Patent Publication Number: US-10763005-B2

Title: Insulation for conductors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 15/589,422, which was filed 8 May 2017, and the entire disclosure of which is incorporated herein by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under DE-EE0007873 awarded by the Department Of Energy. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     The subject matter disclosed herein relates to insulation for conductors, such as insulation in electrical motors and generators, insulation for bus ducts, insulation for busbars, and insulation for cables where the conductor geometry is such that taped insulation may be required and, more specifically, to methods and systems related to stator insulation. 
     Electrical motors and generators may employ an architecture that has a rotor that is magnetically coupled to a stator. In some designs, the stator may have coils that conduct large currents and are responsible for the creation and maintenance of the magnetic fields driving the electrical machine. In order to prevent short-circuit between the coil and the stator core, as well as between the windings in the coil, the coils may be covered with insulating materials. During operation of the electrical machines, these insulating materials may be subjected to large electric fields. 
     The insulation in coils may have imperfections such as air pockets or air gaps. For example, when an insulating tape is wrapped around metal bars that form the coil, undesired air gaps may appear between different layers of the insulating tape, around the edges of the tape, and between the tape and the conductor. During operation of the electrical machines, the large electric potential differences may generate very large electric fields in these air gaps. If the electric field becomes larger than a breakdown electric field of the air gap, partial discharge (PD) events may occur. Ionization of gases and electrical discharges due to PD events often lead to damage in the insulation material, leading to degradation in the performance and eventual failure of the electrical machine. Current solutions to the presence of PD in the insulation material for electrical machines are generally related to few choices of material that resists PD damage, such as mica-based insulation. In electrical machines with large sizes and highly complicated winding structure, insulation applied using a lapped-tape system may be more reliable and cost-effective in spite of the presence of air gaps and voids, and resulting PD activity. 
     BRIEF DESCRIPTION 
     Certain embodiments commensurate in scope with the originally claimed disclosure are summarized below. These embodiments are not intended to limit the scope of the claimed disclosure, but rather these embodiments are intended only to provide a brief summary of possible forms of the disclosure. Indeed, embodiments may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     Methods and systems described herein are related to electrical insulations that present a resistive grading network formed by placing a resistive material between insulation layers, and that may provide electrical stress grading. 
     In one embodiment an electrical machine having a stator is described. The electrical machine may have a plurality of insulating material layers that may cover the stator windings. The resistive material may be disposed between the layers of insulating material to form a resistive grading network. 
     In another embodiment, an electrical insulation is described. The electrical insulation may have a plurality of layers of an insulating film coated with a resistive material. The layers of insulating material form contact with neighboring layers via the resistive material coating. 
     A method is also described. This method may include processes to form a resistive material by embedding a filler in a coating binder. The fillers may have conductive particles and/or semi-conductive particles. The fillers may also be non-linear, i.e., the conductivity of the fillers may vary as a function of an applied electric field. The method may also include a process to form coat a polymer film tape with a resistive material to form an insulation tape. The method may also include a process for wrapping a conductor with the insulation tape such that there are at least two layers of insulation tape around the resistive material. 
     In one embodiment, an insulative assembly includes an insulative mica-based carrier film and first and second resistive grading layers joined to opposite sides of the mica-based carrier film. The first resistive material layer is configured to engage one or more conductors and insulate the one or more conductors from at least one other conductor. 
     In one embodiment, a method for creating an insulative assembly for one or more conductors includes obtaining an insulative mica-based carrier film, depositing a first resistive grading layer on a first side of the mica-based carrier film, and depositing a second resistive grading layer on an opposite, second first side of the mica-based carrier film. 
