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  Electroplating method for sealing liquid-cooled generator stator bar structuresUSPTO Application #: 20060103261Title: Electroplating method for sealing liquid-cooled generator stator bar structures Abstract: The interior of a stator bar clip that is joined to the end of a stator bar is electroplated so that a metallic barrier coating overlies the braze joint between the stator bar and the clip to define a seal at and around the joint that is substantially impermeable to liquid. (end of abstract)
Agent: Nixon & Vanderhye P.C. - Arlington, VA, US Inventor: Yu Wang USPTO Applicaton #: 20060103261 - Class: 310254000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20060103261. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands brazed to one another to form a bar. The ends of the strands are brazed to an end fitting, typically referred to as stator bar clip. A cover is brazed to the clip window. The end fitting serves as both an electrical and a cooling flow connection for the stator bar. [0002] The hollow end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. At one end, the end fitting receives the ends of the strands of the stator bar. The fitting and the peripherally outermost copper strands of the stator bar are brazed to one another. The opposite end of the fitting is connected to a stator cooling conduit. [0003] Liquid cooled stator bar clips have gone through design changes over the years. However, they typically contain mixed solid and hollow strands brazed to one another, and a cover brazed to a clip window. During operation, the hollow strands carry water to cool off the bar. Over time, leaks can develop about the connection between the stator bar ends and the stator bar fitting, between cover and clip as well as between adjacent strands. Leaks may also occur at various plumbing connections. It is believed that the major leak mechanism is a crevice corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices such as micro surface voids formed during brazing, especially at the boundary of strands and braze alloy. Crevice corrosion is initiated by changes in local chemistry within the crevice, such as shift to phosphorous acid conditions in the crevice. Stagnant water in the chamber of the fitting is in contact with the braze alloy and the copper strands. This coolant contact with the braze joint and cooper strands is believed to cause corrosion and consequent leakage. [0004] Field repair of coolant leaks through the stator bar end connections has been successful. A leak site is identified by several different tests, such as vacuum decay and traceable Helium test. [0005] An epoxy barrier coating method has been used as a leak repair and prevention method. An example of an epoxy barrier coating method is disclosed in U.S. Pat. No. 5,605,590, the disclosure of which is incorporated herein by this reference. This epoxy barrier coating has been applied to provide protection against water initiated corrosion mechanisms along the brazed length of the strand package. Epoxy coating is manually injected. The voids and air pockets are possible during injection. Thus, the process is labor intensive and requires 100% inspection. As a result, the process can be labor intensive, takes a long time to complete, and can produce defects. [0006] There are also other leak issues in liquid cooled generators. Indeed, there are many types of leaks associated with water-cooled generators. Stator bar end crevice corrosion is the major one. But other leaks are caused by porosity, cracking and localized damage during manufacturing rather than as a result of corrosion. [0007] There is a need for an improved method for leak repair and prevention in liquid-cooled generators. In particular, there is a need for corrosion protection at the junction between the stator bars and their clips. The corrosion protection should be robust and be applicable to various stator bar clip designs, including clips for recessed braze, flush braze and raised hollow strand braze designs. There is also a need for an improved method to repair and prevent leaks within the generator water pass that arise from other causes. BRIEF DESCRIPTION OF THE INVENTION [0008] The invention proposes an electroplating method to deposit a thin layer of metallic barrier coating on a part of or the entire interior surface of a liquid-cooled stator bar clip. The metallic layer provides a corrosion resistant barrier coating to prevent water access to corrosion susceptible region(s) such as clip-to-strand braze joints. Different materials may be deposited, such as copper or nickel as the metallic barrier coating. [0009] Thus, the invention may be embodied in an apparatus for electroplating an interior of a component, comprising: an inflow tube for flowing electroplating solution to an interior of said component; an outflow tube for flowing electroplating solution out of an interior of said component; a pump for conducting electroplating solution through said inflow tube and for drawing said electroplating solution through said outflow tube; an anode disposed in said electroplating solution and electrically connected to a power supply; and said component being electrically connected to said power supply as a cathode. [0010] The invention may also be embodied in a method of sealing a stator bar with an interior liquid pass for flow of coolant forming at least a portion of said stator bar, comprising electroplating at least a portion of an interior wetted surface of said stator bar. [0011] The invention