Abstract:
A current sensor for measuring medium-voltage currents. The current sensor includes an input terminal configured to receive a current, an output terminal configured to transmit the current, a closed core made from a magnetic material and comprising a gap, at least one conductor operably connected to the input terminal and the output terminal and passing through the closed core, the at least one conductor sized to carry the current, and a molded case of solid dielectric material configured to encapsulate the closed core and the at least one conductor, wherein the gap and the terminals are not encapsulated by the molded case. The molded case is dimensioned such that internal and external spacings defined by the molded case are suitable for continuous operation with a medium voltage current as applied to the terminals and the at least one conductor while the core is at ground potential.

Description:
BACKGROUND 
     The present disclosure relates to a Hall Effect sensor for measuring current in medium-voltage applications. In certain embodiments, the present disclosure relates to a Hall Effect sensor for measuring current flowing from a medium-voltage variable-frequency drive to a load motor. 
     Power supplies configured to control a flow of energy between a first alternating current (AC) system and a second AC system are used in a variety of commercial and industrial applications. For example, a power supply is typically used in AC motor control and operation systems. Various power supplies convert energy from a first frequency and voltage to a second frequency and voltage. The first and/or the second frequency may be variable, for example from −330 Hertz to +330 Hertz, and may include a frequency of 0 Hertz, or DC. 
     In most power supplies, it is necessary to include devices to measure large currents in the first and/or the second AC system. These devices produce a small-scale signal which replicates the behavior of the large measured current, but is galvanically isolated from it. This isolated signal is then used for various purposes in the control circuits of the power supply. Often the current to be measured is the output current of the power supply, so that the sensor is located just before the output terminals. 
     For applications in which the current to be measured has a fixed frequency, or a frequency that varies over a limited range, a current transformer can often be used as the measuring device. However, standard current transformers generally cannot be used below a frequency of 25 Hertz. Custom-designed current transformers may extend the lower limit somewhat, but current transformers cannot be used for 0 Hertz, or DC. 
     Therefore in power supplies with a wide range of the first and/or the second frequency, it is customary to use current sensors based on Hall Effect technology instead of current transformers, Numerous suppliers offer Hall Effect current sensors for low-voltage applications. The principal market for these low-voltage sensors are variable frequency drives for AC motors operating at 1000 volts and below. This range is here designated as “low-voltage”. 
     There is also a growing market for variable frequency drives operating above 1000 volts, typically in the range from 1000 to 69,000 volts, This range is here designated as “medium-voltage”, Most commercially available Hall Effect current sensors are not sufficiently insulated for medium-voltage. A few Hail Effect current sensors are available with very large apertures, which can achieve higher levels of insulation by means of large air spacings. However, such large-aperture sensors are bulky and costly. Another disadvantage is that air tends to break down in an electric field much weaker than can be supported by many solid dielectric materials, and the break-down strength of air becomes even weaker as altitude is increased. 
     For these reasons, manufacturers of medium-voltage power supplies typically use conventional low-voltage Hall Effect current sensors. Because the sensor does not have sufficient insulation, a shielded medium-voltage cable is used to pass current through the low-voltage sensor. The shielded cable confines the electric field inside the cable insulating material, so that large external air spacings are not needed. However, where the shielded cable is terminated, bulky stress-cones are required. If the shielded cable is carrying the output current from the power supply, a separate termination device must be provided to receive both the shielded cable and also the user&#39;s load cables, Another drawback is that safety codes require a metal barrier between medium-voltage and low-voltage circuits, so that it is necessary to surround the low-voltage sensor with a grounded metal barrier box. Shielded cables for medium-voltage have much larger diameter than low-voltage cables of the same current capacity. Typically the largest shielded cable that will pass through the aperture of a low-voltage Hall Effect current sensor can handle only half of the current that the sensor can handle. For higher currents, two cables and a second sensor must be used, even though the first sensor alone could handle the current. These measures generally cost much more than the first Hall Effect sensor alone, and occupy a lot of space in the power supply enclosure. 
       FIG. 1  shows a prior art example to illustrate the drawbacks of using low-voltage Hall Effect current sensors in a medium-voltage power supply with an output current sensing circuit  100 . The circuit  100  includes a metal box  102  as a barrier between the low-voltage sensing components and the medium-voltage drive components. A pair of shielded cables  104   a  and  104   b  pass through the barrier box  102 , with grommets  103   a ,  103   b ,  103   c , and  103   d  to protect the cable from the metal edges. Each of the shielded cables  104   a  and  104   b  also pass through low-voltage Hall Effect sensors  108   a  and  108   b  respectively, mounted inside the barrier box  102 . The shielded cables  104   a  and  104   b  may include various insulations such as cross-linked polyethylene (XLPE). Stress cones  106   a ,  106   b ,  106   c , and  106   d  must be installed to avoid a concentration of the electric field at the ends of the cable shields. The shields in the cables  104   a  and  104   b  must be grounded as shown at  112   a . The shields in the stress cones  106   a ,  106   b ,  106   c , and  106   d  must be grounded as shown at  112   a  and  112   b . The barrier box  102  must be grounded as shown at  112   b.    
