Abstract:
A device for sensing electrical current or voltage in an electrical distribution system using an actively compensated current ratio transformer that includes a first magnetic core having a first permeability and a second magnetic core having a second permeability higher than the first permeability. A primary winding having P turns is coupled with the first and second magnetic cores, a measurement winding having M turns is coupled with the first and second magnetic cores so that current in the primary winding induces current in the measurement winding, and a sense winding having S turns is coupled with the second magnetic core. An amplifier coupled to the sense winding receives a voltage developed across the sense winding and produces a compensation current in response to the received voltage. The amplifier has an output coupled to the sense winding to feed the compensation current through the sense winding to reduce the voltage developed across the sense winding voltage to substantially zero. A burden resistor is coupled to the measurement winding and the sense winding for receiving the sum of the current induced in the measurement winding and the compensation current.

Description:
FIELD OF THE INVENTION 
     This invention relates to precision, alternating current and voltage ratio transformation having primary utility in the accurate measurement of higher current or voltage signals applicable to the field of digital power measurement apparatus having fundamental application in 50-60 Hertz AC power systems. 
     BACKGROUND 
     Traditional digital power meters typically employ conventional passive internal current transformers and resistive potential dividers in order to reduce relatively large input currents and voltages by a defined and calibrated ratio down to lower currents and voltages that are readily sampled and converted into a digital representation for further signal processing. Current transformers and resistive potential dividers additionally provide much needed electrical isolation between the external current and voltage signals being measured. With potential dividers, the isolation is afforded by providing a robust (transient overload) and high divider impedance (typically &gt;1 Meg ohm) between the voltage source and the digital power meter input circuitry. Current transformation ratios of 1:1000 are common with (but not limited to) typical nominal primary current levels of 1, 5, or 20 Amps in the case of transformer-connected power meters. Voltage transformation ratios of 200:1 are common with (but not limited to) typical nominal voltage inputs ranging from 67 to 600 Vac. Accurate current and voltage transformation, in both magnitude and phase, is required, particularly when AC power calculations are being made at low power factors. Amplitude error of less than +/−100 ppm, combined with phase shift errors of less than +/−1 minute, are required by the newest generation of Class 0.1 digital power meters. Accuracies must be maintained over widely varying signal amplitudes and environmental conditions. Accuracy at higher current and voltage signal frequencies well beyond fundamental 60 Hz power signals are becoming common, particularly when harmonic representation, power quality, and transient analysis is required. 
     Conventional current ratio transformers suffer from a fundamental electromagnetic limitation that directly impacts their effective use in modern sophisticated digital power meters, particularly the new class of power quality meters requiring high accuracy (Class 0.1), wide dynamic range, stability, and frequency response. This limitation is due to the fact that a portion of the primary input current being measured is required to magnetize the core. This magnetization current component is complex in magnitude and phase and directly impacts the ratio and phase error of the current ratio transformer output current. Core magetization effects may also impact accuracy by shifting the transformer flux swing operating point. Larger, high permeability cores are typically used in order to minimize the effects of core magnetization loss. These undesirable effects are only reduced and not eliminated through the use of such cores. Tape wound torroidal cores, made of ultra high permeability magnetic alloys, such as Molypermalloy, Supermalloy, and Amorphous Glass, may be required to meet the 60 Hz accuracy specifications, but issues of cost, size, and accuracy often limit their inclusion in new high performance designs. 
     A conventional potential divider used for power meter AC input voltage division typically utilizes high valued resistors in order to safely divide the input signal to low levels compatible with conventional electronic analog to digital conversion circuitry. The divider input resistor values must also be of high value in order to limit power dissipation under nominal and overload conditions while reducing leakage currents to safe levels. Unfortunately, the use of such high value resistor divider chains can result in temperature, humidity, capacitive, and thermal noise induced stability issues. The use of high precision matched resistive dividers (e.g., metal foil) are generally required for high accuracy applications but come at a high cost factor. 
     The continuing trend of increased digital power meter performance, particularly in areas of accuracy and frequency response, requires a new and improved approach. 
     BRIEF SUMMARY 
     The present disclosure provides a device for sensing electrical current or voltage in an electrical distribution system using an actively compensated current ratio transformer that includes a first magnetic core having a first permeability and a second magnetic core having a second permeability higher than the first permeability. A primary winding having P turns is coupled with the first and second magnetic cores and is connected to a source of current to be measured. A measurement winding having M turns is coupled with the first and second magnetic cores so that the current to be measured in the primary winding induces current in the measurement winding, and a sense winding having S turns is coupled with the second magnetic core. An amplifier coupled to the sense winding receives a voltage developed across the sense winding and produces a compensation current in response to the received voltage. The amplifier has an output coupled to the sense winding to feed the compensation current through the sense winding to reduce the voltage developed across the sense winding voltage to substantially zero. A burden resistor is coupled to the measurement winding and the sense winding for receiving the sum of the current induced in the measurement winding and the compensation current. The summing of the compensation current with the current induced in the measurement winding preferably compensates for magnetization losses in the first magnetic core, so that the voltage produced across the burden resistor is substantially proportional to the current to be measured in the primary winding multiplied by the ratio P/M. 
