete flashover of SiR insulators in the worst
environmental conditions [58], [61], [62].
In the coastal area, the soluble salts are mainly NaCl and
Na2 SO4 . These salts are deposited on the insulator’s surface
and make a persistent conductive path with the assistance of
water molecules. LC flows through this conductive path to
the surface of the insulator. Finally, the constant flow of LC
leads to the complete breakdown of the SiR insulators [62].
In addition, the degradation of the SiR insulators is similar to
the tropical rainforest continents [2], [8], [59]. In the tropical
rainforest areas, the pollutant is primarily Kaolin (Al SiO2
O5 (OH)4 ) [59], [62]. The performance of SiR insulators in
desert areas has been analyzed under frequent sandstorms
and temperature fluctuations [52], [54], [63]. Hamza et al.
[63] have reported in their work that due to an electric field,
sand particles become charged and begin to discharge on
the surface of SiR insulators. RTV-SiR and HTV-SiR insulators work well in coastal areas but also degrade due to
high contamination [61], [64]. The ESDD scheme [8], [59],
[60], [62] determines the contamination based on the average
concentration of dissolved salt. The nonsoluble deposit density
(NSDD) [2], [8], [59] is another parameter for measuring surface contamination. As per the recent study, the SiR insulator
surface contamination can be classified into five categories
using ESDD values. The classification of contamination levels
is shown in Table VI.
The procedure for preparing artificial contaminants in the
laboratory is described below. According to IEC-60507, the
slurry is prepared by mixing Kaolin, sodium chloride and
distilled water for research [59], [62]. The slurry of this
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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
TABLE VII
P OLLUTION PARAMETERS B EFORE AND A FTER
THE UV A GING OF S I R I NSULATORS [66]
The depth of the insulator surface is obtained from the
following equation, i.e.:
δP =
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artificial contaminant is applied as a layer on the insulating
surface [shown in Fig. 2(b)] and dried for 24 h. Thereafter,
the intensity of the contamination is determined by the amount
of salt available in the slurry. Table VII shows changes in
salinity levels and ESDD values of SiR insulators before
and after UV aging [66]. The effect of contaminants on the
surface of a SiR insulator depends on where the insulator is
installed.
In addition, the algae, parasitic bacteria, and fungi in rainforest areas may appear on the insulator surface [67]–[70].
Notably, the build-up of algae on the polymeric insulator’s
surface reduces the SiR insulator’s hydrophobicity. The development of any organic layer on the surface of composite polymeric insulators can be reduced by applying flame-retardant
fillers, e.g., Zine Borate [68]. The physical properties of the
SiR insulators are investigated by visual inspection methods, such as cracks in the surface, surface irregularities, surface color change, surface erosion, and shade puncture [71].
In addition, the optical inspection method or scanning electron microscope (SEM) has been used to assess the physical
degradation of SiR insulators [31], [71].
B. Analysis of Chemical Properties by Assessment
Techniques
The significant changes in the chemical properties of SiR
insulators as a result of aging effects are measured using the
following techniques.
1) Fourier Transform Infrared Spectroscopy (FT-IR): FTIR
spectroscopy technique is used to detect organic and inorganic
substances in polymers. FTIR spectroscopy is employed to
obtain a range of wavelengths of electromagnetic radiation
from solid, liquid, and gaseous substances. The various chemical bonds present in the polymeric insulator material can
also be analyzed by FT-IR [72], [73]. In addition, the attenuated total internal reflection (ATR) method detects increasing depolymerization on the surface of SiR insulators after
aging [57], [66], [72]. The chemical bonding of each substance and component type has a unique infrared (IR) signal
absorption frequency. According to the process, if the ratio
of the light energy falling on an object to the light energy
transmitted through the object is zero, it means that the object
has absorbed all the light energy. This ratio analysis determines
the condition of the substances present in the insulator [73].
λ
2 1/2
2
∂
S
2 ∂C sin θ − /∂C
(2)
where λ = wavelength (cm); ∂c = crystal’s refractive index
(KRS5 = 2.38); ∂s = sample’s refractive index (SRI = 1.43);
θ = angle of incident (45◦ ); δp = penetration depth (μm).
In the research works [2], [8], the depth of the SiR surface
was measured using an IR frequency and the range of the IR
wavelengths was found to be 400–4000 cm−1 . Gorur et al. [1]
and [23] reported that after the UV-A aging, the modifications
in the chemical elements in the SiR polymers were observed.
