IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 17, NO. 2, APRIL 2002 417

230 kV Optical Voltage Transducers Using MultipleElectric Field Sensors

Farnoosh Rahmatian, Member, IEEE, Patrick P. Chavez, and Nicolas A. F. Jaeger, Member, IEEE

Abstract—230 kV optical voltage transducers were constructedand tested. These transducers use three electric field sensors whosepositions and outputs are selected and combined, respectively, inaccordance with the quadrature method to obtain a voltage mea-surement. They meet IEC 0.2% class specifications and maintain0.2% class accuracies even in the presence of electric field distur-bances caused by local changes in geometry external to the trans-ducer. The local changes in geometry used in the testing mimicthose that may occur in a substation, e.g., installation or movementof equipment.

Index Terms—Electric field effects, electric field measurement,electric fields, Gaussian quadrature, high-voltage techniques, in-tegration (mathematics), numerical analysis, optics, transducers,voltage measurement.

I. INTRODUCTION

OPTICAL voltage transducers (OVTs) for power deliveryapplications offer a variety of advantages over conven-

tional inductive and capacitive voltage transformers. Amongthese are better electrical isolation, wider bandwidth, larger dy-namic range, lighter weight, and smaller size. Use of opticalfibers to carry the measurement light signal to and from thesensor-head also electrically isolates the observer from the high-voltage (HV) environment and protects the measurement fromelectromagnetic interference.

Here, the results of laboratory tests on 230 kV OVTs thatuse three small electro-optic field sensors to measure voltageaccurately are presented. As with their predecessor [1], [2],these OVTs are unique compared with most other OVTs inthat their internal electrodes are separated by a safe distance,avoidi such as SFgas [3]–[7] or others [8]. This aspect of theirdesign is made possible by the use of the quadrature method[9], which effectively produces an efficient numerical lineintegration of the field using field sampling between an HVelectrode and a grounded electrode to give an accurate measureof the voltage between these electrodes. The sensors are housedinside a hollow “off-the-shelf” HV insulator filled with Nproviding a secure environment to the sensors and makingthe OVT structure mechanically and electrically very robust.Using this approach, external changes in geometry, e.g., the

Manuscript received June 22, 2001. This work was supported in part byfunding from the British Columbia Advanced Systems Institute and the NaturalSciences and Engineering Research Council of Canada.

F. Rahmatian is with NxtPhase Corporation, Vancouver, BC, V5M 1Z4,Canada.

P. P. Chavez and N. A. F. Jaeger are with the Department of Electricaland Computer Engineering, University of British Columbia, Vancouver, BC,Canada.

Publisher Item Identifier S 0885-8977(02)02738-3.

installation of nearby equipment in a substation, do not affectthe OVTs accuracy nor does the OVT require recalibration.

In the next section, the basic operation of the OVTs is de-scribed. Then, an overview of standard HV laboratory tests andof additional tests for the purpose of confirming the accuracy ofthe OVTs in the presence of nearby external changes in geom-etry is given, and results are reported.

II. PRINCIPLES OFDESIGN AND OPERATION

The central concept behind the functionality of the OVT isthe efficient numerical integration of the electric field using thequadrature method [9]. The quadrature method is used to deter-mine the required number of sensors, where to place them, andhow to weight and combine their outputs for a desired voltagemeasurement accuracy, for a particular standoff geometry, andfor an expected worst-case perturbation (“stray field effect”) ofthe field along the path of integration.

Along any path, integrating the electric field component thatis oriented parallel to that path from pointto point gives thevoltage difference between those two points. Lettingand

lie on the -axis with the integration path being a straight lineand approximating the integral by a weighted sum gives

(1)

where is the -component of the electric field along the-axis, is the number of samples, is the position of theth sample, and is the weight of theth sample.

The quadrature method can be used to determineand .This involves defining an unperturbed system and a worst-caseperturbed system. Generally, the unperturbed system refers to aparticular configuration of conductors and media having a par-ticular , between and , referred to as . The of anyperturbed system (a variation from the unperturbed system) isexpressed in terms of a multiplicative scaling factorand ,i.e., . For a given , the quadraturemethod calculates the and so that (1) is exact for anythatcan be exactly represented by a polynomial of degree 2or less [9].

