System and method for sensing voltage in medium-to-high voltage applications

[0001] The present invention relates generally to voltage sensors for use with line conductors and, more particularly, to a compact capacitive divider-type sensor that is easy to manufacture and capable of measuring the potential of a medium-to-high voltage line conductor.

[0002] Capacitive divider-type sensors measure the AC voltage of a line conductor or bus bar. In such a device, a first capacitor (C1) with a known capacitance is connected to the line conductor. A second capacitor (C2) with a capacitance larger than the first is connected in series between C1 and ground. The voltage (V_C2) across C2 is proportional to the line voltage (V_L) between the line conductor and ground. Specifically, V_L may be determined using the following equation:
V_C2=V_L(C1/(C1+C2)).

[0003] When constructing a capacitive divider capable of measuring voltage in medium voltage (MV) to high voltage (HV) applications (e.g., approximately 1 kV to 50 kV), numerous design factors are typically taken into account. For example, both C1 and C2 are typically constructed using a number of individual capacitors to reduce the voltage on both C1 and C2 to a value that is within each capacitor's voltage rating. Such individual components, however, significantly increase complexity, inaccuracy possibilities, and manufacturing costs to the capacitive divider.

[0004] Also, the electric field (E-field) concentration on individual components of the capacitive divider influences the design of the capacitive divider. As is known, E-field concentration is, in part, a factor of the voltage of the line conductor. That is, as the voltage of the line conductor increases, E-field concentration on the individual components of the capacitive divider may increase as well. When E-field concentration is high, dielectric breakdown may occur between the line conductor and the voltage sensor. While E-field concentration may be minimized by increasing the distance between individual components of the capacitive divider, such an increase in component spacing also increases the overall size of the device.

[0005] It would therefore be desirable to design an apparatus and method for measuring the voltage of a medium-to-high voltage line conductor that is small, accurate, and relatively inexpensive to manufacture.

[0006] The present invention provides a system and method for sensing the voltage of a medium or high voltage line conductor. Therefore, in accordance with one aspect of the present invention, a voltage sensor for a line conductor includes an electrical insulator comprising a surface and an edge, the surface having a length and the edge having a thickness defining a thickness of the electrical insulator. The length of the surface of the electrical insulator is substantially greater than the thickness of the edge. The surface of the electrical insulator has an opening therein to allow passage of a line conductor therethrough in a direction generally perpendicular to the surface. The voltage sensor also includes first and second electrodes positioned about the opening on the surface of the electrical insulator and spaced apart from one another to provide a common capacitive divider signal indicative of a voltage potential of the line conductor.

[0007] In accordance with another aspect of the invention, a method of manufacturing a line sensor includes providing an insulating substrate having an attachment surface and having an opening therein to allow a line conductor to pass through in a direction generally perpendicular to the attachment surface. The method also includes coupling first and second electrodes of an electrode assembly to the attachment surface of the insulating substrate, spacing the first and second electrodes of the electrode assembly apart from one another on the insulating substrate, and configuring the electrode assembly to output a signal indicative of a voltage potential of the line conductor.

[0008] In accordance with yet another aspect of the invention, a voltage sensor for a line conductor includes an electrically insulating substrate having a contact surface, the contact surface having an opening sized to allow a line conductor to pass therethrough in a direction generally perpendicular to the contact surface. The line conductor also includes a sensor electrode coupled to the contact surface of the electrically insulating substrate and a grounded electrode coupled to the contact surface of the electrically insulating substrate. The sensor electrode and the grounded electrode form elements of a capacitive divider and produce a common output.

[0009] Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.

[0017] The embodiments of the invention set forth herein relate to a system and method for sensing voltage of a medium or high voltage line conductor. In one embodiment, the invention is implemented with a voltage sensor having two capacitors in series formed therein. A voltage sensor 10 is shown in FIG. 1 and includes a grounded voltage sensor shield 12, which may be constructed of injected-molded plastic plated with a metal, such as, for example, nickel and/or copper. Other alternatives include stamped metal, a metal mesh/screen, or a combination of the aforementioned materials. Sensor shield 12 is a faraday shield having a generally circular shape. In one embodiment, sensor shield 12 may be constructed having a top shield 14 and a bottom shield 16 fastened together with rivets 18. An electrical insulator or electrically insulating substrate 20 having a generally disk shape is positioned between top and bottom shields 14, 16. Electrical insulator 20 has an opening 22 therein to receive a line conductor (not shown in FIG. 1). In a preferred embodiment, electrical insulator 20 is a printed circuit board (PCB). Alternatively, electrical insulator 20 may be a thin (approximately 0.002 in), insulated support sheet comprising a material such as, for example, Kapton.