     In one embodiment, an insulative assembly includes a carrier film including a mica sheet, a first polymeric film on a first side of the mica sheet, and a second polymeric film on an opposite second side of the mica sheet. The assembly also includes a first resistive grading layer on a third side of the carrier film and a second resistive grading layer on an opposite fourth side of the carrier film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the inventive subject matter will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates an electrical motor that may employ stators having resistively graded insulation, in accordance with an embodiment; 
         FIG. 2  illustrates a section of a stator coil with conductive bars covered with resistively graded insulation, in accordance with an embodiment; 
         FIG. 3  illustrates a section of coated tape that may be used to provide resistively graded electrical insulation, in accordance with an embodiment; 
         FIG. 4  illustrates a process to apply the coated tape of  FIG. 3  to provide resistively graded insulation to a conductive bar, in accordance with an embodiment; 
         FIGS. 5A and 5B  illustrate resistively graded networks formed by the coated tape of  FIG. 3  around air gaps that may lead to resistively graded insulation, in accordance with an embodiment; 
         FIG. 6A  and  FIG. 6B  illustrate an electrical circuit that illustrates the resistively graded networks of  FIGS. 5A and 5B , in accordance with an embodiment; 
         FIG. 7A  and  FIG. 7B  illustrate the stress grading effect of the resistive networks around air gaps, in accordance with an embodiment; 
         FIGS. 8A and 8B  illustrate the effect of stress graded networks that may be formed by multiple layers of the coated tape of  FIG. 3 , in accordance with an embodiment; 
         FIG. 9  provides a chart that depicts the effect of the frequency of the voltages across the resistively graded insulation, in accordance with an embodiment; 
         FIG. 10  provides a chart that depicts the effect of the dimensions of air gaps in a resistively graded insulation, in accordance with an embodiment; 
         FIG. 11  provides a chart that illustrates a non-linear material that may be used to provide non-linear resistively graded networks for electrical stress graded insulation, in accordance with an embodiment; 
         FIGS. 12A and 12B  illustrate the effect of the usage of the non-linear material of  FIG. 11  in a resistively graded insulation system, in accordance with an embodiment; 
         FIG. 13  provides a chart for a performance of a resistively graded insulation system with linear and non-linear materials, in accordance with an embodiment; 
         FIG. 14  illustrates a method for producing electrical machines with resistively graded insulation, in accordance with an embodiment; 
         FIG. 15  illustrates a cross-sectional view of one example of an insulative assembly; 
         FIG. 16  illustrates a cross-sectional view of the insulative assembly disposed between conductors; and 
         FIG. 17  illustrates a flowchart of one embodiment of a method for creating an insulative assembly for one or more conductors. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The present description relates to insulation for conductors, such as stators of high voltage electrical machines. Not all embodiments of the inventive subject matter, however, are limited to insulation for stators. The insulation and insulative assemblies described herein can be used to provide insulation for other conductors or devices, such as one or multiple wires, cables, laminated bus bars, bus ducts, printed circuit boards (PCBs), etc. 
     Certain electrical machines such as converter-fed motors and/or generators perform the transformation between electrical and mechanical energy through an electromagnetic coupling between a stator and a rotor. For appropriate coupling, the stator may have conductor wires or bars that generate a magnetic field whenever a variable current is conducted through it. To prevent short-circuits between neighboring windings or to the stator core, the stator may be provided with insulation. Failure in the insulation of the stator is a cause of failure and damage to electrical machines. 
     A cause of failure in the insulation is the presence of partial discharges (PD) within the insulation. As discussed above, PDs may occur in air gaps that may be present within layers of the insulation material. If the electric field becomes larger than the breakdown voltage of the air gap, electrical discharges (e.g., sparks) may occur. Embodiments described herein are related to systems and methods that reduce the incidence of PDs from occurring within the insulation material. Materials and methods that provide a resistively graded insulation that reduces the electrical fields using stress graded networks may be described. 
     In some embodiments, the stress graded networks may be provided by a resistive material coating. The resistive materials may be conductive or semi-conductive materials, and may be non-metallic. The conductivity of the resistive material may, for example, be between the conductivity insulators and that of metallic conductors. In some embodiments, the material employed to produce stress graded networks may present a non-linear conductivity (i.e., not following Ohm&#39;s law). In some embodiments, placement of the resistively graded insulation may be facilitated by the fabrication of an insulation tape that may be used to apply insulation around the stators. In some embodiments, methods for production of the tape and for application of the insulating material to the stators are discussed. 
       FIG. 1  is a schematic diagram that illustrates an embodiment of a rotational system  10  (e.g., rotary machinery such as turbomachinery) with an electrical motor  12  coupled to one or more loads  14 . The electrical motor  12  may include, but is not limited to, a converter-fed motor. In the rotational system  10 , the electrical motor  12  provides a rotational output  16  to the one or more loads  14  via a shaft  18 . The electrical motor  12  receives power (e.g., electric power) from a power source  20  that may include, but is not limited to, a motor drive, an AC power line (e.g., three-phase, single-phase), a battery, or any combination thereof. The one or more loads  14  may include, but are not limited to, a vehicle or a stationary load. In some embodiments, the one or more loads  14  may include a propeller on an aircraft (or other high altitude vehicle), one or more wheels of a vehicle, a compressor, a pump, a fan, any suitable device capable of being powered by the rotational output of the electrical motor  12 , or any combination thereof. The shaft  18  rotates along an axis  22 . Note that, while the description generally discusses applications of the insulation system to electrical motors, the insulation system may also be applied to electrical generators, such as steam, gas, hydro, or wind turbines, tidal turbines, or other rotary electric generators. 
