Abstract:
A groundwall insulation system for use in dynamoelectric machines carrying voltage above 4 kV, and preferably in the order of 13.8 kV, or higher has two layers of insulation wound onto the conductors of the high voltage winding. The first layer of insulation has a first permittivity that is greater then the permittivity of the second layer of insulation wound onto the first layer of insulation. As a consequence, at the juncture between the first and second layers there is a sharp increase in the electric field profile as seen through the groundwall insulation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an insulation system for use in windings of a dynamoelectric machine. In particular it relates to an insulation comprising inner and outer layers having differing permittivities to create a more advantageous stress distribution within the dielectric. 
     BACKGROUND OF THE INVENTION 
     Insulation systems for large AC dynamoelectric machines are under constant development to increase the voltages at which these machines operate while at the same time minimizing the thickness of the insulating material. 
     In such insulation systems it is common to utilize mica in a variety of forms from large flake dispersed on a backing material, to the product known as mica paper. While the low tensile strength of mica paper does not lend itself to use in such insulation systems, mica paper has superior corona breakdown resistance countering the coronal discharge occurring in high voltage windings that tends to shorten the life of the insulation. To compensate for the low tensile strength of the mica paper, the mica paper is bonded to glass fibers which also tends to prevent the shedding of mica flakes from the mica tape during a taping operation. 
     More recently a corona resistant polyimide and composite insulation tape has been employed in the insulation systems. This tape has exceptional insulation qualities and good corona discharge resistance. This film may be used independently or as a backing on a mica paper, glass fiber composite tape. The addition of enhanced corona resistant tape insulation provides an insulation system which is electrically more enhanced than standard systems. 
     However, the magnitude and profile of the local electric field within the groundwall insulation has not been considered to date in the development of insulation systems and tapes for the groundwall. This electric field generated in the groundwall insulation as a result of the high voltage applied to the conductor has a direct effect on the insulation life. As various initiatives are in place to reduce the groundwall insulation thickness it should be understood that the effect of the electric field as it is distributed across the groundwall also has an effect on the performance of the insulation system and the life of the groundwall insulation system. Accordingly, there is a need to develop a groundwall insulation system for use in windings for dynamoelectric machines that takes into consideration the effects of the localized electric field generated in the groundwall insulation as a result of the voltage difference across the insulation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an insulation system for use in windings of dynamoelectric machines that results in a graded or sharp increase in the electric field as distributed through the insulation from the interior of the insulation adjacent the conductor bar or conducting elements to the outer armor or grounded armor of the insulation. 
     It should be understood that the term “graded increase” in the electric field relates a significant change in the electric field profile across a cross section of the groundwall insulation. In accordance with the present invention, the electric field profile across a “flat” cross section exhibits a sharp step increase part way across the cross section compared with a flat electrical field profile in the past. With respect to a corner section of the insulation, the electric field profile gradually decreases away from the conductors and again exhibits a sharp step increase part way across the corner cross section. 
     To accomplish the forgoing aspect of the graded change in the electric field profile of the insulation of the present invention, there is provided an insulation system that has a conductor of a dynamoelectric machine which is insulated with layers of insulation. The insulation has a first inner layer of insulation and a second layer of insulation outer relative to the first inner layer. Both the first and second layers of insulation have predetermined thickness so as to provide for the proper insulation characteristic needed for the insulation itself. However, the permittivity of the first inner insulation layer is chosen to be greater than that of the second insulation layer such that the electric field in the second insulation layer increases sharply at the juncture between the first inner and second layers of insulation. 
     It has been determined that by providing for a relatively higher permittivity on the inner layer, the electric field adjacent the conductor has a reduced magnitude. While the overall electric field distributed across the insulation may not be less, it should be understood that the magnitude of any sharp occurrences of the electric field in the insulating layer adjacent the conductor are reduced. This is a considerable undertaking because the insulation is designed and developed for its weakest areas in the insulation which occur at the corners of the insulation adjacent the conductors where the highest magnitudes of electric field have been experienced in the past. Thus by reducing this magnitude in electric field, the requirements for the thickness of the insulation is reduced thereby minimizing the thickness of the insulation while not adversely effecting the voltages carried by such conductors or the insulation life. It should be understood that in accordance with the present invention it is envisaged that these conductors carry voltages in the order of 4 kV and greater. 
     It is also envisaged that in alternative embodiments of the present invention the insulation may comprise more than two layers of insulation applied over each other in succeeding layers where each succeeding layer has a permittivity less than the preceding layer of insulation. 
     A preferred application of the insulation system of the present invention is as a groundwall insulation for conductors in the winding of a dynamoelectric machine carrying voltages of 4 kV and greater. In applications where the voltage is in the order of 13.8 kV, the thickness of the groundwall insulation is in the order of 3.2 mm. 
