Abstract:
Precision AC voltage, current, phase, power and energy measurements and calibrations with current ranges from 1 uA to 20 kA and voltage ranges from 1V to 1000 kV are now performed with accuracies of better than one part per million. Continued demand for improved accuracy has led the inventors to address improvements to dual stage and multi-stage current transducers that may form the basis of the measuring process within many of the measurement instruments providing the precision AC measurements and calibrations. Additionally, the improvements to dual stage and multi-stage current transducers provide for novel feedback controlled precision AC current sources without requiring measurement of the AC current source output directly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of priority from U.S. Provisional Patent Application US 61/968,557 filed Mar. 21, 2014 entitled “Methods and Devices for AC Current Sources, Precision Current Transducers and Detectors,” the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to precision AC current sources, precision current transducers, and measurements, which include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. In particular it relates to precision AC current sources, precision current transducers, and measurements using enhanced dual stage current transducers. 
       BACKGROUND OF THE INVENTION 
       [0003]    Alternating Current (AC) electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including, for example, voltage, current, capacitance, impedance, frequency, phase, power, energy, and resistance. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment range 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 operating at voltages of hundreds of mV and currents of a few mA. 
         [0004]    Within a variety of AC current applications and AC current test equipment systems AC comparator bridges and AC current transformers are employed to provide the required dynamic range, accuracy, and flexibility. AC current bridge configurations remove many of the issues associated with achieving making measurements at accuracies of a part, or few parts per million, such as insensitivity to lead resistances, excellent ratio linearity, excellent ratio stability, and a high level of resolution. AC current transformers, importantly, isolate the measuring instruments from what may be very high voltage in the monitored circuit and when the current in a circuit is too high to be directly applied to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. They also allow accurate high current generation from precision lower current sources and isolation of the precision source from external variations. 
         [0005]    Accordingly many sources and measurement systems for alternating current power systems have a current transformer at their output and input stages respectively. Over the past approximately 180 years whilst a wide variety of types of electrical transformer are made for different purposes these, despite their design differences, employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts. Over this period many techniques have been developed to improve the accuracy of the current transformer. Among them, the dual stage current transformer, described in the work of Brooks and Holtz in “The Two-Stage Current Transformer” (AIEE Trans., Vol.41, pp382-393, 1922) still forms the basis for a significant proportion of commercial systems. These transformers are generally what is referred to as “step down transformers” for converting high voltage-low current inputs to lower voltage-higher current outputs. 
         [0006]    However, in a range of other applications within electrical systems and measurement systems what is required are precision AC current sources and AC amplifiers. The inventors have found that improvement of the accuracy when designing a precision AC current source is a different problem to measurement systems in that we either wish to remove measuring equipment connected to the output circuit to provide the feedback or wish that the generation and measurement of even very large current AC current sources is performed without requiring the use of a shunt. 
         [0007]    Accordingly, the inventors have established design and circuit methodologies which are applicable to precision AC current sources, amplifiers, and also AC current measurements. Such measurements include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. Similarly, precision AC current sources and amplifiers for test, measurement, and supply applications are desirable over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. 
         [0008]    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 
       [0009]    It is an object of the present invention to provide improvements over the prior art in respect of AC precision current sources, amplifiers, and measurements, which include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. In particular it relates to precision AC current sources, precision current transducers, and measurements using enhanced dual stage current transducers. 
         [0010]    In accordance with an embodiment of the invention there is provided a device comprising:
       a dual stage current transducer comprising at least a primary winding, a first secondary winding and a second secondary winding;   a first four terminal shunt coupled across the first secondary winding; and   a second four terminal shunt coupled across the second secondary winding; wherein   a first voltage generated across the second four terminal shunt is subtracted from a second voltage generated across the first four terminal shunt.       
 
         [0015]    In accordance with an embodiment of the invention there is provided a device comprising:
       a dual stage current transducer comprising at least a primary winding, a first secondary winding and a second secondary winding;   a first four terminal shunt coupled across the first secondary winding;   a second four terminal shunt coupled across the second secondary winding;   an alternating current source disposed between the first secondary winding and first four terminal shunt; and   a third four terminal shunt coupled in series with a load across the primary winding.       
 
         [0021]    In accordance with an embodiment of the invention there is provided a method comprising providing a multi-stage current transducer with a first means to obtain a first voltage proportional to a primary current of said multi-stage current transducer and a second means to obtain a second voltage proportional to a secondary current in a second stage of the multi-stage current transducer, said secondary current being proportional to the magnetizing current of the magnetic core of a first stage of the multi-stage current transducer. 
