Patent Application
20130199837
The subject matter disclosed herein relates to high voltage bushing assemblies, and more specifically, to corona resistant high voltage bushing assemblies applied to a hydrogen-cooled large turbo generator.
When power is provided to a device or structure, a bushing assembly may be used to help isolate the power line from the building or structure. For example, bushings are used to provide high voltages to turbines. Bushings include a conductor, an insulating sleeve around the conductor, and a device to affix the insulating sleeve to the building or structure. The conductor passes through the insulating sleeve and into the building or structure.
According to one aspect of the invention, a bushing assembly comprises an insulating sleeve to surround a conductor; a flange located on an outside surface of the insulating sleeve; and a first band of semiconductive glaze located on the outer surface of the insulating sleeve spaced apart from a first end of the insulating sleeve, the first band of semiconductive glaze including a plurality of sub-bands having different resistivities.
According to another aspect of the invention, a high-voltage bushing system comprises a bushing having an insulating sleeve to surround a conductor and a flange on an outside surface of the insulating sleeve to mount the bushing to a structure, the outside surface of the insulating sleeve having at least one band of semiconductive glaze spaced apart from an end of the insulating sleeve, the at least one band of semiconductive glaze including a plurality of sub-bands having different resistivities; and a current transformer spaced apart from the bushing to monitor a current of the conductor.
According to yet another aspect of the invention, a high-voltage bushing assembly comprises an insulating sleeve to surround a conductor; at least one band of semiconductive glaze on a surface of the insulating sleeve, the at least one band including a plurality of sub-bands having different resistivities; and non-semiconductive glaze on portions of the surface of the insulating sleeve that do not include the at least one band of semiconductive glaze.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
At the first end 2 of the bushing 1, between an exposed portion of the conductor 10 and the flange 30, are a first set of annular ribs or ridges 21 and a first band of semiconductive glaze 61 (or first semiconductive-glazed band). A non-semiconductive-glazed portion 25 is located between the exposed portion of the conductor 10 and the ridges 21. At the second end 3 of the bushing 1, are a second set of annular ribs or ridges 24 and a second band of semiconductive glaze 62 (or second semiconductive-glazed band). A non-semiconductor-glazed portion 26 is located between the second set of ridges 24 and an exposed portion of the conductor 10. Throughout the specification and claims, the portions 21 and 24 are described as ribs, ridges, sets of ribs/ridges, ribbed/ridged portions, annular ribs/ridges, and the like.
The first semiconductive-glazed band 61 includes a plurality of sub-bands having different resistivities. The plurality of sub-bands is arranged to form a resistivity gradient from the non-semiconductive-glazed portion 25 to the first sub-band 63 to the second sub-band 64. In other words, the non-semiconductive-glazed portion 25 has a resistivity greater than each of the sub-bands 63 and 64 of the first semiconductive-glazed portion 61, and the first sub-band 63 has a resistivity greater than the second sub-band 64.
Similarly, the second semiconductive-glazed band 62 includes a plurality of sub-bands having different resistivities. The plurality of sub-bands is arranged to form a resistivity gradient from the non-semiconductive-glazed portion 26 to the third sub-band 65 to the fourth sub-band 66. In other words, the non-semiconductive glazed portion 26 has a resistivity greater than each of the sub-bands 65 and 66 of the second semiconductive-glazed band 62, and the third sub-band 65 has a resistivity greater than the fourth sub-band 66.
Although
The flange 30 includes a base portion 31 having a substantially cylindrical or conic shape, and an extended portion 32 extending from the base portion 31. In one embodiment, the extended portion has a substantially disk-like shape. In some embodiments, the flange 30 includes additional features, such as supporting braces and holes for mounting or fixing the flange 30 to a surface. In another embodiment, the base portion 31 of the flange 30 is parallel to the surface of the insulating sleeve 20. For example, each of the outer surface of the insulating sleeve 20 and the base portion 31 of the flange 30 may be cylindrically or conically shaped, and the base portion 31 of the flange 30 may extend along a portion of the outer surface of the insulating sleeve 20 and surround the insulating sleeve 20.
