Patent Application
20160107795
The present application is directed to a reinforced tank for electrical equipment that is resistant to rupture during overpressure conditions, such as an arc fault.
Internal arc energy in electrical equipment such as power transformers and shunt reactors is generated when insulating fluid inside a transformer tank is vaporized and an expanding gas bubble is created. The pressure increase of the expanding gas during an arc fault event can cause the tank to bulge or rupture.
In the case of tank rupture, the seams and welds of the tank separate. In the case of deformation, the tank walls may bulge. In both situations, objects and particles may be expelled forcefully over a sizeable distance causing damage to persons and property. While pressure relief devices and modification of tank dimensions have been utilized with varying degrees of success, there is room for improvement in the design of a tank for electrical equipment that is able to withstand overpressure during an arc fault and thus, resistant to rupture.
In the accompanying drawings, structural embodiments are illustrated that, together with the detailed description provided below, describe exemplary embodiments of tank for electrical equipment. One of ordinary skill in the art will appreciate that a component may be designed as multiple components or that multiple components may be designed as a single component.
Further, in the accompanying drawings and description that follow, like parts are indicated throughout the drawings and written description with the same reference numerals, respectively. The figures are not drawn to scale and the proportions of certain parts have been exaggerated for convenience of illustration.
With reference to
The tank 10 is rectangular, having a bottom wall 38, side walls 14, 16 and a cover 12. Alternatively, the tank 10 is cylindrical, having a single cylindrical side wall, a bottom wall and a cover. The at least one stiffener 20 is a beam, channel member or bar having first and second ends with chamfered surfaces 25. The at least one stiffener 20, when attached to the tank 10, provides reinforcement to the tank 10. The at least one stiffener 20 is joined to the side walls 14, 16 and/or cover 12 by welds 18 between the flanges 23, as shown in
The tank walls 14, 16 and cover 12 are less ductile than the at least one stiffener 20 attached thereto as determined by measured properties, such as values observed during the tensile testing of certain types of mild steel used to form the tank 10 and stainless steel used to form the at least one stiffener 20 in Table 1 presented below. A transformer having a tank 10 with at least one stiffener 20 formed of a material having properties that exhibit a greater ductility than the material used for the tank 10 allows for increased flexibility in the tank 10 in the event of an arc fault. The tank 10 having at least one stiffener 20, when constructed of the materials described below, can withstand the pressure rise during an arc fault by absorbing arc energy generated from inside the tank 10. More particularly, the at least one stiffener 20 absorbs arc energy from the insulating medium when said arc energy is transferred from the internal space of said tank to said stiffeners.
The power transformers 100 and shunt reactors 200 that utilize the tank 10 designs depicted in
With continued reference to
The at least one stiffener 20 may be bolted using fasteners rather than connected using welds 18 to the tank walls 14, 16 and/or cover 12. The at least one stiffener 20 is formed of a ductile material such as extra low carbon stainless steel. By way of non-limiting example, a material that can be used to form the at least one stiffener 20 meets the ASTM A240 standard and is. Type 304L. It should be understood that the inventor contemplates that other materials having a ductility that is greater than the ductility of the material used to form the tank 10 walls 14, 16 and cover 12 may be utilized for carrying out the present disclosure and that the examples provided herein are by way of non-limiting example.
Additionally, any of the stainless steels of types and sub-types 304, 316, or 201 are used to form the at least one stiffener 20. Alternatively, super-austenitic stainless steel alloys such as 25-6HN sold under the trademark INCOLOY® and C-276 sold under the trademark INCONEL®, both registered trademarks of Huntington Alloys of Huntington, W. Va., are used to form the at least one stiffener 20.
The types of stainless steel used in the at least one stiffener 20 are austenitic alloys containing chromium and nickel (sometimes manganese and nitrogen), and structured around the Type 302 composition of iron, 18% chromium (weight percent), and 8% nickel (weight percent). Austenitic stainless steel may be annealed, hot-worked or cold-worked.