     The electrical motor  12  includes a stator  24  and a rotor  26 . The rotor  26  may be disposed within the stator  24 , offset by an airgap  28  between an interior surface  30  of the stator  24  and an exterior surface  32  of the rotor  26 . As may be appreciated, the interior surface  30  of the stator  24  may be cylindrical. Stator poles  34  receiving power from the power source  20  are configured to generate magnetic fields to drive the rotor  26  and shaft  18  about the axis  22 . The stator poles  34  may be powered so that the generated magnetic fields rotate about the axis  22 . In the illustrated figure, stator poles  34  are placed parallel to axis  22 , along the axial direction. Note that in some implementations, stator poles  34  may be placed along in a circumferential direction revolving the axis  22 . In some embodiments, the stator poles  34  are axially spaced along the stator  24 , opposite to laminations  36  of the rotor  26 . The stator poles  34  are circumferentially spaced about the rotor  2 . The power received from power source  20  may generate strong electric fields between wires in the coils of the stator poles  34 . To prevent short circuits between the stator poles  34  and the body of stator  24  due to insulations degradation for PDs, resistively graded insulation may be provided, as discussed below. The magnetic fields of the stator poles  34  induce magnetic fields in channels of the laminations  36  to drive the rotor  26  and the shaft  18  about the axis  22 . 
     A segment of stator  24  is illustrated in the view  150  of  FIG. 2 . View  150  shows a stator bar with four turns  152  surrounded by a groundwall insulation  153 . Each turn  152  has a conductor part  155  which is insulated from the conductor part of the other turns  152  by the turn insulation  154 . The turn insulation  154  prevents short-circuits between the turns  152  or between stator bars  152  and groundwall insulation  153 . The groundwall insulation  153  may present an electrical stress grading system provided by a resistive network within the insulation. The turn insulation  154  may also present such an electrical stress grading system. In some implementations, as detailed below, this resistively graded insulation  153  may be formed by multiple layers of an insulator coated with a resistive coating. The resistive coating may provide an electrical stress grading system and may decrease the electric fields in undesirable air gaps formed within insulation  153 , as discussed below. 
       FIG. 3  illustrates a section of a tape  180  that may be used to form a resistively graded insulation such as the main insulation or groundwall insulation  153 . Tape  180  may be formed by a core  182  composed of a polymer film that provides a dielectric barrier (i.e., a non-conductive film, an insulating film). The polymer employed may generally be materials that present a breakdown strength higher than 40 kV/mm, a dissipation factor smaller than 1%, and a dielectric constant smaller than 4.5, but other insulating materials may be applied. Examples of materials that may be used to form core  182  includes polyimide, polyether ketone (PEEK), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyether imide (PEI/Ultem), or any of the various fluorinated or perfluorinated materials, or polymeric films that embedded with inorganic insulating nanoparticles such as silica, alumina, silicates, or aluminum silicates that may have high dielectric strength and/or high temperature resistance. As detailed below, the thermosetting (e.g., heat-shrinking) polymer films capabilities may facilitate application of the insulation, which may reduce the presence of air gaps or other imperfections, and may lead to reduction in partial discharge (PD) events. Thermoplastic (e.g., flowable) materials that respond to an applied pressure may also facilitate application of the insulation and reduce further the presence of air gaps within the insulation. The core  182  of tape  180  may be covered with a resistive coating  184  in both sides of tape  180 . The resistive coating  184  may employ materials having conductivity ranging from 10 −8  S/m to 1 S/m. In some embodiments the resistive coating may employ non-linear materials, that is, materials that do not obey Ohm&#39;s law as discussed with respect to  FIG. 11 , and the conductivity could strongly depend on the electric field. The resistive coating may be produced by embedding fillers (e.g., conductive or semi-conductive particles) in a coating binder. Thermosetting and/or thermoplastic binders may be considered to facilitate the application of insulation tape  180 . For example, the binder material used in the resistive coating  184  may have a thermal response comparable to that of the insulation polymer, or it may have a glass transition temperature higher than or lower than that of the insulation polymer, based on an intended behavior during application. A bi-stage resin may be used. In this situation, the resin curing may be incomplete during winding, and may have a sufficiently large molar mass or crystallinity to provide tack-free property. The resin may be completely cured at the time of manufacturing by placing the insulated bar in an oven with appropriate temperature and duration. For some binder materials, a solvent or any other thinner may be employed to facilitate application of the coating. Besides thermoplastic and thermoset polymers, the binder could also be a grease. The filler material embedded in the binder material may satisfy a percolating threshold and may occupy a proportion 10-60% of the total volume of the coating material. Examples of conductive particles include metal particles such as copper, silver, iron, tin, gold or any electrically conductive alloys. Examples of semi-conductive particles include such as silicon carbide, a tin oxide, an antimony oxide, zinc oxide, and other particles may be used. After application, the resistive coating may have a thickness of 0.1-200 μm. 
