FIELD OF INVENTION
[0001] The present invention generally relates to high voltage equipment. More particularly
the present invention relates to a bushing for high voltage applications.
BACKGROUND
[0002] Graphene Oxide is a material that has become of interest to use in a number of different
power system applications.
[0003] One experimental study that has been made of the material is disclosed in
WO 2013/033603, where reduced graphene oxide was distributed in a polymer and the field grading
properties investigated, i.e. the ability of the material to change conductivity in
dependence of the applied electrical field. Due to the way graphene oxide was mixed
with the polymer the electrical field dependency was observed to be isotropic.
[0004] The above-mentioned document also describes a number of application areas where the
material can become useful, one such area being machines bushings.
[0005] In some instances, such as at high voltage ratings, so-called condenser bushings
are frequently used in order to allow a conductor to penetrate a metallic wall and
where the potential difference between conductor and wall is large. In condenser bushings
cylindrical metallic foils, having different radii, are placed concentrically around
an inner conductor. By varying the foil lengths, such that the foils close to the
inner conductor are longer than those further out, a significantly more homogeneous
electric field is created leading in turn to a reduction of the maximum field stress.
[0006] In this manner the probability for breakdown and failure can be drastically lowered.
[0007] As the voltage ratings become higher, the bushing size increases. The footprint,
weight, and cost also increase accordingly. It is therefore highly desirable to limit
the size by distributing the field stress as evenly as possible and to avoid regions
with locally excessive field levels. This fact, together with the observation that
bushings are one of the most highly stressed components in the power distribution
chain, implies that their design is crucial for the economy and reliability of any
high voltage power distribution network.
[0008] A bushing is particularly sensitive in the vicinity of the foil edges. With a given
potential difference between a foil and its closest neighbor, the maximum electric
field is found just at the edge of the foil. The metallic foils in a condenser bushing
are usually very thin, of the order of 10-100 µm, and therefore the field enhancement
can be quite large. It is therefore of interest to reduce these stresses.
[0009] One document that describes the use of graphene oxide in relation to foil edges is
WO 2014/206435. In this document graphene oxide is used in a material where the degree of reduction
varies so that a permanent gradient exists in the electrical conductivity of the material.
Thereby the conductivity in one part of the material becomes higher than in another.
There is thus a step-wise change between the conductivities. A bridging element with
these properties is obtained using an annealing method where graphene oxide is irradiated
with laser light and the bridging element is then used at a foil edge of a bushing.
[0010] There would, in view of what is mentioned above be of interest to reduce the stress
on a foil edge through using graphene oxide that obtains a smoother conductivity variation
beyond the foil surface with a limited effect on the bushing size.
SUMMARY OF THE INVENTION
[0011] One object of the present invention is to provide a bushing that addresses the problem
of reducing foil edge stress without increasing the bushing size.
[0012] This object is according to the present invention obtained through a bushing for
high voltage applications comprising a conductor surrounded by at least one foil of
conductive material, where the foil has an edge over which a layer of semiconductive
paint stretches, where the semiconductive paint comprises flakes of thermally reduced
graphene oxide as a field grading material having a conductivity that varies with
electrical field strength. The layer has a thickness and the flakes have at least
one width, where the thickness of the layer is smaller than the width of the flake.
[0013] The present invention has a number of advantages. In particular, it provides excellent
reduction of foil edge stresses and this is achieved with negligible increase in bushing
size. In addition there is a reduced need of field grading material in the paint.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The present invention will in the following be described with reference being made
to the accompanying drawings, where
fig. 1 schematically shows a bushing with an inner conductor surrounded by a field
reducing volume comprising a number of foils, where one section of the field reducing
volume has been indicated,
fig. 2 schematically shows a side view of parts of two neighboring foils in the indicated
section, where one of the foils has been extended with a layer of semiconductive paint,
fig. 3 schematically shows a flake of graphene oxide used in the semiconductive paint,
fig. 4 shows the dependence of conductivity on electrical field strength for a number
of different types of graphene oxide flakes,
fig. 5 schematically shows a number of interconnected flakes in the layer of semiconductive
paint,
fig. 6 schematically shows how the maximum electrical field strength depends on the
low-field conductivity σo for two different paints at two different lengths of extension layer beyond the foil
edge, and
fig. 7 shows a view from above of solid insulation used in the bushing and on which
a layer of semiconductive paint has been applied.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention concerns a bushing for use in high voltage environments, such
as high voltage power transmission environments. A bushing may as an example be a
transformer bushing used for insulating the high voltage conductor from the grounded
transformer housing. It may also be a wall bushing such as a bushing passing through
a wall of a building, for instance a converter hall.
