21, rue d’Artois, F-75008 PARIS
http : //www.cigre.org
B2-214
CIGRE 2012
ASSESSMENT OF THE CONDITION OF OVERHEAD LINE
COMPOSITE INSULATORS
F. SCHMUCK
Pfisterer Sefag
Switzerland
J. SEIFERT
Lapp Insulators
Germany
I. GUTMAN
STRI
Sweden
A. PIGINI
Consultant
Italy
SUMMARY
Composite insulators have become a highly developed alternative to conventional insulators
in all transmission level voltage classes for AC and DC applications around the world. However, an important issue needs to be addressed: the difficulty in which to assess their conditions in service, especially before the application of live line working (LLW) techniques for the
replacement or repair of insulators, insulator strings, dampers or the conductor. Despite
broad experiences and advanced diagnostic tools are presently available, LLW on overhead
lines with composite insulators has not yet been introduced in the present standards and
international standardization committees work on this matter. This paper presents the current
status of the work in CIGRE Working Group B2.21, which is participating in this topic as well.
It can be shown that the diagnostic tools, first introduced more than fifteen years ago, have
been further developed and if correctly used, provide sufficient information to evaluate the
risk of the insulators subjected to LLW. Examples of how LLW is used in various countries
are presented.
KEYWORDS
Composite Insulators, Diagnostics, Life Estimation, Live Line Work, Corona
frank.schmuck@sefag.ch
1
STATUS QUO OF COMPOSITE INSULATOR USAGE
While the first application of composite insulators for the distribution network was seen in the
nineteen seventies, the first industrial application of transmission class composite insulators
began in the nineteen eighties. In the beginning, composite insulators were rather expensive
and their application was limited to special areas, e.g. for areas requiring pollution performance above that of conventional insulators or areas affected by vandalism. However, with the
definition of the “minimum requirements” for the composite insulator design in 1988 by
CIGRE Working Group (WG) 22.10 [1] (the base for the first IEC 1109 edition in 1992 [2]),
significant technological developments were triggered. At present, composite insulators are
often more competitive in comparison to the conventional glass and porcelain insulators,
because of factors like economies of scale in production. Many advantages of composite
insulators have led to their wider use in transmission lines. New opportunities leading to an
even wider use of composite insulators are as follows:



With the introduction of UHV lines such as already existing 1000 kV AC or 800 kV DC
and even discussed 1200 kV AC, considerable cost savings are possible when light
and relatively short (in comparison to porcelain longrods) composite insulator strings
are used. Insulators with lengths of 10 m and longer can be currently manufactured in
one piece, as well as providing mechanical ratings up to 1000 kN and more.
Due to the shift towards “green energy generation” in many countries, a voltage upgrade of existing transmission lines or voltage conversion of the existing transmission
lines from AC to DC are the most practical ways to comply with the variations in energy flow. Depending on how much of additional power is required to be transported
using the same towers and in some cases the same conductors, such refurbishment
generally requires the use of composite insulators. This is also valid for AC/DC conversion, where composite insulators may provide the preferred solution due to their
advantages in terms of pollution performance [3], [4].
Furthermore, the use of compact lines with aesthetically pleasing designs definitely
requires the thin silhouette of composite insulators [5].
Millions
To measure the composite insulator penetration into the market and the insulator reliability,
CIGRE WG B2.21 is currently working on a questionnaire, which will be sent to utilities
worldwide. Their responses will form the basis for a further survey which will update the previous questionnaires from 1990 [6] and 2000 [7]. Today, based on preliminary and sparse
information, only an estimation of the growth for AC can be made as shown in Fig. 1 [8], indicating an exponential trend.
25
20
Installed Composite Line Insulators ≥ 69 kV AC
15
10
5
0
1975
1980
1985
1990
1995
2000
2005
2010
2015
Year
Fig. 1: Estimated growth in composite insulator use in AC-transmission lines
With regard to DC transmission lines, according to another survey by CIGRE WG 22.03 in
1995 [9], the initial application of composite insulators started in the early nineteen eighties
as well. More than 11`000 composite insulators are presently installed in Chinese ± 500 kV
1
and ± 800 kV transmission lines. Extrapolating the limited information available, the number
of composite insulators installed in DC lines worldwide may be of the order of 100`000 [8].
