Title of Invention

"A METHOD OF REPAIR OF STATOR WINDING"

Abstract

This invention relates to a method of restoration of stator winding for global vacuum pressure impregnation (VPI) wound generator stator comprising the steps of: detecting the fault of winding at the slots in the top or bottom bar location of a stator; removing faulted portion of the top bar only end to end when the fault is in top bar by cutting open the top insulation of the bar and removing the copper strip by cutting on both sides; in case of fault detected at the bottom bar, removing first suitable number of top bars beyond the bend zone on both sides by cutting and removing the strip on cutting; cleaning thoroughly the remaining portion of the winding after removal of bars and testing for high voltage withstanding ability; placing a new bottom bar with resin poor insulation cut only up to the bend, brazing to the strips to strip of incoming bar of the existing strips followed by placement of resin poor mica insulated top bars; globally impregnating the repaired job once again through VPI procedure, on placing bottles of accelerator at the corner of the oven along with spraying of the accelerator on me repaired job during curing to prevent incomplete reaction/polymerization of resins on mica tape and Rotating the job during post curing till the stipulated temperature of the oven is reached to prevent excess deposition of wet resin liquid in any pocket.

Full Text

-2- FIELD OF THE INVENTION The present invention relates to a method of restoration of stator windings for global VPI wound generator and the restored stator thereof. More specifically the invention relates to a method of repair of stator windings by replacing the faulty coil in the post VPI stage on developing repair method in the slot portion of the winding by removal of the top bar and bottom bar. BACKGROUND OF THE INVENTION The insulation system for High voltage stator winding adopted for medium and large capacity Turbo generators up to 200 MVA is Global VPI (Vacuum Pressure Impregnation) sys tern. In this system, the copper strips of stator coils are manufactured as an universal practice in half coil/bar form with 360/540 degrees Roebal transposition. Subsequently, the coils are insulated with resin poor porous mica glass tape to the required number of layers as per the voltage rating of the machine. These coils are placed in the stator slots as per the winding scheme. Normally, three phase star connected double layer lap wound stator winding scheme is followed up to 200 MVA capacity. After the completion of the winding, the complete stator (Core and Winding) is impregnated in a VPI process where in the winding is vacuum dried before being flooded with epoxy resin mixture and place under high pressure for effective penetration of resin in to inner layers. -3- Subsequently, the job is post cured in an oven at elevated temperature. This kind of insulation system makes the job a single monolith with high electrical, mechanical, thermal and chemical properties resulting in high reliability and life of the generator. There is no prior patent reference regarding repair of global VPI wound generator. The repair of VPI wound Turbo generator (TG) stator winding for large capacity turbo generators employing Roebel bars is relatively unknown. M/s SIEMENS has suggested local repair of the winding with replacement of affected coil/bar with pre- insulated conductor and in situ curing. But such method results in loss of integration of winding in post repair stage with regard to mechanical and electrical properties of the stator. The drawback of this system of insulation is that, in case of any inadvertent fault in the winding, the repair becomes cumbersome to replace the faulty coil compared to those of winding with pre-cured individual coils/bars as in the earlier case removal of the bar becomes extremely difficult. To solve the above limitation of poor reliability the VPI wound winding, a novel method was searched for since long for removal of the faulty bar, particularly with Rebel transposition of the strips. -4- The proposed invention is related to solve the difficulties of prior state of arts by pulling out the entire faulty coil, of the VPI wound generator winding, rewinding on placing mica taped bar and subsequently globally impregnating the job. DESCRIPTION OF THE INVENTION According to one object of the invention is to first detect the fault in the slot portion of the winding, at the top bar or bottom bar location. According to another object of the invention when the fault is detected in the top bar location, the top bar is removed from end to end and removing the strip by cutting. According to a further object of the invention when the fault is at the bottom bar location, suitable numbers of top bars are first removed end to end to make adequate space in the winding overhang and removing straight portion of the bottom bar beyond the bend zone on both sides by cutting and removing the strip by cutting. -5- According to a still further object of the invention after removal of the bars, the remaining portion of the winding is thoroughly cleaned, tested for high voltage withstanding ability, placing a new bottom bar with resin poor insulation cut only up to the bend, brazing the strips to strip of incoming bar to the existing strips of cut overhang portion of the bar on bath sides followed by placement of new resin poor mica insulated top bars. A yet another object of the invention is to impregnate the repaired job once again globally impregnated by a modified