Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRIC POWER ASSET HEALTH MONITORING
BACKGROUND
Technical Field
The disclosure generally relates to directly measuring the amount of certain
materials including moisture in an electrical transformer.
Description of the Related Art
The presence of moisture in oil-filled transformers has one of the most
significant effects on the short-term and long-term performance of an
insulation system
within the oil-filled transformers. Moisture is always present in the
insulation system,
to some degree, when it is shipped from the manufacturer due to residual
moisture from
manufacturing and will increase over the life of the transformer due to
ingress from
environment and as a byproduct of the aging process of the cellulose included
in the
insulation system. The presence of moisture in the insulation impacts
transformer life,
dielectric performance, and load limitations.
BRIEF SUMMARY
Moisture in oil-filled electrical transformers is undesirable. The technical
problem in the related art is how to determine how much moisture is present in
the
insulation system of the oil-filled transformers and more specifically, where
in the
complex insulation structure of the transformers that that moisture resides.
An electrical transformer often uses paper as an insulator (also referred to
as
'insulation paper') for windings of the transformer. During operation of the
transformer, moisture will move from the oil to the paper and vice versa
depending on
temperature and each material's moisture absorption characteristics.
Practically, an
operating transformer is never in an equilibrium condition regarding moisture
distribution. This makes it very difficult to use equilibrium estimation
methodology,
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measuring the moisture in one medium and estimating the moisture in the other
medium. In most cases, the interest is in knowing the moisture content of the
solid
insulation at the hottest spot within a winding of the transformer.
Moisture content assessment in the insulation can be conducted directly, e.g.,
using Karl Fischer titration methodology on paper samples taken from an
operating
transformer which requires de-energization and exposure of the windings.
Samples can
be cut off from accessible areas such as leads and quickly wrapped up for
shipping to a
lab. However, taking paper samples from the interior of the transformer,
especially
from hot spot regions, is impractical. Other current methodologies in the
related art for
estimating the amount of moisture in the insulation system are indirect and do
not
provide an accurate value of moisture at the hottest spot within a winding.
These
methods in the related art are predicated on the assumption that, in a steady
state
condition, moisture will reach an equilibrium between the various materials
contained
within any closed vessel. The key assumption in these methodologies is that
the
materials are in equilibrium which often is not the case.
One or more embodiments of the present disclosure provide a system and
method for directly measuring the moisture content in the insulation impel in
an oil-
filled transformer. The direct measuring system and method provides a
technical
benefit of accurately assessing the amount of moisture in the insulation paper
of the oil-
filled transformer. The direct measuring system and method does not rely on
the key
assumptions of the various methodologies in the related art and therefore is
able to
provide an accurate measurement of the amount of moisture at a specific
location
within the transformer. That is, the present disclosure provides a specific
location
calculation of the moisture content in the insulation paper rather than a
general
assessment calculation of the moisture content based on a measured moisture in
the oil
as currently employed in the related art.
In an embodiment, a method provides a fiber optic-based sensing element inside
a transformer at a first location adjacent to an insulator wrapped around a
winding of
the transformer. The method transmits light through the fiber optic-based
sensing
element and senses optical signals based on light reflected by a grating
sensor defined
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along a length of the fiber optic-based sensing element at the first location.
The method
also determines a moisture parameter of the insulator wrapped around the
winding at
the first location based on the sensed optical signals.
In an embodiment, a method provides a fiber optic-based sensing element inside
an insulating oil of an oil-filled transformer. The fiber optic-based sensing
element has
a light-transmitting core and a hygroscopic material at least partially
surrounding the
light-transmitting core. The method transmits light through the fiber optic-
based
sensing element and senses optical signals that are reflected back from a
grating sensor
defined in the light-transmitting core at a location along a length of the
fiber optic-based
sensing element. The method also determines a moisture parameter of the
insulating oil
of the oil-filled transformer based on the sensed optical signals.
In an embodiment, a system, includes a first fiber optic-based sensing element
at
a first location inside an oil-filled transformer, a second fiber optic-based
sensing
element at a second location inside the oil-filled transformer, an
interrogator operatively
coupled to the first and second fiber optic-based sensing elements, and a
processing
circuitry coupled to the interrogator. The first fiber optic-based sensing
element
includes at least one set of Fiber Bragg gratings defined along a length of
the first fiber
optic-based sensing element at the first location. The second fiber optic-
based sensing
element includes at least one set of Fiber Bragg gratings defined along a
length of the
second fiber optic-based sensing element at the second location
The interrogator is configured to transmit interrogating light to the first
and
second fiber optic-based sensing elements. The interrogator is further
configured to
receive optical signals based on light reflected back from the first and
second fiber
optic-based sensing elements.
The processing circuitry is configured to calculate a change in spectral
response
of the optical signals from a first point in time to a second point in time
after the first
point in time. The processing circuitry is further configured to determine a
moisture
parameter at the first location or at the second location based on the change
in the
spectral response of the optical signals.
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BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments are described with reference to
the following drawings, wherein like labels refer to like parts throughout the
various
views unless the context indicates otherwise. The sizes and relative positions
of
elements in the drawings are not necessarily drawn to scale. For example, the
shapes of
various elements are selected, enlarged, and positioned to improve drawing
legibility.
The particular shapes of the elements as drawn have been selected for ease of
recognition in the drawings. Moreover, some elements known to those of skill
in the art
have not been illustrated in the drawings for ease of illustration. One or
more
embodiments are described hereinafter with reference to the accompanying
drawings in
which:
FIG. 1 is a diagram of a fiber optic-based sensing element placed inside a
transformer according to various embodiments of the present disclosure.
FIG. 2 is an example configuration of a transformer and insulators wrapped
around the windings according to various embodiments of the present
disclosure.
FIG. 3 is an enlarged view of a fiber optic-based sensing element placed
adjacent to a winding wrapped with an insulator as shown in FIG. 1.
FIG. 4 illustrates a fiber optic-based sensing element having a hygroscopic
material around the fiber optic-based sensing element inside an oil-filled
transformer.
