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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 4, OCTOBER 2012
High-Voltage Insulators Mechanical Load
Limits—Part II: Standards and Recommendations
IEEE Lightning and Insulator Subcommittee
Working Group on InsulatorsTask Force on Insulator Loading
A. C. Baker, R. A. Bernstorf, E. A. Cherney, R. Christman, R. S. Gorur, R. J. Hill, Z. Lodi, S. Marra,
D. G. Powell, A. E. Schwalm, D. H. Shaffner, G. A. Stewart, and J. Varner
Abstract—This paper reviews and discusses the rated strength of
insulators as defined in ANSI, CSA, and IEC product standards.
The definition of rated strength, which for a particular insulator,
requires reference to the tests and acceptance requirements given
in the appropriate standard for that insulator is discussed. An insulator may not exhibit noticeable changes at loads sufficient for the
initiation of irreversible damage, referred to as its damage limit.
The significance of this is discussed in the paper as insulator application loads which must be below the damage limit of the insulator and be made in accordance with the relevant standard,
including consideration of the allowable variation in strength implied by strict conformance to the standard. Damage limits for ceramic and composite insulators based on the minimum allowable
strength according to the current standards are given.
Index Terms—Ceramic insulators, composite insulators, damage
limit, glass insulators, high-voltage (HV) insulators, insulator standards, polymer insulators, porcelain insulators.
I. INTRODUCTION
HE IEEE Lightning and Insulator Subcommittee Working
Group on Insulators formed a task force to develop
loading recommendations for high-voltage (HV) insulators that
are produced to conform with American National Standards
Institute (ANSI), Canadian Standards Association (CSA),
or International Electrotechnical Commission (IEC) product
standards, and that satisfy ANSI or CSA line load and strength
regulations. In a companion paper, ANSI and CSA overhead
line load and strength requirements were reviewed and discussed and a basis for limiting loads to prevent irreversible
damage to the insulators was presented [1].
Mechanical strength ratings for ceramic and composite high
voltage insulators given in ANSI, CSA, and IEC product standards are defined by the tests and acceptance requirements given
in the appropriate standard for a particular insulator and are not
minimums or guaranteed strengths.
T
Manuscript received February 27, 2012; accepted May 15, 2012. Date of publication June 22, 2012; date of current version September 19, 2012. Paper no.
TPWRD-00200-2012.
A. C. Baker, Task Force Chair, is with K-Line Insulators, Rochester, NY
14624 USA (e-mail: tbaker@k-line.net).
R. A. Bernstorf. Working Group Chair, is with Hubbell Power Systems,
Wadsworth, OH 44281 USA (e-mail: rabernst@hps.hubbell).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPWRD.2012.2200509
Insulators may not exhibit noticeable changes, deformation
or otherwise, at loads sufficient for the initiation of permanent
damage and assignment of damage limits requires that the minimum strengths allowed for individual units by the standards be
considered.
II. INSULATOR STANDARDS
ANSI, CSA, and IEC insulator standards specify minimum
performance requirements for new insulators subjected to specific electrical and mechanical tests, and dimensional requirements for service interchangeability. Required tests are classified as:
• Type or Design Tests performed once for a given design
which depends on the insulator design, materials, manufacturing process, and technology.
• Sample (quality conformance) Tests performed on random
samples taken from an offered lot to verify the characteristics of an insulator that can vary with the manufacturing
process and materials and qualify the lot for acceptance.
• Routine Tests performed on each insulator produced with
the intent of eliminating insulators with manufacturing defects from a lot.
Sample tests include strength rating verification, visual inspection, verification of dimensions, and galvanizing thickness.
Mechanical tests, intended to verify the assigned strength rating
according to the relevant standard, are the most important with
regard to providing recommendations on service loading limits.