[0016] Fig. 1 schematically shows a bushing 10 comprising a central inner conductor 12 surrounded
by a field reducing volume 18 comprising a number of cylindrical foils 14 of electrically
conducting material, such as foils of Aluminum. The inner conductor 12 is thereby
surrounded by at least one foil of conductive material, which may have a thickness
in the range of 10 - 100 µm. The foils 14 may be coaxial with the conductor 12 but
have different radii. Thereby the conductor 12 may also define an axis that the foils
14 surround. The foils 14 may be separated from each other and the inner conductor
12 by gaps, where a gap may as an example have a width of 250 µm. In these gaps electrical
insulation may be provided, which insulation may comprise solid insulation as well
as fluid insulation. Solid insulation may be cellulose, while fluid insulation may
be transformer oil. Thereby the solid insulation may also be immersed in the fluid
insulation. In the figure, there is also a flange 16 connected to the outermost foil.
[0017] The boundary points or edges of the above-mentioned foils are furthermore equipped
with a layer of semiconductive paint, which paint comprises a field grading material.
One region 20 of the field reducing volume 18 comprising parts of two foils where
such a semiconductive paint has been applied is also indicated in fig. 1.
[0018] Fig. 2 schematically shows the region 20 with parts of the two foils 22 and 24, where
the upper foil 22 has been provided with a layer of semiconductive paint 26 comprising
field grading material. The upper foil 22 has a foil thickness FO
T, which thus may be 10 - 100 µm. The layer 26 is applied on top of the upper foil
22, i.e. on a surface facing away from the previously mentioned central conductor
and also extends beyond the edge of this foil 22 in a first direction in parallel
with the axis of the inner conductor. The layer 26 is thereby aligned with an upper
foil surface facing away from the conductor 12 and also stretches over the edge. Moreover,
the layer 26 may stretch away from the foil edge in a direction that is parallel with
the axis defined by the inner conductor. The layer 26 extends a distance L beyond
the edge of the foil 22. It is thereby aligned with and covers the foil edge. The
paint thereby has a first thickness L
T above the foil and a second thickness L
T + FO
T in the area stretching away from the foil. The semiconductive paint comprises a field
grading material (FGM), i.e. a material that has a field-dependent conductivity (or
permittivity). This FGM material redistributes the electric field and reduces the
maximum stress on the foil edge.
[0019] Even though only one foil is shown as being provided with the layer 26 of semiconductive
paint, it should be realized that more foils and with advantage all may be provided
with such a layer. Moreover, a foil will have a second edge at an opposite end of
bushing. It should be realized that this end may in a similar manner be provided with
a layer of semiconductive paint stretching out beyond the edge in a second direction
in parallel with the axis of the inner conductor, which second direction is opposite
to the first direction.
[0020] The semiconductive paint is such that it has a conductivity that increases with the
electrical field strength. Through this property the electrical field strength will
be gradually lowered away from the foil edge leading to lower stresses on it. Due
to the shape of the flakes, the electrical conductivity in the perpendicular direction,
which is radially away from the central axis of the inner conductor, is significantly
lower compared to in the direction along the length of the foil and may even lack
dependence of the electrical field strength.
[0021] According to aspects of the invention the enhanced field stress at the foil end edges
of a high voltage condenser bushing is reduced through the paint being applied in
a thin layer and using thermally reduced graphene oxide as a field grading material
(FGM).
