Abstract:
A method for manufacturing a liquid-cooled stator bar suitable for use in a generator. The method includes providing a stator bar comprising a plurality of conducting strands collectively formed into a desired shape, wherein at least one of the conducting strands is configured to channel a liquid for cooling the stator bar. At least one layer of ground wall insulation is applied to the outer portions of the stator bar, and then at least one layer of a conductive tape is applied to an outer area of the ground wall insulation. The ground wall insulation and the conductive tape are cured.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to a method for applying a conductive material to stator bars, and more specifically, to methods for applying a conductive tape to liquid-cooled stator bars for high-voltage generators. 
   In at least some known high-voltage generators, a stator yoke surrounds an armature core and partially encloses a plurality of stator bars, which are sometimes referred to as “stator windings” or “armature windings”. At least some known stator bars include a plurality of strands of copper conductors that are wound in the armature to form loops. The stator bars are generally positioned in slots of the power generator so that desired voltage and current characteristics can be generated during operation. High-voltage insulation is wrapped around the stator bars to maintain ground insulation between the conductors and the stator core and other grounded objects. 
   While in the slot of the power generator, stator bars are subject to a cross-slot flux produced by the normal load current. If the stator bars are loose within the slots, vibration or “bar bouncing” caused by the magnetic forces can damage the stator bars. Thus, in order to reduce this motion, the final size of the stator bars must be carefully configured so that the bar is tightly wedged into the slot. 
   At least some known methods for fabricating stator bars include adding a conductive material or coating in order to prevent corona discharges between the stator bar and stator core laminations. Corona discharges deteriorate the high-voltage insulation which leads to premature failure of the stator bar. Thus, material having a lower resistance than the insulation is applied to the outer surface of the insulation in order to prevent or limit the corona activity. 
   In one known method, particularly for after-market liquid-cooled stator bars, electrical insulation is first applied around the stator bar. The insulation is then cured in a vacuum-pressure autoclave. After curing in the autoclave, the bar is inspected to confirm that it satisfies dimensional requirements. A conducting adhesive is then applied to the cured ground insulation, followed by wrapping the coated insulation with a glass tape or other material (e.g., fabric, felt, or mat). Typically, the width of the tape is approximately one and a half inches wide, making the process of wrapping the bar labor intensive. After the adhesive and the tape dries, which takes approximately eight hours, a conductive paint is applied to the glass tape. The conductive paint helps obtain the desired surface resistivity for the final stator bar. The paint takes approximately eight hours to dry. In some cases, the bar is held (e.g, on sawhorses) which requires the bar to be moved so that those areas covered by sawhorses can be painted. Another eight hours of curing time is then needed. 
   After the paint cures, the stator bar is then inspected again to verify the bar is appropriately sized. Furthermore, the stator bar can be surface resistance tested, in order to ensure the surface resistivity is within an appropriate range. Bars are then “high potential” proof tested at elevated voltages relative to operation in order to identify any flawed ground wall insulation. 
   However, the armoring process described above can be labor intensive. Also, the process requires substantial time in curing the paints and adhesives applied to the stator bars (approximately 16 hours) and substantial time in manually sizing the bar and applying the armor. Lastly, a significant amount of bar handling is required due to the number of process steps for armoring and this presents the potential for handling damage to occur. 
   Overall, the above process can be inefficient and costly. Furthermore, generators may include a variety of stator bar configurations and/or require a variety of stator bar sizes. As mentioned above, it is important to control the finished size of the bar to prevent them from being loose in the slot. Thus, alternative methods and more cost-efficient methods for manufacturing stator bars and for applying a conductive material are desired. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method for manufacturing a liquid-cooled stator bar suitable for use in a high voltage generator is provided. The method includes providing a stator bar having a plurality of conducting strands collectively formed into a desired shape, wherein at least one of the conducting strands is configured to channel a liquid for cooling the stator bar. At least one layer of ground wall insulation is then applied to outer portions of the stator bar, and at least one layer of a conductive tape is then applied to at least a portion of an outer area of the ground wall insulation. The ground wall insulation and the conductive tape are then processed in an autoclave. 
