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Integrated-optic sensors for high-voltage substation applications. Jaeger, Nicolas A. F. 1998

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Invited PaperIntegrated-OpticSensors for High-Voltage Substation ApplicationsNicolas A. F. JaegerCentre for Advanced Technology in MicroelectronicsDepartment of Electrical and Computer EngineeringUniversity of British ColumbiaVancouver, BC, Canada, V6T 1Z4ABSTRACTIntegrated-optic devices for use in high-voltage substations are reviewed. Specifically, twotypes of integrated-optic Mach-Zehnder and the integrated-optic Pockels cell are described andcompared. A system for monitoring the condition offluid-and-paper insulation systems, such as areused in many current transformers and power transformer bushings, is also described. This conditionmonitoring system measures the dissipation factor ofan insulation system being monitored. It usesan integrated-optic Pockels cell to measure the phase ofthe voltage on the high-voltage transmissionline to which the insulation system is connected. Preliminary results, showing that the system iscapable of measuring the dissipation factor to an accuracy of 0.5%,arepresented.1 .INTRODUCTIONThe potential ofelectro-optic sensors for use in the power industry has long been recognized[1] -[4]. Mostwork has focused on systems that used bulk electro-optic crystals as the sensingmedium. At the University of British Columbia we have proposed using integrated-optic, electro-optic sensors [5]- [11]. Initial work on the use of integrated-optic devices concentrated on theintegrated-optic version of the Mach-Zehnder interferometer as the sensor-head. This workeventually led to the development ofan integrated-optic version ofthe Pockels cell for this purpose.The integrated-optic Pockels cell overcomes several ofthe problems that are associated with bulk-optic Pockels cells and integrated-optic Mach-Zehnders as the sensor-heads in high-voltageenvironments.In this paper, work carried out at the University ofBritish Columbia, toward the developmentof integrated-optic sensors for use in power utility substations, is reviewed. It begins with adiscussion of some of the potential applications of electro-optic sensors within high-voltagesubstations. This discussion is followed by a brief comparison of bulk-optic and integrated-opticsensors, in which the advantages and disadvantages of each are briefly discussed. Then the workdone to develop integrated-optic sensors of both the Mach-Zehnder and integrated-optic Pockels celltypes is reviewed. This is followed by a short comparison of the integrated-optic sensor types.Finally, an actual system for monitoring the quality of fluid-and-paper insulation systems in high-voltage substations, and using integrated-optic Pockels cell sensors, is presented along with somepreliminary results.SPIEVol. 3489 • 0277-786X/981$1O.OO 41Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms2.1 Applications 2. ELECTRO-OPTIC SENSORSApplied FieldFigure 1. A typical transfer function for an ideallybiased Mach-Zehnder or Pockels Cell type device.NHighVoltageCenterGrounded ConductorOuter Gas-InsulatedConductor Transmission LineFigure 2. An electro-optic sensor in a fix-geometry electrode configuration, gas-insulatedtransmission line.Electro-optic sensors can be used asvoltage sensors in those applications in whichthe relationship between the electric field and thevoltage on the transmission line are well known,e.g., when the voltage is applied directly to thesensor-head [12], in a fixed-geometry electrodeconfiguration such as that inside a gas-insulatedtransmission line (see figure 2) [10], or in aspecially designed housing that "conditions" (orstructures) the field [13].Currently, there are three main areas inwhich voltage and current information is used inpower substations, metering, protection, andmonitoring. Metering requires a high degree ofaccuracy, typically the accuracy for meteringmust be within 0.3%. Protection requires loweraccuracy but faster response times, e.g., faultconditions must be identified and circuit breakersopened within a few cycles if serious damage tocapital equipment is to be avoided. Monitoringmay require both accuracy, e.g., the fluid-and-paper insulation monitoring system describedElectro-optic sensors designed for power utility applications are typically electric-fieldsensors. For the Mach-Zehnder and Pockels ceiltype electric-field sensors discussed here, the opticalintensity at the output ofthe sensor-head is usually related to the intensity at the input by a sinusoidalfunction. This transfer-function has the formI =i.!±!cos(+ [10 1 2 2 EE 'where J is the intensity at the output, I is the intensity at the input, Ea j5 the applied field (internalto the crystal), E is the half-wave electric-field,and 4 is the intrinsic phase difference or the biasofthe device (in an electroded device Ea and E,,are replaced by Va and V, the applied voltageand the half-wave voltage, respectively). Ideally= ir/2, allowing the sensor to operate in alinear region. Figure 1 shows a typical transferfunction for a device with an ideal bias.