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Presented
at Tech Advantage 98, March
8, 1998, Nashville,
Tennessee
RF/TVI
AND BLINKING LIGHT COMPLAINTS: COMMON SYMPTOMS OF NON-VISIBLE
INSULATOR AND ARRESTER FAILURE
Insulators
and arresters routinely become electrically defective with no
accompanying visual evidence. Historically, it has been widely
assumed that defective insulators and arresters can be identified
by physical evidence such as flash over burns, chipped, cracked
or broken porcelain and blown disconnects. However, a growing
body of data shows that just because insulators and arresters
look good mechanically, does not mean they are good electrically.
Non-visible
defective insulators and arresters are a common source of such
expensive and time consuming operations difficulties as intermittent
and persistent RF/TVI and blinking light complaints. They also
pose hidden threats to line crew safety when energized maintenance
work is being performed.
Data
on the symptoms, patterns and costs of these non-visible defects
are presented and methods of convenient and cost effective testing
to identify them are described.
ON-VISIBLE
INSULATOR FAILURE
Periodic
failure of insulators is a fact of electric utility operations.
Historically, it has been widely assumed that these failures can
be identified by visual inspection for physical evidence such
as flash over burns and chips, cracks or breaks in the porcelain.
As a result, non-visible failure of insulators is not generally
considered to be a cause of operations difficulties or a threat
to line crew safety.
In
this section of the paper, symptoms and consequences of non-visible
insulator failure at both distribution and transmission levels
are described. Data on patterns and frequency of these non-visible
failures are included to the extent they are available.
Nuisance
Calls and Non-visible Insulator Failure on Distribution Circuits
During
the last half of the 1980's and the early 1990's, Cloverland Electric
Co-operative, Dafter, MI. experienced steady year over year increases
in customer complaints and service reliability problems associated
with:
-
•
radio/TV interference (RF/TVI);
•
linking lights;
• the number of OCR operations coupled with difficulty
in locating problems causing breaker operations;
•
the number of fused TAP lines requiring refusing after lightning
and severe storms;
and
pole top fires.
By
mid 1991, the number of customer complaints had reached crisis
levels. The amount of crew time being spent on nuisance calls
was interfering with routine maintenance and planned construction
activities. Furthermore, failure of the company to quickly resolve
these customer complaints was having a negative impact on customer
relations.
Early
efforts undertaken to resolve the complaints included:
- •
a concentrated tree brushing program;
- •
tightening line hardware wherever RF/TVI persisted;
- •
and extensive RF/TVI patrols.
None
of these time consuming and costly efforts were successful in
resolving the problems.
In
early 1991, the company decided to investigate the possibility
that non-visible insulator failure was the source of its problems.
They purchased a Hi-Test Insulator Tester which would allow them
to test insulators for non-visible defects while the lines remained
energized (see Appendix A). Three suggestions made by the manufacturer
of the tester were useful to the company in structuring its test
program:
- •
non-visible insulator failure has been shown to cause the
problems being experienced by the company;
- •
non-visible insulator failure is often more common than visible
insulator failure (a cutaway example of such a failure is
shown at Plate One);
- •
and a large majority of non-visible insulator failures occur
in suspension insulators used in dead-end applications.
From
mid 1991 to mid 1994, the company tested all suspension insulators
in dead-end applications on a circuit by circuit basis starting
at the substations and proceeding out along the circuits. Approximately
1900 miles of lines containing over 35,000 insulators were tested
A
total of 2,243 defective insulators were identified and removed
from service. The type and size of these defective insulators
are shown at Table One. The year of manufacture of a large random
sample of the defects (approximately 75%) is shown at Table Two.
In 1991, when the insulator test program was begun, the company
experienced 719 outages. In 1994, the year the program was completed,
the number of outages had dropped to 498 and that number was expected
to drop even lower in 1995. Based on these reductions in outages,
the company achieved annual savings of $75,000 on nuisance calls
and a payback period on the insulator test program of slightly
less than three years.
Table
One
|
Type
|
4.25"
bell
|
6"
bell
|
10"
bell
|
TOTAL
|
Aluminum |
1102 |
135 |
0 |
1237 |
Steel |
11 |
933 |
62 |
1006 |
TOTAL |
1113 |
1068 |
62 |
2243 |
Table
Two
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Yr.
of Mfg.
