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High-voltage bushing
EP2180485A1
European Patent Office
- Other languages
German French - Inventor
Jan Czyzewski Jens Rocks Norbert Koch Kenneth Johannsson - Current Assignee
- ABB Research Ltd Switzerland
- ABB Research Ltd Sweden
Worldwide applications
Application EP08460041A events
2008-10-27
2008-10-27
2010-04-28
2011-05-11
Application granted
2011-05-11
Status
Active
2028-10-27
Anticipated expiration
Description
translated from
-
[0001] The subject of the invention is a high-voltage bushing applicable in electric power engineering. -
[0002] Bushings are devices used to lead a conductor under voltage through an opening in a wall or equipment at ground (earth) potential. A typical condenser bushing for medium- or high-voltage applications, that is, from 24kV to 800kV and above, comprises a condenser core with a number of concentric electrically conducting field-grading layers of cylindrical shape arranged around the central conductor so as to form a capacitive divider uniformly distributing the voltage among the field-grading layers. As a result, the electric field generated by the high voltage is also uniformly distributed, both inside the condenser core in the radial direction, and outside, close to the outer surface of the bushing, along its axis. -
[0003] Field-grading layers of the condenser core are usually made of metal foil. Bushings using such field-grading layers are known, e.g. from the following patent descriptions:US 3875327 ,US 4 362 897 ,US 4 338 487 ,US 4 387 266 ,US 4 500 745 andGB 1 125 964 -
[0004] Field-grading layers made of metal foil are characterized by very low surface resistivity, typically 1-3mΩ per square. The geometrical arrangement of the field-grading layers of such low resistivity in the condenser core constitutes a number of interconnected capacitance and inductance elements prone to resonant high-frequency oscillations of large quality factor. Such oscillations are triggered by electric impulses of high frequency and lead to local occurrence of high electric field in the condenser core, with a risk of insulation damage. -
[0005] One of the methods to avoid such oscillations is application of field-grading layers of increased surface resistivity. Increased resistivity of the field-grading layers leads to reducing the quality factor of the oscillation circuits. In consequence, the amplitude of the oscillations is reduced and there is less risk of insulation damage. Numerous ways are known of manufacturing field-grading layers of increased electric resistivity. -
[0006] From a Japanese patent descriptionJP 01283716 -
[0007] Field-grading layers used in a bushing known from patent descriptionW02006/001724 are made on the basis of paper, fabric or nonwoven cloth and contain conducting particles suspended in it and forming a percolating network, electrically conducting in the layer plane. The particles can be e.g. carbon nanotubes, carbon nanofibers, metallic microfibers. Such percolative structures are also characterized by electric resistivity higher then that of metals. -
[0008] Another way of increasing the resistivity of the field-grading layers is using a very thin layer of metal, for example deposited on insulating material. From the unpublished patent applicationEP06460047 -
[0009] The innermost field-grading layer of the condenser core of a bushing is electrically connected to the conductor of the bushing. The outermost, and/or one of the other outer field-grading layers are electrically connected to the ground potential. Connection to the ground potential goes typically via the metallic flange which serves to mechanically fix the bushing to the grounded equipment. -
[0010] In most of the condenser bushing types, at least one of the connections of the outer field-grading layers is arranged in a socket, so that it can be disconnected from the grounded flange and connected to a testing device to perform electrical tests on the bushing. During normal operation, the socket is short circuited by a conductive plug. Depending on the function, the socket is known as test- or measurement tap (when connected to the outermost field-grading layer) or voltage- or potential tap (when connected one of the other outer field-grading layers). -
[0011] During normal operation, the connections of the inner- and the outer field-grading layers carry a relatively small capacitive current flowing through the electric capacitance of the condenser core. In surge conditions caused by a lightning impulse, operation of a surge arrester or switching, a pulse of substantially higher current is carried by the connection. A similar situation occurs during impulse testing of a bushing performed in laboratory conditions. During the pulse, the current is being distributed from the connection into the field-grading layer plane and the highest density of the surface current in the field-grading layer occurs close to the point at which the electrical connection is attached to the layer. For a circular connection point, the current