Abstract
A transformer with improved insulation is provided with an electrically insulating sheet separating the primary winding and a first split-core section from the secondary winding and a second split-core section. The insulating sheet is provided with coatings and the coatings are referenced to the primary side and the secondary side windings and cores to form equipotential surfaces and distribute the dielectric stresses across the transformer. Ridges are optionally added to the insulation sheet to increase creepage and clearance distances. The features described reduce the occurrence of corona and partial discharge in the transformer structure and surrounding space.
Claims
1. A transformer comprising a primary winding, a secondary winding, and a core coupling the windings; wherein the core is partitioned to comprise a primary-side section and a secondary-side section; and wherein an electrically insulating structure comprising an insulating sheet separates the windings and core section positioned on the primary side from the windings and core section positioned on the secondary side; and wherein the insulating sheet is coated with a conductive or semiconductive layer on a substantially central portion of its primary-side surface where the primary-side winding and core are positioned and said layer on primary-side surface is electrically referenced to said windings and core on primary side; and wherein the insulating sheet is coated on its primary-side surface with a semiconductive or resistive or non-linear resistive layer extending from the edge of the aforementioned coating on the central portion to substantially near the periphery of the primary-side surface; and wherein the insulating sheet is coated with a conductive or semiconductive layer on its secondary-side surface and said layer on the secondary-side surface is electrically referenced to the transformer windings and core on the secondary side; and wherein the said semiconductive or resistive or non-linear resistive layer on the primary side surface of the insulating sheet extending to the edge of the said surface is electrically connected substantially near the said edge to the conductive or semiconductive layer on the secondary-side surface of the insulating sheet either through another strip of conductive or semiconductive coating on the surface or through a conductive element.
2. A transformer comprising a primary winding, a secondary winding, and a core coupling the windings; wherein the core is partitioned to comprise a primary-side section and a secondary-side section; and wherein an electrically insulating structure comprising a stack of insulating sheets separates the windings and core section positioned on the primary side from the windings and core section positioned on the secondary side; and wherein the individual sheets comprising the stack of insulating sheets are coated with conductive, semiconductive, resistive, or non-linear resistive layers on their surfaces; and wherein the said conductive, semiconductive, resistive, or non-linear resistive layers on the surfaces of the individual sheets in the stack of insulating sheets are electrically connected or referenced to the conductive, semiconductive, resistive, or non-linear resistive layers on the surfaces of the adjacent sheet or to the primary or secondary side windings or core to control the dielectric stress distribution in the transformer.
3. The transformer of claim 1; wherein the insulating sheet has ridges or fins positioned around the primary or secondary side of the transformer.
4. The transformer of claim 2; wherein the stack of insulating sheets has ridges or fins positioned around the primary or secondary side of the transformer.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements.
[0012] FIG. 1 illustrates a transformer with high isolation according to prior art.
[0013] FIG. 2 illustrates a transformer with an insulating sheet separating the primary and secondary sides with various features according to one embodiment of the present invention.
[0014] FIG. 3 illustrates a transformer with a sandwich of insulating sheets separating the primary and secondary sides with various features, according to another embodiment of the present invention.
[0015] FIG. 4 illustrates a transformer, according to another embodiment of the present invention, with one or more insulating sheets separating the primary and secondary sides and one or more fins or ridges added to the insulation sheet to increase the electrical creepage and clearance distance. Some features described in earlier figures have not been repeated in this figure and can be added to the structure.
DETAILED DESCRIPTION
[0016] Various embodiments and aspects of the inventions will be described with reference to details discussed below, and the accompanying drawings will illustrate the various embodiments. The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of various embodiments of the present invention. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present inventions.
[0017] Reference in the specification to one embodiment or an embodiment or another embodiment means that a particular feature, structure, or characteristic described in conjunction with the embodiment can be included in at least one embodiment of the invention.
[0018] FIG. 1 illustrates a transformer, 100, according to prior art, wherein an insulating sheet, 102, made of a material such as mica with a high dielectric stress separates the primary and secondary sides of the transformer. The magnetic core of the transformer is split into two sections, a primary side core (118) and a secondary side core that is placed below the insulating sheet and not visible in the figure. The insulating sheet separates the primary side core, 118, and primary winding, 110, 112, from the secondary side core and a secondary winding, placed below the insulating sheet, 102, in the figure. Terminals 116 provide connection to the primary windings. The insulating sheet, 102, is provided with coatings, 104 and 106, to distribute dielectric stress.