     In accordance with a preferred aspect of the present invention there is provided a groundwall insulation for use on a conductor of a dynamoelectric machine that has a graded electric field profile across the groundwall insulation. The groundwall insulation comprises a first inner insulation layer and a second outer insulation layer. The first inner insulation layer is applied over the conductor and has a first predetermined thickness and first predetermined permittivity. The second outer insulation layer is applied over the first inner insulation layer and forms a juncture therewith. The second outer insulation layer has a second predetermined thickness and second predetermined permittivity wherein the second predetermined permittivity is less than the first predetermined permittivity of the first inner insulation layer creating the graded increase in the electric field in the groundwall insulation at the juncture of the first inner and second outer insulation layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the present invention reference may be had to the accompanying diagrammatic drawings in which: 
     FIG. 1 shows the cross section of a typical stator bar for a large AC dynamoelectric machine; 
     FIG. 2 shows the cross section of a typical stator coil for a large AC dynamoelectric machine; 
     FIG. 3A shows an insulating system for the stator bar of FIG. 1 using the insulating system of this invention; 
     FIG. 3B shows an insulating system for a stator coil of FIG. 2 using the insulating system of this invention. 
     FIG. 4 is a simplified partial view of the conductor of FIG. 3A showing the location of the corner and flat cross-sections for the electric field profiles of FIG. 5; 
     FIG. 5 is a graph of the electric field profile of the groundwall insulation of the stator bar of FIG. 1; and, 
     FIG. 6 is a graph of the electric field profile of the groundwall insulation of the stator bar of FIGS.  3 A and  4 ; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a cross section of a typical stator bar  10  for a large AC dynamoelectric machine. Bar  10  is composed of a large number of insulated conductors such as  12  which are insulated from each other by the strand insulation  14 . 
     The conductors  12  are formed into a group after having strand insulation  14  applied thereto to provide the necessary isolation. The top and bottom surfaces of the conductor group are filled with an insulating material  13  generally referred to as a transposition filler. The group of insulated conductors  12  are next wrapped with a groundwall insulation material  16 . The number of layers of insulating tape making up insulation may be from 7 to 16 layers of a mica tape insulation wound in half lap or wrapped fashion, depending on the level of operating voltage to which the conductors  12  are being subjected. 
     For high voltage applications, that is for voltages above 4000 volts and, preferably 13.8 kV, the preferred groundwall insulation  16  would be layers of a composite mica tape comprising a corona discharge resistant polyimide bonded to a mica type paper tape. This tape provides a good layer of insulation, and because of its corona resistant properties, provides long service life because of the resistance to corona discharge. The mica paper composites and tapes used in these hybrid systems contain a high percentage of a semi-cured resin (resin rich) which may or may not contain a corona resistant material. The wrapped bar is heated and compressed, in an autoclave or press, to allow the resin to temporarily liquefy so as to evacuate any entrapped air and eliminate any voids. Heat and pressure are maintained on the bar undergoing treatment so that the resin contained in the insulation is driven to gelation, bonding the insulation system together. The surface of the cured bar may next be coated with suitable materials to assure that the entire exposed surface of the bar will form an equipotential surface during machine operation. 
     The cured bar manufactured with the tape types as described above will function acceptably well within the design parameters of the machine for a predetermined period of time. 
     FIG. 2 shows the cross section for a typical coil  10   b.  In this instance, strands  12   b  of copper (six shown) are grouped together so that although strands  12  are separated from each other by the presence of strand insulation  14   b,  the six strands grouped into the turn, must be insulated from the other turns of the coil  10   b  by means of turn insulation  15   b.  The turn package is ultimately covered with groundwall insulation  16   b.    
     FIG. 3A shows the cross section of a stator bar insulated in accordance with the teachings of this invention. Here the conductor bundle is composed of individual conductors  22  separated by strand insulation  24  similar to that as previously shown in FIG.  1 A. The conductor bundle is then wound with several layers of composite tape. Each layer of composite tape will comprise a first inner layer  26  of insulation tape and a second insulation layer  28  of tape. These layers  26 ,  28  of tape each have a predetermined thickness and different permittivities. In particular the permittivity of the first inner layer is greater then that of the permittivity of the outer most layer. It should also be understood that additional third or fourth layers of tape with reduced permittivity may be employed in the present invention. 
     It should be understood that these inner and outer insulation layers may comprise layers of half lapped tape composed of a composite such as mica paper backed on a glass tape backing to form layer  28 . A suitable resin impregnant is present in the mica paper. This standard tape has an excellent voltage withstand capability. 
     The groundwall insulation comprising layers  26  and  28  may be subjected to press curing or an autoclaving curing process to eliminate any voids in the insulation layers  26  and  28  and to subsequently drive the resin impregnant to gelation. 
     Suitable surface coatings may be applied to the external surface of insulation layer  28  before or after cure. 
     FIG. 3B shows the composite groundwall insulation as it applies to coil  20  composed of three turns. In this instance, the copper conductors  22   b  are surrounded by strand insulation  24   b.  The turn insulation  25   b  is applied to each turn and the initial layer of groundwall insulation  26   b  containing the same constituents as layer  26  in FIG. 3A is applied. Finally, the layer of outer groundwall insulation  28   b  is applied. With the exception of the presence of the turn insulation  25   b,  the insulation systems of FIGS. 3A and 3B are very similar. 
