WO2007041002A1

WO2007041002A1 - Voltage sensors and voltage sensing methods for gas insulated switchgear

Info

Publication number: WO2007041002A1
Application number: PCT/US2006/036792
Authority: WO
Inventors: James N. Blake; Patrick Chavez
Original assignee: Nxtphase T & D Corporation
Priority date: 2005-09-29
Filing date: 2006-09-21
Publication date: 2007-04-12
Also published as: KR20080074095A; CN101278201A

Abstract

A gas insulated electrical switchgear with an optical voltage sensor (46) positioned across from a conductor (10) for sensing a second voltage related to a first voltage. At least one conductive element (40 or 42) encircles the one conductor and is disposed between the optical voltage sensor (46) and the one conductor (10).

Description

PATENT APPLICATION

OF

James N. Blake

Patrick Chavez

FOR

VOLTAGE SENSORS AND VOLTAGE SENSING METHODS FOR GAS INSULATED SWITCHGEAR

BACKGROUND

[0001] The present invention pertains to electrical switchgear and, more particularly, to sensors and sensing methods suitable for gas insulated switchgear. [0002] Electrical power transmission/distribution systems typically provide for a switch to be interposed between a transformer and a line in the system. This switch (commonly referred to as "switchgear") provides a mechanism whereby the transformer can be disconnected from the rest of the system in response to, e.g., a power surge or other type of system occurrence. Recently, switchgear has been insulated using an insulating gas, e.g., SF₆, since the insulating properties of such gases enable the overall size of the switchgear to be reduced significantly. Initially, gas insulated switchgear was proposed as a single phase solution, i.e., each of the three phases had a separately packaged switch associated therewith. More recently, to further reduce the size associated with the switchgear, three-phase, gas insulated switchgear has been proposed, an example of which is described in U.S. Published Patent Application No. 2003/0178891, the disclosure of which is incorporated here by reference. Therein, three conductors, each associated with a different phase, extend through an enclosure filled with insulating gas and including switching mechanisms. [0003] In electrical switchgear, it can be useful to measure the current flowing through the switchgear and the voltage between two potentials at the switchgear, e.g. the high voltage potential and ground. Different types of sensors have been used in gas insulated switchgear to perform these functions. For example, current and voltage transformers or optical current and voltage sensors have been placed proximate to each conductor to monitor that phase's current and/or voltage. With respect to optical voltage sensors, however, it has been found that variances in either (or both) of (1) the distance between the conductor whose voltage is being measured and the optical voltage sensor and (2) the pressure of the insulating gas introduce errors into the measured voltage. The former circumstance may arise as a result of mechanical tolerances associated with manufacture of GIS devices. The latter circumstance may arise due to changes in temperature and other factors. [0004] One way to address these errors is to perform field calibration of the processing units and/or optical voltage sensors to attempt to compensate for these factors. However, such calibration is time consuming and my not completely address dynamic fluctuations. Another way to address these errors is to use specific sensors to measure variances in gas pressure, temperature, distance to the conductor, etc. and use such measurements to compensate the voltage readings of the optical sensor. However, such a solution adds significant expense to GIS devices.

[0005] Accordingly, it would be desirable to provide voltage sensors and sensing methods which are able to accurately and automatically detect voltage in GIS devices and which address some or all of the problems described above.

SUMMARY

[0006] According to one exemplary embodiment of the present invention, a gas insulated switchgear device includes an enclosure having two endcaps and at least one conductor extending therethrough, the at least one conductor carrying a first voltage; an insulating gas within the enclosure; an optical voltage sensor positioned across from the at least one conductor for sensing a second voltage related to the first voltage; and at least one conductive element which encircles the at least one conductor and is disposed between the optical voltage sensor and the at least one conductor, wherein the at least conductive element operates to establish the second voltage based on a capacitive division of the first voltage. [0007] According to another exemplary embodiment of the present invention, a method for sensing a voltage associated with a conductor in a gas insulated switchgear includes the steps of providing a first voltage associated with the conductor extending through the gas insulated switchgear, capacitively dividing the first voltage to generate a second voltage associated with the conductor, optically sensing the second voltage and determining the first voltage based on the second voltage.