[0022] Traditionally FGMs have been made based on a percolated network of semiconductive
particles, such as SiC or ZnO, in a polymer matrix. In practice, however, it has been
difficult to physically produce FGM layers sufficiently thin and having the optimal
material parameters required. According to percolation theory, around 17 vol% of spherical
particles are required to create a continuous network in the three-dimensional systems
of interest. Since the density is significantly higher for ZnO (ρ= 5.6 g cm
-3) and SiC (ρ= 3.2 g cm
-3) compared to the typically used polymers (Silicone rubber, EPDM, Epoxy resins; ρ
∼ 1 g cm
-3) this means that the materials are filled with at least 50 wt% of the particles,
resulting in poor mechanical properties (low elongation of break and low tear resistance).
In addition the particle diameter of the SiC ranges between 1 - 30 µm, most commonly
3 - 10 µm. The contact resistance between the grains are responsible for the nonlinear
conductivity. In ZnO microvaristors the nonlinearity is an intrinsic property of the
interior of the filler particles alone, and the particle contact resistance is low.
The diameter of the microvaristors ranges between 10 to several hundred micrometers.
A consequence is that it is not possible to produce very thin layers (<10 µm) that
at the same time are mechanically robust and have homogeneous properties. Furthermore,
any attempt to apply a too thick layer, e.g. in the form of a commercially available
FGM tape for machine insulation (thickness 0.1 - 0.3 mm) will result in "bumps" in
the winding layers near the foil edges. Such irregularities will create small voids
where harmful partial discharges can occur, eventually leading to material fatigue
and local breakdown.
[0023] Therefore, the field grading material of the paint is according to aspects of the
invention based on graphene oxide.
[0024] Graphene oxide is provided in flakes. Fig. 3 schematically shows the generic geometry
of one such flake of graphene oxide having a flake width F
W1, where this width, which may be the diameter of the flake, may be roughly 1 µm. The
flake width may generally be in the range 0.5 - 10 µm. A flake may also have a thickness
F
T that is about 1 nm. The flake may generally have a thickness F
T in the range 0.5 - 10 nm and thereby the flake will be more or less two-dimensional.
It may thus be essentially two-dimensional.
[0025] It has been experimentally shown that FGMs based on graphene oxide indeed exhibit
a strong nonlinear behavior as regards the field-dependent conductivity, see
Z. Wang, J.K. Nelson, H. Hillborg, S. Zhao, and L.S. Schadler, "Graphene Oxide Filled
Nanocomposite with Novel Electrical and Dielectric Properties", Adv. Mater., Vol.
24, No. 23, pp. 3134-3137, 2012. There, 3 - 5 phr (parts by weight per 100 parts of silicone resin mixture) of graphene
oxide was thermally reduced at different temperatures, and subsequently dispersed
in a silicone matrix followed by crosslinking to create a silicone rubber. Fig. 4
shows measured conductivity curves for four FGMs with graphene oxide isotropically
dispersed in a polymer matrix, from the above mentioned document. The FGMs are based
on silicone rubber filled with 3 or 5 wt% of thermally reduced graphene oxide. The
graphene oxide flakes were reduced at 70, 120 or 140 °C. In fig. 4 the diamond shaped
markers are related to flakes with 5 wt% graphene reduced at 70 °C, the square shaped
markers are related to flakes with 5 wt% graphene reduced at 120 °C, the circle shaped
markers are related to flakes with 3 wt% graphene reduced at 120 °C and the triangle
shaped markers are related to flakes with 3 wt% graphene reduced at 140 °C.
[0026] The onset of non-linearity increased with increasing reduction temperature. The investigated
material is based on a high aspect ratio filler which is randomly oriented in a polymer
matrix. Thanks to the high aspect ratio, perculation is obtained at around 3 wt% (1.5
vol%).
[0027] From this investigation of graphene oxide, which is also discussed in previously
mentioned
WO 2013/033603, it can thus be seen that the material is promising to be used as field grading material.
Therefore aspects of the invention are directed towards using graphene oxide in the
semiconductive paint. The paint thus comprises flakes of thermally reduced graphene
oxide as a field grading material having a conductivity that varies with electrical
field strength.