   In another aspect, a liquid-cooled stator bar configured to be easily sized for fitting into a slot of a high-voltage generator is provided. The stator bar includes a plurality of conducting strands collectively formed into a desired shape, wherein at least one of the conducting strands is configured to channel a liquid for cooling the stator bar. The stator bar further includes at least one layer of ground wall insulation coupled to outer portions of the collectively formed strands, and two layers of a conductive tape coupled to at least a portion of an outer area of the ground wall insulation, wherein the conductive tape comprises a glass cloth uniformly impregnated with a conducting organic resin. Each layer of the conductive tape has a half-lap configuration on the portion of the ground wall insulation. 
   In yet another aspect, a method for armoring a liquid-cooled stator bar for a high voltage generator is provided. The method includes providing a conductive tape comprising a glass cloth uniformly impregnated with a conducting organic resin. The method further includes applying two layers of the conductive tape, via a tape machine, to at least a portion of an outer area of the ground wall insulation such that each layer is applied in a half-lapped manner. The total thickness of the two layers is from about 0.035 to about 0.045 inch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective end view of an exemplary generator. 
       FIG. 2  is an enlarged cross-section of a stator bar according to one embodiment of present invention. 
       FIG. 3  is illustrates the conductive tape as it is being applied to the stator bar in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective end view of an exemplary generator  100 . A cylindrical rotor  102  (transparently represented by dashed lines) is placed within the stator core bore. A plurality of stator bars  104  are positioned in slots  106  defined around an inner circumference of a stator core  108 . Each stator bar  104  includes at least one circumferential bend  110  defined between a turbine end  112  and a connection end (not shown) of each bar  104 . 
     FIG. 2  is a partial cross-section of exemplary stator bars  104  in stator core  108 . Stator core  108  includes slots  106  (only one is illustrated) that each include an access opening  114 , and a dovetail slot  116  that is adjacent opening  114  and extends generally along slots  106 . In the exemplary embodiment, two stator bars  104  are mounted in each slot  106  with one or more packing fillers  118  at opening  114 . A dovetail shaped wedge  119  is positioned at least partially within dovetail slot  116 . Packing filler(s)  118  and dovetail shaped wedge  119  operate together in applying a radial force to inner and outer stator bars  104  so that bars  104  are retained in slot  106 . In one embodiment, packing filler  118  includes a tapered wedge for driving packing filler  118  between wedge  119  and inner stator bar  104  to apply a radial force on inner stator bar  104 , which in turn applies a radial force to bottom stator bar  104 . 
   As shown in  FIG. 2 , a side packing filler  134  (e.g., ripple spring) is inserted along one side of the inner and outer stator bars  104  to ensure good contact between stator bar  104  and core  108 . Both packing fillers  134 ,  118  may be flat or ripple-spring shaped. Furthermore, packing fillers  134  include a semiconducting material to assist with the electrical contact to the stator core. In one embodiment, packing fillers  134  includes a resin-filled glass weave material. Packing filler  118  can optionally include a semiconducting material if desired. 
   In the exemplary embodiment, stator bars  104  are formed of a plurality of strands  120  of a conducting material that are bundled together prior to form to a pre-determined winding path through stator bar  104 . Although strands  120  may be fabricated from several conductive materials, in some embodiments strands  120  are fabricated from copper, copper alloys, or stainless steel. In some embodiments, strands  120  may be cooled. Strands  120  may be cooled in any suitable manner, fashion, and/or by any suitable means. For example, in some embodiments strands  120  are cooled by passing a fluid, such as, but not limited to, air and/or hydrogen gas, over strands  120 . Moreover, and for example, in some embodiments some or all of strands  120  are hollow cooling strands  120  that channel a fluid, such as, but not limited to, water, an oil, air, and/or hydrogen gas, for cooling strands  120 . In the exemplary embodiment shown in  FIG. 2 , some of strands  120  are shown as hollow and some of strands  120  are shown as solid. Another example of cooling of strands  120  includes indirect cooling of strands  120 , for example by passing a fluid, such as, but not limited to, air and/or hydrogen gas, over stator core  108  to thereby cool strands  120  through conduction between stator core  108  and strands  120 . 