To begin with, we consider an “off-the-shelf” 230 kV insu-lator supported by a stand that is sitting on a ground plane, as itwould be in a substation. The insulator consists of a fiberglasstube having an inner diameter of 358 mm and a wall thicknessof 8 mm, with silicone rubber shedding on the outside of it, andprovides a structurally robust design. A corona ring is also posi-tioned around the top of the insulator. Inside the insulator tube,there is a smaller fiberglass tube having an inner diameter of198 mm and a wall thickness of 4 mm, and extending from the

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418 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 17, NO. 2, APRIL 2002

Fig. 1. Unperturbed OVT system.

Fig. 2. Axial electric field along the path of integration for an unperturbedOVT and a perturbed OVT with 140 kV applied.

top flange to the bottom flange, to support field sensors. At theends of this inner tube are two electrodes separated by a distanceof approximately 2.2 m. This configuration is defined to be theunperturbed system (see Fig. 1). The path of numerical integra-tion is along the tube axis, or-axis, from the bottom electrode,at point , to the top electrode, at point. is along thispath and is calculated for an applied rated voltage of 140 kV andfor a stand having a height of 2.5 m (see Fig. 2).

Determining the number of electric field sensors needed de-pends on the specified accuracy of the OVT and the behavior of

in perturbed systems. The OVT is specified to maintain goodaccuracy, i.e., to meet IEC 0.2% class specifications, in the pres-ence of the kinds of changes in external geometry that it may ex-perience upon its installation or the installations of neighboring

Fig. 3. � for a semi-infinite vertical ground plane 1.6 m away from theOVT’s axis.

equipment during its operation in a 230 kVsubstation. These areconsidered perturbed systems, and, generally, the nearer that thechanges in geometry are to the OVT, the greater the distortionof the electric field inside the OVT is, resulting in a less accu-rate numerical integration for a given or resulting in a larger

to ensure a given accuracy. This is so because the greater thedistortion in is, the more nonlinear is, and the higher thedegree of the polynomial that accurately approximatesis. Sub-station design safety standards specify minimum clearance dis-tances between any two structures installed on different phasesor between grounded and energized structures depending ontheir voltage class (typically about 3 m or 2 m, respectively, for230 kV), thereby limiting the severity of the perturbations and,consequently, the required number of sensors,. Fig. 2 showsthe axial electric field for a perturbation of a vertical groundplane 1.6 m away from the-axis, and Fig. 3 shows thethatcorresponds to that perturbation.

Using a mixed finite element/boundary integral method [10],various insulators with various perturbations were simulated.Examples of perturbations simulated include neighboring-phasebuses, a nearby, vertical ground plane, and a nearby conductingsphere. For each of these cases, sample positions and weights forvarious numbers of sensors were computed using the quadra-ture method, and the accuracy of the resulting weighted sumswere observed. It was found that three sensors are sufficientto accurately measure voltage (0.2% error) for these types ofperturbations and for insulators with internal electrodes spacedapart by approximately 2 m (length of the integration path).Table I shows simulated errors for computer models of per-turbed systems involving either a ground plane or a groundedsphere near the OVT. The distance of a perturbing object refersto the distance between the edge of the object and the-axis,and a sphere’s height is approximately the height of its centerwith respect to the bottom of the insulator, e.g., “Low” is at theheight of the bottom of the insulator and “High” is at the heightof the top of the insulator. Table II shows the changes inatthe three sample locations inside the OVT for two of these sim-ulated cases.

So, having established the need for three electric field sensors,three s and three s were determined using the quadrature

RAHMATIAN et al.: 230 kV OPTICAL VOLTAGE TRANSDUCERS USING MULTIPLE ELECTRIC FIELD SENSORS 419

TABLE ISIMULATED RATIO ERRORSDUE TO EXTREME PERTURBATIONS

TABLE IISIMULATED CHANGES INE (x ) DUE TO PERTURBATIONS

method with the calculated of the OVT system. Then,electric field sensors were mounted inside the inner tube at these

s, and their outputs were weighted with theses and summedin real-time using digital electronics to give the output of theOVT. The sensors measured the-component of the electricfield in which they were immersed. Table III gives thes and

s that were used in the OVT.For numerical integration, the electric field sensors ideally

measure a single component of the electric field at a point.Here, each sensor basically consists of a miniature, cylindrical,Pockels-effect crystal having a height of 2 cm and a diameter of3.5 mm whose axis is aligned with the insulator’s axis, or the

-axis. Light traveling along the axis of the crystal is modulatedby the axial component of the electric field. Light is transmittedto and from each sensor using optical fibers. The outputs ofthe sensors are then digitally processed giving measures of theelectric field, and further digital signal processing performs thenumerical integration in real-time.

