Abstract:
A high-precision Rogowski current transformer, the Rogowski coil is realized in a single printed circuit board while maintaining both the outside field rejection of a traditional Rogowski coil, and the increased output voltage similar to the multiple printed circuit board Rogowski coil arrangements.

Description:
FIELD OF THE INVENTION 
     This invention relates to a Rogowski current transformer. More particularly, the invention is directed to a Rogowski current transformer realized on a printed circuit board. 
     BACKGROUND SUMMARY 
     In order to measure the AC current passing through a solid bus bar, it is desirable to have a Rogowski-type coil on a printed circuit board that is rigid and may be mounted on the bus bar. 
     Current efforts to realize a Rogowski coil on a printed circuit board employ multiple coils on multiple printed circuit boards. The coils can be arranged to cancel the effects of outside magnetic fields when the coils are wired in series in electrically opposite directions. This method is similar to a traditional Rogowski coil where passing a single Rogowski coil&#39;s conductor back through the coil&#39;s helical windings provides outside field rejection and cancellation. 
     In such Rogowski coils the use of multiple printed circuit boards effectively increases the turns-count of the Rogowski coil, providing the benefit of increased output voltage. However, the use of multiple printed circuit boards also increases the overall cost and complexity. Furthermore, the interconnects required to link the multiple coil printed circuit boards introduce additional failure points in the Rogowski current transducer system reducing reliability. 
     It would be desirable to have a Rogowski coil realized in a single printed circuit board, that provided both the outside field rejection of a traditional Rogowski coil, and the increased output voltage similar to the multiple printed circuit board Rogowski coil arrangements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a Rogowski coil realized in a single printed circuit board having both the increased output voltage of the multiple printed circuit board Rogowski coil arrangements, and outside field rejection of a traditional and multiple printed circuit board Rogowski coil arrangements has surprisingly been discovered. 
     In one embodiment, the apparatus for measuring current in a conductor has a printed circuit board having a plurality of discrete substrate layers, an outer coil wound around at least one of the substrate layers, an inner coil wound around at least one of the substrate layers and an aperture in said printed circuit board. 
     In another embodiment, the apparatus for measuring current in a conductor has a printed circuit board having a plurality of discrete substrate layers, a first coil wound around at least one of the discrete substrate layers, and a conductor traversing the center of the first coil. 
     In another embodiment, the apparatus for measuring current in a conductor has a printed circuit board having four discrete substrate layers, a first surface layer, a first inner layer, a second inner layer, and a second surface layer, an outer coil wound on the first surface layer and the second surface layer, and traversing the first surface layer, the first inner layer, the second inner layer, and the second surface layer and an inner coil wound on the first inner layer and the second inner layer, and traversing the first inner layer and a second inner layer. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective illustration of a portion of the Rogowski Coil printed circuit board according to an embodiment of the invention; 
         FIG. 2  is a plan view of a high performance Rogowski current transformer incorporating the circuit board of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a portion of the high-performance Rogowski current transformer of  FIG. 2 ; and 
         FIG. 4  is a perspective illustration of a basic Rogowski coil commonly known in the prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
     Referring now to the drawings, and particularly  FIG. 1 , there is shown generally at  2  a portion of a multi-layer printed circuit board. In this embodiment, the printed circuit board  2  includes a first surface layer  12 , a second surface layer  14 , a first inner layer  16 , and a second inner layer  18  stacked together. In the embodiment shown, the layers  12 , 14 , 16 , 18  of the printed circuit board  2  are constructed from an insulative circuit board substrate  10  adapted such that conductive traces may be deposited or etched using known photoresitive processes. 
     An outer coil  20  is wound in a first direction by disposing conductive traces on outer surfaces of the first surface layer  12  and the second surface layer  14 . The traces are connected in a helical manner using a series of conductive plated outer coil via holes  25  that traverse the first surface layer  12 , the second surface layer  14 , the first inner layer  16 , and the second inner layer  18  of the printed circuit board  2 . 
     An inner coil  30  is wound in a second direction by disposing conductive traces on surfaces of the first inner layer  16  and the second inner layer  18  facing the layers  12  and  14  respectively. The inner coil  30  traces are connected in a helical manner using a series of conductive plated inner coil via holes  35  that traverse the first inner layer  16  and the second inner layer  18 . The inner coil  30  windings in the second direction are electrically opposite to that of the first direction of the outer coil  20  windings. Additionally, the inner coil  30  is constructed with a higher-turn density than the outer coil  20  in order to compensate for the size difference between the outer coil  20  and inner coil  30 . The outer coil  20  and the inner coil  30  are square-shaped in cross section. It is understood that other windings can be used as desired. 
     An aperture  40  is milled in the printed circuit board  2  as illustrated in  FIG. 2 . The aperture  40  is adapted to receive a solid conductive bus bar. The outer coil  20  and the inner coil  30  extend about the periphery of the aperture  40 . Alternatively, the aperture  40  may be adapted to receive one or more various types of electrical conductors without departing from the scope of this invention. 
     In  FIG. 3 , an integrator circuit  50  is linked to an output  48 . The output  48  includes an electrical connection  46  to the outer coil  20 , and an electrical connection  44  to the inner coil  44 . 
     In operation, the prior art Rogowski coil  100  is placed around one or more electrical conductors  115  whose instantaneous current i(t) is to be measured, as illustrated in  FIG. 4 . The instantaneous current i(t) for alternating current waveforms is given by I max  sin(ωt) where I max  is the amplitude of the current. The voltage v(t) induced in the Rogowski coil  100  is defined by the first equation: 
                     v   ⁡     (   t   )       =       -     ⅆ     ⅆ   t         ⁢     (       ∑     j   =   1     N     ⁢     ϕ   j       )               1   )               
where φ j  is the instantaneous flux for the j-th turn of the total N turns. If the core has a constant cross section S, the wire is wound perpendicularly to a line  120  that is centered in the cross section S, and the wire is wound with a constant density equal to n, where the mutual reactance M is defined by M=u o ηS, then equation 1 may be written as:
 