     In  FIG. 1  each of the shielded cables  104   a  and  104   b  are electrically connected to a medium-voltage bus bar  116  such that the high-current produced by the power supply is divided into two paths through the circuit  100 . The bus bar  116  must be supported by insulating standoffs  120   c  and  120   d . After passing through the low-voltage Hall Effect sensors  108   a  and  108   b , and the barrier box  102 , each shielded cable  104   a  and  104   b  is further electrically connected to a second bus bar  118 , where cables for delivering the power to the load can be operably connected by the user. The bus bar  118  must be supported by insulating standoffs  120   a  and  120   b.    
     Within barrier box  102 , the Hall Effect sensors  108   a  and  108   b  are operably connected to tow-voltage control wires  110   a  and  110   b  respectively. The control wires  110   a  and  110   b  provide control power to the sensors  108   a  and  108   b , and transfer any signals generated by the Hall Effect sensors  108   a  and  108   b  to the control circuits of the power supply. The control circuits may add the signals from sensors  108   a  and  108   b  to obtain a replica of the original current in the bus bar  116 . To provide a barrier between the low-voltage control wires  110   a  and  110   b  and the medium-voltage circuits, a rigid or flexible metal conduit  114  is typically used. 
     In a typical motor drive or power supply, the cost of the second Hall Effect sensor, the shielded cables, the stress cones, the barrier box, the standoffs, the miscellaneous hardware, and the assembly labor can be many times greater than the cost of the first Hall Effect sensor alone. Also the volume required for the complete assembly is many times greater than the volume of the first Hall Effect sensor alone. For a multi-phase motor drive, the high-current sensing circuit  100  as shown in  FIG. 1 , or a similar current sensing circuit, may be duplicated for each phase, thereby further increasing the cost and size. The total volume required for all the current sensing circuits can have a significant impact on the overall size of the drive. 
     SUMMARY 
     The present disclosure is directed to an embodiment of a Hall Effect current sensor improved to measure currents in a medium-voltage power supply or motor drive, while eliminating at least some of the drawbacks of the prior art. In particular, the complexity, the volume required, and the assembly time are substantially reduced. 
     This disclosure is not limited to the particular systems, devices and methods described, as these may vary. The terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope. 
     As used in this document, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Nothing in this document is to be construed as an admission that the embodiments described in this document are not entitled to antedate such disclosure by virtue of prior invention. As used in this document, the term “comprising” means “including, but not limited to,” and “encapsulate” means “completely surround.” 
     In one general respect, the embodiments disclose a current sensor for measuring medium-voltage currents. The current sensor includes an input terminal configured to receive a current, an output terminal configured to transmit the current, a closed core made from a magnetic material and comprising a gap, at least one conductor operably connected to the input terminal and the output terminal and passing through the closed core, the at least one conductor sized to carry the current, and a molded case of solid dielectric material configured to encapsulate the closed core and the at least one conductor, wherein the gap, the input terminal and the output terminal are not encapsulated by the molded case. The molded case is dimensioned such that internal and external spacings defined by the molded case are suitable for co lot s operation with an AC voltage between the range of 1,000 volts and 69,000 volts as applied to the input terminal, the output terminal and the at least one conductor while the core is at ground potential. 
     In another general respect, the embodiments disclose a system including a power supply, a load operably connected to the power supply, and at least one current sensor positioned between the power supply and the load. The current sensor includes an input terminal configured to receive a current, an output terminal configured to transmit the current, a closed core made from a magnetic material and comprising a gap, at least one conductor operably connected to the input terminal and the output terminal and passing through the closed core, the at least one conductor sized to carry the current, and a molded case of solid dielectric material configured to encapsulate the closed core and the at least one conductor, wherein the gap, the input terminal and the output terminal are not encapsulated by the molded case. The molded case is dimensioned such that internal and external spacings defined by the molded case are suitable for continuous operation with an AC voltage between the range of 1,000 volts and 69,000 volts as applied to the input terminal, the output terminal and the at least one conductor white the core is at ground potential. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art example of a system to measure current in a medium-voltage circuit, while using low-voltage Hall Effect sensors. 