The output of each sensor is effectively a measure of the av-erage intensity of the axial component of the electric field, or

, inside the sensor-head and is insensitive to the transversefield component. The error between averagingover a finitepath length and measuring at a point, i.e., at the center of thepath, is a function of the curvature of along the path. It wasfound that using the average values of along 2-cm-longpaths centered at the, in place of the point values of at the

, introduces less than a 0.01% error in (1).

Fig. 4. High-voltage test set-up.

TABLE IIIFORMULA SAMPLE POSITIONS, x , WITH RESPECTTO � AND WEIGHTS, �x ,

NORMALIZED TO THE MIDDLE SENSOR’S(#2’s) CALCULATED WEIGHT.

III. L ABORATORY TEST RESULTS

Four identical three-sensor 230 kV OVTs, as described earlierand illustrated in Fig. 1, were constructed and tested in an HVlaboratory (see Fig. 4). The OVTs were filled with gas at170 kPa. Each OVT weighs approximately 220 kg. During thetesting, they were supported by a grounded platform about 2.5 mhigh (see Fig. 4). The electric field sensors were positioned andtheir outputs were weighted according to thes and s givenin Table III. Cables supported the fibers that transmit light to andfrom each OVT, and the analog and digital electronics residedin the control room, where digital data acquisition took place.The output of the digital electronics was passed through a D/Aconverter and was amplified to give an analog voltage output,which is also required for testing. For an applied rated voltageof 140 kV, the OVTs produce 2 V, corresponding to a voltagetransformation ratio of 70 000 : 1.

Various tests were performed on the OVTs (at least one OVTper test) in accordance with IEC standards [11]–[13], and theyincluded error testing, lightning impulse testing, wet testing,power-frequency withstand testing, partial discharge testing,chopped impulse testing, and mechanical testing. Special testswere also performed to evaluate the accuracy of the OVT inthe presence of “substation-like” changes in local conductorgeometry.

Since the OVT design is essentially that of a standard standoffinsulator with a few extra internal dielectric components, itinherits the advantageous mechanical and electrical propertiesof the insulator, particularly with respect to HV withstand andseismic withstand. The OVT successfully passed all of the stan-dardwithstand tests.Additionally, itwithstoodnegative-polarity,full-wave impulses down to 1211 kV, which exceeds the stan-dard full-wave impulse test voltage magnitude of 1050 kV, with

420 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 17, NO. 2, APRIL 2002

(a)

(b)

Fig. 5. OVT (a) ratio errors and (b) phase errors.

no sign of disruptive discharge or insulation failure. Linearitytests were done on three of the OVTs, one at a time, by observingthe ratio of their voltage measurements to the voltage output ofa reference. The OVTs were tested for IEC 0.2% accuracy class[12], [13], which means that the ratio error should not exceed

0.2% and the phase displacement error should not exceed10 min at 80%, 100%, and 120% of rated voltage. The OVTs

demonstrated ratio errors of less than 0.1% and phase displace-ment errors of 1 min or less, meeting the IEC 0.2% standards. Allthe OVTs had a rated phase delay of 0.95. Future versions of theOVT will have user-defined rated phase delays to be set to anyvalue depending on the requirements of the application, e.g., 0to meet the standard requirement in [12]. Additionally, switchingimpulses were applied to an OVT, and the OVTs output faithfullytraced the applied waveforms demonstrating the high bandwidthand dynamic range of the OVT.

Ratio and phase errors were recorded at various other voltagesoutside of the standard range. Fig. 5 shows ratio and phase errorsfor voltages from 3 kV to 350 kV. The ratio errors do not exceed0.2% and phase errors do not exceed 1 min through this entirerange.

Perturbation tests were also conducted in which the accu-racy of an OVT was tested at rated voltage with various objectsplaced in its vicinity. A suspended, vertical, grounded metallic

Fig. 6. Grounded screen perturbation 1.6 m away.

