EP3826034A1

EP3826034A1 - Electrical bushing having enhanced thermal performance

Info

Publication number: EP3826034A1
Application number: EP19210998.1A
Authority: EP
Inventors: Teresa Gargano
Original assignee: ABB Power Grids Switzerland AG
Current assignee: Hitachi Energy Ltd
Priority date: 2019-11-22
Filing date: 2019-11-22
Publication date: 2021-05-26

Abstract

An aspect of the present disclosure provides an electrical bushing 100 with enhanced thermal performance. The electrical bushing 100 includes a central tube 101 having a first emissivity, and a conductor 102 arranged within the central tube 101 providing a gap G between the conductor 102 and an inner surface of the central tube 101, wherein at least a portion of the inner surface of the central tube 101 is provided with a coating having a second emissivity which is higher than the first emissivity. A further aspect of the present disclosure provides an electrical transformer 200 having at least one said electrical bushing 100.

Description

Field of the disclosure

Embodiments of the present disclosure generally relate to an electrical bushing having enhanced thermal performance, particularly a draw lead bushing or a removable rod bushing. In particular, embodiments of the present disclosure relate to an electrical bushing having a central tube with an inner surface having a low emissivity coating. More particularly, embodiments of the present disclosure relate to a high-voltage transformer having at least one electrical bushing according to embodiments described herein.

Technical background:

High-voltage transformers typically include a number of electrical bushings provided therein to facilitate isolation of conductors passing through a barrier, such as a grounded transformer housing. Electrical bushings for high-voltage applications include a central tube, through which a current-carrying conductor passes, leaving a gap between the outer surface of the conductor and the inner surface of the central tube. In a draw-lead bushing application, the current-carrying conductor is a flexible cable, whereas in a removable rod bushing application, the current-carrying conductor is a solid rod. Typically the current-carrying conductor is made of copper. In some applications using draw lead bushings or removable rod bushings, a lower portion of the gap between the conductor and the central tube may be filled with oil from the transformer, while an upper portion of the gap may be filled with a gas such as air. In the lower portion filled with oil, heat transfer between the current-carrying conductor and the central tube is performed through conduction, while in the upper portion filled with air, heat transfer is performed through convection and radiation. In other applications, an insulating gas such as SF6 may be used in the transformer casing, and the entirety of the gap between the central tube and the conductor may be filled with the insulating gas where heat transfer is performed by convection and radiation.
In order to increase the current-carrying performance of the conductor, it is desirable to transfer heat away from the conductor. The current rating of an electrical bushing is dependent on the maximum allowable temperature as prescribed by the relevant standards (for example, IEC 60137).
One approach for increasing the current rating of a bushing is to improve heat transfer between the conductor and the central tube by coating the conductor with a coating to increase emissivity. Painting of the conductor is common in electrical bushings to minimize the risk of discharge and enhance the dielectric performance of the bushing. Painting the conductor with a dark coating increases emissivity of the conductor, and can improve heat transfer away from the conductor by thermal radiation, improving the thermal performance of the bushing and allowing for higher currents to be conducted. However, problems may arise when painting the conductor, as the conductor is subjected to strong electrical fields. It is critical that the coating performs at all temperatures and exhibits good adherence to the conductor to avoid discharges.
In existing electrical bushings, particularly in draw lead bushings, the current-carrying conductor is typically made of copper, which has a high emissivity, and the central tube is typically made of aluminium, which has a comparably low emissivity. Due to the low emissivity of the aluminium central tube, heat transfer between the conductor and the central tube through radiation in the portion of the bushing filled with gas is limited. In view thereof, it is desired to overcome at least some of the problems in the prior art to further improve the thermal performance of the electrical bushing.

Summary of the disclosure

An aspect of the present disclosure provides an electrical bushing. The electrical bushing 100 includes a central tube having a first emissivity, and a conductor arranged within the central tube providing a gap between the conductor and an inner surface of the central tube, wherein at least a portion of the inner surface of the central tube is provided with a coating having a second emissivity which is higher than the first emissivity.
A further aspect of the present disclosure further provides an electrical transformer including at least one electrical bushing 100 according to the above.
The embodiments described in the present disclosure allow for improved heat transfer through radiation between the current-carrying conductor and the central tube. Thus, the embodiments allow for higher currents to be conducted through the bushing.
Further advantages, features, aspects and details that can be combined with embodiments described herein are evident from the dependent claims, claim combinations, the description and the drawings.

