Abstract:
Accurate measurements of electrical power at various points of a power grid is becoming more important and, at the same time, is getting more difficult as the old power distribution model of a few, large power generating stations and a multitude of relatively linear loads is replaced by a newer model containing a multitude of smaller, and to some degree unpredictable power sources, as well as a multitude of not always linear and often smart (essentially also unpredictable) loads. Embodiments of the invention provide for management of AC current measurements in the presence of a DC current. Such current measurement management including at least alarms, feedback, and forward correction techniques exploiting magnetic field measurements from within the magnetic core or upon the surface of magnetic elements and/or shields within the current transducer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of United States of America Provisional Patent Application U.S. 61/893,415 filed Oct. 21, 2013 entitled “Methods and Systems Relating to AC Current Measurements.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to precision AC measurements, which include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, in the current range from 1 mA or less to 20 kA or greater and voltage range of 1V or less to 1000 kV or greater and in a frequency range from a few hertz to one hundred kilohertz. In particular it relates, but is not limited to AC measurements as applicable in power transmission and distribution networks. 
     BACKGROUND OF THE INVENTION 
     The accurate Alternating Current (AC) measurement of electrical power at various points of a power grid is becoming more and more important and, at the same time, is getting more and more difficult. The old power distribution model of a few, large power generating stations and a multitude of relatively linear loads is being replaced by a newer model containing a multitude of smaller, and to some degree unpredictable power sources, as well as a multitude of not always linear and often smart (essentially also unpredictable) loads. This change deteriorates power quality and makes AC measurements, grid management and troubleshooting more difficult. Also, the increasing cost of electrical power makes precise calculation of delivered energy and monitoring of power quality important. 
     There are three main categories of AC power measurement systems: The highest level of accuracy systems, used typically by the Standard and Calibration Laboratories, are developed to reference measurement to the National Standards. These are typically unique installations, not covered by specific regulatory requirements. The next category is high precision AC power measurement systems. In the important case of AC power measurement instruments, usually referred to as Power Analyzers, these would be units meeting the requirements of standards, such as for example International Standard IEC 61000-4-30 “Electromagnetic Compatibility: Part 4-30 Testing and Measurement Techniques—Power Quality Measurement Methods” which relates to Class A measurement methods. These are used where precise measurements are necessary, for example for contractual applications and disputes, verifying compliance with standards, etc. Two different Class A instruments, when measuring the same quantities, should produce matching results within the specified uncertainty for that parameter. The third main category of the AC power measurement system is general use instruments. Generally it is recommended that this group reflect measurement methods and intervals of Class A instruments, with lower precision and processing requirements. It is then classified as Class S. Other instruments including legacy installations, whose operation doesn&#39;t reflect methods of Class A, but still meet key accuracy requirements, are summarily called Class B. Irrespective of the class of the AC power measurements they require determination of the voltage, current, frequency, phase, and relative timing of the single or multiple phases of the power system in order to perform the measurements. 
     The whole measurement chain of electrical quantity for power analysis consists of measurement transducer, measurement unit and evaluation unit (as is defined in the ICE 61000-4-30 standard). The measurement transducer converts the input quantity to a level and a kind suitable for the measurement unit and typically has some other functionality, for example signal isolation or overload protection. For example, the measurement transducer may reduce a power line voltage of hundreds of kilovolts to tens of volts. The measurement unit then converts the input quantity, typically to a digital form, suitable for evaluation. Then the evaluation unit, which is typically some form of a computing device, receives and combines data streams from different input channels including for example the output of the measurement unit and a reference unit, and does the required calculations to produce results. Test results can be: recorded, aggregated, automatically evaluated in the real time, displayed on the instrument screen, used to generate alarms, placed in system logs, and send out for external evaluation and storage, etc. 
     Generally, AC electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including voltage, current, capacitance, impedance, resistance etc. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment from high voltage electrical transmission lines operating at hundreds of kilovolts (kV) and kiloamps (kA) to industrial/medical/residential electrical and lighting, typically 400V/240V/100V and 30/15 A, to a wide variety of industrial/scientific/medical/consumer electrical and electronic devices. 
     Within a variety of applications and test equipment systems the measurement transducer is often a toroidal transformer. These allow for the measurement system to measure the required parameter(s) with the measurement system electrically isolated from the electrical system being measured. Further, toroidal forms of the core of the transformer provide best magnetic performance of the core, providing low magnetic reluctance, good uniformity of the magnetic field and low flux leakage, resulting in the best electrical parameters of the transformer. In general, the toroidal form of the core of the transformer is an accepted standard for meteorological applications. 
     However, with the continued drive for improved accuracy in calibration, standards, and measurements on circuits and components operating at hundreds of kiloVolts, thousands of Amps, with resistances into Gigaohms accuracies of parts per million is being replaced by parts per billion. At the same time as discussed supra such measurements are being performed upon, for example, electrical power distribution systems at various points of a power grid with a variety of generators, distribution systems, etc. with unknown or variable characteristics. On the other hand even the best toroidal core transformers still have three basic limitations, affecting performance of the transducer, namely saturation of the core, finite value of the permeability and finite width of the hysteresis loop. Each one affects operation of the transformer and may limit overall accuracy of the resulting transducer. The first and most obvious way to improve performance of the measurement transformer is to use highest permeability, lowest losses (narrowest hysteresis loop) magnetic materials for the core. 
     Next, the inventors have established a measurement and correction methodology for AC current transducers designing multi-core, multi-stage transformers compensating effects of finite, changing burden. Similarly, DC compensation was introduced to improve AC operation of the measurement transformer in the presence of the DC components magnetizing the transformer core. Beneficially, such measurement and correction methodologies provide instrument designers with multiple options ranging from low cost alarms through to higher cost automated correction hardware software and firmware based circuits. 
