Transformer assembly with shrinkage compensation

A transformer assembly with shrinkage compensation during drying or curing of the windings including: a core having two yokes and two legs, a winding provided about at least one of the two legs of the core, the winding being insulated by an insulating material, a metal profile per yoke, extending in parallel to the respective yoke and being mounted to it, and two pistons seated in the metal profiles, the pistons being movable along their axial direction which is parallel to the longitudinal axis of the at least one winding, wherein the at least two pistons exert a force on the at least one winding in an axial direction of the windings.

TECHNICAL FIELD

The present invention relates to transformer assemblies, in particular transformer assemblies for power applications, and for improved methods for producing such transformer assemblies.

BACKGROUND OF THE INVENTION

During the manufacturing of transformers, such as for traction, power or distribution purposes, windings are typically exposed to a drying process, mainly to eliminate traces of water from the insulating material. During this process, the insulating material shrinks, thereby leading to a small decrease, inter glia, in the axial length of windings.

The active part of a traction transformer comprises one or more windings and a core assembly. During manufacturing, this active part is usually clamped with a mechanical, insulating structure composed by pressure plates and beams. These parts may, for example, be made in wood or in polyester based materials reinforced with fibre glass. A number of metallic axial tie rods are typically installed between the beams, the tie rods allowing to maintain a certain pre-stress force on the windings.

The pre-stress force is applied via the tie rods to the pressure plates and beams. Thereby, during the drying process to which the winding is exposed during the manufacturing process, the winding shrinks along its axial dimension. Hence, in order to desirably maintain the pre-stress force on the windings during the manufacturing process, conventionally the tie rods have to be re-tightened occasionally. This has typically to be repeated a number of times after a certain amount of time, so that the exerted force on the windings is essentially maintained through the entire drying process.

Furthermore, in order to deliver sufficient stability, the assembly of the pressure plates, beams and tie rods must sufficiently be dimensioned, thus resulting in a mass which contributes considerably to the weight of the transformer assembly.

In view of the above and for other reasons, there is a need for the present invention.

SUMMARY OF THE INVENTION

According to a first aspect, a transformer assembly with shrinkage compensation during drying or curing of the windings is provided. The assembly comprises: a core having at least two yokes and at least two legs; at least one winding provided about at least one of the at least two legs of the core, the at least one winding being insulated by an insulating material; at least one metal profile per yoke, extending in parallel to the respective yoke and being mounted to it; and at least two pistons seated in at least one of the metal profiles, the pistons being movable along their axial direction which is parallel to the longitudinal axis of the at least one winding, wherein the at least two pistons exert a force on the at least one winding in an axial direction of the windings.

In a second aspect, a method of compensating the shrinkage during drying or curing of windings in a transformer assembly is provided. The method comprises assembling a transformer assembly according to the first aspect, and, during a drying process or a curing process, applying a force on the winding through pistons.

Further aspects, advantages and features of the present invention are apparent from the dependent claims, their combinations, the description and the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to 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 other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations.

Within the following description of the drawings, the same reference numbers refer to the same components. Generally, only the differences with respect to the individual embodiments are described. When several identical items or parts appear in a figure, not all of the parts have reference numerals in order to simplify the appearance.

The systems and methods described herein are not limited to the specific embodiments described, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. Rather, the exemplary embodiment can be implemented and used in connection with many other applications.

Generally, embodiments described herein pertain to a transformer assembly, which may be a traction transformer for rail vehicles, or generally a transformer for power conversion applications and for power distribution. The assembly comprises a system able to compensate the shrinkage of the winding insulation when drying. This system comprises a mobile piston, pushing the windings, and by a pre-stress screw, henceforth also called adjustment screw, imposing a force on the windings. A bell is typically also employed to transmit the force from the piston to the metal profiles. Spring elements, typically in the form of a stack of spring washers, compensate for the variations of the winding dimensions due to the drying and thermal cycles. Thereby, a decrease of the average manufacturing time of a transformer is achieved, in particular a decrease if the assembly time needed for the active part mounting and to the drying process. Generally, after the drying or curing process of the winding is completed, the transformer assembly is typically further used in a productive environment, e.g., in a railway train. The above-described system used for the shrinkage compensation is then employed to exert a force on the winding(s) in order to maintain their stability, which is particularly useful in the environment of a railway train or locomotive.