Mechanical strength tests performed on a specified number
of samples, selected at random from a given lot, are loaded to
either the ultimate strength failing load tests or withstand tests
depending on the particular standard and type of insulator. Results of such tests are sufficient to determine if the lot conforms
to the requirements for the relevant strength rating, but an estimation of the expected strength variation of the entire lot requires additional information about the production process for
the insulators. For those with an average strength requirement,
and if produced under conditions that allow statistically relevant conclusions to be made, a normal frequency distribution of
strength is expected and a lower limit for the strength of insulators in a lot can be estimated. Absent such controlled conditions,
a meaningful conclusion regarding the expected strength variation for units in the lot is not possible.
Sample test requirements and lot acceptance criteria for standards published by the three organizations are reviewed here
0885-8977/$31.00 © 2012 IEEE
BAKER et al.: HV INSULATORS MECHANICAL LOAD LIMITS—PART II
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TABLE I
CURRENT ANSI C29 HV INSULATOR STANDARDS MECHANICAL TESTS AND
LOT ACCEPTANCE CRITERIA [2]–[12]
Fig. 1. ANSI C29.2 minimum acceptance requirement for M&E tests.
of insulator has two ultimate strength sample test requirements
as given in Table I.
Lot acceptance criteria based on a historical standard deviation for past production assumes a statistically controlled production process though exact process specifications cannot be
included in ANSI product standards [13], [14]. The strength
variation for units produced in such a process can be represented
by a normal distribution curve and the expected strength variation can be expressed by the area under the curve bounded by a
given number of standard deviations from the sample average.
The standard deviation for any sample from a lot of insulators is
most likely about equal to the historical standard deviation, and
assuming it is equal, the requirement for the average combined
strength of the ten samples
mechanical and electrical
from the lot can be represented by the normal frequency distribution curve as shown in Fig. 1 and for a lot minimally satisfying
the sample test requirement expressed as
Rating
with the objective of estimating the allowable low strength of
units in lots that are in strict conformance with the standards
where possible, and to provide a basis for assigning damage
limits to prevent irreversible insulator damage while satisfying
ANSI and CSA overhead line load and strength requirements.
A. ANSI HV Insulator Standards
The ANSI C29 series of insulator standards includes 13
product standards pertaining to specific types of insulators, and
two test standards providing details for conducting the tests
required in the product standards. Only those standards pertaining to HV insulators, and listed in Table I, are in the scope
of this discussion. Spool, strain, pin, and indoor apparatus types
of insulators are not included.
It is improper to specify commercial terms and conditions in
ANSI standards so lot sizes are not defined [13], and the specific numbers of samples to be taken from a lot offered for acceptance, regardless of the number of units in the lot, for each
type of insulator are given in Table I.
1) Ceramic Insulators:
Suspension Type (ANSI C29.2): ANSI C29.2 defines ceramic suspension-type insulators to be wet-process porcelain or
toughened glass insulators. Lot acceptance criterion for this type
(1)
The factor 1.2 in (1) is based on the current C29.2 standard
[4] and may change with the next revision of this standard. Referring to standard normal curve tables, for a lot that minimally
satisfies (1), the area under the curve below the rated value is
about 11.5% of the total area (representing all of the units in the
lot), so the M&E strength of about 11.5% of the units in a lot
minimally acceptable according to ANSI C29.2 could be below
the rated value.
The possible minimum strength for suspension insulator units
that could be in an acceptable lot according to C29.2 is dependent on the coefficient of variation for the manufacturing
standard deviaprocess. The distribution curve bounded by
basis) includes 99.99% of the units in the lot and so
tions
the possible low strength value
for the lot is
(2)
where is the rated M&E value and is the coefficient of variation
for the manufacturing process. A four standard deviation basis was chosen because it is the current CSA requirement and accommodates a comparison of standards as discussed
later [18]. For ceramic insulator manufacturing, this coefficient
is typically in the 5 to 15% range [15], [16] and units with
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TABLE II
POSSIBLE LOW STRENGTH UNITS AS A PERCENT OF THE M&E RATING IN LOTS
MINIMALLY ACCEPTABLE ACCORDING TO C29.2
TABLE III
POSSIBLE LOW STRENGTH UNITS AS A PERCENT OF RATED STRENGTH IN LOTS
MINIMALLY ACCEPTABLE ACCORDING TO C29.7, C29.8, OR C29. 9
strengths as a percent of the rated value as low as that shown
in Table II could be in insulator lots minimally acceptable according to C29.2.
