Manufacturing process for a current transformer housing, current transformer housing and current transformer

Abstract

According to the invention, the housing (14) of a current transformer (10) is manufactured using the aluminum die casting process, wherein the housing is divided into several housing parts (15, 16) and, if required, further housing parts (41, 42) in order to be able to use the simplest possible die casting mold with no or few movable mold parts (26, 27). The aluminum parts produced by the die casting process are then welded together to form the desired housing parts. The aforementioned method allows for increased productivity and, at the same time, increased product quality.

Description

[0001] The invention relates to a manufacturing method for a housing of a measuring current transformer, as well as the housing (current transformer housing) itself and the current transformer (measuring current transformer) provided with the aid of the current transformer housing.

[0002] Current transformers used in high-voltage systems consist, among other things, of a current transformer and a housing that encloses it. Because the housing is at line potential, it is typically supported by an insulator column. The current transformer housing itself typically provides a hermetic seal between the transformer inside and the environment. It may also contain an insulating fluid, such as oil, SF6, or compressed air. Especially when the insulating fluid is gaseous (e.g., SF6, nitrogen, or compressed air), it is typically kept under pressure within the current transformer housing. The housing must remain permanently sealed to prevent fluid leakage and pressure loss, despite environmental influences and temperature fluctuations. When using compressed air, the housing must be designed to withstand a burst pressure of 75 bar.

[0003] It is common practice to manufacture current transformer housings for oil or SF6 filling from aluminum using sand casting. This allows for the creation of complex shapes and undercuts on the current transformer housing, enabling the integration of flanges for the high-voltage cable as well as one or more outputs for measuring leads. Furthermore, a housing integrally produced using sand casting minimizes the number of joints or seams requiring sealing.

[0004] Nevertheless, the object of the invention is to provide an improved manufacturing process for a current transformer housing, and thus also an improved current transformer housing and an improved current transformer. This is particularly relevant with regard to the intended use of compressed air in the housing and the resulting requirements for the long-term pressure resistance and long-term sealing of the housing components.

[0005] This problem is solved by the manufacturing process according to claim 1:

[0006] The manufacturing process for the current transformer housing according to the invention is based on the fundamental concept of dividing the upper housing unit, which until now has been seamlessly cast in one piece using the sand casting process, into at least two castings that can be produced in steel molds using the die casting process. The steel molds are preferably cooled to achieve rapid cooling of the casting. This results, particularly with silicon-containing aluminum alloys, in a fine-grained microstructure that is less prone to fracture. An AlSiMg alloy, e.g., AlSi7Mg, is particularly suitable as the housing material for the casting.

[0007] The housing unit is preferably divided into individual parts such that the parts to be produced by die casting have few or no undercuts, allowing the corresponding molds to require few or no movable components. The parts produced by die casting are then placed on top of or against each other, forming a joint on or within which a weld is then formed. Preferably, the joint is an annular joint, creating a ring-shaped weld that joins the two castings of the upper housing unit. Preferably, the weld wraps around the upper housing unit like a belt, contributing to its stiffening.

[0008] The first mold is used to cast the upper housing section. The second mold, on the other hand, is used to cast an intermediate housing section. This intermediate section is preferably ring-shaped and approximately hollow-cylindrical. It can have a cylindrical outer contour and a slightly conical inner contour, which facilitates demolding after casting.

[0009] Preferably, a third mold is also provided for carrying out the process to produce a further intermediate part. This mold is preferably also a die-casting mold. The intermediate part can be a hollow cylindrical section that is inserted between the upper housing section and the intermediate housing section. The intermediate part is connected to the upper housing section and the intermediate housing section at the top and bottom by two annular, parallel welds to form the upper housing unit. One or more further, preferably hollow cylindrical, intermediate parts can be provided to enable the production of upper housing units of different heights. The number of annular welds to be formed on the housing unit then increases accordingly. The third mold used to produce the further intermediate parts defines a substantially hollow cylindrical cavity between its mold sections.