     In one embodiment, the measurement winding has a greater number of turns than the sense winding, and an attenuation circuit attenuates the compensation current, before the summing of the compensation current with the current induced in the measurement winding, to compensate for the difference between the number of turns in the sense winding and the number of turns in the measurement winding. 
     In one implementation, the permeability of the second magnetic core is at least three times the permeability of the first magnetic core and is substantially independent of temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an electrical schematic diagram of a current transformer embodying the invention. 
         FIG. 2  is an electrical schematic diagram of a voltage transformer embodying the invention. 
         FIG. 3  is a sectioned perspective view of an actively compensated current or voltage ratio transformer. 
         FIG. 4  is a block diagram of a digital power meter including the current and voltage transformers of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. 
       FIG. 1  shows an actively compensated current ratio transformer device having a lower permeability first “main” core  10 , and a higher permeability second “sense” core  11  physically positioned in a stacked arrangement as shown in  FIG. 3 . The main core is made of a lower cost and lower permeability material (such as a ferrite, e.g., Ferroxcube 3E6 Ferrite), while the sense core  11  is made of a higher permeability metal amorphous core material (such as a base metal composition, e.g., Vacuuschmelze Vitroperm). This combination maximizes accuracy and stability while maintaining a low overall component cost. 
     Referring to  FIG. 1 , a primary winding  12 , having P turns, couples magnetically with both the lower permeability main core  10  and the higher permeability sense core  11 . For current transformer use, the primary winding  12  is nominally, but not limited to, a single-turn conductor. The measurement current of interest is the primary winding current Ip flowing in the primary winding  12 . 
     A measurement winding  13 , having M turns, also couples magnetically with both the lower permeability first main core  10  and the second higher permeability sense core  6 . The turns ratio of the measurement winding  13  to the primary winding  12  is nominally of high value in order to reduce the primary winding current Ip to an acceptable (galvanically isolated) lower level for measurement as a voltage developed across burden resistor R 4  as the result of a burden current Ib flowing through the burden resistor R 4 . Typical values of the primary winding current Ip, for current transformer use, range from 0 to 5 A RMS 50/60 Hz in transformer-connected power metering applications. 
     A sense winding  14 , having S turns, couples electromagnetically with only the second higher permeability sense core  11 . 
     In summary, the first lower permeability main core  10  is electromagnetically coupled to two windings: the primary winding  12  and the measurement winding  13 . The second higher permeability sense core  11  is electromagnetically coupled to three windings: the primary winding  12 , the measurement winding  13 , and the sense winding  14 . 
       FIG. 2  shows an actively compensated voltage ratio device having physical and electrical topology that is substantially the same as that of the current ratio transformer of  FIG. 1 , with the addition of a series voltage dropping resistor R 3 . An increase in the number (P=1000) of primary winding turns is combined with an increase in the impedance of the burden resistor R 4  (e.g., in 100 ohms). An external AC voltage source Vs is applied to the primary winding  2 . The number of turns for all the windings, and the values of all the components, may be selected for specific uses (to accommodate specific input/output voltage and current levels). 
     The output of the sense winding  14  is connected to the high impedance inverting and non-inverting inputs of a high gain voltage operational amplifier  20 , the voltage output of which drives a compensation current Ic through the sense winding  14 . The sense winding compensation current Ic is reduced in level through a divider formed by resistors R 1  and R 2 , and applied as a measurement winding current Im to the measurement winding circuit. A pair of parallel diodes D 1  and D 2  connected across the inputs of the amplifier  20  protect the input of the operational amplifier  20  from possible transient primary winding over-range signal conditions. A capacitor C 1  connected across the inputs of the amplifier  20  provides compensation circuit stability. 
     The operational amplifier  20  is provided with power from a +5 Vdc voltage supply  21  (e.g., +5 Vdc). A virtual ground reference DC voltage supply  22  (e.g., +2.5 Vdc) provides a reference source effectively biasing the static DC operating point of the output of the operational amplifier  20  to a voltage level that centers the output swing within the range of the primary supply  21 . It will be appreciated that other supply and reference source supply configurations are possible without affecting the underlying circuit operation. The example shown here is based on having a single unipolar operational amplifier supply  21 . Dual supply and ground referenced offset voltage sources can easily be accommodated depending on the specific supply voltage level availability. 
     For current transformer operation, the AC current being measured ( FIG. 1 ) is applied to the input of the primary winding  12  having P turns and flows as the primary winding current Ip. 
     Through transformer action, a secondary current Is develops in the measurement winding  5  having M turns and is represented by the following equation: 
     