     Such measurement and correction methodologies would beneficially allowed such devices according to some embodiments of the device to achieve performance approaching that of reference measurements operating in laboratory conditions. It would be further beneficial if the same principles provide power utilities, independent electricity producers, electrical engineers and technicians, and others requiring accurate measurements of power systems with a field deployable power system measurement devices providing up to Class A type performance but in rugged devices of reduced cost and complexity. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide measurement and correction methodologies for DC currents within precision AC measurement instruments, which include precision AC current, voltage, phase, impedance, frequency, power and energy instruments, operating in the current range from 1 mA or less to 20 kA or greater and voltage range of 1V or less to 1000 kV or greater and in a frequency range from a few hertz to one hundred kilohertz. In particular the invention relates to, but is not limited to AC measurements as applicable in power transmission and distribution networks. 
     In accordance with an embodiment of the invention there is provided a method comprising measuring a DC signal, the DC signal generated in dependence upon a DC aspect of a signal, measuring an AC signal, the AC signal generated in dependence upon an AC aspect of the signal, and generating a corrected measurement of the measured AC signal. 
     In accordance with an embodiment of the invention there is provided a device comprising:
     a dual stage current transformer comprising a plurality of magnetic cores, a primary winding, a first secondary winding, and a second secondary winding;   a first resistor coupled across the first secondary winding generating a first voltage;   a second resistor coupled across the second secondary winding disposed in series with the first resistor to add a compensating voltage to the first voltage;   a DC magnetic sensor coupled to a first magnetic core of the plurality of magnetic cores for generating a signal proportional to a DC magnetic field within the dual stage current transformer; and   a flux compensation winding coupled to a second magnetic core of the plurality of magnetic cores for generating a magnetic flux to reduce the DC magnetic field within the dual stage current transformer.   

     In accordance with an embodiment of the invention there is provided a method comprising using a DC magnetic sensor and flux compensation in conjunction with a dual stage current transformer, wherein the dual stage transformer uses resistors to add voltages rather than adding currents. 
     In accordance with an embodiment of the invention there is provided a method comprising integrating a magnetic sensor within a magnetic core of a plurality of magnetic cores within a dual stage current transformer allowing operation of the sensor with small AC flux components and improved AC to DC signal ratio. 
     In accordance with an embodiment of the invention there is provided a device comprising a dual stage current transformer comprising a plurality of magnetic cores, a primary winding, a first secondary winding, and a second secondary winding and a DC magnetic sensor coupled to a first magnetic core of the plurality of magnetic cores for generating a signal proportional to a DC magnetic field within the dual stage current transformer. 
     In accordance with an embodiment of the invention there is provided a device comprising a current comparator comprising a magnetic core, a primary winding wound around the magnetic core, a secondary winding wound around the magnetic core, and a magnetic sensor coupled to a magnetic field generated in dependence upon a first current within the primary winding and a second current within the secondary winding, wherein the primary and secondary windings are wound around the magnetic core directly without a magnetic shield disposed between any of the magnetic core, the primary winding, and the secondary winding. 
     In accordance with an embodiment of the invention there is provided a current comparator based sensor comprising:
     a magnetic core;   a primary winding wound around the magnetic core for connecting to an electrical circuit;   a secondary winding wound around the magnetic core;   a magnetic sensor coupled to the magnetic field within the magnetic core;   a control circuit for generating and applying a magnetization current to at least one of the primary winding and a tertiary winding wound around the magnetic core, wherein the magnetization current sequentially cycles the magnetic core to saturation in opposite directions; and   a measurement circuit coupled to at least the secondary winding for determining timing information relating to the cycling of the magnetic core and establishing a magnetization field strength therefrom and the current flowing in the primary winding due to the electrical circuit.   

     In accordance with an embodiment of the invention there is provided a current comparator comprising:
     a magnetic field sensor;   a primary winding for connecting to an electrical circuit disposed either above or around the magnetic field sensor; and   a secondary winding for generating a current to be employed in determining a current flowing within the electrical circuit disposed below the magnetic field sensor when the primary winding is disposed above and around the magnetic field sensor between the primary winding and the magnetic field sensor when the primary winding is around the magnetic field sensor.   

     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIGS. 1 and 2  depict the measurement ratio, I/I 0  of toroidal single stage and dual stage AC current transformers with varying DC current expressed in ampere-turns (AT); 
         FIG. 3  depicts a current sensing circuit for a measurement probe according to the prior art of U.S. Pat. No. 7,309,980 for a single core transformer; 
         FIG. 4  depicts a transformer correction approach according to the prior art of U.S. Pat. No. 7,348,845 for a single core transformer; 
         FIG. 5  depicts Hall current sensor circuit configurations according to an embodiment of the invention; 
         FIG. 6  depicts a dual stage driver circuit for a Hall current sensor according to an embodiment of the invention; 
         FIG. 7  depicts a single stage driver circuit for a Hall current sensor according to an embodiment of the invention; 
         FIG. 8  depicts a two-stage current transformer and associated electrical interface circuit according to an embodiment of the invention; 
         FIGS. 9A-9C  depict schematically electrical circuits of multi-core current transformers according to embodiments of the invention; 
         FIGS. 10A-10C  depict schematically electrical circuits of multi-core current transformers according to embodiments of the invention; 
         FIG. 11  depicts a two stage current transformer according to an embodiment of the invention which when resistively connected as described in  FIG. 10A  provides an implementation of a single stage, Current Transducer with Hall sensor based DC flux detection; 
         FIG. 12  depicts a two stage current transformer according to an embodiment of the invention utilizing third core for the DC bias detection core which when resistively connected as described in  FIG. 10A  provides an implementation of a two stage Current Transducer with Hall sensor based DC flux detection; 
         FIG. 13  depicts a dual stage current transformer according to an embodiment of the invention with a magneto-strictive element positioned on the shield which when resistively connected as described in  FIG. 10A  provides an implementation of the Current Transducer; 
         FIG. 14  schematically an electrical circuit of a multi-core current transformer according to an embodiment of the invention; and 
         FIG. 15  depicts a two stage current transformer according to an embodiment of the invention utilizing a third core for the DC bias detection core which when resistively connected as described in  FIG. 14  provides an implementation of a two stage Current Transducer with DC flux detection using fluxgate detectors and Hall sensor; 
         FIG. 16  depicts a current comparator according to the embodiment of the invention, utilizing a flux gate detector to detect input and output current—turn balance wherein the prior art magnetic shield between the magnetic sensor and the primary and secondary windings is removed; 
         FIGS. 17A and 17B  depict a current comparator according to the embodiment of the invention utilizing Hall Effect magnetic sensors to detect input and output current—turn balance wherein the prior art magnetic shield between the magnetic sensor and the primary and secondary windings is removed; and 
         FIG. 18  depicts an active current to current transducer according to the embodiment of the invention utilizing current comparator with a magnetic sensor and an amplification block to produce AC and DC output current in precise ratio to the input current. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention relates generally to precision AC measurements, which include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, in the current range from 1 mA or less to 20 kA or greater and voltage range of 1V or less to 1000 kV or greater and in a frequency range from a few hertz to one hundred kilohertz. In particular it relates, but is not limited to AC measurements as applicable in power transmission and distribution networks. 