Brief description of the Figures:

The details will be described in the following with reference to the figures, wherein

Fig. 1: is a schematic cross-sectional view of an electrical transformer according to embodiments of the disclosure;
Fig. 2: is a schematic cross-sectional view of an electrical bushing according to embodiments of the disclosure;
Fig. 3: is a plot showing the theoretical rate of heat transfer per unit length vs emissivity of the central tube of an electrical bushing according to embodiments of the disclosure; and
Fig. 4: is a plot showing the conductor temperature in the longitudinal direction of a simulation model of an electrical bushing according to embodiments of the disclosure.

Detailed description of the Figures and of embodiments:

Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations, with the scope thereof for which protection is sought being defined by the claims.
Within the following description of the drawings, the same reference numbers refer to the same or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment can be applied to a corresponding part or aspect in another embodiment as well.
Fig. 1 exemplarily shows a cross-sectional view of a transformer 200 having at least one electrical bushing 100 according to an aspect of the present disclosure. The transformer may be, for example, a medium-or high-voltage transformer, particularly a high-voltage transformer. In the context of the present disclosure, the term "medium-voltage" may refer to a voltage of at least 1 kV and up to 52 kV. Further, the term "high-voltage" in the context of the present disclosure may refer to a voltage of at least 52 kV.
The transformer 200 may include a housing 202 enclosing an internal volume 203, and at least one transformer element 201. The at least one electrical bushing 100 may be mounted to housing 202 such that a conductor 102 of the at least one electrical bushing 100 may pass through housing 202. The at least one electrical bushing 100 may be of the draw-lead type, wherein conductor 102 is an electrical cable. Alternatively, the at least one electrical bushing 100 may be of the removable rod type, wherein conductor 102 is a solid rod. Transformer 200 may be of the wet type, wherein internal volume 203 is filled with an insulation liquid. For example internal volume 203 may be filled with one of the group comprising mineral-based oil, natural or synthetic ester, silicone oil, fluorocarbon-based oil, and vegetable-based oil. Alternatively, transformer 200 may be of the dry type, wherein internal volume 203 is filled with a gas. For example, internal volume 203 may be filled with one of the group containing air, SF₆, N₂, C₂H₂F₄ and C₄F₈.
As exemplarily shown in Fig. 1, conductor 102 of the at least one electrical bushing 100 includes an upper terminal and a lower terminal, where the upper terminal is on the outside of housing 202 and the lower terminal is on the inside of housing 202. Transformer 200 may further include at least one electrical interconnect 204 connecting the lower terminal of the at least one electrical bushing 100 and the at least one transformer element 201. The at least one electrical interconnect 204 may include, for example, a conductive bar interconnect or a cable interconnect. The electrical interconnect 204 may be solid or flexible. Alternatively, conductor 102 of the at least one electrical bushing 100 may extend from the lower end of the at least one bushing 100 directly to the at least one transformer element 201 such that conductor 102 connects directly to the at least one transformer element 201.
Referring now to Fig. 2, an electrical bushing 100 according an aspect of the present disclosure is provided. Electrical bushing 100 includes a central tube 101 having a first emissivity and a conductor 102 arranged within the central tube 101 providing a gap G between the conductor 102 and the inner surface of the central tube 101. At least a portion of the inner surface of the central tube 102 is provided with a coating having a second emissivity which is higher than the first emissivity. By providing at least a portion of the inner surface of the central tube 101 with a coating, the emissivity of the inner surface of the central tube 101 can be increased from the emissivity of the bare central tube material so that the heat transfer by radiation between the conductor 102 and the central tube 101 is enhanced.
Electrical bushing 100 may further include an insulation layer 103 provided on the outer surface of central tube 101. According to an embodiment, which may be combined with embodiments described herein, electrical bushing 100 is a graded capacitive bushing. Electrical bushing 100 may be an oil-impregnated paper (OIP) bushing or a resin-impregnated paper (RIP) bushing, wherein insulation layer 103 includes a winding of oil-impregnated paper or a winding of resin-impregnated paper, respectively. Optionally, insulation layer 103 may further include one or more capacitive grading plates 104.