Thereby, the spring elements are generally typically configured such that, after the drying or curing is completed, a force exerted on the at least one winding during the further operation of the transformer is dimensioned to be higher than a potentially expanding electromagnetic force of the winding in case of a short circuit condition of the transformer. This ensures that the integrity of the winding is maintained against vibration and shock during normal operation, and is also maintained in case of short circuit. Thereby, the pistons typically move for a distance from about 0.5 mm to about 4 mm, more typically from about 1 mm to about 3 mm during drying or curing. During subsequent transformer operation in the field, the pistons typically move only for a negligible distance in case of a short circuit, as the force exerted by the pistons is configured to be higher than the expanding electromagnetic force of the winding. The pistons typically move in a direction towards the metal profiles only during differential thermal expansion of the winding versus the core during operation (e.g. after cold start-up), which occurs within a time constant of some minutes, e.g. 2 minutes to 10 minutes. Thus, a movement of the pistons towards the metal profiles, away from the center of the winding, may occur due to thermal expansion during normal operation, but not due to the electromagnetic forces during a short circuit condition. Thus, during a short circuit condition, the counteracting force exerted by the spring elements on the pistons hinders the pistons from being moved by the electromagnetic forces caused by the short circuit condition. If the short circuit condition would persist for a longer time span of e.g. 15 seconds or more, the resulting heating and thermal expansion of the copper might, however, lead to thermal expansion of the winding, which may subsequently lead to a movement of the pistons towards the metal profiles. In practice, however, a short circuit condition is typically terminated latest after a few seconds by a protection mechanism, which is part of the railway train or locomotive.

Furthermore, in embodiments, the active part of the transformer is generally clamped with metal profiles, which preferably are aluminium extruded profiles. This allows for weight reduction and material cost savings.

In embodiments, reinforcements are installed along the legs of the transformer and are located between the magnetic core and the windings. The reinforcements are in the following also called mechanical connection elements. These reinforcements or mechanical connection elements rigidify the magnetic core and counteract short circuit forces from the windings. The force is transmitted from the windings to the metal profile via the pistons and then, the stress loop is closed through the reinforcements along the leg. A counter-shape on the base of the metal profile(s) is intended for the installation of these reinforcements.

InFIG. 1, a cross-sectional view on a transformer assembly5according to embodiments is shown. The core10has two yokes12,14and two legs16,18(herein, only leg16is visible due to the perspective). Two windings20,22are provided about the legs16,18, whereby only winding20is visible inFIG. 1. The windings20,22are insulated by an insulating material24, which is only schematically shown inFIG. 1. The insulating material24may for example be an aramid paper or the like.

Above and below each of the yokes12,14, metal profiles8a,8b,8c,8dare mounted. The metal profiles8a,8b,8c,8dextend in parallel to the respective yoke12,14and are mounted to the yokes12,14via bolts9. InFIG. 1, the metal profiles8a,8b,8c,8dextend perpendicular to the drawing plane and are thus only visible as a cross-section.

Pistons26a,26bare seated in the metal profiles8a,8b,8c,8d, wherein the pistons26a,26bare movable along their axial direction. Their axial direction is parallel to the longitudinal axis of the winding(s)20,22. The pistons26a,26bthereby exert a force on the winding20,22in an axial direction of the winding20,22. When the winding20,22shrinks in its length dimension (left-right inFIG. 1) due to a drying process, the pistons26a,26bcompensate for this length difference. To this end, the pistons26a,26bare mounted with spring elements40a,40b. These spring elements40a,40bare typically embodied by a stack of spring washers, such as in the embodiment shown inFIG. 1. The spring elements40a,40bmay also be embodied by using other means, such as spiral springs, or the like.