     The ensuing description provides exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
     Precise AC power measurements require precise determination of the voltage, current and timing of the single or multiple phases. However, as evident in  FIGS. 1 and 2  the ratio of a measured AC current, I, with varying DC current relative to the measured AC current, I 0 , at no DC current is not equal to one, i.e. I/I 0 =1, with varying DC current but varies substantially with DC current and magnitude of the AC current being measured. For an applied AC current of 510 A the ratio exceeds 1.01 at approximately 1 ampere turn (AT). This represents an error of 1% or 10,000 parts per million or 10 million parts per billion. Accordingly, when considering test instrumentation providing accuracies of a few parts per million it is evident that poor signal conditioning of the signal being measured will result in errors that dwarf those from the measurement instrument itself. Clearly significant control of the DC current is required in order to achieve the intrinsic accuracy of the test instrument. 
     Within the prior art techniques these have been techniques presented to combine AC and DC current sensing. One such example is presented within  FIG. 3  according to the prior art of Mende et al in U.S. Pat. No. 7,309,980 for a single core transformer in respect of current sensing circuit for a measurement probe. As depicted, there is a ring-shaped core  312  of magnetic material defining an aperture. A current carrying conductor  314  is coupled in a flux linking relationship with ring-shaped magnetic core  312 . The current carrying conductor  314  is preferably linked to the ring-shaped magnetic core  312  via a multi-turn primary winding  316  that is coupled in series with the current carrying conductor  314 . Alternately, the current carrying conductor  314  may be inserted through the aperture in the ring-shaped magnetic core  312  and act as the primary winding  316 . The current to be measured in the current carrying conductor  314  produces a magnetic flux in the magnetic core  312  and is linked to a multi-turn secondary winding  318 . One terminal of the secondary winding  318  is coupled to ground with the other terminal being coupled to the inverting input terminal of a transimpedance amplifier  320 . The inverting input terminal of the transimpedance amplifier  320  is coupled to the output terminal of the amplifier  320  via a current signal path  322  having a transimpedance resistor  324 . Thus the primary winding  316  or alternately the current carrying conductor  314 , the magnetic core  312  and the secondary winding  318  function as a transformer  326 . A magneto-electric converter  328  is disposed within the magnetic core  312  substantially perpendicular to the lines of flux in the magnetic core  312 . The magneto-electric converter  328  is preferably a thin film semiconductor Hall effect device having a first pair of terminals coupled to a bias source  330  and a second pair of terminals connected to differential inputs of amplifier  332 . In an embodiment of the invention, the amplifier  332  is a high gain differential amplifier having low noise and high common mode rejection the single ended output of the differential amplifier  332  is coupled to the non-inverting input of the transimpedance amplifier  320 . Accordingly, the transimpedance amplifier  320  functions as a power amplifier for DC to low frequency current signals and a transimpedance gain amplifier for higher frequency signals. In this manner the overall circuit acts as a DC to high frequency current probe but no correction of the AC portion of the circuit for DC currents is considered. 
     Referring to  FIG. 4  there is depicted a transformer correction approach according to the prior art of Giovannotto in U.S. Pat. No. 7,348,845 for a single core transformer. As depicted the system comprises an amplifier  410  and variable magnetic flux bias system  450 . Amplifier  410  comprises amplifier circuitry  420 , amplifier signal line  425 , output transformer  430 , primary winding  432 , secondary winding  434 , control winding  480 , and optionally load  440 . Amplifier  410  receives input signals from signal source  405 , and provides amplified output signals to load  440 . Variable magnetic flux bias system  450  comprises magnetic sensor  460 , flux signal line  465 , control circuitry  470 , and control signal line  475 . 
     Amplifier circuitry  420  may be any audio amplifier known in the prior art that uses an output transformer, such as output transformer  430 , and may comprise vacuum tubes in a triode, tetrode, or pentode configuration, or may comprise solid state devices. Amplifier circuitry  420  may be operated in bias modes including, but not limited to, Class A, Class AB(1), Class AB(2), Class B, Class C, or Class D. 
     Variable magnetic flux bias system  450  uses magnetic sensor  460  to sense a first magnetic flux in the proximity of output transformer  430 . The first magnetic flux is a portion of the leakage magnetic flux emanating from output transformer  430 . Magnetic sensor  460  may be a linear-output Hall-Effect sensor. In other embodiments, magnetic sensor  460  may include, but is not limited to a magnetoresistive sensor, a fluxgate sensor, a superconducting quantum interference device (SQUID) sensor, or an electron-spin sensor. 
     By placing magnetic sensor  460  in proximity to output transformer  430 , the first magnetic flux of output transformer  430  may be sensed, generating a flux signal on flux signal line  465 . The first magnetic flux has components representing a portion of the total magnetic flux within the transformer, comprising both the desired higher-frequency amplifier signal from signal source  405  and the undesired DC and low-frequency subsonic components. 
