Patent Publication Number: US-9406433-B2

Title: Sound mitigation for air core reactors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/CA2013/050529 filed Jul. 9, 2013 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. This application claims benefit of the 9 Jul. 2012 filing date of U.S. provisional patent application No. 61/669,317. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to dry type air core reactors of the type used in utility and power applications and, more particularly, to mitigating audible sound produced by such reactors. 
     BACKGROUND OF THE INVENTION 
     Air core reactors are inductive devices used in high voltage power transmission, distribution and industrial applications. Air core reactors are used for a variety of purposes, including for filtering out harmonics, as shunt devices which compensate for introduction of capacitive reactive power, and as devices which limit short circuit currents. Air core reactors are formed with a series of concentrically positioned, spaced-apart winding layers, referred to as packages, in a cylindrical configuration. The packages are positioned between upper and lower current carrying members, sometimes referred to as spider units or spiders. The spider units include a plurality of arms radiating along a plane and away from a central hub position in a star configuration. Among other functions, the spider units may serve as terminals for connecting power lines and for interconnecting the winding layers in an electrically parallel configuration. The reactors are normally installed with the spider units in a horizontal orientation with respect to an underlying horizontal ground plane so that the major axis of the cylindrical configuration extends vertically upward from the ground plane. For a single reactor, or for the lower-most reactor in a stacked configuration of two or more reactors, the winding layers are supported above the ground by the lower spider unit and a series of insulators and structural leg members which extend from the lower spider unit to the ground. 
     Because air core reactors carry alternating current through coiled electrical conductors, they are subjected to mechanical forces created by the interaction of the electrical current and the magnetic field generated by the reactor.  FIG. 1  illustrates the general direction of net forces exerted on a typical air core reactor  1  during power operation. Due to the design of the device, the frequency of the force is twice the frequency of the current, and the magnitude of the force is proportional to the square of the current. Due to the relative flexibility of the cylindrically shaped packages in the horizontal direction, the side surfaces of the reactor tend to vibrate in response to the forces, thereby creating audible sound. 
     Air core reactors are often located in populous areas, and the sound created by the reactors can be a serious irritant to the local population. It is known to locate air core reactors behind walls to isolate them, or to construct a separate sound shield around each reactor to surround it with sound absorbing material. The cost of such structures is high, and they may inhibit physical access to the reactor for maintenance activities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic illustration of the forces generated during operation of an air core reactor which result in the generation of audible sound. 
         FIG. 2  is a partial cut-away view of a prior art air core reactor. 
         FIG. 3  is a partial sectional view of an air core reactor wherein current in some windings is purposefully configured to reduce overall sound level. 
         FIG. 4  is a partial sectional view of an air core reactor wherein a force generated in one package is out of phase with a force generated in another package. 
         FIG. 5  is a schematic winding design illustrating the interconnection of various packages of an air core reactor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While traditional solutions to mitigating the adverse effects of audible sound created by an air core reactor have focused on isolating the reactor from neighboring populations, the present inventors have innovatively looked to the source of the sound itself in order to reduce the amount of sound energy produced by the operation of the reactor. The inventors have recognized that prior art air core reactors have been designed on the basis of their electrical performance, but they are not optimized for their acoustic performance. The inventors have developed improvements in the design of such reactors in order to accomplish a reduction in the sound levels emanating from the device during power operation, thereby reducing or eliminating the need for supplemental sound barriers. 
       FIG. 2  is a partial cut-away view of a prior art air core reactor  10  illustrating components typical of air core reactors. The reactor  10  includes a series of cylindrically shaped, spaced-apart packages  12  concentrically positioned about a central hub axis of symmetry A. The packages  12  typically have a thickness range, as measured in the radial direction, on the order of 0.5 to 3 cm and may vary in thickness. The cylindrical shape and relative thin cross section of the packages renders them susceptible to vibration and sound production caused by forces exerted on the packages in the radial direction. Although various reactor designs may include fewer or substantially more packages than shown in  FIG. 2  (e.g., ranging from fewer than three packages to twenty or more packages), for simplicity of illustration,  FIG. 2  illustrates only three such packages. The reactor  10  includes an innermost package  12   a , an intermediate package  12   b  and an outermost package  12   c . The reactor  10  is shown in a common orientation, positioned above a horizontal ground plane, G, with the hub axis A extending vertically above the ground plane. The reactor  10  includes a hollow reactor cavity  13  extending radially inward from the innermost package  12   a  toward the axis A. The cavity  13  and packages  12  are positioned between an upper spider unit  14  and a lower spider unit  16 . The spider units have horizontal orientations with respect to the underlying ground plane G. 
     The packages  12   a ,  12   b ,  12   c  are separated from one another by a plurality of spacers  18  which are shown to have a vertical orientation extending in a direction parallel to the axis A. The spacers  18  are circumferentially spaced apart about the axis A to provide air gaps  20  between adjacent pairs of the packages  12   a ,  12   b ,  12   c . In other embodiments (not illustrated), the reactor may include an outer sheathing or a rigid support structure to which an electrostatic shield may be attached. (See United States Patent Application Publication 2011/0043320 A1 which is incorporated herein by reference.) For such embodiments, further spacers may be provided to facilitate separation of the sheathing or the rigid support structure from the outermost package to also provide an air gap there between. Each air gap  20  permits a current of air to flow upward along a surface of the respective package. 
     The spider units  14 ,  16  each include a series of arms  24 ,  25  radiating along a plane and away from the axis A in a star configuration. The upper and lower spider units  14 ,  16  illustrated each have four spider arms  24 ,  25 , although the number of arms in the spider units may range from fewer than four to more than twelve. Among other functions, the spider arms  24 ,  25  serve as line terminals (not illustrated) for effecting power connections to and between the packages  12  in an electrically parallel configuration. The spider arms  24 ,  25  extend outward from a central hub  26 ,  27 . Each package  12  may include a plurality of layers of spirally wound electrical conductors (not illustrated), with each conductor connected between an arm  24  of the upper spider  14  and an arm  25  of the lower spider  16 . For a single reactor, or for at least the lower-most reactor  10  when arranged in a stacked configuration of reactors, the packages  12  are supported above the ground by a combination of spider arms  25  of the lower unit  16  and a series of structural leg members  34 . In the illustrated embodiment, all four of the arms  24  of the lower spider unit  16  are supported directly by leg members  34  which each extend from a spider arm  24  to the ground. The leg members  34  of the reactor  10  each include an electrical insulator  32 . The number of structural leg members supporting the reactor  10  can vary from fewer than four to more than twelve. 
     Prior art air core reactors are designed to minimize electrical losses while staying within component manufacturing cost constraints, which in turn minimizes the amount of heat generated and the expense of operation of the device. For example, prior art reactors are configured such that the current passing through each package and each winding of each package is generally in-phase (i.e. ±10 degrees) with the current in other packages and windings at the grid power frequency. The inventors have discovered that such designs can be improved to reduce the amount of audible sound produced by the device while still satisfying overall design specification requirements for the device, including having an acceptable level of loss. Sound is produced by force acting on structures of the reactor resulting in those structures deflecting and creating sound waves in the surrounding air. The inventors have reduced sound by designing a change in a phase relationship of the force generated in at least one winding or package when compared to the phase of the force generated in other windings or packages. In embodiments of the invention, a current in at least one package is greater than 10 degrees out of phase with a current in other packages, or at least 20 degrees out of phase, or at least 45 degrees out of phase, or at least 90 degrees out of phase, or up to 180 degrees out of phase at the power frequency (typically 60 Hz or 50 Hz depending upon the country of use) or at all frequencies of 1 kHz or less. Note that audible sound issues are generally not of concern at current, frequencies higher than 1 kHz. By changing the current phase angles, the phase angles of resulting forces are changed accordingly such that the forces generated in the one package are a corresponding number of degrees out of phase with the forces in the other packages. The result is lower net forces acting on potential significant sound radiating surfaces and lower vibration and sound levels. 
     The out of phase current may be carried by all conductive winding layers in a particular package, or by only some of the conductive winding layers in a package.  FIG. 3  is a partial cross-sectional illustration of an exemplary air core reactor  36  showing portions of two packages  38 ,  40  each having four conductor winding layers  38   a ,  38   b ,  38   c ,  38   d  and  40   a ,  40   b ,  40   c ,  40   c . In this example, the reactor  36  is configured such that two outermost winding layers  38   c ,  38   d  of one package  38  conduct current that is out of phase with the current conducted in the two innermost winding layers  38   a ,  38   b  of that same package  38 , as well as being out of phase with the current conducted in the adjacent package  40 . In this embodiment, the out of phase current is indicated as being opposite (180 degrees out of phase, marked as + and −), but is at least 10, 20, 45, 90 or 180 degrees out of phase at the power frequency or at all frequencies of 1 kHz or less in accordance with other embodiments of the invention. Further embodiments may conduct the out of phase current in at least one to as many as all of the winding layers of a package, or in more than one package. Such designs can be achieved using known design tools and techniques commonly available to one skilled in the art of air core reactor design. 
     It may be appreciated that the magnitude of the current carried by each conductor winding of a reactor is a design variable controlled by the designer. As such, it may be desired that the conductor winding carrying the out of phase current carry a lower current magnitude than other conductors in order to limit stresses imposed on the structure and to control losses during power operation. Thus, it will also be appreciated that the present invention has introduced sound production as a design element that can be balanced against other known design requirements, such as electrical losses, manufacturing and material cost, heat generation and dissipation, dimensions, mechanical stresses, etc. Moreover, in consideration of the mechanical response of the structure and the varying sensitivity of human ears over the frequency spectrum, embodiments of the invention may selectively mitigate sound production in particular frequency ranges, perhaps even at the cost of increasing sound production in other frequency ranges. 
     The desired out of phase current/force may be accomplished by changing a direction of a spiral in the conductor windings to be opposite (e.g. counter clockwise) from a direction of a spiral of the other conductor windings (e.g. clockwise) in an embodiment. It may be particularly advantageous for the out of phase current to occur in the outermost and/or innermost package. In one embodiment, at least one conductor winding carries no current. 
     The magnetic field generated during operation of an air core reactor is a function of the current distribution within the reactor. The inventors have realized that it is possible to control the current distribution to advantageously shape the magnetic field such that it flows in one direction through the bulk of the reactor but in an opposite direction through the outermost package(s). In this manner, a sound generating force is produced in the outermost package that is out of phase with the sound generating force produced in the bulk of the reactor by currents that are flowing in the same direction through all of the packages. When the force induced in one winding or package is out of phase (including being in an opposite direction) to that induced in another winding or package, a sound canceling effect is achieved. This concept is illustrated in  FIG. 4  where lines of total coil magnetic flux  42  are shown turning in space and passing in opposite directions through an outmost package  44  and a more inward package  45  of an air core reactor  47 . The current direction in all windings of this embodiment is in the same direction (+), but because the magnetic field is reversed between the packages, the resulting driving force is in opposite (F + , F − ) directions. Accordingly, in embodiments of the invention, an air core reactor may be configured such that an interaction of a current in an outermost package with a magnetic field generated by the reactor during power operation generates a force that is at least 10, 20, 45, 90 or 180 degrees out of phase with a force generated in a more inward package at the power frequency or at all frequencies of 1 kHz or less, effective to at least partially mitigate sound produced by power operation of the reactor. 
       FIG. 5  is a schematic illustration of design features within a package and/or between packages that may be used to generate an out of phase current or force in a winding or package. Such design features include but are not necessarily limited to:
         disconnect one or more conductor windings (layers)  49  of a package  50  from at least one of the spiders  46 ,  48 ;   increase the number of turns in one  52  or more conductor windings of one  54  or more packages compared to the other packages;   include one  56  or more packages or one or more conductor windings tuned to a relatively lower current than in other packages/windings. This may be accomplished, for example, by constructing the conductor winding(s) to have more turns, or to be formed from a smaller gauge wire (represented in  FIG. 5  by resistor  57 );   change the conductor material in one or more packages, such as using copper conductor material in the outermost package  58  and aluminum conductor material in the other packages;   include one or more dummy packages (i.e. not connected electrically) including an outmost dummy package  58 ;   construct the spiral shape of the conductor in one or more layers of one  60  or more packages in the opposite direction (i.e. clockwise  62  verses counterclockwise  64  relative to the hub axis);   direct the current through the conductor in one or more layers of one  66  or more packages in the opposite direction relative to the top  46  and bottom  48  spiders. This is illustrated schematically by connecting the top of the conductor  68  of a package  66  to a bottom spider  48  and the bottom  70  of that conductor to the top spider  46  using jumper connectors  72 ,  74 .       

     Advantageously, some of the modifications discussed above also function to increase the mass or stiffness of at least some of the packages, for example the innermost and/or outermost packages. For any given driving force, a higher mass and/or a higher stiffness will result in a lower total displacement of the packages. Since the sound energy of concern is generated by the radial movement of the sides of the packages, particularly the innermost and outermost packages, any modification that increases the mass or the stiffness of a package will affect the level of sound produced. An increase in mass will generally lower the natural frequency of the package, while an increase in stiffness will generally raise the natural frequency of the package. Thus, these variables can be used by the designer to tailor the mechanical response of the reactor and to thereby affect its sound production. 
     As illustrated in  FIG. 5 , a package  58  may be closely coupled mechanically to the adjacent package  54  with any known mechanical joining device  76  (clamp, frame, bolt, adhesive, etc.) such that it reduces the extent of excursion of the outer surface of the packages and thus decreases the amount, of sound produced. The inventors have also reduced the sound by the selective addition of non-conductive material to at least one package in order to change the deflection of the package in response to the net driving force. The material may simply increase the weight and thereby lower its natural frequency, such as by simply adding layers of epoxy to make the package thicker. Alternatively, the material may be configured to increase the stiffness of the package, such as by forming the material in the shape of a rib  80  attached to an outer surface of the package. 
     While various embodiments of the present invention have been shown and described herein, it will be apparent that such embodiments are provided by way of example only. Numerous other variations, changes and substitutions may be made without departing from the invention concepts disclosed herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the claims which now follow.