When the at least one stiffener 20 is welded to the tank 10, the at least one stiffener 20 is integrated with the tank 10. The welds 18 are formed using an American Welding Society (AWS) or a Canadian Standards Association (CSA) standard weld known to persons having ordinary skill in the art. For example, based on the thickness of the tank wall 14, 16 plate, the size of the weld will vary based on AWS and/or CSA standards. Typically, the welds 18 used to attach the at least one stiffener 20 to the side walls 14, 16 and cover 12, respectively, are partial penetration welds. In the case of the side wall 14, 16 and cover 12 interface 13, the weld may be a full or a partial penetration weld 13 depending on the application.
As previously mentioned, at least one stiffener 20 is welded to the corresponding tank walls 14, 16 and/or cover 12 by welding the flanges 23 to the outer surface of the tank walls 14, 16 and/or cover 12. The at least one stiffener 20 may form a gap with respect to the corresponding tank wall 14, 16 or cover 12. Alternatively, the gap may be filled with a material such as sand to change the natural frequency of the at least one stiffener 20 during operation of the power transformer 100 or shunt reactor 200. The at least one stiffener 20, when attached to the tank walls 14, 16 is attached vertically or perpendicularly with respect to the plane of the bottom wall 38 of the tank 10. Alternatively, the at least one stiffener 20 is attached horizontally or parallel with respect to the plane of the bottom wall 38 of the tank 10.
The at least one stiffener 20 provides the tank 10 the advantage of stiffness in elastic strain of the material during service conditions and flexibility in plastic straining during high overpressure. A tank 10 having side walls 14, 16 with at least one stiffener 20 formed from a more ductile material than the side walls 14, 16 increases the arc energy absorbed by plastic deformation to reduce the risk of tank 10 rupture. The overall impact is that the tank 10 with ductile at least one stiffener 20a has greater flexibility by reducing the pressure rise gradient as will be explained in further detail below, and thus can contain more arc energy than a tank 10 without the ductility of the at least one stiffener 20.
An example of the material used in the tank side walls 14, 16 and cover 12 is CSA G40.21 grade 50 W steel or another type of mild steel that meets the ASTM A36 standard. Yet another type of material used in the tank walls 14, 16 and cover 12 is a mild steel that meets the A572 standard. Other examples of materials used to form the tank 10 and the at least one stiffener 20, respectively, are presented in Table 1 along with values for the corresponding material properties: yield stress, tensile stress, and elongation percentage at break.
The values for the material properties listed in Table 1 are all minimum values for each particular tensile measurement. A person of ordinary skill in the art will recognize that the possible measured values for each tensile property and material type may be greater than the values listed in Table 1. The mild steel used in the tank 10 and the stainless steel used in the at least one stiffener 20 is in the form of a sheet, strip, plate, beam or flat bar.
In Table 1 below, the ‘Usage’ column refers to whether the material is used to form the tank 10 or the at least one stiffener 20, the ‘General’ column refers to the general classification of the material, the ‘Material Type’ column refers to particular material specifications as defined by ASTM or other standards organizations, ‘Yield’ refers to the minimum yield stress and is the point at which the material begins to deform plastically, ‘Tensile’ refers to the maximum stress that a material can withstand while being stretched or pulled before failing or breaking, and ‘Elongation’ refers to the ‘Elongation at Break’ expressed as a percentage (%) and is the ratio between initial length and changed length of the specimen at the point of material fracture or deformation.
Certain combinations of the above materials for use in forming the tank 10 and at least one stiffener 20 may provide better results than other combinations, according to tests performed by the inventor of the present disclosure. For example, a material used in forming the tank cover 12 and side walls 14, 16 having a yield stress measurement that is equal to or greater than the yield stress measurement of the material used to form the at least one stiffener 20, will result in a tank 10 construction with increased flexibility. In particular, the most flexible tank design using the materials in Table 1 is achieved when the yield stress measurement of the material used to form the side walls 14, 16 is at least 20 MPa greater than the yield stress value of the material used to form the at least one stiffener 20.