density is equal to the total current divided by the circumference of the connection spot. For a typical connection used with field grading layers made of metal foil, this circumference is very small. For example, for a soldering connection, the circumference of the contact point (a drop of solder) is typically of the order of 5-15mm, and the local surface current density in the layer can be very large, much larger than the current densities in all the other layers, to which the connections are not provided. Field-grading layers of increased surface resistivity have smaller ability to conduct large electric current than the conventionally used metal foils. Thus, the application of the field-grading layers of increased resistivity, as the layers to which the connections are attached requires a dedicated, new solution. -
[0012] A bushing with field-grading layers made of material of limited current carrying capacity, and, in the same time, with improved current withstand of the connection during a surge, is known from a patent descriptionGB 539 587 -
[0013] In that technical solution, in a surge condition, the distributed path of the current is formed by the substantial capacitance between the field-grading layer and the surge-draining layer. The resistance of the surge-draining layer is very low, compared to the impedance related to that capacitance. Effectively, during the surge, the current path goes from the connection, along the surface of the surge-draining layer and farther, through the capacitance into the field-grading layer in the direction perpendicular to both layers. Thus, virtually no current flows along the surface of the field-grading layer and even when the layer of increased resistivity is applied, there is no effect of damping of high-frequency oscillations by the resistance of that layer. -
[0014] The problem to be solved is to provide an electric connection to a field grading layer of increased resistivity, made in such a way that a substantial part of the high-frequency surge current flows along the surface of that field-grading layer, thus giving the effect of damping the high-frequency oscillations but in the same time the surface current density in that layer is limited so that the layer is not damaged during the surge. -
[0015] The essence of the high-voltage bushing according to the invention, comprising a condenser core and electrically conducting field-grading layers which are arranged coaxially around the central conductor and are embedded in the insulating material of the condenser core, while an electric connection is provided, by means of a current-collecting member, to at least one of the field-grading layers, and this layer is made in form of a thin metal layer deposited on an electrically insulating substrate layer or in form of a percolating network of conducting particles suspended in a layer of electrically insulating material is that the current-collecting member is located on the surface of the layer to which the electric connection is provided and covers a part of the area of that layer. The surface resistivity of the current-collecting member is many times smaller than the surface resistivity of the layer to which the electric connection is provided. The current-collecting member is shaped so that the length of the contour line of its circumference is greater than the length of the shorter side of the layer on the surface of which the current-collecting member is located. -
[0016] Preferably, the length of the contour line of the circumference of the current-collecting member is selected so that during the impulse test required for the bushing, the root-mean-square average of the surface density of the current flowing across the surface of the layer on which the current-collecting member is located, close to the contour line of the circumference of the current-collecting member, is smaller than the root-mean-square average of the withstand current density for that layer exposed to a pulse of a shape and duration similar to the shape and duration of the current flowing through the electric connection which drains the current during the impulse test of the bushing. -
[0017] Preferably, the layer on whose surface the current-collecting member is located has surface resistivity greater than 100mΩ per square. -
[0018] Preferably, the current-collecting member has an elongated shape, and it is located on the surface of the field grading layer longitudinally with respect to the direction of the longitudinal axis of the bushing. -
[0019] Preferably, the current collecting member is positioned close to the symmetry axis of the field-grading layer. -
[0020] Alternatively, the current-collecting member has an elongated shape, and it is located on the surface of the field grading layer perpendicularly with respect to the direction of the longitudinal axis of the bushing. -
[0021] Preferably, the current-collecting member has a shape similar to a geometric figure consisting of many elongated conducting elements located parallel to one another and connected crosswise by means of another elongated conducting element. -