FIG. 5 is a cross-sectional view of a fiber optic-based sensing element in a
length direction including hygroscopic transducers coupled to the gratings
inscribed in
the core of the fiber optic-based sensing element according to various
embodiments of
the present disclosure.
FIG. 6 illustrates a fiber optic-based sensing element having a hydrogen
sensitive material around the fiber optic-based sensing element inside an oil-
filled
transformer.
FIG. 7 is a cross-sectional view of a fiber optic-based sensing element having
an
insulator wrapped around and affixed to the gratings according to various
embodiments
of the present disclosure.
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FIG. 8 illustrates a fiber optic-based sensing element using gratings for a
pressure sensor according to various embodiments of the present disclosure.
FIG. 9 is a flow chart of a method according to various embodiments of the
present disclosure.
FIG. 10 illustrates a system according to various embodiments of the present
disclosure.
FIG. 11 illustrates an example monitoring system according to various
embodiments of the present disclosure.
DETAILED DESCRIPTION
The following description, along with the accompanying drawings, sets forth
certain specific details in order to provide a thorough understanding of
various
disclosed embodiments. However, one skilled in the relevant art will recognize
that the
disclosed embodiments may be practiced in various combinations, without one or
more
of these specific details, or with other methods, components, devices,
materials, etc. In
other instances, well-known structures or components that are associated with
the
environment of the present disclosure, including but not limited to
interfaces, physical
component layout, etc., have not been shown or described in order to avoid
unnecessarily obscuring descriptions of the embodiments. Additionally, the
various
embodiments may be methods, systems, or devices.
Throughout the specification, claims, and drawings, the following terms take
the
following meanings, unless the context indicates otherwise. The term "herein"
refers to
the specification, claims, and drawings associated with the current
application. The
phrases "in one embodiment," "in another embodiment," "in various
embodiments," "in
some embodiments," "in other embodiments," and other variations thereof refer
to one
or more features, structures, functions, limitations, or characteristics of
the present
disclosure, and are not limited to the same or different embodiments unless
the context
indicates otherwise. As used herein, the term "or" is an inclusive "or"
operator, and is
equivalent to the phrases "A or B, or both" or "A or B or C, or any
combination
thereof," and lists with additional elements are similarly treated The term
"based on"
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is not exclusive and allows for being based on additional features, functions,
aspects, or
limitations not described, unless the context indicates otherwise. In
addition,
throughout the specification, the meaning of "a," "an," and "the" include
singular and
plural references.
FIG. 1 is a diagram of a fiber optic-based sensing element placed inside an
electrical transformer according to various embodiments of the present
disclosure.
An electrical transformer is a device that transfers electric energy from one
alternating-current circuit to one or more other circuits, typically either
increasing
(stepping up) or reducing (stepping down) the voltage. A transformer 100 as
illustrated
in FIG. 1 includes an oil-immersed transformer or an oil-filled transformer.
An oil-
filled transformer is a type of voltage transformation device that utilizes an
oil cooling
method to reduce the transformer temperature. Contrary to a dry type
transformer, the
body of an oil-filled transformer, including the core and the coils, is
immersed in
insulation oil. In operation, the heat of the coils and core is transferred to
the insulation
oil and the oil insulates and cools the coils and core.
In particular, the transformer 100 includes a magnetic core, windings 110
(e.g.,
coils), and bushings. The magnetic core provides a path for magnetic flow. The
windings create a magnetic field and consist of a conductor coil wrapped
around the
core. The bushings connect the transformer windings to external electrical
contacts,
e.g., in a substation.
FIG. 2 is an example configuration of a transformer and insulators wrapped
around the windings according to various embodiments of the present
disclosure.
The transformer 100 includes a press board cylinder 122 and a cylinder 124.
The windings 110 are wrapped around the press board cylinder 122. As shown,
the
windings 110 comprise a conductor 126 wrapped with an insulator 120. The
insulator
120 provides mechanical and dielectric strength. The press board cylinder 122
separates the windings 110 from a core and the cylinder 124 separates high
voltage
windings from low voltage windings. In the figures, in some embodiments, when
reference number 110 is used for windings, it includes the insulator 120
wrapped
around the conductor 126
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Referring to FIG. 1, the windings 110 of the transformer 100 are wrapped using
the insulator 120. A commonly used insulator material for the insulator 120 is
a
cellulose paper.
During the operation of the transformer, as the transformer heats up and is
cooled, moisture in the oil-filled transformer will move from the oil to the
paper and
vice versa depending on temperature and each material's moisture absorption
characteristics. Accordingly, directly measuring the moisture content in the
insulator
(here, the cellulose paper 120) provides an accurate measurement of the amount
of
moisture in the insulator at that particular location.
The various methods for measuring unwanted moisture in the insulation of an
oil-filled transformer in the related art are, as mentioned above, mostly
directed to an
indirect measurement of the amount of moisture. For instance, this may include
approximately measuring the amount of moisture in the oil itself as a proxy
for
estimating moisture in the paper insulator, rather than directly measuring the
amount of
moisture in the insulator. Further, due to their various assumptions (e.g.,
various
materials contained within the transformer will reach an equilibrium) and the
nature of
their indirect measurement, the various methods in the 'elated arL provide a
less reliable
measurement of the amount of moisture in the insulator of the oil-filled
transformer.
In order to provide a direct measurement of the moisture content of the
insulator
at a specific location, according to some embodiments, a fiber optic-based
sensing
element 130 is placed adjacent to the insulator of the windings 110 of the
transformer
100. In various embodiments, the fiber optic-based sensing element 130
includes an
optically-transmissive core (or a light-transmitting core) and a cladding
surrounding the
core. The fiber optic-based sensing element 130 also includes one or more
gratings that
function as a sensing element (i.e., grating sensor). The gratings are
disposed along a
length of the fiber optic-based sensing element 130. The gratings can be
provided at
various locations along the length of the fiber optic-based sensing element
130.
Further, as shown in FIG. 1, more than one fiber optic-based sensing element
130 can
be used to directly measure the moisture content (or moisture parameter
indicative of
moisture content) of the insulator of the windings at multiple locations. In
addition, a
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fiber optic-based sensing element 130 can have different types of sensors
along the
length of the fiber optic-based sensing element 130. The gratings can be
inscribed in
the fiber optic-based sensing element 130 to implement these different types
of sensors
(e.g., pressure sensor, humidity sensor, temperature sensor, etc.). That is,
in various
embodiments, grating sensors can be implemented as pressure sensors, humidity
sensors, temperature sensors, strain sensors, or the like. An example of these
gratings
include Fiber Bragg gratings (FBGs). However, the example is not limited to
Fiber
Bragg gratings.
The fiber optic-based sensing element 130, during operation, is placed
sufficiently close to the insulator such that a change in the refractive index
of the optical
fiber due to the absorption or adsorption of moisture in the insulator can be
sensed
using the FBGs. For instance, when the insulator absorbs moisture, moisture
causes
changes in the physical or optical properties of the insulator which in turn
induces
changes in the optical fiber. In one embodiment, the fiber optic-based sensing
element
130 is placed in immediate proximity to the insulator or is in direct contact
with the
insulator at a specific location to provide a direct and accurate measurement
of the
moisture parameter (one or more parameters indicative of moisture content)
within the
insulator at that location. The specific process of interrogating and
receiving a spectral
response (e.g., amplitude response, frequency response, or the like) from the
FBGs to
calculate the moisture content in the insulator will be detailed later herein.
In various embodiments, the fiber optic-based sensing element 130 may be
placed in the transformer during the manufacturing stage of the transformer.
In these
embodiments, the fiber optic-based sensing element 130 may be attached at
specific
locations of the transformer coil when the transformer is manufactured. This
has some
technical benefits as the manufacturer of a winding will likely know or at
least
anticipate the location of the hottest spot within the winding. Further, it
will have a
benefit of automatically providing spectral responses from the fiber optic-
based sensing
elements 130 to processing circuitry (see FIG. 10) via an interrogator (see
FIG. 10)
which can further lower the monitoring cost.
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Finding the hottest spot within a winding (e.g., the highest temperature of
the
winding, or "winding hottest spot temperature") is a prime concern for
transformer
operators. This variable is ideally known under all loading conditions,
especially
conditions involving rapid dynamic load changes. Accurate knowledge of the
winding
hottest spot temperature is a critical input for calculation of the insulation
aging,
assessment of the risk of bubble evolution, and short-term forecasting of the
overload
capability of the transformer. It is also critical for efficient control of
the cooling banks
to ensure that they can be set in motion quickly when needed.
Traditionally, the hottest spot temperature is provided by a winding
temperature
indicator (WTI) using a thermal image method. These devices typically rely on
a
measurement of the top-oil temperature and a simulation of the winding hottest
spot
temperature rise. An example instrument involves a bulb inserted in a
thermowell and
surrounded by insulating oil. To simulate the winding temperature, the
thermowell is
additionally fitted with a heater element, fed by a current proportional to
the
transformer loading.
This thermal image method has drawbacks, as it assumes that the temperature of
oil at the top of the cooling duct is the same as the top oil temperature
measured in an
oil-filled thermowell near the top of the tank. This may be true under stable
operating
conditions but not necessarily under dynamic rapid load changes.
Accordingly, there are technical benefits of a direct measurement of winding
temperature using a fiber optic-based sensing element 130 as it provides a
more
accurate approach for the determination of winding hot spot temperature during
both
stable loading conditions and dynamic loading conditions. That is, the use of
a fiber
optic-based sensing element 130 as described herein removes several
uncertainties in
the process of winding hot spot temperature determination and is not impacted
by any
loading conditions (both stable and dynamic loading conditions), which is an
improvement compared to the methods used in the related art.
As described herein, an assessment of winding insulation temperature under any
loading condition is a critical step for efficient management of an electrical
transformer.
The traditional model used for hot spot temperature determination has shown
many
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limitations, especially under transient conditions. However, the advantages of
the direct
measurement method described herein, using a fiber optic-based sensing element
130
according to one or more embodiments of the present disclosure, is that it
bypasses
these difficulties and uncertainties. These fiber optic-based sensing elements
can
provide dependable information for each winding, under any loading condition.
Further, a method according to one or more embodiments of the present
disclosure can
accurately measure moisture in the paper insulator using a fiber optic-based
sensing
element 130 as described herein (e.g., an FBG moisture sensor having, for
example, a
wholly polymeric fiber with a fiber grating in the cladding) mounted in the
winding.
This also allows determination of an accurate bubbling temperature
calculation.
The FBGs in the fiber optic-based sensing element 130 are capable of measuring
many parameters based on different applications. For instance, the FBGs can be
used to
sense pressure, temperature, humidity, strain, vibrations, liquid levels,
hydrogen in
transformers, and more, based on different spectral responses of the FBGs in
reaction to
different environmental conditions.
FIG. 3 is an enlarged view of a fiber optic-based sensing element 130 placed
adjacent to a winding wrapped with an insulator as shown in FIG. 1.
In particular, FIG. 3 shows an enlarged view of how a fiber optic-based
sensing
element 130 is placed sufficiently adjacent to the insulator 120 that is
wrapped around
the winding 110, to sense the moisture content of the insulator 120 at that
location.
During a normal operation of the transformer, moisture finds its way into the
transformer, particularly into the insulation system of the transformer.
Further,
moisture is likely to increase over the life of the transformer due to ingress
from
environment and as a byproduct of the aging process of the cellulose in the
insulation
system. The presence of moisture in the insulation impacts transformer life,
dielectric
performance, and load limitations, and therefore obtaining an accurate
measurement of
moisture content within the insulator is beneficial.
FIG. 3 shows moisture 150 permeated and absorbed in the insulator 120 and
further shows the FBGs 140 of the fiber optic-based sensing element 130
sufficiently
close the insulator 120. Here, when an optical interrogator (see FIG. 10)
transmits light
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into the fiber optic-based sensing element 130, a portion of the light leaks
to the vicinity
at the FBGs 140. Some part of the light is absorbed by the insulator 120 and
some part
of the light is reflected by the insulator 120. The refractive index of the
walls of the
fiber optic-based sensing element 130 and the geometry of the FBGs 140 changes
over
time as the insulator 120 absorbs moisture, and the change in the refractive
index and/or
geometry of the sensing element 130 causes the amplitude and/or frequency of
light
reflected by the FBGs 140 to change. This change can be evaluated in
comparison to
reflected light from the same or similar FBGs proximate to an insulator that
does not
include any moisture content or has a known baseline moisture content. Optical
signals
based on the reflected light are coupled back into the core of the fiber optic-
based
sensing element 130 and transmitted in return to the interrogator. The
reflected optical
signals are conveyed to the interrogator and the spectral response of the
optical signals,
including a frequency response and/or amplitude response of the optical
signals, is then
processed at the processing circuitry to determine the moisture content of the
insulator
120. In sum, light injected into the fiber optic-based sensing element 130 is
reflected
back from the FBGs with a specific spectral response (including amplitude
response
and/or frequency iesponse) that allows moisture absorption to be calculated by
measuring changes in the spectral response, e.g., from a baseline or reference
(e.g., "no
moisture") spectral response to a current spectral response.
In some embodiments, the processing circuitry may store various baseline or
reference values of the spectral response of a fiber optic-based sensing
element in order
to assess changes in the spectral response and thereby accurately determine
the moisture
content of the insulator at the location where the moisture is sensed, based
on a known
relationship between moisture content in the insulator and changes in the
spectral
response. These reference values are compared with the spectral response of
the fiber
optic-based sensing element 130 positioned at a location sufficiently adjacent
to or in
immediate proximity of the insulator where moisture content needs to be sensed
or
measured.
In at least one embodiment, the processing circuitry uses a memory coupled to
the processing circuitry to store a baseline spectral response of a fiber
optic-based
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sensing element before it is placed inside a transformer. In another
embodiment, the
processing circuitry stores a spectral response of a fiber optic-based sensing
element
when it is placed adjacent to a dry insulator containing no moisture as a
reference value.
In yet another embodiment, the processing circuitry stores a spectral response
of
a 'first' fiber optic-based sensing element that is placed adjacent to a
'second' fiber
optic-based sensing element which is placed at a location of the insulator
where
moisture content needs to be measured. The first fiber optic-based sensing
element may
include a moisture-isolated sensing portion that is used as a reference and is
placed in
close proximity to the second fiber optic-based sensing element such that
conditions
and factors besides the moisture content in the insulator are substantially
identical. The
interrogator may transmit light to both the first fiber optic-based sensing
element and
the second fiber optic-based sensing element so that the interrogator may
receive the
different optical signals having different spectral responses from the two
fiber optic-
based sensing elements in order to compare, calculate, and determine the
moisture
content of the insulator at the sensed location. Further example methods of
determining
the moisture content of the insulator 120 use a correlation of the amount of
moisture to
the received optical signals.
The benefit of having a 'reference' or 'isolated' fiber optic-based sensing
element is that an insulator such as paper ages over time. The paper will
change as it
gets older and also from the stress and ingress/outgress of moisture. Further,
aging of
the paper can be related directly to the heating and cooling cycles of the
transformer.
Accordingly, a hermetically sealed piece of insulator paper around the optical
fiber can
serve as a reference as the paper in the sealed portion is never in contact
with the oil,
but it is subject to the same heating and cooling cycles of the transformer.
FIG. 4 is a view of using a fiber optic-based sensing element having a
hygroscopic material around the fiber optic-based sensing element inside an
oil-filled
transformer.
FIG. 4 illustrates a different embodiment from FIG. 3 in that the fiber optic-
based sensing element in FIG. 4 has a hygroscopic material 160 around the
fiber optic-
based sensing element whereas the fiber optic-based sensing element in FIG. 3
does not
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have the hygroscopic material. However, the fiber optic-based sensing element
130 in
FIG. 3 and the fiber optic-based sensing element 230 in FIG. 4 are similar in
the sense
that FBGs 140 are provided along a length of the fiber optic-based sensing
element.
According to one embodiment, the hygroscopic material 160 is provided along a
length of the fiber optic-based sensing element 230. When the fiber optic-
based sensing
element 230 is placed inside the oil-filled transformer, the moisture 150
inside the oil
170 is absorbed into the hygroscopic material 160. This causes the spectral
response of
optical signals reflected by the FBGs 140 of the fiber optic-based sensing
element 230
to change due to the absorption of moisture by the hygroscopic material 160.
For
example, the walls of the fiber optic-based sensing element 230 including
hygroscopic
materials 160 may absorb moisture over time, which causes changes in the
spectral
response of the optical signals reflected at the location where moisture is
absorbed.
That is, by choosing hygroscopic materials with stable optical properties for
the fiber, it
is possible to detect the moisture level in the surrounding oil as moisture
permeates into
the walls of the fiber optic-based sensing element 230 thus changing its
refractive index
and/or the geometry of the grating element (e.g., FBGs). This change in the
optical and
geometrical properties of the grating can be measured by measuring changes in
reflected light amplitude and frequency by the grating element.
When the fiber optic-based sensing element 230 is first placed inside the oil-
filled transformer, the interrogator may inject light into the fiber optic-
based sensing
element 230 and receive reflected optical signals in return. A baseline
measurement of
the spectral response of the reflected optical signals may be stored as a
reference. As
time passes and moisture 150 inside the oil 170 is absorbed into the
hygroscopic
material 160, the interrogator may inject light into the fiber optic-based
sensing element
230 at a different point in time and receive optical signals with a different
spectral
response. The amount of moisture content in the insulator at that sensed
location can be
calculated by measuring the delta deviation of the spectral response from the
baseline
spectral response.