The first and second bands of semiconductive glaze 61 and 62 are portions of the bushing 1 in which semiconductive materials are incorporated into a glaze that makes up an outer layer of the insulating sleeve 20. In some embodiments, the portions of the bushing 1 that do not include the bands of semiconductive glaze 61 and 62, such as the ridged portions 21 and 24 and the portions 25 and 26, are glazed with a non-semiconductive glaze. Applying a semiconductive glaze to the insulating sleeve 20 bonds the semiconductive material to the insulating sleeve 20 stronger than if applied as a layer by other means, such as by chemically depositing or coating semiconductive materials on a pre-glazed surface or a non-glazed surface without fixing the material to the surface by glazing. In some embodiments, a semiconductor glaze can be formed in a porcelain making furnace whose firing temperature can be as high as 1200 degrees Celsius.
The semiconductive-glazed bands 61 and 62 are located on either side of the flange 30. In one embodiment, the semiconductive-glazed bands 61 and 62 are located immediately adjacent to the flange 30. In other words, in one embodiment, no non-semiconductive-glazed portion is located between the flange 30 and the semiconductive-glazed bands 61 and 62. By locating the semiconductive-glazed bands 61 and 62 adjacent to the flange 30, the corona and flashover resistance of the bushing 1 is substantially increased.
In the embodiment illustrated in
The semiconductive-glazed bands 61 and 62 are bands that circumscribe the insulating sleeve 20. The glazed portions of the insulating sleeve 20 on either side of the semiconductive-glazed bands 61 and 62 include a normal glaze that does not include semiconductive materials. The normal glaze has a relatively high surface resistivity, such a surface resistivity in the range from 1012-1014 ohms/square (“ohms/sq”). According to one embodiment, the surface resistivity of the first and third sub-bands 63 and 65 is in a range from 108-109 ohms/sq, and the surface resistivity of the second and fourth sub-bands 64 and 66 is in a range from 106-107 ohms/sq. In one embodiment, each sub-band 63, 64, 65, and 66 is homogeneous, or comprising each only one band having one resistivity rather than multiple bands having different resistivities.
According to one embodiment, the semiconductive glaze increases the porcelain surface temperature to several degrees Celsius higher because of the nature of resistivity-based voltage grading, which prevents moisture condensation and ambient pollution deposits, which further improves corona resistance of the bushing 1.
In some embodiments, the semiconductive glaze is made with voltage-grading materials having a surface resistivity that decreases with increased electric fields or temperatures. An example of the voltage-grading materials includes iron-titanium oxide. Other examples include tin oxide, silicon carbide, silicon nitride, aluminum nitride, boron nitride, boron oxide, molybdenum oxide, molybdenum disulfide, Ba2O3, and aluminum carbide. In one embodiment, the linear thermal expansion of the semiconductive glaze is smaller than that of the base material, such as porcelain, of the insulating sleeve 20.
In one embodiment of the present invention, electrically conductive adhesive 40 is applied at both ends of the flange 30 adjacent to the bands of semiconductive glaze 61 and 62. The electrically conductive adhesive 40 electrically connects the flange 30 to the bands of semiconductive glaze 61 and 62.
The flange 30 is mounted to the substrate 27 by a highly thermally-insulating (high thermal rating) epoxy-glass bonding material 52. In one embodiment, the substrate 27 includes a protrusion 28 that abuts a ridge of the flange 30 to hold a position of the flange 30 with respect to the substrate 27. The thermally-insulating epoxy 52 fills a space between the substrate 27 and the base portion 31 of the flange 30 corresponding to the height of the protrusion 28. The flange 30 further includes at least six holes 33 to mount the bushing 1 to a surface.
The bands of semiconductive glaze have lengths of d2 and d1, respectively. In one embodiment, the combined length d1+d2 is less than or equal to 12 inches long. For example, in one embodiment the first band of semiconductive glaze 61 is 5.5 inches long, and the second band of semiconductive glaze is 3.5 inches long. The first sub-band and second sub-band have lengths of d3 and d4, respectively. The third sub-band 65 and fourth sub-band 66 have lengths of d5 and d6, respectively. According to one embodiment, a length d3 of the first sub-band 63 is greater than a length d4 of the second sub-band 64, and a length d5 of the third sub-band 65 is greater than the length d6 of the fourth sub-band 66. It is known to those skilled in the art that the semiconductive band length or width may have a process specification limit or tolerance for the porcelain-making process. However, the specification limit and tolerance are affordable and different semiconductive bands exist to further lower the electric field from triggering corona discharge.