Electrical bushing 100 may further include an outer insulation 105, for example, a porcelain outer insulation 105 or a silicone outer insulation 105. Electrical bushing 100 may further include a head element 106, wherein the head element 106 is configured for sealing the outer end of electrical insulator 100 so that insulating liquid or insulating gas from inner volume 203 is prevented from leaking from transformer 200. Head element 106 may be further configured for locating and/or supporting conductor 102 within central tube 101. Electrical bushing 100 may further include mounting flange 107 configured for mounting electrical bushing 100 to housing 202 of transformer 200. Mounting flange 107 may further include an integrated measuring tap, for example, for measuring a temperature of insulation liquid or insulation gas inside electrical bushing 100.
Central tube 101 of electrical bushing 100 is typically provided as the primary structural element of electrical bushing 100. In the context of the present invention, the term "central tube" may refer to any tube-like element with any cross section. In the preferred embodiment, central tube 101 comprises a tube having a constant circular cross-section. However, the present disclosure is not limited thereto, and central tube 101 may include a tube having a square cross-section or an oval cross-section. For example, an electrical bushing having a conductor with a square cross-section may be provided with a corresponding tube also having a square cross-section. Further, central tube 101 may include a tube having a variable cross-section with respect to its length. For example, central tube 101 may include a tube having a variable circular cross-section in the shape of a hollow, truncated cone.
In order to avoid a dielectric breakdown or excessive electric field gradient across a gap G between the inner surface of central tube 101 and the outer surface of conductor 102, central tube 101 may be configured to be at substantially the same electrical potential as conductor 102. Particularly, central tube 101 may be electrically attached to conductor 102 so as to be at substantially the same electrical potential as conductor 102, but without carrying electrical current.
According to an embodiment, which may be combined with other embodiments disclosed herein, central tube 101 may include a non-magnetic material. Particularly, central tube 101 may include an electrically conductive material. More particularly, central tube 101 may include one of the group consisting of an aluminium alloy, a nickel alloy and a copper alloy. In the preferred embodiment, central tube 101 is manufactured from aluminium alloy so as to provide sufficient strength while minimizing the weight of electrical bushing 100.
Conductor 102 is provided to carry electrical current from inside transformer 200 to outside, or vice versa. In the case of electrical bushing 100 being of the draw lead type, conductor 102 is an electrical cable, particularly a flexible electrical cable. In the case of electrical bushing 100 being of the removable rod type, conductor 102 is a solid rod. The passage of high levels of electrical current though conductor 102 generates heat, which is transferred from conductor 102 to other components of electrical bushing 100 via central tube 101. However, the maximum operating temperature of electrical bushing 100, particularly the temperature of conductor 102, is limited. For example, according to the temperature rise test in international standard IEC 60137, the maximum operating temperatures for a resin-impregnated paper (RIP) bushing may be limited to 140 °C conductor temperature and 120 °C condenser body temperature.
In view thereof, two approaches to reducing operating temperature can be taken. Firstly, it is advantageous that conductor 102 includes a material which has low electrical resistance so that heat generated is minimized. For example, depending on the voltage and current rating of electrical bushing 100, conductor 102 may include copper or a copper alloy for high voltage applications, or an aluminium alloy for medium voltage applications. Secondly, heat transfer from conductor 102 to other components of electrical bushing 100 via central tube 100 allows for heat generated to be dissipated. Gap G provided between central tube 101 and conductor 102 allows for heat to be transferred from conductor 102 to central tube 101 by either thermal convection or thermal radiation. According to an embodiment, which may be combined with other embodiments described herein, at least a portion of the gap G between the conductor 102 and the inner surface of central tube 101 is filled with a gas.