Line Post (ANSI C29.7) and Apparatus Type (ANSI C29.8
and C29.9): The ultimate strength sample test requirements for
line post and apparatus-type wet-process porcelain insulators
are also given in Table I. Lot and specified sample sizes for these
types of insulators tend be much smaller than for suspensions.
Extracting statistically valid conclusions for a lot based on small
sample test results is problematic, but, by definition, about half
, could
the insulators in a minimally acceptable lot
have strengths less than the rated value. The strength distribution for line post and apparatus-type insulators, given the same
assumption about the manufacturing process as before, could
also be expected to be like that shown in Fig. 1. Even though
none of the three test samples can have a value below 85% of
rating, a minimally acceptable lot may contain insulators with
strengths considerably below rating as shown in Table III.
2) Composite Insulators: ANSI C29.12, C29.13, C29.17,
and C29.18 standards relate to composite insulators, defined
as those having a fiberglass-reinforced resin matrix core and
an elastomeric outer housing. These four standards share a
common ultimate strength sample test requirement and conformance is demonstrated by the ability of one to three samples,
depending on the relevant standard, to withstand the rated
strength load without failure. The rated strength is a value,
assigned by the manufacturer, which must be justified by
prototype tests that are performed once for each design of an
insulator.
Suspension Type (ANSI C29.12 and C29.13): ANSI
C29.12 refers to composite suspension insulators intended
for use on transmission lines 70 kV and above. A prototype
core time-load test for this type of insulator requires that three
units be subjected to an ultimate strength tensile test, and three
additional insulators must withstand a 96-h tensile test at 60%
of the average strength obtained for the first three. The manufacturer’s assigned specified mechanical load (SML) rating,
justified by the prototype test, is confirmed by SML withstand
quality conformance tests on three randomly selected samples
from an offered lot. The test is passed if no failure below SML
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 4, OCTOBER 2012
occurs. The test load is then increased to failure to provide
historical data to further justify the manufacturer’s choice of
the SML value. A routine 10 s test at 50% of SML is required
for each insulator in the lot.
ANSI C29.13 distribution class composite suspension insulators are intended for use on lines 69 kV and below. Prototype
tests require that three insulators each must withstand rated tensile and torsion test loads. For the tensile test, after withstand at
the rated value, the load is increased to failure to provide data
to support the manufacturer’s choice of the SML value. Three
samples randomly selected from any lot offered for acceptance
must also be subjected to a tensile test and must withstand the
SML rating.
Sample tests for composite-type suspensions insulators are
load withstand tests and on a minimum basis, other than helping
to verify the manufacturer’s choice of SML rating, cannot be
used to estimate the possible low strengths values in a given lot.
However, an extensive time-load test, discussed in [1], provides
a basis for a damage limit for composite suspension-type insulators.
Line Post Types (ANSI C29.17 and C29.18): ANSI C29.17
composite line posts are intended for use on lines 70 kV and
above. Prototype core time-load tests for these line posts require
that three units be subjected to a cantilever load for 96 h at 40%
of the specified cantilever load (SCL), or the routine cantilever
load (RCL) if it is greater. After the load test, dissection and dye
penetration examination of the units near the base end must not
show any evidence of cracking or delamination in the fiberglass
rod. Three units must also be subjected to, and withstand, the
specified tensile load (STL).
Ultimate strength sample tests include a cantilever strength
verification test and a specified tensile load test. By agreement
a cantilever breaking load (CBL) may be performed. The cantilever strength verification test involves loading the post to 40%
of the SCL and monitoring the line end deflection of the post,
which must not increase more than about 3% after 1 min under
load. If performed, the CBL is the maximum bending loading
attainable and must be greater than or equal to the SCL.
Composite line post insulators exhibit considerable deflection
under bending loads making cantilever routine tests difficult to
carry out on a production line basis, and the routine test specified
is a tension test. The specified tension load (STL) withstand
sample test is specified to provide a basis for the routine test.
