Chemist’s Perspective
C
ondition Assessment
of Oil Circuit Breakers
and Load Tap-Changers
by the Use of Laboratory Testing and Diagnostics
Introduction
The use of laboratory testing is of increasing importance today as it provides the data to diagnose the condition of oil-filled electrical apparatus.
This information can be used to detect and identify incipient faults in
apparatus, provide an indication of their severity, and identify long-term
aging trends. In today’s environment, where out-of-service testing of
apparatus is not always possible, being able to acquire easily-sampled
electrical insulating liquids in service is an advantage.
Current-day practices in electrical substations are migrating more towards condition-based, as opposed to time-based, maintenance in order
to protect valuable assets, extend their lives, and use budgeted money
effectively. Condition-based maintenance used for transformers for many
years is now being applied for load tap-changers (LTCs) and bulk oil circuit breakers (OCBs). Since LTCs and OCBs can fail mechanically, electrically, and from deterioration because of local overheating, it is reasonable
to assume that by-products of the deterioration and overheating could be
found in the oil. In recent years dissolved gas-in-oil analysis and other
insulating liquid tests have been used as effective tools to detect problems in LTCs and OCBs. Specific guidelines and algorithms have been
developed for evaluating the results from normal and abnormal LTCs and
OCBs to aid in condition assessment. The goal of a laboratory diagnostic
program is to provide a consistent, reliable analytical technique to detect
problems and provide a ranking or relative health index.
Laboratory Tests Used for Condition Assessment
of LTCs and OCBs
Oil tests were chosen to be able to diagnose the condition of LTCs and
OCBs, based on previous experience with transformers, knowledge of
the operation of these apparatus, and empirical evidence from failures
and problems. Although the same or nearly the same tests are used for
Summer 2004
Lance R. Lewand
Doble Engineering Company
Paul Griffin
Doble Engineering Company
LTCs and OCBs, the methods of
evaluation and the diagnostic
approach is significantly different. The tests currently used are
described below:
Dissolved gases in oil (DGA):
Great emphasis is placed on this
test, as it is an important diagnostic
tool for detecting localized overheating or excessive arcing as well
as other abnormalities. Localized
overheating of conductors and
surrounding insulation may lead
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to a thermal runaway condition which causes carbonization and by-product polymeric films to form
on conductors. This increases the surface resistance
of the contacts, thereby causing increased heating to
the point of failure unless the cycle is interrupted by
maintenance. This can be detected by observing the
generation of hydrogen and hydrocarbon gases such
as methane, ethane, ethylene, and acetylene. It has
been determined that as an overheating event develops into a thermal runaway condition, the ratio of
the hydrocarbon gases changes and can be used as an
additional diagnostic tool. The analysis is complicated
because:
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Gassing characteristics of oils vary to some degree
with their composition, the amount of oxygen present, and the temperature.
Less soluble gases such as hydrogen and carbon
monoxide will escape to the atmosphere more
quickly than the more soluble gases when the apparatus is venting properly.
A temperature distribution exists around an incipient-fault area, which varies with the source of heating and cooling available.
More than one problem type may coexist in the
same apparatus.
The progression from normal to a fault condition
may vary in time and with operating conditions
such as load.
Catalytic surfaces and the composition of those
surfaces may influence the results.
Regardless of the complications listed above, DGA
has become an invaluable tool in helping to detect
and identify problems and their severity in OCBs
and LTCs.
Particle count and size: The total number of particles by size groupings is used to detect abnormal
quantities of by-products and wear materials. The
ratio(s) of the size groupings provides information as
to the extent a detrimental condition has progressed.
Larger particles are especially important, as the dielectric strength of the insulating oil is more adversely
affected by these particles, and formation of larger
particles are an indicator of advanced deterioration.
Particle typing: Particle typing has been used
successfully in other fields such as lubrication and
hydraulic systems for quite some time. In OCBs and
LTCs, particles are formed from three main mechanisms: wear, arcing and overheating.