     Referring now to FIG. 4 there is shown a simplified drawing of the conductor  25  have including the inner insulation groundwall layer  26  and the second more outer insulation groundwall layer  28  also referred to as the first and second layers  26 ,  28 . The first layer  26  has a permittivity which is chosen to be greater than that of the second layer  28 . In testing that has been done, an inner layer of tape insulation  26  was utilized having a permittivity of 6.5. The permittivity of the second more outer insulating layer  28  was chosen to be 4.2. The predetermined thickness of the layers was 0.096 inches or slightly less then 2.5 mm. The electric field profiles were determined at the corner shown in  40  and the flat at  42 . The result in measurement for FIG. 4 is shown in graph number for FIG.  6 . However, before discussing the graph for FIG. 6, reference may be made to the graph for FIG. 5 which relates to the insulation shown in FIG.  1 . 
     In FIG. 5, it is shown that the profile for the electrical field at the corner  40  diminishes in a curved slope fashion given by curve  55  starting at approximately 4200 volts per mm and this gradually decreases to the 3 mm in thickness for this conductor insulation material. On the flat, the potential electric field is stable at approximately 2600 volts per mm. This is shown by curve  50 . 
     Accordingly, the insulation shown in FIG. 1 has its weakest portion at the corner adjacent to the conductor where the electric field is the greatest and hence the insulation has its weakest portion. Referring to FIG. 6, the graph is shown for the conductor as shown in FIG.  3 A and is compared with the graph of FIG. 5 which is also provided on FIG.  6 . The thickness of the two insulation systems  26  and  28  is shown. In graph  65  the maximum magnitude of the electric field is 4000 volts per mm as compared to about 4200 volts per mm in FIG.  5 . However, the electric field profile decreases gradually along a curve until sharp step  68  where the second layer of insulation is formed at this juncture between layers  26  and  28 . Thereafter the electric field diminishes again in a curved slopping manner. With respect to the electrical field profile across the flat  42 , distribution layer, this is shown at  60  and can be compared to profile  50 . Hence the distribution of the electric field adjacent the conductor is less for both the flat and curved portions  42  and  40  and has a sharp graded step increase at  68  and then is greater then that for curves  50  and  55  respectively. The present invention however does provide for a reduction in the maximum magnitude of the electric field that the groundwall insulation must withstand. 
     It should be understood that the electric field profile as shown in FIG. 6 is for a winding of stator bars and that this electric field profile would be present with a step type function across the juncture of the first and second layer of insulation for stator coils and this pattern can repeat with the addition of subsequent or successive layers of insulation having lower permittivities in each succeeding layer. 
     Further, it should be noted that the thickness of the insulation system used in FIG. 6 has been reduced significantly over that used in the prior art of FIG.  5 . Hence this reduction in insulation results in material cost savings. 
     Referring again to FIGS. 3A and 3B, successive layers of insulation  80  and  82  are shown in ghost lines applied in succession over layer  28  in FIG.  3 A and layer  28   b  in FIG.  3 B. These successive layers  80 ,  82  if used, have declining permittivities for each layer applied further from the turn insulation  24  or groundwall insulation layers  26 ,  28 . 
     It is further envisaged that the inner and outer layers of insulation utilized in the present invention may comprise two tapes made from different types of mica having differing permittivities dependent upon and inherent in the choice of mica for the mica paper tape. The mica papers chosen for these tapes would be such that the difference in permittivities inherent to the mica itself would contribute to an overall resultant permittivity of each tape. In this manner, multiple tapes of differing permittivities can be utilized based on a singe basic tape construction and chemisty. The most common form of mica is Muscovite that has a dielectric constant in the 6 to 8 range. Another form of mica is Phlogopite that has a dielectric constant in the 5 to 6 range. There are many different types of Mica pairings from which to select the advantageous pairing of materials. The mica may be chosen from the following: Anandite, Annite, Biotite, Bityte, Boromuscovite, Celadonite, Chemikhite, Clintonite, Ephesite, Ferriannite Glauconite, Hendricksite, Kinoshitalite, Lepidolite, Masutomilite, Muscovite, Nanpingite, Paragonite, Phlogopite, Polylithionite, Preiswerkite, Roscoelite, Siderophillite, Sodiumphlogopite, Taeniolite, Vermiculate, Wonesite, and Zinnwaldite. 
     It should be understood that alternative embodiments of the present invention may be readily apparent to a man skilled in the art in view of the above description for the preferred embodiments of this invention. For example, while the preferred embodiment relates to groundwall insulation, it is within the realm of the present invention that the turn insulation  24  of FIG. 3A surrounding conductor  22  may comprise the first inner layer of insulation and the second more outer layer may comprise the groundwall insulation layer  26  so long as the second layer  26  has a lower permittivity than the layer  24 . Accordingly, the scope of the present invention should not be limited to the teachings of the preferred embodiments and should be limited to the scope of the claims that follow.