Systems and methods for calibrating current transformers

Abstract

System, comprising: a housing (72) having a first cavity (74) and a second cavity (76); a current sensor (12) arranged in the first cavity (74); a calibration circuit (14) arranged in the second cavity (76); and a shielding assembly disposed in the housing (72) around the current sensor (12), the shielding assembly comprising first and second shielding portions disposed on axially opposite sides of the current sensor (12) and third and fourth shielding portions disposed on radially opposite sides of the current sensor (12); wherein the calibration circuit (14) comprises an adjustable phase shift resistor (22) and an adjustable sensitivity resistor (24) connected in series with each other, and a controller (32) configured to calibrate the current sensor (10) by adjusting the resistances of the phase shift resistor (22) and the sensitivity resistor (24) until a voltage waveform detected at an output terminal (28) of the calibration circuit (14) has phase shift and sensitivity characteristics that correspond to the phase shift and sensitivity characteristics of a desired voltage waveform.

Description

BACKGROUND

[0001] The present invention relates to systems and methods for calibrating current transformers. In particular, the invention relates to the calibration of phase and sensitivity characteristics of a current transformer output signal.

[0002] A current transformer generally comprises a winding around a magnetic core. When the current transformer is placed around a cable or other type of conductor, a time-varying current (e.g., alternating current) flowing through the cable can generate a time-varying magnetic field in the magnetic core. The magnetic field can then induce a current in the winding of the current transformer. The current in the winding can be proportional to the current flowing through the cable. The current in the winding can thus be used to measure the magnitude and direction of the current flowing in the cable.

[0003] However, conventional current transformers have a relatively high tolerance in their electrical (i.e., resistance) and magnetic (i.e., inductance) properties due to the magnetic properties of the materials in the current transformers and the manufacturing process used to manufacture the current transformers. Each current transformer can therefore have its own sensitivity or tolerance depending on its specific properties. This sensitivity introduces a greater uncertainty into the electrical signal output by each current transformer. These specific properties can also introduce a phase shift between the signal output by the current transformer and a measured current. It would therefore be advantageous to calibrate current transformers to operate within specified or known tolerances and phase shifts.

[0004] Conventional current transformers also use a steel tape to shield integrated components such as a current sensor from various types of noise (e.g., electrical or magnetic noise). The steel tape can be wrapped around a piece of metal to create a shield that can be placed around the current sensor. Using four steel tape-wound shields, each side of the current sensor can be shielded against magnetic interference and noise. However, the fabrication of each steel tape-wound shield is a complex process that can easily be performed improperly, compromising the integrity of the entire shielding around the current sensor. It would therefore be advantageous to provide a simpler-to-manufacture device for shielding the components in the current transformer.

[0005] JP 2011 - 243 773 A discloses a system comprising a housing, a current sensor arranged therein and a shielding arrangement arranged around the current sensor, which has annular shielding parts arranged on axially and radially opposite sides of the current sensor in order to shield the current sensor against external magnetic fields.

[0006] DE 10 2012 108 797 A1 discloses a current transformer assembly comprising a housing having a plurality of shielding elements defining a cavity therein in which a current sensor is accommodated. Each shielding element comprises a plurality of tape-wound silicon steel layers to reduce electrical interference transmitted from the housing to the current sensor.

[0007] JP 2004 - 14 685 A discloses a shielding arrangement for a current sensor, which comprises shielding elements that shield a current sensor from all axial and radial sides against external interference.

[0008] GB 1 285 346 A discloses a power meter mounted in a housing, comprising a current transformer and a calibration circuit connected to the current transformer. The calibration circuit includes a capacitor connected across a coil of the current transformer for phase compensation and a calibration resistor whose resistance is selected to suit the meter's pointer deflection to enable a correct reading of the measured power.

[0009] US 2010 / 0 321 032 A1 discloses a current measuring device which, in one embodiment, is designed as a current sensor switch having a housing and a current sensor arranged therein, wherein the housing has a setting control with a rotary knob and a calibration scale arranged around the rotary knob to enable a user to calibrate the current sensor. SHORT DESCRIPTION

[0010] Certain embodiments that are consistent with the scope of the original claims are summarized below. These embodiments are not intended to limit the scope of the claims, but rather to provide a brief summary of possible forms of the presently disclosed systems and methods. Thus, the claims may encompass various forms that may be similar to or different from the embodiments set forth below.

[0011] In one aspect of the invention, a system is provided comprising: a housing having a first cavity and a second cavity; a current sensor disposed in the first cavity; a calibration circuit disposed in the second cavity; and a shield assembly disposed in the housing around the current sensor, the shield assembly comprising first and second shield portions disposed on axially opposite sides of the current sensor and third and fourth shield portions disposed on radially opposite sides of the current sensor.wherein the calibration circuit comprises an adjustable phase shift resistor and an adjustable sensitivity resistor connected in series with each other, and a controller configured to calibrate the current sensor by adjusting the resistances of the phase shift resistor and the sensitivity resistor until a voltage waveform detected at an output terminal of the calibration circuit has phase shift and sensitivity characteristics that correspond to the phase shift and sensitivity characteristics of a desired voltage waveform.;

[0012] The first shielding portion may comprise a first circular ring that may be disposed adjacent to a first side of a current sensor. The second shielding portion may comprise a second circular ring that may be disposed adjacent to a second side of the current sensor opposite the first side, such that the first and second circular rings each contain magnetically permeable material.