According to one embodiment, an inner surface or wall 29 of the substrate 27 is glazed with a semiconductive glaze. The semiconductive glaze of the inner surface 29 has a surface resistivity that is equal to or less than the surface resistivity of the second and fourth sub-bands 64 and 66. For example, while the surface resistivity of the second and fourth sub-bands 64 and 66 is in a range between 106-107 ohms/sq, a surface resistivity of the semiconductive glaze of the inner surface 29 may be in a range between 105-107ohms/sq. The non-conducting glaze, or each glazed portion of the insulating sleeve 20 that does not include the semiconductive glaze, including the portions 25 and 26, and the ribbed portions 21 and 24, may have a surface resistivity in a range between 1012-1014 ohms/sq.
An electrically conductive adhesive 40 is coated on an end surface 35 of the flange 30. The electrically conductive adhesive 40 having a surface resistivity as low as 4×10−3 ohms/sq, electrically connects the flange to the second sub-band 66. In one embodiment, the adhesive is a silicone or epoxy-based matrix filled with carbon black or for more endurance, filled with silver particles to achieve the performance required.
Table 1 illustrates a comparison of electric field distribution on an outer surface of a bushing having a semiconductive-glazed band and a bushing having no semiconductive-glazed band.
The values of Table 1 correspond to a bushing attached to a structure filled with hydrogen (H2), such as a turbo generator, so that the part of the bushing on one side of the flange is exposed to air and the part of the bushing on the other side of the flange is exposed to the hydrogen. The values of Table 1 correspond to the side exposed to the hydrogen and tested at rated voltage of 24 kV.
In the examples illustrated in Table 1, a voltage provided to the conductor 10 of 14.6 kV corresponds to testing voltage which is of 1.05× maximal rated voltage of 24 kV/1.732 per IEC 60137 requirements, and the voltage of 68 kV corresponds to a Hipot testing voltage that simulates potential spike that may occur during operation, which is about three times the rated voltage of the bushing In each example corresponding to embodiments of the present invention in which the bushing 1 includes the sub-bands 65 and 66 having different resistivities to form a resistivity gradient from the non-semiconductive glaze portion 26 toward the flange 30, the electric field generated on the outer surface of the bushing 1 is substantially less than when a non-semiconductive glaze is used, thereby reducing significantly the tendency of flashover and coronal discharge having an inception (triggering) strength of approximately 75 kV/inch
Table 2 illustrates a comparison of electric field distribution on an outer surface of a bushing having a semiconductive-glazed band and a bushing having no semiconductive-glazed band.
The values of Table 2 correspond to a bushing attached to a structure filled with hydrogen (H2), such as a turbine, so that the part of the bushing on one side of the flange is exposed to air and the part of the bushing on the other side of the flange is exposed to the hydrogen. The values of Table 2 correspond to the side exposed to the air.
In the examples illustrated in Table 2, the voltage provided to the conductor 10 of 14.6 kV corresponds to a testing voltage, which is of 1.05× maximal rated voltage of 24 kV/1.732 per IEC 60137 requirements, and the voltage of 68 kV corresponds to a Hipot testing voltage that simulates potential spike that may occur during operation, which is about three times the rated voltage of the bushing. In each example corresponding to embodiments of the present invention in which the bushing 1 includes the sub-bands 63 and 64 having different resistivities to form a resistivity gradient from the non-semiconductive glaze portion 25 toward the flange 30, the electric field generated on the outer surface of the bushing 1 is substantially less than when a non-semiconductive glaze is used, thereby reducing substantially the tendency of flashover and coronal discharge on the air side Without voltage grading of the above-described embodiments, the non-semiconductive glazed bushing would have a high potential to trigger corona discharge as it has electric field more than the corona inception field strength of 75 kV/inch.
In contrast, the utility of this a bushing design according to the above-described embodiments is illustrated in
According to the above embodiments, a bushing has substantially improved resistance to corona discharges and flashovers by glazing the bushing with a semiconductive glaze. The outer surface of the bushing includes bands of semiconductive glaze on either side of a flange, the bands including sub-bands having different resistivities to form a resistivity gradient. The inner surface of the bushing includes a semiconductor glaze having a resistivity different from that of at least one of the bands of the outer surface of the bushing. An electrically conductive adhesive is coated on ends of the flange to electrically connect the flange to the semiconductive-glazed bands.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