-E,/2 0 E,/2Opto Out Ut FiberIE-Field42Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsbelow, and/or wide bandwidths, e.g., partial discharge monitoring systems, this very much dependson the type of monitoring being performed.Traditionally, the metering and protection functions have been separate and each has requiredits own equipment geared towards the particular application. Interest in monitoring is currentlyincreasing, in part due to the age ofthe installed plant, leading to a rapid increase in the number ofsystems under development. Furthermore, new areas ofinterest to the power utilities, such as powerquality, are being introduced which will require new types of monitoring systems. Electro-opticsensors are capable of having the accuracies currently required of metering equipment while stillhaving the bandwidths required of protection equipment. Figure 3 shows the response of anintegrated-optic Pockels cell to an industrystandard 1 .2/50 ts/ts lightning impulse. In fact,the bandwidths ofintegrated-optic devices are sowide [14] that a single sensor could be used forboth metering and monitoring partial discharges.The ability to use a single sensor for multipleapplications would represent a significant costsavings to the industry. Also, optical sensorscan have additional advantages over traditionalvoltage and current transformers, e.g., they canbe non-intrusive, they are inherently insulating,and they are immune to electromagneticinterference.The advantages ofoptical sensors lend them very nicely to some ofthe new areas of interestto the power industry such as monitoring power quality. Being able to monitor frequencies wellabove the fundamental (60 Hz) is ofparticular interest. Figures 4a and 4b show the results of usingan integrated-optic Pockels cell to monitor a 60 Hz signal with significant harmonic content. Figure4a shows the spectrum of the applied signal and figure 4b shows the output signal from the.BIFigure4a. The spectrum of a 60 Hz signal, with Figure 4b. The spectrum of the processed outputsignificant harmonic content, applied to an from the integrated-optic Pockels cell.integrated-optic Pockels cell. 43Integrated-Optic PockelsCefl SignalApplied Signal -• ••—---- .t..:-r----Figure 3. Response of an integrated-optic Pockelscell to a 1.2/50 jis/ts lightning impulse.Frequency (Hz) Frequency (Hz) 1800Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms44integrated-optic Pockels cell sensor (while both figures show the spectral content only up to thethirtieth harmonic, this particular system was capable of monitoring the signals to 10 kHz).2.2 Integrated-Optics vs. Bulk-OpticsIntegrated-optic sensors have several advantages over bulk-optic ones. For example, anintegrated-optic sensor typically requires fewer optical components. lenses, beam splitters, polarizers,etc.. as compared to a bulk-optic one. This is especially true at the point of measurement. Figures5a and 5h illustrate the difference in the number ofoptical components used at the measurement pointfor a typical bulk-optic Pockels cell and an integrated-optic Pockels cell, respectively. Also.integrated-optic sensor-heads typically have optical fibers permanently attached to them. The fibersare rigidly bonded to the sensing crystal, reducing the effect of vibration and the possibility ofmisalignment. Another advantage is that the sensor-heads can be much smaller in size and lower incost. In integrated-optic sensors sensing is confined to the region of the optical waveguides. whichtypically have transverse dimensions on the order of several microns and lengths on the order of a fewmillimeters to a few centimeters. This means that integrated-optic devices require much less materialin their fabrication, usually handling, bonding. and packaging issues force them to be larger than isabsolutely necessary for their sensing function. Small size has several additional advantages beyondthe cost advantage such as integrated-optic sensors can be much less intrusive than bulk-optic ones.This means that they can be more easily incorporated into other pieces of equipment (in a substationsuch a piece of equipment could be a signal-column as shown below). Another advantage of smallsize is that in piezoelectric materials the resonances occur at higher frequencies. One advantage ofthe bulk-optic sensor is that it is easily biased. This can be done, for example, by simply includinga quarter-wave plate in the optical system. Nonetheless, it is possible to manufacture integrated-opticPockels cells with a useable bias, as is discussed below. Another advantage of bulk-optic sensors isthat it is possible, using a large device, to apply the entire voltage across the sensor-head [12].3. INTEGRATED-OPTICSENSORSIntegrated-optics devices are fabricated using methods that were originally developed for thefabrication of integrated electronics. Generally speaking, integrated-optic devices are planar devicesthat use optical channel waveguides to direct light within a circuit in much the way that integratedelectronics devices use wires to direct electricity. The waveguides used typically have transverseElectricField —Output.1. ,Li- FibersOutputBeam-SplitterQuarter-Wave EltoInputetarLens PolarizerInputFiberFigure5a. A typical bulk-optic Pockels cell.ElectricField Polarization-MaintainingOutput Fiber.LLI.Titanium In-DiffusedWaveguidePolarizing orPolarization-MaintainingInput FiberFigure Sb. An integrated-optic Pockels cell.Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsdimensions that are on the order of several microns and lengths that are on the order of a fewmillimeters to a few centimeters. The most commonly used substrates for the fabrication ofintegrated-optic devices are lithium niobate, compound semiconductors (gallium arsenide, indiumphosphide, etc.), silicon, and various glasses. Since silicon and glasses do not exhibit the linearelectro-optic effect, the most commonly chosen substrates for integrated-optic devices are lithiumniobate and the compound semiconductors. Ofthese, the fabrication technology for low-loss opticalwaveguides in lithium niobate is the least expensive. Also, lithium niobate is chemically inert, makingit very suitable for fabricating devices intended for harsh environments. See reference [1 51forsomeof the properties of lithium niobate.In lithium niobate the two principal methods used for the fabrication of optical waveguidesare titanium in-diffusion and proton-exchange. Proton exchange results in an increase of theextraordinary refractive index ofthe lithium niobate and ofa decrease in the ordinary refractive index.This makes proton exchange appropriate only for devices in which extraordinary modes are tolaunched but not ordinary modes. In our work we have typically preferred ordinary modes toextraordinary modes, since there is no surface-guiding and since the photo-refractive effect is smaller,and have usually used titanium in-diffusion to fabricate our channel waveguides.3 .1Integrated-Optic Mach-ZehndersThe integrated-optic Mach-Zelmder is one ofthe most widely used electro-optic devices. Itis widely used in telecommunications systems as well as in sensor systems. Typically, in anintegrated-optic Mach-Zehnder light in a single input waveguide is channeled into two branches andthen recombined into a single output waveguide. The optical path lengths ofthe two branches, andhence the relative phases of the light output from the branches, are controlled via the electro-opticeffect. The controlled coupling ofthe branch outputs into the output waveguide results in a transferfunction such as that given in equation 1.3 .1.1The Mach-Zehnder with a Capacitive DividerIn a Mach-Zehnder with a capacitive divider, the capacitor is monolithically integrated ontothe substrate containing the Mach-Zehnder [6],Elecmc [9]. Figure 6 is an illustration of how theseField Metal Pad devices were used. The metal pad on thesubstrate acquired a potential that wasdetermined by the capacitance from the pad tothe high-voltage conductor and by thecapacitance from the pad to the ground-plane,i.e., the substrate formed part of the capacitivedivider. The potential acquired by the pad wasalso the potential difference across the twoLithium Niobate Connton modulating electrodes of the Mach-Zehnder, oneSubstrate Ground-Plane of the modulating electrodes being integral withFigure 6. An integrated-optic Mach-Zehnder the pad and the other being connected to thewith a monolithically integrated capacitive ground-plane.By positioning one of thedivider. 45Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms46branches of the Mach-Zehnder under the electrode connected to the pad and positioning the otherbranch under the electrode connected to the ground-plane an optical push-pull action was achievedin the device.Our devices were fabricated on z-cut lithium niobate substrates [91.Theyhad x-propagatingwaveguides. This allowed either an ordinary or an extraordinary mode to be launched into thewaveguide; in our case we launched an ordinary or TE-like mode in order to avoid the surface-guiding problems associated with lithium out-difftision, which can occur simultaneously with the in-diffusion oftitanium. Various devices were designed which had half-wave voltages ranging from 21to 1 06 V. Since our sensors operated in the small-signal linear region, these devices required thatonly a few volts be induced on the capacitive divider when the voltage on the transmission line wasat its maximum value.We successfully fabricated devices of this type and tested them in the laboratory. Voltageamplitude measurements at 60 Hz were repeatedly performed, at room temperature, giving rms errorsof<O.3%. Nevertheless, these devices have some significant shortcomings. The first is establishinga bias of it/2, which is necessary ifthe device is to operate in the small-signal linear region. Thisrequires precise control ofthe optical path lengths in each ofthe branches which is very difficult toachieve in a repeatable fashion; even for identical devices, fabricated simultaneously on the samesubstrate, there is a significant variation in the bias, reducing the yield that one can expect [9].Another problem is the bias change as a function of temperature. The devices studied exhibited arelatively large temperature dependent change in bias of about 1 .25 °/°C.3. 