Abstract:
The invention is an electro-optic voltage sensor head designed for integration with existing types of high voltage transmission and distribution apparatus. The sensor head contains a transducer, which comprises a transducing material in which the Pockels electro-optic effect is observed. In the practice of the invention at least one beam of electromagnetic radiation is routed into the transducing material of the transducer in the sensor head. The beam undergoes an electro-optic effect in the sensor head when the transducing material is subjected to an E-field. The electro-optic effect is observed as a differential phase a shift, also called differential phase modulation, of the beam components in orthogonal planes of the electromagnetic radiation. In the preferred embodiment the beam is routed through the transducer along an initial axis and then reflected by a retro-reflector back substantially parallel to the initial axis, making a double pass through the transducer for increased measurement sensitivity. The preferred embodiment of the sensor head also includes a polarization state rotator and at least one beam splitter for orienting the beam along major and minor axes and for splitting the beam components into two signals which are independent converse amplitude-modulated signals carrying E-field magnitude and hence voltage information from the sensor head by way of optic fibers.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States has rights in this invention pursuant to Contract No. DE-AC07-94ID13223 between the U.S. Department of Energy and Lockheed Martin Idaho Technologies Company. 
    
    
     RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 08/570,152 filed Dec. 11, 1995 now U.S. Pat. No. 5,731,597. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains generally to the field of voltage sensors and more particularly to a voltage sensor system which utilizes the Pockels electro-optic effect to measure voltage in a conductor without electrical contact with the conductor. 
     2. Background Art 
     High-accuracy measurement of high voltage in conductors has traditionally been accomplished using iron-core ferro-magnetic potential transformers. These devices have substantially limited dynamic range, bandwidth, linearity, and electrical isolation. During electrical fault conditions these transformers can conduct dangerous levels of fault energy to downstream instrumentation and personnel, posing an additional threat to safety and potential liability. 
     A variety of optic sensors for measuring voltage have been developed in attempts to offer the power industry an alternative to the conventional transformer technology. Generally, these voltage sensor systems require that direct electrical contact be made with the energized conductor. This contact is made necessary by the use of a voltage divider which is utilized to connect the sensing element with the energized conductor on which a measurement is to be made. Direct electrical contact with the conductor may alter or interrupt the operation of the power system by presenting a burden or load. 
     In addition to the disadvantages associated with direct electrical contact with the energized conductor, prior art voltage sensor systems are typically bulky, particularly in extremely high voltage applications. This is true because the size of the voltage divider required is proportional to the voltage being measured. The size of such systems can make them difficult and expensive to install and house in substations. 
     Many prior art sensors are based upon the electrostrictive principle which utilize interferometric modulation principles. Unfortunately, interferometric modulation is extremely temperature sensitive. This temperature sensitivity requires controlled conditions in order to obtain accurate voltage measurements. The requirement of controlled conditions limits the usefulness of such systems and makes them unsuited for outdoor or uncontrolled applications. In addition, interferometric modulation requires a highly coherent source of electromagnetic radiation, which is relatively expensive. 
     Open-air E-field based sensors have also been developed to determine voltage in conductors. Unfortunately, these systems typically lack accuracy when used for measuring voltage because the open-air E-field used varies with many noisy parameters including ambient dielectric constant, adjacent conductor voltages, moving conductive structures such as passing vehicles, and other electromagnetic noise contributions. 
     Systems which utilize mechanical modulation of the optical reflection within an optic fiber have also been developed. Among other drawbacks, the reliance of such systems on moving parts is a significant deterrent to widespread use. 
     It would therefore be an advantage in the art to provide a system which does not require direct electrical contact with the energized conductor. It would also be advantageous to provide such a system which is compact, operates in a variety of temperatures and conditions, is reliable, and is cost effective. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an electro-optic voltage sensor head which does not require contact with a conductor. 
     It is a further object of the present invention to provide such an electro-optic voltage sensor head which is capable of use in a variety of environmental conditions. 
     It is a still further object of the present invention to provide such an electro-optic voltage sensor head which can be employed as part of a system to accurately measure high voltages without use of dedicated voltage division hardware. 
     It is an additional object of the present invention to provide such an electro-optic voltage sensor head which minimizes the electronics needed for implementation, which is light weight, and which is small in size. 
     It is a further object of the present invention to provide a sensor head capable of being integrated with existing types of high voltage power transmission and distribution equipment so as to reduce or eliminate the need for large stand-alone voltage measurement apparatus, and to provide a sensor system with a superior dynamic range of frequencies sensed. 