     The illustration in  FIG. 4  shows an application of a roll  202  of a tape  180  around a bar consisting of multiple turns  152 . An insulation layer  153  may be formed by wrapping bar with tape  180 . The insulation layer  153  may be formed by multiple overlapping layers of the tape  180 , resulting in a stack of layers of tape. An increased number of layers of tape  180  may provide a better nominal range for voltage, as it increases the total breakdown voltage of the insulation. Since tape  180  is formed by an insulating core  182  covered with resistive coating  184 , the resulting insulation  153  may be a stack of insulating layers separated by the resistive coatings, as illustrated in  FIGS. 8A and 8B . 
     The process of wrapping tape  180  around bar may lead to imperfections within insulation layer  153 .  FIG. 5A  illustrates a type of imperfection  220  in which an air gap  222  may be formed between two adjacent layers of tape  180 . Air gap  222  may occur, for example, by an accidental slack in the tension on the tape during the application around bar  152 . Note that coating  184  around the core  182  encircles the entire air gap  222 .  FIG. 5B  illustrates a second type of imperfection  230  within the insulation layer  153 . In this imperfection  230 , an air gap  222  may appear in regions where a layer of tape  180  begins or ends. In these situations, air gap  222  may be formed due to natural limitations in the flexibility of tape  180 . In this example, coating  184  may coat many of the internal surfaces of air gap  222 , but in some portions, the tape  182  polymer may be exposed to the air gap  222 . Air gap  222  in  FIGS. 5A and 5B  may have a dimension ranging from 1 μm to 1 mm. 
     In both air gaps  222  of  FIGS. 5A and 5B , note that the coating  184  may provide a resistive network that may be similar to a Faraday cage around air gap  222 . As a result, coating  184  may provide electrical stress grading across the air gap by providing a resistive route with significantly lower impedance than that of the air gap, which may decrease the differences in the electrical potential between different points of the air gap. As a result, coating  184  may decrease ionization effects within air gap  222  and PD effects.  FIG. 6A  illustrates this effect with an electrical circuit diagram with an electrical model  250  for the resistive network around air gap  222 . In this model  250 , the insulation material  252  is coated with a resistive coating  254  surrounding an air gap  256 . The dielectric of insulation material  252  may be electrically modeled as capacitances  258  and the air gap may be modeled as a capacitance  260 . The resistive coating  254  forms the resistive network  254 . Note that the resistive network  262  is in a parallel circuit to capacitance  260  and, the higher the conductance in resistive network  262 , the lower the difference of potential between terminals  264  and  266 . It should be noted that the resistive coating may provide the above described properties even if the conductivity small. For proper function, the conductivity should be sufficiently high to ensure the electric field in the air gap is below the breakdown voltage. Note also that the lower conductivity may reduce resistive loss in the insulation. 
       FIG. 6B  illustrates an equivalent circuit  270  having a lumped components to model. Lumped capacitance  278  models or represents capacitances  258 , lumped capacitance  280  models or represents air gap capacitance  260 , and lumped resistance  282  models or represents the resistance network  262 . As a difference of potential between terminals  272  and  274  of the equivalent circuit  270  increases, the difference of potential across the capacitance  280  may also increase. This is represented by the difference of potential between terminals  276  and  274 . The presence of the lumped resistance  282  may reduce the voltage between terminals  276  and  274 , which reduces the voltage on capacitance  280 . This may correspond to a reduction in the electric field in air gap  256  due to the presence of the stress grading material  254 , which may decrease the occurrence of PD events. Note further that if the lumped resistance  282  is provided by a stress grading material that has non-linear properties such that the conductivity increases with the difference of potential, the lumped resistance  262  may effectively limit the difference of potential between terminals  274  and  276  while allowing some preventing loss of energy when the difference of potential between terminals  274  and  276  is low. 