The number is much lower than for AC, reflecting the current ratio between DC and AC
transmission lines. However, as with AC, the increase in the use of composite line insulators
for DC lines seems to be exponential too. As a matter of fact, the advantages of composite
insulators in DC are even more evident than in AC, because of the superior performance in
comparison to ceramic insulators under pollution, which is a dimensioning factor for DC
overhead lines [10], [11].
Failure Distribution of totally 315
As far as composite insulator reliability is concerned, an analysis by EPRI on the failures
reported in the USA [12] is shown in Fig. 2. With an estimated number of 3 million insulators
installed at that time, 315 failures were reported with a cumulative failure rate of 0.0105 %.
The failures by brittle fracture and flashunder (i.e. by full or partial interface breakdown between the housing and the rod) can also be related to other primary root causes such as
housing damage and rod exposure. As mentioned, the failure rate is a cumulative one and a
large contribution to it is from the failures of the first generation of composite insulators with
their well known weaknesses. As an example, the high number of brittle fracture failures is
related to the use of rods with E-glass impregnated with polyester resin in the first insulator
generations, which were widely installed in the USA network. With the progress in technology, electrochemically resistant glasses, such as ECR or E-CR, have become the standard
for long rod insulator applications, thus limiting the brittle fracture risk as pointed out in the
recently published IEC 62662 [13] and in [14]. The today’s generation of composite insulators
is characterized by material components and manufacturing processes, which are proven by
long-term outdoor experience.
50%
45%
EPRI Report on Failures in USA
40%
35%
30%
25%
20%
15%
10%
5%
0%
Brittle
Interface Discharge Mechanical Flashover Loss of End
Fracture (Flashunder) Rod Failure Failure
Fitting by
Pullout
Fig. 2: Failures of composite insulators (EPRI survey)
Another survey [15] describes the failure records of composite insulators in China, where
impressive electrification programmes have been launched and where composite insulators
have now become the prevailing insulator type. 58 failures are reported in the survey in relation to a number of installed composite insulators of 2.2 million by the end of 2006. Comparing the data of Fig. 3 with the statistics from the USA (Fig. 2), the range and causes of failures are quite similar. The Chinese cumulative failure rate is only 0.0026 %, which is lower
than that reported from the USA. However, it should be noted that in the report from the
USA, the first generation of composite insulators is included, which is recognized to have a
higher failure rate because of the stage of product development and standard availability
during the time of their installation.
2
Installation
Fault
Sealing
Interface
Failure
Mechanical
Damage
Brittle
Fracture
Product
Failure
Number of Failures: 58
Reporting Period until 2006
Ageing
Distribution in %
100
90
80
70
60
50
40
30
20
10
0
Failure Reason
Fig. 3: Survey from China – failures of composite insulators
The reliability of the today’s generation of composite insulators, manufactured in accordance
to the latest technology including a tight quality control and traceability is on par with that of
ceramic insulators [8]. It is important however to stress that this level of reliability can only be
reached, provided:



2
The insulators characteristics (e.g. specific creepage distance) are correctly selected
taking into account all the parameters which can affect their life (e.g. pollution condition of the site, mechanical stresses foreseen etc.).
They are correctly handled and installed.
Appropriate string design is used, including grading rings for grading of the E-field
and corona and power arc control devices are correctly coordinated.
LIVE LINE DIAGNOSTIC METHODS AND PRINCIPLES
An important issue that limits an even wider application of composite insulators is the challenge to assess their conditions in service and especially before the application of LLW techniques for the replacement or repair of insulators, insulator strings or dampers or to repair the
conductor. In particular, LLW on overhead lines equipped with composite insulators are not
covered yet in the present standards and WG`s in IEC, CIGRE, IEEE are working on that
issue. Some of the aspects under discussion are analyzed in the following and in particular:


The progress in live line diagnostic methods and principles and methodology to evaluate the condition of insulators in service.
The methodology to assess the conditions of the insulators specifically before LLW to
ensure a safe operation.
Currently, both CIGRE WG B2.21 and IEEE 15.09.04.01 are working to update the state-ofthe-art of live line diagnostic techniques for composite insulators. However, it is important to
state that the principles of diagnostic techniques remain those identified in [16]:




Visual inspection and hydrophobicity assessment
Infrared (IR) thermography
Ultra-violet (UV) detection
E-field measurements.