       
         
           
             
               Is 
               ⇀ 
             
             = 
             
               
                 Ip 
                 
                   M 
                   P 
                 
               
               - 
               
                 Im 
                 ⇀ 
               
             
           
         
       
     
     The measurement winding current Im represents the current required to magnetize the lower permeability main core  10  and arises due to transformer and main core losses. The current Im is a complex vector quantity of varying magnitude and phase, having non-linear sensitivity to the characteristics of the core material, operating temperature, and core flux level. Without active compensation, the resulting secondary current and the voltage across the burden resistor R 4  have unacceptable ratio and phase errors, as referenced to the primary current Ip. These errors are unacceptable for high accuracy power metering applications. 
     It will be noted that the standard dot convention is used in  FIGS. 1 and 2  to indicate the direction of each winding relative to the other windings in the transformer. Voltages at the dot end of each winding are in phase, while current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil. 
     The operational amplifier  20  is arranged with its inverting and non-inverting inputs connected directly across the sense winding  14  output which is electromagnetically linked to only the higher permeability sense core  11 . The operational amplifier  20  operates to effectively reduce to zero any voltage appearing across the sense winding  14  through a feedback connection made between the output and the inverting input of the amplifier  20 , and connected to one end of the sense winding  14 . The sense winding  14  compensation current Ic develops to force the sense winding voltage output to zero. By Faraday&#39;s law of induction, having zero output voltage from the sense winding  14  implies a zero time-varying flux condition in the high permeability sense core  11 . The high permeability sense core  11  is therefore operating, through active compensation, at close to zero flux, and thus experiences very low core losses. Non-ideal operational amplifier characteristics (finite gain, noise, offsets), winding copper losses, and flux leakage paths prevent complete reduction of sense core  11  operating flux. The use of a high permeability (and stable) material for the sense core  11  keeps residual losses (and errors) at very low levels. The very low flux density in the sense core  11  allows for a small sense core magnetic cross section, thereby maintaining low overall costs when using more expensive higher permeability materials. 
     The sense winding compensation current Ic effectively removes the primary winding-to-measurement winding ampere-turn imbalance since both these windings also link the higher permeability core  11 . The ampere-turn imbalance is due to the measurement winding current Im required to magnetize the lower permeability core. Im is numerically equal to: 
     
       
         
           
             
               Im 
               ⇀ 
             
             = 
             
               
                 Ic 
                 ⇀ 
               
               
                 M 
                 S 
               
             
           
         
       
     
     S is the number of sense winding turns and M is the number of measurement winding turns. It is advantageous to have a reduced number of sense winding  14  turns in order to reduce both manufacturing costs and winding resistance, especially when higher primary to secondary ratios exist, i.e. when M measurement winding turns are high. The effect of the compensation current Ic through the resistive voltage drop across the sense winding  14  is minimized by using a lower number of sense winding turns. It should be noted that the finite gain and drive capability of the operational amplifier  20  establishes a lower limit to the number S of sense winding turns. 
     The compensation current Ic is reduced by a current divider formed by resistors R 1  and R 2  before injection into the measurement winding circuit as Im. The following equality applies: 
     
       
         
           
             
               S 
               M 
             
             = 
             
               
                 R 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 2 
               
               
                 