     Heat maps  290  in  FIG. 7A and 296  in  FIG. 7B  illustrate the effect of the conductivity of the material employed in the stress graded network. In heat maps  290  and  296 , an insulation layer may have an insulation material  182 , a stress grading material  184  and an air gap  222 . Air gap  222  may be a disc-shaped void diameter of 0.2 mm and 0.02 mm thickness. The heat map  290  illustrates a value of the electric field  292  in each portion of the heat map as the insulation undergoes a 250 kHz high voltage that generates an average field of 10 kV/mm (RMS). The heat map  290  may correspond to a situation in which the conductivity of the stress grading material is of 10 −5  S/m. The resulting electrical fields within the airgap may be as large as 30 kV/mm, which may cause PD events. Heat map  296  may correspond to a situation in which the stress grading material  184  may have a higher conductivity of 10 −2  S/m. In this situation, the electric field inside the air gap  222  is much smaller, in the order of 5 kV/mm. This reduction in the electric field in the air gap  222  from the increase in the conductivity in the stress grading material  184  may be associated with the decrease in the potential difference across different points of the stress grading material  184 . 
       FIG. 8A  shows the large electric fields that may occur when there is no resistive coating employed as a stress grading material in insulation  352 . In this example, the tape thickness is 0.1 mm, and the maximum width of the void is 0.2 mm. Insulation  352  may be formed by several tape layers of an insulation material  182 . Insulation  352  may also have air gaps  222  that may be formed due to imperfections during application of the insulation material  182 . Note that in insulation  352 , the layers of insulation material are not coated with a stress grading material, and therefore the conductivity between layers of insulation material  182  may be very poor. As a result, when an electrical potential difference is applied between the top 354 and the bottom  356  of the insulation layer, very little electrical stress grading occurs across the insulation  352 . As discussed above, this may lead to the very large electric fields  292  in air gaps  222  (˜20 kV/mm in this example), leading to PD processes if this electric field becomes higher than the breakdown electric field. 
       FIG. 8B  shows how the inclusion of a resistive coating may provide better stress grading across an insulation  362  and prevent PDs. In insulation  362 , the layers of insulation material  182  may be coated with a conductive stress grading material  184 . The insulation film thickness is 0.1 mm, and there is a 5 μm thick resistive coating on the each surface of the insulating tape. Insulation  362  may also have air gaps  222  similar to the ones of insulation  352 . However, due to the improved stress grading network from stress grading material  184 , when an electric potential difference is applied between the top 354 and the bottom  356  of the insulation layer, the electric field  292  observed in the air gaps are smaller (˜8 kV/mm in this example) and may be below the breakdown voltage of air gap  222 . As a result, application of a stress grading material  184  to tapes of the insulation material  182 , and subsequent use of the coated tapes to provide insulation for stators may substantially reduce PDs phenomenon in stator insulation. 
     Chart  300  in  FIG. 9  illustrates how changes in the operating frequency of the machine employing the stators with resistively graded insulation affects the behavior described herein. The chart shows a ratio  302  between the electrical field in a 0.2 mm diameter air gap and the breakdown electrical field as function of the conductivity  304  of the stress grading material. When the ratio  302  goes above a threshold  306 , PDs may occur. Threshold  306  may be associated to the ratio  302  being equal to 1 as a ratio above the threshold indicates that the electric field in the air gap is higher than the breakdown electric field. In these conditions, the air gap dielectric ionizes and allows a discharge, which may lead to PD. Ratio  302  is shown for signals having a low frequency  310  (2 kHz), a medium frequency  312  (20 kHz), and a high frequency  314  (250 kHz). Note that for the frequencies tested, there may be a minimum conductivity  304  (e.g., point  320  for low frequency  310 , point  322  for medium frequency  312 , and point  324  for high frequency  314 ) that makes the ratio  302  to become smaller than threshold  306 . Note further that as the frequency increases, the minimum conductivity that prevents PD also increases, as illustrated by the conductivity  304  of points  320 ,  322 , and  324 . 
     The impact of high frequency signals may be particularly important in applications having a pulse-width modulation (PWM) controlled drive. In such systems, the electrical signal may be a low frequency signal being carried in a square wave carrier signal. In such applications, even if the nominal operation frequency of the machine may be low, the PWM carrier signal contains high frequency components that may be applied to the stator coils. Accordingly, in such applications, the resistively graded insulation may be subject to frequencies that are much higher than the nominal operation frequency of the machine. 