     The above objects and others not specifically recited are realized in a specific illustrative embodiment of an Electro-Optical Voltage Sensor System whereby one may measure the voltage difference (or electrical potential difference) between objects or positions. Voltage is a function of the electric field (hereinafter &#34;electric field&#34; shall be indicated &#34;E-field&#34;) and the geometries, compositions and distances of the conductive and insulating matter. Where, as in the present invention, the effects of an E-field can be observed, a voltage measurement can be calculated. 
     Various aspects of the invention employ a sensor head in combination with a transmitter, a detector, and a signal processor. The transmitter produces a beam of electromagnetic radiation which is routed into the sensor head. Although this electromagnetic radiation used in the present invention can comprise any wavelengths beyond the visible spectrum, the term &#34;light&#34; will be used hereinafter to denote electromagnetic radiation for the purpose of brevity. The beam undergoes polarization before it undergoes an electro-optic effect in the transducing material of the sensor. In the polarized beam, the light has at least two components (A and B) which propagate along at least two orthogonal axes, thus forming at least two orthogonal planes within the beam. The electro-optic effect occurs when the sensor head is placed into an E-field, and is observable as a phase differential shift of the orthogonal beam components&#39; differential phase shift causes a corresponding change in the beam&#39;s polarization, which is expressed by a shift from a circular beam of light to an elliptical beam of light which is shifted 45 degrees from the primary axes. By measuring the major and minor axes of the ellipse formed by the beam, one can determine the polarization change which has occurred in the beam, which can ultimately be processed to determine the voltage. 
     In accordance with one aspect of the present invention, the polarized light is then passed through a 45 degree fixed polarization rotator. The two components (A and B) of the beam are rotated 45 degrees and converted into a set of amplitude modulated (AM) signals of opposing polarity that are transmitted out of the sensor. The detector converts the set of optical AM signals into electrical signals from which the voltage is determined by the signal processor. 
     In accordance with still another aspect of the present invention, the sensor is not disposed in contact with the conductor, but is disposed between the conductor and the grounded outer sheath. In such a position, the risk of alteration of the E-field due to conductive objects, such as motor vehicles, is dramatically reduced because of the grounded outer sheath. The sensor can be positioned anywhere between the conductor and the grounded plane (i.e. the grounded sheath) while maintaining a high degree of accuracy is maintained, the monitoring equipment is still optically isolated from the voltage carried by the conductor. This provides a remarkable improvement over optical sensing systems in which the sensor is disposed outside the grounded plane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become apparent from a consideration of the subsequent detailed description presented in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic semi-three dimensional view of the sensor head made in accordance with the principles of the present invention. 
     FIG. 2 shows a partially sectioned side view of the collimator of FIG. 1. 
     FIG. 3 is a diagram of the light polarizing beam splitter of FIG. 1. 
     FIG. 4 is a schematic top-down view of an alternative embodiment of the sensor head made in accordance with the principles of the present invention. 
     FIG. 5 is a schematic top-down view of an alternative embodiment of the sensor head made in accordance with the principles of the present invention. 
     FIG. 6 is a schematic top-down view of an alternative embodiment of the sensor head made in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment in accordance with the present invention is illustrated in FIG. 1, which is a schematic semi-three dimensional view of the electro-optic voltage sensor head depicted generally at 4, which senses an electric field. The sensor head 4 of the present invention may be used for sensing the presence and magnitude of an electric field and for measuring voltage. A voltage measurement is a measure of the voltage difference (or electrical potential difference) between objects or positions. Voltage is proportional to the electric field (hereinafter &#34;electric field&#34; shall be indicated &#34;E-field&#34;) depending upon the geometries, compositions and distances of the conductive and insulating matter. Where, as in the present invention, the effects of an E-field can be observed or sensed, a voltage measurement can be achieved. 
     Prior to discussing the sensor head 4 further, attention should be paid to the collimators 14, 16, 18, shown in FIGS. 1, 4, 5, and 6, which are generally represented by the collimator 20, shown in FIG. 2. A collimator 20 comprises a lens 30 and a transparent end 34 which can pass an electromagnetic radiation beam 12 into or out of the core 32 of an optic fiber 40. 
