Improved Analog Optical Fiber Link for Signal Measuring in a High Power Testing Facility

Transcription

1 Improved Analog Opical Fiber Link for Signal Measuring in a High Power Tesing Faciliy Absrac This paper inroduces a new measuring sysem for highly aggressive EMI environmens based on fiber opic ransmission links and microprocessor-based error compensaion. The analog fiber-opic ransmier-receiver sysem as applied in a high-power mid-volage esing laboraory is presened. The opical fiber links are paricularly well suied o his kind of applicaion because of heir dielecric naure and elecromagneic inerference immuniy properies. Addiionally, he measuring sysem relies on a clever error compensaion sysem, feedback and self-check feaures o monior all vial funcions, including baery charge level, daalink inegriy, recovery and, if necessary, synchronizaion, and sophisicaed daa analysis. This work demonsraes ha he opical fiber approach provides a unique, elecrically isolaed, lighningproof analog daa ransmission sysem, and ha measuring sysems based on opical fibers are now feasible and compuer microprocessors play an imporan role in is inegraion. Keywords Elecromagneic inerference, Gain variaion, High power esing, Measuring sysems, Opical fiber link, Volage offse drif compensaion. I José C.Velázquez, Julio C. Monero, Joaquín H. Rodríguez, Raul Garduno I. INTRODUCTION N Mexico, he CFE (Spanish iniials of he Naional Elecrical Uiliy) owns a large esing faciliy called LAPEM [1], formed by a group of laboraories inended o validae conformance o sandards of all elecrical equipmens before puing hem ino public service. Among he mos imporan laboraories is he High-Power Mid-Tension (HPMT) laboraory [2], which is currenly being modernized by IIE (Spanish iniials of Elecrical Research Insiue). In he HPMT laboraory, devices such as swiches, circui breakers, ransformers, fuses, and ohers, are esed o verify heir response o definie currens and volages, including desrucive shor-circui condiions up o 38 kv and 100 ka. The elecromagneic environmen a he HPMT during he ess is very aggressive due o he inerference cussed by high curren and high volage ransiens. Hence, he major challenge for a measuring sysem is o achieve he highes Manuscrip received February 15, 2007; revised April 27, J.C. Velázquez is wih he Conrol and Insrumenaion Deparmen, Elecrical Research Insiue, Cuernavaca, Mor Mexico (Phone: ; fax: ; jconrado@iie.org.mx). J.C. Monero is wih he Conrol and Insrumenaion Deparmen, IIE, Cuernavaca, Mor Mexico ( jcmc@iie.org.mx). J.H. Rodríguez is wih he Conrol and Insrumenaion Deparmen, IIE, Cuernavaca, Mor Mexico ( jrr@iie.org.mx). R. Garduno is wih he Conrol and Insrumenaion Deparmen, IIE, Cuernavaca, Mor Mexico ( rgarduno@iie.org.mx). Issue 1, Volume 1, immuniy o elecromagneic inerference (EMI) and elecrical isolaion a lighing levels, as well as high precision and large bandwidh. These requiremens preven he use of convenional coaxial cables and circuiry, and sugges he use of opical fibers as ransmission media and specially designed elecronics, for measuring he required elecrical signals during he ess. Opical fibers have been mainly used in digial communicaions applicaions, where high speed and large bandwidh are he main requiremens, bu ransmission of a DC componen is no required. On he oher hand, he accuracy and precision of analog opical fiber ransmission links, for a measuring sysem under very aggressive EMI condiions, is highly dependen on mainaining under conrol he DC volage offse drif due o emperaure change. Consequenly, an error compensaion mechanism has o be implemened for he measuring sysem kind of applicaions. This paper inroduces a measuring sysem for elecrical signals a very aggressive EMI environmens. The sysem, based on fiber opic ransmission links and microprocessorbased error compensaion, is able o monior he daa-link inegriy and o recover links using feedback and self-recovery feaures. Secion II provides a brief descripion of he faciliy and esing requiremens. Secion III presens a descripion of he measuring sysem, including he remoe ransmier and he receiver. Secion IV describes he signal condiioning mechanism in he ransmier. Secions V and VI describe he DC volage drif and he bandwidh compensaion schemes. Secion VII describes he auomaic calibraion sraegy, and Secion VIII shows how o find ou he ransmission error. Finally, conclusions are drawn in Secion IX. II. TEST REQUIREMENTS Measuring he elecrical variables a he HPMT laboraory is a big challenge. Measuremen and signal condiioning of large currens and volages, in he order of ens of ka and kv, have o be considered before ransmiing his informaion o he conrol room for furher analysis. However, he cenral problem is o preserve he inegriy of signals, which have o be ransmied wihou any disorion. To solve his problem, a measuremen sysem capable of measuring and carrying analog signals wih minimum added error and wih sufficien ampliude for pos processing had o be developed [3]. Also, he signals o be measured depend on he ype of es a hand. Every ime a es is configured, he amoun and ype of signals have o be seleced.

2 III. DESCRIPTION OF MEASURING SYSTEM The analog fiber opic link of he measuring sysem is basically a fiber opic ransceiver composed by hree main pars, including he remoe ransmier, he receiver uni and he fiber opic communicaion link, as simply shown in Figure 1. The remoe ransmier locaed in he high EMI esing area a he HPMT laboraory, measures, ses up and sends ou he analog inpu signal. The receiver uni, locaed in he conrol room a he HPMT laboraory, demodulaes and filers he ransmied signal and drives he resulan analog oupu forward for digiizing and furher processing. The communicaion link consiss of wo opical fiber channels ha provide elecrical isolaion and immuniy o he high volage and EMI in he HPMT laboraory environmen. As depiced in Figure 2, four modules inegrae he remoe ransmier: digial remoe conroller, analog condiioning module, analog ransmier and power supply. One fiber opic link is used o receive conrol commands and he oher is muliplexed for conrol command reply and analog signal ransmission. F.O. receiver F.O. ransmier Remoe conroller Analog ransmier Power supply Baery Condiioning module power inpu analog inpu analog inpu Remoe ransmier fiber opic link Fig. 1. Analog fiber opic link block diagram. Receiver uni analog oupu The analog fiber opic link was designed o accomplish DC o 200 KHz fla zone bandwidh, wih maximum DC shifing of 5 mv/ C and no more han 1% gain variaion hroughou he link. Due o he wide range of he analog inpu volages, he remoe ransmier has a condiioning fron-end ha can manage signals from 200 mv o 200 V in nine selecable aenuaion ranges. Once in he remoe ransmier, he analog inpu signal is modulaed in frequency, and hen convered ino an infrared ray o ravel along he fiber channel. A he oher channel end he receiver is in charge of convering back he signal from ligh o volage, amplify, demodulae and condiioning i so i can be used for pos processing. The analog oupu signal is in he range of ±10 vols and can be processed by pracically any digiizer device. Seup feaures allow selecion of inpu range for operaion, diagnosics and auomaic self-calibraion. Also, he versaile power supply inside he ransmier can deec AC power supply and swich o baeries if AC power is disconneced. An inernal digial remoe conroller senses baery volage and conrols baery charge. A. Remoe Transmier The ransmier equipmen consiss of boh digial and analog secions, which are physically separaed bu inerac wih one anoher [8]. The digial secion is in charge of receiving and inerpreing he serial commands upcoming from he digial secion of he receiver equipmen a he conrol room. The analog secion performs signal condiioning. Then, he signal is modulaed and convered ino an infrared ligh signal in order o be ranspored hrough he opical fiber channel. Fig. 2. Transmier block diagram. The cenral elemen of he analog ransmier is a VCO (volage conrolled oscillaor) ha oscillaes a some iniial frequency in accordance wih a volage sepoin V S o generae he oupu of he VCO a frequency ω 0, called he cenral free-running frequency. The oupu frequency of he VCO is: ω() = ω + G V () (1) 0 VCO E where G VCO is he VCO volage-o-radian frequency gain in radians per second per vol and V E is he applied volage o he conrol inpu of he V CO o se he cenral angular frequency and o modulae ω(). The inpu volage is: V () = V () + V + V (2) E m s N where V m () is he modulaiing signal V S is he DC sepoin volage o produce ω 0 and V N is he noise volage a he impu of he VCO. Subsiuion of (2) ino (1) yields: 0 ( ) ω() = ω + G V () + V + V (3) VCO m s N Wih sensiiviy funcions: 1 dω 0 (4) = Vm() + Vs + VN dgvco (5) = GVCO dvs (6) = GVCO dv (7) N Therefore, he uncerainy for ω() is given by: Issue 1, Volume 1,

