Abstract:
An insulated stator bar and method of applying an outer insulation to a bare stator bar to form the insulated stator bar. The outer insulation surrounds a perimeter of the bare bar and extends along a longitudinal length of the bar. The outer insulation comprises at least one extruded member, e.g., a single extruded member, two individual extruded members, etc., containing an electrical insulation material. The at least one extruded member comprises an opposing pair of edges parallel to the longitudinal length of the bar. The edges are attached together so that the perimeter of the bar is entirely enclosed by the at least one extruded member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a Divisional patent application of U.S. patent application Ser. No. 10/605,489, filed Oct. 2, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention generally relates to electric insulation materials. More particularly, this invention is directed to an extruded groundwall insulation material for a stator bar of an electric machine and a process for applying the insulation material to the stator bar.  
         [0003]      FIG. 1  represents an end portion of a stator (armature) bar  10  of the type installed in dynamoelectric machines, such as a generator used in power generation of high-voltage alternating current. The stator bar  10  can be manufactured to have essentially any length, shape and cross section appropriate for a given generator design, voltage, and power. For most high-voltage applications, the stator bar  10  will not have a simple linear shape, but instead will have a complex shape with bends and turns.  
         [0004]     As shown, the stator bar  10  is composed of a number of conducting copper strands  12  that are insulated from each other by strand insulation  13 . The strands  12  are arranged to form two tiers that are separated by a strand separator  14 , all of which may together be termed a “bare bar.” Surrounding the tiers is a stator bar (groundwall) insulation  15  formed by multiple wrappings of a mica paper tape  16 . Typically multiple layers of tape are tightly wrapped around the conductor, usually overlapping by one-half the width of the tape, or “half-lapped.” The groundwall insulation  15  serves to insulate the stator bar  10  from the stator in which it is installed.  
         [0005]     Groundwall insulation of the type shown in  FIG. 1  is widely used in the power generation industry. The mica paper tape  16  is a prepreg composed of a mica paper typically backed by a single woven backing or a pair of backings. A resin composition permeates the mica paper and bonds each backing to the mica paper, thereby forming the prepreg tape. Examples of this type of groundwall insulation include commonly-assigned U.S. Pat. No. 3,563,850 to Stackhouse et al., U.S. Pat. No. 5,618,891 to Markovitz, U.S. Pat. No. 6,043,582 to Markovitz et al., and U.S. Pat. No. 6,359,232 to Markovitz et al. After being wrapped with a sacrificial release film to protect the tape and prevent contamination, the stator bar  10  are placed in an autoclave for vacuum heat treatment and subsequent curing of its tape  16 . Vacuum heat treatment is carried out to remove air, moisture and any solvent or volatile compound present in the resin binder of the tape  16  while curing under pressure serves to consolidate the tape insulation, such that the resin binder bonds the mica paper and each of its backings together to form a void-free solid insulation. Removal of air, moisture, solvents and volatile compounds from the binder is necessary to prevent formation of voids in the cured insulation that would otherwise adversely affect the quality of the insulation and induce premature insulation failure due to breakdown under electrical stress. The latter characteristic of insulation is termed “voltage endurance,” and is normally due to erosion by electrical discharge and electrochemical attack.  
         [0006]     It can be appreciated that groundwall insulation of the type described above is labor intensive and incurs significant process costs. Furthermore, if not properly controlled, the taping process can lead to the presence of voids, resulting in reduced performance reliability. Therefore, improved groundwall insulation and processes have been investigated. For example, extruded groundwall insulation has been proposed, examples of which include commonly-assigned U.S. Pat. No. 5,650,031 to Bolon et al. and U.S. Pat. No. 5,710,475 to Irwin et al. In these approaches, the bare stator bar is passed through an extrusion die, which deposits the groundwall insulation in-situ along the entire length of the bar. The technical challenges associated with extruded groundwall insulation, including the difficulty of passing stator bars with complex shapes through a die, have been significant, such that stator bars equipped with extruded insulation are not currently in production. Accordingly, there remains a demand for groundwall insulation that overcomes the shortcomings of groundwall insulation formed of multiple wrappings of mica paper tape.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention provides an insulated stator bar for an electric machine, and method of applying an outer insulation to a bare stator bar to form the insulated stator bar. The invention is particularly directed to groundwall insulation for stator bars used in dynamoelectric machines that operate at high voltages, such as a generator for power generation of alternating current delivered to a distribution network (e.g., typically in a range of about 13.8 to 1 g kV).  
