Patent Publication Number: US-7902812-B2

Title: Rogowski coil assembly and methods

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application No. 11/593,239, entitled “Shielded Rogowski Coil Assembly and Methods,” filed Nov. 6, 2006, now U.S. Pat. No. 7,564,233, the complete disclosure of which is hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to electrical power systems, and more specifically to devices for measuring current through an electrical conductor. 
     BACKGROUND 
     Rogowski coils provide a reliable means of sensing or measuring current flow at a given point in an electrical system. Current flowing through a conductor generates a magnetic field that, in turn, induces a voltage in the coil. Using the voltage output signal of the coil, actual current conditions in the conductor can be calculated. With the advent of microprocessor-based protection and measurement equipment capable of calculating the current, Rogowski coils are becoming an attractive alternative to conventional current measuring devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates principles of operation of a Rogowski coil. 
         FIGS. 2A and 2B  illustrate embodiments of a Rogowski coil with two electrical loops to cancel external magnetic fields. 
         FIG. 3  illustrates one principal of Rogowski coil design. 
         FIG. 4  illustrates a PCB Rogowski coil design. 
         FIGS. 5A and 5B  illustrate embodiments of Rogowski coils with conventional shielding. 
         FIGS. 6A and 6B  illustrate embodiments of conventional interface shielding for Rogowski coils. 
         FIGS. 7A and 7B  illustrate conventional shielding for Rogowski coils fabricated from printed circuit boards. 
         FIG. 8  illustrates a first embodiment of a shielded Rogowski coil assembly according to the present invention. 
         FIG. 9  illustrates a second embodiment of a shielded Rogowski coil assembly according to the present invention. 
         FIG. 10  illustrates a third embodiment of a shielded Rogowski coil assembly according to the present invention. 
         FIG. 11  illustrates a fourth embodiment of a shielded Rogowski coil assembly according to the present invention. 
         FIG. 12A  illustrates a noise effect algorithm for minimizing noise transmission in a Rogowski coil interface. 
         FIG. 12B  illustrates a noise effect algorithm for determining noise for a Rogowski coil interface. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Electrical generation and power transmission systems typically include a number of protective devices to protect components and equipment from potentially damaging overvoltages and overcurrents. Such protective devices include, among other things, relay devices that open and close portions of the system in response to actual operating conditions. Successful operation of network protection devices in a power distribution system is of course dependent upon accurate sensing and measurement of operating conditions. Microprocessor based equipment, such as digital relay devices, are increasingly being used in electrical power systems, but are prone to inaccurate current measurements due to ambient noise conditions and magnetic fields generated by nearby conductors and equipment. To overcome these and other disadvantages of existing Rogowski coil devices and associated systems, exemplary embodiments of shielded Rogowski coil assemblies and methods for mitigating noise effects are provided according to the present invention. 
     For a full appreciation of the inventive aspects of exemplary embodiments of the invention described below, the disclosure herein will be segmented into sections. Basic construction and operation of Rogowski coils are first discussed in Part I. Conventional magnetic shielding structures for Rogowski coils will be discussed in Part II. Shielded Rogowski coil assemblies according to the present invention will be discussed in Part III. Algorithms and methods for detecting and minimizing noise in Rogowski coil interfaces are disclosed in Part IV. 
     I. Introduction to Rogowski Coils 
     As illustrated in  FIG. 1 , a Rogowski  100  is generally fabricated from a conductor  102 , that may be fabricated from wire, that is coiled or wound on a non-magnetic core, which may be, for example, air or a non-magnetic material. The  102  coil may be placed around a conductor or conductors  104  whose currents are to be measured with the coil  102 . A primary current flowing through the conductor  104  generates a magnetic field that, in turn, induces a voltage in the coil  102 . A voltage output v(t) of the coil  102  is governed by the following Equation: 
                     v   ⁡     (   t   )       =       ⁢       -     μ   o       ⁢     μ   r     ⁢   nS   ⁢       ⅆ     i   ⁡     (   t   )           ⅆ   t                       =       ⁢       -   M     ⁢       ⅆ     i   ⁡     (   t   )           ⅆ   t           ,               
where μ o , is the magnetic permeability of free space, μ r  is the relative permeability (the ratio of the permeability of the coil  102  to the permeability of free space μ o ), n is the winding density (turns per unit length), S is the cross sectional area of the core in the Rogowski coil, and M represents the mutual reactance or mutual coupling between the coil  102  and the conductor  104 .
