All-opticl usr differentil protection scheme for electric power systems M Nsir +, A Dysko, P. Niewczs, G Fusiek Institute for Energy nd Environment, Electronic nd Electricl Enginering Deprtment. University of Strthclyde, Glsgow UK. + Emil: muhmmd.nsir@strth.c.uk Keywords: Frdy rottor, opticl devices model, differentil protection, Jones mtrix formlistion, opticl mesurement. Astrct This pper proposes novel implementtion of differentil protection scheme using mgneto-optic current sensors. The proposed ll-opticl Differentil Protection (ODP) scheme utilizes inherent properties of mgneto-optic sensors connected in series to perform differentil protection functionlity. In order to demonstrte the vlidity of the proposed scheme, ll constituent components such s opticl fire, polrisers nd Frdy rottors hve een modelled using the Jones mtrix representtion. Through selected simultion-sed cse studies, including externl nd internl (high resistive nd solid) fults, the pper demonstrtes tht the proposed novel ODP scheme for usr protection meets the protection relying performnce criteri in terms of discrimintion, sensitivity, stility, s well s ultr-high speed of opertion. Introduction In the conventionl usr differentil protection schemes, iron core current trnsformers (CTs) or equivlent electronic current trnsformers (ECTs) re pplied to mesure current in protected circuits. CTs suffer from mesurement ccurcy issues resulting from the existence of the mgnetizing current. This cn e worsened y sturtion cusing significnt current mesurement errors, potentilly leding to erroneous rely opertion nd flse tripping of the circuit reker (CB). The effect is prticulrly significnt in usr protection pplictions s mny CTs re often connected in prllel nd thus incresing the influence of the mgnetising current. Development of n opticl current trnsducer (OCT) for protection purposes is imed to resolve the CT sturtion prolem nd other technologicl shortcomings [-4]. Significnt progresses hve een mde in recent yers to relize such OCTs. A numer of methods hve een implemented which cn e grouped under two rod types, nmely hyrid opticl sensing [5], exploiting comintion of primry current trnsducers, mgnetostrictive or piezoelectric sensors nd opticl strin sensors tht re interrogted remotely, nd pure opticl sensing [6], utilising rnge of intrinsic or extrinsic opticl sensors sed on the mgnetooptic Frdy effect. Moreover, rnge of reserch ctivities in the re of opticl current sensing for monitoring, metering nd protection purposes re still underwy [7-]. ODP schemes hve een reported extensively in open literture. These schemes pply the generl method of modern numericl protection where currents re mesured y OCT, digitized, nd nlysed using numericl lgorithms sed on the selected protection scheme [-3]. Therefore, such schemes could e termed s mixed opticl-numericl protection. They generlly consist of two steps: conversion process nd process of sustitution into protection scheme lgorithm. The first step (the conversion process) is the conversion of the mesured current from nlogue to opticl modultion tht is performed y n OCT. The second step is to trnslte the opticl signls into digitl vlues which re then used y n lgorithm within numericl rely to perform given protection relying scheme. In contrst to the previous method, this pper introduces new method to perform n ODP scheme tht could e termed s ll-opticl protection. An ODP scheme for usr protection sed on the circulting current differentil protection principle is proposed. The proposed ODP scheme is implemented using crefully designed rrngement of sic opticl devices without the need for complex numericl processing t the second step conversion in the previous method. The proposed scheme utilizes ll opticl elements to perform the opertionl chrcteristic of differentil protection using configurtion of opticl devices such s Frdy rottor, liner polrizer, polrized light source nd opticl detector. In this design, inherent opertionl functionlity is relised using series connection of opticl devices tht