         FIG. 2  illustrates a 3D view of an improved embodiment of a Hall Effect current sensor for medium-voltage applications, according to this invention. 
         FIG. 3  illustrates how the improved embodiment of a medium-voltage Hall Effect current sensor shown in  FIG. 2  could simplify the current sensing system of  FIG. 1 . 
         FIG. 4   a  illustrates a prior art block diagram of an open-loop Hall Effect current sensor. 
         FIG. 4   b  shows an illustrative interior view showing how an open-loop Hall Effect current sensor could be implemented in the embodiment shown in  FIG. 2 . 
         FIG. 4   c  illustrates a cross-section view of the configuration in  FIG. 4   b.    
         FIG. 4   d  illustrates a prior art block diagram of a closed-loop Hall Effect current sensor. 
         FIG. 4   e  shows an illustrative interior view showing how a closed-loop Hall Effect current sensor could be implemented in the embodiment shown in  FIG. 2 . 
         FIG. 4   f  illustrates a cross-section view of the configuration in  FIG. 4   e.    
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to an embodiment of a Hall Effect current sensor improved to measure currents in a medium-voltage power supply or motor drive, while eliminating at least some of the drawbacks of the prior art. In particular, the complexity, the volume required, and the assembly time are substantially reduced. 
       FIG. 2  shows an illustrative embodiment of a medium-voltage Hall Effect current sensor according to this invention. The sensor  200  may be molded inside a case  202  of solid insulating material such as epoxy or polyester resin. The molding process excludes air from all regions exposed to a strong electric field, and substitutes a solid dielectric material with much better breakdown strength. The molding material for the case  202  may be poured under vacuum, to ensure that no air bubbles are entrapped. The air in such bubbles could break down during operation, and the resulting partial discharge could cause gradual degradation of the solid insulation. The case may be dimensioned so that the external spacings between the medium-voltage terminals, and the control circuits or ground, provide the clearances for strike and creep needed for medium-voltage. Inside the case the clearances can be much smaller, due to the high breakdown strength of the solid insulating material. 
     A high-current terminal  206  may be provided to receive the current from the power supply circuits, and a second high-current terminal  208  may be provided to deliver the current to the load after measurement. The case  202  may be mounted on top of a metal box  204 , which contains a printed circuit board carrying some or all of the circuits needed for a Hall Effect current sensor. The box  204  may be equipped with two mounting flanges  210 , each with mounting holes  212  (the right-hand mounting flange is not visible in  FIG. 2 ). When the mounting flanges are bolted to a metal surface inside the power supply enclosure, the box  204  becomes grounded, and the space inside the box  204  becomes surrounded by a grounded metal barrier to separate the low-voltage circuits on the printed circuit board from any medium-voltage circuits. The height of the case  202  may be chosen to ensure that terminals  206  and  208  are spaced away from the box  204  sufficiently to provide the clearances for strike and creep needed for medium-voltage. The box  204  may be equipped with an access hole  214  suitable to attach a rigid or flexible metal conduit, as a barrier for the low voltage control wiring. A plate  216  secured by screws  218  may be temporarily removed to connect the control wiring to the printed circuit board. 
       FIG. 3  illustrates how the prior art assembly  100  of  FIG. 1  could be simplified by using the molded medium-voltage Hall Effect current sensor  200  of  FIG. 2 . In  FIG. 3  the second Hall Effect sensor  108   b , the shielded cables  104   a  and  104   b , the stress cones  103   a ,  103   b ,  103   c , and  103   d , the barrier box  102 , the standoffs  120   a ,  120   b ,  120   c , and  120   d , the miscellaneous hardware, and most of the assembly labor of  FIG. 1  have been eliminated. The bus bar  116  of  FIG. 1  ( 316  in  FIG. 3 ) now connects directly to terminal  206 . The user&#39;s load cables may be connected directly to terminal  208 . The molded medium-voltage Hall Effect current sensor  200  may have enough mechanical strength to support the bus bar  316  and the load cables, without needing the standoffs  120   a ,  120   b ,  120   c , and  120   d  of  FIG. 1 . The molded medium-voltage Hall Effect current sensor  200  may be slightly larger than the first low-voltage sensor  108   a  of  FIG. 1 , but the complete assembly  300  of  FIG. 3  may be substantially smaller than the assembly  100  of  FIG. 1 . 