[0013] The third shielding portion may include a third circular ring disposed adjacent to a third side of the current sensor; and the fourth shielding portion may include a fourth circular ring disposed adjacent to a fourth side of the current sensor opposite the third side.

[0014] The distance between an inner radius and an outer radius of the first circular ring of any aforementioned system may be greater than or equal to a sum of a thickness of the third circular ring, a thickness of the current sensor, and a thickness of the fourth circular ring.

[0015] The distance between an inner radius and an outer radius of the second circular ring of any aforementioned system may be greater than or equal to a thickness of the third circular ring, a thickness of the current sensor, and a thickness of the fourth circular ring.

[0016] The first and second circular rings of any previously mentioned system may be axially spaced from each other, and the third and fourth circular rings may be radially spaced from each other.

[0017] The third and fourth circular rings of any previously mentioned system may be concentric with each other.

[0018] The third and fourth circular rings of any previously mentioned system may be tape-wound shields.

[0019] The third and fourth circular rings of any previously mentioned system may each comprise the magnetically permeable material.

[0020] The magnetically permeable material of any of the aforementioned systems may include Supermalloy, Metglas®, Ultraperm®, MuMETAL®, or any combination thereof.

[0021] The first and second circular rings of any previously mentioned system may each partially cover both the third and fourth circular rings.

[0022] The shielding parts of any of the aforementioned systems may be removably disposed within the housing.

[0023] The housing of any of the aforementioned systems may comprise: a housing portion having an inner wall circumferentially disposed about a central opening, an outer wall circumferentially disposed about the inner wall, and a floor extending between the inner and outer walls; and a lid portion removably connected to the housing portion via one or more fasteners.

[0024] The first and third shielding parts of any previously mentioned system may comprise a single part.

[0025] The first, third and fourth shielding parts of any previously mentioned system may comprise a single part.

[0026] In a further aspect, a method of operating the above-described system according to the first aspect is provided, the method comprising: calibrating the current sensor via the calibration circuit, wherein the calibration comprises: receiving an expected measurement output signal corresponding to the current sensor, the expected measurement output signal comprising expected phase and sensitivity characteristics; sending a signal to a current source for supplying a current to a conductor monitored by the current sensor; receiving an instantaneous measurement output signal from the current sensor, the instantaneous measurement output signal comprising instantaneous phase and sensitivity characteristics; and adjusting the instantaneous phase and sensitivity characteristics by adjusting the resistances of the phase shift resistor and the sensitivity resistor to substantially correspond to the expected phase and sensitivity characteristics;and measuring a current flowing through one or more conductors by means of the current sensor. Measuring a current flowing through one or more conductors by means of the current sensor while absorbing electrical and / or magnetic interference with the shielding arrangement arranged around the current sensor.; BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other features, aspects and advantages of the present invention will be better understood from the following detailed description with reference to the accompanying drawings, in which like reference numerals represent like parts throughout the drawings, in which: Fig. 1 is a simplified diagram illustrating one embodiment of a current transformer calibration system in accordance with aspects of the present disclosure; Fig. 2 an embodiment of a calibration circuit in the current transformer calibration system of Fig. 1 according to aspects of the present disclosure; Fig. 3 is a flow diagram illustrating one embodiment of a method for calibrating a current transformer using the current transformer calibration system of Fig. 1 according to aspects of the present disclosure; Fig. 4 is a perspective top view of an embodiment of a housing for a shield for shielding a current transformer in the current transformer calibration system of Fig. 1 according to aspects of the present disclosure; Fig. 5 an interior view of an embodiment of the housing of Fig. 4 according to aspects of the present disclosure; Fig. 6 an exploded view of an embodiment of the shielding of Fig. 4 according to aspects of the present disclosure; Fig. 7 a plan view of a square-shaped embodiment of the shield of Fig. 4 according to aspects of the present disclosure; Fig. 8 a plan view of an octagonal shaped embodiment of the shield of Fig. 4 according to aspects of the present disclosure; and Fig. 9 a plan view of a hexagonal shaped embodiment of the shield of Fig. 4 according to aspects of the present disclosure. DETAILED DESCRIPTION

[0028] One or more specific embodiments are described below. In order to succinctly describe these embodiments, not all features of an actual implementation are described in the description. It should be recognized that in developing such an actual implementation, as with any engineering or design project, numerous implementation-specific decisions must be made to achieve the designers' specific goals, such as meeting system and business requirements that may differ from one implementation to the next. It should further be recognized that this development effort could be complex and time-consuming for one of ordinary skill in the art having the benefit of this disclosure, but would nevertheless be routine in design, manufacture, and production.