         [0022]    In accordance with an embodiment of the invention there is provided a method comprising:
       providing a current transducer having two stages where current of a first secondary of the current transducer passes through a first four terminal shunt and a current of a second secondary of the current transducer passes through a second four terminal shunt;   summing the voltages from the first and second four terminal shunts to represent the instantaneous value of the primary current within the current transducer; and   at least one of:
           digitizing the resulting summed voltage; and   providing the current transducer comprises providing a first magnetic core of the current transducer in the form of a hollow toroid and a second magnetic core of the current transducer in the form of a toroid core embedded within the first magnetic core.   
               
 
         [0028]    In accordance with an embodiment of the invention there is provided a method comprising providing a bridge for establishing the value of the resistance and the inductance of a load, the bridge comprising a current transducer having two stages and first to third four terminal shunts, wherein a first current within a first secondary of the current transducer passes through a first four terminal shunt, a second current within a second secondary of the current transducer passes through a second four terminal shunt and a third current passing through the load disposed across a primary of the current transducer also passes through the third four terminal shunt. 
         [0029]    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 
         [0030]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0031]      FIG. 1  depicts an AC current transformer according to the prior art using a dual stage current transformer design of Brooks and Holtz; 
           [0032]      FIG. 2A  depicts a circuit using a dual stage AC current transducer with electrical shield and four terminal resistor for improved accuracy; 
           [0033]      FIG. 2B  depicts a circuit using dual stage AC current transducers with a transimpedance amplifier to sum the currents from the first and second stages; 
           [0034]      FIG. 3A  depicts an AC dual stage current transducer using dual independent resistors for improved accuracy according to an embodiment of the invention; 
           [0035]      FIG. 3B  depicts a precision AC current source exploiting a dual stage current transducer, dual independent load resistors, and a programmable controlled current source according to an embodiment of the invention; 
           [0036]      FIG. 4A  depicts a precision AC current source exploiting a dual stage current transducer, dual independent load resistors, and with an adjustable uncontrolled current source according to an embodiment of the invention transducer; 
           [0037]      FIG. 4B  depicts a precision AC current source exploiting a dual stage current transducer with adjustable uncontrolled current source and a transimpedance amplifier to sum the currents from the first and second stages according to an embodiment of the invention; 
           [0038]      FIGS. 5A through 5C  depict high current AC shunt calibrators according to embodiments of the invention exploiting a dual stage current transducers 
           [0039]      FIG. 6  depicts dual stage current transducer designs exploiting core-in-core, dual core, and triple core designs to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2A through 5B ; 
           [0040]      FIG. 7  depicts a dual stage current transducer design exploiting a core-in-core design to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2A through 5B ; and 
           [0041]      FIG. 8  depicts a dual stage current transducer design exploiting a three core design to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2A through 5B . 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    The present invention is directed to improvements over the prior art in respect of AC precision current sources, precision current transducers, and measurements, which include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. In particular it relates to precision AC current sources, precision current transducers, and measurements using enhanced dual stage current transducers. 
         [0043]    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. 
         [0044]    Within the drawings presented in respect of this specification elements having the same number are the same element and may or may not be referenced explicitly in every drawing due to the recurring elements being commonly numbered. 
         [0045]    Dual stage transformers as known by one of skill in the art comprise three windings together with one or more magnetic cores. These three windings are commonly referred to as the primary winding, to which the signal to be transformed by the dual stage transformer is coupled, the first secondary winding, from which the transformed signal is coupled, and the second secondary winding (also known as the correction winding), from which a signal (commonly referred to as the correction signal) is coupled. The primary winding and first secondary winding may in some embodiments of the dual stage transformer be conceptually identical and coupled with the same magnetic flux and can be, for example, swapped to reverse the dual stage transformer operation. In contrast the second secondary winding cannot be swapped with either of the main windings, namely the primary winding and first secondary winding. Within this document, except for the claims and the summary of the invention where the terms first secondary winding and second secondary winding are maintained, the first secondary winding will be referred to as the “secondary winding” (with the current flowing within it referred to as the secondary current) and the second secondary winding will be referred to as the correction winding (with the current flowing within it referred to as the correction current). 