More specifically, when an interrogator (see FIG. 10) transmits light to the
fiber
optic-based sensing element 230, the transmitted light is leaked to the
vicinity and to its
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surroundings due to the FBGs. A part of the light is absorbed by the
hygroscopic
material 160 and a part of the light is reflected by the hygroscopic material
160. The
reflection of light by the hygroscopic material 160 changes over time as it
absorbs
moisture and the refractive index of the hygroscopic material 160 changes. The
reflected light is coupled back to the core of the fiber optic-based sensing
element 130
and is transmitted to the interrogator as optical signals having a measurable
spectral
response (e.g., frequency response and/or amplitude response).
Another method of determining the moisture content of the oil using the
hygroscopic material 160 is to use an isolated fiber optic-based sensing
element with
hygroscopic materials. In at least one embodiment, an isolated fiber optic-
based
sensing element (or also referred to as "reference fiber optic-based sensing
element")
indicates a fiber optic-based sensing element sufficiently adjacent to the
fiber optic-
based sensing element 230 to function as a reference for comparison but is
sealed from
moisture of the surrounding environment. Alternatively, another fiber optic-
based
sensing element with hygroscopic materials that is isolated may be placed in
close
proximity to the fiber optic-based sensing element 230. To be specific, at
least one of
the gratings of the isolated fiber optic-based sensing element is isolated
from the
sensing environment (e.g., transformer oil) and thereby preserving it as a
reference
which is used to detect any changes to the gratings which are exposed to the
transformer oil.
The difference in spectral response (e.g., frequency response and amplitude
response) for both fiber optic-based sensing elements is compared and
calculated by the
processing circuitry to determine the moisture content of the oil at a
specific sensed
location based on the moisture absorbed in the hygroscopic material 160. In
other
words, the difference in spectral response changes between a spectral response
of
reference fiber optic-based sensing element having the hygroscopic material
and a
spectral response of the fiber optic-base sensing element 230 having the
hygroscopic
material can be used to determine the moisture content in the oil at a
specific sensed
location.
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The spectral response of a reference fiber optic-based sensing element may be
obtained in many ways. The processing circuitry may store in the memory and
retrieve
from the memory various reference values of the spectral response of a fiber
optic-
based sensing element. The spectral response of a fiber optic-based sensing
element
having a hygroscopic material can be calculated and stored as a reference
value before
the fiber optic-based sensing element is placed inside a transformer. In
another
embodiment, the reference fiber optic-based sensing element having a sealed
portion of
the hygroscopic material can be placed adjacent to a fiber optic-based sensing
element
having a hygroscopic material that is used for sensing moisture content at a
specific
location within the transformer (e.g., any location within the oil or the
winding). The
reference fiber optic-based sensing element is placed in close proximity to
the other
fiber optic-based sensing element used to sense moisture content such that
other
conditions and factors are not substantially different. The interrogator may
transmit
light to both fiber optic-based sensing elements including the reference fiber
optic-
based sensing element so that the interrogator may receive the different
spectral
responses of the two fiber optic-based sensing elements in order to compare,
calculate,
and detelmine the moisture content of the oil at the sensed location based on
the
moisture absorbed in the hygroscopic material.
Although the hygroscopic material is shown as completely surrounding the fiber
optic-based sensing element along the length of the fiber optic-based sensing
element,
the embodiment of the present disclosure is not limited to the embodiment
shown in
FIG. 4.
In another embodiment, the hygroscopic material may be used in an area only
where the FBGs are located. That is, the hygroscopic material may be disposed
to
surround and overlap the area where the FBGs are located in the fiber optic-
based
sensing element.
If there are several FBGs at sensing locations along the length of the fiber
optic-
based sensing element that are spaced apart from each other, the same or
different
hygroscopic materials may be used to surround the FBGs at each sensing
location.
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Accordingly, the hygroscopic materials may not be located in an area where the
FBGs
are not disposed.
According to some embodiments, the cladding of the fiber optic-based sensing
element may be entirely or at least partially made of hygroscopic materials.
On the
other hand, according to other embodiments, the hygroscopic material may be
coated on
the cladding as a separate layer. However, in both cases, the spectral
response that
changes based on the hygroscopic material absorbing moisture is detected by
the
interrogator in reflected optical signals and the processing circuitry coupled
to the
interrogator may determine the moisture content of the oil based on the
detected change
in spectral response.
Examples of hygroscopic materials include, but are not limited to, Poly(methyl
methacrylate) PMMA, Polysulfonate, or the like.
FIG. 5 is a cross-sectional view of a fiber optic-based sensing element 330 in
a
length direction of the optical fiber, including hygroscopic transducers
coupled to the
gratings inscribed in the core of the fiber optic-based sensing element
according to
some embodiments of the present disclosure.
As shown, FIG. 5 describes an additional method and system of integrating
hygroscopic transducers 300 with the gratings (e.g., Fiber Bragg gratings 140)
to
convert structural changes induced in such materials due to moisture
absorption or
adsorption into stresses which, when applied to the gratings, change the
spectral
response induced by the gratings in reflected optical signals and thus lead to
a
measurable moisture content using the fiber optic-based sensing element 330.
Embodiments of FIG. 5 utilize the concept of using intrinsic hygroscopic fiber
properties for moisture detection in addition to combining multiple
transducers with the
grating.
The moisture content of an insulator 120 may be directly measured when the
insulator 120 is wrapped around gratings (e.g., FBGs 140) in a fiber optic-
based sensing
element. When an insulator 120 is wrapped around the FBGs 140 as shown in FIG.
5,
the optical interface between the FBGs 140 and the insulator 120 is critical.
An optical
coupling and a mechanical coupling between the interface of the FBGs 140 and
the
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insulator 120 allows for measurement of the moisture content in the insulator
120 by
way of changes in the spectral response of optical signals reflected by the
FBGs 140.
It may also be beneficial to use a hygroscopic transducer 300, as shown in
FIG.
5, which has a strong bond between the fiber optic-based sensing element 330
and the
hygroscopic transducer 300. The hygroscopic transducer 300 comprise of
hygroscopic
materials that expands and contracts in response to moisture absorption and/or
adsorption. In one embodiment, the hygroscopic transducer 300 is a wrapping of
the
hygroscopic material 160 at a location as shown in FIG. 5, where the
hygroscopic
transducer 300 overlaps with the location of the grating sensor 140.
