Abstract:
The disclosure relates to a tape with nonlinear field control properties containing ZnO microvaristor particles. The doped ZnO particles are produced by crushing a sintered ZnO block, by desagglomeration or crushing of calcinated granulated particles, or by crushing of a calcined or sintered tape (tape casting). Embodiments, among other things, relate to: hollow ZnO microvaristor particles produced by granulation technique, having reduced average density and having diameters in a range well below 90 μm; and compounding the Zno filler in binders that are used to impregnate tapes. Compared to nonlinear field control tapes with conventional embedded nonlinear filler particles, a stronger and more reliable nonlinear resistivity is achieved and the ZnO filler is simpler to produce and to compound in the binder. The resulting tapes are flexible, preferably self-adhesive and have a strong nonlinear electrical resistivity. The tapes are useful to protect high field-stress regions in electrical components.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to European Patent Application No. 05405395.4 filed in the European Patent Office on 21 Jun. 2005, and as a continuation application under 35 U.S.C. §120 to PCT/CH2006/000315 filed as an International Application on 12 Jun. 2006 designating the U.S., the entire contents of which are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates to the field of high and medium voltage technology and, in particular, to nonlinear electrical materials and devices. The disclosure is based on nonlinear electrical tapes and on electric apparatuses comprising such nonlinear electrical tapes. 
       BACKGROUND INFORMATION 
       [0003]    The disclosure starts from the prior art as described in EP 1 118 086 B1. Therein, a glow protection band or corona shielding band with improved electrical properties is disclosed, that is used in the winding insulation of an electrical machine. The band comprises a fabric-like carrier material that is impregnated with a solution of a reaction resin containing filler particles that are coated with antimony-doped tin oxide (SnO 2 ). Optionally a hardening agent and/or an accelerator can be added. The coated filler particles provide a more reliable and better reproducible nonlinear electrical behaviour compared to the conventionally used silicium carbide (SiC) filler. The coated filler system is specifically designed to be relatively light-weight to avoid particle sedimentation during compounding while providing sufficient nonlinearity in electrical resistivity. For this purpose, inorganic substrate filler materials of low density are chosen, such as Al 2 O 3 , SiO 2 , TiO 2 , BaSO 4 , chalk, talcum, kaolin, mica or titanates, and electrical nonlinearity is added by coating the substrate filler particles with the much heavier doped tin oxide coating. The coating is brought onto the substrate- or seed-particles by means of hydrolyse or thermal decomposition of organic slurry. Obviously, such a complex coated filler system is difficult to manufacture. 
         [0004]    In DE 2 233 204 a glow protection tape of semi-conductive material is disclosed for use in an insulated high-voltage winding of an electric machine. The tape may consist of a woven glass impregnated with graphite particles. The tape can be pre-stretched, heat-treated and is shrinkable in longitudinal direction. The tape is wound around a glass-mica band protecting a copper conductor winding. It is designed to replace a carrier band that is surface-coated with a glow-protection lacquer. 
         [0005]    In U.S. Pat. No. 4,297,250 a method of making a doped zinc oxide (ZnO) varistor powder composition is disclosed. The ZnO powder can be dispersed in a resinous medium to provide a ZnO stress grading varnish composition. The varnish may be applied as paint to coils or other high voltage insulated conductors. The ZnO powder can also be fired onto a ceramic article such as a bushing. 
         [0006]    In the article by R. Strümpler, P. Kluge-Weiss and F. Greuter, “Smart Varistor Composites”, Proceedings of the 8 th  CIMTEC-World Ceramic Congress and Forum on New Materials, Symposium VI (Florence, Jun. 29-Jul. 4, 1994) a composite comprising doped ZnO microvaristor powder embedded in a polymer matrix is disclosed. 
         [0007]    In EP 0 875 087 B1, as well, an electrical stress-controlling composition is prepared from doped zinc oxide (ZnO) filler particles embedded in a polymer matrix. The composition is specified such that only round or smoothly shaped spheroid particles are used and that a majority of the particles has a maximum dimension of between 5 μm and 100 μm. The compound can be processed to form various stress-controlling products, e.g. field grading sleeves in medium voltage cable terminations and joints. 
