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DISTRIBUTION
On-line monitoring of oil dielectric
breakdown strength
by Tim Cargol, Weidemann-ACTI, USA
This paper discusses the current methods of dielectric breakdown strength testing and reports on a new non-destructive method.
Oil dielectric breakdown strength is a valuable
indicator of the insulation condition in virtually
all oil filled equipment. Many damaging
phenomena such as moisture ingress,
particulates, burning, overheating and
carbonization result in changes in oil dielectric
breakdown strength. By monitoring for
changes in breakdown strength, many
problems with oil filled equipment can be
detected and repaired early, before they
become costly failures.
Oil is used in high voltage apparatus for
two primary purposes, to act as a coolant
and to electrically insulate the high voltage
components. Under stress and over time, oil
can lose its ability to electrically insulate. A
dielectric breakdown strength test directly
measures the oil’s ability to electrically
insulate. The test electrically stresses the oil
to the point of failure and records those
conditions. By applying a high voltage stress
to the oil, such tests simulate the stresses the
oil is under in actual use.
In equipment with electrical contacts, such as
on-load tap changers, circuit breakers, and
regulators the oil has an additional function
-to quench the arcs that are produced under
the normal operation of these devices. The
by-products of these switching arcs degrade
the oil’s breakdown strength, and it is the
dielectric breakdown strength of the oil that
determines the oil’s ability to quench the
arc. Thus, in normal operation of contacting
equipment, the oil is a wearing component
that must be checked and maintained for
optimal performance.
While the oil in high voltage equipment is
seldom allowed to deteriorate to the point
where it directly causes a failure, monitoring
the condition of the oil and maintaining its
quality can make a significant difference in
the overall health of the equipment. Unusual
changes in the dielectric breakdown strength
can be indicative of serious problems such
as overheating conductors or moisture
ingress. Continued use of lower quality oil can
have a damaging effect on the cellulose
insulation, especially if the oil has a high
moisture concentration [1]. As a transformer’s
cellulose insulation is not replaceable without
an overhaul, and the oil can be easily
cleaned and dried on site, it can certainly
be considered worthwhile to maintain the oil
in the best condition, lest it contribute to any
premature aging or wear.
Existing test methods
There are several existing methods for the
measurement of dielectric breakdown
strength such as the ASTM D877, ASTM D1816
and IEC 60156 methods. While these methods
can often be performed on-site with portable
equipment and are valuable laboratory tests,
they suffer from poor repeatability, and due
to their destructive nature, cannot be used
on-line. Advancements have been made in
controlling the destructive energy released
by these test devices [2], but not to the
level or cost that would be appropriate for
on-line use.
Like the arcs produced in contacting
equipment, the arcs produced in the
test instruments degrade the breakdown
strength of the oil. Since the dielectric
breakdown strength is the quantity under
measurement, this limits the number of
successive tests that can be run on a given
sample. In the ASTM methods, five test shots
are performed on a given sample before it
must be discarded. With only five samples,
it can be difficult to get a statistically valid
representation of the oil and, in fact, the
ASTM standards allow a range of 92% of the
mean over the five test shots to be considered
valid [3], [4].
With several types of chemical and physical
particulate contaminants commonly present,
oil can be an inhomogeneous media.
Temperature variations can locally influence
the relative saturation of moisture; turbulence
of flow and proximity to sources of pollution
can influence the type and concentration
of particulate contaminants. Thus, it can be
difficult to get a representative sample of
the oil’s true condition with the small sample
sizes that are used in existing test instruments
and the limited number of specimens that
can be drawn from the finite size of the oil
compartments. One can imagine trying to
taste a bowl of soup with a spoon that will
not hold all of the ingredients.
The inhomogeneity of the oil, combined with
the fact that the number of test shots that can
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be administered is limited further reduces
the capability of existing test methods. It is
not surprising that while the importance of
dielectric breakdown strength is recognized,
less faith is placed in the ability of the standard
test methods to measure it accurately.
New developments
The key to both improving the accuracy of
laboratory test methods and enabling on-line
testing is the reduction of energy dissipated
during the breakdown of the oil. If the test
shots do not damage the oil, more test shots
can be performed and a more accurate
statistical sample can be developed.