Further, the elongation percentage at break for the material used in the at least one stiffener 20 is at least 10% higher than the elongation percentage at break for the material used in forming the tank 10 walls 14, 16 and cover 12, although all of the combinations of stiffener 20 material and tank 10 material that can be made from Table 1 data will allow for the difference in elongation percentage requirement to be met.
In regards to the tensile stress measurement, it is important to note that high strength, low alloy (HSLA) steel does not have the desired elongation at break (%) and tensile stress measured values suitable for usage in the tank 10 or at least one stiffener 20 material. HSLA has a greater tensile stress value coupled with a lower elongation % value at break that renders HSLA not suitable for carrying out the present disclosure. Likewise, using a tank 10 material and stiffener 20 material having measured tensile values that are too similar, may prevent the tank 10 from expanding in response to overpressure. It should also be noted that the tank 10 and at least one stiffener 20 should not both be formed of stainless steel in an above ground installation because that arrangement may not block the magnetic field generated during operation of the power transformer 100 or shunt reactor 200. However, the tank 10 and at least one stiffener 20 may both be formed of stainless steel if the transformer 100 is located in a subsea environment.
The chemical composition of various tank 10 and at least one stiffener 20 materials are provided in Tables 2-9, by way of non-limiting example. The chemical compositions of the various exemplary stainless steels and mild steels are provided in weight percent (weight %) in tables 2-9, based on total weight. ‘Min’ (Minimum) and ‘Max’ (Maximum) weight percent values for each element in a composition are provided in tables 2-9. A (-) in the Min column indicates that an element may be present in the compound in trace amounts up to the Max value. A (-) in the Max column indicates that there is no specified Max value for the element in the compound.
The mild steel used to construct the tank 10 has the following composition in weight percent based on total weight:
0%≦carbon 0.29%;
0.5%≦manganese 1.5%;
0%≦phosphorous 0.04%;
0%≦sulfur 0.05%;
0%≦silicon 0.4%; and the remainder being constituted by iron. Additionally, other elements may be present in trace amounts.
Mild steels of CSA standard G40.20/G40.21 grades 44 W and 50 W have, in addition to the composition by weight percent ranges listed above: 0%≦niobium+vanadium 0.1%.
Mild steels meeting the ASTM A36 standard, the ASTM standard A572 Grade 42 Type 1 and Grade 50 Type 1 have, in addition to the ranges listed for the elements C, Mn, P, S and Si above, at least 0.2% by weight percent of copper.
In other words, the mild steel used in the side walls 14, 16 and cover 12, in addition to having the elements C, Mn, P, S and Si, includes in its composition a member selected from the group consisting of: 0% niobium+vanadium 0.1% and at least 0.2% percent by weight copper.
Mild steel meeting the ASTM standard A572 Grade 42 Type 1 and Grade 50 Type 1 have, in addition to the ranges listed for the elements C, Mn, P, S, Si and Cu above: 0.005 niobium 0.05, percent by weight.
The austenitic stainless steel used in the at least one stiffener 20 has the following composition in weight percent based on total weight:
0.03%≦carbon≦0.08%;
0%≦manganese≦2.0%;
0%≦phosphorous≦0.045%;
0%≦sulfur≦0.03%;
0%≦silicon≦0.75%;
8%≦nickel≦14%;
16%≦chromium 20%;
0%≦nitrogen≦0.1%; and the remainder being constituted by iron (Fe). It should be understood that any element listed as 0% may be present in trace amounts and that other elements may be present in trace amounts in any of the steel and stainless steel compositions mentioned herein.
It should be noted that in addition to the elements listed in the ranges above, stainless steel ASTM A666 Type 316 also contains molybdenum, expressed in weight percent based on total weight, as follows: 2% molybdenum 3%.