Similarly, ethylene-propylene-diene monomer (EPDM) has
been studied by artificial weather exposure for 90 days, and
changes in EPDM’s mechanical properties, chemical properties, tensile strength, and structural appearance have been
observed using the FTIR spectroscopy method [64], [72], [73].
In [74], the investigation said that after 2000 h, UV-B aging
in HTV-SiR insulators, visible changes have been observed in
the FTIR spectrum in proportion to Si–CH3 and C–H bond.
Li et al. [50] analyzed the changes in chemical structure after
the aging of FRP rods of SiR insulators by FTIR method. The
study’s results suggest that the FRP rod’s aging directly affects
the number of waves within the chemical group. Here, changes
in wavenumber have been identified from FTIR analysis.
2) Energy Dispersive X-Ray (EDX) Technique: The output of
the EDX method is the spectrum that can identify the exact
change in the chemical composition of any polymeric sample.
It has been reported in [2], [8], and [39] that due to the aging
of SiR insulators, changes in the ratios of aluminum (Al) and
silicon (Si) (filler material) have been satisfactorily evaluated
with EDX analysis. Also, the results showed that the amount of
low molecular weight (LMW) SiR polymer chains decreased
due to the aging impact. Reynders et al. [2] reported in their
article that the density of silicon molecules decreases to a
depth of 100 nm from the surface of aged and contaminated
SiR insulators. The physicochemical analysis is performed on
aged SiR insulators to understand the surface morphology of
the insulators [59], [62]. The outcome of the EDX measurement depends on the surface depth of the SiR insulator. The
thickness of the surface is measured by the increased voltage
of the electron beam [39], [49]. The penetration depth of the
X-ray beam is calculated using the following formula [39]:
Aw
D = 0.033 e1.7 − ec1.7
Z n ρSi
(3)
where D is the penetration depth of the X-ray beam, e is the
beam of the electron energy (keV), ec is the binding energy
to excite the X-ray line of activity, Aw is the atom’s weight,
Z n is the atomic number, and ρSi is the material’s density.
The article reported [74] that the SEM-EDX technique analyzed the changes in their chemical compositions after the
aging of SiR insulators, and the test outcomes are shown
in Table VIII.
Similarly, the development of biological microorganisms on the surface of high voltage (HV) polymeric
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TABLE VIII
C OMPARISON S TUDY OF SEM-EDX R ESULT [74]
TABLE IX
R ESULTS FOR HTV-S I R I NSULATOR S AMPLES A FTER THE AGING T EST
U SING XPS A NALYSIS U NDER D IFFERENT C ONDITIONS [3], [78]
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insulators can be effectively observed by EDX-based
analysis [8], [55], [68], [69].
3) X-Ray Diffraction (XRD) Technique: The XRD technique
can analyze the polymeric materials’ crystal structure [75].
The fundamental law behind the XRD method is Bragg’s diffraction law. In the XRD event, X-rays fall on an element, and
then the intensity of the radiation emitted from the component
and the scattered angle of the irradiated ray is measured. From
these values, the material’s crystal structure can be analyzed.
The RTV-coated SiR polymers are physically corroded due to
thermal aging and dry band arcing [76]. The detailed XRD
analysis has been reported in [75] and [76]. A comparison
study was conducted by Verma et al. [77] between two types of
HTV-SiR insulators [i.e., HTV1S1 (fresh) and HTV2S2 (aged)]
and they implemented artificial UV-C aging on two HTV-SiR
variants. After the XRD analysis plot, the result indicates that
the samples became more crystalline after aging.
4) X-Ray Photoelectron Spectroscopy (XPS) Technique: The
XPS method helps to measure the basic structure of the spectrum of photoelectrons emitted from the surface of an aged
sample. A detailed discussion on the XPS measurement procedure has been reported in [3] and [78] (shown in Table IX).
It has been reported in [32] that with prolonged artificial aging, the concentration of the chemical elements of
SiR insulators such as silicon (Si) and carbon (C) has been
decreased. In contrast, the concentration of oxygen (O2 ) has
increased. Chakraborty and Reddy [3] performed the aging test
on three different specimens under three external conditions
and the result of the XPS analysis is shown in Table IX.
From Table IX, it is concluded that the oxidation process
occurs in the outer layer of the SiR insulator, increasing the
concentration of oxygen elements.