                   R 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 + 
                 
                   R 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Active compensation is completed through the injection of electronically derived current Im into the measurement winding circuit, effectively replacing the magnetization current component lost in magnetizing the lower permeability main core  10 . Under conditions of active compensation, the ampere-turns of the primary winding  14  is in precise balance with the ampere-turns of the secondary measurement winding  13  and therefore the resulting burden current Ib is related to the primary current Ip by a constant factor of M/P (measurement winding to primary winding turns ratio). Ratio and phase errors are therefore essentially removed from the burden current Ib and the resultant output voltage developed across the burden resistor R 4 . It will be appreciated that the correct current or voltage transformer winding polarity relationship is mandatory for proper active compensation to occur. 
     For potential transformer operation ( FIG. 2 ), the primary winding current Ip equals the input voltage Vs divided by the sum of an input resistor R 3  and the reflected burden impedance R 4 /(M/P) 2 . With P=M, the turns ratio is unity and therefore the reflected impedance (varies as the square of the turns ratio) is simply equal to the value of R 4 . The following equation shows the relationship of Ip to input voltage Vs: 
     
       
         
           
             Ip 
             = 
             
               Vs 
               
                 
                   R 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 + 
                 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   
                     
                       ( 
                       
                         M 
                         P 
                       
                       ) 
                     
                     2 
                   
                 
               
             
           
         
       
     
       FIG. 3  shows the physical construction of the actively compensated current or voltage ratio transformer having a stacked toroidal arrangement of the cores  10  and  11  to help minimize leakage flux and improve the self-shielding characteristics of the compensated current or voltage ratio transformer combined with a measurement winding  13  and a sense winding  14 . (Other arrangements are possible, including a toroid-within-a-toroid arrangement but are generally more expensive and complex than needed for a power metering application.) The measurement winding  13  is wound over both cores  10  and  11 , while the sense winding  14  is wound only over the sense core  11 . The primary winding  12  is shown as a single turn in the current transformer embodiment and passes through the central tunnel  30  of the stacked toroidal core combination. 
     The voltage transformer embodiment typically requires many primary turns (not shown in  FIG. 3 ) which are wound over an insulating layer positioned directly on top of an electrostatic and magnetic stamped metal shield  31  that completely covers the current ratio transformer including the internal surface of the axial tunnel  30 . A small air gap  32  prevents the shield from forming a shorted secondary turn. The shield  31  operates to prevent stray electrostatic and higher frequency magnetic fields from coupling to the windings and/or the main and sense cores. The measurement winding  13  and the sense winding  14  are brought out as two conductor pairs  33  through a small opening  34  in the outer shield  31 . A shield ground connection wire  35  is provided with one end physically soldered to the shield  31 . The illustrative embodiment employs a significant number of machine wound turns requiring the use of fine copper wire (e.g., 34 AWG) in order to construct a commercially viable compact and cost effective transducer. 
       FIG. 4  illustrates a typical application of the actively compensated current and voltage ratio transformers as employed in a digital power meter. For schematic simplicity, the block diagram of  FIG. 4  illustrates the key functional sections with a single phase voltage and current pair (phase A) shown. It will be appreciated that for polyphase applications, a simple duplication of the analog circuitry is all that is required (from a hardware standpoint) for additional voltage and current phase pairs. 
     The primary input current Ip is applied to an actively compensated current ratio transformer  40  as previously described and shown in  FIG. 1 . The compensated burden resistor voltage output is applied to a series of fixed gain amplifiers  41  ranging from a high gain CREEP stage to a lower gain OVER_RANGE stage. The outputs of these amplifiers  41  are applied to the multiplexed inputs of a current A/D converter  42 . The specific selected input is controlled by a digital signal processor  43  operating to select the required range based on current signal levels. This auto-ranging capability utilizes the wide dynamic range offered by the actively compensated current ratio transformer topology. 
     The corresponding voltage phase is applied to an actively compensated voltage ratio transformer  44  as previously described and shown in  FIG. 2 . The output drive dual-range-gain amplifiers  45  in a similar fashion to the corresponding current channel. The outputs of the amplifiers  45  are applied to the multiplexed inputs of a voltage A/D converter  46 . Both the current and voltage A/D converters  42  and  46  are simultaneously sampled with the acquired digital waveform representation processed in real time by the digital signal processor  43  and a main CPU  47 . Power measurement quantities, such as real power (watts), reactive power (VARS), energy (watt-hrs), volts (RMS), current (RMS) and power factor, are provided to the user through a display I/O  48  and a digital COMM  49 . A large memory bank  50  is used for storage of variables, waveforms and programs. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.