may also be embodied in a method of sealing interior surfaces of a fitting receiving a stator bar end, to define a seal substantially impermeable to liquid, the fitting having a chamber for receiving a liquid through an opening in the fitting and said chamber being in communication with hollow strands forming at least a portion of said stator bar, said method comprising the steps of: disposing an electroplating solution in said chamber; disposing an anode of the metal or metal alloy to be deposited in said electroplating solution; connecting said anode to the positive terminal of a current source as a cathode; electrically connecting said stator bar to the negative terminal of said current source; and establishing an electrical potential between said anode and said cathode to initiates electrophoretic migration of metal ions from the anode to the electrically conductive inner surface of the component. [0012] The invention is also embodied in stator bar end and a fitting receiving the end to define a seal substantially impermeable to liquid, the fitting having a chamber for receiving a liquid through an opening in the fitting and in communication with hollow strands forming at least a portion of said stator bar and for flow of the liquid through the hollow strands, wherein at least a portion of an interior surface of said fitting has a metallic layer disposed thereon, wherein said metallic layer is deposited by electroplating. BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 is a schematic illustration of a liquid-cooled stator winding arrangement illustrating the stator bars and end fittings coupled to inlet and outlet coolant headers; [0014] FIG. 2 is a schematic cross-section of a plating set-up for plating the interior of a component, such as a stator bar clip; and [0015] FIG. 3 is a schematic illustration of a plating set-up for simultaneous plating wherein the bar is flooded with circulating solution. DETAILED DESCRIPTION OF THE INVENTION [0016] Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands which are brazed to one another and brazed to an end fitting. The end fitting serves as both an electrical and a hydraulic connection for the stator bar. The end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. Another opening of the end fitting receives the ends of the strands of the stator bar, the fitting and peripherally outermost copper strands of the stator bar being brazed to one another. Over time, leaks have variously developed about the connection between the stator bar ends and the stator bar fitting as well as between adjacent strands. It is believed, based on leak analysis results, that the leak mechanism is due to a crevice corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Crevice corrosion is initiated by changes in local chemistry within the crevice, such as shift to phosphorous acid conditions in the crevice. [0017] There are also other leak issues in liquid cooled generators. Examples of other leaks in addition to the clip crevice corrosion leaks are clip crack leaks, clip window leaks, plumbing and fitting leaks, and connection ring leaks. [0018] Field repair of leaks through the stator bar end connections has only been moderately successful. [0019] In an embodiment of the invention, an electroplating method is proposed to deposit a thin layer of metallic barrier coating on the entire interior surface of liquid-cooled stator bars' water pass, or selectively on target areas, e.g., with high corrosion risks or otherwise having a high leak potential. The metallic layer provides a corrosion resistant barrier coating to prevent water access to corrosion susceptible region(s) such as clip-to-strand braze joints. Concurrently the coating has the capability to repair and prevent substantially all other types of leaks. [0020] Referring now to the drawings, particularly to FIG. 1, there is illustrated a liquid-cooled stator winding arrangement used in a typical liquid-cooled generator. A stator core having stator core flanges 12 and core ribs 14 is illustrated, with stator bars 16 passing through radially extending slots and terminating at opposite ends in end fittings 18 and 20. Inlet hose 22 connects inlet fitting 18 to an inlet coolant header 24 and outlet hose 26 connects the outlet fitting 20 to an outlet coolant header 28. Each stator bar 16 includes a plurality of hollow and solid copper strands respectively disposed in side-by-side and superposed relation one to the other. The fitting, for example, fitting 20, includes a clip 30 formed of an electrically conductive material, such as copper. The clip comprises a body having a rectilinear opening at one end for receiving the strands of the stator bar 16. At the opposite end, there is provided an opening which in use is normally engaged with a copper tube 32 which serves as both an electrical connection and a hydraulic connection for flowing liquid coolant, e.g., deionized water, into or from the chamber 34 defined by the stator bar clip 30 and the exposed ends of the hollow and solid copper strands. The liquid in the chamber either flows into the fitting and through the hollow strands for cooling purposes when the fitting comprises an inlet fitting or receives the liquid coolant from the hollow strands for egress when the fitting is employed as an outlet fitting. As mentioned above, the solid and hollow strands are brazed to one another, and a cover is brazed to clip window. These junctions are among the potential sites for leaks over time. Continue reading... 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