In the case of an electrical bushing 100 provided in a wet-type transformer 200, which is exemplarily shown in Fig. 2, at least a portion of gap G provided between central tube 101 and conductor 102 is filled with insulation liquid, while the remaining portion of gap G is filled with a gas, typically air. According to IEC 60137, the insulation liquid may, for example, fill 1/3 of the height of gap G, with the remaining 2/3 of the height of gap G being filled with air. In this case, heat is transferred from conductor 101 to central tube 102 primarily by thermal convection in the portion of gap G filled with insulation liquid, and primarily by thermal radiation in the portion of gap G filled with air.
Alternatively, in the case of an electrical bushing 100 provided in a dry-type transformer 200, gap G provided between central tube 101 and conductor 102 is filled with insulation gas. In this case, heat is transferred from conductor 101 to central tube 102 primarily by thermal radiation.
According to an embodiment, which may be combined with other embodiments described herein, the gap G between the conductor 102 and the inner surface of central tube 101 is at least 1 mm. Particularly, gap G may be in the range of at least 5 mm and up to 8 mm. Increasing the gap G allows for a larger volume of insulating liquid and/or insulating gas to be provided between conductor 102 and central tube 101, thereby enhancing the insulation properties of the bushing, thus gap G is preferably at least 1 mm, more preferably at least 5 mm. However, increasing the gap G increases the overall size and cost of electrical bushing 100, thus gap G is preferably at most 10 mm, more preferably at most 8 mm.
In order to enhance the heat transfer by thermal radiation between conductor 102 and central tube 101, the inner surface of central tube 101 is provided with a coating, wherein the coating has an emissivity which is higher than the emissivity of the inner surface of central tube 101. As previously mentioned, the preferred embodiment of the present disclosure includes a central tube 101 manufactured from an aluminium alloy. However, the emissivity of aluminium alloy is low, reducing the rate of heat transfer between conductor 102 and central tube 101. Increasing the emissivity of the inner surface of central tube 101 by providing a high-emissivity coating thereon allows for increased heat transfer by radiation.
When considering a theoretical model of heat transfer by radiation, the energy transfer between the inner surface of the central tube and the outer surface of the conductor provided therein can be approximated as two infinitely long concentric cylinders. Where the outer surface of the conductor is denoted as the "1" surface and the inner surface of the central tube is denoted as the "2" surface, the rate of heat transfer by radiation from the "1" surface to the "2" surface can be determined with equation (a) below: ${\dot{Q}}_{12} = \frac{A_{1} σ (T) ({_{1}}^{4} - {T_{2}}^{4})}{\frac{1}{ε_{1}} + \frac{1 - ε_{2}}{ε_{2}} {(\frac{r_{1}}{r_{2}})}^{2}}$
where A₁ is the outer surface area of the conductor, σ is the Stefan-Boltzmann constant, r₁ is the radius of the outer surface of the conductor, r₂ is the radius of the inner surface of the central tube, T₁ is the temperature of the conductor in degrees Kelvin, T₂ is the temperature of the central tube in degrees Kelvin, ε₁ is the emissivity of the conductor, and ε₂ is the emissivity of the outer surface of the inner surface of the central tube.
The above approximation of equation (a) can be applied to an exemplary electrical bushing with a central tube having an inner radius of r₂ = 19 mm and a conductor having an outer radius r₁ = 16 mm to determine the rate of heat transfer as a function of the emissivity ε₂ of the inner surface of the central tube. Exemplary values were selected for the temperatures of the conductor and the central tube of T₁ = 120 °C and T₂ = 110 °C, respectively. The emissivity of the conductor was exemplarily selected to be ε₁ = 0.6, representing a conductor made of copper having an oxidized outer surface. The resulting exemplary relationship Q̇₁₂(ε₂) / Q̇₁₂(1) shown in Fig. 3 is the normalized heat transfer relative to an ideal emissivity of ε₂ = 1.
It is clear from the exemplary relationship shown in Fig. 3 that the heat transfer by radiation can be significantly improved by increasing the emissivity ε₂ of the inner surface of the central tube. For example, a central tube made of aluminium with an oxidized inner surface has an emissivity of approximately ε₂ = 0.2, resulting in a normalized heat transfer in the exemplary electrical bushing of Q̇₁₂ = 0.37 relative to an ideal emissivity of ε₂ = 1. However, increasing the emissivity of the inner surface of the central tube to ε₂ = 0.95, which may be achievable by coating the inner surface of the central tube with paint or lacquer, leads to a normalized heat transfer of Q̇₁₂ = 0.98 relative to an ideal emissivity of ε₂ = 1. Even a mild increase in emissivity to ε₂ = 0.55, which is achievable with an anodized coating on the inner surface of an aluminium central tube, results in an almost 2x increase in heat transfer between the conductor and the central tube as compared to an emissivity of ε₂ = 0.2.