1 .2 The Mach-Zehnder with Domain Inversion in One BranchAnother possible way to fabricate an integrated-optic, electro-optic field sensor is to usedomain inversion in one ofthe branches ofa Mach-Zehnder [1 1]. Inverting the ferroelectric domainsinverts the spontaneous polarization and, subsequently, the linear electro-optic effect in the branch[16]. In such a device both branches are subjected to the same electric field and the modulation ofthe output comes about due to the electro-optic effect in the inverted region being opposite to thatin the non-inverted regions. Figure 7 shows anElectric integrated-optic Mach-Zehnder with a domain-Field Output inverted region in one of the branches.One way ofachieving domain inversion isto in-diffuse a large dose of titanium into a z-cutlithium niobate substrate followed by a heattreatment, this was the method that we employed.In our devices, first a large dose of titanium wasin-diffused forming the domain-inverted regionsthen a smaller dose of titanium was in-diffused toform the waveguides.Under controlled conditions these deviceswere capable of measuring 60 Hz signals with a,UfNormalBranchInputWaveguide Lithium NiobateSubstrateDomain-InvertedRegionFigure 7. An integrated-optic Mach-Zehnderwith a domain-inverted region in one branch.Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termssufficiently high degree of accuracy, i.e., with mis errors of O.3% at room temperature. Whiledevices of this type have the advantage that they do not have integrated metal electrodes, but areemersion-type devices, they still have several significant shortcomings. The principal problem thatwas encountered was the completely random biases ofthe fabricated devices. This is to be expectedsince the large dose of titanium changes the optical length of the domain-inverted region in anuncontrolled fashion. Also, the bias change as a function of temperature was much larger than forthe other device types studied, about a factor of four greater than for similar Mach-Zehnders withmonolithically integrated capacitive dividers.3.3 The Integrated-Optic Pockels CellThe sensor-head of the integrated-optic Pockels cell consists of a piece of y-cut lithiumniobate containing az-propagating waveguide [10], see figure 5b. The dimensions ofthe pre-diffusiontitanium strip can be chosen prior to the diffusion process so that the waveguide formed will supportonly the fundamental TE-like and the fundamental TM-like modes. When an electric field is appliedparallel to the y-axis of the crystal the optical indicatrix is changed in such a way that equal butopposite changes occur in the mode indices ofthe TE- and TM-like modes. Polarized light is coupledinto each ofthe modes at the input ofthe waveguide using a polarizing or polarization-maintainingoptical fiber. The input light is polarized so that its electric field is oriented at about 45 °tothe x- andy-axes ofthe substrate, so that nearly equal amounts ofpower at coupled into both modes. Since ina y-cut, z-propagating, waveguide the transverse optical field distributions are determined by theordinary refractive index distribution, the mode-profiles for the TE- and TM-like modes are nearlyidentical. The amount of power in each of the modes can be controlled to obtain nearly circularpolarization across the entire output plane when there is a ic/2 phase difference between the TE- andTM-like modes at the output, i.e.,when the bias is 'it/2 (for other values ofthe bias the output will beuniformly elliptical in the output plane).The applied electric field causes the ellipticity of the polarization ellipse at the output tochange. The output is interrogated using a birefringent, polarization-maintaining fiber. The powersparallel to the principal axes ofthe output polarization ellipse are given by equation 1 where the plussign corresponds to the power parallel to one axis and the minus sign corresponds to the powerparallel to the other. The fast and slow axes ofthe output fiber are aligned with the principal axesof the output polarization ellipse (i.e., at an inclination of about 45 °tothe x- and y-axes of thesubstrate) so as to couple the power parallel to one of the principal axes into the fast mode of thefiber and to couple the power parallel to the other principal axis into the slow mode. In this way theoutput fiber acts as an analyzer. The powers in each ofthe fiber' s modes are separated and measuredindividually.It might be tempting to fabricate the integrated-optic Pockels cell sensor-head in such a wayas to take advantage oflithium niobate' s large r33electro-opticcoefficient, however, there are severalreasons for not doing