[0029] When elements of various embodiments of the present invention are presented, the terms "a," "an," "the," "the," and "the" are intended to mean that one or more of the elements are present. The terms "comprising," "containing," and "having" are intended to be inclusive and mean that additional elements may be present besides those listed.

[0030] Current transformers can be used for various purposes, including measuring the current input and / or output of a device (e.g., generator, motor), determining the amount of leakage current in the device, or similar. A demanding application for a current transformer may involve accurately measuring the leakage current of the stator windings of a motor or generator in real time. Leakage current is a complex signal that has an amplitude and phase (i.e., related to the motor or generator line voltage) with a real (i.e., resistive) component and an imaginary (i.e., capacitive) component. The leakage current signal level is also generally very low compared to the motor line current signal. An accurately calibrated current transformer would therefore be advantageous for capturing this measurement.

[0031] Against this background, embodiments of the present disclosure generally relate to a calibration circuit and methods for using the calibration circuit to calibrate various current transformers so that each current transformer can exhibit similar sensitivity and phase characteristics in its measurements. According to the invention, the calibration circuit comprises two series-connected variable resistors. The first variable resistor is used to adjust the sensitivity of the current transformer's measurement output signal, while the second variable resistor is used to adjust the phase shift of the measurement output signal.By controlling the sensitivity and phase shift of the measurement output signal of each current transformer, the calibration circuit can enable each current transformer to be calibrated to specific specifications, thereby ensuring uniformity between manufactured current transformers and improving the data quality for the measured values ​​acquired by each calibrated current transformer.

[0032] In addition to providing systems and methods for calibrating current transformers, embodiments of the present disclosure also generally relate to a fixed side shield that may be coupled to the sides of a current sensor within the current transformer. The fixed side shield may protect the current sensor from various types of noise that may affect the current transformer's measurement output. With this in mind, in one embodiment, the fixed side shield may be fabricated to form four circular rings that can fit within a housing, which may be used to shield each side of the current sensor. The circular rings may be made of a magnetically permeable material and combined to fit snugly around each axial side of the current sensor.The circular rings can be manufactured to precise dimensions using a laser, waterjet, or similar. Because the circular rings can be manufactured to form a shielding barrier around each axial side of the current sensor, the fixed side shield can provide superior shielding compared to using a conventional steel tape-wound shield.

[0033] For introduction, Fig. 1 shows a simplified graphical representation of a current transformer calibration system (current transformer 10). Current transformer 10 may include a current sensor 12 and a calibration circuit 14. Current sensor 12 may be a current transducer that utilizes a toroidal inductor and a ferrite core to detect a magnetic coupling field from an electric current flowing through a conductor. Calibration circuit 14 may be used to calibrate the measurement output signal of current transformer 10.

[0034] In one embodiment, the current sensor 12 may be coupled around a conductor 16 that can conduct the current I. The ring-shaped inductor may here provide a current or voltage output signal (ie, measurement output signal of the current sensor 12) that is proportional to the amplitude of the current I. Although in Fig. 1, the current sensor 12 is illustrated as being coupled around a single conductor 16, it should be noted that the current sensor 12 may be coupled around multiple conductors. The current sensor 12 may thus measure the total current across all conductors monitored by the current sensor 12.

[0035] The measurement output signal of the current sensor 12 (i.e., the measurement output signal of the current transformer 10) may generally exhibit a phase shift (e.g., degrees) between the measurement output signal and the current I. The magnitude of the phase shift may depend on the characteristic inductance, capacitance, and / or resistance in the current transformer 10 (i.e., in the current sensor 12 and the calibration circuit 14), as well as an impedance of any load circuit coupled to the current transformer 10. Furthermore, since each individual current transformer 10 may have different characteristic inductance, capacitance, and / or resistance properties, each current transformer 10 may have a different sensitivity or tolerance. This means that each current transformer 10 may output a slightly different measurement value for the same input current due to the different sensitivity of each current transformer 10.For example, one current transformer 10 may output 1 volt when 100 amperes of current flow through conductor 16, while another current transformer 10 may output 1.1 volts when the same 100 amperes of current flow through conductor 16. The sensitivity of each current transformer 10 may vary due to various factors, including a number of winding turns on the toroidal inductor, a characteristic resistance of the toroidal inductor, the resistance of a load circuit, and the like.

[0036] With this in mind, the calibration circuit 14 can control the phase shift and / or sensitivity characteristics of the current transformer 10. This means that the calibration circuit 14 can adjust the phase shift and / or sensitivity characteristics of the output signal of the current transformer 10 to correspond to a desired phase shift and / or sensitivity characteristic. In one embodiment, the calibration circuit 14 can be used to calibrate different current transformers 10 so that the measurement output signal for each current transformer 10 can be substantially similar or the same (e.g., less than 1%). For example, the calibration circuit 14 can adjust the phase shift and sensitivity characteristics of the measurement output signal of multiple current transformers 10 so that each current transformer 10 outputs substantially similar or the same values ​​for different input currents (e.g., current I).