Traditionally, existing test methods used a
rather bulky but simple scheme to generate
the high voltage necessary to break down
the oil. These devices consist of basically a
variable autotransformer that is used to raise
the voltage in the test cell until a breakdown
occurs, at which point a relay shuts off current
to the transformer. With all the energy stored
in the magnetics and capacitance of the
transformer, the energy that is released into
the test cell after the relay shuts off (assuming
the relay works in a timely fashion) can be
some tens of joules.
Many of the less expensive dielectric
breakdown test sets available today still rely
on the variable autotransformer approach.
Recently, however, efforts have been made
to reduce the energy dissipated during a
breakdown even with the use of resonant
test sets. These test sets limit the stored
energy available during a breakdown event
and can very quickly detect a breakdown
and de-energize the test set. Such sets are
capable of limiting the energy dissipated
during a breakdown to a mere 20 mJ [2].
Unfortunately, this advanced capability
comes at the price of complexity and cost,
making these test sets excellent laboratory
devices, but not yet suitable for on-line use.
Beginning in the late 1980s work was begun
at MIT to investigate new methods and
devices for monitoring transformers [5].
Among the topics explored in this program
were new methods to measure the dielectric
breakdown strength of oil. Development
continued on dielectric breakdown methods
DISTRIBUTION
Number of
Sample
D-877 Value
Average
breakdown
time to
events
breakdown
EastSt+carb
25 kV
20/20
127 ns
E36St
32 kV
20/20
141 ns
Southport
33 kV
20/20
209 ns
SaltdomeT1
36 kV
17/20
183 ns
AddisT2
40 kV
18/20
212 ns
Clean-barrel
53 kV
0/20
>300 ns
Table 1: Sample laboratory NDBD Data [10].
20 test shots per sample, 300 ns pulse.
Fig. 1: Graphical depiction of the ASTM/IEC methods of slowly raising an ac test
voltage and the NDBD method of applying a high speed pulse
through the 1990s and resulted in the nondestructive breakdown (NDBD) test.
The non-destructive breakdown test
Two factors influence the amount of energy
that is dissipated in a breakdown event during
a breakdown test: first, the speed at which the
test voltage can be shut off, and secondly,
the amount of energy stored in the system
that will be dissipated after the voltage is shut
off. The NDBD test addresses both of these
issues by effectively miniaturizing the test both
in instrument size and in time.
When the voltage is raised in the existing test
methods to cause a dielectric breakdown,
the breakdown event itself is extremely fast
relative to the voltage ramp. Relative to the
50 Hz or 60 Hz sine wave of the test instrument,
a typical breakdown event with a breakdown
transition time of only a few nano-seconds
[6] appears to transition from a static or
DC voltage.
In the NDBD test, only the portion of the sine
wave - under conditions that are known
to cause a breakdown - are applied to
the test cell. Thus, the NDBD test applies a
very short DC pulse to the test cell which
has a geometry and voltage stress that will
cause a breakdown. The measurement that
the NDBD test then performs is the time it
takes for the oil to break down under these
conditions. Whereas the existing methods
measure the voltage of breakdown as their
output variable, the NDBD test measures the
time to breakdown (Fig. 1). With a proper
understanding of the test conditions, the two
can be quite agreeable [7].
In a typical NDBD test scheme, the oil is
placed in a test cell consisting of a needle
to plane gap. The needle is pulsed negative
to a fixed voltage between 20kV and
30 kV depending on the sensitivity desired.
Research has shown that a negatively pulsed
needle to plane gap provides the best
sensitivity [8], [9]. The spacing between the
needle and plane is somewhere between
0,15 mm and 0,30 mm, and pulsed for a
duration between 300 ns and 500 ns. The
gap spacing and pulse width are normally
fixed, but can be changed to bring out a
greater sensitivity or to accommodate the
equipment available.