3 Wr = w1+ w2 + w3 + w4 dω0 dgvco dvs dvn (8) Vin PGA where w 1, w 2, w 3, w 4 are he esimaed or measured errors for ω 0, G VCO, V S, V N respecively. On he oher hand, he V CO can be expressed as: ( ω0 ) VCO () = VAMPL sin + GVC VE () d + VOFF (9) which is he hear of he V CO modeling for ime-analysis and corresponds o an FM modulaor of he form: ( ω0 ) Y () = Asin + k fd () (10) B. Receiver The receiver equipmen is available in a rack-moun configuraion. Up o 32 receiver channels can be plugged in one rack enclosure o ease connecion o he rack-mouned daa acquisiion sysem in he conrol room. As wih he ransmier equipmen, he receiver also has boh analog and digial secions. Figure 3 shows a simplified block diagram of one receiver. The analog secion is in charge of deecing, amplifying and demodulaing he incoming analog signal from he ransmier equipmen, as well as amplifying he demodulaed signal o a level adequae for digiizing and pos-processing. The digial secion is he inerface beween he remoe conrol sysem and he remoe ransmier; i is responsible for sending and receiving commands rough an RS-485 inerface. Fiber Opic Digial eleconrol Analog receiver Power supply Digial calibraion amplifier Fig. 3. Receiver block diagram. RS-485 ransceiver Curren amplifier IV. INPUT CONDITIONING Selecable passive volage dividers condiion inpu volage signal up o 200 vols. Reducing a volage is a simple ask, bu condiioning an analog signal from DC up o 200 KHz wih fla bandwidh and no disorion is much more difficul. Figure 4 shows he block diagram for signal condiioning a he remoe ransmier inpu. A passive aenuaor is designed using he equaion R 1 C 1 =R 2 C 2. Resisors R 1 and R 2 are proposed for a 1 M inpu impedance. Capacior C 1 is also proposed and C 2 is calculaed o ge 200 KHz fla bandwidh. FO Remoe conroller Fig. 4. Signal condiioning in ransmier. Analog ransmier There are seven seps of aenuaion and wo seps of magnificaion signal. Table I shows he combinaion of passive aenuaor and Programmable Gain Amplifier (PGA) for condiioning analog signals from 200 mv up o 200 vols. Addiional ranges of aenuaion can be obained wih an appropriae combinaion of passive aenuaor and PGA. FO TABLE I ATTENUATION COMBINATION. Vin Pasive (Vpp) Aenuaor PGA Aen V. DC DRIFT COMPENSATION Since high power esing implies no only AC signal bu DC signals oo, i is very imporan o have minimum DC shifing in order o have a very small overall ransmission error. The ransmier equipmen is he one exposed he mos o he wo main sources for DC shifing: inernal heaing and ambien emperaure. Inernal heaing is produced by he normal behavior of elecronic devices and exernal influence is due o he size of he high power esing zone, where i is impossible o have a conrolled environmen. Changes in emperaure origin a DC drif of 20 mv per Celsius degree. In order o correc he effecs of inernal heaing and ambien variaions agains he ransmier, firs he emperaure naural response curve was obained. Wha i means is ha he equipmen was esed inside a hermal chamber wih no compensaion circuiry, and afer urning he circuis on and sabilized hem for 30 minues a a 25 C emperaure, he calibraion process is performed. Calibraion consised of grounding he inpu signal and adjused some poins in he ransmier and receiver circuis o obain his very same signal a he receiver oupu. Afer his he chamber emperaure was dropped o a 0 C level and susained here for a 30 minues period, and hen reading and regisering he dc offse level. The nex sep was o program he emperaure chamber wih a ramp from 0 C o 50 C along a 6 hour period (Fig. 5). Issue 1, Volume 1,

