Patent Publication Number: US-2023152353-A1

Title: System and method of improving linearity of current sensor for a circuit interrupter

Description:
FIELD OF THE INVENTION 
     The disclosed concept relates generally to circuit interrupters with improved current sensors, and in particular, to circuit interrupters having current sensors with improved linearity. 
     BACKGROUND OF THE INVENTION 
     Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Older circuit breakers used a single current transformer to both sense load current and harvest and supply power to the electrical components that are attached to the current transformers. Newer circuit breakers have separated current sensing function and power harvesting function, and use one sensor (e.g., Rogowski coils) for sensing the load current and another sensor (e.g., a conventional current transformer) for power harvesting. The separation of functions is advantageous in that, it allows using devices that are better suited for performing each function. For example, Rogowski coils, a current transformer having a non-magnetic core (e.g., plastic, epoxy, air, etc., having relative permeability μr=1), provides more accurate metering than conventional iron-core current transformer does since Rogowski coil are linear as the non-ferromagnetic material cannot saturate at high currents whereas the conventional ferromagnetic-core current transformers saturate at high currents or when a DC component is present in the load current. 
     In an ideal situation, a perfect linearity is expected of a current sensor—every time the current is doubled, the signal coming out of the current sensor exactly doubles. However, there are a few noise factors, which affect the linearity of the current sensor such as external magnetic field generated by near-by coils and/or near-by pole-to-pole interaction to the external magnetic field. Further, it is also known that inside the circuit interrupters near-by current carrying parts may produce a magnetic field due to the presence of ferromagnetic materials. For example, the presence of the power harvester near or right next to the Rogowski coil and other assemblies on other phases tend to affect the output of Rogowski coils such that the linearity is lost. Further, unwanted interactions among near-by poles (e.g., in 3 pole circuit breakers) generate external magnetic fields that affect the output of Rogowski coils. That is, pole B may affect pole A, pole C may affect pole B, pole A may affect pole B, etc. where the output of each Pole sensor need be affected by respective pole only. Further, the current sensor of a circuit interrupter responds to the localized magnetic field. The magnetic field vector produced by the external magnetic field combines with the sensor field vector to produce a resultant or external magnetic field vector which, without calibration or compensation, is unsuitable for reliable and accurate metering. 
     There is considerable room for improvement in current sensing in circuit interrupters. 
     SUMMARY OF THE INVENTION 
     These needs, and others, are met by a circuit structured to electrically connect between a power source coupled to a hot conductor and a load. The circuit interrupter includes separable contacts; an operating mechanism coupled to the separable contacts; an electronic trip unit coupled to the operating mechanism; and a current sensor assembly including a harvester circuit comprising a harvester core, a primary conductor through which input current from the power source flows, and a secondary winding wound around a first part of the harvester core, the harvester circuit structured to harvest power from the power source and supply power to at least the operating mechanism and the electronic trip unit, where the harvester core saturates at a first input current level of a range of input current levels; a Rogowski coil comprising a toroid through the center of which the primary conductor passes, the Rogowski coil structured to measure voltage at an output of the Rogowski coil, the measured voltage indicative of output current of Rogowski coil, wherein linearity of the output of Rogowski coil to the input current deviates more than a deviation tolerance based at least in part on saturation of the harvester core at the first input current level, and a compensation coil wound around a second part of the harvester core, the compensation coil structured to delay saturation of the harvester core until the input current reaches a second input current level higher than the first input current level. The electronic trip unit is structured to receive a signal including the measured voltage from the Rogowski coil and cause the operating mechanism to open the separable contacts and interrupt current flowing through the circuit interrupter based at least in part on the measured voltage. 
     Another example embodiment includes a circuit structured to electrically connect between a power source coupled to a hot conductor and a load. The circuit interrupter includes separable contacts; an operating mechanism coupled to the separable contacts; an electronic trip unit coupled to the operating mechanism; and a current sensor assembly including a harvester circuit comprising a harvester core, a primary conductor through which input current from the power source flows, and a secondary winding wound around a first part of the harvester core, the harvester circuit structured to harvest power from the power source and supply power to at least the operating mechanism and the electronic trip unit, where the harvester core saturates at a first input current level of a range of input current levels; a Rogowski coil comprising a toroid through the center of which the primary conductor passes, the Rogowski coil structured to measure voltage at an output of the Rogowski coil, the measured voltage indicative of output current of Rogowski coil, wherein linearity of the output of Rogowski coil to the input current deviates more than a deviation tolerance based at least in part on saturation of the harvester core at the first input current level, and a magnetic shielding made of magnetic material having a high permeability, the magnetic shielding arranged between the Rogowski coil and the harvester circuit and structured to isolate the non-linearity source from the Rogowski coil over the range of input current levels. The electronic trip unit is structured to receive a signal including the measured voltage from the Rogowski coil, and cause the operating mechanism to open the separable contacts and interrupt current flowing through the circuit interrupter based at least in part on the measured voltage. 