     The molded medium-voltage Hall Effect current sensor  200  of  FIG. 2  may be implemented with either an open-loop or a closed-loop circuit, Both circuits are well known to those of ordinary skill in the art.  FIG. 4   a  shows a generalized prior art block diagram of an open-loop Hall Effect current sensor circuit  400   b . A heavy conductor  430  carrying the current to be measured may pass through the aperture of a magnetic core  426 . The core  426  may be made of any material with high relative permeability, such as steel laminations or powdered iron or ferrite. The core  426  is shown with a toroidal shape in  FIG. 4   a , but any shape which provides a closed path may be used. The current flowing in the conductor  430  creates a magnetic field in the core  426 , with magnitude proportional to the current. A small gap in the core  426  contains a Hall Effect element  428 . The Hall Effect element  428  produces a small signal proportional to the magnetic field passing through it. The small signal may be magnified by the amplifier  425  to produce the output signal  432 . For an open-loop circuit, the output signal is generally a voltage, 
       FIG. 4   b  shows a 2D side view  400   b  of the molded medium-voltage Hall Effect current sensor  200 , as implemented with an open-loop circuit.  FIG. 4   c  shows a 2D view  400   c  of section A-A of the same sensor  200 , as implemented with an open-loop circuit. In both  FIG. 4   b  and  FIG. 4   c , the molded case  402  is shown as transparent. A copper bar  430  carries the current to be measured, and is formed to create terminal  406  to receive the current and terminal  408  to deliver the current to the load. The terminals  406  and  408  may be provided with threaded holes for attaching bus bars or cables. The copper bar  430  may be bent or otherwise deformed so that any current flowing from terminal  406  to terminal  408  passes through a magnetic core  426 , which is grounded to the box  404 . 
     A small gap in the core  426  contains the Hall Effect element  428 . The molding process may leave the gap exposed, so that the Hall Effect element  428  may be installed later. The Hall Effect element  428  may be connected to a printed circuit board  420 , which contains the amplifier  425 . The amplifier output signal  432  is connected to a terminal strip or output device  422  mounted at one end of the printed circuit board  420 , near a removable access cover  416  and a hole  414  for a rigid or flexible conduit. Wires may be connected to the terminal strip or output device  422 , and routed through the rigid or flexible conduit to reach the control circuits as shown in  FIG. 3 . 
       FIG. 4   d  shows a generalized prior art block diagram of a closed-loop Hall Effect current sensor circuit  400   d . This closed-loop circuit  400   d  is similar to the open-loop circuit  400   a . A heavy conductor  430  carrying the current to be measured may pass through the aperture of a magnetic core  426 . The core  426  can be made of any material with high relative permeability, such as steel laminations or powdered iron or ferrite. The core  426  is shown with a toroidal shape in  FIG. 4   d , but any shape which provides a closed path may be used. The current flowing in the conductor  430  creates a magnetic field in the core  426 . A small gap in the core  426  may contain a Hall Effect element  428 . The Hall Effect element  428  produces a small signal proportional to the magnetic field passing through it. The small signal is magnified by the amplifier  425  to produce the output signal  432 . For a closed-loop circuit, the output signal is a current. This current  432  is passed through a winding  424  on the core  426 , in a direction opposite to the current in the conductor  430 . By this means the magnitude of the magnetic field in the core  426  is kept near zero. This allows a smaller gap, a higher gain in the amplifier  425 , and improves accuracy. 
       FIG. 4   e  shows a 2D side view  400   e  of the molded medium-voltage Hall Effect current sensor  200 , as implemented with a closed-loop circuit.  FIG. 4   f  shows a 2D view  400   f  of section A-A of the same sensor  200 , as implemented with a closed-loop circuit. In both  FIG. 4   e  and  FIG. 4   f , the molded case  402  is shown as transparent. A copper bar  430  carries the current to be measured, and is formed to create terminal  406  to receive the current and terminal  408  to deliver the current to the load. The terminals  406  and  408  may be provided with threaded holes for attaching bus bars or cables. The copper bar  430  may be bent or otherwise deformed so that any current flowing from terminal  406  to terminal  408  passes through a magnetic core  426 , which is grounded to the box  404 . 
     A small gap in the core  426  contains a Hall Effect element  428 . The molding process may leave the gap exposed, so that the Hall Effect element  428  can be installed later. The Hall Effect element  428  may be connected to a printed circuit board  420 , which contains the amplifier  425 . The amplifier output signal  432  drives one end of the winding  424 . The other end of winding  424  may be connected to a terminal strip or output device  422  mounted at one end of the printed circuit board  420 , near a removable access cover  416  and a hole  414  for a rigid or flexible conduit. Wires may be be connected to the terminal strip or output device  422  and routed through the rigid or flexible conduit to reach the control circuits as shown in  FIG. 3 . 
     Various of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments.