Patent Publication Number: US-2015069289-A1

Title: Electrically insulating material, particularly for high voltage generator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of the U.S. patent application Ser. No. 13/485,106 filed May 31, 2012, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate generally to the field of electrically insulating materials, particularly of the dielectric type. Embodiments of the present invention may be applied in particular to electrically insulating materials for high voltage generators, used for example in the medical imaging field. 
     2. Description of the Related Art 
     Numerous electrically insulating materials have been developed, particularly to ensure the insulation of high voltage generators, supplying for example, X-ray tubes used in medical imaging. 
     A known polymer based dielectric material in use today is polypropylene combined with talc. These types of insulators can withstand a very strong electric field. For example, they can be subjected to voltages of the order of 80 kV/cm. 
     However, above a certain voltage value (known as “start” voltage), partial discharges begin through the material and the polymer degrades irreversibly, as does its insulation properties. 
     Because the currents dig into the material, certain molecules of the polymer undergo a rearrangement of the polymer molecules, which progressively degrades the insulation capacities thereof, up to its breakdown. 
     However, increases in power are required to increase the frequency at which this type of device may take images. This requires increasing the voltage delivered by the high voltage generators, while minimizing the weight of the generators because they are typically mounted on the scanner. 
     In addition, smaller and smaller dimensions are required for high voltage generators, in order to increase the rate at which images may be taken with the imaging devices on which they are mounted. This requires that the voltage delivered by the generators be increased, while minimizing the weight of the generators, because they are mounted on scanners. 
     Consequently, there exists a need to develop novel electrically insulating materials, capable of withstanding ever stronger electrical fields without overloading the generators on which they are mounted. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the invention, an electrically insulating material for high voltage generators is provided. The electrically insulating material comprises a polymer based dielectric material filled with nanoparticles, wherein the voltage at which partial discharges start in the polymer based dielectric material is greater than the voltage at which partial discharges start in an unfilled polymer based dielectric material. According to another embodiment of the invention a high voltage generator is provided. The high voltage generator comprises an electrically insulating material, the electrically insulating material comprising a polymer based dielectric material filled with nanoparticles, wherein the voltage at which partial discharges start in the polymer based dielectric material is greater than the voltage at which partial discharges start in an unfilled polymer based dielectric material. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other characteristics, aims and advantages of embodiments of the present invention will become more clear from reading the detailed description that follows, with reference to the appended drawings, given by way of non-limiting examples, and among which: 
         FIG. 1  schematically represents a material according to an embodiment of the invention; 
         FIG. 2  illustrates a test assembly to determine the “start” voltage or the life of a given material according to an embodiment of the present invention; and 
         FIG. 3  schematically represents a sectional view of a high voltage generator, the insulation of which is provided by a material according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , an electrically insulating material  10  is shown that may be used, for example, in high voltage generators. This insulating material  10  is a polymer based dielectric  11 , for example, based on polypropylene or instead polyethylene. However, other resins may be envisaged. 
     In addition, the electrically polymer  10  material is filled with electrically conducting nanoparticles  12 . Carbon nanotubes, which are very good electrical conductors, may be utilized and possibly supplemented with other nanoparticles such as, for example, nano-oxides of titanium or nanoparticles of boron nitrite. 
     In the case of electric insulation applications of high voltage generators, the dielectric permittivity of the filled polymer is chosen as close as possible to that of the insulating oil used (dielectric permittivity may be between about 2.2 and about 3.0, preferentially between about 2.3 and about 2.8). 
     The nanoparticles  12  added to the material, which have good thermal properties, also allow it to be a better thermal conductor than a polymer based material alone. For example, for an insulating material  10  comprising less than 1% by weight of carbon nanotubes, the thermal conductivity of the insulating material  10  is greater than or equal to about 0.6 W·m −1 ·K −1 . In comparison, an insulating material based on polymer alone has a thermal conductivity of the order of about 0.2 W·m −1 ·K −1 . The addition of nanoparticles of boron nitrite, for example, enables the thermal conductivity of the material to be further increased. 
     As mentioned above, an unfilled polymer based material undergoes aging when it is subjected to an electric field exceeding a threshold voltage, known as “start” voltage. Voltages above the “start” voltage cause partial electrical discharges to occur in the material, which leads to irremediable breakdown of the polymer. This phenomenon is linked to a rearrangement of the molecules of the polymer  11  under the constraints of the electric field. 