4 Vou (V) 0,2 0,15 0,1 0,05 0-0,05-0,1-0,15-0, Temperaura (ºC) VII. AUTOMATIC CALIBRATION PROCEDURE The calibraion process reduces errors in DC offse and V o /V i Gain. The calibraion program allows inpu selecion a he remoe ransmier and digiizes he oupu volage a he analog receiver. Figure 7 shows he Transmier-Receiver sysem wih channel selecor a he inpu, and offse and gain calibraion a he oupu. Teleconrol commands from receiver conroller o remoe conroller are used o selec he ground GND reference inpu for offse calibraion and high precision volage reference inpu for gain calibraion. The PGA is seleced for uni gain in boh cases. Fig. 5. Transmier DC drif compensaion. VI. BANDWIDTH COMPENSATION Communicaion sysems usually consider a 3 db bandwidh, which implies a gain reducion a of he signal originally being ransmied. Hence, he error is abou 30% of he signal usually originaed by a low pass filer. Figure 6 shows he frequency response of he measuremen sysem ransmission media. The shape corresponds o a Buerworh low-pass filer. The frequency response of he original design is shown as he simulaed plo, wih a -3 db cu-off frequency a 1 MHz. The acual response shows a cuoff frequency a bi below he desired 1 MHz bandwidh. Finally, he opimized response shows a large fla zone wihin 0.05 db deviaion up o 200 KHz, afer ha he cu-off frequency is a 800 KHz. For measuremen purposes he 200 Khz fla zone is he mos imporan requiremen o be saisfied o keep measuremen errors wihin ±0.5 %. The whole fiber opic analog link can be simply modeled wih he second order ransfer funcion: Gs () = s s (11) Tha has he following main properies: DC gain G dc =2.01, Peak gain G p =2.02, naural oscillaion frequency f n = 500 khz, damping coefficien =0.65, and ime consan τ=0.489 s. Fig. 7. Calibraion circui. An RS 485 link enables communicaion beween he Daa Acquisiion Sysem and receiver conroller. The calibraion command can be seleced in configuraion sofware in Daa Acquisiion Sysem. The calibraion program loads a specific file wih a calibraion array for firs calibraion. Afer ha, every new calibraion array is overwrien in he file. Figure 8 shows he calibraion process. DC offse calibraion is necessary before gain calibraion. Fig. 6. Measuremen sysem bandwidh. Fig. 8. Block diagram for calibraion program. Issue 1, Volume 1,

5 For DC offse calibraion he inpu seleced a he Tx is GND, hen 1,000 samples are acquired and digiized, he average value is compared wih GND reference in Daa Acquisiion Sysem. A 256 sep digial poeniomeer in he analog receiver is adjused o minimize he DC offse error. The high precision and low noise inernal DC reference in he Transmier is seleced as an inpu, hen 1,000 samples are acquired and digiized, he average value is compared wih volage reference value in Daa Acquisiion Sysem. A Programmable Gain Amplifier in he analog receiver is adjused o minimize he Vo/Vi Gain error. The calibraion daa array is generaed wih specific values for each channel link. Figure 9 shows he plo of acual curren measuremen. E = R + D + D + R + RS (13) r o o g n n Where E r is he receiver error esimaion, R o is he receiver hermal volage offse drif, D o is he demodulaor volage offse drif, D g is he demodulaor volage gain drif, R n is he receiver hermal noise and RS n is he receiver power supply noise. For he complee measuremen sysem considering he ransmier, receiver and opical fiber link he random error esimaion is: P = E + E (14) 2 2 m r where P m is he precision of he measuring sysem, E r is he receiver error esimaion and E is he ransmier error esimaion. Fig. 9. Plo of curren in a shor circui es VIII. ERROR ESTIMATION The accuracy and precision are measured and esimaed for he ransmier, he receiver and for he complee opical fiber link measuring sysem. The accuracy is deermined by he sysemaic errors and limied by he precision. The precision is deermined by he random errors. The sources of sysemaic error are: Lineariy Deviaion, Frequency response, Volage offse, Signal Gain, and Opical power regulaion for he opical link. The sources of Random error are: Thermal Volage offse drif; Modulaor-demodulaor Thermal Volage offse drif, Modulaor-Demodulaor Thermal Volage gain drif, Thermal noise, Power supply noise. Thermal Volage offse and Thermal Volage gain drif of he modulaor is compensaed locally o limi he error. Thermal Volage offse and Thermal Volage gain drif of he demodulaor is compensaed by mainaining consan he emperaure a he conrol room a 20 ºC. The precision of he ransmier is: E = T + M + M + T + S (12) o o g n n Where E is he ransmier error esimaion, T o is he hermal volage offse drif, M o is he modulaor volage offse drif, M g is he modulaor volage gain drif, T n is he ransmier hermal noise and S n is he ransmier power supply noise. The random error for he receiver is: IX. CONCLUSIONS This paper inroduced an improved analog opical fiber link for signal measuring in a high power esing laboraory. The proposed link includes boh novel ransmier and receiver. This link is very versaile due o is very wide range of inpu signal magniudes (200 mv o 200 V) and bandwidh (DC o 200 khz). Hence, he link can be used in a wide variey of measuring applicaions where boh galvanic isolaion and high EMI immuniy are required. The remoe ransmier circuiry incorporaes a clever hardware and sofware hermal compensaion mehod ha cancels undesired errors due o emperaure variaion. This approach permied accomplishmen of sric inernaional requiremens regarding maximum measuremen errors for measuremen of elecrical signals (< 1%) in high power esing faciliies. Also, he auomaed calibraion procedure dramaically reduces he amoun of ime needed o se he es iniial condiions, as well as he duraion of mainenance periods for he communicaion links. The ransfer funcion was obained from experimenal daa. Wih his model i is possible o analyze he sysem dynamic behavior in boh ime and frequency domains. Alhough he sep response is no shown in he paper, he ime consan of he whole opical link is around 0.5 microseconds, which means he measuring sysem can perfecly deal wih he signals in he required 200 KHz fla zone of he frequency response. Finally, he operaion of he measuring sysem relies on clever error compensaion scheme, feedback and self-check feaures o monior all vial funcions. REFERENCES [1] The ligh of a laboraory. Research and Developmen. Ocober, Ocubre2002/hm/cfe.hml (In Spanish). [2] CFE Comisión Federal de Elecricidad. zados. (In Spanish). [3] User manual and commissioning of elecro-opical links for medium and Issue 1, Volume 1,

