Sensitive DC current imbalance detector and calibration method

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.

FIELD OF THE INVENTION

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

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.

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.

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

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.

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.

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.

DETAILED DESCRIPTION

In accordance with an embodiment of the present invention,FIG. 1shows a depiction of current leakage detectors100incorporated within a pumping system102. It will be appreciated that the current leakage detector100can be incorporated within any electric equipment and that the discussion of the incorporation of the current leakage detector100within the pumping system102is merely an application for the current leakage detector100.

The pumping system102includes a submersible pump104driven by an electric motor106. When energized, the motor106moves the pump104, which forces fluids in the wellbore108to the surface. The motor106is provided with electric power from a surface-mounted power supply110. The power supply110may include electric generators and connections to a power grid. The pumping system102further includes a motor drive112and transformer114that condition and control the power provided to the motor106. In this way, the operational characteristics of the motor106can be controlled and affected by the motor drive112, transformer114and power supply110. Although the pumping system102is depicted as a submersible system used to recover fluids from an underground reservoir, it will be appreciated that the pumping system102might also include a surface pumping system that moves fluids between surface facilities.

A first current leakage detector100is, in an embodiment, incorporated within the motor drive112(as shown inFIG. 1) and used to monitor current passed to the transformer114. A second current leak detector100can be placed within the transformer114and used to monitor current passed to the motor106. Each current leakage detector100is configured to monitor the electric current passing into and out of a load. In the exemplary embodiment depicted inFIG. 1, the load is either the transformer114or the electric motor106. It will be appreciated that the load observed by the current leakage detector100could be any electric load that draws current from a power source. It will be further appreciated that additional or fewer current leakage detectors100may be used in embodiments.

Turning toFIG. 2, shown therein is a circuit diagram of an embodiment of the current leakage detector100. In embodiments, the current leakage detector100includes a power source116, a load118, a coil core120, a first sensing coil122, a second sensing coil124, a giant magneto-restrictive (GMR) sensor126, a sensor amplifier128, a sensor analog-to-digital converter (ADC)130, a bias coil132, a bias coil driver134, a control unit136and a switch138. In the exemplary application of the current leakage detector100inFIG. 1, the load118is the motor106and the transformer114. The power source116is used to provide power to the load118when the switch138is closed. The power source116is also in an embodiment configured to directly or indirectly provide power to the control unit136, bias coil driver134, sensor amplifier128and sensor ADC.

Current is directed to the load118from the power source116through the first sensing coil122. Current returns from the load118to the power source116through the second sensing coil124. The first and second sensing coils122,124are each wound around opposing sides of the coil core120. In an embodiment, the coil core120is formed as a unitary soft ferromagnetic core having a “block C” shape. The first and second sensing coils122,124have substantially the same number of turns and are wound in opposition on the core, but not necessarily on opposing legs of the coil core120so that the net magnetic coercive force produced by first and second sensing coils122,124is substantially eliminated when current passing through the first and second sensing coils122,124is the same.

If leakage current exists between the load118and the first and second sensing coils122,124, the current passing through the first and second sensing coils122,124will not be equal and the coercive magnetic force generated by the first and second sensing coils122,124will not be canceled. The GMR sensor126is magnetically coupled to the first and second sensing coils122,124and 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 coils122,124.

The magnetic field generated by the coercive magnetic force generated by the first and second sensing coils122,124and the response signal generated by the GMR sensor126are both grossly nonlinear. If the coercive magnetic force generated by the first and second sensing coils122,124is small, the GMR sensor126may not produce a representative output signal. The signal may be disproportionately small and may be characterized by an incorrect polarity.

To improve the response of the GMR sensor126, the current leakage detector100utilizes the bias coil132to provide a baseline magnetic field at the GMR sensor126. The bias coil132selectively applies a bias magnetic field that moves the response provided by the GMR sensor126into a more predictable and useful range. From the biased baseline range, the GMR sensor126can more accurately and robustly signal a field imbalance between the first and second sensing coils122,124. To place the response of the GMR sensor126within the biased baseline range, the current leakage detector100includes a bias circuit140. The bias circuit140can be characterized as the collection of the GMR sensor126, the sensor amplifier128, the sensor ADC130, the bias coil132, the bias coil driver134, and the control unit136.

Generally, the control unit136provides a control signal to the bias coil driver134. The bias coil driver134then applies a responsive drive current to the bias coil132. The bias coil132then produces a bias magnetic field that is picked up by the GMR sensor126. The GMR sensor126produces a signal that is representative of the bias magnetic field. The signal output by the GMR sensor126is provided to the sensor amplifier128and then to the sensor ADC130. The digitized signal is then passed back to the control unit136to complete the bias circuit140loop.

In embodiments, the bias circuit140is used to calibrate the GMR sensor126within a selected biased baseline range using algorithms implemented by the control unit136. An embodiment of a method200of calibrating the current leakage detector100is depicted inFIG. 3. The method begins at step202when the control unit136instructs the bias coil driver134to send a bias current (Ib) to the bias coil132. The magnetic field produced by the magnetic coercive force generated by the bias coil132is recognized by the GMR sensor126and registered by the control unit136. At step204, the bias current (Ib) is adjusted to the level at which the GMR sensor126outputs a minimum signal (Vmin). The minimum voltage output by the GMR sensor126is recorded by the control unit136.

At step206, the control unit136adjusts the current supplied to the bias coil132to an extent that produces the maximum voltage (Vmax) output by the GMR sensor126that can be accepted by the sensor amplifier128. The maximum voltage output by the GMR sensor126is recorded by the control unit136. Next, at step208, the control unit136sets an initial bias current (Ib0) at the value that produces a voltage at the GMR sensor126that is approximately at the median value (Vmid) between the minimum voltage (Vmin) and maximum voltage (Vmax) recorded by the control unit136. Because of the combined nonlinearities in the response of the bias coil132and GMR sensor126, the initial bias current (Ib0) that produces a midpoint voltage (Vmid) at the GMR sensor126may not represent a median value between the bias currents used to produce the minimum (Vmin) and maximum (Vmax) voltages at the GMR sensor126.

The method200of calibrating the current leakage detector100is in an embodiment carried out before the power source116is connected to the load118. Once the current leakage detector100is placed in operation, the GMR sensor126can be continuously or periodically recalibrated at step210to account for changes in the system. Such changes may include, for example, changes in the load118and temperatures changes at the first and second sensing coils. Recalibration can be carried out by adjusting the bias current (Ib) supplied to the bias coil132to find the median voltage (Vmid) output by the GMR sensor126.

In operation and after the initial bias current (Ib0) has been determined, the switch138can be closed to direct current from the power source116to the load118through the first and second sensing coils122,124. The bias coil132applies the initial bias magnetic field to place the response of the GMR sensor126within the desired range so that any imbalances between the first and second sensing coils122,124is more accurately detected by the GMR sensor126and reported by the control unit136. In embodiments, the control unit136triggers an alarm or notification if a leakage current condition is detected. The control unit136can also be configured to open the switch138or otherwise disconnect the power source116in the event a leakage current condition is detected.