Patent Abstract:
A current leakage detector for detecting current leakage between a power source and a load including a first sensing coil and a second sensing coil positioned opposite the first sensing coil. The current leakage detector further includes a magnetic field sensor proximate the first sensing coil and the second sensing coil and the magnetic field sensor has a response range. The current leakage detector also includes a bias circuit configured to adjust the response range of the magnetic field sensor. A method for detecting current leakage includes providing a first sensing coil and a second sensing coil. The method continues with the steps of providing a magnetic field sensor in proximity to the first and second sensing coils and providing a bias circuit. The method continues with the step of utilizing the bias circuit to place the response of the magnetic field sensor within a preferred response range.

Full Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of electric devices and more particularly, but not by way of limitation, to a current leak detector and method of calibration. 
       BACKGROUND 
       [0002]    In many conventional electric circuits, electric current flows from a power source to a load and back to the power source. The intended current path is typically achieved through use of insulated conductors and electrical components. If the insulation fails or the circuit is otherwise compromised, electric current may “leak” into unintended areas of the device. Leakage current is current that escapes the intended circuit path and returns to the power supply through an unintended route. 
         [0003]    Leakage current may travel from the circuit into a conductive housing or panel. If the housing or panel is properly grounded, the leakage current is diverted to ground. In some instances, however, the housing or panel may not be grounded or the ground may be insufficient to safely carry the leakage current. In these cases, anyone coming into contact with the housing or panel may be exposed to an electric shock. 
         [0004]    Prior art DC current leakage detectors tend to be difficult to calibrate and lack sensitivity. The deficiencies of the prior art current leakage detectors expose operators of electrical equipment to potential harm. There is, therefore, a need for an improved current leakage detector that can either alert an operator of a current leakage event or remove the power (or both) before the operator comes into contact with the hazardous equipment. 
       SUMMARY OF THE INVENTION 
       [0005]    In present embodiments, a current leakage detector is configured for detecting current leakage between a power source and a load. The current leakage detector includes a first sensing coil and a second sensing coil arranged in opposition to the first sensing coil. The current leakage detector further includes a magnetic field sensor proximate the first sensing coil and the second sensing coil and the magnetic field sensor has a response range. The current leakage detector also includes a bias circuit configured to adjust the response range of the magnetic field sensor. 
         [0006]    In another aspect, embodiments include an electrically powered device that includes a power supply, a load and a current leakage detector for detecting current leakage between the power supply and the load. The current leakage detector includes a first sensing coil and a second sensing coil arranged in opposition to the first sensing coil. The current leakage detector further includes a magnetic field sensor proximate the first sensing coil and the second sensing coil, and the magnetic field sensor has a response range. The current leakage detector also includes a bias circuit configured to adjust the response range of the magnetic field sensor. 
         [0007]    In yet another aspect, embodiments include a method for detecting current leakage between a power source and a load connected to the power source. The method includes the steps of providing a first sensing coil between the power source and the load and providing a second sensing coil arranged in opposition to the first sensing coil between the load and the power source. The method continues with the steps of providing a magnetic field sensor in proximity to the first and second sensing coils and providing a bias circuit. The method continues with the step of utilizing the bias circuit to place the response of the magnetic field sensor within a response range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a depiction of a current leakage detector constructed and installed within an electric submersible pumping system. 
           [0009]      FIG. 2  is a circuit diagram of the current leakage detector of  FIG. 1 . 
           [0010]      FIG. 3  is a process flow diagram for a method of calibrating the current leakage detector of  FIG. 1   
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In accordance with an embodiment of the present invention,  FIG. 1  shows a depiction of current leakage detectors  100  incorporated within a pumping system  102 . It will be appreciated that the current leakage detector  100  can be incorporated within any electric equipment and that the discussion of the incorporation of the current leakage detector  100  within the pumping system  102  is merely an application for the current leakage detector  100 . 