In the present disclosure, the inner surface of central tube 101 having a first emissivity is provided with a coating having a second emissivity, which is higher than the first emissivity. According to an embodiment, which may be combined with other embodiments described herein, the second emissivity is at least 0.3 at 120 °C. Particularly, the second emissivity is at least 0.5 at 120 °C, more particularly the second emissivity is at least 0.9 at 120 °C.
The coating applied to the inner surface of central tube 101 may include any suitable coating which has a higher emissivity than that of central tube 101, while also being able to withstand operating temperatures inside electrical bushing 100. Preferably, the coating may be a high temperature coating capable of withstanding an operating temperature of at least 80 °C, preferably at least 100 °C, more preferably at least 120 °C. According to embodiments, which may be combined with other embodiments described herein, the coating may include a polymeric material. Particularly, the coating may include an epoxy coating, an enamel coating or a silicone coating. For example, emissivity values for a lacquer or varnish coating may be in the range of 0.8 to 0.95. Alternatively, the coating may include an oxide coating. Particularly, the coating may include an anodized coating or a natural oxide layer. For example, an aluminium oxide layer may have an emissivity of at least 0.4, while an anodized aluminium coating may have an emissivity of at least 0.5.
Further, the color and/or finish of the coating may be selected to further increase emissivity of the inner surface of central tube 101. For example, a black lacquer may have an emissivity of at least 0.8, while a white lacquer may have an emissivity of at least 0.95. Further, a shiny black lacquer may have an emissivity of at least 0.8, while a flat black lacquer may have an emissivity of at least 0.95.
According to an embodiment, which may be combined with embodiments described herein, the coating may have a thickness of at least 35 µm. Particularly, the coating may have a thickness of at least 70 µm, more particularly at least 100 µm. Since the main function of the coating is to increase emissivity, the coating can be a low thickness coating. However, if additional properties are desired such as improved corrosion resistance, then a thicker coating may be applied. It is preferable that the coating thickness is at least 35 µm so as to provide a sufficiently resilient coating which does not flake or degrade during operation of electrical bushing 100.
According to an embodiment, which may be combined with other embodiments described herein, conductor 102 has a third emissivity, and at least a portion of the conductor is provided with a second coating having a fourth emissivity which is higher than the third emissivity. By providing a high emissivity coating on the conductor, the heat transfer by thermal radiation between conductor 102 and central tube 101 may be further enhanced.
The coatings described above with respect to central tube 101 may be applied to conductor 102, such that the emissivity of the conductor is enhanced. For example, in the preferred embodiment, conductor 102 may include a copper alloy, which may have a third emissivity of 0.15 for a copper alloy having a dull surface, up to 0.6 for a heavily oxidized copper alloy. A black lacquer having an emissivity of at least 0.8 or a white lacquer having an emissivity of 0.95 may be applied to the conductor to enhance the heat transfer by radiation between conductor 102 and central tube 101.
However, due to conductor 102 extending out from the upper and lower ends of electrical bushing 100, for example for attachment to electrical interconnects, conductor 102 is subjected to significantly higher electric field gradients than central tube 101. Thus, if a coating which is applied to conductor 102 is not adequately adhered to conductor 102, the high electric field gradient may cause the coating to degrade or flake. As such, in the present disclosure, the preferred embodiment includes a coating on the inner surface of central tube 101 instead of conductor 102. However, the present disclosure is not limited thereto, and conductor 102 may also be provided with an emissivity-enhancing coating, for example, in reduced voltage applications.
In addition to the theoretical model discussed above, an expanded model of a bushing according to embodiments of the present disclosure was further considered, the results of which are exemplarily shown in Fig. 4.