5) Secondary Ion Mass Spectroscopy (SIMS) Methodology:
The method is utilized for analyzing the modification in the
synthetic structure of SiR protectors after aging like XPS
analysis [2], [8]. The photoelectron effect plays an essential
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role in this process [79]. SIMS is used to examine the emitted photoelectron’s amount and energy from the elements of
SiR insulators (excluding hydrogen) after aging. It is noted
that each element has a unique bond energy value and that
bond energy depends on the electron induction present in
that element. This technique can also estimate the nature of
the chemical bonds between the elements. Xilin et al. [79]
conducted an aging experiment on HTV-SiR samples and the
chemical analysis of insulators’ samples was performed using
SIMS methodology. Test results showed that Si, C, and Al
molecules disappeared from the surface of the SiR insulator
after the aging process. In addition, the aging process causes
the Si–O bond to become fragile and as a result, the Si
molecules begin to loosen rapidly. Similarly, the amount of
C molecules decreases, and CO2 is released due to oxidation.
The Al particles migrate from the surface of the insulator as
they begin to react with acidic substances in the environment
after aging, resulting in the formation of Al2 O3 particles.
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6) Surface Roughness Methodology/Scanning Electron
Microscopy Technique: The morphology of the harshness
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of the surface of SiR insulators is analyzed by scanning
electron microscopy [3]. Pollutants deposited on the surface
of the SiR insulators also deteriorate the surface’s chemical
contents [80]. In [81], after artificial UV-C radiation,
the changes in the SiR insulator were investigated by
SEM spectroscope between pure and aged samples. It is
noteworthy that the surface degradation of the sample
of aged SiR insulators was greater than that of the pure
sample because oxidation occurred in aged SiR samples.
Moreover, the surface degradation of SiR insulator samples
is higher in UV-C radiation than in other UV radiation [81].
Habas et al. [80] reported that the SEM technique was
performed on the SiR samples at high vacuum mode to avoid
charging. Similarly, Chakraborty and Reddy [3] reported that
the gold sputtering method helped to eliminate the charging
effects. The findings revealed that the SiR insulators’
sample was smooth, homogeneous, and had less porosity
on the surface before UV aging. But the overall structural
morphology has changed after aging [3], [48]. In addition,
in [53], investigations have shown that any type of brittle
fracture and decay-like fracture of the FRP rod of 500-kV
composite insulator due to aging and corrosion-like fractures
can be analyzed through the SEM method.
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7) Gas Chromatography (GC)/Mass Spectroscopy Technique: The procedure of GC is the technique for isolating
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chemical compounds in analytical chemistry. The chemical
compounds used to analyze in GC are those compounds that
can evaporate without decomposition. This method also helps
to determine the mass of molecules of various chemical elements of a gas. The GC method can analyze the impact of
surface pollution levels on SiR insulators’ LMW elements
[82], [83]. Gustavsson et al. [82] reported that the spot discharge on the SiR surface directly depends on LMW dimethylsiloxanes. The GC method has been used in studies [7], [83]
to understand the properties of LMW molecules scattered
from SiR insulator polymers due to contamination. In [83],
GC assessed the RTV-coated SiR insulator’s surface erosion. The GC test was performed using two RTV-coated SiR
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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
TABLE X
R ESULTS OF SE M ETHOD OF HTV-S I R [84]
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insulators (i.e., aged sample and unaged sample). After the
GC test, it was found that the aged sample had a continuous
peak of LMW siloxane due to depolymerization. As a result
of aging, these LMW siloxanes move away from the surface
of the insulator, causing the RTV-coated SiR insulator surface
to deteriorate.
8) Solvent Extraction (SE) Technique: SE is the widely used
procedure to determine the chemical analysis of the aged
SiR substance. SE method is conducted through four stages.
Initially, the solvent enters the solid Structure; then, the dissolved substances are diffused into the solvent; subsequently,
the dissolved material is spread around the surface of the
SiR insulator. Finally, the extracted solution is collected for
analysis [7], [8]. In this technique, the LMW components
are extracted from the surface of the SiR insulator. A more
accurate idea of surface erosion of the aged SiR insulator can
be obtained using these extracted components in the FTIR
and GC methods [82], [83]. In [84], the post-aging properties of the surface of the HTV-SiR insulator were analyzed
by the SE method. The HTV-SiR component comprises two
groups of materials: high compatible rubber (HCR) and liquid
silicone rubber (LSR). In this process, at room temperature,
the sample was immersed in toluene for 96 h. Afterward, the
LMW particles were extracted once the solvent had evaporated
entirely. The change in sample weight before and after evaporation measures LMW. After the SE technique, a comparative
analysis of LMW between HCR and LSR has been shown in
Table X.