     As shown in the embodiment of FIG. 1, a beam 12 is routed into the sensor head 4 along a first movement path 100 by at least one translucent element, which is shown as an optic fiber 8, although other light transmission devices could be used. The PM fiber 8 directs the beam 12 into the sensor head 4, after which the beam 12 is passed sequentially through elements 22, 24 and 26, then backwardly through elements 26 and 24 and into elements 44, 46 and 47. The beam 12 is then routed from the sensor head elements 46 and 47 by a pair of single-mode or multi-mode optical fibers 42 and 45. The optical fibers 8, 42 and 45 electrically isolate the sensor head 4. The optic fiber 8 may use polarization maintain fibers to deliver light to the transducer 26, or other means such as low-birefringence fiber, single-mode fiber, multi-mode fiber, and a steered collimated beam. 
     In the preferred embodiment, the sensor head 4 has a cross sectional area of only approximately fifty millimeters squared (50 mm 2 ) or less, and a length of approximately twenty five centimeters (25 cm) or less. The sensor head 4, when placed in an E-field (not shown) causes a differential phase modulation of the components in the orthogonal planes of a light beam 12. 
     FIG. 1 shows the preferred embodiment of the sensor head 4. The sensor head 4 comprises a polarizing means which is shown as a polarizer 22; a translucent means shown as a translucent medium 24; a transducing means shown as a transducer 26; a reflecting means shown as a retro-reflector 27; a polarization rotating means shown as a half-wave plate 44; a polarizing beam splitting means shown as a polarizing beam splitter 46; and beam reflector 47. The polarizer 22 re-polarizes the beam 12 emerging from the optic fiber 8 through the collimator 14. 
     Because light from optic fiber 8 may be conveyed via a polarization maintaining fiber or some other transmission mechanism which supplies a polarized beam of light, the polarizer 22 is an optional device that can be used to ensure proper and stable polarization alignment with the electro-optic axes of transducer 26. The polarizer 22 linear-polarizes the beam 12 such that the beam comprises at least two beam components which are propagating in orthogonal planes. In practice, the polarizer 22 may be eliminated or placed anywhere between the transducer 26 and the source (not shown) of the beam 12, including anywhere along the optic fiber 8. 
     The beam 12 passes from the polarizer 22 through the translucent medium 24 and into the transducer 26. The translucent medium 24 is a translucent means which comprises a non-conductive, non-birefringent material, such as fused quartz or a similar substance. The sensor head is designed to be installed in several varieties of high voltage transmission and distribution apparatus in which an E-field is naturally produced. This apparatus is typically co-axial, producing an intense, tightly-controlled radial electric flux between inner and outer conductors. In accordance with fundamental principles of electromagnetics, an E-field accompanies this electric flux as well. By mounting the transducer within this electric flux, an E-field proportional to voltage is established within the transducer, which in turn undergoes the electro-optic effect. As the behavior and durability of the polarizer 22 and optic fibers 8, 42, 45 in the presence of intense E-fields are in some cases either undesirable or not well known, a translucent medium 24 can be used to provide a pathway for the beam 12 from the polarizer 22, which can in such cases be located outside of the intense E-field, to the transducer 26, which is positioned directly in the intense E-field where the Electro-optic effect takes place. Due to the tightly controlled nature of this E-field, voltage measurement based upon E-field magnitude as described herein is highly accurate and impervious to external influences. 
     The transducer 26, when in an E-field (not shown), induces a differential phase shift between the orthogonal planes of the beam 12 through the Pockels electro-optic effect. The differential phase shift varies in magnitude responsive to the presence of an E-field, meaning that the differential phase shift which is induced in the absence of an E-field differs in magnitude from the differential phase shift which is induced in the presence of an E-field. 
     The Pockels linear electro-optic effect, commonly called the electro-optic effect for short, is observed in Pockels transducing crystals and similar media. The Pockels electro-optic effect is observed as a shift between the relative phases of the beam components. This shift is induced in the beam 12 by the transducer 26, also called the transducing medium. The magnitude of the effect typically corresponds proportionally to the magnitude of the E-field. 
     In the preferred embodiment the transducer 26, or transducing medium, comprises a material which exhibits the Pockels electro-optic effect. In the present invention the transducer 26 is preferably Magnesium Oxide-doped Lithium Niobate (MgO-LiNbO 3 ), although other materials, such as Ammonium Dihydrogen Phosphate (NH 4  H 2  PO 4 ), Ammonium Dideuterium Phosphate (NH 4  D 2  PO 4 ), Potassium Dideuterium Phosphate (KD 2  PO 4 ), Lithium Niobate (LiNbO 3 ), Lithium Tantalate (LiTaO 3 ), electro-optic polymers, organic materials, and others can be used. The detector (not shown) employs two photo detectors and a two-channel signal processor to determine voltage. 
