Note: Descriptions are shown in the official language in which they were submitted.
TITLE: ENERGY EFFICIENT BUSHING FOR A TRANSFORMER
FIELD
[0001] Various embodiments are described herein that relate to an oil-
free bushing,
also referred to as a dry bushing, which is energy efficient in character, for
a transformer.
More particularly, at least one embodiment described herein relates to a
transformer
including an oil-free bushing with low resistive losses, low eddy current
losses and low
leakage current losses.
BACKGROUND
[0002] When used with reference to electrical devices or systems, the term
"bushing"
refers to an insulated device that allows an electrical conductor to pass
safely through a
conducting barrier, which is usually earthed. An example of such a conducting
barrier is an
enclosure, or wall, of a transformer.
[0003] In a power transformer, bushings serve to connect the windings
of the
transformer to a supply line external to the transformer, while insulating an
incoming or
outgoing conductor from the enclosure of the transformer.
[0004] A bushing includes a conductor made of a conductive material,
which
connects the windings of the transformer to a supply line, and insulation
partially
surrounding the conductor. Bushings employing various types of insulating
materials have
been developed, including porcelain, paper, resin and fibreglass, such as that
shown in
Figure 5, which typically uses oil inside a resin.
[0005] Existing bushings have a number of drawbacks, including:
1. Oil-impregnated porcelain bushings tend to suffer from fractures, fires
and/or
explosions, potentially leading to injuries or fatalities. Similarly, oil-
impregnated
paper bushings may catch fire or develop oil leaks, and are also prone to
moisture
ingress. These bushings are also dependent on the availability of oil.
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2. A drawback of resin bushings is that the insulation in such bushings may be
relatively brittle and lack adequate resistance to shock, thus increasing the
risk of
failure.
3. Further, bushings having insulation provided by fibreglass may be prone to
delaminate due to high electric stress, moisture ingress, temperature extreme
fluctuations and/or as a result of pollution. Delamination is also a concern
in resin
bonded paper designs.
SUMMARY OF VARIOUS EMBODIMENTS
[0006] In accordance with at least one embodiment of the invention, there
is
provided a bushing for a transformer, the bushing comprising: an elongate
enclosure body
to accommodate a conductor extending along a longitudinal axis, the conductor
having a
first terminal end and a second terminal end, the ends extending from opposite
sides of the
enclosure body; a mounting flange fitted to the enclosure body to enable the
bushing to be
mounted to an enclosure of the transformer; the enclosure body comprising two
electrically
insulating layers partially surrounding the conductor, a first layer of the
electrically insulating
layers being substantially provided by a first polymeric material, which
includes co-axial
energy efficient screens, and a second layer of the electrically insulating
layers being
substantially provided by a second polymeric material, the first and second
layers being
arranged about the conductor in such a manner that the bushing is
substantially cavity-free.
[0007]
In at least one embodiment, the first layer including the energy
efficient
screens defines an inner core, with the second layer providing an outer cover
which at least
partially covers the inner core.
[0008]
In at least one embodiment, the energy efficient screens comprise high
relative permeability materials including one or more of nanocrystalline grain
structure
ferromagnetic metal coatings, Permalloy or Mumetal.
[0009]
In at least one embodiment, the energy efficient screens have low
magnetic
anisotropy and low magnetostriction.
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[0010] In at least one embodiment, the energy efficient screens have
a low coercivity
so that they saturate at low magnetic fields.
[0011] In at least one embodiment, the inner core includes a
condenser screen
arrangement, in the form of fine layers of metallic screens included or
inserted in the inner
core to perform two functions including voltage control by capacitance grading
and
magnetic decoupling.
[0012] In at least one embodiment, the two electrically insulating
layers may be
attached directly to the conductor, thereby providing a substantially cavity-
free bushing.
[0013] In at least one embodiment embodiments, the first layer may
be moulded
directly onto the conductor.
[0014] In at least one embodiment, the second layer may be moulded
directly onto
the first layer.