By using a very short pulse, the total energy
stored in the system is very small. Even when
a breakdown event occurs at the beginning
of the pulse, the remainder of the pulse
width is less than 500 ns, which is much
faster than any existing method can switch
off. Depending on the impedances used
to generate the pulse, the overall energy
available to be dissipated in the test cell can
be less than 1 µJ [10]. The energy released
in the NDBD test is so small that even after
many thousands of test shots, no measurable
dissolved gas or change in dielectric strength
can be detected [9].
The output of a NDBD test is the time it takes
for the oil to breakdown under the pulse. This
value will be anywhere from 0% to 100% of
the width of the pulse (300 ns or 500 ns). If the
oil is very clean, it may not break down even
under the full pulse width. Thus, the NDBD test
method has an upper limit on the quality of oil
it can measure, much like the upper limit on
the test voltage that can be applied through
existing instruments. Changes to gap spacing
and pulse voltage can increase or decrease
this limit as needed.
In addition to measuring the time to
breakdown, the number of breakdown
events versus non-breakdown test shots
can be used as a simpler means to classify
measurements. Laboratory analysis [10]
of several oil samples revealed a strikingly
good correlation between the ASTM D877 breakdown strength and the NDBD
energize - May 2006 - Page 24
number of breakdown events and time to
breakdown (Table 1).
By not damaging the oil under test, the NDBD
method can perform many more shots than
the five limited by existing methods. Typically
a NDBD test regimen will perform between
20 and 32 test shots per sample. With this
larger number of shots a smaller standard
deviation can be developed with the large
range of data values possible from the oil’s
inhomogeneous consistency and stochastic
characteristics.
Bringing the NDBD test to the field
Taking the NDBD test method out of the
laborator y and into a field deployable
monitor required some careful consideration
to both the durability and cost of such a
sophisticated instrument. Additionally, a
remote monitor must not be able to fail in a
way that could accidentally result in damage
to the oil. The key to assuring this safety,
durability, and reasonable cost is in how the
pulse is generated.
Creating and controlling a fixed width
high voltage pulse can be a technically
complex endeavor. However, as long as
only a fixed width is required, a transmission
line pulse generator is an elegant solution. In
a transmission line pulse generator, a piece
of coaxial cable transmission line is charged
up to the pulse voltage and then discharged
through a switch or trigger device. In this
scheme, the pulse width is determined by
the length of the transmission line and thus,
it cannot fail in a way that could cause
additional energy to flow into the test cell. The
only modes of failure for the coaxial cable
involve it becoming shorter and so it can
never produce a pulse that is too long.
In an actual NDBD test instrument such as the
Weidmann Centurion, careful attention must
be paid to avoiding any stray capacitances
DISTRIBUTION
that can unnecessarily store energy that
could be dissipated in the test cell during
breakdown. In addition to the inherent
energy limitation of the NDBD test method,
the Centurion incorporates special circuitry to
reduce the current available to flow thorough
the test cell. This special circuitry reduces the
energy dissipated to virtually immeasurable
levels, below 1 µJ. This is not only necessary
to protect the oil from any possible damage,
but also to ensure the longevity of the device
since any electrode erosion cannot be
repaired once the device is installed.
Field experience
In the fall of 2001 the first prototype of a NDBD
based dielectric breakdown strength monitor
was installed on the on load tap changer of
a 40 MVA transformer. Much more recent
data has been collected from this and other
installations, but the data from the first few
months of operation has proven the most
interesting and dynamic seen thus far in
dielectric breakdown strength monitoring.
This particular transformer had suffered a
leak that allowed rainwater to enter the
OLTC compartment and saturate the oil with
moisture. In response to an indication of
low dielectric strength from the monitoring
device, oil samples were taken in early August
of 2001 and laboratory testing indicated an
ASTM D-877 breakdown voltage of 13.9 kV
and a moisture content of 85 ppm. Later
in August, repairs were made and the oil
in the LTC compartment was replaced with
new oil. As can be seen in Fig. 2, the relative
dielectric strength (a number representing the
normalized time to breakdown) rebounded
with the change in oil.
As can also be seen in Fig. 2, the day-to-day
and week-to-week dielectric strength of this
installation continued to oscillate somewhat.