this. First, if a Pockels cell were designed so that one of the modes was anordinary mode and the other was an extraordinary mode, as in an x- or y-propagating waveguideformed in a z-cut substrate, the differences in the refractive index distributions would lead tosignificantly different mode-profiles for the TE- and TM-like modes. This would result in thepolarization state across the output plane of the sensor-head being non-uniform which, in turn, would47Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms48lead to a degradation in the on/off ratio of the device. Such a device would also forfeit two of itsprincipal advantages over the Mach-Zehnder type devices discussed above, the ability to bias it andits much reduced sensitivity to temperature, see the discussion below. Other reasons for not doingso have been mentioned above, e.g., it avoids surface-guiding and the photo-refractive effect issmaller.In the laboratory, under controlled conditions, these devices were able measure 60 Hz signalswith accuracies in excess ofO.3%. They had the lowest bias changes with temperature ofthe devicesstudied, about 0.014 °/°C/mm (or about 0. 14 0/°Cfor a one centimeter long device) [1 7]. Theintegrated-optic Pockels cell has the advantage ofhaving two complementary output signals whichallows the signals to be normalized, the bias to be tracked, and the output to be relinearized. Anothersignificant advantage of the integrated-optic Pockels cell is that it is possible to fabricate deviceshaving desirable biases. This can be done because there is a small modal birefringence exhibited bythe in-diffused channel waveguides used in the Pockels cells. The amount ofmodal birefringence isdependent on the transverse shape ofthe waveguide. Therefore, by fabricating numerous waveguidesnext to each other, in the same substrate, having slightly different pre-diffusion titanium strip-widths,each waveguide will have a unique bias when the crystal is cut and polished [10]. Provided that asufficient number ofwaveguides are fabricated in the substrate, it is possible to pick one that has auseful bias. This is done without adding to the cost ofthe finished device since, as mentioned above,the finished sample is many times larger than is necessary for the sensing function, e.g., for handling,bonding, and packaging purposes.3.4 The Integrated-Optic Pockels cell vs the Integrated-Optic Mach-ZehndersThe integrated-optic Pockels cell has two major advantages over the Mach-Zehnder typesensors discussed above. First, in it the amount by which the bias changes with temperature is aboutan order of magnitude less than for the Mach-Zehnder with capacitive divider and forty times lessthan for the Mach-Zehnder with domain inversion. Second, it is possible to manufacture integrated-optics Pockels cells with waveguides that have useful biases. Furthermore, the Pockels cell providestwo complementary optical output signals that can be used to normalize the signals, to track the bias,and to relinearize the output, knowing the bias may also provide a means for estimating thetemperature ofthe sensor-head. Two complementary output signals can also be achieved for Mach-Zebnder type devices, however, to obtain them would require a significantly more complex design,taking up much more real-estate on the substrate, and two output fibers to interrogate the twosignals. 4. A DISSIPATION FACTOR MONITORING SYSTEMAt the University of British Columbia there has been considerable work conducted towarddeveloping a condition monitoring system for the fluid-and-paper insulation systems that are used inmany current transformers and power transformer bushings [18]; dissipation factor is widely believedto be the best indicator of the health of fluid-and-paper insulation systems. This monitoring systemmeasures the dissipation factor of an insulation system on-line and in real-time. It does this byaccurately measuring the phase of the voltage on the high-voltage line and the phase of the currentDownloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsthrough the insulation system. The current through the insulation system can be measured at thecapacitance-taps (cap-taps) of current transformers and bushings.4 -7z'Cap-TapUnitThe system has several components; theprincipal ones being an integrated-optic Pockelscell, several cap-tap units containing low-dissipation factor capacitors (more than oneinsulation system can be monitored using a singlePockets cell), and an optoelectronics and signalprocessing unit. The optoelectronics and signalprocessing unit accepts the input signals andstores the calculated values of the dissipationfactor. As shown in figure 8, the integrated-opticPockels cell is connected to the high-voltage lineat the top of a high-voltage signal-column. Lightis supplied to the Pockets cell from a laser andlight from the Pockels cell is interrogated byphoto-detectors, all located on an optoelectronicsboard. The optical signals are carried to andfrom the high-voltage environment, where thePockels cell is located, on