A thermal model of an electrical bushing according to the present disclosure was constructed, the electrical bushing model including a 36 mm diameter solid rod conductor, a central tube having an internal diameter of 40 mm and a wall thickness of 4 mm, and a resin-impregnated insulation layer provided to a diameter of 110 mm. A silicone insulation was modelled having dielectric shed features typical of a high-voltage bushing. The total conductor length was modelled at approximately 2100 mm long, of which 1000 mm of the conductor length is in contact with air. Emissivity of the conductor was maintained at 0.6, while the emissivity of the central tube was varied. The gap between the conductor and the central tube was modelled to be filled with mineral oil up to the 500 mm Z-axis level, such that the portion of the gap in the range of -450 mm to 500 mm in the Z axis is filled with mineral oil, and the portion of the gap in the range of 500 mm to 1300 mm is filled with air. Nominal operating conditions of 123 kV and 1700 A flowing through the conductor were applied.
The plot shown in Fig. 4 shows the temperature of the conductor on the Y axis along the length of the conductor on the X axis. Two models were considered, a first model in which the central tube is not provided with a coating and has an emissivity of 0.2, and a second model in which the central tube is provided with a coating and has an emissivity of 0.95. As can be seen in Fig. 4, the temperature of the conductor in the model having a coated central tube is significantly lower. In some areas, the temperature difference between the electrical bushing having a coated central tube and the electrical bushing having a non-coated central tube can be over 8 °C. With such a temperature difference, the electrical bushing having a coated central tube can be rated at a higher operating current as compared to the electrical bushing having a non-coated central tube.
While the foregoing is directed to aspects and embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Electrical bushing (100) comprising
a central tube (101) having a first emissivity; and
a conductor (102) arranged within the central tube (101) providing a gap (G) between the conductor (102) and an inner surface of the central tube (101),
wherein at least a portion of the inner surface of the central tube (101) is provided with a coating having a second emissivity which is higher than the first emissivity.
Electrical bushing (100) according to claim 1, wherein the second emissivity is at least 0.3 at 120 °C, particularly at least 0.5 at 120 °C, more particularly at least 0.8 at 120 °C.
Electrical bushing (100) according to one of claims 1 to 2, wherein at least a portion of the gap (G) between the conductor (102) and the inner surface of the central tube (101) is filled with a gas.
Electrical bushing (100) according to one of claims 1 to 3, wherein the coating comprises a polymeric material.
Electrical bushing (100) according to one of claims 1 to 3, wherein the coating comprises at least one of the group comprising an acrylic coating, an epoxy coating, a polyurethane coating, an enamel coating, a silicone coating, an anodized coating or an oxide coating.
Electrical bushing (100) according to any one of claims 1 to 5, wherein the coating has a thickness of at least 35 µm, particularly at least 70 µm, more particularly at least 100 µm.
Electrical bushing (100) according to any one of claims 1 to 6, wherein the gap (G) between the conductor (102) and the inner surface of the central tube (101) is at least 1 mm, particularly in the range of at least 5 mm and up to 8 mm.
Electrical bushing (100) according to any one of claims 1 to 7, wherein the central tube (101) comprises a non-magnetic material.
Electrical bushing (100) according to claim 8, wherein the central tube (101) comprises one of the group consisting of an aluminium alloy, a nickel alloy and a copper alloy.
Electrical bushing (100) according to any one of claims 1 to 9, wherein the conductor (102) has a third emissivity, and at least a portion of the conductor (102) is provided with a second coating having a fourth emissivity which is higher than the third emissivity.
Electrical bushing (100) according to any one of claims 1 to 10, wherein the electrical bushing (100) is a graded capacitive bushing, particularly a resin-impregnated paper bushing or an oil-impregnated paper bushing.
Electrical transformer (200) comprising at least one electrical bushing (100) according to any one of claims 1 to 11.
Electrical transformer (200) according to claim 12, further comprising a transformer housing (202) forming an internal volume (203) which is filled with insulating liquid.
Electrical transformer (200) according to claim 12, further comprising a transformer housing (202) forming an internal volume (203) which is filled with an insulating gas.