[0010] A fourth mold can be used to produce a lower housing section. Depending on requirements, the lower housing section can be a flat plate, a cup-shaped part, or another shape. If the lower housing section is simply a flat plate, the flange of an insulator column can also be used. The upper housing unit and the lower housing unit formed by the lower housing section can be bolted together to complete the current transformer housing.

[0011] It is also possible to create the current transformer housing by placing the upper housing unit onto a flange plate of an insulator and bolting it to the insulator. In this case, the flange plate then completes the current transformer housing.

[0012] After casting, at least those surfaces of the castings where the weld seam is to be created are preferably machined. This preferably concerns the end faces, i.e., the lower end face of the upper housing part, as well as the upper end face and, if necessary, the lower end face of the (vertically positioned) intermediate housing part. These are the surfaces where the upper housing part and the intermediate housing part are joined to form a joint. By machining these surfaces so that they form a chamfer or a conical shape extending almost the entire wall thickness, the weld seam can be designed as an annular weld.

[0013] The annular weld is preferably produced exclusively from the outside of the current transformer housing. This weld is preferably produced with the welding head stationary, while the current transformer housing rotates around a vertical axis in front of the welding head. The weld is preferably produced over several rotations of the upper housing unit, resulting in an annular weld lying in a horizontal plane. The weld can be a V-weld, particularly a multi-layer V-weld.

[0014] Preferably, the weld is produced using the metal inert gas (MIG) or metal active gas (MAG) welding process. In a preferred embodiment, the weld is produced using a tandem welding process. In this process, two welding filler wire strands are fed to the weld at two points spaced apart in the direction of weld advance, under shielding gas and current. While the first wire encounters the still-cold casting, the second wire encounters the (pre-)weld weld formed by the first wire, which has just solidified. This initially agitates the casting structure at the first weld point and then, at the subsequent second weld point, relieves stress and welds with a homogeneous microstructure thus prepared. This ensures that the weld and its heat-affected zone are permanently tear-resistant.The two wires used in tandem welding can be made of the same welding filler material. They can also have the same diameter. Wires made of an aluminum-silicon alloy, e.g., with the chemical designation S ALSi5(A) (also known by the numerical designation S 4043 A), are particularly suitable as welding filler material.

[0015] Preferably, the housing is tempered after welding. For this purpose, it can be stored for several hours or days at a temperature between 100 °C and 300 °C, preferably 150 °C to 200 °C, which relieves internal stresses.

[0016] Further details of advantageous embodiments of the invention will become apparent from the drawing and the following description referring to the drawing. The drawing contains several figures showing the following: Fig. 1. A current transformer housing mounted on an insulator for use in a high-voltage system, in side view. Fig. 2. A modified current transformer housing with a larger interior, also for high-voltage use and mounted on an insulator. Fig. 3 a current transformer housing according to the invention with a further enlarged interior, in side view, Fig. 4 a current transformer housing based on the model of the current transformer housing according to Fig. 3, however, in an upside-down arrangement, in side view, Fig. 5 a first mold for producing a current transformer housing top, in schematic sectional view, Fig. 6 a second mold for producing a current transformer housing intermediate part, in schematic sectional view, Fig. 7 and Fig. 7a the current transformer housing during welding of the housing top part to the housing intermediate part, Fig. 8 the joint between the upper and lower housing parts before applying the weld, Fig. 9 the joint after Fig. 8 with the finished weld, Fig. 10 the joint after Fig. 8 in a modified embodiment before the application of the weld, in a partial vertical section view, and Fig. 11 the joint after Fig. 10 after the welding process, in a partial vertical section view.

[0017] Fig. Figure 1 illustrates a current transformer 10, which is set up to measure the current flowing in a high-voltage line 11. The current transformer 10 is at the same potential as the high-voltage line 11 and provides a signal, in particular an electrical signal, at a measuring output 12, which indicates the current flowing in the high-voltage line 11. The high voltage can be several hundred volts. The signal output at the measuring output 12 can also be an optical signal.

