Abstract
A gapped core leg for a shunt reactor, comprising magnetic core elements separated by spacers cast directly between the core elements. Accordingly, a rigid core leg construction is achieved.
Claims
1. A gapped core leg for a shunt reactor, the gapped core leg comprising: a plurality of core elements arranged in a stacked manner, and a spacer arranged in a gap between adjacent core elements, characterized in that the spacer is directly cast between the adjacent core elements; and the direct cast spacer has two main surfaces and a side surface, the side surface comprising through holes across the direct cast spacer.
2. The gapped core leg according to claim 1, wherein the direct cast spacer comprises a polymer composite.
3. The gapped core leg according to claim 2, wherein the polymer composite is a polymer concrete.
4. The gapped core leg according to claim 1, wherein the through holes are running in two levels adjacent to each main surface of the direct cast spacer.
5. The gapped core leg according to claim 1, wherein the spacer fills the whole volume between the adjacent core elements.
6. The gapped core leg according to claim 1, further comprising a mould with a plurality of radial gates between adjacent core elements for introducing spacer material.
7. The gapped core leg according to claim 1, further comprising a mould with a common gate for introducing spacer material between adjacent core elements, at least one core element with a through hole to connect gaps on both sides of the core element.
8. The gapped core leg according to claim 1, wherein the core elements are in the form of radial laminated core steel sheets.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention will be explained in greater detail with reference to the accompanying drawings, wherein
(2) FIG. 1 shows a typical prior art shunt reactor core frame with a gapped core leg installed between two yokes and two side legs,
(3) FIG. 2 shows a cylindrical core element of a prior art shunt reactor with ceramic spacers glued on one face of the core element,
(4) FIG. 3 shows a gapped core leg according to one embodiment of the present invention,
(5) FIG. 4 shows a direct cast spacer element according to one embodiment of the invention,
(6) FIG. 5 illustrates a casting arrangement wherein the mould is provided with an individual radial gate for each gap between adjacent core elements, and
(7) FIG. 6 illustrates a casting arrangement wherein the mould is provided with a common gate for several gaps between adjacent core elements.
DETAILED DESCRIPTION OF THE INVENTION
(8) In a prior art shunt reactor core frame 14 of FIG. 1, a gapped core leg 1 is positioned between two yokes 15 and two side legs 16. The core leg 1 comprises a plurality of core elements 2 arranged in a stacked manner. The core elements 2 are spaced apart by a large number of cylinder-shaped ceramic spacers 17 provided in each gap between adjacent core elements 2. The magnetic connection between the yokes 15 and the core leg 1 is obtained via so-called cross flux plates 18. The core elements 2 comprise radial laminated core steel sheets 19 according to FIG. 2, the lamination blocks being moulded in epoxy resin to form solid pieces. The ceramic spacers 17 are glued on one face of the core elements 2 before stacking the core elements 2.
(9) FIG. 3 shows a gapped core leg 1 according to one embodiment of the invention with a plurality of core elements 2 being separated by direct cast spacers 3. In FIG. 3 one of the direct cast spacers 3 appears to be loose, but this is only for the purpose of illustrating that the whole volume between two core elements 2 is filled with the spacer material. In reality the direct cast spacers 3 have a strong adhesion with the core elements 2 as a result of the direct casting method. In one preferred embodiment all the spacers 3 are of the direct cast type, but using other types of spacers in some of the gaps might turn out to be desirable. This could e.g. be because of worsened cooling properties of the core leg 1 when the gaps are completely filled with material. Ceramic spacers 10 and other prior art solutions may be used in some of the gaps when desired.
(10) The outermost core elements 2 of the core leg 1 may be machined after casting in order to bring the dimensions of the core leg 1 within desired tolerances. It is also possible to allow direct cast spacers 3 to be the outermost elements of the core leg 1, especially if this is preferable from the machining point of view.
(11) FIG. 4 shows a direct cast spacer 3 according to one embodiment of the invention. The direct cast spacer 3 has two main surfaces 7 and a side surface 6. The spacer material is preferably a polymer composite such as polymer concrete. In order to improve the rigidity of the direct cast spacers 3 and the core leg 1 as a whole, the spacer material can be reinforced with appropriate material such as glass fibre or carbon fibre. The side surface 6 of the direct cast spacers 3 is provided with through holes 5 in order to improve the cooling properties. The through holes 5 are accomplished by, before casting, providing the corresponding gaps between adjacent core elements 2 with tubes or pipes across the gap through a surface corresponding to the side surface 6 of the direct cast spacer 3. Preferably the tubes or pipes function at the same time as reinforcement such that no additional reinforcement is needed. The through holes 5 are preferably located close to the core elements 2, and they are preferably running in two levels adjacent to each main surface 7 of the direct cast spacer 3.
(12) With an appropriate casting arrangement a plurality of direct cast spacers 3, preferably all of them, can be cast in one shot. This means in practice that the gaps are filled in parallel and no pressure difference between the gaps can occur. This has significance if an excessive pressure is used during the casting which might cause deformation or displacement of the core elements 2. Casting in one shot entails an additional advantage of a fast manufacturing cycle.
(13) FIG. 5 shows a casting arrangement according to one embodiment of the invention, wherein the mould 8 is provided with an individual radial gate 9 for each gap between adjacent core elements 2 which is to comprise a direct cast spacer 3. The casting is done by arranging the core elements 2 in a mould 8 in a stacked manner and filling any predetermined gap between adjacent core elements 2 with the spacer material 13. Individual gates 9 enable a fast casting cycle and complete filling of the gaps. In this casting arrangement the axis 4 of the core leg lies preferably substantially horizontally during casting.
(14) The distances between the core elements 2 may be defined before casting by arranging distance pieces 10 in the gaps between adjacent core elements 2, and by keeping the stack tight during casting by applying an appropriate axial force at the outermost core elements 2. Three distance pieces 10 in each gap ensure a steady support for the core elements 2. The distance pieces 10 may be manufactured from the same material as the direct cast spacers 3, but they may also consist of other suitable insulating material.
(15) FIG. 6 shows a casting arrangement according to another embodiment of the invention, wherein the mould 8 is provided with a common gate 11 for several gaps between adjacent core elements 2. The gaps on both sides of a core element 2 are connected by providing the dividing core element 2 with a through hole 12. All the gaps of the core leg can be connected by through holes 12 when desired, but some gaps may be isolated in order to use an alternative type of spacer in them. In this casting arrangement the axis 4 of the core leg is preferably substantially vertical during casting, and the common gate 11 is placed in an axial end of the mould 8. Placing the gate 11 at the top end can be chosen in order to allow gravity to contribute to filling the gaps, and placing the gate 11 at the bottom end can be chosen in order to enhance the extraction of air, whichever placement turns out to be more advantageous. This casting arrangement enables the use of a simple mould 8 with a single gate 11, but the number of gates 11 may be increased when desired. Increasing the number of gates 11 may involve providing both axial ends of the mould 8 with a gate 11, or combining axial gates 11 with radial ones 9.
(16) Vacuum casting can be applied if the presence of air bubbles is considered critical. However, small air bubbles are not expected to be a problem since the mechanical strength is ensured by the massive direct cast spacers 3 and small air bubbles do not affect the electrical properties of the spacer.
(17) The invention is not limited to the embodiments shown above, but the person skilled in the art may modify them in a plurality of ways within the scope of the invention as defined by the claims. For example, while the drawings only show core legs with a circular cross section, any other suitable cross section shapes are possible without departing from the inventive concept of the invention.