[0018] A first or sensor electrode 24 and a second or grounded electrode 26 are affixed to or formed on support plate 20 about the opening of 22 electrical insulator 20. In one embodiment, first and second electrodes 24, 26 are formed using a printing, painting, electro-deposition, or metal spraying process to deposit a layer of conductive material such as copper, for example, on electrical insulator 20. Wired connection 28 connects first electrode 24 to a contact pad 30 of a readout circuit 32, which will be described in more detail with respect to FIGS. 2 and 3. Wired connection 34 connects second electrode 26 to grounded voltage sensor shield 12.

[0019] As shown in FIG. 1, radius corners or edge portions 36, 38 of top and bottom shields 14, 16 have a curved profile, which minimizes electric-field concentration and arcing on edge portions 36, 38 and helps to prevent dielectric breakdown. Alternatively, curled edge portions 36, 38 may be formed by attaching a large diameter wire ring to top and bottom shields 14, 16. The curvature of edge portion 36, 38 also allows first and second electrodes 24, 26 to be placed closer to a line conductor, therefore decreasing the overall size of voltage sensor 10.

[0020] Referring now to FIG. 2, a cross-sectional view of a voltage sensor 40 is shown according to one embodiment of the invention. Voltage sensor 40 includes a shield assembly 42 having a first shield 44 and a second shield 46 connected to ground 48. A printed circuit board (PCB) 50 is positioned between first shield 44 and second shield 46. PCB 50 has a top edge 52, a bottom edge 54, a first side surface or attachment surface 56, and a second side surface or attachment surface 58. As shown in FIG. 2, PCB 50 has an overall length 60 much larger than a thickness 62 thereof. An opening 64 in PCB 50 allows a line conductor 66 to pass therethrough. PCB 50 is positioned such that top edge 52 and bottom edge 54 are oriented substantially parallel to a central axis 68 of line conductor 66. First side surface 56 and second side surface 58 of PCB 50 are oriented substantially perpendicular to central axis 68 of line conductor 66 and have respective contact locations 70, 72 and a number of respective non-contact locations 74, 76 positioned thereon. A sensor electrode 78 is affixed to first side surface 56 of PCB 50 at contact location 70, and a grounded electrode 80 is affixed to second side surface 58 of PCB 50 at contact location 72. As shown in FIG. 2, sensor electrode 78 and grounded electrode 80 are located equidistant to opening 64 of PCB 50 and line conductor 66. Alternatively, electrodes 78, 80 may be offset from each other on PCB 50 such that one of sensor electrode 78 and grounded electrode 80 is closer to line conductor 66 than the other.

[0021] Also affixed to PCB 50 is a readout circuit 82 having a contact pad 84 positioned thereon. Sensor wire 86 electrically connects sensor electrode 78 to contact pad 84. Ground wire 88 grounds electrode 80 via a connection to first shield 44. Readout circuit 82 buffers the high impedance of voltage sensor 40 and includes a buffer and amplifier circuit 90 to amplify signals from sensor electrode 78 for a voltage sensor output 92. Because dielectric constants are temperature sensitive, a thermocouple 94 and a microprocessor 96 may also be included on readout circuit 82 such that a dielectric constant may be corrected as a function of temperature.

[0022] A schematic circuit diagram of voltage sensor embodiments of the invention is shown in FIG. 3. Referring to FIGS. 2 and 3, a capacitive divider 98 is formed between line conductor 66, sensor electrode 78, and grounded electrode 80. A first capacitor (C1) 100 is formed between line conductor (V_L) 66 and sensor electrode 78. As is known, the capacitance of C1 100 may be controlled by selectively sizing sensor electrode 78 and setting the distance between sensor electrode 78 and line conductor 66. In one embodiment, the capacitance of C1 100 may be approximately 10 fF. A second capacitor (C2) 102 is formed between sensor electrode 78 and grounded electrode 80. Similar to that explained with respect to C1 100, the capacitance of C2 102 may be selected based on the size and position of both sensor electrode 78 and grounded electrode 80. In one embodiment, the capacitance of C2 102 may be approximately 100 pF. A buffer and amplifier circuit 90 mounted on readout circuit 82 amplifies a sensed voltage potential signal 104 proportional to the sensed voltage from capacitive divider 98 and transmits a signal indicative of a voltage potential of line conductor 66 to an external trip unit (not shown). In one embodiment, the ratio between the sensed voltage signal 104 and V_L 66 is 1:10,000 or 100 mV/kV.

[0023] FIG. 4 shows a cross-section of a voltage sensor 106 according to another embodiment of the invention. A sensor shield or shield ring 108 is connected to ground 110 and forms a continuous piece of metal or metal-plated plastic that surrounds a PCB 112. PCB 112 has a passage 114 formed therein to receive a line conductor 116. An electrode assembly 118 comprising a sensor electrode 120 and a grounded electrode 122 are positioned on a common planar contact surface 124 of PCB 112. Wired connection 126 grounds electrode 122 via a connection to grounded shield ring 108. Wired connection 128 electrically connects to contact pad 130 of a readout circuit 132.

[0024] As shown in FIG. 5 according to one embodiment of the invention, a voltage sensor 134 may be incorporated within a sensor assembly 136. Voltage sensor 134 includes a voltage shield or housing 138, a PCB 140, and first and second electrodes 142, 144. In addition to voltage sensor 134, sensor assembly 136 also includes a current sensor assembly 146 having a housing or shield 148 to prevent dielectric breakdown from a line conductor 150. Shield 148 surrounds a current sensor 152 and first and second parasitic power current transformers (CT) 154, 156. Accordingly to one embodiment, current sensor 152 is a Rogowski coil that detects the current in line conductor 150. CTs 154, 156 parasitically generate power from either the line current or line voltage and provide power to voltage sensor 134 and to the external trip unit (not shown).

[0025] Voltage sensor 134 and current sensor assembly 146 are positioned in a housing or common structure 158 with spacers 160. While spacers 160 are shown as solid cylinders, alternate embodiments may only have a few minimal support points for equal effectiveness. A passage 162 in housing 158 is sized to receive line conductor 150. Wired connections 164, 166 from voltage sensor 134 and current sensor assembly 146, respectively, connect to a multi-pin connector 168, which allows transmission of sensor signals from the voltage sensor 134 and current sensor 152 to an external trip unit (not shown).

[0026] In one embodiment, an insulating material or encapsulant 170, such as an epoxy with a high dielectric breakdown strength, for example, forms a dielectric within housing 158. Insulating material 170 minimizes dielectric breakdown between line conductor 150 and the shields 138, 148, and also between voltage sensor 134 and current sensor assembly 146. Insulating material 170 is injected into housing 158 to encapsulate voltage sensor 134 and current sensor assembly 146 and fill any air gaps within voltage sensor 134 and current sensor assembly 146. Vent holes 172 in both current shield 148 and voltage shield 138 allow air to escape during encapsulation.

[0027] Therefore, in accordance with one aspect of the present invention, a voltage sensor for a line conductor includes an electrical insulator comprising a surface and an edge, the surface having a length and the edge having a thickness defining a thickness of the electrical insulator. The length of the surface of the electrical insulator is substantially greater than the thickness of the edge. The surface of the electrical insulator has an opening therein to allow passage of a line conductor therethrough in a direction generally perpendicular to the surface. The voltage sensor also includes first and second electrodes positioned about the opening on the surface of the electrical insulator and spaced apart from one another to provide a common capacitive divider signal indicative of a voltage potential of the line conductor.

[0028] In accordance with another aspect of the invention, a method of manufacturing a line sensor includes providing an insulating substrate having an attachment surface and having an opening therein to allow a line conductor to pass through in a direction generally perpendicular to the attachment surface. The method also includes coupling first and second electrodes of an electrode assembly to the attachment surface of the insulating substrate, spacing the first and second electrodes of the electrode assembly apart from one another on the insulating substrate, and configuring the electrode assembly to output a signal indicative of a voltage potential of the line conductor.

[0029] In accordance with yet another aspect of the invention, a voltage sensor for a line conductor includes an electrically insulating substrate having a contact surface, the contact surface having an opening sized to allow a line conductor to pass therethrough in a direction generally perpendicular to the contact surface. The line conductor also includes a sensor electrode coupled to the contact surface of the electrically insulating substrate and a grounded electrode coupled to the contact surface of the electrically insulating substrate. The sensor electrode and the grounded electrode form elements of a capacitive divider and produce a common output.

[0030] The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.