As for other sample tests, after withstand at the rated tension
value is demonstrated, the load may be increased to failure to
provide data to support the choice of the STL.
ANSI C29.18 distribution class composite line posts are intended for use on lines 69 kV and below. A prototype tensile
test, with withstand at the STL load, followed by a load increase
to failure to support the choice of STL is required. The ultimate
strength sample test for this class line post requires that a cantilever breaking load test on one randomly chosen sample from
the lot to be greater than, or equal to, the SCL.
Sample tests for composite-type post insulators, like for suspensions, are rated load withstand tests and on a minimum basis,
other than helping to verify the manufacturer’s choice of SCL
rating, cannot be used to estimate the possible low strengths
values in a given lot. Extensive time-load tests similar to those
BAKER et al.: HV INSULATORS MECHANICAL LOAD LIMITS—PART II
TABLE IV
CANADIAN HV INSULATOR STANDARDS MECHANICAL
TESTS AND LOT ACCEPTANCE CRITERIA
2345
load strength
the condition that
and standard deviation
must satisfy
(3)
for suspensions provide a basis for a damage limit for composite
post-type insulators.
B. CSA HV Insulator Standards
CSA HV insulator standards apply to ceramic, toughened
glass, annealed glass, and composite insulator types. New CSA
standards for distribution voltage class composite insulators
under development will be based on current Canadian Electrical
Association (CEA) purchase specifications LWIWG-01(96)
and LWIWG-02(96). Current CSA insulator standards, including the two CEA specifications, are listed in Table IV
[17]–[24]. The number of samples required for lot acceptance
tests depends on the lot size since there is no restriction regarding lot size specification for Canadian standards. However,
a minimum number of samples, regardless of lot size, is required for ceramic or glass suspensions, and for station posts,
the number of samples to be taken is by agreement between
manufacturer and purchaser. The specified lot size ranges for
composite insulators are given in the table.
1) Ceramic and Glass Insulators: Ceramic-type insulators
refer only to wet-process porcelain insulators in CSA standards.
If made of glass, suspension-type insulators must be toughened
glass but station post types may be toughened or annealed glass.
Suspension Types (CSAC411.1 and C1325): CSA standards for ac and dc porcelain and toughened glass suspension
insulator require randomly selected samples from lots offered
for acceptance to be subjected to a failing load test. A minimum
of 15 samples is equally divided into three groups for the various sample tests, with 10 of the units subjected to the failing
load test. For the lot to be acceptable, the sample average failing
and no individual value may be below the rated value. In this
that
case, for a minimally acceptable lot, the minimum value
could be expected in such a lot, regardless of the coefficient of
variation for the production process, is at least the rated value;
a significant difference compared to ANSI C29.2 requirements.
Station Post Type (CSA C156.1): The standard for porcelain and glass station post insulators specifies that the number
of samples to be selected at random from an offered lot for ultimate strength tests be by agreement between the manufacturer
and purchaser. For lot acceptance, each sample so chosen must
at least withstand the specified failing load. A minimum withstand requirement precludes a determination of the minimum
strength of units that might be contained in such a lot.
2) Composite Insulators: Composite insulators in CSA and
CEA standards currently include transmission class suspensions, and distribution class suspensions and line posts.
Transmission Suspension Type (CSA C411.4): CSA
C411.4 covers composite suspensions intended for use on
lines 69 kV and greater and requires a core time-load test
as described before for ANSI transmission class composite
suspension insulators to confirm the SML rating assigned by
the manufacturer. Randomly chosen samples from a lot offered
for acceptance must pass an SML withstand test.
Distribution Class-Type Insulators: In both CEA
LWIWG-01 (suspensions) and CEA LWIWG-02 (line posts),
a maximum lot size of 3000 units is specified with a sample
of two selected for ultimate strength sample tests; for lots less
than 300 units, the number of samples is to be negotiated.