     Yet another example embodiment includes a method of improving linearity of a current sensor and accuracy of a circuit interrupter including a Rogowski coil for sensing load current, a harvester circuit having a ferromagnetic core, a primary conductor, and a secondary winding, and a compensation coil arranged 180 degree opposite to the secondary winding. The method includes passing input current through the primary conductor, where the ferromagnetic core saturates based on input current reaching a first input current level of a range of input current levels, output current of the Rogowski coil being linear to the input current; delaying, by the compensation coil, saturation of the ferromagnetic core until the input current reaches a second input current level; measuring, by the Rogowski coil, voltage at an output of Rogowski coil, the measured voltage indicative of the output current of the Rogowski coil, the output current exhibiting no deviation of linearity of the output of the Rogowski coil greater than a deviation tolerance over the range of input current levels; receiving, by an electronic trip unit of the circuit interrupter, a signal comprising the measured voltage; determining that the signal indicates a detected fault based on the measured voltage; and in response to determining that the signal indicates the detected fault, tripping the circuit interrupter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG.  1    is a schematic diagram of a circuit interrupter in accordance with an example embodiment of the disclosed concept; 
         FIG.  2    illustrates a current sensor assembly of a circuit interrupter in accordance with an example embodiment of the disclosed concept; 
         FIGS.  3 A-B  illustrate graphs of error rates of Rogowski coil in relation to input current in accordance with an example embodiment of the disclosed concept; 
         FIGS.  4 A-B  illustrate 2 degree error in winding of Rogowski coil and error rates of Rogowski coil in relation to input current in accordance with an example embodiment of the disclosed concept; 
         FIG.  5    is an exploded view of a current sensor assembly in accordance with an example embodiment of the disclosed concept; 
         FIG.  6    illustrates a geometrical arrangement of a circuit interrupter including a magnetic shielding in accordance with an example embodiment of the disclosed concept; 
         FIG.  7    illustrates error rates of Rogowski coil in accordance with an example embodiment of the disclosed concept; and 
         FIGS.  8 A-B  illustrate Rogowski coil having a non-uniformity of winding and error rates of Rogowski coil in accordance with an example embodiment of the disclosed concept; 
         FIG.  9    is a flowchart of a method of improving linearity of current sensors and accuracy of a circuit interrupter in accordance with an example embodiment of the disclosed concept; and 
         FIG.  10    is a flowchart of a method of improving linearity of current sensors and accuracy of a circuit interrupter in accordance with an example embodiment of the disclosed concept. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Directional phrases used herein, such as, for example, left, right, front, back, top, bottom and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
     As employed herein, the statement that two or more parts are “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. 
     Older circuit breakers used conventional iron-core current transformers to both sense load current and harvest power to supply power the electrical components that are attached to the current transformers. As a current measuring device, the current transformer produces a low level current that accurately represents a higher current level for the purpose of metering and protection. It is a closed loop device including a ferromagnetic core, a primary conductor (a primary winding) and secondary winding around the core. The transformer consists of two or more coils wrapped around the ferromagnetic core and connected by the magnetic flux present within the core. The primary winding has a coil with the current to be measured passing through the center of the ferromagnetic core. The coil produces the magnetic field that drives the current on the secondary winding. The current on the secondary winding is proportional to the current flowing through the center of the core. A single current transformer is needed for each phase and/or neutral to monitor current. As a power harvesting device, the current transformer provides AC power, which is rectified and then regulated to a stable DC voltage before being fed to electrical components (e.g., without limitation, an operating mechanism, an electronic trip unit, or a controller of circuit interrupter). The primary winding is connected to the source, and the secondary winding is connected to the electrical components. The core supports both windings and magnetically couples the primary and secondary windings. The performance of the current transformer depends on the amount of the magnetic flux in the core. The current transformer operates in linear region and saturation region. In the linear region, magnetic flux density is approximately proportional to field intensity, but in the saturation region the increase in flux density slows down and eventually becomes constant. Saturation is a state in which the current transformer is no longer able to produce an output proportional to its primary current or according to its ratio. Saturation causes distortion in the secondary voltage and and should be avoided or delayed for applications requiring both current sensing and energy harvesting. 
     Newer circuit breakers use a separate current sensor (e.g., a Rogowski coil) for sensing load current and a harvester circuit (e.g., a conventional iron-cored current transformer) for power harvesting for the electrical components attached thereto in order to achieve more reliable metering and protection. For example, a Rogowski coil may be a better metering mechanism than a conventional current transformer having ferromagnetic core due to the coil linearity—the coil current output to input current relationship is linear for all currents. A typical Rogowski coil is a toroid with a non-magnetic core (e.g., plastic, epoxy, air, etc., having relative permeability μ r =1). Since non-ferromagnetic material cannot saturate, the output of Rogowski coil is proportional to the sum of time rate of change of the flux within its windings. The presence of the power harvester near or right next to the Rogowski coil and other assemblies on other phases tend to affect the output of Rogowski coil such that the linearity of the output of the Rogowski coil deviates more than a deviation tolerance (e.g., without limitation, less than or equal to 0.25% where 0%=perfect linearity). For example, due to winding and construction imperfections, the output of the Rogowski coil are affected by nearby conductors (e.g., the harvester coils) that carry high currents. Further, unwanted interactions among near-by poles (e.g., in a 3 pole circuit breaker) generate external magnetic fields which affect the output of Rogowski coils. That is, pole B may affect pole A, pole C may affect pole B, pole A may affect pole B, etc. where the output of each pole sensor needs be affected by respective pole only. Thus, linearity of the current sensor may become non-linear beyond the deviation tolerance. The non-linear output of Rogowski coil then reduces the accuracy and reliability of the current sensor and the circuit interrupter. Such non-linearities can be avoided by perfecting the construction and the winding of the Rogowski coil. It is difficult to produce perfect constructed coils. Other approaches can address the non-linearity by reducing the size of the non-linear fields coupled into an imperfect Rogowski coil. 