With reference now to
The at least one stiffener 20a, 20b, 20c, 20d, 20e is attached to the tank 10 by welding the flanges 23 or sides of the respective stiffeners, along the length of the flanges 23, to the 20a, 20b, 20c, 20d, 20e to the respective side wall 14, 16 and/or cover 12. The width (the X-dimension in
The at least one stiffener 20a, 20b, 20c, 20d, 20e first and second ends are generally spaced apart from the cover 12 and bottom wall 38, respectively. In some cases, the at least one stiffener 20a first and second ends are flush with the cover 12 and bottom wall 38, respectively. Alternatively, the at least one stiffener 20a, 20b, 20c, 20d, 20e is attached directly to the cover 12 using a cylindrical gusset 32 or a plate gusset 44 as will be described later in reference to
The at least one stiffener 20a, 20b, 20c, 20d are metal beams and the at least one stiffener 20e is a metal bar. All of the stiffeners have 20a, 20b, 20c, 20d, 20e first and second ends. At least one of the first and second ends a chamfered edge 25. The chamfered edges 25 of the at least one stiffener 20 are generally positioned proximate to the seam (where two plates used to form the side walls 14, 16 meet) of the tank side wall 14, 16 or cover 12, proximate to the interface 13 between the side walls 14, 16 and cover 12, or proximate to the interface between the side walls 14, 16 and bottom wall 38. It should be understood that number and type of the at least one stiffener 20a, 20b, 20c, 20d, 20e joined to the side walls 14, 16 and/or cover 12 vary depending on the application.
With continued reference to
Lastly,
Referring now to
In an autotransformer, the primary voltage is applied across two of the terminals and the secondary voltage is taken from two terminals. A first end of the winding is connected to a bushing 24 extending from the cover 12 of the tank 10. It should be understood that while the power transformer 100 example provided is an autotransformer, the mild steel tank 10 having at least one stiffener 20 formed of stainless steel attached thereto, may be applied to any power transformer having dielectric fluid as an insulating medium.
The power transformer 100 has at least one stiffener 20a, 20e welded to tank walls 14, 16 and the tank cover 12 as shown. The at least one stiffener of the type 20a are u-shaped beams that are attached to the outside surface of tank walls 14, 16 by welding the flanges 23 of at least one stiffener 20a to the corresponding tank walls 14, 16. One of the at least one stiffener of the type 20a is welded to side wall 14 and two of the at least one stiffener of the type 20a is welded to the side wall 16.
Each one of the at least one stiffener 20a is positioned perpendicularly with respect to the plane of the bottom wall 38. At least one stiffener of the type 20e is attached to side wall 14 along with the arcuate stiffener 22 and is used to reinforce the bushing chamber 26 and distribute the stress acting on the bushing chamber 26 to the side walls 14, 16 of the tank 10.
The arcuate stiffener 22 surrounds the circumference of bushing chamber 26 and is welded or otherwise fastened to side wall 14 and the bushing chamber 26. The bushing chamber 26 and thus the arcuate stiffener 22 are shaped so as to reduce space and the amount of insulating fluid inside the power transformer 100. Also, shown on side wall 14 are cooling system connections 28. It should be understood that opposing side walls 16 have the same or similar location and number of at least one stiffener 20a and that the opposing side walls 14 have the same or similar location and number of the at least one stiffener of the types 20a, 20e in the present example, however, that may not be the case in other applications.
Additionally, at least one stiffener 20e is attached to the tank cover 12 to reinforce the connection 21 between the cover 12 and the active part of the transformer such as the core and at least one coil winding.
