Abstract:
The present disclosure relates to electrical machines. The teachings thereof may be embodied in a corona shielding system for an electrical machine. For example, a corona shielding system may include: an outer corona shield and an overhang corona shielding. The outer corona shielding and the overhang corona shielding may comprise a first corona shielding material having a nonlinear field strength-dependent electrical resistance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2015/067785 filed Aug. 3, 2015, which designates the United States of America, and claims priority to DE Application No. 10 2014 219 439.3 filed Sep. 25, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to electrical machines. The teachings thereof may be embodied in a corona shielding system for an electrical machine. 
       BACKGROUND 
       [0003]    Electrical machine may include high-voltage machines, e.g., a generator for generating electrical energy and/or an electric motor and/or another electrical operating means having a relatively high rated voltage, such as, in particular, a transformer, a bushing, or a cable. These machines of ever higher power are being developed, since advancing technology is requiring ever higher power densities. 
         [0004]    A high-performance generator, such as a turbo generator, has a stator with a (stator) laminated core and a plurality of generator slots in which the generator winding is located. The main insulation of the generator winding against the laminated core is under high electrical load. High voltages arise during operation and must be dissipated in the insulation volume between the conductor bar under high voltage and the laminated core which is at ground potential. This leads to excessive increases in the field at the edges of the laminations in the laminated core, and these in turn cause partial discharges. The partial discharges, when they meet the insulation system, lead to very intense local heating. This gradually breaks down the organic materials of the insulation system, including the materials of the outer corona shielding system, into volatile products of low molecular weight, for example into carbon dioxide. 
         [0005]    An important constituent part of the insulation system is what is called the outer corona shielding. In relatively large generators and electric motors, it is applied directly to the surface of the generator winding insulation. Typically, the outer corona shielding comprises carbon black- and graphite-containing tapes or coatings. The outer corona shielding must have a certain square resistance which lies within a specific range. If it is too low, the laminated cores can be electrically short-circuited, which can lead to high induced circulating currents, occurring across the ends of the laminated core and the outer corona shielding and lead to high-current arcs. In the case of excessive resistance, by contrast, high-voltage spark erosion can occur. 
       SUMMARY 
       [0006]    Capacitive/resistive field control may reduce the increase in field strength in the region of the overhang of the corona shielding. Capacitive control is implemented by the insulating materials of the insulation system, while resistive control is implemented by the overhang corona shielding. The overhang corona shielding is conductive surface coverings at the outlets of the conductor bars from the laminated core. The surface coverings typically have a square electrical resistance of approximately 10 8  to 10 10  Ohm. A more homogeneous electrical field distribution is achieved by means of the overhang corona shielding. A severe nonlinearity of the resistance of the materials used in the overhang corona shielding is used in an attempt to force the electrical field out of the regions of high field strengths. This nonlinearity leads to a reduction in the specific resistance as the electrical field strength increases. Surface coverings of this kind can be realized either as coats with drying and curable resins, which are applied directly onto the insulating material surface, or together with tapes. 
         [0007]    The teachings of the present disclosure may provide an improved corona shielding system for an electrical machine. In particular, they may enable manufacture of the corona shielding system in a particularly simple manner. In addition, overhang corona shielding and outer corona shielding may be particularly well electrically matched to one another in an expedient manner. In particular, the corona shielding system may have a long service life. 
         [0008]    Some embodiments may include a corona shielding system for an electrical machine comprising an outer corona shielding and an overhang corona shielding, in which both the outer corona shielding and the overhang corona shielding are formed with the same corona shielding material which has a nonlinear field strength-dependent electrical resistance. 
         [0009]    In some embodiments, the outer corona shielding, at least partly, and the overhang corona shielding, at least partly, are formed from the same corona shielding material. 
         [0010]    In some embodiments, at least a portion of the outer corona shielding and at least a portion of the overhang corona shielding are designed such that they can be handled in one piece. 
         [0011]    In some embodiments, the corona shielding material has an electrical resistance which becomes lower the greater an electrical operating field of the electrical machine in which the corona shielding material, at least during operation, is located. 
         [0012]    In some embodiments, the corona shielding material has a matrix and a filler which is arranged within the matrix. 
         [0013]    In some embodiments, the matrix is a polymeric matrix. 
         [0014]    In some embodiments, the filler comprises globular and/or planar particles. 
         [0015]    In some embodiments, the filler is formed with mica. 
         [0016]    In some embodiments, the matrix has glass fibers. 
         [0017]    In some embodiments, at least a portion of the corona shielding material forms a tape and/or a coating. 
         [0018]    Some embodiments may include an electrical machine comprising a corona shielding system as described above. 
         [0019]    In some embodiments, the electrical machine comprises a high-voltage machine. 
         [0020]    In some embodiments, the corona shielding material, at least partly, is arranged on a main insulation of the electrical machine. 
         [0021]    Some embodiments comprise at least one conductor bar and a corona shielding system in which the corona shielding material, at least partly, is applied to the or at least one of the conductor bars and/or the electrical insulation thereof or is arranged on the or at least on one of the conductor bars and/or the electrical insulation thereof. 
     
    
     DETAILED DESCRIPTION 
       [0022]    The teachings of the present disclosure may be embodied in a corona shielding system for an electrical machine, e.g., a generator for generating electrical energy and/or an electric motor and/or another electrical operating means having a relatively high rated voltage, such as, in particular, a transformer, a bushing, or a cable. In some embodiments, the corona shielding system includes an outer corona shielding and an overhang corona shielding. In some embodiments, both the outer corona shielding and the overhang corona shielding are formed with the same corona shielding material which has a nonlinear field strength-dependent electrical resistance. 