[0019] FIG. 2 illustrates, according to one embodiment of the present invention, an improved electrical insulation system, 200, wherein the magnetic core of the transformer is split into two portions, 202 and 204. Split-core section 202 is associated with the primary winding 210, while split-core section 204 is associated with the secondary winding, 212. The primary-side and secondary-side split-core sections and windings are separated by an insulating sheet 220 made of a material with high dielectric strength such as mica. In the illustration, the primary side is assumed to be the high voltage side and the secondary side is assumed to be at a lower voltage. The insulating sheet, 220, is provided on the secondary-facing side with a layer or coating 230 of a moderately electrically conductive or semiconductive material. The conductivity of the material for 230 is selected to limit eddy current losses due to transformer magnetic flux, while providing sufficient conductivity. An example material that can be used is a 1-5 mil thick layer of conductive carbon 838AR supplied by MG Chemicals. A protective coating, 240 can be used to prevent damage to layer 230 during assembly or operation of the transformer. On the primary-facing side of the insulating sheet, 220, a moderately conductive or semiconductive coating, 232, is provided in the central portion where the primary-side split-core and winding are located. A second coating, 234, of a semiconductive or resistive or non-linear resistive material is placed on the periphery of 232and extending towards the edge of the insulating sheet 220. The material of coating 234 is more resistive than that of coating 232 and is used to control the dielectric stress across the sheet, 220, from the central portion towards the periphery. A conductive coating or band, 236 and 237 is placed on the edge of the sheet 220 and provides an electrical connection, 254 between the primary-side coating 234 and the secondary-side coating, 230. The length of the coating 234 along the surface of sheet 220 can be 4-10 inches for handling voltages up to 15 kV that are typically used in electric distribution grids or industrial equipment. A protective layer 242 is provided to prevent damage to the coatings 232, 235 and 237 during the transformer assembly or operational life. The primary-side windings and core section are electrically referenced/tied, 250, to the coating 232. In a similar fashion, the secondary-side windings and core section are electrically referenced/tied, 252 to the coating 230. This electrical referencing places the primary-to-secondary voltage potential gradient substantially across the insulating sheet 220, and reduces dielectric stress in the voids or air gaps near the windings. This reduces the possibility of corona or partial discharge in these voids or air gaps and improves the reliability of the transformer.
[0020] FIG. 3 illustrates a transformer, 300, with improved insulation according to another embodiment of the present invention. The structure is similar to the one in FIG. 2, with the enhancement being in the repeated stacking of the insulation and stress-grading coatings. In the illustration, conductive/semiconductive layers 342, 332, 330, and 322, and semiconductive/resistive layers 338 and 328 along with the electrical ties 350 and 352, divide the total potential difference between the primary and secondary windings into two halves with insulation sheets 324 and 332 each withstanding half the voltage. This allows the use of two thinner insulation sheets versus one single sheet to block the full voltage. Thinner insulation sheets typically have a better dielectric strength/mm than thicker sheets. This is due to the difficulty in manufacturing thick insulation sheets without voids and material defects. The structure shown in FIG. 3 shows two distributed layers of insulation stacking, but this can be extended to a higher number of layers to allow the use of even thinner insulation sheets with improved performance and cost.
[0021] FIG. 4 illustrates, according to another embodiment of the present invention, a transformer, 400, with improved insulation. An insulating sheet, 422, separates primary-side split-core, 402 and primary winding, 412 from the secondary-side split-core, 404, and secondary winding, 414. Ridges, 430, 431, 432, 433, 434, and 435 of a high insulation strength material are placed on the insulating sheet, 422 to increase the creepage and clearance distance and reduce the possibility of dielectric breakdown. The ridges can be an integral part of the insulating sheet, 422.
[0022] The foregoing description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. It will be recognized by those skilled in the art that many modifications and variations are possible without departing from the essential scope of the invention. It is, therefore, to be understood that the scope of the invention is not limited to the particular embodiments disclosed, and that the invention will include all embodiments falling within the scope of the claims appended hereto.