[0015] In at least one embodiment, the first layer may be
substantially provided by
resin and the second layer may be substantially provided by a hydrophobic
material. For
example, the hydrophobic material may be a polymer. In such cases, the polymer
may be
an elastic polymer.
[0016] In at least one embodiment, the first layer is substantially
provided by resin
and the second layer is substantially provided by silicone rubber. In such
embodiments,
the second layer may thus be provided by a substantially shock resistant
material.
[0017] In at least one embodiment, the coefficient of thermal expansion of
the
conductor and the first layer may be selected so as to be closely aligned with
one another,
thereby reducing the possibility or extent of delamination due to mechanical
stress caused
by a temperature gradient between the conductor and the first layer, in use.
[0018] In at least one embodiment, the second layer may provide a
plurality of
.. coaxial sheds spaced apart along the length of the bushing.
[0019] In at least one embodiment, the conductor may be provided by
a tube. In
other embodiments, the conductor may be a solid, rod-like conductor.
Alternative design
dimensions and materials may be used to minimise resistive losses.
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[0020]
In at least one embodiment, the first terminal end of the conductor may
be
configured for operative connection to an electrically active component of the
transformer
and the second terminal end of the conductor may be configured for operative
connection
to an electrically active external component.
For example, the electrically active
component of the transformer may be transformer windings and the electrically
active
external component may be a supply line.
[0021]
In at least one embodiment, the conductor is magnetically isolated from
a
transformer tank of the transformer using the energy efficient screens.
[0022]
The conductor may be manufactured from any suitable conductive material,
e.g. Aluminium or copper.
[0023]
The bushing is preferably a high voltage bushing, for use in phase-to-
phase
voltages greater than 100 kV and in current ratings ranging from approximately
1250 A to
2700 A. In at least one embodiment, the bushing is a 132 kV bushing. The
bushing may
be configured for use as a generation, transmission or distribution
transformer.
[0024]
In at least one embodiment, the bushing may include a condition monitoring
sensor. The condition monitoring sensor may be configured to monitor one or
more
predefined condition parameters associated with the bushing and to communicate
values of
one or more monitored parameters to a receiving module remote from the
bushing.
[0025]
In such embodiments, the measured condition parameter may be leakage
current in both of the two electrically insulating layers, with the sensor
taking the form of a
coupling capacitor.
[0026]
In such embodiments, the condition monitoring sensor may be arranged to
take measurements externally on a surface of the bushing and is not connected
to any
inner parts of the bushing.
[0027]
In such embodiments, the condition monitoring sensor may obtain
measurements on a single phase instead of for all three phases simultaneously.
[0028]
In an embodiment, the bushing includes a transmitter that is coupled to
the
condition monitoring sensor and is configured to transmit the measured
condition
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parameter or any other measurements made by the condition monitoring sensor to
a
remote controller, typically in an online manner.
[0029] In another aspect, at least one embodiment of the invention
extends to a
transformer, which includes at least one bushing as hereinbefore described.
[0030] Other features and advantages of the present application will become
apparent from the following detailed description taken together with the
accompanying
drawings. It should be understood, however, that the detailed description and
the specific
examples, while indicating preferred embodiments of the application, are given
by way of
illustration only, since various changes and modifications within the spirit
and scope of the
application will become apparent to those skilled in the art from this
detailed description.
BRIEF DESCRIPTION OF DRAWINGS
[0031] For a better understanding of the various embodiments
described herein, and
to show more clearly how these various embodiments may be carried into effect,
reference
will be made, by way of example, to the accompanying drawings which show at
least one
example embodiment, and which are now described. The drawings are not intended
to limit
the scope of the teachings described herein.
[0032] FIGURE 1 shows an equivalent circuit diagram of an electrical
bushing.
[0033] FIGURE 2 shows a phasor diagram that is related to the
equivalent circuit
diagram of an electrical bushing of Figure 1.
[0034] FIGURE 3 is a schematic diagram showing how to determine a
bushing's
dissipation factor DF and capacitance by summing leakage current for bushings
at one side
of a transfer.