[0022] Preferably, the current collecting member is made of metal foil. -
[0023] Alternatively, the current collecting member is made of a braid or a woven or unwoven fabric containing metal wires, fibers or metal foil strips. -
[0024] Preferably, the current collecting member is electrically connected with the field grading layer using electrically conducting adhesive or electrically conductive paint. -
[0025] A high-voltage instrument transformer comprising a bushing according to the invention. -
[0026] The bushing according to the invention is highly resistant to high-frequency voltage oscillations or impulses since the high frequency oscillations are damped by the electric resistance of the field-grading layer to which the connection is provided. In the same time, the current density in the field grading layer is limited so that the bushing is not prone to failures due to a pulse of high current occurring during surge condition. -
[0027] The invention is presented as an embodiment in the drawing wherefig. 1 shows schematically the longitudinal section of the high-voltage bushing,fig. 2 - the same bushing in cross-section along the line A-A,fig. 3 - the unwound outer field-grading layer together with the current-collecting member in the first embodiment of the invention,fig. 4 - the unwound outer field-grading layer together with the current-collecting member in the second embodiment of the invention,fig. 5a - the unwound outer field-grading layer together with the current-collecting member in the third embodiment of the invention,fig. 5b - the field-grading layer offig. 5a with the relevant surface areas indicated,fig. 6 - an example of the waveform of the current flowing through the connection of the field-grading layer during the impulse test, andfig. 7 - the outer field-grading layers, in cross-section along the line A-A, in the first embodiment of the invention with the distribution of the current from the connection to the field-grading layers indicated in the drawing. -
[0028] The high-voltage bushing according to the invention comprises acondenser core 1 which is arranged around a centralcylindrical conductor 2. Thecondenser core 1 is placed inside a standard insulating casing intended for high-voltage bushings, which is not shown in the drawing. Thecondenser core 1 is comprised of many field-gradinglayers 3 which are placed cylindrically, coaxially around the centralcylindrical conductor 2 and are embedded in insulatingmaterial 4 of thecondenser core 1. To one of thelayers 3, for example theoutermost layer 3a, there is connected, by means of a current-collectingmember 5, an externalelectric connection 6 which connects thelayer 3a with anexternal conducting flange 7 by means of which the bushing is fixed to the earthed wall of the electric equipment, not shown in the drawing. Theelectric connection 6 can be also connected to one of theother layers 3, typically layers located nearer theflange 7, which is not shown in the drawing. Theconnection 6 can also be connected to thelayer 3 located closest to thecentral conductor 2, and theconnection 6 is then connected to thecentral conductor 2, which is not shown in the drawing either. Theconnection 6 connected to one of thelayers 3 located nearer theflange 7 or to theouter layer 3a can be also connected to a test- or voltage tap in form of the contact of a socket located in theflange 7, which is not shown in the drawing. Such socket makes it possible to connect measuring instruments to theappropriate layer flange 7, allows the earthing of the givenlayer grading layer 3a of the exemplary embodiment of the invention is made of paper filled with a percolating network of metallic fibres and its surface resistance is 10Ω - 20Ω per square. Alternatively, the field-grading layer 3a is made as a metallic film applied on an electrically insulating substrate layer made of insulating paper and its surface resistance is 5Ω - 15Q per square. The current-collectingmember 5 is placed on theoutermost layer 3a and is in electric contact with it over the whole area of the current-collectingmember 5. The current-collectingmember 5 has a shape similar to a rectangle with rounded corners and in the first embodiment of the invention it is located on thelayer 3a in such way that the longer sides of the rectangle are located parallel to the direction of the longitudinal axis of the bushing , marked by anarrow 8 in the drawing. The current-collectingmember 5 is located near the axis of symmetry of thelayer 3a. The current-collectingmember 5 is made as a flat braid consisting of copper wires, whose surface resistance is at least 1000 times less than the surface resistance of the field-grading layer 3a. The contour line of the circumference "L" of the current-collectingmember 5 is a line, substantially perpendicularly to which the current flow distribution takes place in plane of thelayer 3a from themember 5 to thelayer 3a. The current is supplied to the current-collectingmember 5 through theconnection 6. The length of the contour line of the circumference "L" is approximately twice longer than the length of the shorter side of thelayer 3a. -