     Example embodiments of the disclosed concept address these issues. In some example embodiments, a compensation coil is added to the harvester circuit to improve linearity of the current sensor (e.g., Rogowski coil). Conventionally, a manual calibration is needed to compensate the effect of external magnetic field in sensor linearity performance. One form of calibration involves correcting a minor current deviation without affecting linearity. For example, assume that when 100 A is input, a trip unit (e.g., an electronic trip unit) reads 101 A as the current sensor output and when 200 A is input, the trip unit reads 202 A. While the output is different by a minor amount, the linearity remains intact. That is, the sensor output doubled as the input doubled. In this example, the current sensor is calibrated to calculate one calibration factor to correct the 101 A to 100 A. Another form of calibration involves correction or compensation of non-linearity of the sensor output. Assume that the trip unit reads 101 A when 100 A is input, but the trip unit also reads 205 A when 200 A is input, etc. In this example, the linearity of the current sensor output deviates greater than the deviation tolerance. As such, the Rogowski coil cannot be calibrated to reflect the linearity within the deviation tolerance by simply multiplying the output result by one number at 100 A. Rather, multiple measurements across the range of input current being measured must be performed in order to obtain different calibration factors to correct the non-linearity. Such manual corrections at multiple current levels increase production time and reduce productivity and efficiency. 
     In order to countermeasure, correct, compensate, countermeasure, mitigate, or preempt the deviation beyond the deviation tolerance and prevent having to obtain multiple calibration factors for calibrating the Rogowski coil exhibiting non-linear relationship, the compensation coil is added to the harvester circuit, arranged at 180 degree opposite to the secondary winding of the harvester circuit. The compensation coil in accordance with the present disclosure is structured to generate magnetic flux in the direction of the ferromagnetic core of the harvester circuit, and thus delays the saturation of the harvester core. Such delay prevents the magnetic linking of the additional magnetic flux generated by the harvester coils to Rogowski coil output until a wider range (e.g., without limitation, 800 A-1000 A) of continuous current is reached. This delay has been shown to improve the linearity of the current sensor. For example, the maximum deviation from linearity reduces significantly when the compensation coil is added (e.g., 0.74% without the compensation coil to 0.15% with the compensation coil, where 0% means a perfect linearity) as shown in Table 1. 
     In some example embodiments, a magnetic shielding with magnetic material having a high relative permeability is incorporated to improve linearity of the Rogowski coil. Typically, circuit breakers do not include a magnetic shielding between the harvester circuit and the Rogowski coil, thereby allowing the magnetic flux to link the Rogowski coil and return to their source (i.e., the primary winding) via air. Rather, there is only a plastic plate used to assemble both Rogowski coil and harvester coils. Incorporating a magnetic shield made of magnetic material having a high permeability (e.g., without limitation, μ r ≥10,000) in accordance with the present disclosure effectively isolates a source of non-linearity (e.g., the saturated harvester core, a near-by current carrying components within the circuit interrupter, a near-by pole-to-pole interaction, etc.) from the linear Rogowski coil so as to preserve and/or improve the linearity of Rogowski coils. In some examples, by changing the material of the conventional plastic plate to assemble the Rogowski coils and harvester coils to magnetic steel in accordance with the disclosed concept, this plate may also serve the purpose of magnetic shielding. 
     It is well-known that the higher the permeability of the material the more the magnetic flux pass through the material. Thus, the magnetic shielding with magnetic material having a high permeability creates a low reluctance return path for magnetic flux of external magnetic fields generated by the source of non-linearity. As such, the magnetic shielding absorbs the external magnetic fields as the magnetic field lines travel through the magnetic material of the magnetic shield in the return path to the source of the non-linearity, never finding it more efficient to permeate the air space around the magnetic shield. Thus, shielding either the source of the non-linearity (e.g., the saturated harvester core, a near-by current carrying components within the circuit interrupter, a near-by pole-to-pole interaction, etc.) or the component to be protected (e.g., the Rogowski coil) not only improves significantly the linearity of the current sensor and the accuracy of the circuit interrupter, but also offers the most effective use of the magnetic shielding material and the most cost efficient way of improving linearity of the Rogowski coils. For example, a percentage error measured over a range of input current levels ranging from 30 A to 1800 A without the magnetic shield present has been shown to range from 0.34 to 0.41% with a bending observed between 400 A to 1200 A input current levels whereas a percentage error measured over the same range of input current levels with the full magnetic shield with the material having high permeability has been shown to remain constant at around 0.23% as illustrated in  FIG.  7   . That is, the Rogowski coil output exhibits an improved linearity with the magnetic shielding (i.e., no bending observed). 
     Therefore, the example embodiments of the disclosed concepts improve the linearity of the output of the Rogowski coil and the accuracy of the circuit interrupter by incorporating a compensation coil structured to delay saturation of the harvester core or a magnetic shield structured to isolate the non-linearity source from the linear Rogowski coil. Such simple addition of the compensation coil or the magnetic shielding (e.g., a magnetic plate) that are commonly available results in signification production increases at a minimum construction cost and satisfies the customer demand for high accuracy and reliability of current sensors and circuit interrupters for metering and protection. 
       FIG.  1    is a schematic diagram of a circuit interrupter  10  (e.g., without limitation, a circuit breaker) in accordance with an example embodiment of the disclosed concept. The circuit interrupter  10  is structured to be electrically connected between a power source (now shown) via HOT conductors  12  and a load(s)  18  via LOAD conductors  14 . The circuit interrupter  10  is structured to trip open or switch open to interrupt current flowing to the load  18 , for example, in the case of a fault condition (e.g., without limitation, an overcurrent condition) to protect the load  18 , circuitry associated with the load  18 , as well as the components within the circuit interrupter  10 . While a 3-phase circuit breaker  10  is shown in  FIG.  1   , it will be appreciated that a single phase circuit breaker or any other number of phases may be employed without departing from the scope of the disclosed concept. 
     The circuit interrupter  10  includes separable contacts  100 , an operating mechanism  200 , an electronic trip unit  300 , a current sensor assembly  400 , an AC/DC converter  500  and DC/DC converter  600 . The operating mechanism  200  is structured to physically open and close the separable contacts  100 . The electronic trip unit  300  is structured to control the operating mechanism  200  to open the separable contacts  100  based on a signal including voltage measured at an output of the Rogowski coil  515 . The electronic trip unit  300  includes a controller  305  structured to monitor for faults based on power flowing through the circuit breaker  10  and output a trip signal to the operating mechanism  200 . For example, in a mechanical circuit interrupter, the separable contacts are designed to interrupt current flowing through the circuit interrupter and have associated components such as an arc chute to manage arcing as a result of circuit interruption. In some example embodiments, the separable contacts  100  are closed with manual intervention by a user through, for example, a reset switch. In some example embodiments, the operating mechanism  200  is structured to close the separable contacts  100  in response to a close signal from the electronic trip unit  300 . The controller may be further structured to control the operation of the current sensor assembly  400  (e.g., the harvester circuit). 
     The current sensor assembly  400  includes a harvester circuit  410  and a current sensor  415 . In some example embodiments, the harvester circuit  410  includes a compensation coil  425  arranged 180 degree opposite to the harvester coils as described with reference to  FIGS.  2 - 4 B . The harvester circuit  410  may be, e.g., without limitation, an iron-cored current transformer capable of harvesting and powering the electric devices (e.g., the operating mechanism  200 , the electronic trip unit  300  via the controller  305 , or a controller (not shown) of the circuit interrupter  10 ) attached to the harvester circuit  410 . Upon passing input current through a primary conductor of the harvester circuit  410 , the harvester circuit  410  is structured to supply power to the electronic devices after the reduced AC current from secondary winding is rectified by the AC/DC converter  500 , regulated to a stable low DC voltage by the DC/DC converter  600 , and filtered by a filter (not shown). The harvester coil is discussed further in detail with reference to  FIG.  2   . 
     The current sensor  415  may be a Rogowski coil helically wrapped around a non-ferromagnetic core and structured to measure voltage at an output and transmit a signal to an electronic trip unit  300 , the measured voltage indicative of output current of the Rogowski coil  415 . The Rogowski coil  415  is discussed further in detail with reference to  FIG.  2   . 
     The electric trip unit  400  is structured to receive a signal including the measured voltage from the Rogowski coil  415 . Based on the signal, the electric trip unit  400  determines whether the measured voltage indicates a detected fault and in response to determining the measured voltage indicates the detected fault, trips the circuit interrupter  10  by causing the operating mechanism  200  to open the separable contacts  100 . As such, the accuracy and reliability depend on an accurate metering of the output current of Rogowski coil  415 , which in turn depends on preserving linearity of the output current of Rogowski coil to the input current at least within a deviation tolerance (e.g., up to 0.25% where 0%=perfect linearity). 
     However, there are noise factors that affect linearity of Rogowski coil  415 , thereby decreasing the accuracy of the circuit interrupter  10 . For example, the near-by ferromagnetic core of the harvester circuit  410  or near-by current carrying components (e.g., without limitation, an AC/DC converter  500 , a DC/DC converter  600 , etc.) generate external magnetic field that negatively affect the linearity of Rogowski coil  415 . The ferromagnetic core of the harvester circuit  410  typically saturates about from 200 A to 300 A, thereby generating the external magnetic field upon saturation. In other examples, a near-by pole-to-pole interaction may affect the output current of Rogowski coil (e.g., without limitation, pole A affecting pole B, thereby affecting the output current at pole B), thereby affecting the linearity of the Rogowski coil  415 . In order to mitigate or countermeasure these notice factors, the compensation coil  425  is incorporated at a part of the harvester circuit and arranged at 180 degree opposite to the harvester coil (e.g., the secondary winding of the harvester circuit  410 ). The compensation coil  425  is structured to delay saturation of the ferromagnetic core of the harvester circuit  410 , thereby preventing linking magnetic flux of the external magnetic fields generated at least by the saturation of the ferromagnetic core of the harvester circuit  410  until a wider range (e.g., without limitation, 800 A-1000 A) of continuous current. As such, the compensation coil  425  improves the linearity of Rogowski coil  415  and accuracy of the circuit interrupter  10  as discussed further with reference to  FIGS.  2 - 4 B . 
