Electro-optic voltage sensor head

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.

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 "electric field" shall be indicated 
"E-field") 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 "light" 
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' differential phase 
shift causes a corresponding change in the beam'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.

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 
"electric field" shall be indicated "E-field") 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.sup.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.sub.3), although other materials, 
such as Ammonium Dihydrogen Phosphate (NH.sub.4 H.sub.2 PO.sub.4), 
Ammonium Dideuterium Phosphate (NH.sub.4 D.sub.2 PO.sub.4), Potassium 
Dideuterium Phosphate (KD.sub.2 PO.sub.4), Lithium Niobate (LiNbO.sub.3), 
Lithium Tantalate (LiTaO.sub.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 (.pi./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 
.pi./2 retardation within the retro-reflector 27 biases the sensor'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.degree. 
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'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'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.degree. 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.