[0037] The calibration circuit 14 can control the phase shift and / or sensitivity characteristics of the current transformer 10 by adjusting the resistance of two variable resistors in the calibration circuit 14. Fig. For example, Figure 2 illustrates a circuit diagram 20 of the current transformer 10 showing the current sensor 12 coupled to the calibration circuit 14. As in Fig. 2, the calibration circuit 14 includes a phase shift resistor 22 connected in series with a sensitivity resistor 24, which can be used to control the phase shift and sensitivity characteristics of the measurement output signal of the current transformer 10, respectively. The phase shift resistor 22 and the sensitivity resistor 24 can be variable resistors that can be adjusted to calibrate the phase shift and sensitivity characteristics of the measurement output signal of the current transformer 10. In certain embodiments, the phase shift resistor 22 and the sensitivity resistor 24 can be adjusted until the measurement output signal of the current transformer 10 substantially corresponds to a desired or predetermined output measurement value.In this way, multiple current transformers 10, each having a different current sensor 12 and a different calibration circuit 14, can be calibrated such that each calibrated current transformer 10 can exhibit the same measurement characteristics, even though each current transformer 10 may have different self-inductance, capacitance, resistance, and the like. If an active current transformer 10 fails in the field, the calibration circuit 14 can further be used to calibrate a backup current transformer 10 so that the failed current transformer 10 can be replaced with a current transformer 10 having the same phase shift and sensitivity characteristics as the current transformer 10 being replaced. The backup current transformer 10 can thus continue to provide a user or system with measurement data similar to or equal to that of the previously used current transformer 10, thereby preserving the integrity of all subsequent measurement data acquired.

[0038] In one embodiment, the calibration circuit 14 may be coupled to a secondary winding of the current sensor 12. The calibration circuit 14 may also be coupled to a protection circuit 26, which may include a plurality of resistors, diodes, Zener diodes, and the like. The protection circuit 26 may protect the phase-shifting resistor 22, the sensitivity resistor 24, and the current transformer 10 from voltage spikes, short circuits, and the like. The protection circuit 26 may also protect additional equipment connected to the current transformer 10 or the calibration circuit 14 from excessive energy. Examples of the additional equipment may include signal conditioning equipment, various types of monitoring units, plant control equipment, and the like.

[0039] The calibration circuit 14 has an output terminal 28 that can measure a voltage across the sensitivity resistor 24. The output terminal 28 can correspond to the measurement output signal of the current transformer 10 and can also be used to calibrate the current transformer 10. Thus, the voltage waveform sensed via the output terminal 28 can be used to determine whether the phase shift or sensitivity characteristics of the current transformer 10 should be adjusted to target values ​​provided by a user, a model, a table, or the like. Furthermore, in certain embodiments, the resistance of the phase shift resistor 22 and the sensitivity resistor 24 can each be adjusted until the voltage waveform sensed via the output terminal 28 substantially corresponds to a desired voltage waveform having specified phase shift and sensitivity characteristics.

[0040] As previously mentioned, the measurement output signal of current transformer 10 may be proportional to the current I flowing through conductor 16. With this in mind, the voltage waveform detected at output terminal 28 is also proportional to the current I. In one embodiment, current I may be supplied to conductor 16 from a current source 30. Current source 30 may be a precise current source capable of supplying current I, such that current I may accurately reflect a current value specified or input to current source 30. In this way, current transformer 10 may be accurately calibrated based on a known current value supplied to conductor 16.

[0041] The phase-shift resistor 22, the sensitivity resistor 24, and the current source 30 can generally be individually controlled and operated by a user / operator, who can use the calibration circuit 14 to calibrate the current transformer 10. According to the invention, the phase-shift resistor 22, the sensitivity resistor 24, and the current source 30 are coupled to a controller 32. The controller 32 can control the phase-shift resistor 22, the sensitivity resistor 24, and the current source 30 using a data transmission component 34, a processor 36, a memory 38, a data storage 40, input / output (I / O) channels 42, and the like.

[0042] The data transmission component 34 may be a wireless or wired data transmission component that can facilitate the transmission of data between various components (e.g., the power source 30) in the power converter 10. The processor 36 may be any type of computer processor or microprocessor capable of executing computer-executable code. The memory 38 and the data storage 40 may be any suitable article of manufacture capable of serving as a tangible machine-readable medium for storing code or instructions executable by a processor. These articles of manufacture may be computer-readable media (e.g., any suitable form of memory or data storage) capable of storing processor-executable code executable by the processor 36 to perform the presently disclosed methods.