 
     For an ideal Rogowski coil  102 , M is independent of the location of the conductor  104  within the coil  102 . As is evident from Equation 1, the Rogowski coil output voltage v(t) is proportional to the rate of change of the measured current i(t) flowing in the conductor  104 . The coil output voltage v(t) is therefore typically integrated to determine the current i(t) in the conductor  104 . 
     To prevent undesirable influence of a nearby conductor  106  carrying high currents, the coil  100  may include, as shown in  FIG. 2A , first and second wire coils or loops  102 ,  108  wound in electrically opposite directions. The two coils  102 ,  108  effectively cancel all electromagnetic fields coming from outside the coil  100 . In such an embodiment one or both loops  102 ,  108  may be fabricated from a wound wire on the core. If only one loop wire wound on a non-magnetic core is utilized, then the other loop may be returned the center of the coil  100  to cancel undesirable effects of external magnetic fields. 
     In an embodiment illustrated in  FIG. 2B , both loops  102  and  108  may include wound wires, with the second winding  108  being wound in the opposite direction. In this way, the voltage induced in the Rogowski coil  100  from the conductor passing through the coil will be doubled. 
     Conventionally, and as illustrated in  FIG. 3 , Rogowski coils have been fabricated from flexible, nonmagnetic cores  110  such as cores commonly used in known coaxial cables. Insulating jackets and shielding from such cables, may be stripped to obtain the cores, and after cutting the cable core to size, the coil  102  (and  108 ) may be wound over the plastic cable core  110 . Existing conductors extending through the center of the coaxial cable core  110  may serve as the return loop for cancellation of external magnetic fields as described above. In lieu of such flexible cores  110 , high performance Rogowski coils alternatively been fabricated from relatively rigid and straight rods that may be manufactured with a more uniform cross sectional area than the flexible cores. In such a construction, magnetic shielding of the ends of the rods where they connect to one another has been found to be necessary. 
     U.S. Pat. No. 6,313,623 discloses high-precision Rogowski coil designs of various shapes that are fabricated on printed circuit boards (PCBs), as shown in the coil  150  shown in  FIG. 4 . In the coil  150 , two wound coils  151 ,  152  are formed on separate PCBs  154 ,  156  located next to each other. Each PCB  154 ,  156  defines one of the coils  151  and  152 , and the coils  151  and  152  are wound in opposite directions (right-handed and left-handed), respectively. The coil  151  is formed with traces extending on opposing sides of the circuit board  154  interconnected, for example, by plated through-holes, and the coil  151  has a right-handed configuration that progresses in a clockwise direction around the center of the board  154 . The left-handed coil  152  is designed in a similar manner on the board  156  except that it has a left-handed configuration that progresses in a counter-clockwise direction around the center of the board  156 . The coils  151 ,  152  may be made on multi-layer PCBs as desired. Further details of such coils and PCBs are described in U.S. Pat. No. 6,313,623, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     The Rogowski coils such as those disclosed to U.S. Pat. No. 6,313,623 may be fabricated with a high degree of precision using computer controlled fabrication techniques for forming the coils on the PCBs. Highly sensitive and highly accurate coils for current sensing and measuring applications may therefore be provided. Inaccuracies in the signal output from such coils, however, remains a concern. While the coils on the PCBs are designed to cancel external magnetic fields, the output of the coils have nonetheless been found susceptible to noise, signal distortion and undesirable influences by surrounding conductors and equipment in the vicinity of the coils To address such issues, various shielding features have been proposed for coils with varying degrees of success. 
     II. Conventional Rogowski Coil Shielding 
     Conventional approaches to shield Rogowski coils and secondary leads interfacing with measuring devices is shown in  FIGS. 5A and 5B  (single-shielded) and  FIGS. 6   a  and  6   b  (double-shielded). In  FIG. 5A , a shielded coaxial cable  160  is connected to a coil  162 .  FIG. 5B  illustrates a twisted pair wire  164  connected to the coil  162 . As is known in the art, the twisted wires carry equal but opposite signals and are less susceptible to noise issues and cross talk issues from adjacent signal conductors. While the shielded cable  160  and the twisted pair wire  164  provide some degree of protection against noise and electromagnetic influences in the environment of the coils  162 , the level of protection afforded by them is inadequate for high precision Rogowski coils. 