         [0023]    In some embodiments, the corona shielding material has a nonlinear field strength-dependent electrical resistance. The material may include a partial discharge-resistant overhang corona shielding tape, as is disclosed in laid-open specification DE 10 2010 009462 A1. The overhang corona shielding tape may be applied to a main insulation of a stator winding of the electrical machine, wound around the main insulation, over the entire original length of the outer corona shielding and overhang corona shielding. Therefore, the overhang corona shielding tape may have a nonlinear electrical resistance characteristic to allow efficient potential control at the end of the outer corona shielding. 
         [0024]    In some embodiments, the electrical resistance may be in a working range of from 0.1 to 0.3 kV/mm is 10 4  to 10 7  Ωm. There is a pronounced nonlinear field strength-dependent electrical resistance of the overhang corona shielding tape, so the tape can also be used in the slot region of the stator winding. In the region of the laminated core and/or an external outer corona shielding conductive nonwoven, the radial field strength over the main insulation of the stator winding may be up to approximately 3 kV/mm, depending on a position of the stator winding within the laminated core. 
         [0025]    In such embodiments, the overhang corona shielding tape may be very conductive and function as an additional outer corona shielding layer at high operating field strengths and therefore close to the phase connections. At low operating field strengths, e.g., at a distance from the laminated core, the overhang corona shielding tape has a very high resistance and acts as an additional insulating layer. The overhang corona shielding material may be formed as an overhang corona shielding coating instead of an overhang corona shielding tape. 
         [0026]    The corona shielding system may therefore be formed in a particularly simple manner. In some embodiments, both the outer corona shielding and also the overhang corona shielding can be formed by means of the same corona shielding material. The corona shielding system may be of particularly simple construction. In addition, particularly close electrical coupling between the overhang corona shielding and the outer corona shielding is realized by means of forming both the outer corona shielding and also the overhang corona shielding with the same corona shielding material. An outer corona shielding conductive nonwoven layer is preferably formed together with the overhang corona shielding. 
         [0027]    In some embodiments, the outer corona shielding, at least partly, and the overhang corona shielding, at least partly, are formed from the same corona shielding material. Particularly simple manufacture and particularly close electrical coupling of the outer corona shielding and the overhang corona shielding are ensured specifically with this development. In some embodiments, at least a portion of the outer corona shielding and at least a portion of the overhang corona shielding are formed with an overhang corona shielding tape and/or with an overhang corona shielding coating as described above. 
         [0028]    In some embodiments, at least a portion of the outer corona shielding and at least a portion of the overhang corona shielding can be handled in one piece. Manufacture and handling during the manufacturing process may be made easier with the design of at least a portion of the outer corona shielding and a portion of the overhang corona shielding as overhang corona shielding tape. 
         [0029]    In some embodiments, the corona shielding material has an electrical resistance which becomes lower the greater an electrical operating field of the electrical machine in which the corona shielding material, in particular during operation, is located. 
         [0030]    A particularly strong electrical operating field prevails specifically in the region of the outer corona shielding during operation. In the case of a severe nonlinearity of the corona shielding material, the electrical resistance of the corona shielding material in the region of the outer corona shielding is sufficiently low to prevent or to reduce potential peaks along the length of a conductor bar of the stator winding. In the region of the ends of a conductor bar of this kind however, the electrical resistance of the corona shielding material is sufficiently high, so that the corona shielding material forms an additional insulating layer which effectively reduces or prevents potential peaks in this region. 
         [0031]    In some embodiments, the corona shielding material includes a matrix and a filler arranged within the matrix. The matrix may include a polymeric matrix, e.g., a plastic matrix and/or resin. The polymeric matrix may include a thermoplastic and/or a thermoset and/or an elastomer. 
         [0032]    In some embodiments, the filler is formed by particles with dimensions of, at least on (in particular arithmetic) average, at most one millimeter and preferably of, at least on (in particular arithmetic) average, at most 100 micrometers. The filler expediently comprises partial discharge-resistant and electrically conductive particles. In a suitable manner, the particles comprise a partial discharge-resistant core and a partial discharge-resistant coating which is conductive. 
         [0033]    In some embodiments, the filler, in a suitable manner, comprises globular and/or planar particles. Globular particles refer to those particles in which the dimensions in different spatial directions differ by less than a factor of at most 3, e.g., at most 1.5. Planar particles refer to those particles of which the dimensions in at least one spatial direction differ by a factor of at least 3, e.g., at least 5, from the dimensions in a direction, and/or two directions, perpendicular thereto. 
         [0034]    In some embodiments, the filler comprises mica. In some embodiments, the matrix comprises glass fibers. In some embodiments, at least a portion of the corona shielding material forms a tape and/or a coating in the corona shielding system. 
         [0035]    In some embodiments, an electrical machine includes a corona shielding system as described above. In some embodiments, the electrical machine comprises a high-voltage machine, e.g., a generator for generating electrical energy and/or an electric motor and/or another electrical operating means having a relatively high rated voltage, such as, in particular, a transformer, a bushing, or a cable. 
         [0036]    In some embodiments, the corona shielding material is arranged, at least partly, on a main insulation of the electrical machine. The main insulation may be arranged on a conductor bar and/or on a stator winding of the electrical machine. In some embodiments, the electrical machine includes at least one conductor bar and a corona shielding system as described above, wherein the corona shielding material of the corona shielding system, at least partly, is applied to the or at least one of the conductor bars and/or the electrical insulation thereof or is arranged on the winding bar or at least on at least one of the conductor bars and/or the electrical insulation thereof.