     In some example embodiments, the current sensor  400  includes a harvester circuit  410 , a current sensor  415 , and a magnetic shielding (e.g., a magnetic shielding  450  of  FIG.  5   ) arranged between the current sensor  415  and the harvester circuit  410 . In these example embodiments, the harvester circuit  410  may not include the compensation coil  425 . The magnetic shielding  450  is made of a magnetic material (e.g., without limitation, a magnetic plate) with high relative permeability as described with reference to  FIGS.  5 A- 8 B . The magnetic shielding  450  is structured to isolate a source of non-linearity (e.g., without limitation, the saturated core of the harvester circuit  410 , a near-by current carrying components, a near-by pole-to-pole interaction, etc.) from the Rogowski coil  410 . The magnetic shielding  450  isolates the source of non-linearity by generating a return path for magnetic flux of the external magnetic field generated by the source, and accommodating magnetic flux lines of the external magnetic field to travel through the magnetic material of the magnetic shielding to the source via the return path. As such, the magnetic shielding  450  isolates the source of non-linearity from the Rogowski coil  415 , effectively shielding the source of the external magnetic field, and thus, improving the linearity of the Rogowski coil output and accuracy of the circuit interrupter  10  as described in further with reference to  FIGS.  5 - 8 B . 
       FIG.  2    illustrates a current sensor assembly  400  in accordance with an example embodiment of the disclosed concept. The current sensor assembly  400  includes a harvester circuit  410 , a current sensor  415 , and a compensation coil  425 . 
     The harvester circuit  410  is a current transformer including a harvester core  420 , a primary conductor  430  passing through the center of the harvester core  420 , a secondary winding  435  wound around a first part of the harvester core  420 , and a magnetic burden  440 . The harvester circuit  410  is structured to harvest power from the power source and supply power to electric devices coupled to the harvester circuit  410 . A single current transformer is needed for each phase and/or neutral to monitor current. The harvester core  420  is a ferromagnetic core. While in  FIG.  2    the harvester core  420  is shaped rectangular, it may be circular in some example embodiments. The primary conductor  430  has the current to be measured passing through. 
     The primary conductor  430  is connected to the HOT conductors  12  for receiving AC power, and the secondary winding  435  is connected to the electric devices (e.g., without limitation, the operating mechanism  200 , the electronic trip unit  300 , or a stand-alone controller (not shown) of the circuit interrupter  10 ) within the circuit interrupter  10 . The harvester core  420  supports both primary conductor  430  and secondary winding  435  and magnetically couples the primary conductor  430  and secondary windings  435 . The primary conductor  430  produces the magnetic field that drives the current on the secondary winding  435 . The current on the secondary winding  435  is proportional to input current flowing through the center of the harvester core  420 . The harvester coil  410  is controlled by a controller (e.g., the controller  305  in the electronic trip unit  300 , or a standalone microcontroller (not shown)) and is structured to generate AC voltage and current based on the AC power received from the HOT conductors  12 . The AC voltage is then rectified to a regulated DC voltage by the AC/DC converter  500 , the regulated DC voltage is regulated to a stable DC voltage by a DC/DC converter  600 , and the voltage ripple of the regulated DC voltage is removed by a filter (not shown) before being supplied to the electric devices (e.g., without limitation, the operating mechanism  200 , the electronic trip unit  300 , or a stand-alone controller (not shown) of the circuit interrupter  10 ). 
     The performance of the harvester circuit  410  depends on the amount of the magnetic flux in the harvester core  420 . The current transformer  415  operates in linear region and saturation region. In the linear region, magnetic flux density is approximately proportional to field intensity, but in the saturation region the increase in flux density slows down and eventually becomes constant. Saturation is a state in which the current transformer is no longer able to produce an output proportional to its primary current or according to its ratio. The harvester core  420  saturates at a first input current level of a range of input current levels upon flowing of input current through the primary conductor  430  from the power source. The first input current level may be, e.g., without limitation, about 200 A. Saturation causes distortion in the secondary voltage and negative effect on the linearity of the output of Rogowski coil  415 , and should be delayed for metering. 
     Rogowski coil  415  includes a toroid of a coil(s) helically wrapped around a non-magnetic core (e.g., plastic, epoxy, air, etc.) having relative permeability μ r =1. The Rogowski coil  415  is structured to measure voltage at an output of the Rogowski coil  415 , the measured voltage indicative of output current of the Rogowski coil  415 , the output current being linear to the input current passing through the primary conductor  430 . The linearity of the output of Rogowski coil  415  may deviate more than a deviation tolerance (e.g., without limitation, up to 0.25% where 0%=perfect linearity) based at least in part on saturation of the harvester core  420  at the first input current level (e.g., without limitation, about 200 A). Since non-ferromagnetic core cannot saturate, the Rogowski coil outputs remain substantially linear even at high currents. However, the output of Rogowski coil  415  is subject to noise factors including external magnetic fields generated by a source of non-linearity. For example, presence of the harvester circuit  410  near or right next to the Rogowski coil  415  and other assemblies on other phases tend to affect the output of Rogowski coils such that linearity of the output of Rogowski coil  415  deviates beyond the deviation tolerance. Further, unwanted interactions among near-by poles (e.g., in a 3-pole circuit breaker) affect the linearity of the output of Rogowski coil  415 . That is, pole B may affect pole A, pole C may affect pole B, pole A may affect pole B, etc., where the Rogowski coil output of each pole needs be affected by respective pole current only. Thus, the output of the current sensor  415  may become non-linear beyond the deviation tolerance as a result of the external magnetic fields generated by the non-linearity source. The non-linear output of Rogowski coil  415  then reduces the accuracy and reliability of the current sensor  415  and the circuit interrupter  10 . 