     Flux signal line  465  is coupled to control circuitry  470 . Control circuitry  470  is configured to receive the flux signal from flux signal line  465  and to generate a control signal on control signal line  475  representing the undesired DC and low-frequency subsonic components of the first magnetic flux of output transformer  430 . 
     Control winding  480  is coupled to control circuitry  470  via control signal line  475 , and thereby receives the control signal. Using the received control signal, control winding  480  induces a second magnetic flux in output transformer  430  that may set a non-zero quiescent magnetic bias level in output transformer  430 . Alternatively, control circuitry  470  may generate a control signal that causes control winding  480  to induce a second magnetic flux that substantially cancels out or nulls the undesired DC and low-frequency subsonic components of the first magnetic flux in output transformer  430 . Control winding  480  may be a spare or unused winding in output transformer  430 , or may be added after output transformer  430  is manufactured. Control winding  480  may be a primary winding or a secondary winding of output transformer  430 . Control winding  480  may be multiple individual windings coupled to control signal line  475 . 
     In one embodiment, control circuitry  470  may be adjusted so that a quiescent magnetic bias level is maintained within output transformer  430 . The quiescent magnetic bias level may be maintained at a level different from zero. In another embodiment, control circuitry  470  may be adjusted so that the second magnetic flux substantially cancels or nulls out the DC and low-frequency subsonic components of the first magnetic flux, and thus minimizes the magnetic saturation within output transformer  430 . Control circuitry  470  may be implemented using operational amplifiers, or alternatively using a proportional integral (PI) or proportional-integral-derivative (PID) control loop comprising a digital signal processor or microcontroller. 
     Referring to  FIG. 5  there is depicted a Hall sensor circuit board according to an embodiment of the invention wherein first and second Hall sensors Hall # 1   510  and Hall # 2   520  respectively which detect opposite sense magnetic fields, (+) and (−) respectively. As depicted in  FIG. 5 , each Hall sensor is connected to +5V and GND power supply rails and generates an output signal coupled to output ports  530 A and  530 B respectively for (+) and (−) field directions respectively. Whilst a pair of Hall sensors are depicted a single Hall sensor, or multiple hall sensors may also be employed. Similarly, non-differential configurations of a pair of Hall sensors may also be employed. 
     In each the Hall sensors are inserted within holes in the circuit board in order to reduce the vertical dimensions of the Hall sensor circuit board as this impacts the performance of the magnetic core of the transformer within which it is to be inserted as minimizing the profile of the sensor/circuit board reduces the size of the slot that has to be cut into the core of the current transformer transducer. Surrounding the Hall sensor devices and the circuit board which is inserted into the transformer core is a protective film or layer which may be wrapped, such as in the example of using a protective film or tape or deposited such as for example by dip coating. The circuit board may be formed from one or more standard circuit materials known within the prior art including, but not limited to, FR-4, FR-6, CEM-3, CEM-4, G-10, alumina, and aluminum nitride. It would be evident that other circuit board designs may be employed as well as that the number and orientation of the Hall effect sensors may be varied together with their integration into different numbers of packages. For example, a custom Hall sensor package may employ 4 Hall effect sensors orientated at right angles to one another with 2 measuring (+) fields within the core and the others measuring (−) fields within the core relative to the sensors. Similarly, placement may be adjusted in respect of the design of the core. Beneficially pre-packaged sensors allow for pre-screened components in hermetic packages if appropriate although non-hermetic and discrete die options may be considered as well as a discrete ceramic package having internally the sensors and appropriate circuit tracks. 
     Now referring to  FIG. 6  there is depicted a dual-stage driver circuit for use in conjunction with first and second Hall sensors Hall # 1   610  and Hall # 2   620  respectively according to an embodiment of the invention. As depicted two operational amplifiers (op-amps) such as Texas Instruments THS4521 Fully-Differential Amplifiers are employed with an output generated across output resistor RI in proportion to the field measured.  FIG. 7  depicts a corresponding single stage driver according to an embodiment of the invention. 
     Referring to  FIG. 8  there is depicted an exemplary circuit according to an embodiment of the invention to generate a digital representation of an input analog signal applied across the L and N terminals  800 A and  800 B respectively. As depicted a current transformer (CT)  810  with primary winding of N 0  turns is coupled to the L and N terminals  800 A and  800 B. A first secondary winding of N 1  turns is coupled to a first load resistor, R LOAD1 =50Ω and a second secondary winding of turns is coupled across a second load resistor, R LOAD1 =50Ω, which is serially connected to the first load resistor. The outer connections of the first and second load resistors are coupled to the + and − inputs of a differential operational amplifier (OpAmp)  820  via resistors, R A =100 KΩ. The differential outputs of the differential OpAmp  820  are each fed back via feedback resistors R B =50 kΩ and coupled via anti-aliasing circuitry to ADC  830 , such as for example an Analog Devices ADS1271 which provides a 24-bit delta-sigma analog-to-digital converter (ADC) at 105 kSPS and 51 kHz bandwidth. The ADC  830  output is coupled to output  800 D. The reference voltage, ADC  830  power, and differential OpAmp  820  power are supplied via third input  800 C, +V IN . 
     Now referring to  FIG. 9A  there is depicted an embodiment of a Current Transducer (CT)  900  according to the prior art exploiting a dual-stage design wherein the signal induced within a first secondary windings N 1  has a corrective signal applied which is generated by second secondary winding N 2 . CT  900  being a dual stage CT without DC bias compensation. CT  900  consists of a dual stage current transformer CT R    929  containing primary winding N 0  and first and second secondary windings N 1  and N 2  respectively. The Current Transducer  929  primary input terminal I IN  is connected to the start connection of the primary winding N 0 , while the end connection of N 0  is connected to the primary output terminal I OUT . An electrical shield S 930  is placed between the primary and the secondary sides and connected to a dedicated shield terminal Sh  900 C. Winding N 1  is loaded with a precise resistance R 1    931  and second stage winding N 2  is loaded with a precise resistance R 2    932 . The High output terminal H  900 A of the Current Transducer  900  is connected to the start connection of secondary winding N 1 , while the end connection of winding N 1  is connected to the start connection of second stage winding N 2 . End connection of second stage winding N 2  is connected to the Low output terminal L  900 B of the transducer. Accordingly, current passing through the primary winding N 0  produces a proportional voltage between output terminals H  900 A and L  900 B wherein the winding N 1 /precise resistance R 1    931  combination provides a correction current applied to that generated by second stage winding N 2 /precise resistance R 2    932 . The High and Low output terminals H  900 A and L  900 B together with shield terminal Sh  900 C are coupled to processing circuit  930 . 