       SUMMARY 
       [0008]    A nonlinear electrical tape showing favourable nonlinear electrical properties and being easy to manufacture is disclosed, an electrical component comprising such a nonlinear electrical tape, and a medium or high voltage apparatus comprising such a component. 
         [0009]    In a first aspect the disclosure consists in a tape with nonlinear electrical properties, for electrical devices, in particular for electrical high-voltage apparatuses, comprising a substrate that is impregnated with a binder containing inorganic filler particles, wherein the filler particles comprise microvaristor particles containing doped zinc oxide (ZnO). The nonlinear electrical tape based on ZnO microvaristors has several advantages over known field control tapes. Compared to conventional tapes with embedded SiC particles, a stronger and more reliable nonlinear resistivity is achieved. Compared to tapes containing doped-SnO coated particles, ZnO microvaristors are much simpler to produce, have a markedly reduced density and hence improved processability, and have a clearly more pronounced electrical nonlinearity. 
         [0010]    An exemplary embodiment can show properties and applications of the field grading tape. In other exemplary embodiments, favourable design parameters of the ZnO microvaristor particles are identified. These encompass measures to obtain light-weight particles and specific choices of particle size distributions and particle shapes for optimising nonlinear field grading tapes based on doped ZnO microvaristors. 
         [0011]    In another aspect the disclosure consists in any electrical component or device making use of the above specified nonlinear electrical tape for dielectric insulation, overvoltage protection and/or field control purposes. 
         [0012]    Further embodiments, advantages and applications of the disclosure will become apparent from claims or claim combinations and when consideration is given to the following detailed description. 
     
    
     DETAILED DESCRIPTION 
       [0013]    Field control is needed for coils of motor and generator, cables, bushings and whenever an insulated conductor passes through a grounded concentric screen. For such and other purposes, the disclosure provides a nonlinear electrical tape for electrical devices, in particular for electrical high-voltage apparatuses, which tape comprises a substrate that is impregnated with a binder, e.g. an epoxy resin, containing inorganic filler particles, wherein the filler particles comprise microvaristor particles containing doped zinc oxide (ZnO). 
         [0014]    The tape containing ZnO microvaristors according to disclosure compares favourably in many respects with conventional tapes based on embedded SiC particles or doped-SnO coated particles. The nonlinear properties of the SiC tapes depend on contact phenomena between SiC—SiC particles and are thus strongly dependent on SiC grade, on tape processing and handling conditions, and on tape degradation under voltage overload. In contrast, the nonlinearity of the doped ZnO is an effect produced by the built-in grain boundaries, is hence an intrinsic property and is therefore robust against processing conditions and aging effects. ZnO filler particles are less abrasive than silicium carbide filler particles. Furthermore, ZnO based materials have higher nonlinearity coefficients α&gt;10 than SiC based materials having a in the range of 3 . . . 5. This means that dielectric losses can be reduced at normal operating conditions without losing overvoltage protection at impulse, or, alternatively, dielectric losses can be kept constant at normal operating conditions and impulse withstand capability, in particular impulse withstand voltages, can be increased. 
         [0015]    Compared to tapes containing doped-SnO coated particles, doped ZnO microvaristors are obviously simpler to produce, in that standard processes of doping and sintering can be applied and tedious processing steps, such as coating of seed particles with doped and sintered varistor material, are avoided. Furthermore, ZnO microvaristors have a lower material density than SnO 2 , namely 5.6 g/cm 3  instead of 7.0 g/cm 3 . In addition, ZnO microvaristor particles can be produced to be more or less porous and, in particular, can be in the shape of hollow spheres such that their apparent or average density, defined as the weight of the particles divided by their enclosed volume, may substantially be further reduced. This results in reduced danger of sedimentation and demixing during compounding filler and binder and during impregnating the tape with the compounded binder. Finally, ZnO microvaristors have a superior electrical nonlinearity. Reported nonlinearity coefficient α range from 3 to 34 for SnO 2 , compared to 50 to 200 for ZnO. Overall, doped ZnO microvaristor tapes are simpler to produce and show more favourable electrical properties than hitherto known nonlinear field control tapes. 