     In order to correct, compensation, mitigate, countermeasure or event preempt the effect of these non-linearity components on the linearity of Rogowski coil  415 , the compensation coil  425  is added to the harvester circuit  410 . The compensation coil  425  may be any coil capable of carrying current. The compensation coil  425  is arranged 180 degree opposite to the harvester coils (e.g., secondary winding  435 ) and structured to delay saturation of the harvester core  420 . By generating the magnetic linking of additional magnetic flux generated by the harvester coils  430 , 435  to the Rogowski coil output until a wider range of continuous current (e.g., without limitation, about 800 A to 1000 A) and improve linearity of the circuit interrupter  10 . That is, the compensation coil  425  provide fully automatic compensation of any deviation of linearity of Rogowski coil  415  beyond the deviation tolerance by delaying the saturation of the harvester core  420  until a second input current level (e.g., about 200 A) is reached. The fully automatic compensation of deviation of Rogowski coils  415  includes simplifying conventional complex manual calibration of non-linear output of the Rogowski coils. 
     Typically, the current sensor of a circuit interrupter responds to a localized magnetic field. However, the magnetic field vector produced by the external magnetic field combines with the sensor filed vector to produce a resultant or external magnetic field vector which, without calibration or compensation is unsuitable for reliable and accurate metering. Generally, a manual calibration is needed to compensate or correct the effect of external magnetic field in sensor linearity performance. One form of calibration involves correcting a minor current deviation, which does not include deviation of the linearity of Rogowski coil. For example, assume that when 100 A is input, a trip unit (e.g., an electronic trip unit  300 ) reads 101 A as the Rogowski coil output and when 200 A is input, the electronic trip unit  300  reads 202 A. While the output are different by a minor amount, the linearity remains intact. That is, the sensor output doubled as the input current doubled. In this example, the manual calibration requires obtaining a calibration factor to correct 101 A to 100 A and calibrating Rogowski coil  415  to adjust its output based on the calibration factor. This calibration is simple and requires only one calibration using a single calibration factor. Another form of calibration involves correction or compensation of non-linearity of the sensor output. Assume that the electronic trip unit  300  reads 101 A when 100 A is input, but the trip unit  400  also reads 205 A when 200 A is input, etc. As such, Rogowski coil  415  exhibits a non-linear relationship between the input current and current sensor output. In this example, the linearity cannot be obtained by multiplying the output result by one number at 100 A, rather, multiple measurements across the range of input current being measured must be performed in order to obtain different calibration factors. Such manual calibration requiring multiple measurements performed at multiple current levels to obtain respective calibration factors increase production time and reduce productivity and efficiency. By a simple addition of the compensation coil  425 , such complex manual calibration can be avoided. 
     By generating magnetic flux, the compensation coil  425  delays the saturation of the harvester core  420  until higher currents are reached, and prevents the magnetic linking of the additional magnetic flux generated by the harvester coils  430 , 435  to the Rogowski coil output until a wider range (e.g., without limitation, 800 A-1000 A) of continuous current is reached. As such, any deviation that would have occurred as a result of the magnetic flux affecting linearity during saturation of the harvester core  420  is automatically avoided, corrected or compensated until the wider range of the current is reached. Such delay of saturation significantly improves the linearity of the output of the Rogowski coil  415 .  FIGS.  3 A-B  illustrate graphs of linearity of Rogowski coil  415  in accordance with an example embodiment of the disclosed concept. For  FIGS.  3 A-B  0% error rate is assumed for the purpose of better illustrating the non-linearity or linearity of Rogowski coil  415 .  FIG.  3 A  shows that the linearity of Rogowski coil  415  remains constant as the graph shows no bending.  FIG.  3 B  shows a bending observed over the input current ranging from 400 A to about 1400 A. The bending shows that the presence of the harvester coils  410  significantly affects the linearity of Rogowski coil  415  over the aforementioned input current range. Such bending requires a complex manual calibration measuring multiple calibration factors at different input current. The deviation from linearity, however, can be effectively corrected or compensated by adding the compensation coil  425  as described with reference to  FIG.  2   . With only the harvester coils  430 , 435  present, the harvester core  420  of the harvester circuit  410  saturates early with the input current of 200 A to 300 A. With the harvester coils  435 , 435  and the compensation coil  425 , the saturation of the harvester core  420  is delayed to the input current ranging from 800 A to 1000 A. Non-linearity can be improved if the saturation of the harvester core  425  is delayed or reduced by the compensation coil  425 . 
     As shown in Table 1 below, maximum deviation from linearity reduces significantly when the compensation coil  425  are added (e.g., 0.74% without the compensation coil to 0.15% with the compensation coil, where 0% means a perfect linearity). Further, as shown in  FIG.  4 B , an error rate of the Rogowski coil  415  having a two-degree error in winding measured with the compensation coil incorporated exhibits no sudden significant change of the error rate, meaning no deviation of the linearity beyond the deviation tolerance. 
                     TABLE 1                  Maximum Deviation from Linearity        with or without Compensation coil                                 ximum Deviation            Case   (Perfect Linearity = 0)                       Without the    0.74           Compensation coil               With the    0.15           Compensation coil                        
In some examples, the compensation coil  425  has a higher current than the harvester coils  430 , 435  as there is no magnetic burden for the compensation coil  425 . As such, the current in the compensation coil  425  can cause power loss (e.g., without limitation, 2-4 Watt), which can be regulated by adding an external resistance (e.g., a resistor) over the compensation coil  425 .
 