         [0012]    The pumping system  102  includes a submersible pump  104  driven by an electric motor  106 . When energized, the motor  106  moves the pump  104 , which forces fluids in the wellbore  108  to the surface. The motor  106  is provided with electric power from a surface-mounted power supply  110 . The power supply  110  may include electric generators and connections to a power grid. The pumping system  102  further includes a motor drive  112  and transformer  114  that condition and control the power provided to the motor  106 . In this way, the operational characteristics of the motor  106  can be controlled and affected by the motor drive  112 , transformer  114  and power supply  110 . Although the pumping system  102  is depicted as a submersible system used to recover fluids from an underground reservoir, it will be appreciated that the pumping system  102  might also include a surface pumping system that moves fluids between surface facilities. 
         [0013]    A first current leakage detector  100  is, in an embodiment, incorporated within the motor drive  112  (as shown in  FIG. 1 ) and used to monitor current passed to the transformer  114 . A second current leak detector  100  can be placed within the transformer  114  and used to monitor current passed to the motor  106 . Each current leakage detector  100  is configured to monitor the electric current passing into and out of a load. In the exemplary embodiment depicted in  FIG. 1 , the load is either the transformer  114  or the electric motor  106 . It will be appreciated that the load observed by the current leakage detector  100  could be any electric load that draws current from a power source. It will be further appreciated that additional or fewer current leakage detectors  100  may be used in embodiments. 
         [0014]    Turning to  FIG. 2 , shown therein is a circuit diagram of an embodiment of the current leakage detector  100 . In embodiments, the current leakage detector  100  includes a power source  116 , a load  118 , a coil core  120 , a first sensing coil  122 , a second sensing coil  124 , a giant magneto-restrictive (GMR) sensor  126 , a sensor amplifier  128 , a sensor analog-to-digital converter (ADC)  130 , a bias coil  132 , a bias coil driver  134 , a control unit  136  and a switch  138 . In the exemplary application of the current leakage detector  100  in  FIG. 1 , the load  118  is the motor  106  and the transformer  114 . The power source  116  is used to provide power to the load  118  when the switch  138  is closed. The power source  116  is also in an embodiment configured to directly or indirectly provide power to the control unit  136 , bias coil driver  134 , sensor amplifier  128  and sensor ADC. 
         [0015]    Current is directed to the load  118  from the power source  116  through the first sensing coil  122 . Current returns from the load  118  to the power source  116  through the second sensing coil  124 . The first and second sensing coils  122 ,  124  are each wound around opposing sides of the coil core  120 . In an embodiment, the coil core  120  is formed as a unitary soft ferromagnetic core having a “block C” shape. The first and second sensing coils  122 ,  124  have substantially the same number of turns and are wound in opposition on the core, but not necessarily on opposing legs of the coil core  120  so that the net magnetic coercive force produced by first and second sensing coils  122 ,  124  is substantially eliminated when current passing through the first and second sensing coils  122 ,  124  is the same. 
         [0016]    If leakage current exists between the load  118  and the first and second sensing coils  122 ,  124 , the current passing through the first and second sensing coils  122 ,  124  will not be equal and the coercive magnetic force generated by the first and second sensing coils  122 ,  124  will not be canceled. The GMR sensor  126  is magnetically coupled to the first and second sensing coils  122 ,  124  and is configured to output an analog signal in response to the magnetic field generated by the presence of the coercive magnetic force generated by the current imbalance in the first and second sensing coils  122 ,  124 . 
         [0017]    The magnetic field generated by the coercive magnetic force generated by the first and second sensing coils  122 ,  124  and the response signal generated by the GMR sensor  126  are both grossly nonlinear. If the coercive magnetic force generated by the first and second sensing coils  122 ,  124  is small, the GMR sensor  126  may not produce a representative output signal. The signal may be disproportionately small and may be characterized by an incorrect polarity. 