Fig. 7. Grounded sphere perturbation 1.2 m away.

screen at various distances from the OVT (see Fig. 6), a sus-pended, grounded metallic sphere (1 m in diameter) at variousheights and distances with respect to the OVT (see Fig. 7), agrounded truck near the OVT (see Fig. 8), and a neighboringenergized OVT 120out of phase with and 3 m away from (less

RAHMATIAN et al.: 230 kV OPTICAL VOLTAGE TRANSDUCERS USING MULTIPLE ELECTRIC FIELD SENSORS 421

Fig. 8. Truck perturbation.

TABLE IVRATIO ERRORSDUE TO SAFE PERTURBATIONS

TABLE VRATIO ERRORSDUE TO UNSAFE PERTURBATIONS

than minimum phase spacing) the tested OVT were used. Theratio errors for the cases of the grounded sphere and groundedscreen at safe distances from the OVT and for the cases of thetruck and the neighboring phase are less than 0.1%. The ratioerrors for the unsafe cases do not exceed 0.3%. Table IV showsthe errors for some of the safe cases, and Table V shows the

TABLE VICHANGES INE (x ) DUE TO PERTURBATIONS

Fig. 9. OVTs at the Ingledow substation, Surrey, BC, Canada.

errors for some of the unsafe cases. Here, the distance of a per-turbing object is defined as the distance between the edge of theobject and the OVTs central axis. The minimum safe distancefor a grounded object is defined as approximately 2 m or more(the exact number depends on the local substation safety reg-ulations). Heights of perturbing objects are with respect to thebottom of the insulator. The phase displacement errors are lessthan 1 min for all of the perturbed cases.

Demonstrating the effectiveness of the weighted sum usedin the OVT design to determine voltage accurately, Table VIshows the percentage changes in the measured electric field atthe sensor locations in the perturbed cases with respect to theunperturbed case. Table VI also shows phase changes in themeasured electric field for the case of the neighboring phase.It should be noted that for the cases of the suspended sphere,the sphere was vertically supported by a grounded cable, whichresults in a more severe perturbation compared to the simulatedgrounded-sphere perturbations.

Three of the tested OVTs have been installed as a three-phasevoltage measurement system at BC Hydro’s Ingledow substa-tion in Surrey, BC, Canada. They have been in operation sinceJune 2000 and are being evaluated by BC Hydro (see Fig. 9).

IV. CONCLUSION

Four 230 kV OVTs were constructed and tested. The OVT de-sign is based on computer modeling and the quadrature method.It performs an accurate numerical line integration of the electricfield with only three electric field sensors to give a measure ofthe voltage. One or more of the OVTs were subjected to var-ious IEC electrical and mechanical withstand tests, and all tests

422 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 17, NO. 2, APRIL 2002

were passed successfully. They demonstrated 0.2% class accu-racies in accordance with [13], i.e., ratio errors not exceeding

0.2% and phase displacement errors not exceeding10 min.The OVTs have a fixed rated phase delay of nearly 1, whichwill be set by the user in future versions of the OVT to meetthe requirements of specific applications, e.g., 0to meet therequirement of [12]. Further tests demonstrated that the OVTs’voltage measurements are accurate to within 0.1% and 1 minfor various cases of electric field perturbations caused by nearbymetallic objects and a neighboring OVT. The OVTs were alsoshown to be very accurate (within 0.3%) even when the metallicobjects were placed dangerously close to the OVT. Significantmeasured changes in the local electric field measurements at thefield sensor locations due to perturbations were also shown, dis-playing the effectiveness of the implementation of the quadra-ture method.

REFERENCES

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[2] P. P. Chavez, N. A. F. Jaeger, F. Rahmatian, and C. Yakymyshyn, “In-tegrated-optic voltage transducer for high-voltage applications,” inAp-plications of Photonic Technology 4, Proceedings of SPIE, vol. 4087, R.A. Lessard and G. A. Lampropoulos, Eds., 2000, pp. 1229–1237.

[3] T. Sawa, K. Kurosawa, T. Kaminishi, and T. Yokota, “Development ofoptical instrument transformers,”IEEE Trans. Power Delivery, vol. 5,pp. 884–891, Apr. 1990.

[4] L. H. Christensen, “Design, construction, and test of a passive opticalprototype high voltage instrument transformer,”IEEE Trans. Power De-livery, vol. 10, pp. 1332–1337, July 1995.