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No.
of Defects
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No.
of Defects
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In
summary, the results of this program were:
-
• substantial reduction in RF/TVI complaints;
-
• a substantial reduction in blinking light complaints;
-
• a substantial reduction in the number of breaker
operations and the elimination of a large number of time
consuming and
costly complaint calls;
-
• a dramatic reduction in the number of fused TAP
lines needing refusing after lightning and severe storms;
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• reduction of pole fire losses to virtually zero;
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• reduction of line losses from 10.42% in 1990 (the
year before the program was begun) to 9.63% in 1993 (the
year before the
program was completed) with further reductions anticipated.
NOTE: The company also operates 163 miles of transmission
line and attributes at least one-third of its line losses
to these circuits; and
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• savings of approximately $75,000 annually on nuisance
complaints alone.
The
Operations Manager at Cloverland Electric Co-op during the time
of this program said, "The insulator inspection and maintenance
program has had more immediate impact on service reliability than
any other program I have ever adopted."
Non-Visible
Insulator Failure on Transmission Circuits
The
presence of non-visible insulator failure on transmission circuits
is known to cause at least three operations problems:
- •
unexplained ground fault trips;
- •
RF/TVI complaints; and catastrophic failure of insulator strings,
- •
often accompanied by the conductor falling to the ground
1)
Unexplained Ground Fault Trips
Unexplained
ground fault trips while not common are not unusual. Where no
obvious explanation for the interruption exists and subsequent
patrol of the circuit turns up nothing, it is typically attributed
to birds, squirrels, loggers, firewood collectors, or weather.
However, it is now known that non-visible insulator failure is
a significant source of these unexplained ground fault trips.
A narrative example follows:
"Until
the early 1970's, this circuit had a good reliability record.
At that point, during the spring and fall seasons of the next
several years, the line began to experience occasional unexplained
ground fault trips. The circuit would be restored by reclosing
the relay and then patrolled. No apparent cause for these trips
was discovered and they were generally attributed to imprudent
loggers, firewood collectors, and lightning strikes. It was
observed, however, that these problems were invariably encountered
after precipitation. On one occasion, the circuit could not
be restored and a pole fire and structure loss occurred. This
event was clearly the result of insulator failure, although
upon subsequent inspection there was no visible evidence of
defective insulators on the structure.
In
late 1975, a ground fault relay tripped the circuit and service
could not be restored. Numerous attempts to reclose over a period
of several hours resulted in a timed ground fault trip. After
an extensive foot and helicopter patrol and just prior to darkness,
service people were stationed along the line in an attempt to
observe flash, noise, or any other symptom that might pinpoint
the problem.
A
rural customer telephoned the company to report that a pole
behind his house had made a noise and he thought he had seen
a flash. A service crew was dispatched to this neighborhood
and another reclose was attempted. The centre string on a running
vertical corner was observed to flash and was heard to be noisy.
This
string was replaced and service was then restored - after a
ten hour outage. Upon removal of the suspect string, a small
burn mark was observed on the pole behind the curved washer.
All the insulators, however, appeared normal. At this point,
the company was convinced that non-visible insulator defects
were causing the majority of the unexplained trips.
Every
structure on the line had already been climbed and each insulator
visibly inspected closely. It was concluded that a means other
than visual inspection would have to be employed to 'test' suspension
insulators."
Subsequent
testing of the insulators on this circuit using early prototypes
of the Hi-Test Insulator Tester revealed numerous 6 bell strings
in which 4, 5, or all 6 bells were non-visibly defective. The
subject circuit is 75 miles in length and is operated at 69 KV
nominal. Replacement of the defective insulators completely resolved
the problem. Prior to this insulator test program, the company
was planning to replace all the insulators on the circuit - a
project which would have cost in excess of $1,000,000 in their
estimation.
2)
RF/TVI Complaints
RF/TVI
complaints associated with transmission circuits are not as common
as those emanating from distribution circuits due to the difference
in population density commonly found around these two types of
circuit. However, they are not unknown as evidenced by the following
example:
"Since
1983, more than 60 electrically punctured 9 inch diameter porcelain
suspension insulators have been found on Commonwealth Edison's
(CECO) 345 KV transmission system as a result of television
interference (TVI) complaints from customers living near R/Ws
... (we) suspect the problem is more widespread than otherwise
indicated by the number of TVI customer complaints. Many miles
of transmission lines are located in rural areas far away from
residential developments. Therefore, we would not expect to
receive TVI complaints in these areas if punctured insulators
were present."