step of impregnation after repair to prevent incomplete reaction/ polymerization of resins on mica tape due to less content of accelerator and incomplete curing during curing after impregnation stage of repaired winding, bottles of accelerator are placed in the corners of the oven along with spraying accelerator on the job and during post curing in the oven, the job is rotated till the stipulated temperature is reached. High voltage Insulation system of large capacity synchronous Turbo generators, operating in various power plants play very important role of withstanding the voltage stress called the dielectric stress of the machine. In order to serve this primary function, a commercial insulation has to ensure good mechanical, chemical and thermal properties also. The combination of all the above will ensure long life of insulation and thus the machine reliability. -6- The high voltage insulation, has undergone several changes from bituminous Class-B thermoplastic system more than five decades before to present system of thermosetting global VPI (Vacuum pressure impregnation) today. The insulated bars are placed in the slots of the generator stator core and connected electrically as per the winding scheme. Later on the stator is processed in a VPI cycle where in the job is first vacuum dried at an elevated temperature followed by flooding with epoxy resin and hardener system supported by pressurization by a neutral gas for effective penetration of resin through all the radial layers of porous mica tape. The accelerator content in the tape helps in reaction /polymerization of the resin mixture and formation of gelling. Subsequently, the job is post cured at an elevated temperature in an oven for total consolidation and drying. Followings are the typical characteristics of the detailed global VPI insulation: ADVANTAGES: (i) Higher dielectric strength, lesser insulation thickness ad thus machine size, -7- (ii) Lesser dielectric loss, void free insulation, better electrical life of the machine, (iii) better mechanical rigidity, lesser vibration and wear and tear, (iv) higher thermal conductivity of insulation, machine is cooler, (v) higher resistance to moisture, acids and alkalis, lesser aging of insulation, (vi) lesser flammability, lesser damage to insulation in case of eventual fire in winding faults, (vii) higher power/ weight of the machine, Disadvantages: (i) Poor reparability. According to the invention there is provided a method of restoration of stator winding for global vacuum pressure impregnation (VPI) wound generator stator comprising the steps of: detecting the fault of winding at the slots in the top or bottom bar location of a stator; removing faulted portion of the top bar only end to end when the fault is in top bar by cutting open the top insulation of the bar and -8- removing the copper strip by cutting on both sides; in case of fault detected at the bottom bar, removing first suitable number of top bar beyond the bend zone on both sides by cutting and removing the strip on cutting; Cleaning thoroughly the remaining portion of the winding after removal of bars and testing for high voltage withstanding ability; placing a new bottom bar with resin poor insulation cut only up to the bend, brazing to the strips to strip of incoming bar of the existing strips followed by placement of resin poor mica insulated top bars; globally impregnating the repaired job once again through VPI procedure, on placing bottles of accelerator at the corner of the oven along with spraying of the accelerator on the repaired job during curing to prevent incomplete reaction/polymerization of resins on mica tape and rotating the job during post curing till the stipulated temperature of the oven is reached to prevent excess deposition of wet resin liquid in any pocket. The proposed invention will be better understood from the following description with reference to the accompany drawings in which: Figure 1 represents details of TG Stator bars with Roebel Transposition with global VPI insulation systems as practiced in the existing art. Figure 2 represents transposition details of VPI insulation systems of strips of conductors in the top and bottom bar positions placed on the slots of generator stator core with overhang and transposition zone of a winding design of a TG stator. -9- Figure 3 shows graphically the repairing process of faulted winding detected in slot portion of the stator according to the present invention. Figure 4 shows in actual photograph showing bottom bar cut at second bend of copper strip according to the present invention. Figure 5 shows in photograph removal of copper strips of failed bar according to the present invention. Figure 6 shows in photograph a view of insulation through after removal of copper strip according to the present invention. Figure 7 shows a view of brazed bottom bar after repair according to the present invention. Figure 8 shows a brazed repaired bar under insulation by a modified VPI process according to the invention. In the VPI system of insulation, the copper conductors are insulated with resin poor (less binder) porous mica glass based tape for the required number of layer as per rated terminal voltage of the generator. Necessary transposition of the strips of the conductors is made to equalize the induced voltage among the strips as represented in Figures 1 and 2. -10- Figures 1 & 2 show the main components of Roebel stator bars and windings and manner of insulation. Normally the Roebel Bar design of the stator bars are used for medium and large