[0035] FIGURES 4a-4c shows measurements of a main insulation (Cl), a
tap
insulation (C2) and an insulation of bushings (Cl) when the tap is not
included,
respectively.
[0036] FIGURE 5 shows a perspective view of a bushing for a
transformer,
according to at least one embodiment of the invention;
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. '
[0037] FIGURE 6 shows a cross-sectional side view of the bushing
shown in Figure
5.
[0038] FIGURE 7 shows sensors that may be used for condition
monitoring attached
on the bushing shown in Figure 5.
DESCRIPTION OF THE EMBODIMENTS
[0039] The following description of various embodiments of the
invention is provided
as an enabling teaching of the invention. Those skilled in the relevant art
will recognise
that many changes can be made to the embodiments described herein, while still
attaining
the beneficial results of at least one embodiment of the present invention. It
will also be
apparent that some of the desired benefits of the embodiments of the present
invention can
be attained by selecting some of the features of the embodiments of the
present invention
without utilising other features. Accordingly, those skilled in the art will
recognise that
modifications and adaptations to the present invention are possible and can
even be
desirable in certain circumstances, and are a part of the present invention.
Thus, the
following description is provided as illustrative of the principles of the
present invention and
not a limitation thereof.
[0040] It should also be noted that, as used herein, the wording
"and/or" is intended
to represent an inclusive-or. That is, "X and/or Y" is intended to mean X or Y
or both, for
example. As a further example, "X, Y, and/or Z" is intended to mean X or Y or
Z or any
combination thereof.
[0041] It should be noted that terms of degree such as
"substantially", "about" and
"approximately" as used herein mean a reasonable amount of deviation of the
modified
term such that the end result is not significantly changed. These terms of
degree may also
be construed as including a deviation of the modified term, such as by 1%, 2%,
5% or 10%,
for example, if this deviation does not negate the meaning of the term it
modifies.
[0042] Furthermore, the recitation of numerical ranges by
endpoints herein includes
all numbers and fractions subsumed within that range (e.g. 1 to 5 includes 1,
1.5, 2, 2.75, 3,
3.90, 4, and 5). It is also to be understood that all numbers and fractions
thereof are
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presumed to be modified by the term "about" which means a variation of up to a
certain
amount of the number to which reference is being made if the end result is not
significantly
changed, such as 1%, 2%, 5%, or 10%, for example.
[0043]
In one aspect, at least one embodiment of the present invention aims to
provide a transformer bushing that addresses the shortcomings of conventional
bushings
discussed in the background section, at least to some extent. It is also an
aim of the
invention to enable the condition of the bushing to be readily and accurately
determined. In
this regard, it is known that one way of determining the condition of a
bushing is to
calculate the bushing condition assessment variable of power factor (PF) (or
the related
dissipation factor (DF)) value, for quantifying the condition of bushing
insulation systems.
The PF and DF values are related by equations (1) and (2) below:
DF = PF
¨(PF)2
(1)
PF = DF
+ (DF)2
(2)
[0044]
Electrically bushings can be represented by the equivalent circuit
diagram
shown in Figure 1) and the related phasor diagram shown in Figure 2, which
show the
components of the total current and the applied voltage across the insulation
material of the
bushing.
[0045]
The cosine of the power angle (A) is called the power factor. The
complement
of 0 is called the loss angle and is denoted by 6 in Figure 2. If 0 decreases,
more resistive
current will flow through the insulation, and thus the power factor will
increase.
[0046]
The power factor, PF, is the ratio of the real power in watts, W,
dissipated in a
material, to the complex power which is a product of the effective sinusoidal
voltage, V, and
current, I, in volt-amperes (VA). The power factor may be expressed as the
cosine of the
phase angle (0) (or the sine of the loss angle (6)).
[0047] Equation (3) below thus provides the power factor:
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=
'r V = I P
PF = cos 0 = sin g == __ ' =
/ V=/ S VG2 ____________________________________________________________
+0).02
(3)
where:
I= total current (mA), where 12 = Ic2+1r2;
lc = capacitive current (mA);
Ir = leakage current (mA);
V = voltage applied across the insulation (V);
S is the complex power = Voltage (V) x Current (I) (Volt-Amperes (VA))
P is the real power, as follows:
P=Vx1 Watts (W)
P=Vxlx cosine (A) Watts (W)
C = equivalent parallel capacitance (F); and
G = equivalent ac conductance.