RF/TVI
complaints associated with non-visible failure of suspension insulators
used in strings will be intermittent in nature. The critical element
is moisture. As long as the external surface of the insulators
in the string remains dry, the punctured insulator(s) will not
'arc'. However, once the external surfaces of the insulators are
damp enough to become conductive, those insulators which are internally
punctured will 'arc' thereby causing the RF/TVI. It is this intermittent,
weather dependent nature of the problem which makes location and
identification of these defects so problematic.
3)
Catastrophic Failure of Insulator Strings
Catastrophic
failure of a string of insulators characterized by a high proportion
of the insulators being badly damaged or totally destroyed is
described in the following example:
"In
October, 1984, a live line crew was called out to change a dead-end
string of insulators which were initially believed to have been
damaged by rifle fire. Only five unbroken bells remained on
the string, the other seven having shed their porcelain. Close
examination of the fragments on the ground indicated they had
not been shot as the porcelain skirts were broken almost exactly
in half and, in some cases, much of the bonding material was
still attached to the porcelain fragments.
In
this company, suspension insulators are routinely tested to
determine that they can be safely hot sticked before any live
line procedures take place. Following normal operating procedures,
the crew tested the string using the Hi-Test Insulator Tester
and discovered that only three insulators (one of which was
a cob) were electrically effective. The other nine were demonstrated
to be defective by the tester. Testing of the neighboring insulators
on the structure (all of which appeared good) located a further
five defective insulators."
The
common characteristics of this type of failure include: progressive
physical damage along a string of insulators ending with an insulator
for which only a metal cap remains; charred, blackened and/or
twisted metal caps and pins on one or more insulators; separation
of the insulator string resulting in the conductor being dropped;
and pieces of the porcelain with bonding material attached to
them lying on the ground beneath the subject string.
The
mechanism for such failures is hypothesized to be non-visible
defective insulators in the string in conjunction with moisture
inside one or more of these defects. If a string in this condition
flashes (as a result of lightning or a switching surge), then
the moisture inside the insulator(s) would be vaporized and the
insulator(s) blown apart. The explosive force originating inside
the insulator would explain the pieces of porcelain on the ground
with bonding material attached to them.
Patterns
of Non-visible Insulator Failure
Non-visible
insulator failure is not distributed randomly across the system.
Data collected from the field over the past ten years shows two
strongly non-random patterns to these failures which pose hidden
threats to line crews performing energized work.
- They
occur primarily on dead-end structures (see Table
Three)
There seem to be three reasons for the concentration of these
defects on dead-ends: mechanical, electrical and weather stress.
Insulators installed on dead-ends are known to be subject
to more mechanical and electrical stress than those installed
on tangents or running corners. In addition, insulators installed
on dead-ends have the bonding material much more exposed to
wetting from rain or snow than those installed on tangents
or running corners. This moisture is known to invade the bonding
material of some insulators and can cause internal pressure
in those insulators in two ways: i) by producing a chemical
change in the bonding material, known as cement growth; and
ii) moisture present in the bonding material will expand when
subjected to freezing weather; this pressure can be relieved
through a crack in the porcelain head of the insulator.
-
Table
Three
-
They
"cascade" within a string of insulators
(see Table Four)
The reason(s) for the "cascading" pattern of failure
is not known. However, the implications for line crew safety
are clear from the data in Table Four and the discussion of
that data which follows.
Table
Four
No.
of Non-visible Defects
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This
data is from a test program in which a total of 414 strings of
insulators (with 12 bells per string) were tested for non-visible
insulator failure. Of that total, 115 strings (or slightly over
one-quarter of them) had at least one non-visible failure. Furthermore,
once a string has been found to have a non-visible defect, the
likelihood of finding additional defects in that string was more
than double the likelihood of finding the first defect in the
next string to be tested.
Insulators
were tested on a total of eighty-four structures in this program.
Examination of the data structure by structure identified forty-two
(i.e. one-half) which required maintenance attention to one or
more insulator strings. Examination of the data from dead-end
structures only, identified forty-one of the fifty-four structures
tested (i.e. three-quarters) requiring maintenance attention to
one or more insulator strings looked at another way, 41 of the
42 structures needing maintenance attention to insulators where
dead-ends. Finally, of the 115 strings in which non-visible failures
were found, 31 had levels of insulation which were reduced by
one-third or more.