The suspension test samples are subjected to an SML test as
previously described, and the line posts to a cantilever load
test. In each case, the test is continued until failure, with the
requirement that failure does not occur before reaching the
rated load value.
As for ANSI, lot sample tests in CSA composite insulator
standards are rated load withstand tests not suitable for establishing damage limits. Time-load tests discussed in [1] provide
the basis for damage limits for this type of insulator.
C. IEC HV Insulator Standards
IEC HV insulator standards include porcelain, toughened
glass, annealed glass, and composite-type insulators. A list of
the standards, with the ultimate strength sample test requirements and acceptance criteria, and routine mechanical tests for
each type, is given in Table V [25]–[30].
According to IEC insulator standards given in Table V, the
number of randomly chosen test samples from a lot offered for
acceptance depends on the particular standard and the number
of units in the lot. The minimum number where specified, or
a number of samples for typical lot sizes, are assumed in the
following discussion. Porcelain and glass suspensions and line
posts sample tests are ultimate strength failing test and an evaluation of the expected strength variation in a given lot can be
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 4, OCTOBER 2012
TABLE V
IEC HV INSULATOR STANDARDS MECHANICAL TESTS
AND LOT ACCEPTANCE CRITERIA
TABLE VI
CONSTANTS c
TABLE VII
POSSIBLE LOW STRENGTH UNITS AS A PERCENT OF SFL IN LOTS MINIMALLY
ACCEPTABLE ACCORDING TO IEC 60383 OR IEC 1325
made. For porcelain and glass station posts, and all composite
insulators, samples are only required to withstand the specified
ultimate strength rating for strict compliance with the relevant
standard, and a statistical evaluation of the possible strength
variation for units in the lot cannot be made for these types.
1) Porcelain and Glass Insulators: Ceramic-type insulators
refer only to electrical-grade porcelain insulators in IEC standards, and two types of suspension insulators are included; Class
A, in which the length of the shortest puncture path through solid
insulating material is at least equal to half the arcing distance,
and Class B, in which the shortest puncture path through solid
insulating material is less than arcing distance. A long rod insulator, not commonly used in North America, is an example of
Class A, and a cap and pin suspension insulator is an example
of Class B. Only Class B suspension insulators are considered
here.
Suspension and Line Posts (IEC 60383 and IEC 1325):
IEC 60383 for ac porcelain, toughened glass, or annealed glass
suspension or line post-type insulators, and IEC 1325 for dc
porcelain and toughened glass suspension insulators, require
that samples for ultimate strength tests have an average strength
so that
and standard deviation
(4)
where SFL is the specified failing load and is a constant dependent on the number of samples required to be tested. If this
requirement is not met, a re-test of the lot is allowed, provided
the average does not fail to satisfy (4) by a factor no more than
with the values for the two constants for different
sample sizes given in Table VI.
Acceptance of a lot that only meets the minimum requirement of (4) leads to a possibility of having some units in the lot
with strengths significantly below the rated value. As before,
assuming a statistically controlled manufacturing process and
coefficients of variation of 5%, 10%, and 15%, the possible low
basis,
strengths of units as a percent of the rated value on a
that could be in such a lot according to IEC 60383 or 1325 are
given in Table VII.
Station Posts (IEC 60168): Ultimate strength sample tests
for porcelain and glass station posts are satisfied if the specified mechanical failing load is reached in the test, and statistical
analysis of the distribution of strengths for lots minimally acceptable is not possible.
2) Composite Insulators: Composite insulators, as defined
in IEC, consist of a fiberglass rod core, a polymeric housing,
and end fittings. As in ANSI and CSA composite insulator
standards, ratings in IEC standards are values assigned by the
manufacturers that must be confirmed by various design tests.
A number of randomly chosen samples from insulator lots, as
specified in IEC 61109 (suspensions) or 61952 (line posts), are
subjected to the appropriate test with a minimum requirement
that the samples must withstand the rated load.
III. OVERHEAD LINE COMPONENT STRENGTH COORDINATION
Strength coordination for overhead line components is aided
by referencing component strength characteristics on a compatible basis, which, except for insulators, is generally minimum
breaking strength [1]. Currently, HV insulators are applied on
the basis of rated mechanical strength only having meaning with
respect to the standard the insulators were produced to meet.