[5] S. Weikel and G. Stranovsky, “Application of an electro optic voltagetransducer at 345 kV,” inProc. EPRI Optical Sensors for Utility T&DApplicat. Workshop, Portland, OR, July 20–21, 1995.

[6] J. C. Santos, M. C. Taplamacioglu, and K. Hidaka, “Pockels high-voltagemeasurement system,”IEEE Trans. Power Delivery, vol. 15, pp. 8–13,Jan. 2000.

[7] C. P. Yakymyshyn, M. Brubaker, P. Johnston, and C. Reinhold, “Manu-facturing challenges of optical current and voltage sensors for utility ap-plications,” inProc. SPIE Conf. on Sensors and Controls for AdvancedManufact., Oct. 14–17, 1997.

[8] K. Bohnert, J. Kostovic, and P. Pequignot, “Fiber optic voltage sensorfor 420 kV electric power systems,”Opt. Eng., vol. 39, no. 11, pp.3060–3067, Nov. 2000.

[9] P. P. Chavez, F. Rahmatian, and N. A. F. Jaeger, “Accurate voltage mea-surement by the quadrature method,”IEEE Trans. Power Delivery, Nov.8, 2000.

[10] B. H. McDonald and A. Wexler, “Finite-element solution of unboundedfield problems,” IEEE Trans. Microwave Theory Tech., vol. 20, pp.841–847, Dec. 1972.

[11] “High-voltage Test Techniques—Part 1: General Definitions and TestRequirements,” Int. Electrotechnical Commission (IEC), Geneva,Switzerland, International Std. IEC 60 060-1.

[12] “Instrument Transformers—Part 2: Inductive Voltage Transformers,” In-ternational Electrotechnical Commission (IEC), Geneva, Switzerland,International Std. IEC 60 044-2, 1997.

[13] “Instrument Transformers—Part 7: Electronic Voltage Transformers,”International Electrotechnical Commission (IEC), Geneva, Switzerland,International Std. IEC 60 044-7 FDIS.

Farnoosh Rahmatian (S’89–M’91) was born inTehran, Iran, in 1969. He received the B.A.Sc.(Hon.), M.A.Sc., and Ph.D. degrees from theUniversity of British Columbia, Vancouver, BC,Canada, in 1991, 1993, and 1997, respectively, allin electrical engineering.

Since 1997, he has been the Director of Re-search and Development at NxtPhase Corporation,Vancouver, BC, Canada, working on precisionhigh-voltage optical instrument transformers for usein high-voltage electric power transmission systems.

He is also an adjunct professor at the Department of Electrical and ComputerEngineering at the University of British Columbia, Vancouver, BC, Canada.

Dr. Rahmatian is also a member of the IEC TC38 Working Group on instru-ment transformers, the Standards Council of Canada, IEEE Power EngineeringSociety, and IEEE Lasers and Electro-Optics Society.

Patrick P. Chavez was born in Vancouver, BC,Canada, in 1971. He received the B.A.Sc. andM.A.Sc. degrees in electrical and computer engi-neering from the University of British Columbia,Vancouver, BC, Canada, in 1995 and 1997, respec-tively, where he is currently pursuing the Ph.D.degree in electrical and computer engineering.

He is also an advisor to NxtPhase Corporation,Vancouver, BC, working on optical high-voltage in-struments. His fields of interest include high-voltageinstrumentation, computer-aided design in electro-

magnetics and optics, and numerical analysis in industrial applications.

Nicolas A. F. Jaeger (M’89) was born in NewRochelle, NY, in 1957. He received the B.Sc. degreefrom the University of the Pacific, Stockton, CA,in 1981, and the M.A.Sc. and Ph.D. degrees fromthe University of British Columbia, Vancouver,BC, in 1986 and 1989, respectively, all in electricalengineering.

Since 1989 he has been a Faculty Member inUBC’s Department of Electrical and ComputerEngineering, where he is now a Professor, and since1991 he has been the director of the University’s

Centre for Advanced Technology in Microelectronics.Dr. Jaeger is a past recipient of the Canadian Institute of Energy’s “Research

and Development” Award, the BC Advanced Systems Institute’s “TechnologyPartnership” Award, and the Natural Sciences and Engineering ResearchCouncil of Canada and the Conference Board of Canada’s “Synergy” Award.