[0029] In the second embodiment of the invention, the current-collectingmember 5, which is located on theoutermost layer 3a, has a shape similar to a rectangle with rounded corners and it is located on thelayer 3a in such way that the longer sides of the rectangle are located perpendicularly to the direction of the longitudinal axis of the bushing, marked by abi-directional arrow 8 in the drawing. The current-collectingmember 5 is made as a flat braid, consisting of copper wires, whose surface resistance is at least 1000 times less than the surface resistance of the field-grading layer 3a. The contour line of the circumference "L" of the current-collecting member is a line, substantially perpendicularly to which the current flow distribution takes place in plane of thelayer 3a from the current-collectingmember 5 to thelayer 3a. The current is supplied to themember 5 through theconnection 6. The length of the contour line of the circumference "L" is approximately 4 times longer than the length of the shorter side of thelayer 3a. -
[0030] In the third embodiment of the invention, the current-collectingmember 5, which is located on theoutermost layer 3a has a shape similar to a geometric figure comprised of many elongated, preferablyrectangular conducting elements 9 with rounded corners, located parallel to one another and interconnected crosswise by means of another rectangular conductingelement 10 with rounded corners, whose longer sides are located perpendicularly to the direction of the longitudinal axis of the bushing, marked with abi-directional arrow 8 in the drawing. The conductingelements grading layer 3a. The contour line of the circumference "L" of the current-collecting electrode is a line, substantially perpendicularly to which the current flow distribution takes place in plane of thelayer 3a from the current-collectingmember 5 to thelayer 3a. Current is supplied to the conductingelement 10 of the current-collectingmember 5 through theconnection 6. The length of the contour line of the circumference "L" is approximately 10 times longer than the length of the shorter side of thelayer 3a. -
[0031] In all the embodiments, the length of the contour line "L" of the circumference of the current-collectingmember 5 is so selected that during the impulse test required for the bushing, the root-mean-square average of the surface density of the current flowing across the surface of thelayer 3a near the contour line "L" is smaller than the root-mean-square average of the withstand current density for thelayer 3a exposed to a pulse of a form and duration similar to the form and duration of current flowing through theconnection 6 during the impulse test of the bushing. -
[0032] In the embodiment, the bushing undergoes a chopped lightning impulse test. An exemplary waveform of the surge current I CLI flowing through theconnection 6 during such test is shown infig. 6 . -
[0033] As shown infig. 7 , the surge current I CLI runs through theconnection 6 to the current-collectingmember 5. From there, a part I CC of the surge current I CLI, flows to thelayer 3a in the direction perpendicular to its surface and farther, capacitively to the other field-gradinglayers 3. The other part of the surge current, I NC, flows into the part oflayer 3a not covered by the current-collectingmember 5 in the direction parallel to its surface and substantially perpendicular to the contour line "L" (fig.3 ) of the circumference of the current-collectingmember 5. This part of the surge current flows farther from thelayer 3a capacitively to the successive field-gradinglayers 3 as well. Therefore, since the capacitive impedance is the main part of the high-frequency impedance of the circuit, the ratio of the values of the currents I CC and I NC corresponds to the ratio of the respective capacitances, which in turn are proportional to the surface area Scc of the current-collectingmember 5 and the surface area SNC of thelayer 3a not covered by the current-collectingmember 5, respectively, with the exception of the surface area of thezone 11 on which fragments of thewound layer 3a overlap. Hence the other part of the surge current is:
where S3a is the surface area of thelayer 3a except for the surface area of thezone 11 on which fragments of the rolled uplayer 3a overlap. -
[0034] The surface density ρL of the current flowing across the surface of thelayer 3a, near the contour line "L", is, on average, I NC/L, where L is the length of the contour line "L", hence
-
[0035] Using the above equation, the length L is selected so that the root-mean-square average current density ρL does not exceed the average withstand current density for the material of which thelayer 3a embedded in insulating material used in the bushing is made. The root-mean-square average withstand current density for the material of thelayer 3a is defined for a pulse of a form similar to the form of the pulse I CLI, or the form of its envelope indicated by a dashed line, both indicated infig. 6 and of duration identical with or longer than the duration of the pulse I CLI. -