       FIGS. 6A and 6B  illustrate other conventional approaches for preserving the integrity of the Rogowski coil output signals.  FIG. 6A  a illustrates a double shielded cable  170  having concentric layers of insulation around the signal conductors in the cable.  FIG. 6B  illustrates a shielded twisted pair wire  172 . While the double shielding features shown in  FIGS. 6A and 6B  are more effective than the single shielding features shown in  FIGS. 5A and 5B , they remain inadequate for some installations of high precision coils. 
       FIGS. 7A and 7B  illustrate high precision coils  180  similar to the coils  150  shown in  FIG. 4  that are fabricated from PCB materials. The coils  180  are provided with a protective shield  182  of a non-magnetic material for added isolation of the coils  180  from the ambient environment noise and electromagnetic factors that may distort the output voltage signal. The shielded coils  180  may be interfaced to a measuring device with a shielded coaxial cable  160  ( FIG. 7A ) or a twisted pair wire  164 . A double shielded cable  170  ( FIG. 6A ) and shielded twisted pair wire  172  ( FIG. 6A ) may likewise be utilized with the shielded coils  180 . Regardless, the shielded cable  160  (or double shielded cable  170 ) and the twisted pair wire  164  (or shielded twisted wire pair  172 ) must be terminated to the coil  162  at their ends where they meet the coil  162 . 
     III. Shielded Interface Assemblies 
     One vulnerability of convention coils lies in the electrical connections and the interfaces between the PCBs of the coil and measuring equipment, such as protective relay devices. That is, while the coils formed on the PCBs are designed to cancel external magnetic fields, the connections of the coils and interfacing wires, conductors, or cables to measuring equipment and devices is susceptible to noise and undesirable influence in the signal from nearby conductors and other magnetic fields present in the vicinity of the coil. 
     For example, and referring to  FIG. 7A , to terminate the cable  160 , the shielding of the cable must be partly removed at the end to expose the conductors, and to terminate the twisted pair wire  164  shown in  FIG. 7B , the twisted wires must be untwisted at their ends to establish the connection to the coil. In either case, the terminated ends of the cable  160  and the twisted pair wire  164  are generally unprotected and may provide points of ingress for environmental noise and external magnetic fields that may result in inaccuracies in the coil output voltage signal. That is, voltage may be induced at the unprotected termination ends of the cable and the twisted pair wire that may distort the output voltage signal of the shielded coil  182  when received by the measuring device. 
       FIG. 8  illustrates a Rogowski coil assembly according to the present invention wherein first and second PCBs  200  and  202  and respective precision coils  204 ,  208  are formed in a similar manner to the coils  151 ,  152  shown in  FIG. 4 . Unlike the coils  151 ,  152 , that are directly connected in series to one another to produce a single voltage output with a single interface lead, the coils  204  and  208  define separate, independent, and distinct coil loops that each provide a respective output voltage signal v 1 (t) and v 2 (t). The outputs v 1 (t) and v 2 (t) may be interfaced with respective shielded cables  210  and  212 , In turn, the cables may be connected in series as shown in  FIG. 7 . 
     Current measured by the Rogowski coils  200  and  202  will induce voltage in each coil and generate output signals v 1 (t) and v 2 (t). Because the cables  210  and  212  are connected in series and because the coils  204  and  208  are wound in opposite directions, voltages v 1 (t) and v 2 (t) add to each other as shown in  FIG. 8 . In an ideal Rogowski coil, v 1 (t) and v 2 (t) are equal. In an actual Rogowski coil, due to manufacturing tolerances and production constraints, a slight difference between v 1 (t) and v 2 (t), may result. If voltages are inducted in the interface cables  210 ,  212  because of external magnetic fields, the voltages will be induced in each interface cable  210  and  212  in the same direction, but because the two coils are connected in series induced voltages from external fields in the output signal will be canceled. 
     To address signal integrity issues, each PCB  200  and  202  and each cable  210  and  212  are provided with a respective magnetic shield  220 A,  220 B,  220 C,  220 D surrounding the coils  200 ,  202  and the cables  210  and  212 . Consequently, the coils  204  and  208  are doubly protected from induced voltages and noise that are unrelated to current flow in the conductor passing through the coils  204  and  208 . The cables  210  and  212  are also protected from induced voltages and noise that are unrelated to the output voltage signals supplied by the coils  200  and  202 . The coil shields  220 A and  220 B are interfitted with the cable shields  220 C and  220 D so that the terminated ends of the cables  210 ,  212  are fully protected and shielded from induced voltages that may present errors and inaccuracies in the coil voltage signal outputs, thereby eliminating a point of vulnerability to signal contamination issues in conventional coil assemblies. 