At
the time of this test program the circuit was operating with no
interruptions or other symptoms of these defects. These data graphically
demonstrate the hidden threat to line crew safety which can be
produced by these two patterns of non-visible insulator failure.
NON-VISIBLE
SURGE ARRESTER FAILURE
As
with insulators, operations and maintenance personnel often assume
that an arrester which looks good is good. This is generally coupled
with the belief that a defective arrester will identify itself
quickly either through the operation of the disconnect or by way
of a fragmentation failure (in the case of porcelain housed arresters).
However,
arresters do not always identify themselves quickly once they
become electrically defective and there is evidence that non-visible
failure of distribution class surge arresters are also a source
of blinking lights and RF/TVI complaints.
These
difficulties are frequently treated as transient problems and
the service procedure is often to wait for the problem arrester
to identify itself through operation of the disconnect or by fragmentation
failure. In areas of high lightning activity, where the number
of arresters installed across the system is large, such a maintenance
strategy can produce circumstances where operations difficulties
associated with non-visible failure of arresters become an on-going
problem.
In
this section of the paper, types of arrester failure are discussed
with particular attention given to those types of failure which
occur gradually. It is these gradual failures which produce the
operations difficulties described above. Where available, data
on the causes of these failures are presented.
Pathways
to Distribution Class Arrester Failure
Three
types of distribution class arresters are commonly employed today:;
i) the metal oxide (MOV) type which consists of metal oxide blocks
in series between line and ground with no spark gap spaces; ii)
a recent variant on the MOV type which includes resistance gaps
together with metal oxide blocks; and iii) the older silicon carbide
type which consists of a series of silicon carbide blocks and
spark gap spaces. Both metal oxide and silicon carbide blocks
have non-linear resistance such that they are highly resistant
to electric current up to a specified voltage and then very rapidly
become conductive as that voltage is exceeded.
Failure
of any of these types of arresters can occur either suddenly or
gradually.
A)
Sudden failure occurs for the following reasons:
- •
puncture of the blocks due to heat channeling caused by excessive
energy release from either a single large surge or repeated
operations in rapid succession;
- •
internal flashover of the blocks due to contact arcing;
- •
internal flashover of the gaps and valve elements due to moisture
invasion; or
- •
dielectric failure of the collar material of the blocks.
Sudden
failure of distribution arresters does not pose nearly the degree
of operations difficulty or maintenance expense which gradual
failures pose. Sudden failures are generally accompanied by ample
physical evidence of the failure either through operation of the
disconnect or fragmentation of the arrester housing. As a result,
the time and cost of replacement is generally quite small since
they are quickly and easily located (assuming there is no personal
injury or property damage if the failure is a fragmentation type).
B)
Gradual failure occurs for the following reasons:
- •
flashover of the spark gap spaces due to moisture invasion
or metallization of electrode spacers due to heavy surge currents;
- •
repeated sparkover of gap spaces due to external contamination
or repetitive switching surges;
- •
failure to interrupt the power follow curve;
- •
operation in the presence of abnormal voltage overload;
- •
thermal runaway of gap assembly grading resistors (silicon
carbide type) or excessive energy absorption, block element
degradation or abnormal system voltages (metal oxide types);
- •
dielectric failure of one or more of the block elements (metal
oxide types); or
- •
irregularities at the surface interface between blocks; or
- •
minor moisture invasion creating a conductive path parallel
to the block elements (metal oxide types).
Gradual
failure of arresters can take anywhere from several days to several
weeks to reach a point where blown disconnects or fragmentation
failure occur. In high lightning activity areas, where the number
of arresters installed is large, gradual failure of arresters
can produce ongoing blinking lights and RF/TVI complaints. The
absence of visual evidence of the defect in the field makes traditional
trouble shooting activities cost in-effective. The Hi-Test Surge
Arrester Tester allows maintenance personnel to address these
problems in a cost-effective manner (see Appendix B).
Causes
of Arrester Failure in the Field
Collection
of failed distribution class arresters over several months by
field personnel at Ontario Hydro produced several hundred arresters
subsequently examined by R&D staff at Ontario Hydro. Their
study showed that these failures were due to the following causes:
- •
moisture
invasion - 86% of failures
- •
lightning - 6% of failures
- •
surface contamination- 5% of failures
- •
misapplication - 2% of failures
- •
unknown - 1% of failures
The
arresters included in this study were all porcelain housed, silicon
carbide types. The results show very clearly the sensitivity of
these arresters to moisture invasion. It requires the smallest
failure of an end seal gasket to allow air to pass in and out
of the arrester as ambient air and arrester temperatures change.
Extremely small quantities of moisture inside arresters produce
failure. Such moisture invasion failures can occur even on clear
days as the failure is ultimately triggered by the redistribution
of the moisture inside the arrester due to temperature changes
in the air. Frequently such failures are triggered by the thawing
of frozen moisture in the arrester or re-energization of the arrester
during maintenance work.
The
shift to polymer housed arresters and metal oxide technology is
a method for reducing the air space inside arresters thereby reducing
the likelihood of moisture invasion. Widespread field application
of polymer housed MOV arresters will change the pattern of failure
away from moisture invasion toward thermal problems as the most
common cause of failure.
This
will not change the operations difficulties experienced in the
presence of gradual, non-visible failure of arresters. Blinking
lights and RF/TVI are both complaints known to be caused by gradual
failure of arresters for thermal reasons.
SUMMARY
Non-visible
failure of insulators and surge arresters are sources of operations
problems which are difficult to locate in the field and costly
to the utility. RF/TVI and blinking light complaints are common
symptoms of the presence of these non-visible failures on distribution
circuits.
Data
showing the frequency of these failures and some of the non-random
patterns which are characteristic of these failures are presented
and discussed. Attempts to resolve these problems through traditional
trouble shooting activities are generally non-productive and costly.
Two
pieces of equipment developed specifically to assist in the identification
of non-visibly defective insulators and arresters are described.
APPENDIX
A
TEST
EQUIPMENT: THE HI-TEST INSULATOR TESTER
This
piece of equipment was developed in response to the types of service
reliability problems and threats to line crew safety posed by
non-visible insulator failure described in this paper. It allows
line crews to test insulators quickly, safely and reliably while
they remain in service and the line remains energized.
It
includes the following features.
- the
tester can be used on any AC circuit from the lowest distribution
to the highest transmission voltage with the circuit either
energized or de-energized;
- it
fits on the end of a standard hotstick and is light enough
for any lineman to use unassisted - weight is approximately
2.7lbs or just over 1kg;
- it
is a self contained DC power source which is unaffected by
high AC voltages imposed across the test probes, thus allowing
a lineman to "meg" insulators in service;
- it
imposes a 10 KVDC potential on any object placed between two
probes extending from the rear of the tester and indicates
the condition of the insulator by way of an LED display and
an audible warning buzzer on the front of the tester;
- the
10 KVDC potential is current limited so that no shock hazard
is present for the user and the test is non-destructive to
insulators; and
- it
is powered by a rechargeable 8 VDC battery with the recharger
built into the tester.
APPENDIX
B
TEST
EQUIPMENT: THE HI-TEST SURGE ARRESTER TESTER
This
piece of equipment was developed in response to the problem of
gradual failure of distribution class surge arresters described
in this paper. It can be used either in a stores yard/warehouse
setting to test materials being recycled or it can be taken to
the field and used in a trouble shooting activity to locate non-visible
defective arresters which are causing operations difficulties.
It
includes the following features:
- •
it
imposes a current limited voltage on the arrester under test;
- •
the voltage imposed on the arrester can be varied continuously
from zero to the maximum output of the tester;
- •
it displays the voltage withstand capability of the arrester
under test in both actual voltage DC and the RMS equivalent
AC voltage;
- •
it displays the current leakage of the arrester under test
in microamps;
- •
the test is non-destructive to arresters;
- •
it is equipped with both a pivoting handle and a belt clip
for convenient portability and ease of use in the field (weight
is approximately 1 kilogram or a little over two pounds);
and
- •
it is powered by a 12 VDC rechargeable battery with the recharger
built into the tester.
by:
John A. Farquhar, Ph.D
President, Hi-Test Detection Instruments Inc., Blaine, WA.
Copyright, 1998
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to the HiSat Arrester/Leakage Tester product page
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