Establishing a minimum strength limit for insulators in a particular lot requires an evaluation of the allowable strength variation in the relevant standard, and, depending on the standard,
can require a consideration of manufacturing process variations.
A summary of the minimum requirements for ultimate strength
sample tests for insulators in strict compliance with ANSI, CSA,
and IEC insulator standards is given in Table VIII.
1) Ceramic Insulators: Porcelain and glass suspension-type
insulators conforming to ANSI and CSA standards, and suspensions and line posts conforming to IEC standards, must have, for
randomly chosen samples from a lot offered for acceptance, an
average strength and standard deviation so that
(5)
where is the rated strength value, and is a constant specified
in the relevant standard. If the coefficient of variation for the
BAKER et al.: HV INSULATORS MECHANICAL LOAD LIMITS—PART II
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TABLE VIII
ANSI, CSA, AND IEC INSULATOR STANDARDS MINIMUM STRENGTH
REQUIREMENTS FOR SAMPLE TESTS
TABLE IX
DAMAGE LIMITS FOR CERAMIC INSULATORS
• Long-term service continuity for weather-related loads.
• Coordination of line component failure limits to ensure
safety and contain damage.
The first objective requires that the highest anticipated loading
event must not initiate irreversible damage to the insulator. For
the second, the loading event must not exceed the minimum
ultimate strength expectation for the insulators. In both cases,
the recommendations must be made with respect to the relevant
standards for the rated strength of the insulators.
A. Ceramic Insulators
process that produced the insulators is then, on a
basis,
the lowest expected strength for units in the lot for a lot that
minimally meets the standard is
(6)
As the constant approaches a value of 4, the variation becomes
of less importance provided the process is capable of producing
units with normally distributed strengths.
Ceramic apparatus and line post-type insulators conforming
to ANSI standards must have, for randomly chosen samples, an
average strength at least equal to the rated value. For CSA and
IEC, none of the samples can have strengths below the SCL
rating, but for ANSI, strengths less than rating are acceptable
provided no sample is below 85% of the rated value. The lowest
expected strength for units in a lot minimally acceptable on a
basis in these cases is
(7)
The lower limit of expected strengths from (6) or (7), provided
the coefficient of variation is known or can be adequately
approximated, can be a basis for coordinating the minimum
breaking strength of porcelain and glass insulators with other
line components.
2) Composite Insulators: The common requirement in composite insulator standards that samples chosen from offered lots
must at least withstand the rated strength level suggests that the
lower limit for expected strengths, if is known, is also minimally given by (7) and could provide a basis for coordinating the
minimum breaking strength of composite insulators with other
line components.
IV. LOAD LIMIT RECOMMENDATIONS
Mechanical load limit recommendations for HV insulators
should recognize two objectives:
Irreversible damage to porcelain and glass insulators can be
expected if loads exceed more than 70–80% of the strength of
the insulator (the damage limit) [1]. The strength of a particular insulator must be inferred from the assigned rating and the
requirements to satisfy that rating. General recommendations
must be based on an assumption that the insulators meet the minimum requirements for a particular standard, and for ceramic
insulators, the damage limit
can be defined as
(8)
is defined by (6) or (7). For those standards where
where
a value of the constant is not defined, a value of zero for the
constant was assigned as indicated in (7).
The damage limits according to (8) for ceramic insulators, as
a percent of rating, in lots minimally meeting ANSI and CSA
standards, with manufacturing process variations in the 5–15%
range, are given in Table IX, where is the rated strength according to the relevant standard. The constant is equal to either
the allowed variation, expressed as a number of standard deviations, specified by the standards or assigned a value of 0 when
not specified. Higher values for can be required by individual
customer specifications, and Table IX would be modified accordingly. This subject will be covered in a future paper.