     The shields  220 A,  220 B,  220 C and  220 D may be fabricated from magnetic shielding materials known in the art, including but not limited to silicon steel laminates and the like, to provide electromagnetic shielding and isolation of the coils  204 ,  208  and associated interface cables  210 ,  212  from undesirable environmental factors that may otherwise produce noise and inaccuracies in the output signals of the coils  204  and  208 . The shielding material  220 A,  220 B,  220 C and  220 D may be fabricated as separate pieces that are assembled to the coils  204 ,  208  and cables  210 ,  212 . 
       FIG. 9  illustrates a coil assembly similar to that shown in  FIG. 8 , but with twisted pair wires  222 ,  224  utilized within the shielding materials  220 C and  220 D in lieu of the cables  210  and  212  shown in  FIG. 8 . 
       FIG. 10  illustrates another coil assembly wherein the PCBs  200 ,  202  and the respective coils  204 ,  208  are protected into a single shield  230 A fabricated from a non-magnetic material. The twisted pair wires  222 ,  224  are likewise protected by a single shield  230 B. The interface shield  230 B interfits with the coil shield  230  and surrounds the terminations of the wires  222 ,  224  to avoid voltages being induced in the terminated ends of the wires  222 ,  224 . 
     IV. Noise Effect Algorithms 
     While the shielded coil assemblies shown above in  FIGS. 8-10  are believed to be less vulnerable to noise and external magnetic fields than known Rogowski coils, in another aspect of the invention signal processing techniques are provided that allow any noise in the coil output signals to be detected and mitigated. The signal processing techniques, explained briefly below, may be implemented in algorithm form and may be executable by an electronic device interfaced with the coils. 
       FIG. 11  illustrates an exemplary arrangement of PCB coils  204 ,  208  being interfaced with a measuring device such as a protective relay  230  monitoring current flow i(t) in a conductor  228  passed through each of the coils  204  and  208 . In one embodiment, the relay  230  is a digital processor based device, although it is understood that other known devices may alternatively be used in lieu of a relay  230 . Coaxial cables  210  and  212  may be connected to the relay  230  on separate channels thereof as shown in  FIG. 11 . 
     As shown in  FIG. 12A , the coil voltage outputs v 1 (t) and v 2 (t) that are input into the relay channels are in part true voltage outputs of the Rogowski coils VRC 1 (t) and VRC 2 (t) and in part are voltages attributable to noise. While the voltage outputs of the coils RC 1  and RC 2  are of opposite polarity due to the coils being wound in opposite directions, the noise components are not, and consequently by subtracting the voltage input v 2 (t) from v 1 (t), the noise component is effectively cancelled or eliminated in the resultant output signal v(t) that is the sum of VRC 1 (t) and VRC 2 (t). It may therefore be ensured that the relay  230  is receiving an accurate signal v(t) from which it may reliably make control decisions, or from which the monitored current may be calculated or otherwise determined, such as via known integration techniques. 
     As illustrated in  FIG. 12B , by summing the coil voltage outputs v 1 (t) and v 2 (t) that are input into the relay channels, the level of noise in the input signals to the relay may be readily determined and calculated. Because the voltage outputs of the coils RC 1  and RC 2  are of opposite polarity due to the coils being wound in opposite directions and the noise components are not, when the signals are summed the coil output voltages VRC 1 (t) and VRC 2 (t) cancel one another and the resultant output value v(t) is substantially entirely noise. Because the noise in each of the interface conductors is added, the output value v(t) is twice the actual noise level. Using event recoding functionality in the relay  230 , external noise conditions may be monitored to identify noise issues for resolution. 
     Still further, utilizing separate secondary interface conductors for each PCB coil facilitates diagnostic functions and features in the relay. If, for example, the coils  204 ,  208  ( FIG. 11 ) are in satisfactory operating condition, the output voltages of the coils transmitted through the interface conductors should be approximately the same. If, however, one coil has open wires turns or shorted turns, the output voltages of the coils will be markedly different, and the relay  230  may identify a problem in the coil for immediate attention. 
     Having now explained the operating principles of the invention, it is believed that relay device  230  may be programmed to perform signal processing algorithms to minimize noise effects in the input signals from the Rogowski coils, determine a level of noise in the signals, and perform diagnostic functions and procedures. The algorithms may be implemented using conventional programming techniques that are within the purview of those in the art. Further explanation of associated algorithms, methods and techniques associated with such programming is not believed to be necessary. 