[0008] According to yet another exemplary embodiment of the present invention, a system for sensing a voltage associated with a conductor in a gas insulated switchgear includes means for providing a first voltage associated with the conductor extending through the gas insulated switchgear, means for capacitively dividing the first voltage to generate a second voltage associated with the conductor, means for optically sensing the second voltage, and means for determining the first voltage based on the second voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings illustrate exemplary embodiments of the present invention, wherein:

[0010] FIG. 1 depicts a single phase, gas insulated switch device;

[0011] FIG. 2 shows a ring including an optical voltage sensor;

[0012] FIG. 3 illustrates the ring of FIG. 2 inserted between two portions of a GIS device in position to measure a voltage of an associated conductor;

[0013] FIG. 4 depicts an optical voltage sensor according to an exemplary embodiment of the present invention;

[0014] FIG. 5 is a flowchart depicting a method of optical voltage sensing according to an exemplary embodiment of the present invention; and

[0015] FIGS. 6(a)-6(c) depict an optical voltage sensor according to another exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0016] The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

[0017] Figure 1 illustrates a single-phase, gas insulated switchgear (GIS) device 1 in which exemplary embodiments of the present invention can be implemented. Therein, the GIS device includes bus bars 2 for each pole, disconnection switches 3, grounding switches 4, 5 and circuit breaker 6. Each of the three enclosures 11 is associated with a single phase of the GIS device 1, is filled with an insulating gas, e.g. SF₆ gas and include an electrical conductor 10 (not shown in Figure 1) extending therethrough. The enclosures also include mechanical parts of the switches, e.g., contacts, drive mechanisms, etc., associated with the GIS device 1 and the electrical conductor 10 which conducts its respective phase's current through the switchgear 1. [0018] One way to optically measure voltage associated with the electrical conductor

10 is to position a Pockels cell in the electrical field generated by the conductor, pass light through the Pockels cell and monitor polarization changes in the light. An example of this technique is illustrated in Figures 2 and 3. Therein, a ring 13 is inserted between two portions 14 and 14' of an outer conductor which forms enclosure 11 in the region of the circuit breaker 6. Figure 3 is a cross section of the ring 13 and outer conductor of enclosure

11 taken along the section line B-B in Figure 2. The ring 13 and outer conductor portions 14 and 14' can be connected together via flanges 12 and bolts 27. The ring 13 includes a metallic L-shaped portion 15 and a non-metallic inner ring 16 disposed in the hollow of the L-shaped portion 15. The non-metallic inner ring 16 can be fabricated from a dielectric material, e.g., a resin. The Pockels cell 25 is mounted in a bore 17 fabricated in the inner ring 16. An optical fiber 24 extends from the control unit 26 through the bore 17 and Pockels cell 25, the control unit 26 including an optical source and processing components and circuitry for measuring the returned light (not shown). It should be noted that this configuration is purely illustrative of one arrangement for placing an optical voltage sensor proximate a conductor in a GIS device and that the present invention can also be used in GIS devices having different configurations.

[0019] Pockels cells operate on the principle that the polarization of light changes as it passes through an electro-optic crystal (Pockels cell), e.g., from circularly polarized to elliptically polarized, based on the intensity of an electric field in which the Pockels cell is located. An example of a Pockels cell voltage sensor is found in U.S. Patent No. 5,029,273, the disclosure of which is incorporated here by reference. Thus, the light returned from Pockels cell 25 is evaluated in control unit 26 to determine its respective change in polarization, from which the magnitude of the electric field (and hence voltage) associated with electrical conductor 10 can be determined.