[0018] The current transformer 10 is preferably mounted in a fixed position on an insulator column 13 and is insulated to withstand high voltage. It is typically, though not necessarily, located outdoors and thus exposed to changing temperatures and weathering throughout the day and year.

[0019] The current transformer 10 has a housing with an upper housing unit 14, which includes a housing top 15 and at least one housing intermediate 16. The housing unit 14 is closed at the bottom by a flange plate 17 of the insulator column 13. Screws 18 can be used for a sealed connection between the flange plate 17 and the upper housing unit.

[0020] The housing unit 14 consists of an aluminum alloy, preferably an aluminum silicon magnesium alloy AlSiMg, for example AlSi7Mg or another alloy suitable for die casting. With regard to silicon content, the alloy is preferably hypoeutectic.

[0021] The components described in the following are used to manufacture the upper housing part 15 and the intermediate housing part 16. Fig. 5 and Fig. Figure 6 illustrates molds 19 and 20. The first mold 19, used to produce the upper housing part 15, is a steel die. It has a first recess 21 that corresponds to the outer contour of the upper housing part 15. A first mold core 22 is arranged in the recess 21, the outer contour of which corresponds to the inner shape of the upper housing part 15. This creates a mold cavity 23 into which liquid aluminum is poured to produce the upper housing part 15. The mold 19 is cooled, preferably by water cooling. This is intended to ensure rapid solidification of the melt and a fine-grained casting. After the aluminum or aluminum alloy poured into the mold cavity 23 has solidified, the mold 19 can be opened by removing the mold core 22. The upper housing part 15 can then be removed. The mold core 22 can also be cooled, preferably water-cooled.

[0022] Similarly, the second mold 20 is a steel die. It has a second recess 24 that corresponds to the outer shape of the intermediate housing part 16 to be produced. A second mold core 25 is arranged in the recess 24, the outer contour of which corresponds to the inner contour of the intermediate housing part 16. The mold core 25 can also be cooled, preferably water-cooled.

[0023] To create feedthrough openings 45, 46 for the high-voltage line 11, movable mold cores 26, 27 can be provided in the second mold 20. These cores are preferably arranged radially movably at diametrically opposite locations in the mold 20 in order to penetrate or release the second mold cavity 28. The mold cores 26, 27 are arranged radially with respect to a central axis M of the second mold 20.

[0024] To produce the cavity 16, a liquid aluminum alloy is introduced into the mold cavity 28 and demolded after cooling and solidification. Preferably, this aluminum alloy is the same alloy as the aluminum alloy poured into the first mold 19.

[0025] Cooling the molds 19, 20 results in very rapid solidification of the aluminum alloy, thus preventing the formation of a coarse-grained microstructure, unlike in sand casting. This is particularly true for hypoeutectic AlSi alloys with less than 12% silicon content, which are preferred here. Furthermore, smooth inner and outer surfaces are produced, offering little surface area for dirt deposits and corrosion.

[0026] After demolding, the upper housing part 15 and the intermediate housing part 16 are, as shown in Fig. Figure 8 shows a section of the end faces 29, 30, which are machined, for example by milling or turning, to form dimensionally accurate and clean surfaces. The end faces 29, 30 then define an annular joint 31 between each other. Preferably, the end faces 29, 30 are inclined in the radial direction, i.e., they are slightly conical.

[0027] Fig. Figure 7 illustrates this with the upper housing part 15 placed on the intermediate housing part 16, with the annular joint 31 lying in a horizontal plane. Stacked one above the other in this way, the upper housing part 15 and the intermediate housing part 16 are arranged on a rotary table 32, by means of which the two parts can be rotated in a controlled manner about a vertically oriented axis of rotation D. The axis of rotation D coincides with the central axes of the upper housing part 15 and the intermediate housing part 16.

[0028] At least one welding head 33 is arranged at the level of the annular joint 31, which, for example, with a continuous supply of an aluminum wire, a suitable shielding gas and a suitable welding current, preferably in the metal inert gas welding process (MIG), welds the area in Fig. 9 illustrated weld seam 34 is produced. The aluminum wire can have an alloy composition that differs from that of the upper housing part 15 and the intermediate housing part 16. For example, it can be used as a welding filler material with the numerical designation S Al 4043 A and the chemical name S AlSi5(A) (e.g., available from Drahtwerk Elisental W. Erdmann GmbH & Co).

[0029] Preferably, the weld seam 34 (and also the weld seam 34') is produced using the tandem welding process, which is described in Fig. Figure 7a illustrates the tandem welding process. Two welding heads 33, 33a are used to carry out the tandem welding process. Each welding head continuously feeds a welding wire made of suitable filler material, with an arc burning continuously or intermittently between the respective welding wire and the housing. While the housing 14 rotates about its central axis D in the direction of the Fig. As the first welding head 33 rotates in the direction of the arrow indicated in 7a (or in the opposite direction), the welding wire creates a weld pool that initially agitates the cast structure of the upper housing part 15 and the housing part 16 at the weld seam. The rotational speed of the housing 14, the speed of the wire feed and the welding current, as well as the distance between the two welding heads 33, 33a, are coordinated so that the weld pool solidifies but is still hot before it reaches the second welding head 33a. The second weld thus relieves stress in the cast structure and welds it with a prepared homogeneous microstructure. This results in a very high weld seam 34, particularly when performed as in Fig. As shown in Figure 9, a belt arches over both the outside of the housing and, at least optionally, also over the inside of the housing, absorbing radial forces occurring at this point. This contributes significantly to the compressive strength of the housing 14. The belt forms a zone of increased tensile strength in the circumferential direction.

[0030] The weld seam 34, which acts as a tensile-resistant belt, can be used particularly in the embodiments according to Fig. 2, Fig. 3 and Fig. 4. The parts held together by it are decoupled with respect to pressure to such an extent that these parts can be considered as individual components with regard to pressure load. This allows the housing 14 according to the invention to be used for current transformers 10, which contain compressed air as an insulating medium and have burst pressures of 75 bar (or even more), despite a possibly reduced wall thickness. The weld seam 34 can be used like the Fig. 9 and Fig. 11 illustrates multi-layered or single-layered designs, which can be executed in a single turn.

[0031] After welding, the housing 14 is preferably stored at an elevated temperature (tempered or "aged"). This relieves internal stresses, which also contributes to the long-term compressive strength of the housing.

[0032] The weld seam 34 is preferably produced during several revolutions of the rotary table 32, so that it ultimately consists of several superimposed layers 35, 36, 37, 38. In particular, the weld seam 35 arches over the outer surface of the housing upper part 15 and the outer surface of the housing intermediate part 16, which is aligned with it. The weld seam may also have an inward curvature at its root. The multi-layered design of the weld seam 34 minimizes the local heat input into the housing upper part 15 and the housing intermediate part 16 and ensures rapid cooling of molten wall components. This ensures that the weld seam 34 does not ultimately constitute a weak point, but rather provides a housing-stiffening function similar to that of the housing-enclosing belt.

[0033] During the Fig. 8 and Fig. In the illustrated embodiment 9, the upper housing part 15 and the intermediate housing part 16 have end faces 29, 30 inclined symmetrically to each other, i.e., they have conical angles that open in opposite directions. The end face 29 rests on a cone whose apex points downwards, while the end face 30 rests on a cone whose apex points upwards. At their radially inner edges, the end faces 29, 30 may each have a planar section 39, 40 that is narrow enough to allow the first layer 35 of the weld to form a root extending to the inner surface. On the other hand, the planar sections 39, 40 are preferably wide enough to allow the upper housing part 15 to be securely placed onto the lower housing part 16.

[0034] While the end faces 29, 30 in the embodiment according Fig. While 8 each have the same cone angle, this angle can also be defined differently, such as Fig. Figure 10 illustrates this. In particular, the upper end face 30 of the intermediate housing part 16 can have a larger conical angle than the end face 29. The end face 30 can also be flat or, in extreme cases, slightly inclined inwards, i.e., lying on a cone whose apex points downwards. The resulting asymmetrical annular joint 31, however, lies in a horizontal plane, as does the previously described annular joint 31. Due to the reduced outward inclination of the end face 30, the weld pool can easily be retained in the annular joint 31. Again, the weld seam 34 is formed, at least preferably, during several rotations of the upper housing part and the intermediate housing part 16, resulting in the weld seam 34 having several layers 35 to 38. Otherwise, the previous description applies accordingly.

[0035] The presented method can be used not only to measure the current transformer 10 according to Fig. 1, but provide an entire family of additional current transformers. Fig. Figure 2 illustrates an embodiment in which the housing 14 (i.e., the upper housing unit) has an annular, and thus essentially hollow cylindrical, intermediate part 41 between the upper housing part 15 and the housing intermediate part 16. This intermediate part forms the annular joint 31 with the housing part 16, on which the weld 34 is formed. The upper housing part 15 then connects to the intermediate part 41 by a weld 34', for which the explanations given in connection with the weld 34 apply.

[0036] Alternatively, the upper housing part 15 and the intermediate part 41 can also be cast in one piece, if the mold 19 is used for this purpose. Fig. 5 is dimensioned accordingly higher.

[0037] In Fig. Figure 3 illustrates another embodiment of a current transformer 10, in which the upper housing unit is designed according to Fig. 2 is combined with a lower, approximately cup-shaped housing unit 42. This lower housing unit 42, which is also manufactured, for example, by die casting, has a base 43 at the bottom that transitions into a cylindrically rising wall 44 in a curve. The wall 44 has a flange at the top, which can be screwed to a lower flange of the upper housing unit 14. Otherwise, the previous description applies accordingly.

[0038] Fig. Figure 4 illustrates another variant of the current transformer 10, the special feature of which is that the housing, consisting of housing units 40 and 42, is arranged upside down. The "upper" housing unit 14 is located at the bottom and is directly connected to the insulator column 13. The "lower" housing unit 42 forms the upper end. The measuring terminal 12 can also be provided at this point.

[0039] According to the invention, the housing 14 of a current transformer 10 is manufactured using the aluminum die casting process, wherein the housing is divided into several housing parts 15, 16 and, if required, further housing parts 41, 42 in order to be able to use the simplest possible die casting molds 19, 20 with no or few movable mold parts 26, 27. The aluminum parts produced by the die casting process are then welded together to form the desired housing units 14, 42.

[0040] The aforementioned process allows for increased productivity and simultaneously improved product quality. By using die casting, the overall housing weight can be reduced despite the higher pressure resistance compared to sand casting. Without time-consuming post-processing of the castings, the surface roughness can be reduced to R max=10, without the need for complex surface grinding. The smooth housing surface increases protection against arcing. The wall thickness of the housing according to the invention can be reduced compared to housings manufactured by sand casting, because the weld seam 34 acts like a belt that spans and holds the housing 14 together. Reference symbol: 10 current transformers 11 High-voltage line 12 measuring connection 13 Insulator column 14 Housing / upper housing unit 15 Housing top 16 Housing intermediate part 17 Flange plate 18 screws 19 first mold 20 second mold 21 first recess with inner contour 22 first mold core 23 first mold cavity 24 second recess with inner contour 25 second mold core 26, 27 movable mold cores 28 second mold cavity M Central axis of the second mold20 29, 30 end faces 31 Ring joint 32 Rotary table 33 Welding head 34 weld seam 35 - 38 layers of the weld seam 34 39, 40 Plan sections of the end faces 29, 30 41 Intermediate part 42 lower housing unit 43 Bottom section of the lower housing unit 42 44 Wall of the lower housing unit 42 45, 46 through-holes

Claims

[1] Method for manufacturing a housing unit (14) for a current transformer (10), comprising the following steps: Providing a first mold (19) having a first recess (21) corresponding to an outer contour of a housing top part (15) and a first mold core (22) with an outer contour corresponding to the inner shape of the housing top part (15), Providing a second mold (20) having a second recess (24) corresponding to the outer contour of a housing intermediate part (16) and a second mold core (25) with an outer contour corresponding to the inner shape of the housing intermediate part (16), Closing the molds (19, 20) by inserting and positioning the mold cores (22, 25) in the molds (19, 20), thereby providing mold cavities (23, 28), Manufacturing the upper housing part (15) and the intermediate housing part (16) by filling the mold cavities (23, 28) with liquid aluminium material, cooling and demolding the upper housing part (15) and the intermediate housing part (16), Placing the housing intermediate part (16) on a workpiece holding table (32), Placing the upper housing part (15) or an intermediate part (41) on the intermediate housing part (16) so that an annular joint (31, 31') is formed between the upper housing part (15), or the intermediate part (41), and the intermediate housing part (16), forming a weld seam (34, 34') along each annular joint (31, 31') to form the upper housing unit (14). [2] Method according to claim 1, characterized bythe provision of a third mold which has a third recess corresponding to the outer contour of a further intermediate part (41) and a third mold core with an outer contour corresponding to the inner shape of the further intermediate part (41), as well as producing the further intermediate part (41) by filling liquid aluminium material into the third mold cavity, cooling and demolding the further intermediate part (41). [3] Method according to claim 1 or 2, characterized by the provision of a fourth mold, which has a fourth recess corresponding to the outer contour of a lower part (42) and a fourth mold core with an outer contour corresponding to the inner shape of the lower part (42), as well as producing the lower part (42) by filling liquid aluminium material into the third mold cavity, cooling and demolding the further lower part (42). [4] Method according to any of the preceding claims, characterized by, that the housing upper part (15) and the housing intermediate part (16) and, if present, the further intermediate part (41) are machined to the surfaces (29, 30) bordering the ring joint (31, 31'). [5] Method according to claim 4, characterized by , that the surfaces (29, 30) are machined conically. [6] Method according to claim 5, characterized by that they have different cone angles. [7] Method according to any of the foregoing claims, characterized by , that the housing intermediate part (16) is placed upright on the workpiece holder (32). [8] Method according to any of the foregoing claims, characterized by , that the first and second molds (19, 20) and the first and second mold cores (22, 25) are made of steel. [9] Method according to any of the foregoing claims, characterized by, that the weld seam (34, 34') is produced seamlessly in the MIG process or in the MAG process, preferably in tandem welding. [10] Current transformer housing comprising an upper housing unit (14) with a housing top part (15) and a housing intermediate part (16) which are welded together at an annular butt joint (31) forming an annular weld seam (34), wherein the housing top part (15) and the housing intermediate part (16) are each seamless one-piece aluminium castings. [11] Current transformer housing according to claim 10, characterized by , that it has a lower housing unit (42) with a bottom section (43) and with a wall (44) rising from it, which is cylindrical on the outside, wherein the wall (44) has an increasing thickness towards the bottom section (43). [12] Current transformer housing according to claim 10 or 11, characterized by, that the intermediate housing part (16) has through-holes (45, 46) at two opposite locations [13] Current transformer housing according to one of claims 10 to 12, characterized by , that the upper part of the housing (15) is a dome-shaped roof section with a wall extending down from it, which is cylindrical on the outside. [14] Current transformer housing according to claim 13, characterized by that the wall has an increasing thickness towards the roof section. [15] Current transformer (10) with a current transformer transformer arranged in the current transformer housing (14) according to one of claims 10 to 14.

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