     Optionally a switchable resistor, i.e. a resistor switchable into the circuit or selectable between a first fixed resistance value and no resistance, is coupled between the winding N 1  and point A during manufacturing testing. Accordingly, if a variation in the signal at the H and L terminals  1000 A and  1000 B is measured for constant input when the switchable resistor is toggled between its two states then the polarity of the correction circuit is incorrect in assembly. Accordingly, as discussed supra in respect of  FIGS. 1 and 2  DC currents on the input side will impact the measurements such that an incorrect AC current will be measured. Referring to  FIGS. 9B and 9C  two simple embodiments of DC current sensing are depicted wherein in  FIG. 9B  first circuit  900 D includes a Shunt R S    934  allowing a measurement of the DC current to be made thereby allowing, for example, an alarm to be triggered when the DC current exceeds a predetermined threshold. However, this DC offset may be difficult to observe and the Shunt R S    934  may limit the operating range of the measurement instrument including first circuit  900 D to provide the Current Transducer. In second circuit  900 E a Hall effect sensor  935  is added to monitor the input to primary winding N 0  and provide sensing of any DC current present on the input. Whilst this removes the loading issue of first circuit  900 D the Hall effect sensor  935  induces an inherent offset that must be accounted for and corrected for. Depending upon conductor design to the primary winding N 0  a configuration such as presented within the prior art of Seitz in U.S. Pat. No. 4,749,939 may be employed for example. Rather than a Hall effect sensor  935  a Flux Gate Detector (FGD) may be employed but these have the drawback that they operate with AC signals themselves, typically at 700-800 Hz and thereby generate noise within the second circuit  900 E. 
     Now referring to  FIGS. 10A and 10B  there are depicted first and second circuit schematics  1000 A and  1000 B depicting variants of the Current Transducer (CT) according to an embodiment of the invention. The CT now consists of a dual stage current transformer CT R    1050 A containing primary winding N 0  and first and second secondary windings N 1  and N 2  respectively together with an electrical shield S  930  placed between the primary and the secondary sides and connected to a dedicated shield terminal Sh  900 C. First secondary winding N 1  is loaded with a precise resistance R 1    931  and second secondary winding N 2  is loaded with a precise resistance R 2    932 . The High output terminal H  900 A of the Current Transducer  1000 A is connected to the start connection of secondary winding N 1 , while the end connection of winding N 1  is connected to the start connection of second stage winding N 2 . End connection of second stage winding N 2  is connected to the Low output terminal L  900 B of the transducer. Accordingly, current passing through the primary winding N 0  produces a proportional voltage between output terminals H  900 A and L  900 B wherein the winding N 1 /precise resistance R 1    931  combination provides a correction current applied to that generated by second stage winding N 2 /precise resistance R 2    932 . 
     In first circuit  1000 A, unlike CT R    929  in  FIG. 9A , the CT R    1050 A now has a Hall sensor  1010  embedded within it which couples via Magnetic Field (MF)  1040 A to Processing Circuit  1020  which also receives the output from the modified CT R    929 . Accordingly, Processing Circuit  1020  may determine in some embodiments of the invention that the DC current is beyond a threshold established in dependence, for example, upon the magnitude of the AC current and the desired accuracy of the AC current reading. Accordingly, a measurement instrument may allow coarse low accuracy measurements on poorly conditioned input signals but prevent high accuracy measurements until the input signal has been conditioned to the required degree. 
     In second circuit  1000 B ( FIG. 10B ), unlike the CT R    929  in  FIGS. 9A through 9C  and CT R    1050 A in  FIG. 10A , the CT R    1050 B now has a Hall sensor  1010  and a tertiary winding  1070 . The Hall sensor  1010  is embedded within the CT R    1050 B and couples via Magnetic Field (MF)  1040 A to Processing Circuit  1030  which also receives the output from the modified CT R    1050 B. Accordingly, Processing Circuit  1020  generates a correction current which is coupled to a tertiary winding  1070  with N 3  turns also coupled to the CT R    1050 B. Accordingly, the Processing Circuit  1030  now generates a current in dependence upon the measured DC field from Hall sensor  1010  and number of turns N 3  in order to generate within the CT R    1050 B a field negating or reducing the DC field present within the CTR  1050 B as a result of the conditioning or lack of conditioning applied to the input signal being analyzed. 
     Referring to  FIG. 10C  there is a third circuit  1000 C which is very similar to second circuit  1000 B except that in addition to the tertiary winding N 3  coupled to the CT R    1050 C there is a quaternary winding N 4  coupled together with the second secondary winding N 2 , these being upon a different core of the Current Transducer to that of the first secondary winding N 1  and tertiary winding N 3 , Tertiary winding N 3  and quaternary winding N 4  provide Correction Winding  1   1070  and Correction Winding  2   1060  for the two cores of the Current Transducer. Accordingly, corrective magnetic fields may be induced if necessary in multiple cores of a Current Transducer. 
     According to the design of the Current Transducer that the Hall sensor  1010  may be embedded into one core of a plurality of cores or alternatively multiple Hall sensors  1010  may be embedded such that a Hall sensor  1010  is disposed within each core of the Current Transducer or a predetermined subset of the cores of the Current Transducer. 