follow the current differentil connection pttern. Such design is intended to minimize the complexity of the ODP configurtion while retining the qulity of the differentil protection scheme. Due to the purely opticl nture of the scheme, the need for digitistion of the sensor outputs nd digitl signl processing within the rely re eliminted; therefore, the proposed scheme hs the potentil for the reduction of the totl operting time. This pper egins with theoreticl overview of the circulting current protection scheme nd modelling of opticl components in Section 2. A methodology to design novel ll-opticl differentil protection scheme configurtion is
explined in detil in Section 3. Moreover, the results of computer simultions re given in Section 4. Finlly, Sections 5 nd 6 cover the discussion nd conclusions, respectively. 2 Theoreticl ckground 2. Circulting current differentil protection The sic principle of circulting current differentil protection scheme is presented in Fig.. I CT- i i 2 Loop i diff = i + i 2 Loop 2 CT-2 Fig.. Principle of circulting current differentil protection. The scheme is sed on Kirchhoff s current lw which mintins tht the directed sum of ll currents in the helthy element is equl to zero (under idel mesurement conditions), Therefore, current sum which is not equl to zero would indicte n existence of fult in the protected element. Rely threshold setting is typiclly introduced in the differentil protection to cover ny non-idel conditions. The differentil rely genertes no tripping signl when the differentil current i diff is smller thn the threshold setting, nd the tripping signl is produced only when differentil current is greter thn the setting [4-6]. The differentil rely opertion logic cn e stted s follows: { i diff = I + I 2 ; No trip is generted, if: i diff i setting ; () Trip is generted, if: i diff > i setting } where i diff is the differentil current in the rely rnch, i nd i 2 re phsors derived from secondry currents of CT- nd CT-2 flowing in loop nd loop 2 respectively. 2.2 Opticl component modelling using Jones principles For the purposes of modelling the opticl devices nd the ODP scheme configurtion, polrized light nd ll opticl prts (sensors, opticl fire, polrizers, wve-pltes, etc.) cn e expressed in mtrix which use [2x2] mtrix representtion. Since in this method polrized light will e used s crrier trnsverse wve to interrogte Frdy rottors, the polrized light hs to use representtion which cn descrie its chrcteristics in terms of two liner orthogonl polriztions. As consequence, the polrized light could e expressed y the two element vector, E, to descrie its stte of polriztion (SOP). All opticl prts such s the frdy rottor, liner polrizer, nd opticl fire should comply with the modelling process sed on the Jones mtrices [7-2]. I 2 In order to incorporte ll models of opticl devices in n integrted frmework, Jones formliztion hd een introduced s forml system for nlysing light nd opticl mteril interction. The electric field component of the trnsverse wve, E o, exiting single device tht introduces n opticl effect J when incident light, E i, psses through this medi, is given s E o = J E i (2) The effect of multiple opticl devices cn e determined y simple multipliction of the mtrices where output of the first device ecomes input for the next device. This principle cn e descried y E o = J m J m- J m-2 J m-3... J 3 J 2 J E i (3) where m is numer of opticl components in the pth. The Jones formliztion is n efficient method in numericl modelling of polrized light, opticl devices, nd interction etween light nd opticl mterils [7]. 