One example of bonding the hygroscopic transducer 300 to the fiber optic-based
sensing element 330 includes, but are not limited to, 3D printing, gluing,
fused
interface, PVD, CVD, or the like.
FIG. 6 is a view of using a fiber optic-based sensing element having a
hydrogen
sensitive material around the fiber optic-based sensing element inside an oil-
filled
transformer.
FIG. 6 is a different embodiment from FIG. 4 in that the fiber optic-based
sensing element in FIG. 6 has a hydrogen sensitive material 180 around the
fiber optic-
based sensing element whereas the fiber optic-based sensing element in FIG. 4
has a
hygroscopic material 160 instead. However, the fiber optic-based sensing
element 230
in FIG. 4 and the fiber optic-based sensing element 430 in FIG. 6 are similar
in the
sense that FBGs are provided along a length of the fiber optic-based sensing
element.
As described in connection to the different embodiments in FIGs. 3 to 5, a
fiber
optic-based sensing element having FBGs can be used to measure various
environmental parameters, such as moisture. The fiber optic-based sensing
elements
can be extended to measure other parameters or additional parameters such as
pressure,
vibrations, liquid levels, and hydrogen in electrical transformers.
Internal arcing in an oil-filled electrical transformer can instantly vaporize
surrounding oil, generating gas pressures that can cause rupture in the tank
and
potentially spread flaming oil over a large area. Oil preservation system
malfunctions
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can increase the operating pressure above the threshold. Therefore, it is
beneficial to
monitor pressure inside a transformer to avoid catastrophic failures and
outages.
In some embodiments, this pressure when applied to FBG gratings, changes the
spectral response of light reflected by the FBG gratings, and thus lead to a
measurable
pressure parameter based on the reflected light in the fiber optic-based
sensing element.
Another cause of transformer failure is dielectric breakdown of the
transformer' s insulation system. These failures are often preceded by partial
discharge
activity (e.g., electrical sparks). Partial discharges in oil produces
hydrogen dissolved
in the oil.
According to various embodiments as shown in FIG. 6, a coating with a
hydrogen sensitive material 180 can be applied to the outer surface of the
optical fiber
where the FBGs are located. In some cases, hydrogen sensitive material can
expand or
contract in response to the presence of hydrogen. When the hydrogen sensitive
material
expands and creates strain, this changes the geometry of the FBGs, which
changes the
spectral response of light reflected by the FBGs, and lead to a measurable
parameter
that is processed to determine hydrogen concentration in transformer oil
According to at least one embodiment, the hydrogen sensitive material 180 is
provided along a length of the fiber optic-based sensing element 430. Although
the
hydrogen sensitive material 180 is shown as completely surrounding the fiber
optic-
based sensing element 430 along the length of the fiber optic-based sensing
element
430, the present disclosure is not limited to the embodiment shown in FIG. 6.
For instance, in another embodiment, the hydrogen sensitive material 180 may
surround only an area where the FBGs are located. That is, the hydrogen
sensitive
material 180 may be disposed to surround and overlap the area where the FBGs
are
located in the fiber optic-based sensing element 430.
If there are several FBGs along the length of the fiber optic-based sensing
element 430 that are spaced apart from each other, the hydrogen sensitive
material 180
may surround each of the FBGs at their respective locations. Accordingly, the
hydrogen sensitive material 180 may not be located in an area where the FBGs
are not
disposed.
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According to some embodiments, the cladding of the fiber optic-based sensing
element may be entirely or at least partially made of a hydrogen sensitive
material 180.
On the other hand, according to other embodiments, the hydrogen sensitive
material
180 may be coated on the cladding as a separate layer. However, in both cases,
the
hydrogen sensitive material 180 absorbing hydrogen 200 affects the geometry of
the
FBGs which causes changes in the spectral response of optical signals
reflected by the
FBGs. This change in spectral response is sensed by the interrogator and
communicated
to the processing circuitry coupled to the interrogator to determine the
hydrogen content
of the oil in the transformer.
FIG. 7 is a cross-sectional view of a fiber optic-based sensing element having
an
insulator wrapped around and affixed to gratings (e.g., FBGs) according to
various
embodiments of the present disclosure.
As shown, a transformer insulation paper (one example of an insulator 120) is
wrapped around and affixed to an FBG inscribed into a silica fiber. As the
optical
properties of the insulation paper change due to moisture adsorption from the
surrounding oil, the spectral response (amplitude and/or frequency) of light
reflected
and transmitted by the fiber optic-based sensing element 430 changes,
producing
measurable parameters which in turn indicate the amount of moisture in the
transformer
oil.
The fiber optic-based sensing element 430 can implement different type of
sensors based on the FBGs 140. Further, different types of sensor may be
implemented
along different portions of the same fiber optic-based sensing element. In the
example
fiber optic-based sensing element 430 shown in FIG. 7, humidity sensors HS are
implemented using FBGs 140 that are inscribed into the fiber optic-based
sensing
element 430 at two locations as shown, where the insulator is wrapped around
and
affixed to the FBGs. The fiber optic-based sensing element 430 also includes a
temperature sensor TS between adjacent humidity sensors HS. Here, in this
example,
the temperature sensor TS is not surrounded by the insulator 120. In at least
one
embodiment, the interrogator can determine which reflected optical signals are
received
from which of the sensors (the two humidity sensors HS or the temperature
sensor TS)
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by evaluating time of transmission of the injected interrogating light to time
of receipt
of the reflected optical signals. Light reflected by a sensor that is farther
along the
length of the fiber optic-based sensing element 430 will take a longer time
than light
reflected by a sensor that is closer to the interrogator.
In at least one embodiment, the insulator 120 can be positioned on and affixed
to the cladding of the fiber optic-based sensing element 430. In another
embodiment,
the insulator 120 can be positioned on and affixed to the core of the fiber
optic-based
sensing element 430. That is, a bottom surface BS of the insulator 120 may
directly
contact the core (and the FBGs in that location) and a side surface SS of the
insulator
120 may directly contact the cladding of the fiber optic-based sensing element
430.
FIG. 7 illustrates both embodiments.
The method of measuring the moisture content is determined in a similar
manner to the method described in connection with FIGs. 3 or 4. That is, the
fiber
optic-based sensing element 430 receives interrogating light from an
interrogator. At
least a portion of the light injected into the fiber optic-based sensing
element 430 from
the interrogator is reflected by the FBGs 140. The amplitude response and the
frequency response of the reflected light, as they change according to
moisture
absorption, can be detected and calculated by the interrogator (or by the
processing
circuitry connected to the interrogator). By comparing the spectral response
of the
reflected optical signals with a reference or baseline spectral response, and
measuring
the delta deviation of the spectral response of the reflected optical signals,
the moisture
content can be determined.
As shown, FBGs can be implemented in many ways to sense different
parameters including pressure. FIG. 8 illustrates a fiber optic-based sensing
element
using gratings for a pressure sensor according to some embodiments of the
present
disclosure.
In a new transformer, the windings are assumed to be under a clamping force
that is equal or greater to the forces developed during a through-fault.
However, as
aging takes place, the insulation 120 of the transformer 100 loses some of its
mechanical properties and shrinkage takes place. This reduces gradually the
windings'
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initial clamping force resulting in mechanical looseness of the windings 110
making the
windings 110 prone to fail or subject to deformations under through-fault
conditions.
Thus, as long as the transformer's clamping system (not shown) maintains the
clamping
pressure, the windings may remain tight during a through fault event and
should
therefore not sustain any damage, due to movement of the conductors. The issue
becomes problematic to transformer operators in that when a transformer ages,
the
winding relaxes and becomes loose.
Generally, transformers have a rigid clamping system to compress the winding
to a specified pressure. Any change in the thickness of the materials in the
winding 110
and associated insulation may change the pressure on the winding 110. The
thickness
of the conductor material will not change except for the thermal expansion and
contraction due to changes in temperature. The cellulose insulation material,
being
organic, will change in thickness and elasticity over time resulting from the
effect of
moisture, temperature, and aging. Therefore, it is beneficial to measure the
clamping
pressure to detect in a timely manner the looseness of windings 110 to avoid a
failure of
a transformer 100 during a through fault.
Additionally, it is note that twelve to fifteen percent of tiansfoonei
failures are
caused by winding deformations. These geometric variations lead to an increase
of
winding vibration and, consequently, to an increase of the solid insulation
mechanical
fatigue. The insulation can be degraded and short circuits between turns may
appear.
These winding deformations can also change the distance between conductors,
changing the windings series and shunt capacitances. In these cases, the
voltage
distribution in case of lightning or switching over voltages are changed and
is different
from what the transformer was designed to withstand, and therefore increases
the risk of
failure. Also, measuring the vibration of the core provides information on
whether the
vibration of the coils is caused by the core, which can be caused by the core
being
loose. Therefore, there are several technical benefits to using a fiber optic-
based
sensing element as described herein to directly measure vibrations to detect
winding
deformations and/or a loose core.
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In some embodiments, FBG sensors 140 can be disposed directly in the
windings and the core inside an oil-filled power transformer tank to measure
the
vibration of the core and the windings 110 by measuring strain force on the
FBGs. FIG.
8 shows a pressure sensor PS implemented based on FBGs 140 in a fiber optic-
based
sensing element 530 which can measure clamping pressure of the winding. For
instance, clamping pressure can be measured by affixing the FBGs 140 to the
section of
the winding near the clamp. Vibrations causing changes in the geometry of the
FBGs
140 can be detected by changes of spectral response in light reflected by the
FBGs, as
described herein, and these spectral changes can be correlated with changes in
pressure
on the FBGs 140, thus allowing the measurement system to measure vibrations
over
time. Another example method for measuring clamping pressure is to measure
strain at
the clamp by affixing the FBGs 140 of the fiber optic-based sensing element
530 to the
body of the clamp.
The application of the fiber optic-based sensing element 530 having pressure
sensors PS can be used in different settings to detect different signals and
parameters.
For instance, failures in load tap changers are frequently caused by faults
that are
mechanical in nature. These include failures of springs, bearings, shafts, and
drive
mechanisms. In some embodiments, measuring the strain forces using FBGs 140
which
change their spectral response and thus lead to a measurable changes in light
reflected
by the FBGs to an interrogator can be used to monitor vibrations in load tap
changers.
Some embodiments of the present disclosure can also measure clamping pressure
of an
on-line transformer. Different types of fiber materials and inscriptions can
be used such
as FBG, LPG and Fabry¨Perot to achieve the various objectives described
herein,
according to different applications of the fiber optic-based sensing element.
For
example, the type of optical fiber used for measuring strain forces may be
different than
the type of optical fiber used for measuring temperature and/or humidity. The
optical
fiber used for strain, for example, may be a silica or sapphire fiber coupled
with a strain
gauge.
FIG. 9 is a flow chart of a method according to various embodiments of the
present disclosure.
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The method 600 includes providing, at 610, a fiber optic-based sensing element
inside a transformer at a first location adjacent to an insulator wrapped
around a
winding of the transformer. The fiber optic-based sensing element includes at
least one
set of gratings (e.g., Fiber Bragg gratings) along a length of the fiber optic-
based
sensing element. At 620, the method includes transmitting light through the
fiber optic-
based sensing element using an interrogator. At 630, the method includes
sensing
optical signals reflected from the set of gratings. At 640, the reflected
optical signals
are sensed by the interrogator. The method 600 concludes at 650 by determining
a
sensed parameter (e.g., amount of moisture, hydrogen, or the like) based on
the sensed
optical signals, including for example changes in spectral response of the
sensed optical
signals.
As described throughout the present disclosure, embodiments of the present
disclosure provide methods for directly measuring an environmental parameter,
such as
moisture content, in paper insulation or the insulating oil in the
transformer. This
method allows for measurement of environmental parameters (e.g., moisture
content) at
multiple specific locations (e.g., point by point assessment) in the
transformer.