•
2
Metal particles are formed by wear as two metallic surfaces move against each other. Roylance and
Hunt have determined that the size, morphology,
and types of particles are dependent upon the severity of the pressure applied to the surface and the
angle at which the surfaces intercept.
Arcing can also form metal particles, but these particles are quite different in morphology and topography as they are not wear-induced. Arc-produced
metal particles are formed from molten metal being
quenched by the cooler surrounding oil and, thus,
are produced as a somewhat teardrop-shaped particle. Arcing will also produce organic particles such
as carbon fines and larger conglomerations from the
breakdown of the oil. (See Peelo.)
• Overheating increases the rate of decay of other
materials and induces the formation of by-products
such as polymerized oil films. The examination of
the filter provides a qualitative identification of the
types of particles. In the process of particle typing
there is an attempt to relate the particles to specific
materials of construction. Along with microscopic
examination of the particles trapped on a filter, a
Doble carbon-coding process has been introduced
to aid in quantifying the carbon loading in the
sample.
•
Oil Quality: Several oil quality tests can be performed to check for dielectric strength and aging.
The program described here presently uses water
content, dielectric breakdown voltage (D 1816, 1 mm
gap), and neutralization number. The three oil-quality
tests provide the essential information necessary to aid
in a diagnosis without being overly complicated. The
dielectric breakdown voltage test provides information on the insulating capability of the oil. The water
content aids in determining how wet or dry the system
is and if free water exists. The neutralization number
provides information on the extent of the degradation
of the oil and is important since high concentrations of
organic acids can exacerbate an already deteriorated
condition.
Total Metals: The metals test, consisting of both
particulate metals and those dissolved in the oil, is
an extremely meaningful test. It provides an indication of the amount of material that has been worn or
sublimated from the moving and/or stationary contacts and is now present in the oil. It also provides a
quantitative analysis as to composition of the metals
found in the oil.
Diagnostic Methods for LTCs
The dissolved gas-in-oil test is most often the key
test for detecting and diagnosing problems with LTCs.
LTCs are a crucial element of utility networks, as they
must operate repeatedly in a precise fashion in order
to maintain a constant voltage output. This must be
achieved regardless of variations on input or load.
LTCs have been a weak link in many networks, since
they deteriorate over time due to mechanical problems
or contact wear from repeated operations. Erosion of
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the contacts over time is expected
due to the nature of their function.
Coking of the contacts causes overheating which can cause thermal
runaway.
There are three main types of
LTCs: reactive with arcing contacts
in oil, resistive with arcing contacts
in oil, and arcing contacts in a
vacuum bottle. The resistive LTC
mechanism is a European design.
It has been determined that the
difference between resistive and
reactive types is important, as the
shorter time of arc extinction of
the resistive type (five to six milliseconds after contact separation)
should lower the concentrations of
gases generated (see Griffin). The
vacuum-type models break under
vacuum and so the arcing gases
should not be present at all or in
very small quantities.
Doble has developed guidelines for testing of LTCs which
are model-specific on the basis
of empirical evidence of gassing
behavior and problems detected.
(See Asche.) Lau et al determined
that on-line filters can change the
relative composition and quantities of gases generated. The on-line
filters are used by many utilities to
remove carbon and other particles
from the oil, hence lengthening
maintenance intervals as coking
is reduced.
The LTC diagnostics are then
used to determine a condition code
providing a relative ranking that
can be used for condition-based
maintenance. A similar system
used for OCBs is described later
in this article.
Diagnostic Methods for OCBs
OCBs, consisting of moving and stationary contacts and ancillary
components involved in making and breaking the circuit, can wear out,
losing the ability to perform their intended function. This could happen
because of misalignment, poor contact surfaces, wear, improper timing
of contact movement, and a thermal runaway condition as well as other
factors. Dielectric failure may occur from excessive localized moisture
and/or excessive amounts of conductive particles. In addition, even
stationary components such as the arc-chute materials can breakdown
which results in inadequate arc quenching and carbon buildup.