3
Visual inspection and IR/UV methods can be used both from the ground and from the air [1721]. Usually a helicopter is used for aerial inspections; however many more exotic devices
from mini-helicopters to Zeppelins are on trial [21]. Despite the fact that the diagnostic technique principles are mostly the same as in [16], the diagnostic tools, like different camera
systems, have been dramatically improved, making diagnostics much easier today. Furthermore, R&D activity has provided much better methods for interpretation of the measurements, which was a previous bottleneck, reducing their usage.
2.1
VISUAL INSPECTION
Visual inspection is presently the most commonly used inspection technique. It can be employed remotely from a long distance as well as by close-up visual inspections. Binoculars or
telescopes are used to perform remote visual inspections. Better efficiency may be obtained
when the inspections are made as close as possible to the insulator, e.g. operating from a
tower, from a helicopter (including mini-helicopters), or from a bucket truck. A number of
practical guides for visual inspection are available from CIGRE, EPRI and STRI. The guides
typically include detailed descriptions of different types of possible defects with carefully selected colour photographic examples, enabling field personnel to quickly locate the photograph(s) and definition(s) of interest with respect to insulator deterioration and/or damages.
CIGRE WG B2.21 collected and summarized the gained experience in the Technical Brochure TB 481, published in December 2011 [14]. In particular, it is important to define the
criticality of a damage/defect, enabling required actions to be selected. The TB 481 can also
assist in this regard.
A simpler defect classification is necessary regarding LLW, as only conductive or semiconductive defects are recognized to be critical. This assumption is valid for the enforced rule
that LLW is only permitted under dry weather conditions. While visual inspection allows most
of the outside defects to be detected, internal defects, which might lead to flashunder, cannot
be observed. Furthermore, visual inspection can generally provide rather qualitative information, which can be better quantified by the other diagnostic methods examined in the following sections.
2.2
INFRARED THERMOGRAPHY (IR)
A thermal emission is associated with local heating caused by a current flowing along a defective part of the insulator, characterized by relatively high conductivity in comparison to the
intact insulating material. An example would be in the presence of tracking or other semiconductive paths as shown in Fig. 4.
Fig. 4: Examples of internal defects investigated [17] (left: tracking mark) and in [18] (right:
channel with low conductive moisture)
With IR, the temperature distribution along the insulator axis is measured by means of an
infrared camera, searching for hot spots associated with possible local defects. Compact
4
cameras with high sensitivities and excellent performance were developed which currently
permit a fast and reliable inspection of the insulators. An example of defect detection by IR is
shown in Fig. 5. Guidelines for IR inspection from a helicopter are available [18]. The method
is particularly sensitive to defects developing between the housing and the core, leading
possibly to a flashunder. In this case the fault current is passing through the defective zone
causing a significant temperature increase. The phenomenon is particularly evident when the
tracking affects large parts of the insulator or when a sufficient conductivity is present all
along the insulator. This could be because of humidity ingress in the interface or because of
wetting of the part of the insulator not yet damaged [17], [18], [22]. On the contrary, temperature measurements by means of an infrared camera are not suitable to detect conductive or
semi-conductive defects developing on only a small section of the insulator with the remaining part sound and characterized by high resistivity, especially if the measurements are
made in a low humidity condition (with the insulator dry). In this case, corona may instead
occur on the tip of the defect, with a very low current associated with it, thus leading to very
limited temperature increase, hardly detectable in service [19], [20]. Corona measurements
are thus an important tool as well, as discussed in the following chapter.
Fig. 5: Examples of clear IR detection of internal conductive defects (from left-to-right: in laboratory; at test station; in service)
2.3
ULTRA-VIOLET DETECTION (UV)
The possibility of localizing initial corona activity constitutes an interesting technical challenge, especially in daylight conditions, thus different techniques are available for day and
night measurements. For daylight corona cameras, the diagnostic indicator considered is the
emission generated by the defects in the UV-C range (i.e. with wavelength in the range 240280 nm), a bandwidth in which the solar light is filtered by the atmosphere. Corona emission
intensity is calculated using the number of pulses of light emission (named “blobs” - Fig. 6). A
counter gives a number proportional to the quantity of “blobs” received by the sensor.