         [0046]    Referring to  FIG. 1  there is depicted a dual stage transformer  100  according to the prior art of Brooks and Holtz, see “The Two-Stage Current Transformer” (AIEE Trans., Vol. 41, pp382-393, 1922). Within this the current transformation is effected in two stages, the first generated by secondary winding N 1    110 B in response to the signal coupled to the primary winding N 0    110 A, which is approximately correct in magnitude and phase. The second stage is the generation of an auxiliary corrective current via correction winding N 2    110 C which, when combined with the secondary current, gives a resultant current which very closely approximates to the secondary current which would be furnished by an ideal current transformer having no errors. As depicted the secondary and correction windings  110 B and  110 C respectively are coupled across load resistor  120  generating a potential across first and second outputs  100 A and  100 B respectively which are coupled to circuit  130 , which for example contains one or more analog-to-digital converters (ADCs) as part of measuring the converted signal. According to the ratio of the turns in the primary winding  110 A to the secondary and correction windings  110 B and  110 C the resulting output may be scaled up from the input signal, scaled down, or even be simply equal such that the measurement circuit, e.g. circuit  130 , is buffered from the input signal carrying circuit. 
         [0047]    Referring to  FIG. 2A  there is depicted a dual stage transducer  200 A wherein a current transducer CT R    2000  again comprises primary winding  2000 A and secondary and correction windings  2000 B and  2000 C but now a shield  130  is disposed between the primary winding  2000 A and the magnetic core of the current transducer  2000  and coupled via shield port Sh  200 C to circuit  230 . Also within dual stage transducer  200  the load resistor  120  is replaced by a non-inductive four terminal shunt R  210  thereby increasing the accuracy of the reproduced voltage across the signal output ports  200 A and  200 B respectively. The non-inductive four terminal shunt R  210  may, for example, be a “Kelvin” configuration resistor with four terminals (via leads) allowing a current to be applied though a pair of opposite leads and the voltage to be sensed across the other pair of opposite leads. The “Kelvin” configuration effectively eliminates the resistance and temperature dependence of the leads. 
         [0048]    Referring to  FIG. 2B  there is depicted a dual stage transducer  200 B wherein a current transducer CT R    2000  again comprises primary winding  2000 A and secondary and correction windings  2000 B and  2000 C respectively with the shield  130  disposed between the primary winding  2000 A and the magnetic core of the current transducer  2000  and coupled via shield port Sh  200 C to circuit  230 . However, in dual stage transducer  200 B the non-inductive four terminal shunt R  210  employed within dual stage transducer  200 A in  FIG. 2A  is replaced by a transimpedance amplifier (TIA)  250  with feedback resistor  220 . As depicted one side of each the secondary and correction windings  200 A and  200 B respectively are coupled to the positive input port of the TIA  250  whilst the negative input port of the TIA  250  is coupled to the other side of each the secondary and correction windings  200 A and  200 B respectively. The output of the TIA  250  being coupled to first output port  200 A whilst the sides of the secondary and correction windings  200 A and  200 B respectively are coupled to the second output port  200 B. 
         [0049]    However, in many test and measurement applications even the enhanced current reproduction and error reduction of dual stage transducers  200 A and  200 B is insufficient. 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 at accuracies of parts per million is being replaced by accuracies of parts per billion. Accordingly, referring to  FIG. 3A  there is depicted a dual stage transducer  300  according to an embodiment of the invention employing a CT R    2000  in conjunction with first and second four (4) terminal resistors (4TeR)  310  and  320  respectively. As depicted the secondary current from the secondary winding  2000 B is connected to the first 4TeR  310  and the second secondary current from the correction winding  2000 C is connected to the second 4TeR  320 , e.g. the magnetizing current of the first stage transducer is coupled to this resistor. If, now the first and second 4TeR  310  and  320  are serially connected on their voltage measurement terminals then the sum of these two voltages are an accurate replica of the current being measured. This arises in part due to the fact, that the required induced voltage in the second core of the CT R    2000  is much smaller and consequently the remaining error of the magnetizing current of the second stage is negligible. 
         [0050]    Within some embodiments of the invention the second stage (correction) current and voltage within the dual stage transducer are small and accordingly, depending upon the precision of the source, measurement circuit, etc. that they form part of, the precision 4TeR  320  may be replaced with a suitably tolerance two terminal resistor. 