     Referring to  FIG. 11  there is depicted a Current Transducer according to an embodiment of the invention such as described supra in respect of Current Transducer (CT)  1000 A in  FIG. 10A  exploiting a dual-core transformer architecture. Accordingly, first image  1100 A depicts the CT sequentially stripped from the outermost layer towards the centre whilst second image  1100 B depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1130 A through  1130 E respectively and shielding  1160 . Accordingly, as shown the CT comprises first and second cores  1110  and  1120  respectively. First core  1110  has embedded within it Hall sensor  1180 . Second core  1120  then has first tape layer  1130 A separating the first winding  1140  from it which is then overwound with second tape layer  1130 B. The first and second cores  1110  and  1120  with their respective surrounding layers are then overwound with third tape layer  11300 . Atop third tape layer  1130 C second winding  1150  is wound around both the first and second cores  1110  and  1120  respectively. Second winding  1150  is then overwound by fourth tape layer  1130 D, shielding  1160 , fifth tape layer  1130 E and third winding  1170 . As depicted first winding  1140  corresponds to second secondary winding N 2  of  FIG. 10A , second winding  1150  corresponds to first secondary winding N 1  of  FIG. 10A , and third winding  1170  corresponds to the primary winding N 0  of  FIG. 10A . Optionally a second shielding may be disposed between the first and second windings  1140  and  1150  respectively such as between second and third tape layers  1130 B and  1130 C respectively. 
     Second image  1100 B depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1130 A through  1130 E respectively and shielding  1160  removed thereby showing how the first to third windings  1140 ,  1150  and  1170  respectively are wound around the closed magnetic elements forming the first and second cores  1110  and  1120  respectively. Also depicted within first core  1110  is Hall sensor  1180 , for example, within a slot machined within the closed magnetic element forming first core  1110 . It would be evident to one skilled in the art that the number of windings for each of the first to third windings  1140 ,  1150 , and  1170  respectively and geometries of the first and second cores  1110  and  1120  respectively may be adjusted according to the electrical voltage, current and power of the signal being measured and design of the Asynchronous Power Measurement System within which the Current Transducer forms part. Accordingly, a Hall sensor such as described supra in respect of  FIG. 6 , and other variants not depicted, may be inserted into the first core  1110  as depicted or alternatively second core  1120  in order to provide the determination and/or management of a DC field within the Current Transducer. Optionally, multiple Hall sensors  1180  may be embedded into one or more cores. 
     Referring to  FIG. 12  there is depicted a Current Transducer according to an embodiment of the invention such as described supra in respect of Current Transducer (CT)  1000 A in  FIG. 10A  employing a three core transformer architecture. Accordingly, first image  1200 C depicts the CT sequentially stripped from the outermost layer towards the centre whilst second image  1200 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1230 A through  1230 E respectively and shielding  1260 . Accordingly, as shown the CT comprises first, second, and third cores  1210 A,  1220 , and  1210 E respectively. Second core  1220  then has first tape layer  1230 A separating the first winding  1240  from it which is then overwound with second tape layer  1230 B. The first, second, and third cores  1210 A,  1220 , and  1210 B respectively with their respective surrounding layers are then overwound with third tape layer  1230 C. Atop third tape layer  1230 C second winding  1250  is wound around first, second, and third cores  1210 A,  1220 , and  1210 B respectively. Second winding  1250  is then overwound by fourth tape layer  1230 D, shielding  1260 , fifth tape layer  1230 E and third winding  1270 . As depicted first winding  1240  corresponds to second secondary winding N 2  of  FIG. 10 , second winding  1250  corresponds to first secondary winding N 1  of  FIG. 10 , and third winding  1270  corresponds to the primary winding N 0  of  FIG. 10 . Optionally, a second shielding may be disposed between the first and second windings  1240  and  1250  respectively such as between second and third tape layers  1230 B and  1230 C respectively. Embedded within third core  1210 B is Hall sensor  1280 . 
     Second image  1200 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1230 A through  1230 E respectively and shielding  1260  removed thereby showing how the first to third windings  1240 ,  1250  and  1270  respectively are wound around the closed magnetic elements forming the first, second, and third cores  1210 A,  1220 , and  1210 B respectively. Also depicted within second image  1200 D is Hall sensor  1280  which may be inserted into a slot machined within the third core  1210 B. It would be evident to one skilled in the art that the number of windings for each of the first to third windings  1240 ,  1250 , and  1270  respectively and geometries of the first, second, and third cores  1210 A,  1220 , and  1210 B respectively may be adjusted according to the electrical voltage, current and power of the signal being measured and design of the Asynchronous Power Measurement System within which the Current Transducer forms part. Accordingly, a Hall sensor  1280  such as described supra in respect of  FIG. 6 , and other variants not depicted, A through  6 C and  FIGS. 11A through 11C  may be inserted into the first, or the third core  1310 A, or  1310 B in order to provide the determination and/or management of a DC field within the Current Transducer. 
     Referring to  FIG. 13  there is depicted a Current Transducer according to an embodiment of the invention such as described supra in respect of Current Transducer (CT)  1000  in  FIG. 10A  employing a dual-core current transformer architecture. Accordingly, first image  1300 E depicts the CT sequentially stripped from the outermost layer towards the centre whilst second image  1300 F depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1330 A through  1330 E respectively and shielding  1360 . Accordingly, as shown the CT comprises a first core comprising first to fourth core elements  1310 A to  1310 D respectively surround a second core  1320 . Second core  1320  then has first tape layer  1330 A separating the first winding  1340  from it which is then overwound with second tape layer  1330 B. The first core (first to fourth core elements  1310 A to  1310 D) and second core  1320  respectively with their respective surrounding layers are then overwound with third tape layer  1330 C. Atop third tape layer  1330 C second winding  1350  is wound around first core (first to fourth core elements  1310 A to  1310 D) and second core  1320 . Second winding  1350  is then overwound by fourth tape layer  1330 D, shielding  1360 , fifth tape layer  1330 E and third winding  1370 . As depicted first winding  1340  corresponds to second secondary winding N 2  of  FIG. 10 , second winding  1350  corresponds to first secondary winding N 1  of  FIG. 10 , and third winding  1370  corresponds to the primary winding N 0  of  FIG. 10 . Optionally a second shielding may be disposed between the first and second windings  1340  and  1350  respectively such as between second and third tape layers  1330 B and  1330 C respectively. 