     The differential phase shift between orthogonal components of the beam 12 produces a corresponding alteration of the polarization of the beam 12, thus allowing determination of the original E-field intensity from the Pockels effect by analyzing magnitude of the polarization change. The magnitude of the shift is proportional to the magnitude of the E-field, and thus the magnitude of the voltage. Hence, the polarization state of the beam 12 is directly representative of E-field magnitude and voltage. 
     From the transducer 26, the beam 12 enters into the retro-reflector 27. The retro-reflector 27 comprises a reflector material 29 which turns the beam 12 back into the transducer 27. The reflector material of the embodiment shown in FIG. 1 has two functions; one is to reflect the beam 12 and the other is to cause a quarter-wave shift between the components in orthogonal planes of the beam 12. In the embodiment shown in FIG. 1, the reflector material 29 has an index of refraction and incident angle that together facilitate total-internal reflection of beam 12. An alternative embodiment would include a reflective coating disposed on the surfaces of the reflector material 29 for causing the beam 12 to reflect at the boundaries of the reflector material 29. 
     The quarter-wave retardation property of the retro-reflector 27 induces a differential 1/4 wavelength shift between the orthogonal planes of the beam 12 through either using a reflector material 29 possessing intrinsic birefringence, or by inducing a phase shift upon reflection of the beam 12. The reflector material 29 thus in the preferred embodiment includes phase shifting means for shifting the phase of at least one of the beam components to thereby achieve a differential phase shift between said beam components of 1/4 of a wave length. This could be accomplished by shifting the phase of only one of the beam components by 1/4 of a wave length, or by shifting the phase of both components such that the collective result is a differential phase shift of 1/4 of a wave length. 
     In the embodiment employing a reflector material 29 containing intrinsic birefringence, the birefringence is not dependant upon the E-field. In the preferred embodiment, the reflector material 29 induces a 1/8 wavelength differential phase shift upon the beam at each reflection, producing a collective 1/4 wavelength (π/2 radians) shift following the two reflections within the retro-reflector 27. 
     Those skilled in the art will appreciate that reflective coating may further serve to induce, or partially induce, the 1/4 wave length shift of the beam 12, as phase shifts are known to occur in electromagnetic radiation upon reflection from a suitable surface. One skilled in the art could further induce a 1/4 wave length shift in beam 12 by combining a reflection and intrinsic birefringence. 
     The polarization of the beam 12 entering the retro-reflector 27 depends upon the E-field magnitude present when the beam 12 makes a first pass through the transducer 26. If there is an E-field present then there will be some differential phase shift already present in the beam 12. The π/2 retardation within the retro-reflector 27 biases the sensor&#39;s overall resultant polarization such that zero E-field (and hence zero voltage) corresponds to circular-polarized light, as no differential phase shift is present in the beam 12 from either pass through the transducer 26. However, due to the location of the retro-reflector in the sensor head, if the transducer 26 is in a non-zero E-field and induces a differential phase shift in the beam 12, then the retro-reflector 27 will not convert light from linear to circular-polarization. Rather, it will induce elliptical-polarization upon the beam 12. The ellipticity of this polarization will modulate between -1 and +1 in proportion to the voltage. While a laser is used in the preferred embodiment, other sources of electromagnetic radiation could also be used in the practice of the invention. 
     The reflection of the beam 12 in the retro-reflector 27 is in accordance with the principle of the angle of incidence being the same as the angle of reflection. In the practice of the preferred embodiment of the present invention, the retro-reflector 27 is configured to cause a 180° change in the direction of the beam 12, thereby sending the beam backwardly into the transducer 26 along a second movement path 101. The second movement path 101 is preferably, but not necessarily, parallel to the first movement path 100. 
     When the beam 12 reenters the transducer 26 from the retro-reflector 27, it undergoes further differential phase shift from the Pockels electro-optic effect. The beam 12 then passes through the transparent medium 24 and enters into the polarization rotating means, which is provided by the half wave plate 44. 
     In the sensor head 4, the half-wave plate 44 is positioned so that is optical axis is at an angle of 22.5 degrees with respect to the direction that the electric field is applied to the transducer. This effectively rotates the direction of the major axis of the elliptical polarization so that it will align with one of the polarizing beam splitter&#39;s axes. 