How a Condition Assessment is Determined
In order to provide a condition assessment for OCBs and LTCs, a numeric ranking is determined through the use of the four separate sections
of the analytical data: DGA, oil quality, particle count, and metals. The
rankings from the four groups are summed or otherwise manipulated
to provide a numeric ranking. The ranking system is further reduced
to a “Condition Code” from which specific maintenance functions are
recommended. This ranking system is shown in Table 1.
TABLE 1
Condition Codes and Maintenance Assessment
Condition
Code
1
Assessment
2
INVESTIGATE now to determine problem
3
Monitor — Resample in 3 months or 1 month depending on
loading
Monitor — Resample in 12-15 months (the resampling
interval for LTCs would be less)
Of No Concern — Resample in 3 years (the resampling
interval for LTCs would be less)
4
5
Remove from service now — Remedial action needed
A Condition Code 1 indicates an OCB or LTC in the worst possible
condition. A Condition Code 5 would indicate an apparatus in good
condition. A large emphasis is placed on the ethylene to acetylene ratios
above certain levels, since it clearly distinguishes the severity of the overheating. In general, the gassing results and ratios are given more weight
than the oil quality, particle count, or metal results, as it most often detects
the problems in the earlier stages, detects a wide range of problems, and
is the most reliable indicator.
Conclusions
In economic terms, condition-based maintenance of LTCs and OCBs
makes practical sense. It focuses resources on intervention to prevent LTC
and OCB failures and can save a substantial amount of money not only
in terms of replacement and installation costs but also in terms of lost
revenue. The cost of the program is much less than continuing to service
LTCs and OCBs on a prescribed maintenance schedule, which is currently
being further extended in most utilities and industrial sites. This type of
diagnostic approach using analytical data from insulating-oil test results
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helps to identify quickly apparatus in poor condition
and then focuses attention in a more timely fashion.
It facilitates ranking of apparatus as to condition so
that a priority hierarchy can be established. This permits maintenance activities to be more thoughtfully
developed and managed, thus saving time, money,
and unplanned outages due to failure.
References
Asche, R. G., “Dissolved Gas Analysis of Load Tap
Changers, Subcommittee Project Report,” Proceedings of the 2002 Annual International Conference of
Doble Clients, Transformer Test and Maintenance,
2002, Sec. 13E.
Griffin, Paul J., “Field Testing — Laboratory Diagnostics, Acquiring Information on Transformer
Health,” The Life of A Transformer Seminar, Doble
Engineering Company, Clearwater, Florida, USA,
February 16-20, 2003.
Lau, M., Horn, W., Schellhase, H., Dominelli, N., and
Ward, B., “Successful Application of Filtration System on On-Load Tap-Changers,” Proceedings of the
2002 Annual International Conference of Doble Clients,
Transformer Test and Maintenance, 2002, Sec. 13C.
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Peelo, D., “Tutorial T3: Circuit Breaker Basics,” presented at the Sixty-Ninth Annual International
Conference of Doble clients, 2002.
Roylance, Brian J. and Hunt, Trevor M., Wear Debris
Analysis, 1999.
Lance Lewand received his Bachelor of Science degree from
St. Mary’s College of Maryland in 1980. He has been employed
by the Doble Engineering Company since 1992 and is currently
the Laboratory Manager for the Doble Materials Laboratory and
Product Manager for the DOMINO. product line. Prior to his
present position at Doble, he was Manager of the Transformer
Fluid Test Laboratory and PCB and Oil Services at MET Electrical Testing in Baltimore, MD. Mr. Lewand is a member of ASTM
Committee D 27.
Paul J. Griffin received his BS degree at the American International College and his MS at the University of Rhode Island.
He has been employed by the Doble Engineering Company for
the past 24 years and is currently Vice President of Laboratory
Services. He is secretary of the Doble Oil Committee; a member
of ASTM committee D 27, subcommittee chair of Analytical Tests,
section chair of Gases in Oil; US Technical Advisor to IEC TC10
for Fluids for Electrotechanical Applications; member of the IEEE
Insulating Fluid subcommittee of the Transformer committee, and
a member of the CIGRE Working Group 15.01 Fluid Impregnated
Insulating Systems.
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