sized high voltage generator stator winding. In such case, either single or double layer star connected lap-winding design is adopted. In case of the Roebel bars, several insulated copper strips are transposed, i.e. bent traversed from top to bottom in single/two planes similar to plaiting to minimize the copper losses due to any possible difference in the induced electro motive force (EMF) among the copper strips. The bars after transposition and stacking of strips, are insulated with several layers of resin poor (less binder) porous mica tape with glass fabric as base layer. Necessary outer corona and end corona protection tapes are applied as outer layers for high voltage machines. During winding assembly, the overhang winding is braced with conformable resin poor mats and sleeves. Figures 3 to 8 show clearly how the proposed invention has developed an improved method of restoration of stator windings for global VPI wound TG generator and the restored stator through repairing course. The turbo generator stator winding is electrically tested for Insulation resistance, High voltage and dielectric loss after the VPI process. Any kind of weakness in the winding such as damage to winding due to external metallic object, VPI process deviation etc are revealed resulting in failure at this stage. (Winding can also fail in operation at power plant due to any inadvertent foreign object during overhaul etc.) In such a case, if the fault is detected in the slot portion of the winding, following repair process is suggested based on the two possible situations according to the procedure as laid down in figure 3. -11- (i) Fault is in the top bar. (ii) Fault is in the bottom bar. In case the fault is located in the top bar, only the top bar is removed from end to end. In case the fault is in the bottom bar, suitable number to top bars are first (6- 10) removed end to end to make provision of adequate space in the winding overhang for removal of straight portion by cutting of the bottom bar and later on brazing a suitable portion (beyond the winding involute bend). Removal of TOP Bar: a. Debraze the eye joints on both side of the winding. b. Remove the wedge form slot portion and cut open the top insulation of bar and make the copper visible. c. Pull the copper strip form eye end up to the transposition bend. d. Cut and remove the strip. e. Follow (c) and (d) above for the next strip till the transposition bend. -12- f. Repeat the above steps from the other end to remove the strips of second half till all copper strips are removed from the bar. g. Remove the U-shape of left out insulation from overhang and slot part by chiselling followed by buffing. Removal of Bottom Bar: (In case of bottom bar fault) a. First remove the suitable number of top bars as detailed above. b. Make a cut of the bar on overhang beyond the bend zone on both side. c. Remove the copper strips in the same manner as detailed in the top bar removal. After removal of the bars, the remaining portion of the winding is thoroughly cleaned and tested for high voltage withstanding ability. Place a new bottom bar with resin poor insulation cut only up to the bend (Not Eye to eye). Braze the strips to strip of incoming bar to the existing strips of cut overhang porting of the bar on both sides. This will follow placement of new resin poor mica insulated top bars. Electrical connection of top bars are made on end eye joints and insulated with resin poor tapes. Figures 4 to 8 show the manner of repair of the top or bottom bar of the stator and replacement of the faulted portion of the strips. -13- Impregnation after repair: The global VPI process as detailed hereinbefore needs slight modification for a repaired winding due to the reason that the number of bars to be impregnated are very few and thus the total content of accelerator for the total job is less. Secondly, excess wet resin during the process may get settled in various pockets and crevices of already large portion of cured winding and may not get cures. This may result in tracking and electrical discharge during subsequent stage of testing of the winding after repair. To prevent this from happening, two precautions are taken: i. During the curing after impregnation stage of repaired winding, bottles of accelerator are placed in the corners of oven as well as spray of the accelerator on the job. ii. During post curing in the oven, the job is rotated till the stipulated temperature to prevent excess deposition of wet resin liquid in any pocket. Rest part of the VPI process including the parameters are same as original stator. -14- Following the repair of the winding, the same is electrically tested for Insulation resistance, absorption coefficient, polarization Index, Dielectric losses (Tan Delta and tip up) and High voltage at original level to ensure the quality of repair. The illustrated invention as herein narrated above has developed a method of removal of the failed bar from an impregnated generator stator by removing / pulling out strips after strips from both ends of the machine following the Roebel transposition bending of the bar which is unique in repair of large capacity Globally impregnated Turbo Generator stators. Advantages: The proposed method of repair of winding of turbo generator with global vacuum pressure Impregnation ensures the following advantages: a. Ease of removal of failed coils from impregnated stator winding b. Fast repair process. c. Ensure original properties of Global VPI insulation. d. Better reliability of repair and longer life of machine. -15- The