     Second image  1300 F depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  1330 A through  1330 E respectively and shielding  1360  removed thereby showing how the first to third windings  1340 ,  1350  and  1370  respectively are wound around the closed magnetic elements forming the first, second, and third cores  1310 A,  1320 , and  1310 B respectively. It would be evident to one skilled in the art that the number of windings for each of the first to third windings  1340 ,  1350 , and  1370  respectively and geometries of the first core (first to fourth core elements  1310 A to  1310 D) and second core  1320  respectively may be adjusted according to the electrical voltage, current and power of the signal being measured and design of the Asynchronous Power Measurement System within which the Current Transducer forms part. Further, a Hall sensor  1390  as described supra in respect of  FIGS. 6A through 6C  and  FIGS. 11A through 11C  is disposed within the second core  1320  in order to provide the determination and/or management of a DC field within the Current Transducer. 
     Also depicted in  FIG. 13  disposed upon third first core element  1310 C is a magneto-strictive film  1380  which adjusts a dimension in respect to a magnetic field. Accordingly, the magneto-strictive film  1380  will increase/decrease in length along the axis of third first core element  1310 C when orientated appropriately such that the DC resistance of a thin-film upon the surface of the third first core element  1310 C or the third first core element  1310 C itself varies with the DC field within the third first core element  1310 C. Optionally, magneto-strictive elements may be disposed upon each of the first to fourth first core elements  1310 A through  1310 D respectively, and second core  1320  respectively and coupled to a Processing Circuit for processing in order to define an action, such as an alarm or provisioning of a compensation signal such as described above in respect of  FIGS. 11A through 11C  for example. Optionally, the magneto-strictive element  1380  may be employed in conjunction with a Hall sensor disposed within the second core  1320 . Optionally, multiple Hall sensors  1390  and magneto-strictive elements  1380  may be employed in conjunction with one another within/upon one or more magnetic cores of a Current Transformer. 
     Accordingly, it would be evident that Current Transducers as depicted in respect of  FIGS. 11 through 13  may be amended to incorporate either a tertiary winding N 3  in isolation or a tertiary winding N 3  and quaternary winding N 4  such as described supra in respect of  FIGS. 10A through 10C  for example. Such a configuration is depicted in  FIG. 14  by electrical circuit  1400  of a multi-core current transformer according to an embodiment of the invention. As depicted a CT R    1050 C, as described supra in respect of  FIG. 10C , is augmented with first and second fluxgate coils  1430 A and  1430 B respectively. As depicted each of the first and second fluxgate coils  1430 A and  1430 B respectively are coupled to fluxgate driver  1420  which provides square wave and inverted square wave signals and the output signals from the first and second fluxgate coils  1430 A and  1430 B respectively are coupled to a summation circuit and demodulator (DEMOD)  1410 . Each of the DEMOD  1410  and driver  1420  are coupled to Processing Circuit  1440 . As depicted first and second fluxgate coils  1430 A and  1430 B respectively are excited with equal currents but in opposite directions thereby cancelling the overall effect upon the core of CT R    1050 C. Processing circuit  1440  may provide processing of the DEMOD  1410  in hardware and/or software or a combination thereof. For example, according to an embodiment of the invention processing circuit  1440  provides a square wave signal which comprises only odd harmonics such that effect of any magnetic field within the associated core of CT R    1050 C is to generate distorted output signals with even order harmonics which are filtered from the output of DEMOD  1410  by a second order low pass filter prior to being amplified and coupled to an integrator which also receives the output from the dual-stage current transformer within CT R    1050 C. 
     Within  FIGS. 10C and 14  there are depicted Correction Winding  1   1070  and Correction Winding  2   1060  in conjunction with the first and second secondary windings respectively and their associated cores within the transformer. It would be evident to one skilled in the art that only one or other of the Correction Winding  1   1070  and Correction Winding  2   1060  may be employed. 
       FIG. 15  depicts a two stage current transformer in first and second images  1500 A and  1500 B respectively according to an embodiment of the invention utilizing a third core for the DC bias detection core which when resistively connected as described in  FIG. 14  provides an implementation of a two stage Current Transducer with DC flux detection. Accordingly, the majority of the structures depicted in first and second images  1500 A and  1500 B respectively are common to the descriptions supra in respect of first and second images  1200 C and  1200 D in  FIG. 12  reflecting the third circuit  1000 C in  FIG. 10C . However, in addition to the elements in common with these first and second images  1200 C and  1200 D the first and second images  1500 A and  1500 B also depict first and second fluxgate coils  1430 A and  1430 B respectively together with Compensation Coil  1   1020 . As depicted is second image  1500 B the Compensation Coil  1020  is disposed around first core  1510 , second core  1220 , and third core  1210 B as is primary winding, third winding  1270 . Hall sensor  1280  is depicted disposed within third core  1210 B. Accordingly, in first image  1500 A the Compensation Coil  1020  is now formed upon the fifth tape layer  1230 E upon which is wound second Shield  1530 , sixth tape layer  1540 , and third winding  1270 . 
     Now referring to  FIG. 16  there is depicted a current comparator in first and second images  1600 A and  1600 B respectively according to the embodiment of the invention, utilizing first and second fluxgate coils  1620 A and  1620 E respectively to detect input and output current—turn balance wherein there is no magnetic shield between the magnetic sensor and the primary and secondary windings in contrast to prior art toroidal transformers. Accordingly, as depicted in second image  1600 B the primary coil  1630 , with turns N 1 , and secondary coil  1640 , with turns N 0 , are wound around a single core  1610  together with first and second fluxgate sensors  1620 A and  1620 B respectively. As depicted in first image  1600 A the primary winding  1630 , secondary winding  1640 , and first and second fluxgate sensors  1620 A and  1620 B are wound around the single core  1610  with first tape layer  1650 A. Surrounding all of these are second tape layer  1650 B and shield  1660 . The inventors have established that other magnetic shield(s) can be removed where the toroidal transformer establishes the magnetic flux from the primary winding  1630  primarily through the magnetic core  1610  which is achieved through precision control of the windings in conjunction with a high quality magnetic core and low loading from the secondary winding  1640 . Alternatively, the magnetic core if the current comparator depicted within first and second images  1600 A and  1600 B of  FIG. 16  may be a dual-core or multi-core design. 