2.3 Frdy mgneto-optic principle When linerly polrized light psses through mgneto-optic medium, such s opticl glss or TGG crystl, the direction of polriztion is rotted in proportion to the mgnetic field component prllel to the light propgtion direction (refer to Fig. 2.). The ngle of rottion of the polrized light (θ) is proportionl to the mgnetic field intensity vector (H) nd the cosine of the ngle etween the field nd the direction of the light wve propgtion. In the trnsprent mteril the effect is descried y Becquerel's formul (4) which is well known s the Frdy effect [22, 23]: θ = V L μ H dl (4) where, V is the mteril Verdet constnt relted to mteril chrcteristics, wvelength, nd temperture; dl is the differentil vector long the direction of light propgtion; nd L is the distnce of the polrized light trversing the Frdy mgneto-optic mteril. Since μh cn e sustituted y B (mgnetic field strength), (μ is the permeility of the mgneto-optic mteril), eqution (4) cn e expressed s θ = V L B dl = V B L (5) 3 Design of ll-opticl protection scheme 3. Sensor design The proposed trnsducer exploits Frdy rottor (FR) element centred in solenoid used to induce mgnetic field in the rottor s result of current flowing through the coil. This is illustrted in Fig. 2. 2
The stte of polristion (SOP) of light exiting the Frdy rottor cn e expressed y the ngle of rottion : θ = V L k n i (6) polrized light which hs to trvel long the protection scheme configurtion (Fig. 3). This condition is fulfilled when reltively short sections of fire (metres) re employed. where k is fctor for mgnetic field., n is the numer of turns per unit length, i is the mesured current. Thus, the ngle of rottion is proportionl to the mesured current on primry side. E P FR- i E i / E o : light input / output P / P2: polrizer-/2 FR-2 P2 : FO link, FR-/2: opticl sensor-/2 E o. Frdy effect Eo. FR sensor design B FR sensor Fig. 3. Opticl differentil protection scheme configurtion for 2 rnches In order to nlyse the polrized light interction within opticl components, y following eqution (3) nd Fig. 3, the input nd output reltionship of polrized light in the complete ODP scheme cn e expressed using the Jones mtrices formlism: Fig. 2. Opticl current trnsducer employing Frdy mgneto-optic effect 3.2 Design nd principle of ODP scheme c. Design of FR sensor (inside the coil) s OCT The ODP scheme cn e relized y comining the circulting current differentil principle with oth Frdy rottor sensors (illustrted in Fig 2) connected in series using section of opticl fire. The comined opticl rottion is sum totl of the outputs produced y the two sensors. This is descried y the following eqution: θ diff = θ θ 2 (7) where θ nd θ 2 re the light stte outputs from Frdy rottor sensor dn 2 respectively. To successfully implement the scheme, three conditions must e fulfilled. Firstly, polrized light is required for opertion of the scheme. Secondly, the differentil current detection should e relised y the opticl circuit s much s possile, with miniml involvement of electronic signl processing. Thirdly, the scheme must provide dequte performnce in terms of sensitivity of the differentil current detection. The three requirements of the ODP design cn e fulfilled y the pproprite rrngement of sic opticl devices s shown in Fig. 3. It is noted tht two liner polrizers in Fig. 3 must e plced with the trnsmission xis (TA) t 9 or crossed position with respect to ech other. The Frdy rottor sensors re connected in series nd plced etween the two polrisers (P nd P2). Their electricl terminls must e connected in such wy tht the polristion rottions of the individul sensors cncel out when there is no fult condition (no differentil current present). Such is n ODP scheme follows the differentil relying connection requirement. The scheme cn e extended to more thn two Frdy rottor sensors, for exmple pplied to multi-terminl circuit. Criticlly, the opticl fire is required to mintin the SOP of E o = FO5 9 P2 8 FO4 7 FR2 6 FO3 5 FR 4 FO2 3 P 2 FO E i (8) where FO, FO2, FO3, FO4 nd FO5 re mtrices representtion of fire optic sections, 2, 3, 4 nd 5 respectively, P nd P2 re mtrices representtion of polrisers nd 2, FRl nd FR2 re mtrices representtion of Frdy rottors nd 2, nd E i is mtrices representtion of the polrized light input. Since ech opticl Jones mtrix in eqution (8) is in the similr Crtesin coordinte system tht re linked together y E o nd E i ; therefore, Crtesin Jones mtrix trnsformtion must e pplied to the opticl device if it hs een rotted