         [0016]    In the following, exemplary embodiments of the disclosure are discussed. 
         [0017]    The tape can be a flexible tape, e.g., with at least one surface being self-adhesive, for applying the tape on electrical components. The tape can be applied in field-stress regions of electrical components and provides a nonlinear electrical field control by means of its embedded doped ZnO microvaristor particles. 
         [0018]    The doped Zno microvaristor particles are at least partially hollow particles with an average density below the specific material density of Zno, which is 5.6 g/cm 3 . The doped ZnO microvaristor particles can be hollow particles that are produced by granulation techniques, e.g. spray-drying with or without blowing agent, sol-gel processing, spray pyrolysis, coating of preprocessed polymer, or fluidised bed process technique. Production of such granulated hollow ZnO microvaristor particles is disclosed e.g. in the aforementioned article by R. Strümpler et al., the content of which is in its entirety herewith enclosed in this application. According to this article, particles produced by spray-drying have sizes up to 300 μm and are sieved to particle sizes up to 200 μm. 
         [0019]    In contrast, in the disclosure, particularly small-sized hollow ZnO microvaristors shall be produced and used such that the smoothness and flexibility of the tape is not compromised. Advantageously, the doped ZnO microvaristor particles have a particle size distribution with maximum dimensions smaller than 90 μm, preferred smaller than 80 μm, more preferred smaller than 70 μm, further more preferred smaller than 60 μm, most preferred smaller than 50 μm. In particular, the doped ZnO microvaristor particles have a particle size distribution with maximum dimensions smaller than 40 μm, preferred smaller than 30 μm, more preferred smaller than 20 μm. 
         [0020]    According to the disclosure, the production parameters of the granulation, e.g. rotary disk atomisation, shall be adjusted to such an extent that small hollow particles, preferentially with maximum dimensions below 40 μm, are generated. It is further anticipated that the production yield will decrease for very small diameters in the range of 30 μm to 10 μm or below. Thus, taking into account a decreasing powder production yield and an improved tape smoothness and flexibility with decreasing particle size, an optimum ZnO microvaristor particle size distribution for nonlinear field grading tapes shall be in the range of 10 μm to 50 μm, preferred 20 μm to 40 μm, particularly at 30 μm. 
         [0021]    With advantage, in the binder, at least 70%, preferred at least 80%, more preferred at least 90% of the inorganic filler particles are doped ZnO microvaristor particles. The doped ZnO microvaristor particles have a Gaussian or bimodal particle size distribution. Such Gaussian and bimodal particle size distributions are disclosed in EP 0 992 042 (WO 99/56290), the content of which is in its entirety herewith enclosed in this application. In EP 0 992 042 it is also disclosed that the filler can comprise electrically conductive particles fused to the surface of the microvaristor particles, wherein the electrically conductive particles form direct electrical low resistance contacts between the microvaristor particles. Furthermore, two filler components with different nonlinear current-voltage characteristic may be used, as disclosed in EP 1 274 102 A1, the content of which is in its entirety herewith enclosed in this application. In the present disclosure, at least one of the filler components must be doped ZnO microvaristors and preferably all filler components shall be based on ZnO particles with different dopings. In contrast to EP 1 274 102 A1, however, doped SnO 2  shall not be added as a filler component because of its too large density causing sedimentation and demixing during compounding. 
         [0022]    It is possible to specify the Zno powder for compounding in the binder such that it comprises at least two ZnO particle fractions. A first fraction of the doped ZnO microvaristor particles can have smooth, preferably spherical shapes and the particles for the first fraction have been calcinated and subsequently separated, in particular the necks are broken up, such that the particles retain their original, predominantly spherical shape. A second fraction of the doped ZnO microvaristor particles can have irregular, in particular spiky shapes and the particles for the second fraction have been produced by calcinating or sintering and subsequently fracturing or produced in a way such that the particles have irregular, in particular spiky, shapes. Such multi-fractional ZnO microvaristor particle compositions are disclosed in the not yet published European Patent Application No. 04405210.8, the content of which is in its entirety herewith enclosed in this application. 