     Within an embodiment of the invention operation of the current comparator depicted in  FIG. 16  exploits the magnetic core  1610  as part of a magnetic field sensing apparatus continuously magnetized back and forth from saturation in one direction to saturation in the other direction wherein the time required to drive the magnetic core from saturation to saturation is used as a measure of the magnetic field strength. Within another embodiment of the invention two magnetic cores are employed in conjunction with a push-pull drive circuit for driving them from saturation to saturation thereby producing a differential output signal which beneficially reduces the coupling effects of the higher power magnetic drive circuit on the lower level output signal. 
     Referring to  FIGS. 17A and 17B  there are depicted current comparators according to an embodiment of the invention utilizing a Hall Effect magnetic sensor  1710  embedded within the magnetic core  1720  of the current comparator to detect input and output current—turn balance wherein the prior art magnetic shield between the magnetic sensor and the primary and secondary windings has been removed. As depicted in the cross-section of the current comparator comprises the Hall Effect magnetic sensor  1710  “around” which are wound the primary coil  1630  and secondary coil  1640  with the assembly then surrounded by magnetic shield  1730  which shields the current comparator from external magnetic fields. Optionally, a magnetic circuit may be employed in conjunction with the configuration depicted in  FIG. 17A  in order to concentrate magnetic field on the Hall effect magnetic sensor  1710  depending upon the geometry of the Hall effect magnetic sensor  1710  and the primary and second coils  1630  and  1640  respectively. However, adding such a magnetic element introduces hysteresis and impacts accuracy. 
     In contrast in  FIG. 17B  the primary coil  1630  is formed below the Hall Effect magnetic sensor  1710  and the secondary coil  1640  is formed above it. In this manner the primary and secondary coils  1630  and  1640  respectively may be manufactured and characterized independent from the overall transformer. Optionally, as with  FIG. 17A  magnetic field concentrator(s) may be employed to concentrate the magnetic field on the Hall effect magnetic sensor  1710 . 
     The current comparator depicted in  FIG. 16  represents a design wherein the primary and secondary coil windings are implemented directly on the magnetic core. In contrast the current comparator depicted in  FIGS. 17A and 17B  exploits a magnetic sensor (Hall Effect) and may be implemented as a “planar” design although it may also be made as a toroid and may employ a number of Hall Effect (or other) sensors, or a single sensor with the magnetic field concentrator, for example a magnetic core with a cut slot. 
     Referring to  FIG. 18  there is depicted an active current to current transducer (AC-CT)  1800  according to the embodiment of the invention utilizing current comparator with a magnetic sensor  1810  within magnetic core  1870  and an amplification block  1820  to produce AC and DC output current in precise ratio to the input current. Accordingly, an input current I I  within a primary coil  1850  induces a magnetic flux within the magnetic core  1870  which is detected by magnetic sensor  1810 . The output of the magnetic sensor  1810  is amplified by amplification block  1820  and coupled to the secondary coil  1840 . Accordingly, the operation of the AC-CT  1800  may be viewed as an AC amplifier with transformer feedback although the operation is significantly different in that within the AC-CT  1800  the aim, rather than compensate the input voltage with the transformed output voltage, is to compensate a first magnetic flux generated by the current flowing within the input winding with a magnetic flux generated in the output winding, such that the overall induced magnetic flux as measured by the magnetic sensor  1810  is approximately equal to zero. It would be evident to one skilled in the art that this scheme is good for both AC current transduction as well as DC transduction. The concept of the AC-CT  1800  is similar to that employed within DC comparator resistance bridges. The physical implementations of AC-CT  1800 , in common with the current transducers depicted in  FIGS. 16 and 17 , are absent magnetic shield(s) except external to the overall assembly in order to protect the current transducers from external magnetic fields only. However, such external magnetic shields are not essential from the conceptual viewpoint although they will be beneficial in reducing external electromagnetic interference fields do lower the “noise” level of the implementations. 
     Alternatively, with respect to embodiments of the invention, the transformer may be shell form or a combination of core and shell forms. Shell form designs may be more prevalent than core form designs for distribution transformer applications due to the relative ease in stacking the core around the winding coils. Core form designs tend to, as a general rule, be more economical, and therefore more prevalent, than shell form designs for high voltage power transformer applications at the lower end of their voltage and power rating ranges. At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra high voltage and higher MVA applications because, though more labor intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage. However, it would be evident that embodiments of the invention may be applied to core form, shell form, and combination core-shell form transformers. 
     Within the descriptions presented supra in respect of  FIGS. 10A through 14  the determination of corrections and alarms has been presented based upon determinations of DC magnetic fields arising from DC currents in respect to measurements of AC currents. In respect of corrections these are described primarily as being applied through the generation of opposing magnetic fields within the Current Transducer or the triggering of an alarm in respect of terminating a measurement, providing a warning, or truncating the measurements to a predetermined accuracy for example. However, as depicted in  FIGS. 1 and 2  there is a surface or plurality of surfaces relating the error in an AC current measurement to the DC current and the AC current. Accordingly, within another embodiment of the invention the Processing Circuit depicted within  FIGS. 11A through 11C  may digitize the measured AC current and apply one or more corrections based upon one or more corrective algorithms to the digitized AC current based upon characterisation of these one or more surfaces. Such algorithms may be common to all measurement systems exploiting common coefficients or may be common algorithms exploiting coefficients derived from a characterisation of the Current Transducer wherein the derived coefficients are stored within a memory associated with the Processing Circuit. 
     Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
     Implementation of the techniques, blocks, steps and means described above may be done in various ways. For example, these techniques, blocks, steps and means may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described above and/or a combination thereof. 
     The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.