[0020] However, as mentioned in the Background section of this specification, the accuracy of the voltage sensing mechanism and method illustrated in Figures 2 and 3 may be impacted by several variables. First, the electric field measurement made by the Pockels cell 25 is dependent upon the distance between the Pockels cell 25 and the electrical conductor 10. Thus, the voltage measurement will vary if this distance varies, which will depend upon (among other things) the mechanical tolerances of manufacture employed to secure the electrical conductor 10 within its respective enclosure 11. Second, the electric field measurement made by the Pockels cell 25 is also dependent upon the pressure of the insulating gas (e.g., SF₆) contained within the enclosure 11. Since this pressure may also vary, due to changes in temperature or the like, this introduces another potential inaccuracy to the voltage sensing arrangement depicted in Figures 2 and 3.

[0021] Exemplary embodiments of the present invention provide for voltage sensing arrangements and methods which compensate for variances in (1) the distance between an optical voltage sensor and the conductor within a GIS device whose voltage is being measured and/or (2) pressure in the insulating gas (e.g., SF₆) within the enclosure of the conductor whose voltage is being measured. An example is illustrated in Figure 4, which is a conceptualized lengthwise cross-section of an enclosure 11 of a GIS device according to an exemplary embodiment of the present invention.

[0022] Therein, two wire rings 40 and 42 are embedded in the dielectric material 44, in which an optical voltage sensor 46 (e.g., a Pockels cell) for detecting a voltage V_out associated with center conductor 10 is also embedded. The wire rings 40 and 42 can be connected by a wire 43 in the region of the optical voltage sensor 46 and can be disposed on either side of the optical voltage sensor 46. As shown in Figure 4, the rings 40 and 42 can be spaced an equal distance from the optical voltage sensor 46, although this symmetry is not required. In Figure 4, reference numerals 10, 11, 12, 14, 14' and 15 refer to similar elements to those described above with respect to Figure 3 and are not, therefore, further described here. Additionally, some elements associated with the complete device have been omitted, e.g., the optical fiber, optical source and digital processing/control unit associated with the optical voltage sensor 46, to simplify the figure. [0023] Capacitances associated with the wire rings 40 and 42 are used to automatically compensate for differences in either (or both) of a distance between the optical voltage sensor 46 and the center conductor 10 and changes in the insulating gas pressure, e.g., due to changes in temperature. For example, since the two wire rings 40 and 42 encircle the center conductor 10, the overall capacitance Cl between the center conductor 10 and the two wire rings 40 and 42 will not vary significantly due to minor changes in the position of the center conductor 10 relative to the optical voltage sensor 46. This is because if a particular GIS device's center conductor 10 is closer to (or farther away from) the illustrated side of the rings 40 and 42 proximate the outer (ground) conductor 14, 14' due to mechanical tolerances, it will be farther away from (or closer to) the opposite side (not shown) of the rings. Capacitance Cl will also be affected by the pressure of the insulating gas. [0024] A second capacitance C2 will exist between the rings 40, 42 and the metallic,

L-shaped portion 15 and the flange/outer conductor 12, 14, respectively. This second capacitance C2 will also vary based on the gas pressure of the insulating gas which operates as the dielectric between outer metallic surfaces of the capacitor. Moreover, capacitances Cl and C2 establish a capacitive voltage divider such that the voltage sensed by the optical sensor 46 will be a function of the capacitances Cl and C2 and the voltage V associated with the center conductor 10. The optical sensor 46 will thus detect a voltage Vout as:

Vout'= V*Cl/(Cl+C2) (1)

Since the capacitances Cl and C2 are both a function of the gas pressure (and temperature), and therefore remain proportional to one another when the gas pressure changes, the detected voltage Vout is substantially independent thereof, i.e., this exemplary embodiment of the present invention automatically compensates for gas pressure and temperature (as well as the distance between the optical sensor 46 and the center conductor 10 due to the encircling of the center conductor by rings 40 and 42, described above).