         [0051]    Optionally, to obtain an even more accurate voltage proportional to the magnetizing current of the second stage an amplifier, e.g. an electronic amplifier, may be employed such that the voltage across the correction winding  2000 C is reduced even further. Accordingly, the error due to the magnetizing current of the second stage, which is related to the voltage drop on the impedance of that correction winding, denoted Z 2 , is negligible because this current is small but the error due to the voltage on the prior art four-terminal shunt resistor R  210  is significant. 
         [0052]    In addition to improved accuracy in calibration, standards, and measurements on circuits and components arising from the measurement circuits themselves a corresponding drive in improved accuracy exists in the design and implementation of precision sources of alternating current within test and measurement instrumentation. Whilst this may appear a different problem to that of the measurement circuit the inventors have realized that actually the technique to solve it is similar to that depicted in  FIG. 3A  in respect of enhanced accuracy current measurements. Accordingly, referring to  FIG. 3B  there is depicted a precision AC current source (PACCS)  350  according to an embodiment of the invention exploiting a current transducer CT R    2000  such as described supra in respect of  FIG. 3A . Accordingly, a controlled current source  410  has been inserted into the circuit loop comprising secondary winding  2000 B and first 4TeR R1  310 . The controlled current source  410  is coupled to the control circuit  430  via control port  400 C. Beneficially the PACCS  350  allows a voltage proportional to the output current of an AC transconductance amplifier to be precisely obtained without the measuring equipment being connected to the output circuit. As the controlled current source  410  is coupled to the secondary winding  2000 B and the load Z  420  is coupled across the primary winding  2000 A then strictly the secondary winding  2000 B is the “primary winding” and primary winding  2000 A the “secondary winding” of the PACCS  350  whilst the correction winding is essentially unchanged. 
         [0053]    Accordingly, feedback information for the regulation of the controlled current source  410  within the PACCS  350  is derived from the output of the PACCS  350 , this being the voltage on the first 4TeR R1  310  from which is subtracted the voltage on the second 4TeR R2  320  generated by the current flowing within a second current loop comprising second 4TeR R2  320  and correction winding  2000 C of the CT R    2000 . Accordingly, this output voltage V across terminals  400 A and  400 B is proportional to the output current and hence can be used as feedback information. It would be evident that measuring this output voltage V using an analog-to-digital converter (ADC) would allow the value of the output secondary current to be obtained in digital form for use within a digital feedback loop to the programmable current source  410 . Alternatively, an analog feedback loop may be employed but it should be emphasized that in either instance the current is measured without connecting any measuring device in the output circuit, a very significant feature against prior art precision current sources with feedback. Further, the problem of generating and measuring even very large currents are addressed without the requirement for using shunts. 
         [0054]    Within some embodiments of the invention, such as depicted by first and second PACCS  400 A and  400 B respectively in  FIGS. 4A and 4B , the controlled current source  410  which is part of a feedback control loop for the PACCS  350  may be replaced by adjustable and programmable AC current sources  440  and  460  respectively. As depicted in  FIG. 4A  first PACCS  400 A which exploits first 4TeR R1  310  and second 4TeR R2  320  in conjunction with adjustable AC current source  440  is not coupled to the control circuit  430 . However, the signal level of the adjustable AC current source  440  may be set, thereby setting the output current supplied to load Z  420 , such that subsequently the value of the current is measured, and this is then used for calibration by the control circuit  430 . In contrast in  FIG. 4B  second PACCS  400 B exploits programmable AC current source  460  which is programmed via a digital control word through data port  400 C allowing the control circuit  430  to establish multiple settings for the PACCS  400 B. Accordingly, the load Z  420  is driven at multiple output currents/voltages and feedback to the control circuit  450  is achieved through TIA  250 . 
         [0055]    Accordingly, referring to  FIGS. 3B ,  4 A and  4 B there are depicted precision AC current sources (PACCS)  350 ,  400 A and  400 B respectively according to an embodiment of the invention exploiting a current transducer CT R    2000  such as described supra. Beneficially each of PACCS  350 ,  400 A, and  400 B allow a voltage proportional to the output current of the PACCS to be precisely obtained without requiring that precision measuring equipment is connected to the output circuit together with the load Z  420   s.    