Clouds of blobs
Fig. 6: Example of “blob” counting by a daylight corona camera (245 kV tension insulators)
5
Very sensitive portable cameras are now available, and some of them are combined “multicameras”, which can also provide IR and visual observations with one instrument. The
method is particularly efficient in detecting conductive/semi-conductive defects only developing on a section of the insulator, with the remaining part of the insulator in good condition and
characterized by a very high resistivity, especially in dry conditions and low humidity.
Under these conditions, the IR measurements indicated a very low temperature increase (in
the order of 1-2 degrees) [19]. The sensitivity of the method was investigated by simulating
conductive defects (using metal wires located on the insulator surface) at the live side,
ground side and at a floating potential (in the middle of the insulator) [19]. The results are
summarized in Fig. 7, where the minimum detectable defect length (in % of the insulator
length) is presented for different insulator ratings. The UV camera was generally capable of
detecting all conductive defects longer than about 20-30% of the insulator length.
Similar results were obtained by substituting the metallic wire with a semi-conductive tape to
reproduce typical tracking values [19].
Ld / L [%]
40
35
floating potential defects
30
ground potential defects
live potential defects
25
20
15
10
5
0
100
150
200
250
300
350
400
Un [kV]
Fig. 7: Minimum conductive defect length detectable by UV for different defect positions and
for insulators of different ratings Un
The question remained about the capability of the method to detect defects located under the
insulator surface, possibly leading to insulator flashunder. While the UV method was not able
to detect the defects when fully covered by the housing, removing a small part of the housing
and thus uncovering the conductive end of the conductive defects (usual condition in case of
severe defects [20]), the sensitivity of the method was seen again and very similar to that
obtained with defects occurring on the surface.
In general, IR thermography and UV measurements principally detect different physical
properties (heat and enhancement of the electric field in the form of corona respectively),
thus a combination of these two methods/cameras or use of a multi-camera would be the
optimum solution for the remote inspection of composite insulators, especially when a certain
failure tendency is known for the age or vintage of the insulators in question.
2.4
E-FIELD MEASUREMENTS (EF)
When a composite insulator is electrically defective, the electric field changes in the vicinity
of the defective area. A portable, in-service and manually-operated diagnostic probe was
developed from the version used for cap-and-pin insulators and has been tested in the laboratory and in the field to check its suitability. The principle of operation is based on the measurement and recording of the AC axial electric field along the insulator. The EF probe can
easily be used by skilled LLW personnel. Defects that generate a distortion of the EF (i.e.
conductive or semi-conductive defects) can be detected by comparing the EF pattern ob6
tained on the defective insulator with a reference fingerprint obtained on the sound insulator
(see example in Fig. 8). To enhance the detection of defects, the data can be normalized by
dividing the electric field value measured for each shed by the corresponding value of the
plot taken as reference [19], [20].
Fig. 8: Examples of E-field probe application on a 420 kV insulator): left: defect at the fitting;
right: defect in the middle
The maximum EF variation found between the corresponding values on defective and sound
insulators (in p.u. of EF of the sound insulator) are shown in Fig. 9 as a function of the defect
length and position. It can be shown that even relatively small conductive and semiconductive defects can be identified by this method. In the attempt to derive general rules,
measurements were performed on insulators having different defects, generated during the
manufacturing process (lack of primer, carbonization on the rod, break of the rod) or resulting
from a long duration ageing test in different environmental conditions. Electrical field deviations higher than the intrinsic sensitivity of the methodology were confirmed for all types of
defects investigated [19].
MAX Electric Field Deviation [p.u.]
LIVE
GROUND
FLOATING
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
Ld / L [p.u.]
Fig. 9: Maximum electric field deviation (p.u) as a function of the defect length and position
for conductive type defects
Apart from being very sensitive, the advantage of this method relies on its capability of indicating defect size and location, as shown in Fig. 8. However, the method is quite demanding
in terms of time/cost and expertise required and thus it is not economical to scan an entire
overhead line. On the contrary, it may be very effective to ensure the safety of LLW on a
specific insulator or insulator vintage. It was found empirically, that the EF-probe may not
detect low severity level defects near the end fitting, due to electric field shielding by the corona ring. This by the way does not allow measurements closer than approximately 15 cm
from the end fitting with corona rings. The atmospheric humidity during EF-probe measurements must be recorded. If an insulator is covered by a hygroscopic pollution layer, this
situation must be considered when taking the reference measurement and analyzing the
results.