     One embodiment of a Rogowski coil assembly is disclosed herein which comprises a first Rogowski coil surrounding a conductor and generating a first voltage output signal, and a second Rogowski coil surrounding the conductor and generating a second voltage output signal wherein the first voltage output signal and the second voltage output signal are processed to address noise components in the first and second output voltage signal. 
     Optionally, each of the first coil portion and the second coil portion may be fabricated on printed circuit boards, and the first and second coils may be electrically connected in opposite directions. First and second interface conductors for the respective first and second coils may be provided, and the first and second conductors may be terminated to the respective coils at one end thereof. The terminations may be magnetically shielded to prevent voltages from being induced in the interface conductors at the terminations. The first and second interface conductors may be surrounded by a common magnetic shield. The conductors may be a coaxial cable or a twisted pair wire and the interface conductors may be connected in series. The first and second interface conductors may be connected to a protective relay device, and the relay device may be programmed to determine a level of noise in the first and second output voltage signal. The relay device may likewise be programmed to cancel noise in the first and second output voltage signal. 
     An embodiment of a Rogowski coil assembly is also disclosed herein. The assembly comprise a first Rogowski coil comprising a first printed circuit board and a first conductive winding thereon, the first coil surrounding a conductor and generating a first voltage output signal proportional to a rate of change of current flowing through the conductor. A second Rogowski coil is also provided and comprises a second printed circuit board and a second conductive winding thereon, the second coil surrounding the conductor and generating a second voltage output signal proportional to the rate of change of current flowing through the conductor. A pair of interface conductors are also provided, with each of the pair of interface conductors transmitting one of the first and second voltage output signals. 
     Optionally, the first and second coils extend in opposite directions. Each of the first and second conductors are terminated to the respective first and second coils, and the terminated conductors are magnetically shielded to prevent voltages from being induced in the interface conductors at the termination locations. The first and second interface conductors may be surrounded by a common magnetic shield. At least one of the first and second interface conductors may be a coaxial cable or a twisted pair wire. The first and second interface conductors are connected in series. A microprocessor based device may be connected to each of the first and second interface conductors, and the microprocessor based device may be programmed to determine a level of noise in first and second output voltage signal. The microprocessor based device may also be programmed to cancel noise in first and second output voltage signal. 
     An embodiment of a Rogowski coil system is also disclosed herein. The system comprises a conductor and a first Rogowski coil comprising a conductive circuit board and a right handed coil formed thereon, the right handed coil receiving the conductor generating a first voltage output when the conductor is energized. A second Rogowski coil comprises a conductive circuit board and a left handed coil formed thereon, the left-handed coil receiving the conductor and generating a generating a second voltage output when an energized conductor is passed through the coil. A first interface conductor is terminated to the first coil and a second interface conductor, separately provided form the first interface conductor, is terminated to the second coil. 
     Optionally, magnetic shields protecting an area of termination of the respective interface conductors to each of the first and second coils. The first and second interface conductors may be surrounded by a common magnetic shield. The interface conductors may be selected from the group of a coaxial cable, a twisted pair wire, and combinations thereof. The first and second interface conductors may be connected in series. A microprocessor based device may be connected to each of the first and second interface conductors, and the microprocessor based device may be programmed to determine a level of noise in first and second output voltage signal. The microprocessor based device may be programmed to cancel noise in first and second output voltage signal. 
     A method of monitoring current with a Rogowski coil assembly is also disclosed. The method comprises providing a first Rogowski coil having a first winding; providing a second Rogowski coil having a first winding extending opposite to the first winding; connecting an interface conductor to each of the first and second coils; obtaining distinct voltage outputs from each of the first and second Rogowski coils; and applying a noise effect algorithm, using a microprocessor based device, to the voltage outputs. 
     Optionally, the method may also comprise applying a noise effect algorithm comprises determining an amount of noise in the voltage outputs, which may comprises adding the voltage outputs conducted through the first and second interface conductor. The noise effect algorithm may also comprise minimizing an amount of noise in the voltage outputs, and minimizing an amount of noise in the voltage outputs may comprise subtracting the voltage outputs conducted through the first and second interface conductor. 
     An embodiment of a Rogowski coil system is also disclosed. The system comprises first and second sensing coils formed on respective circuit boards and configured for induced voltage measurements corresponding to current flow in a conductor passing through the coil; and means for monitoring noise conditions during operation of the coils. 
     The system may also comprise means for shielding the first and second sensing coils. Means for interfacing the first and second sensing coils with the means for monitoring may also be provided. Means for shielding the connection of the means for interfacing with the means for monitoring may also be provided. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.