[0036] For such selection of the contour line L length, the material of thelayer 3a is not damaged during the applied impulse test. At the same time, a large part I NC of surge current I CLI flows across the surface of thelayer 3a and the electric resistance of this surface contributes to the attenuation of the high-frequency oscillations. -
[0037] The above described division of the surge current I CLI into the currents I CC and I NC applies to a case where theconnection 6 is provided to the outer field-grading layer 3a. For a case where theconnection 6 is provided to another layer, the division of the surge current takes place according to the relation between other corresponding surfaces. In particular, for the innermost layer closest to thecentral conductor 2, the division of currents takes place in proportion to the corresponding surface areas of the next, neighbouring it on the outside, field-grading layer. -
[0038] -
[0039] -
[0040] In all embodiments of the invention, the current-collectingmember 5 can be alternatively electrically connected with the field-grading layer 3a by means of a layer of conductive adhesive and/or paint, which is not shown in the drawing. -
[0041] Alternatively, the bushing according to the invention is an element of a high-voltage instrument transformer.
Claims (11)
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Claims (11)
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- A high-voltage bushing comprising a condenser core (1) and electrically conductive field-grading layers (3) which are arranged coaxially around the central conductor (2) and are embedded in insulating material (4) of the condenser core (1), while an electric connection (6) is provided to at least one layer (3a) of the field-grading layers (3) by means of a current-collecting member (5), and the layer (3a) is made in form of a thin metal layer deposited on an electrically insulating substrate layer or in form of a percolating network of conductive particles suspended in a layer of electrically insulating material, characterised in that the current-collecting member (5) is located on the surface of the layer (3a) and it covers a part of the surface area of the layer (3a) and it has a surface resistivity many times smaller than the surface resistivity of the layer (3a) and it is shaped so that the length of the contour line of its circumference (L) is greater than the length of the shorter side of the layer (3a).
- A bushing according to claim1, characterised in that the length of the contour line of the circumference (L) of the current-collecting member (5) is selected so that during the impulse test required for the bushing, the root-mean-square average of the surface density of the current flowing across the surface of the layer (3a) close to the contour line (L) is smaller than the root-mean-square average of the withstand current density for the layer (3a) exposed to a pulse of a shape and duration similar to the shape and duration of the current flowing through the connection (6) during the impulse test of the bushing.
- A bushing according to claim 1 or 2, characterised in that the layer (3a) has surface resistivity greater than 100mΩ per square.
- A bushing according to any of the previous claims, characterised in that the current-collecting member (5) has an elongated shape, and it is located on the surface of the layer (3a) longitudinally with respect to the direction of the longitudinal axis of the bushing.
- A bushing according to claim 4, characterized in that the current collecting member (5) is positioned close to the symmetry axis of the field-grading layer (3a).
- A bushing according to claims 1, 2 or 3, characterised in that the current-collecting member (5) has an elongated shape, and it is located on the surface of the layer (3a) perpendicularly with respect to the direction of the longitudinal axis of the bushing.
- A bushing according to claims 1, 2 or 3, characterised in that the current-collecting member (5) has a shape similar to a geometric figure, composed of many elongated conductive elements (9), positioned parallel to one another and connected crosswise by means of another elongated conducting element (10).
- A bushing according to any of the previous claims characterized in that the current collecting member (5) is made of metal foil.
- A bushing according to any of the claims 1 to 7, characterized in that the current collecting member (5) is made of a braid or a woven or unwoven fabric containing metal wires, fibers or metal foil strips.
- A bushing according to any of the previous claims, characterized in that the current collecting member (5) is electrically connected with the field grading layer (3a) using electrically conducting adhesive or electrically conductive paint.
- A high-voltage instrument transformer, characterized in that it comprises a bushing according to the invention.