7
3
EXPERIENCES WITH DIAGNOSTIC METHODS AND PRINCIPLES APPLIED
TO INSULATOR STRINGS SUBJECTED TO LIVE LINE WORK
It is an important rule that LLW is always performed in fair weather, excluding conditions that
could lead to pollution flashover, thus only the dielectric performance under transient voltages is of concern. If lightning is probable, LLW is usually forbidden or interrupted. So, only
long front overvoltages are of concern, which are simulated in the laboratory by Switching
Impulses (SI) [19], [20]. Many types of possible deteriorations of composite insulators in service do not affect the SI dielectric strength and are not of importance with respect to LLW
feasibility. Examples of these defect types are pure external surface phenomena (chalking,
color changes etc. [20]). Even shed damages (punctures, splitting etc.) may not be critical
enough to cause a reduction of the dielectric strength under dry conditions required for LLW
[19], [20]. Only conductive or partially conductive defects can affect the SI strength. Tracking
type defects will always be potentially critical, since they are conductive or partially conductive even in dry conditions and are often hidden inside the insulator. However, the insulator
can also become partly conductive following moisture ingress in the core and at the interface.
This occurs, for example, when moisture penetrates through the punctures/splitting of the
sheath towards the rod, thus increasing the conductivity of the rod or part of it [20]. Therefore, prior to LLW, the condition of the insulators of the span(s) subjected to maintenance are
to be evaluated in order to detect any possible risk of flashover. Specific procedures are applied for this requirement, which is focused only on personnel safety. More specifically, the
goal is to detect the quite large conductive or semiconducting type defects, since only large
defects (e.g. 20-30% of the HV EHV insulator [19], [20]) create risk during the LLW. This
specific requirement is very important for the assessment of the feasibility and suitability of
the available diagnostic methods.
LLW on composite insulators is carried out by many companies in the world, adopting different diagnostic tools, selected on the basis of their specific experience and taking into account
the characteristics and the known weaknesses of the insulator generations installed in their
system. In spite of the fact that R&D activities are going on to verify the efficiency of the different diagnostic methodologies, in many countries, like Australia and South America, LLW
Example of the Italian
Transmission Network up
to 420 kV
Example of the Canadian
Transmission Network up
to 115 kV
Visual Inspection
from Ground
IR-Inspection from
Ground
UV-Camera Corona
Measurements
Temperature Increase measured?
no
No Action taken.
yes
Tower Climbing
Tower Climbing
Close Visual Inspection
(from Tower)
Noise, Heat or Visual
Defect?
no
Replace under Voltage.
yes
EF-Measurement
Reschedule. Replace
de-energized.
Fig. 10: Examples of diagnostic procedures before LLW
8
on composite insulators is still based mainly on careful visual inspection [24], [25], [26], [27],
[28], [29]. Combined methodologies are proposed in other countries. As an example (Fig. 10)
the Italian procedure [30] supplements visual inspection from ground level with both UVcamera and electric field measurements while in Canada, visual inspection is supplemented
by thermal observation [22], [23]
4
CONCLUSIONS






The many advantages of composite insulators have led to their wide use in transmission lines all over the world. The reliability of the present generation of composite insulators is considered to be similar to that of ceramic cap-and-pin insulators.
An essential issue that limits an even wider application of composite insulators is the
concern about the assessment of their conditions in service and especially before the
application of live line working (LLW) techniques.
The applicable diagnostic principles did not significantly change during the last 15
years. However, significant progress has occurred in the development of diagnostic
tools and in the interpretation of measurements.
On-line diagnostics, together with periodical tests on samples taken from service, enable reliable indications regarding the insulator conditions in service and regarding
their life estimation to be obtained.
Diagnostics is easier for LLW, since its aim is to identify only large conductive or
semi-conductive defects which may be critical during the specific linemen activity.
A well chosen combination of available diagnostic methodologies make it possible to
identify the absence of critical defects of composite insulators and to carry out LLW
safely on overhead lines equipped with composite insulators, in a similar way as it is
for ceramic and glass insulators.
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10