Also, the aged surface of the SiR insulator sometimes contains negatively charged or positively charged particles. These
particles can be detected by COV analysis [85].
9) Terahertz and Laser-Induced Spectroscopy Technique:
The aging conditions of the composite insulators can be
assessed after measuring dielectric properties by the terahertz
technique. Research from the last decades revealed another
diagnostic tool, terahertz spectroscopy, which can predict
material aging, potential defects in the insulators, and so on
[86], [87]. Terahertz spectroscopy is based on the terahertz
wave, which ranges from 0.1 to 10 THz (i.e., wavelengths
range from 3 mm to 30 μm) [88]. Reasons behind the use of
terahertz spectroscopy are good resolution both in time and
frequency domain than other nondestructive testing (NDT),
easy to operate, suitable for complex test structural material,
less ionizing effect, and strong penetrability and that is why
this spectroscopy can be used for aging assessment of SiR
composite insulators [86], [88]. Despite the aforementioned
advantages, only independent terahertz spectroscopy analysis
is not suitable for minor errors due to its bipolar pulse and
limited pulsewidth that creates overlapping of the signals [86].
Three types of artificially made defects, such as single air gap
defect, inclusion defect, and double-layer air gap defect, were
investigated and their impacts on composite insulators were
analyzed by terahertz spectroscopy with the combination of
deconvolution method (to overcome the overlap), reported in
[88] and [100]. The authors also suggested that these methods can be used for the internal defects of the composite
insulators. Mei et al. [88] reported in their article that the
properties of five types of composite insulators (i.e., epoxy
resin, epoxy glass fiber, XLPE, porcelain, and HTV-SiR) were
measured using terahertz spectroscopy and the broadband
dielectric spectrometer (with the relatively low-frequency band
(i.e., 1 Hz–1 MHz) and a comparative analysis was carried out
for both the techniques.
Nowadays, it is an area of concern for the researcher to
diagnose the degradation of the insulators remotely. For this
purpose, laser-induced breakdown spectroscopy (LIBS) can
be employed to estimate the degradation of the insulator
electrically and chemically [89]. This spectroscopy has some
advantages like noncontact analysis, high spatial resolution,
and multielement analysis, and qualitative and quantitative
analysis can be carried out [89], [92]. The application of this
spectroscopy includes detecting the contaminants present in
the insulator’s surfaces and identifying the changes in the
structural morphology after aging [90]. Thus, early flashover
of the insulator can be predicted by detecting the accurate
contamination in the insulator surface [91]. A detailed study on
LIBS has been demonstrated in [91]. Wang et al. investigated
the spectral properties of extinct components of HTV-SiR
insulators from its surface after aging and the change in
temperature properties from plasma was also reported. That
will help to predict the service life of HTV-SiR insulators [91].
Also, the spread of fungi and algae on the surface of the
SiR insulators is clearly understood by applying laser-made
Florence spectroscopy [55], [68], [69], [73].
C. Assessment Techniques for Analyzing Electrical
Properties
Different electrical methods help to analyze the electrical
properties of SiR insulators. These are: 1) LC; 2) partial
discharge (PD); 3) corona discharge; and 4) flashover voltage (FOV) analysis. A detailed description of these analyses
is described in Sections I–IV.
1) LC Analysis: Surface LC investigation is a
well-recognized strategy for identifying the differences
within the electrical parameters of the SiR insulators. The
surface condition of the SiR insulator has been responsible for
the amount of LC flow [92]. Due to the high hydrophobicity,
LCs become negligible in the early stages of aging. As the
aging progresses, the hydrophobicity of the insulator decreases
and the flow of LCs increase [66], [92]. SiR insulators are
installed in outdoor applications. The environmental impact
on them is also severe. At the early phase of activity, the
SiR insulator behaves as a capacitor and the profile of
surface LC is like a capacitive sinusoid. When the surface
of the insulator becomes contaminated, the surface becomes
conductive. As a development, the resistive surface LC begins
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SIT et al.: REVIEW ON CHARACTERISTICS AND ASSESSMENT TECHNIQUES OF HIGH VOLTAGE SiR INSULATOR
TABLE XI
C OMPARATIVE A NALYSIS ON LC F LOW, P OWER D ISSIPATION , AND
S URFACE R ESISTANCE OF AGED S I R I NSULATOR [97]
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to flow and the wave shape of the surface LC current is
distorted as time progresses [94]. It has been observed that
the deformation of LC is related to the surface condition and
the chemical property of the contaminant layer, environmental
temperature, humidity, and acid rain [6], [8], [45], [94]. The
presence of odd harmonics in the LC signal distorts the
nature of the signal, which is related to the poor condition
of the surface of the SiR insulator [66], [69], [93]. It is
reported in the articles [66], [94] that the fifth harmonic
component is more responsible for this distortion of LC than
the third harmonic component. It has also been reported
that in light surface contamination, the total harmonic
distortion (THD) of the LC gradually increases with relative
humidity [95].