or is under coordinte rottion. As the representtion of circulting current differentil protection in the electricl domin is stted y eqution (), the similr expression in opticl domin is stted y eqution (8). For usr with more thn 2 rnches / circuits, eqution (8) could e extended y dding numer of FR nd FO links sed on Kirchhoff Current Lw. In ddition, the polriztion sttes or the light intensity t the exit of P2 or FO is then monitored y detector unit. The output signl cn e descried s [24, 25] I = S = ( E o E o * ) / Z (9) where E o is electric field component of the trnsverse wve, E o * is trnspose of the complex conjugte E o nd Z is totl impednce of the medium which is the rtio of the electric field to the mgnetic field. The detector unit consists of photo-detector nd optoelectronic threshold detector. A comprison process is crried out y simple optoelectronic threshold detector to detect fult occurrence through ll-opticl comprison tht compres etween power modultion outputs nd the threshold level. The output then informs decision either to trip or no-trip s prt of opticl protection relying opertion. 3
Trip Signl Modultion (W) Current (A) Trip Signl Modultion (W) Current (A) Trip Signl Modultion (W) Current (A) In prcticl term, the first condition, no-trip signl is generted y the ODP when the opticl power modultion is less thn the threshold setting. This condition tkes plce when rted current flows t the primry side (no-fult condition) or fult occurs t outside of protected circuit (externl fult) which cuse oth Frdy rottor outputs to hve the sme sttes ut in opposite direction therefore the sum of light rottions is zero. Conversely, trip signl is generted y the ODP when the opticl power modultion is greter thn the threshold setting. This condition occurs when n internl fult occurs t protected circuit which cuse oth Frdy rottor outputs to hve different sttes - therefore the sum of light sttes is greter thn zero. 4 Cse studies To ssess performnce of the proposed ODP for usr protection, Mtl / Simulink model of the scheme ws uilt. The model consists of test power system (Fig. 4), the light source wvelength of 55 nm, differentil protection model sed on eqution (8) nd the output detector using eqution (9). Internl fult G: 2 MVA, 2kV T: 2 MVA, 2/32kV Externl close-end fult TL: 32kV, km Busr protection zone G2: 25 MVA, 2kV T2: 25 MVA, 2/32kV Fig. 4. system model for opticl differentil protection scheme In generl, two min fult conditions re considered, i.e. internl nd externl fult with respect to the protected zone. Internl fult lso includes high resistive fult with R f =2 Ω. The fult scenrios re summrised in Tle. The fult inception time is ssumed to e.2 second nd the power of opticl source is mw. FO ttenution is.2 db/km nd other losses such s Fresnel reflection is clculted using the Mtl progrm. Threshold setting is chosen to 23.328 μw s n verge vlue of mesured opticl power modultions etween solid externl fult nd high resistive internl fult. This ensures tht there is comfortle mrgin for oth sensitivity nd stility of the scheme. Additionlly, smll stilising dely of.5 ms hs een pplied to prevent spurious tripping on rndom noise. Cse Fult Fult Fult loction type resistnce Remrks. internl L-G R f =2 Ω Fig. 5. internl L-G R f = Ω (solid) Fig. 6 2 externl L-G R f = Ω (solid) Fig. 7 Tle : Loction, type nd resistnce of simulted fults For verifying protection sensitivity, Cse-. with high resistive fult condition ws used. The simultion results indicted tht the ODP could detect n occurrence of fult which provides power modultion with pek vlue of 23.55 μw. By pssing this opticl power modultion into simple threshold comprison, the protection genertes trip signl s illustrted in Fig. 5. c Fig. 5. ODP Scheme simultion for internl high resistive fult:. Mesured fult current,. Opticl power modultion, nd c. Trip signl. c c 5-5.2.4.6.8. x -5 2.35 2.34 2.33 2.32 2.3.2.4.6.8..2.4.6.8. 2 - x 4.2.4.6.8. x -4 5.2.4.6.8..2.4.6.8. Fig. 6. ODP Scheme simultion for internl solid fult:. Mesured fult current,. Opticl power modultion, nd c. Trip signl. 2-2 2.33 2.32 2.3 x 4.2.4.6.8. 2.34 x -5.2.4.6.8..2.4.6.8. Fig. 7. ODP Scheme simultion for externl solid fult:. Mesured fult current,. Opticl power modultion, nd c. Trip signl. In cse of solid internl fult (refer to Fig. 6) the power modultion is much higher thn the ssumed threshold [.332 mw (-8.75 db) compred to 23.328 μw (-26.32 db)] which demonstrtes very good level of dependility. 4