[0025] It will be appreciated that the present invention can be implemented in many different variations of the afore-described exemplary embodiment. For example, the physical location of the optical voltage sensor 46 or the manner in which it is mechanically integrated into the outer conductor of the GIS device may vary. Moreover, the conductive element used to establish the capacitive voltage divider need not be formed from two interconnected rings, but can take any desired shape. More generally, a method for optically sensing a voltage associated with a conductor in a GIS device is described in the flowchart of Figure 5. Therein, at step 500, the GIS device provides a first voltage associated with the conductor extending through a gas insulated enclosure. At step 502, the first voltage is capacitively divided to generate a second voltage associated with the conductor. This second voltage is then sensed optically by, e.g., a Pockels cell at step 504. Then, the first voltage carried by the center conductor can be determined based on the second voltage, e.g., by using equation (1) above, at step 506.

[0026] As will be appreciated from the foregoing description, voltage sensors according to exemplary embodiments of the present invention can be generally modeled as shown in Figure 6(a). Therein a voltage sensing arrangement, generally labeled by reference number 600 includes the two conductors of different electric potentials (e.g., outer, ground conductor 602 and inner conductor 604 at a positive potential), two contiguous layers 606 and 608 of substantially dielectric material with different real and/or imaginary permittivities el, e3, respectively, sandwiched between the two conductors 602 and 604 (e.g., layer 606 can be a dielectric layer 44 described above and layer 608 can be an insulating gas, e.g., SF₆). A bounded region 610 includes an optical electric field sensor (not shown in this Figure) located inside layer 606 with a real and/or imaginary dielectric permittivity e2 which may be different than the permittivity el of surrounding layer 608. The region 610 may contain a gel surrounding the electric field sensor or simply be a pocket of air in which the sensor is connected as described above. The dimensions of bounded region 610 are such that changes in the electric field at the sensor location caused by changes in the real and/or imaginary dielectric permittivity of the surrounding layer with respect to permittivities of said bounded region and of other said layer are minimized.

[0027] If the permittivity of layer 606, el, changes, then the electric field in layer 608,

E2, will change through satisfying the boundary conditions of the Laplace equation governing layers 606, 608 and 610. More specifically, the defining Laplace equation has boundary conditions indicating that the tangential electric field Et across a boundary is continuous, and that the normal electric flux Dn = eEn across the boundary is also continuous, based on electrostatic field theory. This means that the electric field sensor disposed in region 610 may be sensitive to changes in permittivity caused by, e.g., changes in temperature.

[0028] In order to better understand the relationship between the dimensions of the bounded region 610 and the behavior of E2 with respect to a change in permittivity el, the voltage sensor can be deconstructed into two fundamental building-block structures shown in Figures 6(a) and 6(b). Considering first Figure 6(b), where the height of the bounded region 610 is extended completely across the layer 606 between the outer conductor 602 and the layer 608, as el is increased, E2 will decrease. Furthermore, decreasing the width of region 610 will amplify this effect. Now considering Figure 6(c), where the width of the bounded region 610 is extended to a maximum, as el is increased, E2 will increase. Moreover, decreasing the height of region 610 will amplify this effect.

[0029] The effects discussed above for the cases shown in Figures 6(a) and 6(b) are opposing. Since the voltage sensor example of Figure 6(a) is a combination of these two cases, the height and width of the bounded region 610 can be chosen so that the aforementioned effects substantially cancel each other out, minimizing the sensitivity of the electric field E2 in bounded region 610 to changes in the permittivity of layer 606. With the dimensions so chosen, the electric field sensor (e.g., Pockels cell) located in bounded region 610 provides a measure of the voltage (potential) between the conductors 602 and 604 that is substantially insensitive to changes in the permittivity of layer 606, e.g., due to changes in temperature.

[0030] The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article "a" is intended to include one or more items.