                     v   ⁡     (   t   )       =       -     μ   o       ⁢   nS   ⁢       ⅆ     ⅆ   t       [       ∑   j             ⁢     i     j   ⁡     (   t   )           ]               2   )               
According to the above equation the electrical conductor will induce a voltage on the Rogowski coil  100  proportional to the rate of change of the measured current i(t) in the electrical conductor. The Rogowski coil output voltage v(t) can be measured at output  48 .
 
     The voltage output of the Rogowski coil v(t) is a function of the time-rate-of-change of the AC current passing within the perimeter of the coils. The integrator circuit  50  is necessary in order to attain the actual waveform of interest which is an output voltage v out  that is proportional to the measured current i(t). Output voltage integration may be performed using a variety of means commonly known in the art. The output voltage signal v out  can be measured at the integrator output  52 . 
     As shown in  FIG. 1 , there is a difference in size between the outer coil  20  and inner coil  30 . This difference in size of the outer coil  20  and inner coil  30  results in a difference in induced voltage, both by the current of interest i(t) and by magnetic fields created by conductors outside the printed circuit board aperture  40 . In order to compensate for this difference in induced voltage, it is desirable that the inner coil  30  is constructed with a higher turns density than the outer coil  20 . The effective electrical geometries of the outer coil  20  and inner coil  20  can be made substantially identical by constructing the inner coil  30  with a higher turns density than the outer coil  20 . 
     The maximum effective turns density is largely determined by the minimum achievable hole size during printed circuit board fabrication, and thicker printed circuit boards impose a larger minimum hole size constraint for plated vias. Therefore, the inner coil  30  can be constructed with a higher turns density, because the inner coil vias  35  traverse two substrate  10  layers, while the outer coil vias  25  traverse all four of the substrate  10  layers. 
     The outer coil  20  and inner coil  30  are wound in electrically opposite directions to cancel the undesirable electrical fields from outside of the aperture  40 . This maximizes the accuracy of the measurement capability of the Rogowski Coil. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.