It is noteworthy that under the influence of wet conditions,
the magnitude of LC in EPDM, glass, and porcelain type
insulators is significantly lower [6], [62], [93]. The flow of
LC current in SiR insulators is higher in humid environments in comparison with dry environments [96]. In [96],
it has been reported that the resistive LC is more destructive
to the surface of the SiR insulator than the capacitive LC.
The ESDD method detects the total amount of contaminants
deposited on the surface of the insulator [62], [92]. In [97],
under humid conditions, the permeation of water molecules
in the FRP rod and SiR interface has been studied through
rotation wheel tests and analysis of surface LC flow. Also, the
authors performed an FFT analysis of LC data to understand
the development of the arc. Also, the impact of arcing was
analyzed by calculating the relative magnitudes of the third
and fifth harmonic components. A comparative study among
the three parameters from the initial period to the final period
of aging is shown in Table XI. It is relevant to mention
that the aging condition of SiR insulators can be analyzed
by observing the actual displacement of the phase angle of
LC and the nature of the LC. The nature of LC waveforms
has been monitored in the laboratory by adopting various
schemes [42], [62], [94].
Some well-established feature extraction and classification
techniques have been used on laboratory LC as well as field
LC measurement data to detect contamination levels and aging
of insulators [42], [59], [62], [94]. Also, some research has
been reported on the influences of different electrical voltage
levels (e.g., ac or dc) during the study of the LC measurement
[66], [82]. Gustavsson et al. [82] reported that the creepage
distance of the SiR insulator and the amount of ATH filler used
in the SiR insulator are two more necessary parameters that
control the flow of LC on the surface of the SiR insulator. The
magnitude of the LC for both porcelain (IP) and SiR insulators
depends on three environmental factors like illuminance,
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TABLE XII
P ORCELAIN AND S I R I NSULATORS LC’ S D EVIATIONS W ITH
E NVIRONMENTAL C ONSTRAINTS [93]
Fig. 4. Patterns of the LC waveform vary with the contamination levels.
(a) Very Light, (b) light, (c) moderate, (d) high, and (e) extremely High
contamination, respectively.
temperature, and relative humidity [93]. The comparative findings among the aforesaid factors are shown in Table XII.
In addition, In Fig. 4, the LC waveform of an 11-kV rated
SiR insulator for 10-kV applied voltage under five types of
contamination levels is shown [42], [62].
Pylarinos et al. [94] showed that in their study, any progressive LC waveform could ultimately lead to the flashover of
the SiR insulators. The time-based monitoring scheme can be
used to clean the contaminated surface of SiR insulators, consequently reducing the effect of surface LC. In contrast, this
monitoring method is not useful for the completely degraded
surface of the SiR insulator. For this reason, the LC measurement method is the early indication for detecting degraded
conditions of the SiR insulator.