This particular transformer experienced large
swings in load current which roughly correlated
with the changes in dielectric strength. It is
believed that the small changes in dielectric
strength were due to moisture migrating in
and out of the cellulose insulation with the
loading induced changes in temperature.
Fig. 2: Plot of the relative dielectric strength, a normalized average
of the time to breakdown, from the prototype monitor
non-destructive breakdown testing in the field,
and further engineering development has led
to an inexpensive monitor implementing the
NDBD technology.
The ability to continuously and accurately
measure oil dielectric strength on a live
apparatus makes way for a whole new level
of apparatus care and maintenance. The
liquid insulation of all oil filled apparatus
can be knowledgably maintained in its best
condition, preventing any unnecessary wear
or aging; and in contacting equipment online
measurements of dielectric strength can
be used to indicate the need for necessary
preventative maintenance.
As additional dielectric strength monitors
are deployed and laborator y research
is continued, the exciting field of nondestructive breakdown testing can only be
expected to grow.
References
[1]
[2]
Conclusion
The non-destructive method of dielectric
breakdown strength testing promises to
be a great improvement over the existing
methods. By not damaging the oil sample
under test, the new method can repeat
its diagnostic many more times than the
existing methods and produce much
greater repeatability and accuracy. This new
NDBD method also enables the dielectric
strength to be measured on-line. Prototype
experience has demonstrated the efficacy of
[3]
Fabre, J., Pichon, A., “Deteriorating
Processes and Products of Paper in Oil.
Application to Transformers.”, Paper 137,
CIGRE Session 1960, Paris, June 15-20,
1960
Mathis, H.-J., Baur, M., Blank, R., Woschitz,
R., and Überfall, T., “New Ways of Disruptive
Discharge Recognition in Insulation Testing
Devices,” Electrical Insulating Materials:
International Issues, ASTM STP 1376, M.M.
Hirschler, Ed., American Society for Testing
and Materials, West Conshohocken, PA,
1999
American Society for Testing and Materials.
“D877-02e1 Standard Test Method for
Dielectric Breakdown Voltage of Insulating
Liquids Using Disk Electrodes” ASTM
International, West Conshohocken, PA,
2005
energize - May 2006 - Page 25
[4]
American Society for Testing and Materials.
“ D 1 8 1 6 - 0 4 S t a n d a r d Te s t M e t h o d
for Dielectric Breakdown Voltage of
Insulating Oils of Petroleum Origin Using
VDE Electrodes” ASTM International, West
Conshohocken, PA, 2005
[5]
Kirtley, J.L., Jr. Hagman, W.H. Lesieutre, B.C.
Boyd, M.J. Warren, E.P. Chou, H.P. Tabors,
R.D. “Monitoring the health of power
transformers”, IEEE Computer Applications
in Power, Volume: 9 , Issue: 1, pp. 18-23,
Jan. 1996
[6]
Grzesik, R. G., Utreja, L. R., “The Charge
Carrier Velocity Model of the Spark Gap”,
Conference Record, 7th International
Pulsed Power Conference, pp. 522-526,
1989
[7]
Cooke, C. M., Hagman, W., “NonDestructive Breakdown Test for Insulating
Oil” EPRI Substation Equipment Diagnostics
Conference III, New Orleans, Nov. 1994.
[8]
Mazzetti, C., Pompili, W., Forster, E. O.,
“Study of the Time to Breakdown Under
Impulse Conditions” Proceedings, Second
International Conference on Properties
and Applications of Dielectric Materials,
pp. 67-69, 1988.
[9]
Cooke, C. M., Hagman, W. H., Final
Technical Report, A Non-Destructive
Breakdown Measurement for Oil
Dielectric Strength Testing, Laboratory
for Electromagnetic and Electronic
Systems and Electric Utility Program, M.I.T.,
Cambridge, MA, April 1994
[10] Cargol, T., A Non-Destructive Transformer
Oil Tester, Massachusetts Institute of
Technology, Cambridge, MA, June 2000
Contact Dave Prentice, Whiteleys - a division
of Powertech Caldus, Tel (011) 614-2231,
[email protected] 