It was determined through numerical simulation that during overpressure conditions inside the tank 10, such as greater than 69 kPa, the upward displacement of the cover 12 was too high. Therefore, the at least one stiffener 20e were welded to the tank cover 12 to further support and protect the connection 21 between the cover 12 and active part. It should be understood that the arrangement of at least one stiffener of the types 20a, 20e as depicted in
The power transformer 100 may also have c-shaped clamps (not shown) to reinforce the side wall 14, 16 seam welds. It should be understood that the c-shaped clamps may also be used to reinforce tank cover 12 welds 13 that fuse the cover with the tank side walls 14, 16 at the outermost edge of the side walls 14, 16 and slightly inward from edges of the cover 12.
With reference now to
The shunt reactor 200 tank 10 has two of the at least one stiffener 20a attached to each of the side walls 16 and at least one stiffener 20a attached to each of the side walls 14. In particular, at least one stiffener 20a is joined to the edge of the side wall 16 where a seam is formed between side walls 14, 16 and another at least one stiffener 20a is joined to the side wall 16 so that an edge of the stiffener 20a is aligned proximate to a midpoint of side wall 16. Further, at least one stiffener 20a is attached to side wall 14 at a midpoint of side wall 14 and additionally provides reinforcement to manhole 28. It should be understood that in the present example, there are two opposing side walls 14 that are mirror images and two opposing side walls 16 that are mirror images in terms of dimensions and the at least one stiffener 20a affixed thereto.
It should be understood that the predetermined position and number of stiffeners may vary depending on the application and desired operating parameters as previously mentioned and that the location and number of stiffeners described herein are provided by way of non-limiting example.
With reference now to
For example, the overall volume inside the tank 10 is able to increase by about 28% at 400 kPa pressure which is the pressure determined by a numerical simulation software at the point of tank rupture. The 28% increase in volume at 400 kPa allows for gas expansion inside the tank 10 and represents a comparison between the expansion volume (in m3) of a tank formed of mild steel having mild steel stiffeners joined thereto 50 versus a tank formed of mild steel with stainless steel stiffeners joined thereto 60. The arc energy contained by a power transformer 100 having a mild steel tank 10 with at least one stiffener 20 formed of stainless steel joined thereto 60 is at least 11 mega Joules (MJ).
With reference now to
On average, a mild steel tank 10 having the at least one stiffener 20a formed of stainless steel attached thereto provides a withstand of thirty percent overpressure in relation to the maximum rated operating pressure for power transformers 100 and shunt reactors 200.
The inventor's process of optimizing the tank 10 first accounted for side wall 14, 16 and cover 12 thickness, the at least one stiffener 20 dimensions, position of at least one stiffener 20, and quantity of the at least one stiffener 20 using regular, mild steel for both the at least one stiffener 20 and tank 10 in a numerical simulation as mentioned above. Then, the at least one stiffener 20 material was changed to stainless steel and the numerical simulation was repeated.
With reference now to
The gussets 32, 44 are formed of steel or stainless steel and distribute localized stress experienced by the side walls 14, 16 and respective cover 13 interface welds or bottom wall interface with the side walls 14, 16. While the gussets 32, 44 are constructed to withstand a vacuum service load of −101.3 kPa and an overpressure of at least 69 kPa experienced by the tank 10, the gussets 32, 44 are designed to deform before the at least one stiffener 20, side walls 14, 16, bottom wall 38 and cover 12 of the tank 10.
To the extent that the term “includes” or “including” is used in the specification or the claims, it is intended to be inclusive in a manner similar to the term “comprising” as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term “or” is employed (e.g., A or B) it is intended to mean “A or B or both.” When the applicants intend to indicate “only A or B but not both” then the term “only A or B but not both” will be employed. Thus, use of the term “or” herein is the inclusive, and not the exclusive use. See, Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995). Also, to the extent that the terms “in” or “into” are used in the specification or the claims, it is intended to additionally mean “on” or “onto.” Furthermore, to the extent the term “connect” is used in the specification or claims, it is intended to mean not only “directly connected to,” but also “indirectly connected to” such as connected through another component or components.
While the present application illustrates various embodiments, and while these embodiments have been described in some detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative embodiments, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant's general inventive concept.