6 low frequency. Deparmen of Communicaions. Elecrical Research Insiue (In Spanish). [4] Daa acquisiion sysem for elecrical signals of he Shor-Circui Laboraories a LAPEM. Technical Specificaion. July, (In Spanish). [5] Shor-circui curren calculaion. Technical Noebook Num B. de Mez-Nobla, F. Dumas, G. Thomasse. Schneider Elecric. Sepember, [6] IEEE Sd , IEEE Sandard High Volage Tesing Techniques. [7] Fiber Opic Transmission. C. Henn. Applicaion Bullein AB-192 Burr Brown Co [8] J. Monero, J. Velazquez, M. Gomez, R. Garduno and T. Aguilar, Measuremen sysem for elecrical variables in a high EMI environmen, 5h WSEAS Inernaional Conference on Applied Elecromagneics, Wireless and Opical Communicaions (ELECTROSCIENCE '07), Tenerife, Spain, December 14-16, Jose C. Velazquez. Obained a BSc degree in Elecronics Engineering from he Orizaba Insiue of Technology, Mexico, and a MSc degree in Digial Sysems from he Naional Polyechnic Insiue, Mexico. He has been a researcher and projec manager in he Conrol and Insrumenaion Deparmen a he Elecrical Research Insiue since His research ineress are digial communicaions and ineroperabiliy of digial communicaion sysems. He has been responsible for many research projecs wih he elecrical indusry of Mexico. He is currenly working in he modernizaion projec of he high power ess laboraory of he Mexican Naional Elecric Company. Julio C. Monero Cervanes. Obained he Elecronics Engineer degree from he Insiuo Tecnológico de Piedras Negras in Coahuila, México, in 1988 and he Maser of Science degree in Elecronics from he Cenro Nacional de Invesigación y Desarrollo Tecnológico (CENIDET), in Cuernavaca, Mexico, in Since 1992, he has been a research engineer in he Conrol and Insrumenaion Deparmen a he Elecrical Research Insiue. He has paricipaed in several R&D projecs relaed o digial and analog communicaion sysems, especially in srong EMI environmens, and has published several papers in his area. Joaquin H. Rodriguez-Rodriguez. Graduaed as a Mechanical and Elecrical Engineer from Universiy of Nuevo Leon a Monerrey, Mexico in 1974, and obained a Masers of Science degree in Elecrical Engineering from he Universiy of Torono, Canada in He also go a Diploma in Technology Markeing from Insiuo Tecnologico de Esudios Superiores de Monerrey (ITESM) in 2001, and a Diploma in Micro Elecro-Mechanical Sysems (MEMS) coordinaed by he Mexico-US Foundaion for Science in As par of his professional experience, he worked as a research engineer for Trench Elecric Ld. of Torono, Canada from 1979 o He joined he Elecrical Research Insiue in Mexico as a researcher and head of he elecommunicaions laboraory, where he developed projecs relaed o elecronics, opical fibers and elecommunicaions sysems for he elecrical uiliies in Mexico from 1982 o In he las four years, he has been working on MEMS Applicaions and opical fiber sensors for on-line monioring of Power elecric apparaus. He was professor a ITESM, he Universiy of Morelos and he Cener for Research and Technological Developmen, all in Mexico. Has published many echnical papers and paricipaed in naional and inernaional echnical meeings. From 1985 o 2007 was recognized as Naional Researcher in Mexico by he Naional Research Sysem (SNI). Raul Garduno-Ramirez. Is an Elecrical Engineer by ESIME-IPN (1985), Mexico, a Maser of Science by CINVESTAV-IPN (1987), Mexico, and go a Docor of Philosophy degree from he Pennsylvania Sae Universiy (2000), USA, as a Fulbrigh Scholar. In 1986 worked a he Naional Laboraory of Mechanical Engineering, Japan, developing indusrial robo conrols. Since 1987 works a he Elecrical Research Insiue, Mexico, on power plan auomaion and conrol sysems developmen. His areas of ineres include inelligen conrol sysems, urbogeneraor conrol and muliobjecive opimizaion of power plan operaion. Has published more han seveny echnical papers in inernaional conferences and journals, and four book chapers. Dr. Garduno is member of he Mexican Naional Research Sysem since 1989 and Senior Member of IEEE (2003). Issue 1, Volume 1,