[0048] The dissipation factor, (DF), is the ratio of the resistive
current (Ir) to the
capacitive current (lc) which is equal to the tangent of its loss angle (6) or
the cotangent of
its phase angle (0) (see Figures 1 and 2). The DF is also called loss tangent,
tan6, tanD or
tan delta, and is calculated using equation (4) below:
X
DF = tan(g) = ¨I = cot(0) = = G = 1
I c R coC coC = R
(4)
where:
C = equivalent parallel capacitance (F), with C = c, ;
R = equivalent ac parallel resistance (Ohm);
G = equivalent ac conductance;
Xc = parallel reactance; and
w = 27cf (assuming a sinusoidal wave shape).
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[0049] The reciprocal of the dissipation factor DF is the quality
factor, Q, sometimes
called the storage factor. When the dissipation factor DF is less than 0.1,
the power factor
PF differs from the dissipation factor by less than 0.5 %.
[0050] One way of determining the condition of a bushing is to
measure leakage
current. The underlying principle is that all insulating dielectric materials
have some power
losses due to leakage current, which will vary depending on:
1. the type of insulation;
2. the amount of dielectric material;
3. the temperature of the dielectric material;
4. the voltage and frequency applied across the insulation;
5. the frequency of the applied voltage;
6. the humidity during operation;
7. the extent of water immersion of the bushing;
8. the extent of weathering;
9. the age of the bushing in operation;
10. the quality of the manufacturing process and
11. conditioning while in operation, as described in ASTM D150 (2011),
Standard
Test Methods for AC Loss Characteristics and Permittivity (Dielectric
Constant) of
Solid Electrical Insulation.
[0051] As losses increase due to any or all the above causes, the power
factor PF
will also increase, reflecting deterioration in insulation ability. This
deterioration is caused
by changes in the dielectric material due to:
1. aging of material;
2. inclusion of contaminants during production;
3. absorption of moisture while in service;
4. breakdown of bubble inclusions under voltage stress; and
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5. other factors as explained further below.
[0052]
With reference to Figure 3, the sum of the leakage currents for three
bushings
at one side of a transformer allows the bushing's dissipation factor DF and
capacitance
(Cl) to be determined, where the C in the above DF equation (i.e. Equation 4)
is the total
capacitance corresponding to the sum of Cl and C2 in Figure 3.
[0053]
One way of measuring leakage current is to use a sensor in the form of a
coupling capacitor. In addition, the phase angles and the frequency are also
measured,
using the same sensor. To compensate for the assumptions used in the
calculation of PF
and capacitance (Cl), algorithms for filtering and smoothing are implemented.
The
assumptions are that the line voltage at the bushing terminals is constant on
all three
phases, and that the phase angles between the phase voltages are constant.
[0054]
Figures 4a-4c show how Cl and C2 are defined, as well as offline
measurement methods, done with 10kV.
[0055]
In terms of other factors that may affect the deterioration of the
bushing's
insulation abilities, these include the following:
1. Exposure of the insulation to a range of frequencies results in
permittivity and
loss index, as a result of dielectric polarizations which exist in the
material. The two
most important are dipole polarization due to polar molecules and interfacial
polarization caused by inhomogeneities in the material. It is expected that
bushing
insulation in a substation may be exposed to the entire electromagnetic
spectrum,
from direct current frequencies (OHz) to radar frequencies of at least 3x101
Hz.
There are only very few materials, such as polystyrene, polyethylene, and
fused
silica, whose permittivity and loss index are even approximately constant over
this
frequency range.