FIG. 10 illustrates a system according to various embodiments of the present
disclosure.
The system 700 includes a transformer 100, an interrogator 210, and processing
circuitry 750. Other elements not shown in FIG. 10 may also be included in the
system
700 such as a memory (not shown) coupled to the processing circuitry 750.
A fiber optic-based sensing element 130 is positioned adjacent to a location
in
the transformer 100 where a desired parameter is to be measured. An
interrogator 210
coupled to an end of the fiber optic-based sensing element 130 sends
interrogating light
through the fiber optic-based sensing element 130 to a grating sensor, such as
an FBG
sensor 140, positioned at the location in the transformer 100 to be measured.
The
interrogator 210 receives reflected light from the grating sensor. Measurement
data
based on quantified characteristics of the reflected light (e.g., spectral
response) as
determined by the interrogator 210 may be processed at the processing
circuitry 750 to
provide information on physical parameters of the transformer (or an
environment
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within the transformer) at the sensed location, for generating maintenance
reports and
the like. In some embodiments, measurement data may refer to raw data measured
from light reflected by each grating sensor, which could be changed in the
frequency
content or amplitude of the reflected light from each grating sensor. The
measurement
data is then correlated to, for example, the stress exerted by pressure
sensitive material
wrapped around the grating sensor or it could be changed in the frequency
content or
amplitude of the reflected light as a result of the physical changes in paper
insulation
120 wrapped around or directly adjacent to the grating sensor, as the paper
insulation
120 absorbs moisture for example.
The processing circuitry 750 may be electrically connected (e.g., by a USB
cable) to the interrogator 210 and receives measurement data (e.g., measured
light
characteristics) from the interrogator 210. The processing circuitry 750
determines the
physical parameters based on the measured light characteristics. These
parameters may
be used to calculate valuable maintenance data such as PPM (parts per million)
of
moisture content in the transformer oil or moisture content in the paper
insulation 120
of windings 110 in the transformer 100.
In various embodiments, measurement data from an inteitogator 210 may be
communicated wired or wirelessly to the processing circuitry 750.
One or more embodiments of the direct measurement method using fiber optic
grating sensors according to the present disclosure reduces the need for
additional
cables to measure each parameter (e.g., as used in current fiber optic sensors
with
semiconductor terminations). The direct measurement method using fiber optic
grating
sensors also provides better immunity to electromagnetic interference EMI
(e.g.
electrical sensing elements). It also provides higher accuracy (using spectral
measurements) and reduces overall cost of installation.
Further, the ability to measure multiple parameters simultaneously presents
the
possibility to diagnose transformer faults more holistically based on
stochastic models.
Embodiments of the present disclosure can advantageously reduce false alarms
and
human errors in diagnosing asset health, and ultimately automate predictive
maintenance and proactively repair and replace assets before they fail.
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FIG. 11 illustrates an example of a monitoring system according to various
embodiments of the present disclosure.
A fiber optic-based sensing element 130 in a transformer 100 is coupled to an
interrogator 210. The interrogator 210 receives reflected optical signals from
the fiber
optic-based sensing element 130 and based thereon, the interrogator 210
determines a
physical parameter of the transformer 100 or an environment within the
transformer
100. Such physical parameter may include, for example, a level of moisture or
hydrogen in the transformer oil or insulating paper. The interrogator 210 is
operatively
coupled to a communication unit 310 so that parameter information (e.g., the
moisture
content information or the hydrogen content information) can be wirelessly
transmitted.
A monitoring system 800 also includes a gateway 410 communicatively coupled to
a
communication tower 810 so that the information from various plants (e.g.,
wind power
plant 910, solar power plant 920, industrial plants 930, or the like) using
transformers,
such as the transformer 100, can be transmitted real-time to the monitoring
system 800.
In some embodiments, communication technologies such as LTE/4G, 5G, and
Ethernet may be used to provide an TOT (Internet of Things) gateway solution
for
'emote monitoring as shown in FIG. 11. In sonic embodiments, the interrogator
210
may include an edge computing device for localized decisions.
One or more embodiments of the present disclosure relate to online
measurement of moisture in paper insulation, temperature, and clamping
pressure, or a
combination of these parameters, in a transformer using a fiber optic-based
sensing
element as described herein. For instance, a polymer optical fiber with a
grating sensor
as described herein may be used to assess the health of electrical
transformers and other
assets in high voltage environment. The embodiments described herein can
further be
utilized to measure rapid pressure rise in the transformer tank, liquid level,
vibration in
windings, hydrogen levels, and etc.
Embodiments of the present disclosure may facilitate providing significant
improvements in asset monitoring and maintenance, including computational
costs and
reduced delay.
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Some embodiments may take the form of or comprise computer program
products. For example, according to an embodiment of the present disclosure,
there is
provided a computer readable medium comprising a computer program adapted to
perform one or more of the methods or functions described above. The medium
may be
a physical storage medium, such as for example a Read Only Memory (ROM) chip,
or a
disk such as a Digital Versatile Disk (DVD-ROM), Compact Disk (CD-ROM), a hard
disk, a memory, a network, or a portable media article to be read by an
appropriate
drive or via an appropriate connection.
Furthermore, in some embodiments, some or all of the methods and/or
functionality may be implemented or provided in other manners, such as at
least
partially in firmware and/or hardware, including, but not limited to, one or
more
application-specific integrated circuits (ASICs), digital signal processors,
discrete
circuitry, logic gates, standard integrated circuits, controllers (e.g., by
executing
appropriate instructions, and including microcontrollers and/or embedded
controllers),
field-programmable gate arrays (FPGAs), complex programmable logic devices
(CPLDs), etc., and various combinations thereof.
The various embodiments described above can be combined to provide further
embodiments. These and other changes can be made to the embodiments in light
of the
above-detailed description. In general, in the following claims, the terms
used should
not be construed to limit the claims to the specific embodiments disclosed in
the
specification and the claims, but should be construed to include all possible
embodiments along with the full scope of equivalents to which such claims are
entitled.
Accordingly, the claims are not limited by the disclosure.
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