         [0018]    To improve the response of the GMR sensor  126 , the current leakage detector  100  utilizes the bias coil  132  to provide a baseline magnetic field at the GMR sensor  126 . The bias coil  132  selectively applies a bias magnetic field that moves the response provided by the GMR sensor  126  into a more predictable and useful range. From the biased baseline range, the GMR sensor  126  can more accurately and robustly signal a field imbalance between the first and second sensing coils  122 ,  124 . To place the response of the GMR sensor  126  within the biased baseline range, the current leakage detector  100  includes a bias circuit  140 . The bias circuit  140  can be characterized as the collection of the GMR sensor  126 , the sensor amplifier  128 , the sensor ADC  130 , the bias coil  132 , the bias coil driver  134 , and the control unit  136 . 
         [0019]    Generally, the control unit  136  provides a control signal to the bias coil driver  134 . The bias coil driver  134  then applies a responsive drive current to the bias coil  132 . The bias coil  132  then produces a bias magnetic field that is picked up by the GMR sensor  126 . The GMR sensor  126  produces a signal that is representative of the bias magnetic field. The signal output by the GMR sensor  126  is provided to the sensor amplifier  128  and then to the sensor ADC  130 . The digitized signal is then passed back to the control unit  136  to complete the bias circuit  140  loop. 
         [0020]    In embodiments, the bias circuit  140  is used to calibrate the GMR sensor  126  within a selected biased baseline range using algorithms implemented by the control unit  136 . An embodiment of a method  200  of calibrating the current leakage detector  100  is depicted in  FIG. 3 . The method begins at step  202  when the control unit  136  instructs the bias coil driver  134  to send a bias current (I b ) to the bias coil  132 . The magnetic field produced by the magnetic coercive force generated by the bias coil  132  is recognized by the GMR sensor  126  and registered by the control unit  136 . At step  204 , the bias current (I b ) is adjusted to the level at which the GMR sensor  126  outputs a minimum signal (V min ). The minimum voltage output by the GMR sensor  126  is recorded by the control unit  136 . 
         [0021]    At step  206 , the control unit  136  adjusts the current supplied to the bias coil  132  to an extent that produces the maximum voltage (V max ) output by the GMR sensor  126  that can be accepted by the sensor amplifier  128 . The maximum voltage output by the GMR sensor  126  is recorded by the control unit  136 . Next, at step  208 , the control unit  136  sets an initial bias current (I b0 ) at the value that produces a voltage at the GMR sensor  126  that is approximately at the median value (V mid ) between the minimum voltage (V min ) and maximum voltage (V max ) recorded by the control unit  136 . Because of the combined nonlinearities in the response of the bias coil  132  and GMR sensor  126 , the initial bias current (I b0 ) that produces a midpoint voltage (V mid ) at the GMR sensor  126  may not represent a median value between the bias currents used to produce the minimum (V min ) and maximum (V max ) voltages at the GMR sensor  126 . 
         [0022]    The method  200  of calibrating the current leakage detector  100  is in an embodiment carried out before the power source  116  is connected to the load  118 . Once the current leakage detector  100  is placed in operation, the GMR sensor  126  can be continuously or periodically recalibrated at step  210  to account for changes in the system. Such changes may include, for example, changes in the load  118  and temperatures changes at the first and second sensing coils. Recalibration can be carried out by adjusting the bias current (I b ) supplied to the bias coil  132  to find the median voltage (V mid ) output by the GMR sensor  126 . 
         [0023]    In operation and after the initial bias current (I b0 ) has been determined, the switch  138  can be closed to direct current from the power source  116  to the load  118  through the first and second sensing coils  122 ,  124 . The bias coil  132  applies the initial bias magnetic field to place the response of the GMR sensor  126  within the desired range so that any imbalances between the first and second sensing coils  122 ,  124  is more accurately detected by the GMR sensor  126  and reported by the control unit  136 . In embodiments, the control unit  136  triggers an alarm or notification if a leakage current condition is detected. The control unit  136  can also be configured to open the switch  138  or otherwise disconnect the power source  116  in the event a leakage current condition is detected. 
         [0024]    It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of embodiments of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention. 
         [0025]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Technology Classification (CPC): 7