2. The major electrical effect of temperature on an insulating material is to
increase
the relaxation frequencies of its polarizations. They increase exponentially
with
temperature at rates such that a tenfold increase in relaxation frequency may
be
produced by temperature increments ranging from 6 to 50 C. The temperature
coefficient of permittivity at the lower frequencies would always be positive
except
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for the fact that the temperature coefficients of permittivity resulting from
many
atomic and electronic polarizations are negative. The temperature coefficient
will
then be negative at high frequencies, become zero at some intermediate
frequency
and positive as the relaxation frequency of the dipole or interfacial
polarization is
approached.
3. Voltage stress causes dielectric polarizations, except interfacial
polarization,
which are nearly independent of the existing potential gradient until such a
value is
reached that ionization occurs in voids in the material or on its surface, or
that
breakdown occurs. In interfacial polarization, the number of free ions may
increase
with voltage and change both the magnitude of the polarization and its
relaxation
frequency. The DC conductance is similarly affected.
4. Humidity has the electrical effect on an insulating material of increasing
greatly
the magnitude of its interfacial polarization, thus increasing both its
permittivity and
loss index and also its DC conductance. The effects of humidity are caused by
absorption of water into the volume of the material and by the formation of an
ionized water film on its surface. The latter forms in a matter of minutes,
while the
former may require days and sometimes months to attain equilibrium,
particularly for
thick and relatively impervious materials.
5. Water immersion is the effect of water on an insulating material
approximates
that of exposure to 100 % relative humidity. Water is absorbed into the volume
of the
material, usually at a greater rate than occurs under a relative humidity of
100 %.
However, the total amount of water absorbed when equilibrium is finally
established
is essentially the same under the two conditions. If there are water-soluble
substances in the material, they will leach out much faster under water
immersion
than under 100 % relative humidity without condensation. If the water used for
immersion is not pure, its impurities may be carried into the material. When
the
material is removed from the water for measurement, the water film formed on
its
surface will be thicker and more conductive than that produced by a 100 %
relative
humidity without condensation, and will require some time to attain
equilibrium.
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6. Ageing means that under certain operating conditions of voltage,
temperature
and mechanical shocks, an insulating material may deteriorate in electric
strength
because of the absorption of moisture, physical changes of its surface,
chemical
changes in its composition, and the effects of ionization both on its surface
and on
the surfaces of internal voids. In general, both the insulating material's
permittivity
and dissipation factor will be increased, and these increases will be greater
the lower
the measuring frequency.
7. Weathering, is a natural phenomenon, which includes the effects of varying
temperature and humidity, of falling rain, severe winds, impurities in the
atmosphere,
and the ultraviolet light and heat of the sun. Under such conditions, the
surface of an
insulating material may be permanently changed, physically by roughening and
cracking, and chemically by the loss of the more soluble components and by the
reactions of the salts, acids, and other impurities deposited on the surface
of the
insulating material.
[0056] In another aspect, at least one embodiment of the present invention
thus also
aims to provide a transformer bushing that, when viewed holistically, is the
best possible
bushing when taking into account all of the factors mentioned above.
[0057]
Referring to Figures 5 and 6, a bushing 10 for a transformer is shown,
the
bushing 10 comprising an elongate enclosure body 12 to accommodate a conductor
14
extending along a longitudinal axis. The conductor 14 has a first terminal end
16 a second
terminal end 18, the ends 16, 18 extending from opposite sides of the
enclosure body 12.
In some embodiments, the conductor 14 comprises a tube. For example, the
conductor 14
may preferably comprise a solid, rod-like conductor. The conductor 14 may be
manufactured from any suitable conductive material, e.g. Aluminium or copper.
[0058] A mounting flange 20 is fitted to the enclosure body 12 to enable
the bushing
10 to be mounted to an enclosure of the transformer. In addition to a test tap
33, condition
monitoring sensors 34, 35 are attached at the flange (as shown in Figure 6).
[0059]
The enclosure body 12 comprises two electrically insulating layers 22,
24
partially surrounding the conductor 14. The first layer 22 of the insulating
layers is
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substantially provided by a first polymeric material and the second layer 24
of the insulating
layers being substantially provided by a second polymeric material. The layers
22, 24 are
arranged about the conductor 14 in such a manner that the bushing is
substantially cavity-
free (and substantially devoid of oil and paper).