Mechanical connection elements50a,50b,50c,50dpreferably extend in parallel to the windings20,22(and to the legs16,18) between the metal profiles8a,8b,8c,8dwhich are provided on both ends of the windings20,22. The mechanical connection elements50a,50b,50c,50dcomprise two elongated metal bars per leg, extending in parallel to each other and in parallel to the legs16,18in a space between the legs16,18and the respective windings20,22. Thereby, the elongated metal bars are mounted at their respective ends to at least one of the metal profiles8a,8b,8c,8d. InFIG. 2, a part of the transformer assembly is shown, wherein only the windings20,22are left out for illustrational purposes.

Thus,FIG. 2shows a perspective view on the assembly ofFIG. 1without windings. The arrows symbolize the force which the (left out) windings would exert on the metal profiles8a,8b,8c,8dvia the pistons26a,26b. InFIG. 3, the same transformer assembly5is shown in complete form, that is, including windings20,22, and plates32band32a.

In embodiments, the metal profiles8a,8b,8c,8dcomprise or consist of aluminium or an aluminium alloy. Thereby, two profiles8a,8b;8c,8dextend in parallel to each other on opposite sides of each of the yokes12,14. The pistons26a,26bare each seated in the metal profiles8a,8b,8c,8dvia a bell60a,60b(not shown inFIG. 1). The bell60a,60balso preferably comprises or consists of aluminium. The bell60a,60bis intended to transmit the force from the piston26a,26binto the metal profile8a,8b,8c,8d. The bell60a,60bis particularly useful in the case of extruded metal profiles8a,8b,8c,8d, as schematically shown inFIG. 4. For each piston26a,26b, an adjustment screw34a,34bis used for adjusting a force of the piston26a,26bonto the respective winding20,22. The adjustment screw34a,34bis seated in an insert36having an inner thread. The insert36is fastened to the bell60a, preferably also via a thread. A plate32bis typically located between the pistons26a,26band the windings20,22, in order to transmit the force from the pistons26a,26bto the windings20,22. In embodiments, the plate32bcomprises or consists of a polymer or a fiber-enforced resin. The plate32bmay be ring-shaped, such that it substantially covers the cross section of the winding20,22. The plate32bmay also comprise further functionality, for example oil conducts when the plate32bis used for transporting/circulating insulating oil into the transformer5. The plate32bshall be able to withstand the punctual force from the pistons26a,26bon one side.

As can be seen inFIG. 2, on the other side of the windings20,22, there are no pistons, but a further plate32a, which transmits the force from the windings20,22(being pushed by the pistons26a,26bfrom the other side) onto the metal profiles8a,8d. As is shown inFIG. 2, a total number of four pistons26a,26a,26b,26bact on each winding20,22. Thereby, two pistons per winding are located in metal profile8band two pistons per winding are located in metal profile8c, all on one end of the windings20,22. In other embodiments, the number of pistons per winding may differ. It goes without saying that the skilled person may, based on this disclosure, find other variants to provide the pistons26a,26bin the metal profiles8a,8b,8c,8d, which are regarded to fall under the present disclosure.

To sum up, the method of compensating the shrinkage during drying or curing of windings of a transformer assembly comprises: assembling a transformer assembly5as described above, and then, during a drying process or a curing process of the windings, applying a force on the winding through pistons. The force is obtained by spring elements, and a shrinkage of the windings during drying or curing is compensated by the pistons.

The transformer assembly may be one of a traction transformer for rolling stock, a distribution transformer, or a power transformer. It is preferably immersed in an insulating fluid, such as mineral oil or oil from organic sources.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that the spirit and scope of the claims allows for equally effective modifications. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims, if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.