Routine mechanical tests should not, of course, be conducted
at the damage limit level and, therefore, except as a guide for
normal everyday loads, cannot be used to determine maximum
loading capability. The strength percentage of insulator strength
ratings allowed for use in ANSI C2 (NESC) given in [1, Table
1] should not be increased without revising the relevant ANSI
standards to support an increase. In general, the past practice of
limiting maximum service loads to not exceed the routine test
load for insulators has given satisfactory performance [31].
B. Composite Insulators
Ultimate strength QC tests for composite insulators in
ANSI,CSA and IEC standards for lot acceptance are withstand
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 4, OCTOBER 2012
tests on small sample sizes which precludes a statistically-based
recommendation. However, the concept of a damage limit below
which irreversible damage does not occur, is well established
for composites [32].
1) Suspension Type: The damage limit for suspension-type
composite insulators is about 65% of the short-term tensile
strength, and the manufacturer-assigned SML rating is less
than, but based on, this strength. As discussed in [1], a minimum short term of 105% of the SML was assumed to develop
a guaranteed 1-min withstand strength curve. This corresponds
to a damage limit
(9)
for suspension-type composite insulators.
2) Post Type: Manufacturers typically provide a maximum
design cantilever load (working load limit) for composite posttype insulators. As discussed in [1], to establish a damage limit
for posts, it is recommended that a design test should be a 96-h
cantilever test at 110% of the MWL with the post then dissected
and examined to ensure no damage to the rod occurred. This
corresponds to a damage limit
(10)
V. CONCLUSIONS
The rated mechanical strength of HV insulators for use on
overhead lines manufactured to conform to ANSI, CSA, or IEC
product standards only has meaning in the context of the individual standard upon which the rating is based. It does not imply
minimum or guaranteed strength.
According to the standards, insulators are judged to be in conformance with the requirements for a mechanical strength rating
of a particular standard based on samples taken at random from
a given lot and subjected to ultimate strength tests. Individual
units in a lot could have strengths below the rated value, and the
lot could still satisfy requirements for the lot to be accepted.
If a normal distribution of strength for the insulators in a lot
can be assumed, and the sampling rate and acceptance requirements are suitable for a statistical evaluation, the possible low
strength limit for units in the lot can be estimated.
The sampling requirements for ceramic (electrical-grade
porcelain and toughened glass) HV insulators in ANSI, CSA,
and IEC insulator standards are suitable for statistical evaluation of the strength distribution for lots of these types of
insulators. The standards for composite insulators only require
that a small sample withstand the rated mechanical load, and a
statistical evaluation of the strength distribution in a lot is not
possible.
To provide long-term service continuity, the highest anticipated mechanical loading event for an insulator must not initiate irreversible damage to the insulator. The load at which irreversible damage is initiated is the damage limit of the insulator.
For ceramic insulators, the damage limit is about 70% of the
possible low strength for insulators based on a statistical evaluation of sample test results for a given lot. Depending on the
standard, and the coefficient of variation for the process that produced the lot, this damage limit for suspension insulators corresponds to 34% to 70% of the insulator rated strength.
The damage limit concept for composite insulators utilizing
a resin-reinforced fiberglass rod is well established and corresponds to 65% of the rated strength for composite suspension
insulators and 55% of the rated strength for composite post-type
insulators.
The methods for determining allowable in-service loads discussed in the companion paper and the avoidance of irreversible
damage in HV insulators will provide long-term service continuity and strength coordination with other line components for
safety and damage containment.
REFERENCES
[1] A. C. Baker, R. A. Bernstorf, E. A. Cherney, R. Christman, Z. Lodi,
R. S. Gorur, R. J. Hill, S. Marra, D. G. Powell, A. E. Schwalm, D. H.
Shaffner, G. A. Stewart, and J. Varner, “High voltage insulators mechanical load limits—Part I, overhead line load and strength requirements,” IEEE Trans Power Del., vol. 24, no. 3, Jul. 2012.
[2] ANSI C29 High Voltage Insulator Standards Nat. Elect. Manuf. Assoc.,
Roslyn, VA.
[3] ANSI C29 Electrical Power Insulators—Test Methods, C29.1-1988
(R2009).