optical fibers inside thesignal-column. The capacitors that interrogatethe insulation currents are connected in serieswith the insulation systems being monitored.They are connected to the cap-taps of the devicesbeing monitored and supply measurement signalsto the system via shielded, twisted pairs of wires.The capacitors in the cap-tap units have beenchosen so as to provide low-voltage signals to thesignal processing unit. Figure 9 shows the systemin the High-Voltage Laboratory at PowertechLabs Inc., where the testing was performed (twointegrated-optic Pockels cells were testedsimultaneously). Also shown in figure 9 is ahigh-voltage fluid-and-paper capacitor that waspart of a capacitive divider used to monitor theoutput voltage from the resonant test-set.Since the capacitors in the cap-tap unitsare in series with the insulation systems. thevoltage signals measured across them,representing the insulation currents, are in phasewith the applied voltage. This can becompensated for by accurately shifting the phaseof one of the signals. Here the signal from the49H igh-VoltagcLinc integrated-Optic—— -Poceis CellSensor-HeadCurrent iranstormerlug it-Vö tapeSienal-CdluninOptoelectronicsandSignal ProcessingTo the (lontrolRoomFigure8. An integrated-optic Pockets cell baseddissipation factor monitoring system.Figure 9. Experimental set-up used to measurethe dissipation factor of fluid-and-paper insulationsystems. Shown are a resonant test-set. twosignal-columns containing integrated-opticPockels cells, a high-voltage fluid-and-papercapacitor. and a current transformer.Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsintegrated-optic Pockels cell, representing the applied voltage, is shifted backward in phase by aquarter-cycle (to shift the voltage signal first its frequency is accurately calculated and then a timedelay is applied). It is now possible to use the standard equation (see for example [19])I (tanô =tanlarcsinl fv(t)i(t)dt [2]I 7CATIvIL ' 0where N is an integer, v(t) is the applied voltage (the shifted signal), and i(t) is the insulation current,to calculate the dissipation factor.Typically, fluid-and-paper insulation systems degrade relatively slowly. However, when asystem fails in the field it typically fails catastrophically, exploding and causing damage to adjacentequipment. The signature of an impending failure is a relatively rapid increase in an insulationsystem' 5 dissipation factor (taking place over hours or days as opposed to weeks or months). Duringsuch an episode the insulation system' s dissipation factor increases from 1% or below to severalpercent (10%)in a few tens ofhours [20]. The system described here takes a data point about onceevery 5 seconds. Figure 10 shows data collected from a high-voltage current transformer over a 1800mm (30 hr) period by the system. Also shown in figure 10 is data collected concurrently using a high-precision, high-voltage capacitive divider. The maximum difference between the measurements takenby the two systems is 0.5%, which meets the requirements for an effective monitoring system of thistype. Figure 11 shows daily-average data collected from a particular current transformer over a 90-day period by the same system (data was not collected every day but only at certain times, mostlyevenings and weekends).LI. 005— — — — — — .— —.• ., .0 10 20 30 40 50 60 70 80 90DayFigure10. Dissipation factor data taken from a Figure 11. Daily average dissipation factor datahigh-voltage current transformer over a 1 800 mm taken from a high-voltage current transformer(30 Kr) period. Data taken using an integrated- over a 90-day period.optic Pockels cell —solidcurve. Data takenusing a high-precision capacitive divider —dashedcurve.it-0.01-0.02-003404-0.050 600 1200 1800Time(minutes)50Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms5. SUMMARYIn this paper, work being carried out at the University ofBritish Columbia, toward developingintegrated-optic sensors for high-voltage substation applications, has been reviewed. The advantagesof integrated-optic, electro-optic sensors, as compared to bulk-optic ones, have been discussed.Three types ofintegrated-optic, electro-optic sensor studied have been described and their respectiveadvantages and shortcomings discussed. Finally, a specific application of and integrated-opticPockels cell, as part ofa fluid-and-paper insulation system monitoring system, has been discussed andpreliminary results have been presented.6. ACKNOWLEDGEMENTSThe author wishes to acknowledge the Electric Power Research Institute for its key role inmaking the work on the development of the dissipation factor monitoring system possible and theNational Research Council of Canada, the Science Council of British Columbia, and the BritishColumbia Advanced Systems Institute for their financial support. He is also indebted to hiscolleagues, collaborators, and co-workers who have contributed to this work.7. REFERENCES1 .G.A.Massey,D.C. Erickson, and R.A. 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