P-mod & Set-reset P-Mod & Set - reset The ODP stility/security ttriute is tested y simultion Cse 2 with externl solid fult (refer to Fig. 7). In this cse the power modultion level is 23.56 μw (-26.35 db) which is elow the ssumed threshold. Another protection ttriute is the speed of opertion. The ODP totl operting time consists of three consecutive time intervls which re: the differentil output, opticl power comprison, nd decision processing time (including ny stilising time delys nd tripping logic). The differentil output processing time is very short ecuse it uses polrized light to conduct instntneous differentil processing. In the presented cse it is pproximtely 3.28 μs with FO length round 5 m nd the light speed in the medium c/n (where c is the light constnt nd n is refrctive index of opticl medium). The remining processes which re the photodetection nd comprison of the opticl power modultion nd decision processes re similr to typicl numericl rely processing time, which in this cse could e chieved in less thn 2 ms (refer to Fig. 8 nd Tle 2). It is lso worth mentioning tht the ppliction of Frdy rottors which sustitute iron core CTs helps in eliminting potentil sturtion prolems. 2.338 x -5 2.336 2.334 2.332 modultion Drop-out Pick-up.27.28.29.22.22 Fig. 8. Time trces simultion of ODP scheme for different fult resitypes:. Internl high resistive fult, nd. Internl solid fult. Cse Incipient Differentil sttus (ms) ODP Trip t Fult t No. Pickuout time (ms) Drop- response (s) (s) Attempt..2.75.825 2.85.22 2...2.5.25.5 2.2 Tle 2: Pick-up, drop-out nd tripping time of ODP scheme 5 Discussion nd Conclusions Trip signl Trip Signl 2.5 x -5 2.4 modultion Pick-up Trip signl Trip 2.3 signl.2.2.22.23.24.25.26 In this pper, sic model of the ll-opticl differentil protection scheme for usr protection sed on comintion of Frdy rottors nd other pssive opticl devices modelled using Jones principles hs een developed. The proposed ODP model hs the ility to generte opticl modultion output s the SOP sum of ll FR outputs tht corresponds with the differentil of mesured current. Therefore, the ODP scheme is n lterntive of n electromgnetic rely type of circulting current differentil protection scheme. The proposed ODR scheme consists of simple opticl rrngement nd need no complex procedure to conduct its function s differentil protection. Thus, the proposed technique of ll-opticl usr differentil protection scheme does not merely use the opticl sensing techniques to derive the numericl replic of the current ut lso utilises opticl devices to directly implement the fundmentl principles of differentil protection. The scheme is simple series connection of opticl components without the need for complex signl processing. The required processes re conducted y direct sutrction of polristion rottions s distinctive primry process of the ODP scheme to shorten the rection time. Through systemtic simultionsed cse studies, the proposed novel configurtion hs met the protection relying key performnce criteri such s selectivity, sensitivity, security, dependility nd speed of opertion. The simultion hve demonstrted tht n immedite response to n increse in differentil current cn e chieved using opticl system nd the ll-opticl protection scheme providing fst-cting nd highly discrimintive fult detection system for us r protection. The method tht ws presented in this pper could provide contriution to sic development of opticl differentil protection frmework. Even though the ODP scheme hs een demonstrted on usr protection, the proposed method hs potentil to e pplied s differentil protection for short trnsmission lines (up to