[0060] The first layer 22 typically defines an inner core 26, with the
second layer 24
providing an outer cover 28 which at least partially covers the inner core 26.
The two
electrically insulating layers 22, 24 may be attached directly to the
conductor 14, thereby
providing a substantially cavity-free bushing. In some embodiments, the first
layer 22 may
be moulded directly onto the conductor 14, with the second layer 24 being
moulded directly
onto the first layer 22.
[0061] The first layer 22 may be substantially provided by resin and
the second layer
24 may be substantially provided by a hydrophobic material. The hydrophobic
material
may be a polymer. The polymer may be an elastic polymer. In one embodiment,
the first
layer 22 is substantially provided by resin and the second layer 24 is
substantially provided
by silicone rubber. The second layer 24 may thus be provided by a
substantially shock
resistant material.
[0062] The coefficient of thermal expansion of the conductor 14 and
the first layer 22
may be selected so as to be closely aligned with one another, thereby to
reduce the
possibility or extent of delamination due to mechanical stress caused by a
temperature
gradient between the conductor 14 and the first layer 22, in use. The society
for materials
engineers and scientists (ASM) lists typical values of linear and volumetric
expansion (10-6
mirn.K-1) for various materials at 20 C and 101.325 kPa as follows: Water 69
and 207;
Aluminium 23.1 and 69; Copper 17 and 51; PVC 52 and 156; Polypropylene 150 and
450.
[0063] In an embodiment, the inner core 26 includes a condenser
screen
arrangement, typically in the form of very fine layers of high relative
permeability metallic
foil screens 30 included or inserted in the inner core 26. A condenser screen
arrangement
is generally only required at voltages above 88kV, and although three screens
30 are
shown in Figure 6, the exact number, arrangement and layout of the screens 30
may vary
depending on the application. The screens 30 produce a capacitive effect which
dissipates
the electrical energy more evenly throughout the inner core 26 and reduces the
electric
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field stress between the energised conductor 14 and any earthed material. It
does this by
distributing the electric field optimally in the radial and tangential
directions, so as to
lengthen the lifespan of the insulation materials. If the capacitances due to
the screens 30
are equal, then the voltage is distributed as shown in Figure 3. A lower and
uniformly
distributed voltage within the dielectric materials reduces the electric
stress in the bushing
10. The inner core 26 may be assembled so as to minimise electric stress in
the bushing
and/or on a surface of the bushing 10. Accordingly, the screens 30 can provide
for
voltage control by capacitance coupling and they can also provide for magnetic
decoupling.
[0064] The energy efficient screens 30 comprise high relative
permeability materials
10 including one or more of nanocrystalline grain structure ferromagnetic
metal coatings,
Permalloy or Mumetal. These materials have low magnetic anisotropy and low
magnetostriction. The energy efficient screens 30 also have a low coercivity
so that they
saturate at low magnetic fields.
[0065] The outer cover 28 of the second layer 24 may include a
plurality of coaxial
sheds 32 spaced apart along the length of the bushing.
[0066] The first terminal end 16 of the conductor 14 may be
configured for operative
connection to an electrically active component of the transformer and the
second terminal
end 18 of the conductor 14 may be configured for operative connection to an
electrically
active external component. The electrically active component of the
transformer may be
transformer windings and the electrically active external component may be a
supply line.
[0067] The bushing 10 is preferably a high voltage bushing 10, for
use in phase-to-
phase voltages greater than 100 kV and in current ratings ranging from
approximately 1250
A to 2700 A. In one embodiment, the bushing 10 is a 132 kV bushing. The
bushing 10
may be configured for use as a generation, transmission or distribution
transformer.
[0068] In some embodiments, the bushing 10 may include at least one
condition
monitoring sensor 34, 35. The condition monitoring sensor 34, 35 may be
configured to
monitor one or more predefined condition parameters associated with the
bushing 10 and
to communicate values of one or more monitored parameters to a receiving
module remote
from the bushing 10. In an embodiment, the measured condition parameter is
leakage
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current in both of the two electrically insulating layers, with the sensor 34,
35 taking the
form of a coupling capacitor with detection ranging from 80 pF up to 10 nF.