         [0056]    The embodiments of the invention described above in respect of  FIGS. 2A through 4B  assume that the voltage induced in the uniformly wound coil on the toroidal magnetic core of the CT R    2000  is proportional to the total ampere-turns passing through the opening of that magnetic core and consequently that only the magnetizing current is causing the error. However, with the development of the current comparator, see for example Miljanic et al. in “The Development of the Current Comparator: A High Accuracy AC Ratio Measuring Device” (IEEE Part 1: Comm. &amp; Elect., Vol 81(5), pp 359-368), it was shown that the voltage induced in the winding wound on the toroidal magnetic core measures the total ampere-turn passing through its opening only if it is shielded from stray magnetic and electric fields. Accordingly, for embodiments of the invention as described in respect of  FIG. 6  below a shield, for example a hollow toroid of the magnetic material which surrounds the measuring core situated in its interior and/or a copper tape/box for electrical shielding. 
         [0057]    It would be evident to one skilled in the art that the PACCS  400  may be considered as a combination of a dual stage current transducer and a shielded current comparator wherein the magnetic shield of the current comparator is used as the magnetic core of the first stage of the dual stage current transducer, and the detection winding of the current comparator is actually the second stage of the dual stage current transducer. 
         [0058]    Referring to  FIG. 5A  there is depicted an AC shunt calibrator  550 A, particularly for their calibration at high current. Accordingly, as depicted a PACCS 5000, which is depicted as PACCS  350  in  FIG. 3B  with controlled current source  410  disposed within the circuit comprising the secondary winding of a CT R    2000  in conjunction with first 4TeR R1  310 . The calibration AC shunt R3  510  is a 4 terminal resistor wherein the other terminals are coupled to the control circuit  530  as Out 1 and Out 2 at ports  500 A and  500 B respectively. The H and L outputs from PACCS  350  are now depicted as Out 3 and Out 4 at ports  500 C and  500 D respectively which are also coupled to the control circuit  530 . Also connected to PACCS  350  from the control circuit  530  are Shy  200 C for the shield within CT R    2000  and control  400 C whilst a microprocessor  540  is coupled to the control circuit  530 . Accordingly, the AC shunt calibrator  550 A can be calibrated over a range of test conditions, established through the setting of the controlled current source  410  under the action of the control circuit  530 , by determining the current via Out 3 and Out 4 on ports  500 C and  500 D together with the voltage across the calibration AC shunt  510  via Out 1 and Out 2 on ports  500 A and  500 B. These may be measured using two independent voltmeters (or ADCs). This configuration provides flexibility in where and if a common ground connection is made. 
         [0059]    Now referring to  FIG. 5B  there is depicted an AC shunt calibrator  550 B according to an embodiment of the invention wherein PACCS 5500, which is depicted as comprising PACCS  400 B in  FIG. 4B  except that the programmable AC current source  460  has been replaced with adjustable AC current source  440  such that there is no control/data signal to the current source within the PACCS 5500 from the control circuit  530 . Referring to  FIG. 5C  there is depicted a variant of AC shunt calibrator  550 A in AC shunt calibrator  550 C wherein output Out 3  500 C, coupled to first 4TeR R1  310  is still coupled to control circuit  530  but is grounded. However, now Out 4  500 D, which is coupled to second 4TeR R2  320 , is coupled with Out 2  500 B and therein the control circuit  530  rather than directly to the control circuit. Accordingly, variations in the output of the PACCS 5000 which are applied to the Load Z  420  under test are automatically applied to the measured current flowing in the load circuit via third 4TeR R3  510 . 
         [0060]    Within the embodiments of the invention depicted supra in respect of precision AC current sources  FIGS. 3A ,  4 A and  4 B and exploited within the AC shunt calibrators  550 A to  550 C in  FIGS. 5A to 5C  respectively current sources are employed in conjunction with the secondary winding. However, in other embodiments of the invention these current sources may be an AC power source in order to drive power shunts during calibration. Such AC power sources may range from 50 W to 1000 W, for example. 
         [0061]    Referring to  FIGS. 6  there are depicted first to third dual stage current transducers (2SCT R )  600 A to  600 C respectively designs exploiting core-in-core, dual core, and triple core designs respectively to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2 through 5 . Referring to first 2SCT R    600 A a magnetic shield, the first stage magnetic core  610 , has wound around it primary winding  620  and secondary winding  630 . Disposed within the magnetic shield are electric shield  650  and correction winding  640  which surround measuring toroid, second stage magnetic core  660 . 