km). Further investigtion is needed to rigorously ssess such pplictions. Although fult detection system cn e relised for sic protected circuit, such s us r, due to the smll vlue of opticl power modultion during high resistnce fults, the intensity of opticl power modultion my not e sufficient to detect such fult. These issues will e ddressed y the future reserch. Simultion results tht re presented in this pper were chieved with ssumption tht the opticl sensors hd similr chrcteristics. It should e noted tht opticl power ttenution is neglected due to ending of opticl fire. Future work will concentrte on ddressing these issues nd will ssess the influence of these effects on the proposed system performnce. This will e chieved oth through improved modelling nd lortory testing of the system prototype. Acknowledgment The uthor would like to thnk Directorte Generl Higher Eduction of Repulic of Indonesi (k Dikti) for their support through BPPLN scholrship progrmme. References [] IEEE, "Opticl current trnsducers for power systems: review," Delivery, IEEE Trnsctions on, vol. 9, pp. 778-788, 994. [2] J. D. P. Hrluik, "Opticl current sensors eliminte CT sturtion," in Engineering Society Winter Meeting, 22. IEEE, 22, pp. 478-48 vol.2. 5
[3] K. Bohnet, et l., "Fier-Optic Current nd Voltge Sensor for High-Voltge Susttions," 6th Interntionl conference on Opticl Fier Sensors, p. 3, 3-7 octoer 23 23. [4] S. Kucuksri nd G. G. Krdy, "Experimentl Comprison of Conventionl nd Opticl Current Trnsformers," Delivery, IEEE Trnsctions on, vol. 25, pp. 2455-2463, 2. [5] L. Dziud, et l., "Lortory evlution of the hyrid fieroptic current sensor," Sensors nd Actutors A: Physicl, vol. 36, pp. 84-9, 27. [6] M. H. Smimi, et l., "Open Core Opticl Current Trnsducer: Modeling nd Experiment," Delivery, IEEE Trnsctions on, vol. PP, pp. -, 25. [7] C. T. Lw, et l., "Fier-Optics-Bsed Fult Detection in Systems," Delivery, IEEE Trnsctions on, vol. 23, pp. 27-279, 28. [8] A. P. Steer, et l., "Opticl fire current sensor for circuit protection," in Developments in Protection, 989., Fourth Interntionl Conference on, 989, pp. 296-3. [9] R. B. Amin Moghds, Mehdi Shdrm, "An Innovtive Fire Brgg Grting Sensor Cple of Fult Detection in Rdil power Systems," presented t the 4th Annul IEEE System Conference 2, 2. [] P. Orr, et l., "Flexile protection rchitectures using distriuted opticl sensors," in Developments in Systems Protection, 22. DPSP 22. th Interntionl Conference on, 22, pp. -6. [] H. Y. Li nd P. A. Crossly, "Design nd evlution of current differentil protection scheme incorporting fier opticl current sensor," in Engineering Society Summer Meeting, 2. IEEE, 2, pp. 396-4 vol. 3. [2] M. Nsir, et l., "Development of System Differentil Protection Bsed on Opticl Current Mesurement," in 48th Interntionl Universities' Engineering Conference, Dulin, 23. [3] G. Fusiek, et l., "All-opticl differentil current detection technique for unit protection pplictions," in Interntionl Instrumenttion nd Mesurement Technology Conference 23, Minnepolis, USA, 23. [4] J. Holch, "Comprison etween high impednce nd low impednce us differentil protection," in Systems Conference, 29. PSC '9., 29, pp. -6. [5] P. M. Anderson. (999). System Protection. [6] GEC. Protective Relying Appliction Guide. [7] R. C. Jones, "New Clculus for the tretment of Opticl Systems: I-VII," Journl of the Opticl Society of Americ, vol. 3,32,37,38, 94-948. [8] A. J. Rogers, Polriztion in opticl fiers: Norwood, MA : Artech House, 28. [9] R. M. A. Azzm, Bshr, N. M, Ellipsometry nd Polrized Light. Amsterdm: Elsevier Science BV, 987. [2] S. Kucuksri nd G. G. Krdy, "Complete Model Development for n Opticl Current Trnsformer," Delivery, IEEE Trnsctions on, vol. 27, pp. 755-762, 22. [2] Hndook of opticl systems: Weinheim ; Gret Britin : Wiley-VCH, 25. [22] E. Hecht, Physics: Brooks/Cole Pu., 994. [23] R. D. Guenther, Modern optics: New York : Wiley, 99. [24] F. L. Pedrotti, et l., Introduction to Optics: Person Prentice Hll, 27. [25] C. A. Dimrzio, Optics for engineers: Boc Rton, FL : CRC Press, 22. 6