Title of Invention

NOVEL PROCESS FOR DESIGN OF CONVERTOR TRANSFORMER VALVE WINDING .

Abstract

Novel process for design of converter transformer valve winding comprising a protective coating disposed between the copper conductor (1) and a first layer of paper insulation (2) and a second layer of Enamel insulation (3) of the valve winding to protect the valve winding from insulation failure due to formation of a conductive layer, the conductive layer being formed by the chemical reaction between the copper conductor (1) and the transformer oil.

Full Text

This Invention relates to an improved valve winding of a converter transformer. BACKGROUND Of THE INVENTION. In most of the filed converter transformers, it has been found that a shining layer is formed between copper conductor and first layer of paper insulation in the valve winding. This compound is conducting in nature and doubts have been expressed that it may deteriorate the insulation strength of the covered paper insulation. Such conducting material is likely to lead to partial discharges between strands of copper-paper Insulation system and eventually an internal fault. The formation of conducting layer Is due to chemical reaction between sulphides/mercaptans present in transformer oil & copper, which produces copper sulphide & thereby deteriorating the insulation properties of paper insulation. Such deterioration eventually leads to the Insulation failure. OBJECTS OF THE INVENTION: An object of this invention is to propose en improved veto winding of a convenor transformer. Another object of this invention is to propose an improved valve winding of a converter transformer having a protective layer of enamel Insulation. Further object of this invention is to propose an Improved valve winding of a convenor transformer having no chemical reaction between the sulphides/mercaptans present in transformer oil and copper. Still further object of this Invention is to propose en Improved valve winding of a converior transformer having proper insulation properties. SUMMARY OF THE INVENTION According to this invention, there is provided an improved valve winding of a converter transformer comprising a protective coating disposed between the copper conductor and a first layer of paper insulation of the valve winding to protect the valve winding from insulation failure due to formation of a conductive layer, the conductive layer being formed by the chemical reaction between the copper conductor and the transformer oil. BRIEF DESCRIPTION OF ACCOMPANYING DRAWING Fig-1 New concept applied on normal PICC Fig-2 New concept applied on glued bunch PICC Fig-3 EDAX Analysis of copper conductor from Rihand Fig-4 EDAX Analysis of Chandrapur copper conductor DETAILED DESCRIPTION OF THE INVENTION In the new design, direct contact between winding copper (especially valve windings) and transformer oil is avoided thereby eliminating the chance of formation of conducting layer due to chemical reaction between conductor and transformer oil. In this design, a protective layer of enamel insulation is used over all the valve winding copper to prevent the direct chemical reaction between copper & sulphides/mercaptans present in transformer oil. Usually for valve windings, Paper insulated copper conductor (PICC) (1) or Glued Bunched PICC (4) are used. Conventionally both these type of conductors without enamel (3) were being used since last many years for all applications as per Fig la & 2a. Paper (2) used to be the insulating material for these type of conductors. However the new invention makes the use of protective layer of enamel (3) as a mandatory requirement over bare copper as per Figure 1b & 2b to prevent the direct chemical reaction between copper (1) & sulphides/mercaptans present in transformer oil. The thickness of protective layer of enamel used in valve winding depends on the type of winding. The protective layer of enamel used in valve winding comprises of Polyvinyl acetal enamel class 120 of IEC 317-18-1990. Published information regarding detailed analysis carried out on refined transformer oil samples indicates that there are different types of sulphur compounds present. Some of them are corrosive or reactive. Further analysis reveals that five basic groups of sulphur and sulphur compounds are found in crude oil as indicated below: R- Paraffin with straight or branched chain hydrocarbon or cyclic hydrocarbon. As shown in table above, the elemental sulphur and sulphur containing mercaptans are very reactive, sulphides are also reactive. Reactive sulphur is mainly in form of organic sulphur compounds like R-SH, where the sulphur is attached at the end of an organic molecule. When the molecule is more complex, for instance when the sulphur is surrounded or contained within the molecule then the sulphur compounds are more stable and less reactive like in R-S.S-R. Thiophenes are the most stable of all the sulphur compounds. It has been found that deterioration in insulation properties is due to reaction of mercaptan sulphur and elemental sulphur with exposed copper surface of conductor leading to formation of conducting layer of copper sulphides and other compounds. Probable Phenomena of Failure The reactive sulphur, once in contact with copper surface, starts reacting with it, forming copper sulphide even at operating temperature. The rate of reaction increases with temperature. The reaction is likely to be faster in hot spot region of the winding and ducts. It is likely that presence of voltage could accelerate the formation of copper sulphide. However no experimental evidence has been published yet. The copper sulphide thus formed may create craters/protrusions in localized area of conductor giving rise to onset of partial discharge or might permeate through the paper, reducing the breakdown strength of the insulation. Such occurrence could be the reason of several failures observed in the converter transformers. The reactive sulphur, once in contact with copper surface, starts reacting with it, forming copper sulphide even at operating temperature. The rate of reaction increases with temperature. The reaction is likely to be faster in hot spot region of the winding and ducts. It is likely that presence of voltage could accelerate the formation of copper sulphide. The copper sulphide thus formed may create craters/protrusions in localized area of conductor giving rise to onset of partial discharge or might permeate through the paper, reducing the breakdown strength of the insulation. Such occurrence could be the reason of several failures observed in the converter transformers. Analysis and Field of Invention There are two major projects related to HVDC in India, one is Rihand Dadri & the other is Chandrapur-Padghe. One sample (copper conductor & paper sample) was drawn from Rihand Transformer, which had failed & the other (copper conductor & paper sample) was drawn from the Chandrapur Transformer, which was withdrawn form operation due to high Dissolved gases. These samples were subjected to detailed tests & analysis. The results are shown in Fig.3 & 4 respectively. The sample of Rihand was found to contain precipitates of copper sulphides, (Fig.3). Whereas in the sample of C-P project, oxide deposits were found along with traces of copper sulphide (Fig.4). Thus the copper conductor of valve windings when coated with thin layer of enamel insulation totally eliminates the contact of Mercaptan sulphur and elemental sulphur with base copper of valve windings which in turn also eliminate the failure of converter transformers. This invention relates to elimination of contact of oil & base copper surface by use of enamel coating over copper surface. This excludes the formation of conducting layer of copper sulphides and other compounds thereby preventing the failure of valve winding. WE CLAIM 1. A novel process for design of convertor transformer valve winding comprising a protective coating disposed between the copper conductor (1) and a first layer of paper insulation (2) and a second layer of Enamel insulation (3) of the valve winding to protect the valve winding from insulation failure due to formation of a conductive layer, the conductive layer being formed by the chemical reaction between the copper conductor (1) and the transformer oil. 2. An improved valve winding of a converter transformer as claimed in claim 1, wherein said protective layer is a thin coating of enamel (3). 3. An improved valve winding of a converter transformer as claimed in claim 1, wherein the said coating of enamel comprises of Polyvinyl acetal enamel Class 120 of IEC 317-18-1990. Novel process for design of converter transformer valve winding comprising a protective coating disposed between the copper conductor (1) and a first layer of paper insulation (2) and a second layer of Enamel insulation (3) of the valve winding to protect the valve winding from insulation failure due to formation of a conductive layer, the conductive layer being formed by the chemical reaction between the copper conductor (1) and the transformer oil.

Documents:

394-KOL-2005-FORM-27-1.pdf

394-KOL-2005-FORM-27.pdf

394-kol-2005-granted-abstract.pdf

394-kol-2005-granted-claims.pdf

394-kol-2005-granted-correspondence.pdf

394-kol-2005-granted-description (complete).pdf

394-kol-2005-granted-drawings.pdf

394-kol-2005-granted-examination report.pdf

394-kol-2005-granted-form 1.pdf

394-kol-2005-granted-form 18.pdf

394-kol-2005-granted-form 2.pdf

394-kol-2005-granted-form 3.pdf

394-kol-2005-granted-gpa.pdf

394-kol-2005-granted-reply to examination report.pdf

394-kol-2005-granted-specification.pdf