[0028] The paint may more particularly comprise the above-mentioned flakes dispersed in
a suitable polymer resin (binder) and solvent, where it is possible that the amount
of graphene oxide is 1 - 5 wt %.
[0029] The paint may be deposited onto the foil and possibly also the solid insulation between
it and a neighbouring foil by spraying or printing, for instance with a thickness
of 10 - 200 nm. The first thickness L
T may thus be 10 - 200 nm. Since the two-dimensional (2D) graphene oxide flakes are
very small (typically 1 nm in thickness and 1 x 1 µm in length and width) compared
to SiC and ZnO particles it is possible to produce thin and homogeneous paint layers.
It is possible that the flake width is at least twice the layer thickness. It may
be even be preferred that the flake width is at least four times the layer thickness.
As can be seen in fig. 2, it is possible that this relationship is only valid for
the part of the layer of paint 26 that is provided on top of the foil 22, but not
for the part that stretches out over the edge. The relationship may thus be valid
at least above the foil. Moreover, it can be noted that the graphene oxide flakes
can be individually dispersed, but also in the form of multiple layers (graphitic
structure). Thereby the size increase of the bushing caused by the introduction of
the paint is negligible.
[0030] It is also possible that the layer thickness L
T is at least twice the flake thickness F
T. It is more particularly possible that the layer thickness L
T is 2 to 200 times larger than the flake thickness F
T in order to obtain the graphitic structure. It is here possible that the range is
only valid for the part of the layer of paint 26 that is provided on top of the foil
22 and not for the part that stretches out over the edge.
[0031] The part of the layer of paint 26 that stretches out from the foil edge would then
have the second thickness L
T + FO
T, which can be considered to be equal to FO
T. In this area, the layer thickness would thus essentially have the same thickness
as the foil. Moreover, the thickness of this part of the layer 26 would be 1000 -
200000 times larger than the flake thickness F
T. The layer thickness might in this area also be 1 - 200 times higher than the flake
width F
w.
[0032] Furthermore, due to the fact that the flake width is considerably larger than the
layer thickness L
T, the flakes 28 will be essentially oriented along the length of the layer of paint
within this layer 26. The graphene oxide flakes may thus be partially aligned along
the surface due to the thin layer thickness and the larger flake size. This is schematically
indicated in fig. 5. Thereby the non-linear field-dependent conductivity is obtained
along this surface. Since the conductivity perpendicular to the layer is likely to
be lower, an anisotropic conductivity results. Consequently a lower filler fraction
is required, compared to randomly oriented graphene oxide flakes, thus the perculation
threshold may be < 1.5 vol%.
[0033] This semiconductive paint can then be used for reducing the electric field stress
along the foil edges in HV bushings. The semiconductive paint can be applied in desired
patterns using printing or spraying techniques.
[0034] This kind of semiconductive ink or paint can also be very useful for grading the
electric stress along printed conductive electrodes.
[0035] Furthermore, when allowing for detailed tailoring of the conductivity parameters
σ
o (the low-field constant conductivity), E
c (the threshold field above which nonlinear increase starts), and α (the exponent
of nonlinear increase), it becomes possible to further reduce the field stress and
to make this reduction more robust.
[0036] An example is shown in Fig. 6, illustrating the results from a simulation study.
This example is related to a considerably thicker layer of semiconductive paint than
described in the invention. However, the general behavior is the same, as are the
conclusions. Fig. 6 shows the maximum field stress, max (E), along the surface of
the layer of semiconductive paint plotted against σ
o for two different values each of E
c (E
c = 0.4 kV/mm and E
c = 6.0 kV/mm) and L (L = 1 mm and L = 5 mm), where L is the length of the layer of
paint beyond the foil edge. The curves C
1 and C
2 correspond to E
c = 6.0 kV/mm, where C
1 is for L = 1 mm and C
2 for L = 5 mm. Because E
c is higher than the actual electric field, the material basically behaves as a linear
FGM, i.e. the conductivity is approximately constant and equal to σ
0. The curves C
3 and C
4, on the other hand, represent E
c = 0.4 kV/mm, where C
3 is for L = 1 mm and C
4 for L = 5 mm, implying that the layer is driven far into the nonlinear regime. In
the limit of very small σ
o the FGM layer has no effect and the maximum field stress occurs at the foil edge.