invention as described herein above with an exemplary embodiment should not be read and construed in a restrictive manner as various modifications, alterations and adaptations are possible within the scope and ambit of the invention as defined in the encompassed appended claims. -16- WE CLAIM 1. A method of restoration of stator winding for global vacuum pressure impregnation (VPI) wound generator stator comprising the steps of: - detecting the fault of winding at the slots in the top or bottom bar location of a stator; - removing faulted portion of the top bar only end to end when the fault is in top bar by cutting open the top insulation of the bar and removing the copper strip by cutting on both sides; - in case of fault detected at the bottom bar, removing first suitable number of top bar beyond the bend zone on both sides by cutting and removing the strip on cutting; - Cleaning thoroughly the remaining portion of the winding after removal of bars and testing for high voltage withstanding ability; -17- - placing a new bottom bar with resin poor insulation cut only upto the bend, brazing to the strips to strip of incoming bar of the existing strips followed by placement of resin poor mica insulated top bars; - globally impregnating the repaired job once again through VPI procedure, on placing bottles of accelerator at the comer of the oven along with spraying of the accelerator on the repaired job during curing to prevent incomplete reaction/polymerization of resins on mica tape and - rotating the job during post curing till the stipulated temperature of the oven is reached to prevent excess deposition of wet resin liquid in any pocket. 2. The method of restoration of stator winding as claimed in claim 1, wherein in case fault detected at top bar, the top bar is removed in the following sequential steps of: a. debrazing the eye joints on both side of the winding; b. removing the wedge from the slot portion and cutting open the top insulation of the bar and make the copper strips visible; -18- c. pulling the copper strip from eye end up to the transposition bend; d. cutting and removing the strip; e. following the steps (c) and (d) for the next strip till the transposition bend; f. repeating the steps a) to (e) from the other end to remove the strips of second half till all the copper strips are removed from the bar; g. removing the U-shape of left out insulation from overhang and slot part by chiselling followed by buffing. 3. The method of restoration of stator winding as claimed in claims 1 and 2, wherein in case of fault at bottom bar 6 to 10 numbers of top bars are removed. 4. The method of restoration of stator winding as claimed in claim 1, wherein electrical connection of top bars are made on end eye joints and insulated with resin poor tapes. -19- 5. The method of restoration of stator as claimed in claim 1, wherein on impregnation of the repaired stator winding the faulted stator is restored to original stator ensured with all the electrical and mechanical parameters in original level on characterize evaluation of insulation resistance, absorption coefficient, polarization index, dielectric losses (Tan Delta and tip up) and high voltage. 6. The method of restoration of stator winding for global vacuum pressure impregnation (VPI) wound generator stator as herein described and illustrated. 7. The restored faulted stator made according to the method of restoration of stator winding of claim 1. This invention relates to a method of restoration of stator winding for global vacuum pressure impregnation (VPI) wound generator stator comprising the steps of: detecting the fault of winding at the slots in the top or bottom bar location of a stator; removing faulted portion of the top bar only end to end when the fault is in top bar by cutting open the top insulation of the bar and removing the copper strip by cutting on both sides; in case of fault detected at the bottom bar, removing first suitable number of top bars beyond the bend zone on both sides by cutting and removing the strip on cutting; cleaning thoroughly the remaining portion of the winding after removal of bars and testing for high voltage withstanding ability; placing a new bottom bar with resin poor insulation cut only up to the bend, brazing to the strips to strip of incoming bar of the existing strips followed by placement of resin poor mica insulated top bars; globally impregnating the repaired job once again through VPI procedure, on placing bottles of accelerator at the corner of the oven along with spraying of the accelerator on me repaired job during curing to prevent incomplete reaction/polymerization of resins on mica tape and Rotating the job during post curing till the stipulated temperature of the oven is reached to prevent excess deposition of wet resin liquid in any pocket.

Documents:

00485-kol-2007-abstract.pdf

00485-kol-2007-claims.pdf

00485-kol-2007-correspondence others.pdf

00485-kol-2007-description complete.pdf

00485-kol-2007-drawings.pdf

00485-kol-2007-form 1.pdf

00485-kol-2007-form 2.pdf

00485-kol-2007-form 3.pdf

00485-kol-2007-gpa.pdf

485-KOL-2007-(07-11-2012)-ABSTRACT.pdf

485-KOL-2007-(07-11-2012)-AMANDED PAGES OF SPECIFICATION.pdf

485-KOL-2007-(07-11-2012)-CLAIMS.pdf

485-KOL-2007-(07-11-2012)-CORRESPONDENCE.pdf

485-KOL-2007-(07-11-2012)-DESCRIPTION (COMPLETE).pdf

485-KOL-2007-(07-11-2012)-DRAWINGS.pdf

485-KOL-2007-(07-11-2012)-FORM-1.pdf

485-KOL-2007-(07-11-2012)-FORM-2.pdf

485-KOL-2007-(07-11-2012)-OTHERS.pdf

485-KOL-2007-(15-05-2013)-FORM-13.pdf

485-KOL-2007-CORRESPONDENCE.pdf