     Chart  330  in  FIG. 10  illustrates how dimensions in the air gap may affect PD phenomenon. In this example, spherical voids with different diameters are considered. The chart shows how a ratio  302  between the electrical field and the breakdown electrical field varies as a function of the conductivity  304  of the stress grading material in an insulation under a 250 kHz electric field. Ratio  302  is shown for small voids  342 , medium voids  344 , and large voids  346 . As in chart  300  in  FIG. 8 , there may be a minimum conductivity  304  (e.g., point  342  for small voids  332 , point  344  for medium voids  334 , and point  336  for large voids  346 ) at which ratio  302  is smaller than threshold  306 . Note further that, as the voids dimensions increase, the minimum conductivity that prevents PD also increases, as can be seen by the conductivity  304  of points  342 ,  344 , and  346 . Charts  300  and  330  shows that, for a linear stress grading material (i.e., a material that obeys Ohm&#39;s law), increased signal frequency and increased dimensions in the voids may require a more conductive coating to prevent PDs. 
     The results above in  FIGS. 9 and 10  are associated with linear stress grading material. However, coatings that may have non-linear resistive behavior may also be employed. Chart  400  in  FIG. 11  illustrates the electrical behavior of one such material. Specifically, chart  400  shows that for such material, as the electric field  402  increases, the conductivity  404  may increase as well (curve  406 ). If such material is disposed between layers of insulation, as illustrated above, the resulting resistive network may show low conductivity in regions where the electric field is low and a high conductivity in regions where the electric field is large, such as around air gaps. The low conductivity in regions with smaller electric fields may increase the efficiency of the electrical machine as it decreases the currents in the resistive network. The above-discussed effect of employing a non-linear stress grading material  184  with the behavior of curve  406  as coating for insulation layers  182  in an insulation  420  is illustrated in  FIG. 12A . As discussed above, an electric field may become very large around the air gaps  222 . As a result, the conductivity  422  of the stress grading material  184  around air gaps  222  may become very large, such as in sections  422  of the stress grading material  184 . By contrast, sections  424  of the stress grading material that are not near the air gaps  222  may have a low conductivity, which may decrease the electrical currents. 
     Chart  430  in  FIG. 12B  shows the transient response in an insulation layer. Specifically, the chart shows how the ratio  432  between the electric field in air gap  222  and the breakdown electric field varies as a function of time  434 , and how the conductivity  435  of the non-linear stress grading material  184  varies as a function of time  434 . The transient is measured with respect to a voltage  436  that increases from 0 to a voltage V MAX  with a 1 μs rise time. Note that the electric field  438  in the air gap increases to a peak  439  below a ratio  432  of  1 , and stabilizes in a lower value. The transient observed in electric field  438  may be a consequence of the change in the conductivity of the non-linear material  440  observed. The conductivity of the non-linear material  440  may reach a peak  441  before stabilizing in a lower conductivity. Note that the conductivity of the non-linear material  440  may reach ˜10 −3  S/m to prevent PD during the transient, before settling at a much lower ˜10 −5  S/m. This adjustment behavior of the conductivity of the non-linear material  440  allows large currents to alleviate high electric fields and prevent PDs without allowing large currents when the electric fields are low. As a result, the effective DF of the insulation is low, improving the losses and self-heating compared to the use of a linear material. 
     The tradeoff between PD and DF discussed above in linear stress grading materials, and the improvement provided by the use of non-linear stress grading materials is illustrated in chart  450  of  FIG. 13 . This chart provides the ratio  452  between the electric field in air gap  222  and the breakdown electric field as a function of the DF  454 . Dissipation factor (e.g., DF  454 ) is defined as the ratio between the energy loss and the total energy in the insulation system. The DF provides a measure of energy losses (e.g., thermal losses) that may occur in an insulation of an electrical machine under an oscillating signal. Quadrant  456  illustrates a region of chart  450  with an insulation with low losses and high PD incidence (e.g., ratio  452  may be higher than 1 and DF  454  is low). Quadrant  458  illustrates a region of chart  450  with an insulation with high losses and high PD incidence (e.g., ration  452  may be higher than 1 and DF  454  is high). Quadrant  460  illustrates a region of chart  450  of an insulation with low PD incidence, but high losses (e.g., ratio  452  may be lower than 1 and DF  454  is high). Quadrant  462  illustrates a region of chart  450  of an insulation with little PD, low losses (e.g., ratio  452  may be lower than 1 and DF  454  is low). Generally, an insulation layer in quadrant  462  may provide a better tradeoff, as discussed above. Region  463  of the chart illustrate how linear stress-grading material with different conductivities may behave. At low conductivity, linear stress-grading material provides insulation in quadrant  456 . As the conductivity increases, stress-grading materials show less PD incidence, but the DF increases as a tradeoff (quadrant  460 ). The use of a non-linear stress grading material  464  may lead to an insulation in quadrant  462 . Accordingly, the use of a non-linear stress grading materials may be lead to an insulation with little losses and little partial discharges, which may result in a highly efficient and highly reliable electrical machine. 
     Flow chart  500  of  FIG. 14  illustrates a method to provide an electrical machine with an insulation that may have stress grading materials over the layers. A process  502  may include forming a conductive coating by embedding filler particles in a coating binder. A solvent or a thinner may be added to the coating binder and this binder may be applied to a tape produced from an insulation material. Process  502  may have a step to coat both surfaces of the tape with the conductive coating. As discussed above, the resistive material may be a linear conductive material or non-linear conductive material. In some implementations, the tape may be coated through a gravure coating and/or a roll coating process. In other implementations, the coating may be applied to a long sheet of the insulation material, which may be later cut and rolled into produced the tape. The coating may also be an inorganic thin film applied using vacuum deposition techniques, such as sputtering, evaporation, or chemical vapor deposition. The insulation material may be a polymer film, as discussed below. In some implementations, the resistive material may be deposited on the surface of the polymer film, or it may be added to the polymer film during its formation. 
     The tape having the coated resistive material may be then applied to a conductive bar that may be used in a stator (process  504 ). During the application, the tape may be wrapped such that multiple layers of the tape overlap, resulting in a stator bar covered by an insulation having multiple tape layers. The interface between the multiple tape layers may be formed by the resistive coating. Wrapping may be performed employing automated processes for composite tapes. The coatings in the multiple insulation layers may create a stress grading network which may decrease the incidence of PD effects, as discussed above. PDs may be further reduced by employing a process that prevents the formation of air gaps or pockets where PD occurs during process  504 . For example, if the tape is formed from a thermosetting, thermoplastic, or heat-shrinking materials, process  504  may include adjustments to the temperature and/or pressure during and after the application of the tape to decrease the dimensions of potential air gaps. The stator bars may be then placed adjacent to other stator bars to form stator coils and/or wires (process  506 ). The stator may be coupled to a rotor and a casing to assemble a motor and/or a generator (process  508 ). 
       FIG. 15  illustrates a cross-sectional view of one example of an insulative assembly  1500 . The insulative assembly  1500  can represent one embodiment of the tape  180  shown in  FIG. 3 . Alternatively, the insulative assembly  1500  can represent another insulator, such as an insulative sheet. The insulative assembly  1500  includes a carrier film  1501  having stress grading layers  1508  on opposite sides of the carrier film  1501 . The insulative assembly  1500  can represent the tape  180  shown in  FIG. 3 . The carrier film  1501  can represent one embodiment of the core  182  shown in  FIG. 3 , and the stress grading layers  1508  can represent the resistive coatings  184  shown in  FIG. 3 . 
     The carrier film  1501  is formed from a sheet  1502  of insulative material. In one embodiment, the sheet  1502  is formed from mica. The mica can be impregnated with polymer resin. Optionally, the sheet  1502  can be formed from mica impregnated with nanoparticles of polymer resin. As described above, the polymer resin can include one or more of polyimide, polyether ketone (PEEK), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyether imide (PEI/Ultem), or any of the various fluorinated or perfluorinated materials, or polymeric films that embedded with inorganic insulating nanoparticles such as silica, alumina, silicates, or aluminum silicates. 
     Polymeric films  1504  are disposed on opposite sides of the insulative (e.g., mica) sheet  1502 . These polymeric films  1504  may sandwich the mica sheet  1502  to form the carrier film  1501 . The resistive grading layers  1508  are on opposite sides of the carrier film  1501 . Examples of materials that may be used for the resin in the sheet  1502 , in the polymeric films  1504 , and/or the resistive grading layers  1508  include polyimide, polyether ketone (PEEK), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyether imide (PEI/Ultem), or the like. 