At very high σ
o, on the other hand, the FGM layer is highly conducting and effectively acts as an
extension of the foil. The corresponding maximum field is therefore found at the edge
of the FGM layer. In the example Fig. 6 refers to, the FGM layer is thicker (50 µm)
than the foil (5 µm), resulting in different stress values in the two limits. It is
seen that by varying σ
o, E
c, and L it is possible to reach very low field stress values at minima which are broad,
i.e. rather insensitive to changes in the material parameters. By choosing σ
o, or actually the product of σ
o and the thickness of the layer, a few orders of magnitude smaller than the value
corresponding to the minimum for the linear case (curves C
1 and C
2 in Fig. 6), a nearly optimal and robust stress reduction is achieved.
[0037] It should be mentioned that the three relevant material parameters σ
o, E
c, and α can all be controlled by changing the material composition and/or the manufacturing
process. The theoretical optimization thus described can consequently be employed
to find physically realizable solutions that correspond to the desired electrical
and thermal performance of the bushing.
[0038] Moreover, as the material is provided as a paint or an ink, it may be applied in
an industrial printing process. It may as an example be printed on the solid insulating
material before assembly of the bushing. An example of a layer of paint 26 being printed
on solid insulation 30 is schematically shown in fig. 7.
[0039] A successful application of the above concept would imply the possibility to manufacture
high voltage bushings that can withstand higher stress levels, both in regular operation
and under transient conditions. Alternatively, for the same voltage rating the bushing
can be made smaller, lighter, and less expensive. Moreover, through the use of a paint
it is possible to obtain the desired field-grading properties in a simple way using
established industrial production techniques, such as printing.
[0040] From the foregoing discussion it is evident that the present invention can be varied
in a multitude of ways.
[0041] It shall consequently be realized that the present invention is only to be limited
by the following claims.
1. A bushing (10) for high voltage applications comprising a conductor (12) surrounded
by at least one foil (22) of conductive material, said foil having an edge over which
a layer (26) of semiconductive paint stretches, said semiconductive paint comprising
flakes (28) of thermally reduced graphene oxide as a field grading material having
a conductivity that varies with electrical field strength, said layer (26) having
a thickness (LT) and said flakes having a width (Fw,), wherein said thickness (LT) of the layer (26) is smaller than said width (Fw) of the flakes at least above the foil (22).
2. The bushing (10) according to claim 1, wherein said flake width (Fw1) is at least twice the layer thickness (LT).
3. The bushing (10) according to claim 2, wherein said flake width (FW1) is at least four times the layer thickness (LT).
4. The bushing according to any previous claim, wherein the flakes have a flake thickness
(FT) and said layer thickness (LT) is at least twice the flake thickness (FT).
5. The bushing according to claim 4, wherein the layer thickness (LT) is two to two hundred times larger than the flake thickness (FT).
6. The bushing (10) according to any previous claim, wherein the layer thickness (LT) is in the range 2 - 100 nm and the flake width (Fw) is in the range 0.5 - 10 µm.
7. The bushing (10) according to any previous claim, wherein the flakes (28) have a thickness
in the range 0.5 - 10 nm thereby making them essentially two-dimensional and generally
oriented within the layer of paint along the length of the layer (26).
8. The bushing (10) according to any previous claim, wherein the paint comprises the
graphene oxide at 1 - 5 wt%.
9. The bushing (10) according to claim 8, wherein the paint comprises a resin and a binder.
10. The bushing (10) according to any previous claim, wherein the layer (26) of paint
is aligned with an upper foil surface facing away from the conductor (12) and stretches
away from the foil edge in a direction that is parallel with an axis defined by the
conductor.
11. The bushing (10) according to any previous claim, further comprising solid insulating
material (30) adjacent the foil (22), wherein the paint (26) is printed on the solid
insulating material.
12. The bushing (10) according to any previous claim, wherein the foil (22) has a thickness
in the range 10 - 100 µm.