       FIGS.  4 A-B  illustrate error rates of Rogowski coil  415  having two degree of error winding in accordance with an example embodiment of the disclosed concept. A toroidal Rogowski coil is ideally wound with a constant number of turns per centimeter around its perimeter with  20  turns per cm being a typical example. The number of turns per centimeter is referred to as the wire pitch. Deviations from the nominal wire pitch result in the coil being susceptible to pick up from external fields. It is inevitable that in manufacturing the wire pitch will change around the perimeter. One way to characterize a deviation in pitch is to measure what part of the coil&#39;s perimeter differs from nominal. For example, a 2 degree error says the pitch has deviated over 2 degrees out of the 360 degrees. The deviation could mean that there are additional turns or missing turns over these two degrees. The coil&#39;s susceptibility to external fields depends on both how many degrees of deviation there are and on how much the pitch differs from nominal over this region. One of the purposes of the disclosed concept is to make the Rogowski coil  415  less susceptible to external fields given a certain error in the pitch.  FIG.  4 A  depicts Rogowski coil  415  with Rogowski turns represented as slits  416 . There is an error  417  in winding shown on the y-axis. In this example, total of 670 numbers of Rogowski turns were considered and the error spanned two degrees comprising seven turns densely wound or no turns. The error rate B associated with two degree of error winding without the compensation coil  425  ranged from about 2.1 to 1.5% over the input current from 30 A to 1800 A. The error rate A of Rogowski coil  415  with two degree error winding was reduced significantly to about 1.1 to 1.2% when the compensation coil  425  were incorporated to the harvester circuit  410 . Further, the linearity of Rogowski coil  415  improves significantly when the compensation coil  425  is added, showing the error rate A remaining constant or substantially constant throughout the input current ranging from 30 A to 1800 A whereas the error rate B shows that there is a significant bending over the input current ranging from 100 A to 600 A when the compensation coil  425  is not incorporated. 
       FIG.  5    is an exploded view of a current sensor assembly  400  in accordance with an example embodiment of the disclosed concept. The current sensor assembly  400  includes a harvester circuit  410 , Rogowski coil  415 , a magnetic shielding  450 , and the housing covers  10 A,B. As described with reference to  FIGS.  1 - 2   , Rogowski coil  415  are structured to sense the load current by measuring voltage at its output, the measured voltage proportional to the load current input to the primary conductor  430  and indicative of output current at the output of the Rogowski coil  415 . The harvester circuit  410  is structured to harvest and power, e.g., without limitation, electric devices (e.g., without limitation, the operating mechanism  200 , the electronic trip unit  300 , a controller of the electronic trip unit  300 , or a standalone controller of the circuit interrupter  10 ). The magnetic shielding  450  is made of magnetic material, e.g., without limitation, a magnetic plate with high relative permeability. The magnetic plate may be made of mu alloy with relative permeability μ r  ranging from 80,000 to 100,000. The high permeability includes permeability greater than 10,000. By incorporating the magnetic shielding  450  with the magnetic material having a high relative permeability, the non-linear parts (e.g., without limitation, the harvester circuit  410 ) are effectively isolated from the linear Rogowski coil  415 , thereby preventing the magnetic flux of the external magnetic field generated by, e.g., without limitation, the harvester coils  430 , 435  from affecting the output of Rogowski coil  415 . 
     As stated previously, there are noise factors that affect linearity of Rogowski coil  415 , thereby decreasing the accuracy of the circuit interrupter  10 . For example, the primary conductor  430  of the harvester circuit  410  has the current passing through the center of the core, and the primary conductor  430  produces the magnetic field that drives the current on the secondary winding  435 . The harvester core  420  is linear until the magnetic intensity reaches a certain level at which the harvester core becomes saturated. Upon saturation, the harvester core  420  becomes non-linear and the magnetic flux produced by the primary conductor  430  start to affect the linearity of near-by Rogowski coil (e.g., located at about 8 mm distance)  415 . Further, near-by current carrying components (e.g., without limitation, an AC/DC converter, a rectifier, etc.) may generate external magnetic field that negatively affect the linearity of Rogowski coil  415 . In addition, a near-by pole-to-pole interaction may affect the output current of Rogowski coil (e.g., without limitation, pole A affecting pole B, thereby affecting the output current at pole B), thereby affecting the linearity of the Rogowski coil  415 . Typically, circuit breakers either do not include a magnetic shielding between the harvester circuit and Rogowski coil, thereby allowing the magnetic flux to link Rogowski coils. Rather, there is only a plastic plate used to assemble both Rogowski coil and harvester coils. By changing the material of this plastic plate to magnetic steel in accordance with the disclosed concept, this plate may also serve the purpose of magnetic shielding. 
     Incorporating the magnetic shielding  450  made of magnetic material having a high relative permeability between Rogowski coil  415  and the harvester circuit  410  significantly improves the linearity of Rogowski coil  415 . The high permeability allows the magnetic shielding  450  to create a return path with low magnetic reluctance for the magnetic flux of the external magnetic fields generated by a source of non-linearity, e.g., without limitation, the harvester circuit  410 , near-by current carrying components, near-by pole-to-pole interaction, etc. As such, the magnetic shielding  450  absorbs the external magnetic fields as the magnetic field lines travel through the material of the magnetic shielding  450  in the return path to the source, never finding it more efficient to permeate the air space around the magnetic shield. Thus, shielding either the non-linearity source (e.g., the harvester circuit) generating the external magnetic field or components (e.g., Rogowski coils) to be protected offers the most effective use of the magnetic shielding material. This magnetic shielding  450  significantly improves the linearity of Rogowski coil  415  as shown in  FIG.  7    in which a reduction of the error rate with no bending is observed over the entire range of input current levels from about 30 A to 1800 A. 
       FIG.  6    illustrates an example geometric arrangement of a current sensor assembly  400  incorporating a magnetic shielding  450  made of material with high relative permeability in accordance with an example embodiment of the disclosed concept. The magnetic shielding  450  is placed between the harvester circuit  410  and Rogowski coil  415 . The distance between the harvester circuit  410  and Rogowski coil  415  is, e.g., without limitation, 8 mm, and the magnetic shielding  450  is placed in between the harvest circuit  415  and Rogowski coil  415  so as to be, e.g., without limitation, equidistant (4 mm) from the harvest circuit  415  and Rogowski coil  415 . 
       FIG.  7    illustrates current sensor error rates in relation to input current in accordance with an example embodiment of the disclosed concept. The percentage error A without the magnetic shielding  450  ranges from 0.34 to 0.41% with a bending observed in between 600 A to 1200 A input currents. The percentage error B with the full magnetic shielding  450 , however, remains constant at around 0.23% with no bending observed over the entire range of current from 30 A to 1800 A. This clearly shows that incorporating the magnetic shielding  450  to shield Rogowski coil  415  from the non-linearity source significantly improves the linearity of Rogowski coil  415 . 
       FIGS.  8 A-B  illustrate current sensor error rates in relation to input current where Rogowski coil  415 ′ have non-uniform winding in accordance with an example embodiment of the disclosed concept.  FIG.  8 A  depicts a current sensor assembly  400  with Rogowski coil with non-uniform winding in accordance with an example embodiment of the disclosed concept.  FIG.  8 B  depicts percentage errors of Rogowski coil  415 ′ with non-uniform winding with or without the magnetic shielding incorporated in accordance with an example embodiment of the disclosed concept. The percentage error A without the magnetic shielding  450  ranges from 1.3 to 2.1% with a bending observed in between 30 A to 600 A input currents. The percentage error B with the full magnetic shielding  450  incorporate, however, remains constant at around 1.1% with no bending observed. Table 2 below shows that maximum deviation (where 0=perfect linearity) is 0.97% without the magnetic shielding  450 . Table 2 also shows that the maximum deviation is 0.08 when the magnetic shielding  450  is incorporated. As such, even the linearity of Rogowski coil  415 ′ with non-uniform winding improves significantly with the incorporation of the magnetic shielding  450 . 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Maximum Deviation from Linearity of Rogowski coil having  
               