[0043] The controller 32 may also be coupled to the output terminal 28 so that it can monitor the voltage waveforms output by the current transformer 10. In one embodiment, the controller 32 may receive phase shift and sensitivity characteristics from a user and then automatically calibrate the current transformer 10. This means that the controller 32 may instruct the current source 30 to supply a current value to the conductor 16 and then monitor the voltage waveform at the output terminal 28. The controller 32 may then calibrate the current transformer 10 by adjusting the resistance of the phase shift resistor 22 and the sensitivity resistor 24 until the voltage waveform sensed at the output terminal 28 exhibits phase shift and sensitivity characteristics that substantially match the received phase shift and sensitivity characteristics.Additional details regarding a method for calibrating the current transformer 10 using the calibration circuit 14 are described in more detail below with reference to FIG. Fig. 3 described.

[0044] Fig. Figure 3 illustrates a flowchart of a method 50 that may be used to calibrate the current transformer 10 using the calibration circuit 14. The method 50 may be performed by the controller 32.

[0045] In block 52, the controller 32 may receive an input current waveform and an expected or desired current transformer measurement output signal. The input current waveform may include an amplitude and frequency for a current waveform representing the current I that may be supplied to conductor 16 via current source 30. The expected current transformer measurement output signal, in turn, may correspond to an expected output value associated with the input current waveform for the current I flowing through conductor 16 in current transformer 10. Furthermore, the current transformer measurement output signal may also specify a particular phase shift by which a current transformer measurement output signal waveform may be shifted from the supplied current I waveform.

[0046] After receiving these values, the controller 32 may send a signal to the current source 30 in block 54 to supply the current I to the conductor 16 that corresponds to the input current waveform received in block 52. The current source 30 may then supply the corresponding current I to the conductor 16.

[0047] In block 56, the controller 32 may receive the current transformer's measurement output signal via the output terminal 28. In one embodiment, the controller 32 may receive a voltage waveform that may be proportional to a current waveform associated with the current I flowing through the conductor 16.

[0048] Using the expected current transformer measurement output received in block 52 and the instantaneous current transformer measurement output received in block 56, the controller 32 may determine in block 58 whether the instantaneous current transformer measurement output substantially matches the expected current transformer measurement output. For example, the controller 32 may determine whether the amplitude of the instantaneous voltage waveform matches the amplitude of the expected voltage waveform to the same degree or within the same tolerance (e.g., less than 1%). The controller 32 may also further determine whether the instantaneous voltage waveform is in phase with the expected voltage waveform.If either the amplitude of the instantaneous voltage waveform does not match the amplitude of the expected voltage waveform or if the instantaneous voltage waveform is not in phase with the expected voltage waveform, the controller 32 may proceed to block 60.

[0049] In block 60, the controller 32 may send a signal to the phase-shift resistor 22, the sensitivity resistor 24, or both to adjust their respective resistances so that the amplitude and phase of the instantaneous voltage waveform detected at the output terminal 28 correspond to the amplitude and phase of the expected voltage waveform. In one embodiment, the controller 32 may adjust the resistance of the phase-shift resistor 22 and the sensitivity resistor 24 according to the transfer function of Equation 1, as indicated below: V¯oI¯t=NpNs[ω2Lm2RL(RS+RF+RL)2+(ωLm)2+jωLmRL(RS+RF+RL)(RS+RF+RL)2+(ωLm)2] where V¯o corresponds to the actual voltage detected at output terminal 28, It corresponds to the current I flowing through the conductor 16, N p the number of primary windings in the current transformer calibration system 10, N s the number of secondary windings in the current sensor 12, ω corresponds to the angular frequency of the current I, L m corresponds to the inductance of the current sensor 12, R S corresponds to the resistance of the windings in the current sensor 12, R F corresponds to the resistance of the phase shift resistor 22 and R L corresponds to the sensitivity resistance 24.

[0050] Furthermore, the phase angle can be described according to the following equation 2: θ(phase angle)=ArctanRS+RF+RLωLm

[0051] The control 32 can thus adjust the resistance of the phase-shift resistor 22 and the sensitivity resistor 24 based on the actual voltage sensed via the output terminal 28 and Equations 1 and 2. The control 32 can then return to block 56 and receive an updated measurement output signal from the current transformer and continuously process blocks 56, 58, and 60 until the instantaneous voltage waveform sensed via the output terminal 28 substantially matches the expected voltage waveform (e.g., less than 1% difference). The control 32 can thus adjust the resistance of the phase-shift resistor 22 and the sensitivity resistor 24 until the instantaneous voltage waveform substantially matches the expected voltage waveform.

[0052] Referring to block 58, if the instantaneous voltage waveform matches the expected voltage waveform, the controller 32 may proceed to block 62 and terminate the method 50. This means that the current transformer 10 may be considered calibrated with respect to the parameters received in block 52 because, in block 62, the instantaneous voltage waveform matches the expected voltage waveform. In certain embodiments, the method 50 may be performed on multiple current transformers shortly after manufacture. Each of the manufactured current transformers may thus have substantially similar phase shift and sensitivity characteristics. Current transformers may therefore be manufactured using less expensive (e.g., replaceable) components because their effects on the capacitance, inductance, and resistance of the current transformer can be compensated for using the calibration circuit 14.