Claims

WHAT IS CLAIMED IS:

1. A gas insulated switchgear device comprising: an enclosure having at least one conductor extending therethrough, said at least one conductor carrying a first voltage; an insulating gas within said enclosure; an optical voltage sensor positioned across from said at least one conductor for sensing a second voltage related to said first voltage; and at least one conductive element which encircles said at least one conductor and is disposed between said optical voltage sensor and said at least one conductor, wherein said at least conductive element operates to establish said second voltage based on a capacitive division of said first voltage.

2. The gas insulated switchgear device of claim 1, wherein said at least one conductive element includes two rings, each of which encircle said at least one conductor.

3. The gas insulated switchgear of claim 2, wherein a wire connects said two rings in a region proximate said optical voltage sensor.

4. The gas insulated switchgear device of claim 1, wherein a first capacitance (Cl) is established between said at least one conductor and said at least one conductive element and wherein a second capacitance (C2) is established between said at least one conductive element and an outer conductor which forms said enclosure, and wherein said second voltage (Vout) is related to said first voltage (V) by: Vout = V*C1/(C1+C2).

5. The gas insulated switchgear device of claim 4, further comprising: a processing unit, connected to said optical voltage sensor, for determining said first voltage based upon said second voltage.

6. The gas insulated switchgear device of claim 5, wherein said determined first voltage is substantially independent of changes in pressure of said insulating gas.

7. A method for sensing a voltage associated with a first conductor in a gas insulated switchgear, the method comprising the steps of: providing a first voltage associated with said first conductor extending through said gas insulated switchgear; capacitively dividing said first voltage to generate a second voltage associated with said first conductor using a second conductor encircling said first conductor; optically sensing said second voltage; and determining said first voltage based on said second voltage.

8. The method of claim 7, wherein said step of capacitively dividing further comprises the step of: providing two rings as said second conductor, each of which encircle said first conductor, proximate an optical sensor.

9. The method of claim 8, wherein said optical sensor includes a Pockels cell.

10. The method of claim 8 further comprising the step of: electrically connecting said two rings in a region proximate said optical voltage sensor.

11. The method of claim 7, further comprising the steps of: establishing a first capacitance (Cl) between said first conductor and said second conductor; and establishing a second capacitance (C2) between said second conductor and an outer conductor; wherein said second voltage (Vout) is related to said first voltage (V) by: Vout = V*Cl/(Cl+C2).

12. The method of claim 6, wherein said determined first voltage is substantially independent of changes in pressure of said insulating gas.

13. A system for sensing a voltage associated with a first conductor in a gas insulated switchgear, the system comprising: means for providing a first voltage associated with said first conductor extending through said gas insulated switchgear; means for capacitively dividing said first voltage to generate a second voltage associated with said conductor using a second conductor encircling said first conductor; means for optically sensing said second voltage; and means for determining said first voltage based on said second voltage.

14. The system of claim 13, wherein said means for capacitively dividing further comprises: two rings operative as said second conductor, each of which encircle said first conductor, proximate an optical sensor.

15. The system of claim 14, wherein said optical sensor includes a Pockels cell.

16. The system of claim 14, further comprising: means for electrically connecting said two rings in a region proximate said optical voltage sensor.

17. The system of claim 13, further comprising: means for establishing a first capacitance (Cl) between said conductor and said second conductor; and means for establishing a second capacitance (C2) between said second conductor and an outer conductor; wherein said second voltage (Vout) is related to said first voltage (V) by: Vout = V*Cl/(Cl+C2).

18. The system of claim 13, wherein said determined first voltage is substantially independent of changes in pressure of said insulating gas.

19. A gas insulated switchgear device comprising: an enclosure having at least one conductor extending therethrough, said at least one conductor carrying a first voltage; an insulating gas within said enclosure; and a dielectric element having a pocket formed therein, said pocket containing an optical voltage sensor positioned across from said at least one conductor for sensing a voltage associated therewith; wherein a height and a width of said pocket are selected to reduce a sensitivity of said optical voltage sensor to changes in a permittivity of said dielectric element.

20. The gas insulated switchgear device of claim 19, wherein said optical voltage sensor is an electric field sensor.