   Adjacent strands  120  are electrically-insulated from each other by an insulating material  124 . Although strands  120  may have any shape, in the exemplary embodiment strands  120  are generally rectangular in cross section. Transposition putty material  126  may surround radially inward portions  125  and/or radially outward portions  127  of strands  120  for each stator bar  104  within slots  106 . Each stator bar  104  may be surrounded by multiple layers of an electrical insulation  128 . The number of layers of insulation  128  and their particular arrangement are variably selectable based upon a design specification for generator  100 . Although insulation  128  may include other insulation (e.g., extruded insulation), in one embodiment insulation  128  is fabricated from mica-based materials which include a binder. In one embodiment, insulation  128  is an epoxy-mica system, such as Micapal II™ (a trademark of General Electric Company). In some embodiments, insulation  128  may initially be flexible enough to be wound or wrapped around strands  120 , but after curing, may be relatively hard. Although strands  120  may have any shape, in the exemplary embodiment strands  120  are generally rectangular in cross section. 
   As shown in  FIGS. 2 and 3 , stator bars  104  include a conductive tape or armor  130 . Conductive tape  130  is used on stator bars designed for operation above 1,200 volts in order to provide corona protection. In one embodiment, conductive tape  130  is used on stator bars  104  designed for operation above 2,400 volts AC. More specifically, conductive tape  130  is used on stator bars  104  designed for operation above 5,000 volts AC. 
   Conductive tape  130  includes cloth and a conductive or semiconductive substance. The cloth may be organic or inorganic, and the substance is an organic resin. In one embodiment, the cloth is inorganic. In one embodiment, conductive tape  130  is a woven glass cloth uniformly impregnated with a conducting epoxy resin. 
   Prior to curing conductive tape  130  and insulation  128  (discussed below), conductive tape  130  has an average resistivity from about 20,000 to about 200,000 ohms/sq. This average resistivity is calculated with two or four stacked thicknesses of conductive tape  130  (discussed below) on one side of stator bar  104 . In one embodiment, conductive tape  130  has about 36% to about 49% binder/organic conductive filler content. In one embodiment, conductive tape  130  has a breaking strength of approximately 140 lbs./in., min., and a roll winding tension of approximately 8 lbs. min. In an exemplary embodiment of conductive tape  130 , the thickness, breaking strength, and construction of conductive tape  130  must satisfy the standards in ASTM D579 (Specification for Greige Woven Glass Fabrics). 
   Conductive tape  130 , as applied to insulation  128  of stator bar  104 , must have a substantial thickness so as to allow an operator or machine to manipulate the size dimensions of the final stator bar product by removing a portion of conductive tape  130 . For example, an operator can remove a few mils of the conductive tape from a length of a side of the stator bar (e.g., by sanding the bar) so that the stator bar will fit into the respective slot of the generator. Because only a small amount or portion of conductive tape  130  is removed, conductive properties of tape  130  are not significantly altered and insulation  128  (and consequently stator bar  104 ) is still protected. 
   Unlike previous known methods, conductive tape  130  is applied to insulation  128  on stator bar  104  before insulation  128  is cured. A tape machine (not shown) applies one or more layers of conductive tape  130  to stator bars  104 . As shown in  FIG. 3 , in one embodiment, tape machine applies two separate half-lapped layers of conductive tape  130 , around stator bars  104  onto insulation  128 . Because each layer is half-lapped, the final stator bar has four layers (or thicknesses) of tape per side, or eight total layers of conductive tape  130  as the total build for stator bar  104 . In alternative embodiments, conductive tape  130  is applied side by side, in that each layer of tape butts against another layer of tape. Those skilled in the art and guided by the teachings provided herein know that many arrangements and thicknesses of tape  130  may be used in order to achieve the desired resistivity. 
   In one embodiment, one half-lapped layer of conductive tape  130 , which has two thicknesses of conductive tape  130 , has a total thickness from about 0.008 inch to about 0.011 inch. In this embodiment, the thickness of conductive tape  130  is from about 0.004 inch to about 0.006 inch. 
   The total thickness of the layer(s) of conductive tape  130  is substantial enough so as to allow an operator or machine to manipulate the size dimensions of the final stator bar product by removing a portion of conductive tape  130 . In one embodiment, the total thickness of the layer(s) of conductive tape  130  is about 0.035 to about 0.045 inch. More specifically, in one embodiment, the total thickness of the layer(s) of conductive tape  130  is about 0.040 to about 0.045 inch. 
   In one embodiment two half-lapped layers on one side of stator bar  104  have a total thickness from about 0.016 inch to about 0.022 inch. The total build (i.e., the total thickness of conductive tape  130  used on both sides) has a total thickness from about 0.035 inch to about 0.045 inch. 
   The width of conductive tape  130  is amenable for applying conductive tape  130  to insulation  128 . In one embodiment, the width of conductive tape  130  is amenable for applying conductive tape  130  half-lapped onto insulation  128 . In one embodiment, the width of conductive tape  130  is approximately one inch. 
   Stator bar  104  with insulation  128  and conductive tape  130  is then cured in an autoclave by known processes. In one embodiment, the duration of curing is approximately 20 hours. After curing, stator bar  104  is stripped of sacrifice material. 
   The curing process alters the resistivity range of conductive tape  130  and, consequently, stator bar  104 . Those skilled in the art and guided the teachings herein provided know that the composition of conductive tape  130  and the curing process can be altered to reach a final desired resistivity. The desired resistivity range of the outer surface of stator bar  104  must not be too low so that a current and voltage is induced in the armor by the magnetic field. But the desired resistivity range must not be so high that it prevents discharge of the gas at the surface of stator bar  104 . The acceptable resistance range is a function of the distance between a grounding point of conductive tape  130  and the core laminations. 
   In one embodiment, the desired surface resistivity range is from about 500 to about 100,000 ohms/square. In another embodiment, the desired resistivity range is from about 1,500 to about 100,000 ohms/square. 
   However, if portions of the cured conductive tape  130  or if all of the cured conductive tape  130  do not meet the desired resistance range, those skilled in the art can alter the resistance through known methods. For example, if the resistance is too high, then coating the bar with a special paint can reduce the resistance. Also, if the resistance is too low, conductive tape  130  can be removed and known glass tape processes discussed above can be used. 
   After curing, stator bar  104  is inspected for proper size dimensions, the existence of any flaws, and the armor resistivity is tested. 
   By curing insulation  128  with conductive tape  130 , unlike previously known processes, a significant amount of time can be saved and costs reduced. 
   Several tests known by those skilled in the art, including high potential, dissipation factor and tip-up, voltage endurance, thermal aging, impact damage test, comparative wear, compressive creep tests, and winding assembly simulations, were conducted to ensure that embodiments of the present invention would perform successfully in high-voltage generators. 
   Exemplary embodiments of methods and stator bars are described and/or illustrated herein in detail. The methods and stator bars are not limited to the specific embodiments described herein, but rather, components of each stator bar and steps of each method may be utilized independently and separately from other components and steps described herein. Each stator bar component and method step can also be used in combination with other stator bar components and/or method steps. 
   When introducing elements/components/etc. of the methods and assemblies described and/or illustrated herein, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the element(s)/component(s)/etc. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional element(s)/component(s)/etc. other than the listed element(s)/component(s)/etc. 
   While the methods and assemblies described herein have been described and/or illustrated in terms of various specific embodiments, those skilled in the art will recognize that the methods and assemblies described and/or illustrated herein can be practiced with modification within the spirit and scope of the claims.