         [0008]     The outer insulation of this invention surrounds the perimeter of the stator bar and extends along a longitudinal length of the bar, preferably forming a void-free barrier surrounding the perimeter of the bar. The outer insulation comprises at least one extruded member (e.g., a single extruded member, two individual extruded members, etc.) containing an electrical insulation material. The at least one extruded member comprises an opposing pair of edges parallel to the longitudinal length of the bar. The edges are joined together so that the perimeter of the bar is entirely enclosed by the at least one extruded member.  
         [0009]     The method of the present invention generally entails the steps of extruding the at least one extruded member that will form the outer insulation. The at least one extruded member comprises an opposing pair of edges that are parallel to the longitudinal length of the at least one extruded member. In addition, the extrusion process creates an inner cavity that extends the longitudinal length of the at least one extruded member. A bare stator bar is then inserted into the inner cavity of the outer insulation so that the outer insulation surrounds the perimeter of the bar and extends along a longitudinal length thereof. The opposing pair of edges of the at least one extruded member are then attached together so that the perimeter of the bar is entirely enclosed by the at least one extruded member.  
         [0010]     A significant aspect of the present invention is that extruding and then assembling the outer insulation with a bare stator bar is simpler and less costly than prior art mica tape processes, as well as previous attempts to form an in-situ extruded insulation. Known processes such as thermoforming can be utilized to cause the insulation to conform to the outer perimeter of the bar, thereby enabling the forming of a void-free barrier surrounding the bar.  
         [0011]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows in perspective a cross-section of a stator bar wrapped with a prepreg tape in accordance with the prior art.  
         [0013]      FIG. 2  is a perspective end view of a two-piece extruded insulation surrounding a stator bar in accordance with a first embodiment of the present invention.  
         [0014]      FIGS. 3 and 4  are perspective views showing stator bars being inserted into one-piece extruded insulation in accordance with second and third embodiments of the invention.  
         [0015]      FIGS. 5 and 6  are partial end and plan views, respectively, of an embodiment in which mechanical supports are inserted into opposing edges of an extruded insulation to strengthen the joint formed by and between the edges in accordance with a fourth embodiment of this invention.  
         [0016]      FIG. 7  represents a process for welding the abutting edges of an extruded insulation in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 2  schematically represents a cross-sectional view of an insulated stator bar  20  for a generator of a type used in power generation of high-voltage alternating current delivered to a distribution or transmission network. The stator bar  20  includes stator bar (groundwall) insulation  24  surrounding a bare bar  22 . The construction of the latter can be the same or similar to the bare bar ( 12 ,  13  and  14 ) of the stator bar  10  shown in  FIG. 1 , though any suitable bare bar construction is within the scope of this invention.  
         [0018]     The stator bar  20  of  FIG. 1  differs from the stator bar  10  of  FIG. 1  with respect to the construction of its groundwall insulation  24 . In contrast to the insulation  15  represented in  FIG. 1  as being formed by multiple wrappings of a mica paper tape  16 , the groundwall insulation  24  of  FIG. 2  comprises two extruded members  26  and  28 . Each extruded member  26  and  28  is generally C-shaped or U-shaped, which as used herein include the cross-sectional shapes of the extruded members  26  and  28  shown in  FIG. 2 , namely, a base  30  and two parallel sides  32  that are each perpendicular to the base  30 . Such shapes can be readily extruded and then subsequently assembled with the bare bar  22  to result in the stator bar  20  shown in  FIG. 2 .  
         [0019]     Each extruded member  26  and  28  is represented in  FIG. 2  as having a multilayer construction that includes an electrical insulation layer  34  sandwiched between a pair of inner and outer layers  36  and  38 , respectively. Suitable materials for the insulation layer  34  include electrometric and filled thermoplastic materials having sufficiently high electrical resistivity. An optional but preferred property for the insulation material is the ability to undergo thermoforming in order to more closely conform to the bar  22 . Examples of suitable filled thermoplastic materials include polysulfones, polyimides, bismaleimides, cyanate esters, polysulfides, and silicones filled with about 1 to about 50 weight percent of ceramic and/or oxide particles. Suitable thicknesses for the insulation layer  34  will depend on the particular material from which it is formed.  
         [0020]     The inner and outer layers  36  and  38  may be co-extruded with the insulation layer  34 , or laminated or painted onto the insulation layer  34  after extrusion. Suitable materials for the inner and outer layers  36  and  38  include various conductive materials, such as those used to form conductive slot armoring and internal grading for stator bar groundwall insulation. The presence of the conductive inner and outer layers  36  and  38  is optional, but can be beneficial to allow small relative motions between the conductors and the insulation layers of the bare bar  22  and to reduce any electrical stresses induced at the interface between the bar  22  and insulation  24 . Alternatively or in addition, thin semiconductive tapes may be applied inside and/or outside the extruded insulation layer  34 . Suitable thickness for the layers  36  and  38  will depending on the particular materials of which they are formed.  