[0053] Furthermore, as previously mentioned, a challenging aspect associated with the use of current transformer 10 may involve accurately measuring a leakage current from the stator windings of a motor or generator in real time. Excessive leakage current in the stator windings can cause damage to the motor or generator. However, leakage current is a complex signal that has amplitude and phase (i.e., related to the motor's line voltage), and both a real (e.g., resistive) and an imaginary (e.g., capacitive) component, and its signal level can be very low compared to the motor's line current signal. A precision, high-performance current sensor can therefore be advantageous in consistently capturing accurate leakage current measurements.

[0054] Using the calibration circuit 14 described above, multiple similarly designed current transformers can consistently output similar or identical measured values ​​for the same input currents. The calibration circuit 14 can therefore calibrate various toroidal current transformers 10, which may have high tolerances in their electrical (e.g., resistance) and magnetic (e.g., inductance) properties due to the magnetic properties of the materials and the manufacturing process used to manufacture them. The calibration circuit 14 can also compensate for a phase shift between an output signal of the current transformer 10 and a measured leakage current. The calibration circuit 14 described above can therefore enable current transformers 10 to be manufactured with much larger variations in component specifications.As a result, manufacturing costs associated with current transformer manufacture can be reduced and each current transformer 10 can be calibrated to meet more stringent performance specifications.

[0055] In addition to using the calibration circuit 14 to produce current transformers 10 with similar output measurements, a solid side shield can be used to uniformly shield each manufactured current transformer 10. As previously mentioned, conventional tape-wound shields may not be manufactured uniformly due to the complexity of the manufacturing process. Thus, each current transformer 10 shielded using a tape-wound shield may be differently affected by electrical and magnetic noise, thereby affecting the measurement output of each current transformer 10 differently.

[0056] Against this background, Fig. 4 is a top perspective view of a fixed side shield assembly 70. In one embodiment, the fixed side shield assembly 70 may include a housing 72 that may generally form a ring shape 73 and may include an annular cavity 74 that may receive a magnetically permeable material used to shield the current transformer 10. The magnetically permeable material may absorb magnetic fields. In certain embodiments, the permeability of the magnetically permeable material may be measured relative to free space.

[0057] The housing 72 may be made of plastic, aluminum, any polymer, glass-fiber reinforced plastic, non-ferritic metals, or the like. In one embodiment, the housing 72 may include an extended cavity 76 that may enclose the calibration circuit 14. Thus, the housing 72 may also include contact pins 78 that may be used to access the output terminal 28 of the calibration circuit 14, allowing a user to calibrate the current transformer 10 via the calibration circuit 14.

[0058] Against this background, Fig. 5 shows the annular cavity 74 and the enlarged cavity 76 from an interior view 80 of the housing 72. The housing 72 may have a bottom 73, an inner annular cavity wall 75, and an outer annular cavity wall 77. The annular cavity 74 may thus occupy the space between the bottom 73, the inner annular cavity wall 75, and the outer annular cavity wall 77. The housing 72 may be generally shaped such that the bottom 73, the inner annular cavity wall 75, and the outer annular cavity wall 77 are made of the same material and may be formed from a single piece of that material. In one embodiment, the calibration circuit 14 may fit into the outer wall 77 of the annular cavity within the extended cavity 76 so that it is accessible via contact pins 78 that allow the connection of wires, cables, and the like to the calibration circuit 14.

[0059] As previously mentioned, the annular cavity 74 can accommodate four circular rings so that each circular ring can shield one side of the current sensor 12. Fig. Figure 6 illustrates an exploded view 90 of the fixed side shield assembly 70 in cross-section, taken vertically along line I in Fig. 4 cut. Fig. 6 thus shows how any circular ring can fit around the current sensor 12. As in Fig. 6, the fixed side shield assembly 70 may include a circular inner ring 92 (e.g., radially inner ring or radially inner disk), a circular outer ring 94 (e.g., radially outer ring or radially outer disk), a circular top ring 96 (e.g., axially upper ring or axially upper disk), and a circular bottom ring 98 (e.g., axially lower ring or axially lower disk). Thus, the circular inner ring 92 and the circular outer ring 94 may be radially spaced from one another, and the circular top ring 96 and the circular bottom ring 98 may be axially spaced from one another. Additionally, the circular inner ring 92 and the circular outer ring 94 may be concentric with one another.

[0060] In one embodiment, a radial distance D between the inner and outer radii of the circular top ring 96 and the circular bottom ring 98 may be approximately greater than or equal to a radial thickness T1 of the circular inner ring 92 plus a radial thickness T2 of the current sensor 12 plus a radial thickness T3 of the circular outer ring 94. The circular top ring 96 and the circular bottom ring 98 may thus effectively protect the upper and lower portions of the current sensor 12 from electrical or magnetic noise that may be present outside the housing 72.

[0061] With reference to the circular inner ring 92 and the circular outer ring 94, a length L1 (e.g., axial height) of the circular inner ring 92 and a length L3 (e.g., axial height) of the circular outer ring 94 may be approximately equal to or greater than the length L2 (e.g., axial height) of the current sensor 12. The inner and outer radii of the current sensor 12 can thus be effectively protected from electrical or magnetic noise that may be present outside the housing 72. Combined, the circular inner ring 92, the circular outer ring 94, the circular top ring 96, and the circular bottom ring 98 can shield each side of the current sensor 12, so that the entire current sensor 12 is effectively shielded from various sources of magnetic and electrical noise.

[0062] While the fixed side shield assembly 70 has been described as having four circular rings, it should be noted that the fixed side shield assembly 70 may include only two circular rings. This means that in certain embodiments, the fixed side shield assembly 70 may include the circular top ring 96 and the circular bottom ring 98 for shielding the largest surface area sides of the current sensor 12. In this case, the circular inner and outer rings may be tape-wound shields.

[0063] The housing 72 may also include a cover 100. The cover 100 may be connected to the inner wall 75 of the annular cavity, the outer wall 77 of the annular cavity, the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, or the circular lower ring 98 using fasteners such as screws and the like. Once the housing 72 is connected to the cover 100, the fixed side shield assembly 70 can effectively shield the current transformer 10 from electrical or magnetic noise that may be present outside the housing 72.

[0064] In certain embodiments, the circular inner ring 92, the circular outer ring 94, the circular upper ring 96 and the circular lower ring 98 may each be made of a magnetically permeable material such that its permeability can be measured relative to free space, for example, a Supermalloy alloy (e.g., nickel-iron alloy), Metglas ®, Ultraperm®, MuMETAL®, or the like. Thus, the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, and the circular lower ring 98 may each be solid pieces of material manufactured to very precise dimensions using a laser cutter, a waterjet cutter, or the like. Generally, the thickness of the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, and the circular lower ring 98 may be at least 1.905 mm (0.075 in). In certain embodiments, the circular inner ring 92, the circular outer ring 94, the circular top ring 96, or the circular bottom ring 98 may include multiple pieces stacked together to form a complete circular inner ring 92, circular outer ring 94, circular top ring 96, or circular bottom ring 98.This means, for example, that the circular top ring 96 may be made up of ten pieces of 0.1905 mm (0.0075 inch) magnetically permeable material stacked together to form the single 1.905 mm (0.075 inch) circular top ring 96.

[0065] In certain embodiments, two or three of the circular inner ring 92, circular outer ring 94, circular upper ring 96, and circular lower ring 98 may be molded together as a single part. Thus, two or three of the circular inner ring 92, circular outer ring 94, circular upper ring 96, and circular lower ring 98 may be made from the magnetically permeable material, so that two or three of the circular inner ring 92, circular outer ring 94, circular upper ring 96, and circular lower ring 98 are molded together. For example, the circular inner ring 92 and the circular lower ring 98 may be made from a single piece of the magnetically permeable material, thereby forming a first single shielding part.In the same manner, the circular outer ring 94 and the circular upper ring 96 can be made from a single piece of the magnetically permeable material, thereby forming a second single shielding member. These two members can then be placed around the current sensor 12 such that the current sensor 12 can be shielded on each of its sides. In the same manner, the circular inner ring 92, the circular outer ring 94, and the circular upper ring 96 can be made from a single piece of the magnetically permeable material, thereby forming a single shielding member that can fit around three sides of the current sensor 12. The circular lower ring 98 can then be placed on top of the resulting three-sided shielding member so that the current sensor 12 is shielded from noise in every direction.

[0066] Although the fixed side shield assembly 70 has been described as including an annularly shaped housing 72, an annularly shaped cover 100, and four circular shield rings (i.e., the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, and the circular lower ring 98), it should be noted that the fixed side shield assembly 70 may also have other shapes so that the current transformer 10 can fit within the fixed side shield assembly 70. For example, the fixed side shield assembly 70 may have a square-shaped outer edge and an annular-shaped inner edge, as shown in Fig. 7. The annular shaped housing 72, the annular shaped cover 100, the circular outer ring 94, the circular upper ring 96 and the circular lower ring 98, shown in Fig. 6, can thus be shaped such that each aforementioned part has a square-shaped outer edge and an annular-shaped inner edge, while the circular inner ring 92 can have an annular-shaped inner and outer edge.

[0067] In another example, the fixed side shield assembly 70 may have an octagonal shaped outer edge and an annular shaped inner edge, as in Fig. 8, or have a hexagonal shaped outer edge and a ring-shaped inner edge, as in Fig. 9. In this way, the annular shaped housing 72, the annular shaped cover 100, the circular outer ring 94, the circular upper ring 96 and the circular lower ring 98, shown in Fig. 6, have outer edges corresponding to the shapes of the outer edges in Fig. 8 and Fig.9, while the circular inner ring 92 may have an annularly shaped inner and outer edge.

[0068] By precisely manufacturing the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, and the circular lower ring 98 to dimensions that fully enclose the current sensor 12 within the shielding material, manufacturers and assemblers can consistently ensure the same level of shielding in every current transformer 10 produced. Furthermore, the annular inner ring 92, the annular outer ring 94, the annular upper ring 96, and the annular lower ring 98 can be manufactured for assembly with the current transformer 10 and ready for assembly, thereby making the manufacturing process for the current transformer 10 more efficient.Since the calibration circuit 14, the housing 72, the cover 100, the circular inner ring 92, the circular outer ring 94, the circular upper ring 96, and the circular lower ring 98 are each replaceable parts, the assembly and repair of the current transformer 10 or the fixed side shield assembly 70 can be performed more efficiently.

[0069] In this written description, examples are used to disclose the invention, including the best mode, and also to enable one skilled in the art to practice the invention, including making and using devices or systems and performing methods incorporated therein. The patentable scope of the invention is defined by the claims and may include other examples that occur to one skilled in the art. These other examples are intended to be within the scope of the claims if they include structural elements that do not depart from the precise language of the claims or if they include equivalent structural elements with insubstantial differences from the precise language of the claims.

[0070] A shield for protecting a current transformer from noise may include a first circular ring disposed adjacent to a first side of a current sensor. The shield may also include a second circular ring disposed adjacent to a second side of the current sensor opposite the first side, such that the first and second circular rings each contain magnetically permeable material.

Claims

[1] System comprising: a housing (72) having a first cavity (74) and a second cavity (76); a current sensor (12) arranged in the first cavity (74); a calibration circuit (14) arranged in the second cavity (76); and a shielding assembly disposed in the housing (72) around the current sensor (12), the shielding assembly comprising first and second shielding portions disposed on axially opposite sides of the current sensor (12) and third and fourth shielding portions disposed on radially opposite sides of the current sensor (12); wherein the calibration circuit (14) comprises an adjustable phase shift resistor (22) and an adjustable sensitivity resistor (24) connected in series with each other, and a controller (32) configured to calibrate the current sensor (10) by adjusting the resistances of the phase shift resistor (22) and the sensitivity resistor (24) until a voltage waveform detected at an output terminal (28) of the calibration circuit (14) has phase shift and sensitivity characteristics that correspond to the phase shift and sensitivity characteristics of a desired voltage waveform. [2] The system of claim 1, wherein: the first shielding portion comprises a first circular ring (96) disposed adjacent a first side of the current sensor (12); and the second shielding portion comprises a second circular ring (98) disposed adjacent a second side of the current sensor (12) opposite the first side, wherein the first and second circular rings (96, 98) each comprise magnetically permeable material. [3] The system of claim 2, wherein: the third shielding portion comprises a third circular ring (92) disposed adjacent to a third side of the current sensor (12); and the fourth shielding portion comprises a fourth circular ring (94) disposed adjacent a fourth side of the current sensor (12) opposite the third side. [4] The system of claim 3, wherein a distance (D) between an inner radius and an outer radius of the first circular ring (96) is greater than or equal to a sum of a thickness (T1) of the third circular ring (92), a thickness (T2) of the current sensor (12), and a thickness (T3) of the fourth circular ring (94); and / or wherein a distance (D) between an inner radius and an outer radius of the second circular ring (98) is greater than or equal to a thickness (T1) of the third circular ring (92), a thickness (T2) of the current sensor (12), and a thickness (T3) of the fourth circular ring (94); and / or wherein the first and second circular rings (96, 98) are axially spaced from one another, and the third and fourth circular rings (92, 94) are radially spaced from one another. [5] The system of claim 4, wherein the third and fourth circular rings (92, 94) are concentric with each other. [6] The system of claim 3, wherein the third and fourth circular rings (92, 94) are tape-wound shields. [7] The system of claim 3, wherein the third and fourth annular rings (92, 94) each comprise the magnetically permeable material. [8] The system of claim 2 or 7, wherein the magnetically permeable material comprises Supermalloy, Metglas®, Ultraperm®, MuMETAL® or any combination thereof. [9] The system of claim 3, wherein the first and second circular rings (96, 98) each partially overlap both the third and fourth circular rings (92, 94). [10] A method of operating the system according to claim 1, comprising: Calibrating the current sensor (12) via the calibration circuit (14), wherein the calibration comprises: receiving an expected measurement output signal corresponding to the current sensor (12), wherein the expected measurement output signal comprises expected phase and sensitivity characteristics; sending a signal to a current source (30) for supplying a current to a conductor (16) monitored by the current sensor (12); receiving an instantaneous measurement output signal from the current sensor (12), wherein the instantaneous measurement output signal comprises instantaneous phase and sensitivity characteristics; and adjusting the instantaneous phase and sensitivity characteristics by adjusting the resistances of the phase shift resistor (22) and the sensitivity resistor (24) to correspond to the expected phase and sensitivity characteristics; and Measuring a current flowing through one or more conductors by means of the current sensor (12) while absorbing electrical and / or magnetic interference with the shielding arrangement (70) arranged around the current sensor (12).

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