2) PD and Corona Analysis: It has been reported that the
onset of PD event is due to uneven voltage stress, production error, contamination intensity, humidity, dry band effect,
and accumulation of water droplets on the surface of SiR
insulators. Compared with the HVDC system, the PD inception electric field strength intensifies for the high voltage
ac (HVAC) system. The time duration of the PD event lies
in-between nanoseconds to microseconds [8]. Using suitable
micro or nanofillers can enhance the electrical properties of
SiR insulators, such as dielectric strength, tolerance to induction voltage, and preventing PD [7]. Ullah and Akbar reported
in [98] that the surface degradation of SiR insulators could
be determined by analyzing the nature of the PD. Ullah and
Akbar [98] performed a PD test in the laboratory (according
to IEC-60507 standards) on an 11-kV rated SiR insulator
with variable ESDD values. The test consequences exhibited that the magnitude of PD raised with increasing values
of ESDD, but during the experiment, the relative humidity
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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
value was unchanged [98]. Schneider et al. [99] observed that
the average PD value for artificial UV-aged HTV-SiR insulator samples increased with increasing applied voltage. Also,
Schneider et al. [99] reported a comparative PD analysis
between the pure and aged (UV aged approx. 5000 h)
HTV-SiR samples, and the results showed that surface degradation was more pronounced in the aged samples. Similarly,
the nature of the PD pulses can be measured by video apparatus [100], image strengthening UV videography [2], [8], and
thermographic video recorder [2], [8], [9]. Sometimes, the PD
can be detected using audio amplifiers [2] and communication
devices [100]. In addition, the video thermography measurement helps to calculate the power loss from different types of
surface discharges [101]. The arc resistance measurement is
another methodology to analyze the SiR insulator’s localized
discharges [102]. It is reported [103] that an alternating electric
field under the HVAC system predominates in the perimeter of
the insulator and this helps to strengthen the PD phenomenon
even faster. Physical degradation of the insulator begins when
PD begins. The authors suggest that analyzing the PD event
at the early stage of aging with the cleaned surface of the SiR
insulators is very difficult. Another symptom of local discharge
on the surface of the insulator is corona discharge. It is also
worth noting that corona or surface discharge problems are
more severe in HV-ac systems than in HVDC [104], similarly, the other factors, such as relative humidity, temperature,
deposited water droplet’s size on the surface of SiR insulators,
the diameter of conductors, and the surface area of SiR insulators [104], [105], intensify corona discharge. At higher relative
humidity, the frequency of corona events increases rapidly;
corona emission power also increases. Lan et al. [32] revealed
that RTV-SiR insulators had better corona resistance than
HTV-SiR insulators. Extensive research has been conducted on
the effect of electrical discharge on HTV-SiR insulators, and it
has been found that adding nano/microsilica or ATH fillers can
reduce the effect of electrical discharge on SiR insulators [81],
[105]. Studies have shown that the effect of the corona is first
seen in water droplets that accumulate on the surface of the
insulator [106], [107]. Zhu et al. [106] reported in their article
that the corona discharge phenomenon was investigated on
HTV-SiR insulators by a laboratory-made parallel needle plate
electrode system, and the changes in the physico-chemical
properties of the HTV-SiR insulators were reported in this
work. After corona discharge, the hydrophilic state of the
SiR insulator develops, resulting in erosion on the surface
of the insulators [107]. It has been reported in [101]–[106]
that any surface discharge phenomenon degrades the surface
resistance of SiR insulators. The surface discharge is one
of the most threatening agents that creates the depletion of
the surface elements (up to 20-nm depth) [107]. Any longterm surface discharge permanently damages the SiR insulator’s properties [104], [105]–[107]. The chemical analysis
of corona aged surface of SiR insulators can be analyzed
using nuclear magnetic resonance (NMR) detection, scanning
electron microscopy, GC and FTIR techniques [107]. It is
clearly understood that the corona/surface discharge analysis
depends on various uncontrollable factors.
TABLE XIII
R EPRESENTATION OF THE O DD LC H ARMONICS IN S URFACE
F LASHOVER B EFORE AND A FTER THE UV E XPOSURE [48]
3) Analysis of FOV: FOV analysis is the most common
electrical diagnosis method to measure the aging state of SiR
insulators. The power frequency pollution FOV test can be performed either at the ac or dc voltage level [2], [8], [66], [82].
The insulator’s power frequency pollution flashover test is
greatly affected by the insulator’s surface properties. Any
electrical discharge on the surface of the SiR insulator can
ultimately lead to the flashover event. After the flashover
event, SiR insulators start losing their physical, chemical, and
electrical properties [48], [108]. Khatoon and Khan [108] that
the value of FOV obtained after testing at ac-voltage levels was
higher than that of dc-voltage levels and performed a comparative analysis of pollution FOV measurement in dry and wet
type porcelain and SiR insulators. After the pollution FOV test,
the result said that the SiR insulator performs well (i.e., 5%
better) under a highly polluted climate than the porcelain insulator. Ahmadi-Jonidi et al. [48] reported that the surface of SiR
deteriorates after artificial UV-aging and the long-term effects
of artificial UV-aging lead to the complete breakdown of the
SiR insulator. Ahmadi-Joneidi et al. [48] performed three types
of FOV tests with the effect of artificial UV exposure. The
comparative analysis based on odd LC harmonic values is
shown in Table XIII. There are two types of FOV testing methods used to analyze the electrical property of SiR insulators
such as the rapid-flashover clean fog (RFOCF) method and the
quick-flashover salt fog (QFOSF) method [109]. It has been
reported [99], [109] that salinity levels are directly correlated
with the surface resistivity and aging condition of SiR insulators. Due to the high salinity level, a uniform conductive layer
is formed on the surface of the SiR insulator. As a result, the
dry band is created around the circumference of the insulators.
After the dry band arcing, the Si–CH3 and C–H bonds of the
SiR insulator become fragile. Also, the filler particles (mainly
ATH) disappear from the surface after the dry band arcing,
resulting in degradation of the hydrophobicity property. The
combined effect of all types of surface discharge eventually
leads to the FOV event [48], [109].
Albano et al. [110] investigated the dry band flashover
phenomena on SiR insulators by the clean-fog test along
with the IR image analysis and surface LC measurement. The
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SIT et al.: REVIEW ON CHARACTERISTICS AND ASSESSMENT TECHNIQUES OF HIGH VOLTAGE SiR INSULATOR
TABLE XIV
C OMPARATIVE A NALYSIS OF T RACKING AND E ROSION
P ROPERTIES OF VARIOUS S I R A DDITIVES [115]
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highlighted some of the information that artificially contaminated and wet surfaces of SiR insulators are the leading cause
of dry band formation. Also, they concluded that the rapid
formation of dry bands might cause a flashover event.
4) Analysis of Erosion and Tracking Resistance: Erosion and
tracking resistance are the two essential parameters for investigating the electrical characteristics of SiR insulators. These
two tests can be used in SiR polymeric insulators as per
IEC TR 62730:2012 standard [123]. The eroded SiR insulator
means reducing the weight of the materials during the manufacturing process. In contrast, the carbonization process helps
to construct conductive paths on the surface of the insulator
by electrical tracking. It has been observed that LC flows in
the tracks, which causes insulation breakdown. The continuous
surface discharges cause thermal degradation, leading to the
erosion and tracking of SiR insulators. The comparative index
and dry band arcing tests are the two well-known techniques
used to analyze the tracking and erosion properties of SiR
insulators [110], [111]. Nazir et al. [112] proposed an inclinedplane test (IPT) method that can measure the resistance of
erosion and tracking of the SiR insulators. The standards of
IEC-60587 and ASTM-D-2303 were followed during the IPT
measurement [111], [113]. Asad et al. [114] described the IPT
method step by step in their article. The minimum voltage for
conducting the test should be more than 6 kV [113]. Dutta
and Dwivedi [115] presented a comparative analysis among
ATH, silica, and melamine cyanurate filler-based SiR samples
for investigating the tracking and erosion resistance. They
also reported that silica (100 phr) and melamine cyanurate
(15 phr) filler-based SiR samples hold good tracking and
erosion resistance. The findings of the aforesaid assessment
are tabulated in Table XIV.
Also, it has been found that samples made of melamine can
extinguish the electrical arcing. Schmidt et al. [116] reported
that ureido-modified MQ silicone resin (DIPUPES-MQ) could
perform well than addition-cure liquid silicone rubber (ALSR)
because DIPUPES-MQ has boosted tracking and erosion resistance. The aforesaid techniques are not applicable at lowvoltage applications [8] but are used to analyze the tracking
and erosion properties as well as to analyze the impact of
dry-band arcing on SiR insulators [117].
The fillers play a vital role in controlling erosion and
tracking event in SiR insulators. Applying nanoscale fillers
such as alumina makes SiR insulators better surface resistant to contaminant circumstances [103]. Nazir et al. [22]
and Guo et al. [117] investigated and concluded that SiR
1899
TABLE XV
C OMPARATIVE A NALYSIS OF THE P ERFORMANCE OF VARIOUS
I NORGANIC F ILLERS OF S I R I NSULATORS
insulators are made with three types of economical fillers.
These are ATH, AlN and BN [22], [26], [27]. A way
to add more silicone bonds to the crosslinking fashion
that improves the tracking and erosion resistance properties
of SiR insulators [116], [117]. Similarly, the nanodoping
of silica (SiO2 ) is used in the SiR insulator to improve
ac corona resistance, enhance the tracking resistance, and
decrease erosion [103], [104]. Adding barium titanate as
a filler to the RTV-SiR component may reduce erosion
and increase tracking resistance [111]. El-Hag et al. [118]
observed that the SiR insulators could achieve higher tracking
and corrosion resistance if the insulator material was fabricated
by the electrospinning method. In addition, the tracking and
erosion resistance of SiR insulators can be improved by adding
BN fillers with ATH components [119]. The effectiveness of
inorganic fillers in SiR insulators can be evaluated based on
thermal conductivity, thermal resistance, mechanical strength,
electrical conductivity, hydrophobicity, and electrical trace
resistance. The detailed evaluation study on the effectiveness
of inorganic fillers is tabulated in Table XV.
VII. S UMMARY AND S UGGESTIONS
This manuscript reports a comprehensive study of degradation in the physical, chemical, and electrical properties because
of the aging effects on SiR insulators. In this section, the
authors have summarized their views on the aging assessment
study on SiR insulators and present some insights about some
research areas that further help to plan for future exploration.
According to the above discussion, it may be suggested that
HTV-SiR insulators are best suited for outdoor transmission
and distribution line insulator applications compared to other
SiR insulators [77]–[81], [84], [88]. As per the literatures [6],
[10], [12], [16], [28], the influence of the aging of SiR insulators affects the housing of SiR insulators and erodes the FRP
rods of SiR insulators. It should be mentioned here that the
deterioration of the FRP rod is a consequence of the inclusion
of water molecules in the FRP rod through a vacuum in the
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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, VOL. 29, NO. 5, OCTOBER 2022
SiR housing or due to the weakening of the interfacial surface
resistance and the formation of leakage paths in the metal ring
sealant [6], [53], [96]. The factors that rapidly trigger aging
are pollution severity, snow, rainfall, sandstorms, fog, mist,
humidity and moisture in the air. Among the factors mentioned
above, the review suggests that the two most vital factors that
should be prioritized in the aging assessment study are the
pollution severity and the moisture content in the air. In this
article, the comparative analysis among the fillers used in
SiR insulators is based on electrical, thermal, mechanical, and
chemical properties, as shown in Tables I and XV. In addition,
according to the research studies, the authors recommend that
ATH [20]–[22], [104], [116], SiO2 (5 wt.% nanosilica) [105],
hybrid nanofillers (0.6 wt.%) [114], and BN [22], [26], [27]
exhibit a more efficient electrical, mechanical, and chemical
performance under contaminated conditions irrespective of the
level of voltages. As per the research developments, it may
be further suggested that incorporating nano-sized silica in
SiR insulators may be the most effective approach to enhance
corona resistance, tracking resistance, and erosion resistance
[72], [104], [118].
According to recent research studies, it is noteworthy that
hydrophobicity [46], [47], [57] and ESDD analysis [59]–[66]
are the most effective techniques for estimating pollution levels
and for the aging estimation of SiR insulators. Moreover,
it should be mentioned that the chemical studies of the aging
assessment of SiR insulators used in dc and ac transmission
systems are very effective. It is recommended that a few chemical analysis methods like FTIR, SEM, EDX, LIBS, XRD, and
terahertz spectroscopy can effectively be used to estimate the
lifespan and aging state of SiR insulators.
As reported in [92]–[97], surface LC analysis can be a good
indicator for the aging assessment study of SiR insulators.
The reason behind this fact is that the effect of LC can
be minimized after cleaning the contaminated surface of the
SiR insulators through a time-based monitoring scheme if LC
is predominantly due to surface conditions. However, if the
inherent condition of the insulator is degraded, any surface
treatment would not improve the assessment results. It is
noteworthy that the LC measurement can detect the degraded
state earlier than the other electrical methods for this type of
deteriorated insulator.
VIII. C ONCLUSION
This article is a detailed research survey on the effectiveness
of HV overhead line SiR insulators in different environmental
conditions. The conclusions of this review article are summarized below.
The chemical analysis of the SiR insulators has been investigated. Also, the role of fillers as primary agents has been
revealed. A comparative analysis among the fillers has also
been studied to understand how they are employed to enhance
the electrical, thermal, mechanical, and chemical properties of
the SiR insulators.
It is noteworthy that any progressive aging in SiR insulators
initiates changes in the chemical composition and surface
morphology of the SiR insulators (including insulators housing
and FRP rod). Surface roughness, and methods of measuring
contaminants on insulator surfaces, such as hydrophobicity and
measuring CA, analyzed by ESDD, are presented here through
various research articles.
The changes in the chemical properties of SiR insulators
after aging have been described in detail in this article. Analytical techniques to identify chemical changes are FTIR spectroscopy, EDX, XRD, XPS, SIMS, surface roughness, GC,
and SE. Also, the findings of the research work for different
chemical analyses are reported in this article.
In Section VI-C, the various electrical analytical methods of
SiR insulators after aging have been discussed. The variations
in the electrical characteristics of the SiR insulators due to
aging and contamination have been investigated in various
studies by LC, PD, corona discharge analysis, and FOV measurement, which are highlighted in this article.
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