         [0021]     As evident from  FIG. 2 , each extruded member  26  and  28  has a pair of edges  40  and  42 , each opposing pair of which are shown as have complementary interlocking features  44  and  46 . The features  44  and  46  are represented as interlocking tongue and grooves  44  and  46  in  FIG. 2  though other configurations are possible, such as butt joints, lap joints, etc. The interlocking features  44  and  46  serve to mechanically lock together the opposing edges  40  and  42  of the extruded members  26  and  28 . The interlocking features  44  and  46  are preferably continuous along the entire length of their respective extruded member  26  or  28 , though it is foreseeable that the features  44  and  46  could be discontinuous.  
         [0022]     As noted above, the C-shaped cross-section of the members  26  and  28  facilitate forming the members  26  and  28  by extrusion. A suitable extrusion technique would be a profile extrusion technique, by which the different layers of materials are co-extruded with multiple extrusion machines feeding into a specially-designed die head. After assembly of the extruded members  26  and  28  with the bar  22 , members  26  and  28  formed of a thermoplastic material may undergo a thermoforming operation, in which both heat and pressure are applied to the extruded members  26  and  28  to soften the members  26  and  28 , force the members  26  and  28  to closely conform to the exterior perimeter of the bar  22 , and close gaps and voids between the extruded members  26  and  28  and the bar  22 . Thermoforming can also result in a more dense material by reducing any porosity within the extruded members  26  and  28  in the unlikely event that such defects are formed during the extrusion process. Suitable thermoforming techniques and parameters will depend on the particular materials used to form the extruded members  26  and  28 , and would generally be within the skill of those familiar with thermoforming processes.  
         [0023]      FIGS. 3 and 4  represent alternative embodiments of the insulation  24  of  FIG. 2 , as well as techniques for their assembly with a bare bar  22 . In  FIG. 3 , the insulation  24  is formed by a single extruded member  126  having a rectangular shape that defines a central passage  128  and a single pair of opposing edges  140  and  142  located at one of the corners of the rectangular shape.  FIG. 4  also shows the insulation  24  as formed by a single extruded member  226  having a rectangular shape that defines a central passage  228 , but with a single pair of opposing edges  240  and  242  located along one of the sides of the rectangular shape. In the embodiments of  FIGS. 3 and 4 , the edges  140 / 142  and  240 / 242  can be joined together by welding or with interlocking features (not shown) during or following insertion of the bar  22  into their passages  128  and  228 .  
         [0024]      FIGS. 5 and 6  represent features that can be incorporated into any one of the embodiments of  FIGS. 2 through 4  to mechanically secure together the edges  40 ,  42 ,  140 ,  142 ,  240 , and  242  of the extrusion members  26 ,  28 ,  126  and  128 .  FIG. 5  represents a portion of the edge  40  of one of the extruded members  26  and  28  of  FIG. 2 , modified to have slots  46  formed therein.  FIG. 6  is a plan view showing the seam defined by opposing edges  40  and  42  of the extruded members  26  and  28 , in which each edge  40  and  42  has been modified to include opposing slots  46  of the type represented in  FIG. 5 .  FIG. 6  further represents the presence of a peg  48  partially received in the opposing pair of slots  46 . An interference fit between the pegs  48  and the slots  46  creates a mechanical interlocking effect between the opposing edges  40  and  42  of the extruded members  26  and  28 . The interlocking effect can be supplemented by welding the edges  40  and  42  together, as discussed previously.  
         [0025]     A weld can be used with or in lieu of interlocking features placed along the edges  40 ,  42 ,  140 ,  142 ,  240 , and  242  of the extruded members  26 ,  28 ,  126 , and  226 .  FIG. 7  represents an insulated stator bar  20  produced by the technique of  FIG. 4  undergoing a welding operation, in which a weld  52  is being formed along the joint (located along one of the sides of the extruded member  226 ) using a plastic seam welding method. The weld material can be a filled or unfilled resin, suitable examples of which include those materials previously noted for the insulation layer  34 . The welding process is schematically represented in  FIG. 7  as being carried out with a movable weld head  54  and a fixtured stator bar  20 . The weld head  54  is preferably carried on a multi-axis robotic arm (not shown) so that the weld  52  can be accurately formed along the length of the stator bar  20 , which as depicted in  FIG. 7  has a complex geometric shape.  
         [0026]     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.