The sensor 34,
35 is typically placed at the flange 20 by means of a circumferential strapped
band
attachment or a threaded bolt-in device into a connection point that is
similar to a test tap
that is present on most high voltage bushings.
[0069]
In an embodiment, the sensor 34, 35 includes a transmitter to transmit
the
measured condition parameter to a remote controller, typically in an online
manner.
Communications of measured data is network neutral or network independent. The
sensor
can thus use any available network such as a powerline carrier, a fibre
telecommunications
network or a wireless network. On-line monitoring and alarming systems allow
for the
uploading measured data to a server for remote analysis. This feature saves
customers the
costs associated with bringing in an expert and paying its staff to accompany
someone at
the local site to perform advanced diagnostics.
[0070]
The bushing described herein provides increased safety and a
significantly
lower risk to consumers. Particular advantages of at least one embodiment of
the bushing
of the invention including at least one feature from the following non-
exhaustive list:
1. The bushing is waterproof and paperless.
2. The design may eliminate or reduce the risk of bushing explosions and
reduce
the probability of burn out fires on power transformers.
3. The bushing is sustainable and environmentally friendly as it does not
utilize or
depend on fossil fuels, e.g. oil, which is a depleting natural resource and
which
fluctuates in cost.
4. The bushing is environmentally friendly and meets the requirements of
international specifications, which require transformer bushings to "be of
technology
that provides safe operation of the transformer, maintenance free or require
minimum maintenance, environmentally friendly, and as far practically possible
does
not add fire risk".
5. In some embodiments, the bushing can be monitored and maintained from a
remote location.
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6. The remote access component optimizes maintenance of the bushing and
reduces risk to employees who are hired to service bushings, as physically
attending
to a bushing would not be required frequently.
7. The design aids in providing the least possible level of partial discharges
and also
provides mechanical strength.
8. The design can be customised and is suitable for a wide range of
transformer
application.
9. Polymeric dry bushings can withstand extreme operating conditions,
including
temperatures ranging from -400 to 60 C, which significantly reduces
maintenance
and storage costs.
10. The design can be used in many different applications, e.g. generation,
transmission and distribution transformers that require increased levels of
reliability
and safety.
11. In at least one embodiment, the bushing may use shock resistant resin that
is
housed in elastic polymer in order to provide cushion against shock.
12. The use of a polymer as a main component significantly prolongs the life
of the
bushing and reduces the probability of combustion over the lifespan of the
bushing.
13. Unlike fibreglass composition bushings which delaminate under high
electric
stress, water ingress and pollution, the proposed dual polymer bushing is
highly
reliable.
14. Oil impregnated porcelain designed bushings are susceptible to explosion
and
fires which can result in injury or fatalities of personnel, which the dual
polymer
bushing embodiments of the invention addresses.
15. Most resin bushings suffer from brittle fractures as they are not shock
resistant,
so under seismic loading such bushings fail, whereas vibration simulations
based on
data sheet specifications of shock resistant resin types that may be used in
at least
one embodiment of this invention of the 132kV polymeric bushing greatly
reduces
this risk.
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16. This gives the invention an overall operating advantage in performance, as
opposed to oil insulated paper, resign impregnated paper or oil cooled resign
impregnated paper which has a higher probability of combustion over time.
17. The elimination of fibreglass and porcelain increases reliability and
reduces or
eliminates the risk of fractures, explosions causing fires, as well as
delamination.
18. The bushing can withstand a relatively high thermal load.
[0071] While the applicant's teachings described herein are in
conjunction with
various embodiments for illustrative purposes, it is not intended that the
applicant's
teachings be limited to such embodiments. On the contrary, the applicant's
teachings
described and illustrated herein encompass various alternatives,
modifications, and
equivalents, without departing from the embodiments described herein, the
general scope
of which is defined in the appended claims.
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