               
                 Non-uniform Winding with or without Magnetic Shielding 
               
            
           
           
               
               
               
            
               
                   
                   
                 Maximum Deviation  
               
               
                   
                 Case 
                 (Perfect Linearity = 0) 
               
               
                   
                   
               
               
                   
                 Without Magnetic Shielding 
                 0.97 
               
               
                   
                 With Magnetic Shielding 
                 0.08 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  9    is a flow chart of a method  900  of improving linearity of a current sensor of a circuit interrupter in accordance with an example embodiment of the disclosed concept. The method  900  may be performed by the circuit interrupter  10  as described with reference to  FIGS.  1 - 4 B  or any components thereof. 
     At  910 , the circuit passes input current through a primary conductor of a harvester circuit, where a ferromagnetic core of the harvester circuit saturates based on the input current reaching a first input current level of a range of input current levels, and output current of Rogowski coil is linear to the input current. 
     At  920 , the compensation coil delays saturation of the ferromagnetic core until the input current reaches a second input current level. Delaying the saturation of the harvester core includes generating magnetic flux in a direction of the harvester core and preventing linking magnetic flux of external magnetic field generated by at least the harvester core to the Rogowski until the second input current level is reached. As such, the compensation coil provides a fully automatic compensation for deviation of linearity. The fully automatic compensation includes an automatic measurement of a single calibration factor for correcting an error in the output of the Rogowski coil based at least in part on the saturation of the harvester core at the second input current level. 
     At  930 , the Rogowski coil measures voltage at an output of Rogowski coil, the measured voltage indicative of the output current of the Rogowski coil, the output current exhibiting no deviation of linearity of the output of the Rogowski coil greater than a deviation tolerance over the range of input current levels. The deviation tolerance may be, e.g., without limitation, 0.25% where 0%=perfect linearity). In some examples, the Rogowski coil includes a plurality of number of turns and two degree error in winding, each turn being represented as a slit and the two degree error in winding including seven slits two degree error in winding, where a first error rate of the Rogowski coil with the compensation coil is lower than a second error rate of the Rogowski coil without the compensation coil, the first error rate exhibits a sudden change in error rate over a first range starting from the first input current level, and the second error rate remains constant over the range of input current levels, the sudden change indicative of the deviation of the linearity greater than the deviation tolerance. 
     At  940 , an electronic trip unit of the circuit interrupter receives a signal comprising the measured voltage. 
     At  950 , the circuit interrupter determines whether the signal indicates a detected default. If yes, the method  900  proceeds to  960 , and at  960  the circuit interrupter is tripped, at  970  the user turns on the circuit interrupter after clearing the detected fault and the method  900  returns to  910 . If no, the method  900  returns to  910 . 
       FIG.  10    is a flow chart of a method  1000  of improving linearity of a current sensor of a circuit interrupter in accordance with an example embodiment of the disclosed concept. The method  1000  may be performed by the circuit interrupter  10  as described with reference to  FIGS.  1  and  5 - 8 B  or any components thereof. 
     At  1010 , the circuit interrupter passes input current through a primary conductor of a harvester circuit, where a ferromagnetic core of the harvester circuit saturates based on the input current reaching a first input current level of a range of input current levels, and output current of Rogowski coil is linear to the input current. 
     At  1020 , a magnetic shielding of magnetic material isolates a source of non-linearity generating external magnetic field from the Rogowski coil over a range of input current levels. Isolating the source from Rogowski coil includes generating a return path for magnetic flux of an external magnetic field generated by a source including at least the harvester circuit, a near-by pole-to-pole interaction, or a near-by current carrying components within the circuit interrupter. In some examples, the magnetic the magnetic flux lines of the external magnetic field to travel through the magnetic material to the non-linearity source via the return path. In some examples, the magnetic shielding absorbs the external magnetic field by preventing the magnetic flux lines to return to the non-linearity source via the air around the magnetic shielding. The magnetic material has high permeability, e.g., without limitation, greater than or equal to 10,000. The magnetic material may be mu alloy. In some example, the Rogowski coil includes non-uniform winding, where a first error rate of the Rogowski coil measured with the magnetic shielding present is lower than a second error rate of the Rogowski coil without the magnetic shielding, the first error rate exhibits a sudden change in the error rate over a first range starting from the first input current level, and the second error rate remains constant over the range of input current levels, the sudden change indicative of the deviation of the linearity greater than the deviation tolerance. 
     At  1030 , the Rogowski coil measures voltage at an output of Rogowski coil, the measured voltage indicative of the output current of the Rogowski coil, the output current exhibiting no deviation of linearity of the output of the Rogowski coil greater than a deviation tolerance over the range of input current levels. The deviation tolerance may be, e.g., without limitation, 0.25% where 0%=perfect linearity). 
     At  1040 , an electronic trip unit of the circuit interrupter receives a signal comprising the measured voltage. 
     At  1050 , the electronic trip unit determines whether to the signal indicates a detected default. If yes, the method  1000  proceeds to  1060 , and at  1060  the circuit interrupter is tripped, at  1070  the user turns on the circuit interrupter after clearing the detected fault and the method  1000  returns to  1010 . If no, the method  1000  returns to  1010 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.