         [0023]    The production method of the ZnO filler is explained in more detail in this application. The granular powder is typically produced by spray drying a slurry comprising ZnO and doping additives. This production step brings about solid or hollow granules or particles with predominantly spherical shape. The green granules are heat treated to obtain microvaristor granules with nonlinear electrical properties. The term calcination refers to a heat treatment of a bed of loose granules which are more or less baked together, typically by forming necks bridging the particles. Alternatively the granules can be calcinated in a rotary kiln, which reduces the neck formation. The calcinated agglomerate, and in particular the necks, can be broken-up by using little force only. This preserves the original spherical shape of the particles. In contrast, the term sintering relates to a compacted and heat treated fully densified ceramic block. This block must be crushed to get fractured irregular particles. The irregular particles can also be obtained from a calcinated powder by feeding the spherical microvaristor powder particles for example through a double-disc mill with a slit smaller than the smallest dimension of the intact particles. The preferred sizes of the particles are selected typically by classification techniques, e.g. sieving or air separation. The process of mixing the microvaristor powder into the binder is called compounding. Solvent and/or hardening agent and/or accelerator can be added to the binder. 
         [0024]    In summary, it is possible to produce ZnO microvaristor powder by granulation methods and in particular spray drying, as discussed above, subsequent calcinating and then desagglomeration or crushing. It is also possible to produce the particles by crushing sintered ZnO varistor ceramic blocks or, alternatively, by crushing of a calcined or sintered tape (tape casting). The particles shall be full spheres or preferentially hollow spheres, may comprise a mix of spherical and spiky particles or of particles with different nonlinear current-voltage characteristics and have a granulometry from approximately 10 μm to below 90 μm. 
         [0025]    With respect to the tape, the substrate can be in the form of a sheet and preferably a band. The substrate shall be flexible and can be made in the form of a film, a perforated film, a woven fabric or a fleece. Typically, the substrate is chosen to be electrically insulating. It can contain glass and/or polymer, preferably polyester. In an exemplary embodiment, the substrate is made of a polymer and is heat-shrinkable. 
         [0026]    The binder can be among the group of epoxies and silicones. It can be a thermoplastic or a duromer. The binder in the tape can be in a pre-cured B-stage or in a fully-cured C-stage. The binder being in B-stage may add stickiness to the tape. The binder being in C-stage facilitates handling of the tape for further processing. In a further step the tape is applied to an electrical component. This component is further treated, e.g. by further impregnation and heat treatment. Examples are resin rich processing and vacuum pressure impregnation. If the tape has binder in B-stage, a precuring step is needed in order to prevent loosing microvaristor particles during the further impregnation of the electrical component. 
         [0027]    The disclosure relates also to an electrical component or device, in particular a medium- or high-voltage component or device, such as an electric insulation device, electrical overvoltage device or electrical field control device, that comprises a nonlinear electrical tape, in particular a field control tape, as disclosed above. Examples of such electrical components are a conductor bar, e.g. motor bar or generator bar, or a cable termination, machine insulation, transformer insulation, support insulator, bushing or a field control means. Examples of such electrical devices are medium- or high-voltage apparatuses, e.g. a disconnector, circuit breaker, transformer, capacitor, inductor, instrument transformer, cable, or electrical machine. Such component or device according to the disclosure contains a nonlinear field control tape that comprises a substrate that is impregnated with a binder containing inorganic filler particles that comprise doped zinc oxide (ZnO) microvaristor particles. 
         [0028]    In an exemplary embodiment, the tape is a flexible tape with at least one surface being self-adhesive for applying the tape in a field-stress region of the electrical component or device and there performs a nonlinear electrical field control by means of its embedded doped ZnO microvaristor particles; and/or the doped ZnO microvaristor particles are hollow particles having a particle size distribution with maximum dimensions smaller than 70 μm, preferred smaller than 50 μm, more preferred smaller than 30 μm. 
         [0029]    It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.