     The polymeric films  1504  may be much thinner than the grading layers  1508 . For example, each of the polymeric films  1504  may be no thicker than 0.5 mils while each of the stress grading layers  1508  can be at least twice as thick as one of the polymeric films  1504 . The resistive grading layers  1508  are electrically resistive (but not insulative) layers. Each of these layers  1508  can represent the resistive coating  184  described above. Each of the layers  1508  can be formed from a polymer resin composite that is embedded with conductive, semiconductive, and/or non-linear resistive particles, as described herein. The film  1501  may be insulative and not conduct current, while the grading layers  1508  may be resistive to conduction of current, but not electric insulators. 
     The insulative assembly  1500  can be wrapped around one or more conductors as a wrap (e.g., like the tape  180 ), can be placed as a single layer on a conductor and/or between two or more conductors, can be folded back-and-forth between two or more conductors, or the like. The insulative assembly  1500  can be wrapped around elongated conductive bodies such as cables or wires, or can be placed onto or between conductive bodies, such as on or between conductive layers of a bus bar, bus duct, PCB, etc. 
       FIG. 16  illustrates a cross-sectional view of the insulative assembly  1500  disposed between conductors  1702 ,  1704 . In one embodiment, the conductors  1702 ,  1704  can be parallel conductive layers in a laminated bus bar or PCB. Alternatively, the conductors  1702 ,  1704  can be another type of conductive body, such as conductive plates. The insulative assembly  1500  is folded back-and-forth onto itself between the conductors  1702 ,  1704 , as shown in the lower half of  FIG. 16 . 
     A void or air gap  222  can form or be present between portions of the insulative assembly  1500  that fold back on itself. The grading layers  1508  can provide a resistive network like a Faraday cage around the gap  222 , as described above. As a result, the grading layers  1508  may provide electrical stress grading across the gap  222  by providing a resistive route with significantly lower impedance than that of the air gap  222 , which may decrease the differences in the electrical potential between different points of the gap  222 . The resistive layers  1508  may decrease ionization effects within the gap  222  and PD effects. 
     Alternatively, the insulative assembly  1500  may not be folded back-and-forth onto itself between the conductors  1702 ,  1704 . For example, the insulative assembly  1500  may be provided as a single, planar layer without having any folds that cause one portion of the insulative assembly  1500  being located between another portion of the insulative assembly  1500  and the conductor  1702  or the conductor  1704 . Optionally, multiple, single layers of the insulative assembly  1500  may each be provided as a single, planar layer between the conductors  1702 ,  1704  without one or more (or any) of the layers of the insulative assembly  1500  being folded back on itself or folded around a layer of another insulative assembly  1500 . For example, a first insulative assembly  1500  can be disposed on the conductor  1702 , a second insulative assembly  1500  (having separate mica layers  1502  and separate polymeric layers  1504 ) can be disposed on the first insulative assembly  1500 , and so on. 
     Optionally, instead of wrapping the tape  180  around a conductor, the tape  180  also can be folded back-and-forth on itself as shown with the insulative assembly  1500  in  FIG. 16 . Alternatively, the tape  180  can be placed as one or more planar layers between conductors  1702 ,  1704  without folding the tape  180 . 
       FIG. 17  illustrates a flowchart of one embodiment of a method  1800  for creating an insulative assembly for one or more conductors. The method  1800  can be used to manufacture the insulative assembly  1500 . At  1802 , a mica sheet is obtained. At  1804 , a polymeric film is deposited on a first side of the mica sheet. For example, one of the polymeric films  1504  can be deposited onto one side of the mica sheet  1502 . At  1806 , another polymeric film is deposited on an opposite, second side of the mica sheet. This forms the carrier film  1501  described above. 
     At  1808 , a resistive grading layer is deposited on one side of the carrier film. For example, one of the polymer grading layers  1508  can be deposited on one side of the carrier film  1501 . At  1810 , a resistive grading layer is deposited on the opposite side of the carrier film. For example, another of the polymer grading layers  1508  can be deposited on the opposite side of the carrier film  1501 . 
     The systems and methods described herein may allow for electrical insulation methods and systems that may improve the reliability and efficiency of electrical machinery. The insulation systems may be applied to stators in high-voltage, high-frequency generators and/or motors, and may present a smaller incidence of PD events and formation of ionized free radicals in the air gaps. This reduction in PDs may be achieved through by the presence of electrical stress grading network around air gaps and/or other imperfections in the insulation. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.