         [0062]    Second 2SCT R    600 B depicts the same elements except that now the first stage magnetic core  610  and second stage magnetic core  660  are a pair of parallel toroids wherein the primary winding  620  and secondary winding  630  surround both as does the electrical shield  650 . The correction winding  640  then surrounds only the second stage magnetic core  660 . Similarly in third 2SCT R    600 C depicts the same elements except that now the first stage magnetic core comprises first and second core elements  610 A and  610 B respectively and these, in conjunction with the second stage magnetic core  660  are a triplet of parallel toroids. Accordingly, in third 2SCT R    600 C the primary winding  620  and secondary winding  630  surround the first and second core elements  610 A and  610 B and second stage magnetic core  660 . The electrical shield  650  surrounds only the second stage magnetic core  660  as does the correction winding  640 . Other embodiments of a 2SCT R  may be envisioned without departing from the scope of the invention. 
         [0063]      FIG. 7  depicts a dual stage current transducer (2SCT R ) design exploiting a core-in-core design to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2 through 5  and first 2SCT R    600 A. Accordingly first image  700 C depicts the 2SCT R  sequentially stripped from the outermost layer towards the centre whilst second image  700 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  730 A through  730 E respectively and shielding  760  removed for clarity. Accordingly as shown the 2SCT R  comprises a first core comprising first to fourth core elements  710 A to  710 D respectively surround a second core  720 . Second core  720  then has first tape layer  730 A separating the first winding  740  from it which is then overwound with second tape layer  730 B. The first core (first to fourth core elements  710 A to  710 D) and second core  720  respectively with their respective surrounding layers are then overwound with third tape layer  730 C. Atop third tape layer  730 C second winding  750  is wound around first core (first to fourth core elements  710 A to  710 D) and second core  720 . Second winding  750  is then overwound by fourth tape layer  730 D, shielding  760 , fifth tape layer  730 E and third winding  770 . 
         [0064]    As depicted first winding  740  corresponds to correction winding N 2  of  FIG. 10 , second winding  750  corresponds to secondary winding N 1  of  FIG. 10 , and third winding  770  corresponds to the primary winding N 0  of  FIG. 10 . Optionally a second shielding may be disposed between the first and second windings  740  and  750  respectively such as between second and third tape layers  730 B and  730 C respectively. Second image  700 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  730 A through  730 E respectively and shielding  760  removed thereby showing how the first to third windings  740 ,  750  and  770  respectively are wound around the closed magnetic elements forming the first, second, and third cores  710 A,  720 , and  710 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  740 ,  750 , and  770  respectively and geometries of the first core (first to fourth core elements  710 A to  710 D) and second core  720  respectively may be adjusted according to the electrical voltage, current and power of the signal being measured and/or generated. 
         [0065]      FIG. 8  depicts a dual stage current transducer (2SCT R ) design exploiting a three-core design to provide AC devices according to embodiments of the invention as described in respect of  FIGS. 2 through 5  and third 2SCT R    600 C. Accordingly first image  800 C depicts the CT sequentially stripped from the outermost layer towards the centre whilst second image  800 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  830 A through  830 E respectively and shielding  860 . Accordingly as shown the CT comprises first, second, and third cores  810 A,  820 , and  810 B respectively. Second core  820  then has first tape layer  830 A separating the first winding  840  from it which is then overwound with second tape layer  830 B. The first, second, and third cores  810 A,  820 , and  810 B respectively with their respective surrounding layers are then overwound with third tape layer  830 C. Atop third tape layer  830 C second winding  850  is wound around first, second, and third cores  810 A,  820 , and  810 B respectively. Second winding  850  is then overwound by fourth tape layer  830 D, shielding  860 , fifth tape layer  830 E and third winding  870 . As depicted first winding  840  corresponds to correction winding N 2  of  FIG. 10 , second winding  850  corresponds to secondary winding N 1  of  FIG. 10 , and third winding  870  corresponds to the primary winding N 0  of  FIG. 10 . Optionally a second shielding may be disposed between the first and second windings  840  and  850  respectively such as between second and third tape layers  830 B and  830 C respectively. 
         [0066]    Second image  800 D depicts a three dimensional quarter-cut sectional view with first to fifth tape layers  830 A through  830 E respectively and shielding  860  removed thereby showing how the first to third windings  840 ,  850  and  870  respectively are wound around the closed magnetic elements forming the first, second, and third cores  810 A,  820 , and  810 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  840 ,  850 , and  870  respectively and geometries of the first, second, and third cores  810 A,  820 , and  810 B respectively may be adjusted according to the electrical voltage, current and power of the signal being measured and/or generated. 
         [0067]    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. 
         [0068]    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. 
         [0069]    Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.