     However, the insulating material  10  containing electrically conducting nanoparticles  12  allows partial discharge currents to propagate through the insulating material  10  when it is subjected to strong electrical fields, which reduces the stresses applied to the molecules of the polymer  11 . 
     This results in several interesting properties of the nanoparticle  12  containing insulating material  10 . 
     Firstly, the nanoparticles  12  “shunt” the electrical insulation and the insulating material  10  is in fact less insulating than an unfilled polymer based material. For example, for a proportion of nanoparticles  12  less than 1% by weight of the total insulating material  10 , the insulation resistance of the filled material  10  is two orders of magnitude less than the insulation resistance of the unfilled polymer. However, the insulation resistance of the filled material  10  remains sufficient to ensure the insulation of high voltages. As a result the filled insulating material  10  may still be used for the insulation of high voltages, for example, in high voltage generators. 
     On the other hand, the “shunt” that the nanoparticles  12  allow, particularly those based on carbon, make it possible for the first partial discharges to take place at voltages that are higher for the insulating material  10  filled with nanoparticles  12  than for the same unfilled polymer based material. The discharge currents permitted by the nanoparticles  12  increase the resistance of the polymer  11  to strong electric fields. In this case, utilizing the test that is described below with reference to  FIG. 2 , the “start” voltage at which the first discharges were detected was of the order of about 30 kV or greater for a material filled with carbon nanoparticles (proportion by weight of 1% or less). In comparison, the “start” voltage was of the order of about 27 kV for the same unfilled material. Moreover the filled material has a dielectric strength 30% higher than that of the same unfilled material. Furthermore, the life before breakdown, after the first discharges started, was considerably increased for nanoparticle filled materials. 
     The tests for determining the “start” voltage at which the discharges start, as well as for determining the life of the polymer once the “start” voltage is reached, for example, are carried out by utilizing an oil insulation tester, such as the “BAUR Prüf- and Messtechnik GmbH” by the Baur Company. The assembly is shown in  FIG. 2 . A sample EM of the material to be tested is placed between two metal electrodes E facing the tester. The sample EM is a square plate with width of about 2 mm and sides of several centimetres. The electrodes E are spherical electrodes with a diameter of about 12 mm at the end of two arms B that plunge into a glass tank C filled with insulating oil. The two electrodes E and the sample of material EM are thus immersed in the insulating oil, and supplied with high voltage (50 Hz true) via the two arms B. 
     A current transformer T (sensitivity 100 mV/A), which is mounted on one of the arms, is equipped with threshold detection means and returns an input signal to an oscilloscope O. The test for determining the “start” voltage consists of increasing the voltage applied to the electrodes with a gradient of about 0.5 Kv/s. The “start” voltage is the point at which a current is detected for the first time above an intensity threshold given by the oscilloscope O (first significant discharges). In a first experiment, the voltage at the terminals of the electrodes E rises with the same gradient of about 0.5 kV/s to determine the voltage, known as the “stop” voltage, at which breakdown occurs. In another experiment, the supply voltage is set at the “start” voltage and the life between the start of the first discharges and the definitive breakdown of the sample is determined. 
       FIG. 3  illustrates a high voltage generator  1  in which the insulating material  10  described above is used. The generator  1  comprises one or more transformer(s)  20  placed in an oil bath  30 , the whole assembly being enclosed in one or more housing(s) made of insulating material  10  that form for example an insulating sock. 
     The insulating material  10  of the casing(s) is of the type described above. For example, an insulating material  10  based on polypropylene or polyethylene, or even other resins, filled with less than 1% by weight of nanoparticles  12 , for example carbon nanotubes, and having a “start” voltage at which the first electrical discharges appear in the material greater than the same unfilled material. Such a material makes it possible to increase the image taking frequency while minimizing the weight of the generator and maintaining efficient insulation. Furthermore, the cooling of the generator  1  is more efficient with this insulating material  10 , which further extends its lifetime. The more efficient cooling and insulation provided by the insulating material  10  make it possible to reduce the insulation volume in the generator  1 , thereby reducing the weight of the generator  1 . 
     For medical type applications such as, for example, the use of high voltage generators for X-ray tubes mounted on scanners, the ability to reduce the weight of the generator makes it possible to accelerate the rate of rotation of the scanners, and therefore, to reduce the examination time for a patient. 
     Of course, the insulation material  10  and the high voltage generator  1  described above are not restricted to medical applications.