[4] ANSI C29 Wet-process Porcelain & Toughened Glass Suspension Type,
C29.2-1992 (R2009).
[5] ANSI C29 Wet-Process Porcelain Insulators-High Voltage Line Post
Type, C29.7-1996 (R2010).
[6] ANSI C29 Wet-Process Porcelain-Apparatus Cap & Pin Type, C29.81985 (R2010).
[7] ANSI C29 Wet-Process Porcelain Apparatus Post Type, C29.9-1983
(R2010).
[8] ANSI C29 Composite Insulators-Test Methods, C29.11 (Under Revision).
[9] ANSI C29 Insulators-Composite-Suspension Type, C29.12-1997
(R2002).
[10] ANSI C29 Insulators-Composite-Distribution Deadend Type, C29.132000.
[11] ANSI C29 Insulators-Composite-Line Post Type, C29.17-2002.
[12] ANSI C29 Insulators-Composite-Distribution Line Post Type, C29.182003.
[13] “Standardization policies and procedures of the National Electrical
Manufacturers Association,” Jul. 7, 2008.
[14] “ASTM Manual on Presentation of Data and Control Chart Analysis,”
Special Tech. Publ. (STP) 15D, 7th ed.
[15] E. A. Cherney, “Failures of porcelain line post insulators on the Ontario
Hydro distribution system,” presented at the Can. Elect. Assoc. Spring
Meeting, Montreal, QC, Canada, Mar. 1988.
[16] R. S. Gorur, E. A. Cherney, and J. T. Burnham, Outdoor Insulators.
Phoenix, AZ: Ravi S. Gorur Inc., 1999.
[17] Canadian Standards Assoc. (CSA), “CAN/CSA high voltage insulator
standards,” Etobicoke, ON, Canada.
[18] CAN/CSA AC Suspension Insulators: Wet-Process Porcelain and
Toughened Glass, CSA-C411.1-2010, 2010.
[19] CAN/CSA Ceramic or Glass Insulator Units for dc Systems,
CSA-C1325-1999, 1999.
[20] CAN/CSA Ceramic and Glass Station Post Insulators, CSA-156.11986, 1986.
[21] CAN/CSA Composite Suspension Insulators for Transmission Applications, CSA-C411.1-1998, 1998.
[22] Canadian Elect. Assoc., Canadian Electrical Association (CEA) Purchase Specifications. Montreal, QC, Canada.
[23] Dead-End/ Suspension Composite Insulator for Overhead Distribution
Lines, LWIWG-0.1, CEA, 1996.
[24] Line Post Composite Insulator for Overhead Distribution Lines,
LWIWG-0.2, CEA, 1996.
[25] IEC, “IEC International Standard,” Geneva, Switzerland.
[26] IEC International Standard Insulators for Overhead Lines-Ceramic or
Glass Insulator Units for a.c. Systems-Definitions, Test Methods and
Acceptance Criteria, IEC 60383.
[27] IEC International Standard Ceramic or Glass Insulator Units for d.c.
Systems, IEC 1325.
BAKER et al.: HV INSULATORS MECHANICAL LOAD LIMITS—PART II
[28] IEC International Standard Tests on Indoor and Outdoor Post Insulators of Ceramic Material or Glass for Systems With Nominal Voltages
Greater Than 1 000 V, IEC 60168.
[29] IEC International Standard Insulators for Overhead Lines—Composite Suspension Insulators for a.c. Systems With a Nominal Voltage
Greater Than 1 000 V—Definitions, Test Methods and Acceptance
Criteria, IEC 61109.
2349
[30] IEC International Standard Insulators for Overhead Lines-Composite
Line Post Insulators for a. c. Systems With a Voltage Greater Than 1
000 V—Definitions, Test Methods and Acceptance Criteria, IEC 61952.
[31] CIGRE, “Study and conclusions from the results of the inquiry on insulators: Information and damages,” CIGRE SC22-03, Oct. 1981.
[32] IEEE Guide for Application of Composite Insulators, IEEE Standard
987.