     After rotation, the beam 12 passes from the half-wave plate 44 to the polarizing beams splitter 46. The polarizing beam splitter 12 can then easily measure both the major and minor axes of the elliptical polarization state because the major axis of the elliptical polarization will be parallel to one polarization state of the polarizing beam splitter and the minor axis of the elliptical polarization state will be parallel to the other polarization state of the polarizing beam splitter 46. 
     In the polarizing beam splitter 46, (also called an analyzer in the art), the beam 12 is separated in accordance with the respective axes of its polarization ellipse into AM signals 48 and 52. The said polarization ellipse will exhibit an ellipticity ranging between -1 and +1, in proportion to voltage at any given time. As shown in FIG. 3, the major and minor axes of the polarization ellipse of beam 12 can be represented by two orthogonal components, indicated generally at 83. The polarizing beam splitter 46 then separates the beam 12 into two components 84 and 85 corresponding to the optic intensities of the beam 12 along the axes of the polarization ellipse. The intensity of beam components 84 and 85 will modulate conversely to one another in response to modulations in the ellipticity of the beam&#39;s polarization state. The beam components 84 and 85 are two AM signals shown as 48 and 52, respectively, which contain the information needed to determine voltage. 
     The AM signals 48 and 52 then pass through the collimators 16 and 18, shown in FIG. 1, (also shown in FIGS. 4, 5, and 6) and are routed through at single-mode or multi-mode optic fibers 42 and 45. A beam reflector 47 may be used to aid in routing one of the AM signals 48 or 52. 
     FIGS. 4 and 5 show alternative embodiments of the present invention. In these alternative embodiments the beam 12 is reflected by a retro-reflector 27; however, the beam only passes through the transducer 26 once. As can be seen, in FIG. 4 the beam 12 may pass through the transducer 26 before being reflected by the retro-reflector 27, or as in FIG. 5 the beam 12 may pass through the transducer 26 after being reflected by the retro-reflector 27. FIG. 6 depicts an alternative embodiment in which no translucent medium 24 is used. In these three alternatives, the reflection of the beam 12 at a desired angle, shown as 180° enables the fibers 8, 42 and 45 to be aligned as desired, which in the preferred embodiment is parallel to one another. 
     In accordance with FIG. 4, the retro-reflector 27 is essentially a reflecting means for receiving the beam 12 from the transducer 26 and reflecting the polarizing beam into the half-wave plate 44 which rotates the beam prior to entry of the beam into the beam splitter 46. Of course, numerous modifications to the various configurations shown will be apparent to those skilled in the art in light of the present disclosure. 
     It is noted that many of the advantages of the present invention accrue do to the simplified structure of the sensor head, which is sufficiently small so as to be conveniently placed or installed in devices in which electric-fields arise, or built in as part of the device. 
     Although the prior art apparatus and methods for high voltage measurement have attempted to use the electro-optic effect in materials having either a Pockels or Kerr coefficient, they have typically required a separate compensator crystal with a known reference voltage or a separate voltage divider directly connected to the energized conductor in order to make a voltage determination. The result has been devices which were bulky and required additional electronics for measuring the known reference, or required extra hardware which presents size, weight, reliability, and other problems. 
     By integrating the sensor head into the described high voltage transmission and distribution apparatus, voltage measurement is achieved without the use of a large, dedicated, stand-alone device. Because real estate within power substations is at a premium, this sensor offers a substantial economic advantage due to space savings. In addition, contact with the energized conductor is substantially reduced and in most cases altogether eliminated with the practice of the present invention. This is advantageous, as an energized conductor can present significant life and health risks among other hazards and problems associated with the use of the voltage dividers in the prior art. 
     In accordance with another aspect of the present invention, it has been found that it is highly preferable to disposed the sensor in the grounded plane, i.e. between the conductor and the grounded outer sheath. In such a position, the risk of alteration of the E-field due to conductive objects, such as motor vehicles, is dramatically reduced because of the grounded outer sheath. The sensor can be positioned anywhere between the conductor and the grounded plane (i.e. the grounded sheath) while maintaining a high degree of accuracy. The monitoring equipment is still optically isolated from the voltage carried by the conductor. This provides a remarkable improvement over optical sensing systems in which the sensor is disposed outside the grounded plane. 
     Those skilled in the art will appreciate from the preceding disclosure that the objectives stated above are advantageously achieved by the present invention. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements.