chemical sensors

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Pd/Ag coated fiber Bragg grating sensor for hydrogen monitoring in power transformers G. M. Ma, J. Jiang, C. R. Li, H. T. Song, Y. T. Luo, and H. B. Wang Citation: Review of Scientific Instruments 86, 045003 (2015); doi: 10.1063/1.4918802 View online: http://dx.doi.org/10.1063/1.4918802 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in CO2 sensing at room temperature using carbon nanotubes coated core fiber Bragg grating Rev. Sci. Instrum. 84, 065002 (2013); 10.1063/1.4810016 Improved low concentration gas detection system based on intracavity fiber laser Rev. Sci. Instrum. 82, 023104 (2011); 10.1063/1.3534077 A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement J. Appl. Phys. 103, 053107 (2008); 10.1063/1.2890156 Optical fiber sensors for monitoring ingress of moisture in structural concrete Rev. Sci. Instrum. 77, 055108 (2006); 10.1063/1.2200744 Cryogenic Fiber Optic Temperature Sensors Based on Fiber Bragg Gratings AIP Conf. Proc. 823, 267 (2006); 10.1063/1.2202425 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 119.40.120.194 On: Sun, 31 May 2015 08:19:08

Transcript of chemical sensors

  • Pd/Ag coated fiber Bragg grating sensor for hydrogen monitoring in powertransformersG. M. Ma, J. Jiang, C. R. Li, H. T. Song, Y. T. Luo, and H. B. Wang

    Citation: Review of Scientific Instruments 86, 045003 (2015); doi: 10.1063/1.4918802 View online: http://dx.doi.org/10.1063/1.4918802 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/4?ver=pdfcov Published by the AIP Publishing

    Articles you may be interested in CO2 sensing at room temperature using carbon nanotubes coated core fiber Bragg grating Rev. Sci. Instrum. 84, 065002 (2013); 10.1063/1.4810016

    Improved low concentration gas detection system based on intracavity fiber laser Rev. Sci. Instrum. 82, 023104 (2011); 10.1063/1.3534077

    A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement J. Appl. Phys. 103, 053107 (2008); 10.1063/1.2890156

    Optical fiber sensors for monitoring ingress of moisture in structural concrete Rev. Sci. Instrum. 77, 055108 (2006); 10.1063/1.2200744

    Cryogenic Fiber Optic Temperature Sensors Based on Fiber Bragg Gratings AIP Conf. Proc. 823, 267 (2006); 10.1063/1.2202425

    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:119.40.120.194 On: Sun, 31 May 2015 08:19:08

  • REVIEW OF SCIENTIFIC INSTRUMENTS 86, 045003 (2015)

    Pd/Ag coated fiber Bragg grating sensor for hydrogen monitoringin power transformers

    G. M. Ma,1,2,a) J. Jiang,1,2 C. R. Li,1,2 H. T. Song,1,2 Y. T. Luo,3 and H. B. Wang31State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North ChinaElectric Power University, Beijing 102206, Peoples Republic of China2Beijing Key Laboratory of High Voltage and EMC, North China Electric Power University,Beijing 102206, Peoples Republic of China3Electric Power Research Institute of Guangdong Power Grid Co., Ltd.,Guangzhou 510080, Peoples Republic of China

    (Received 6 February 2015; accepted 12 April 2015; published online 23 April 2015)

    Compared with conventional DGA (dissolved gas analysis) method for on-line monitoring of po-wer transformers, FBG (fiber Bragg grating) hydrogen sensor represents marked advantages overimmunity to electromagnetic field, time-saving, and convenience to defect location. Thus, a novelFBG hydrogen sensor based on Pd/Ag (Palladium/Silver) along with polyimide composite film tomeasure dissolved hydrogen concentration in large power transformers is proposed in this article.With the help of Pd/Ag composite coating, the enhanced performance on mechanical strength andsensitivity is demonstrated, moreover, the response time and sensitivity influenced by oil temperatureare solved by correction lines. Sensitivity measurement and temperature calibration of the specifichydrogen sensor have been done respectively in the lab. And experiment results show a highsensitivity of 0.055 pm/(l/l) with instant response time about 0.4 h under the typical operatingtemperature of power transformers, which proves a potential utilization inside power transformers tomonitor the health status by detecting the dissolved hydrogen concentration. C 2015 AIP PublishingLLC. [http://dx.doi.org/10.1063/1.4918802]

    I. INTRODUCTION

    Power transformers, as the voltage converters, are thekey components in power grids and extremely high reliabilitymust be guaranteed. While aging and degradation of insulationpaper and oil may result in a malfunction of the oil-immersedtransformer. Still indeed, once some critical transformerbroke up, tremendous impacts on society, economy, andenvironment would be brought.13 Experience shows thatdissolved hydrogen is produced in most oil thermal andelectrical faults inside the power transformers.1,4 Hence,it is meaningful to monitor the degradation process andhealth status of transformers by detecting dissolved hydrogen.However, the fault gases must be separated from the oilbefore measurement in existing techniques. The polymermembrane separation is frequently used because of its simpleapparatus set-up.5 Whereas, typically, the equilibrium timeof membranes including polyimide (mainly for hydrogenseparation), FEP (Fluorinated Ethylene Propylene, e.g., F46),and PTFE (Poly Tetra Fluoro Ethylene) require 24 72 h,6so the separation process leads to long response time andcomplex structure, even worse, defects gases cannot be locatedany longer.

    Since optical techniques provide excellent approaches todetect hydrogen, many kinds of optical hydrogen gas sensorshave been researched.710 Whereas, only a few attempts havebeen carried out to detect the dissolved hydrogen in oil

    a)Electronic mail: [email protected]

    directly by embedding the hydrogen sensors into the oilwithout inducing any insulation risks. Butler puts forwarda micro-mirror chemical sensor by coating the material onthe end of an optical fiber and measured the reflectivity ofthe light transmitted through the fiber to detect the hydrogenin oil.11 While the detection was susceptible to fiber andconnector losses. Bodzenta proposed to detect hydrogenconcentration in transformer oil by measuring changes ofthe electrical resistance and the optical reflectance of the Pdfilm simultaneously in 2002.12 However, the sensor mentionedwas not so sensitive as to trace dissolved hydrogen in oil.Besides, Mak illustrated a PTFE-Pd-capped Mg-Ti thin filmbased on fiber optic sensor with high resolution of dissolvedhydrogen.13 Although high sensitivity was achieved, dissolvedhydrogen concentration upper range of 130 ppm at 60 C waslimited as an example. As for a suitable hydrogen sensor foron-line monitoring of power transformers, requirements ofa limitation-of-detection (LOD) below 150 l/l and dynamicrange at least 2000 l/l should be met.14 In addition, the sensorneeds to detect in an oil operating temperature ranging from50 C to 80 C.13,15

    Unfortunately, all of sensors mentioned above are focusedthe attention on a point measurement. As a matter of fact,it is unacceptable to monitor dissolved hydrogen in powertransformers using numerous fibers for field application inpower system. Due to obvious advantages in wavelengthdivision multiplex (WDM) and immunity to intensity varia-tion of light source,16,17 FBG (fiber Bragg grating) sensoris the most suitable solution to monitor dissolved hydrogenconcentration. Moreover, benefiting from WDM, FBG-based

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  • 045003-2 Ma et al. Rev. Sci. Instrum. 86, 045003 (2015)

    FIG. 1. Different layers of the FBG-based hydrogen sensor.

    hydrogen sensors are convenient to fault location. Thus, ourteam has put forward to apply FBG technology in detectingdissolved hydrogen.18,19 With the aim to enhance performanceof hydrogen sensor in sensitivity, response time, and reli-ability, FBG hydrogen sensor based on Pd/Ag along withpolyimide composite film is proposed and investigated atvarious temperatures in this article.

    II. FABRICATION OF FBG HYDROGEN SENSOR

    The structure of the FBG-based hydrogen sensor isshown in Fig. 1. Several layers have been made around thefiber cladding (125 m): polyimide layer (1 m), titanium(Ti) layer (20 nm), palladium/silver (Pd/Ag) composite layer(400 nm), and Pd layer (160 nm). The thick Pd film andPd/Ag composite film are used to absorb hydrogen. Stressinduced by volume expansion of Pd absorbing hydrogenresults in a shift in Bragg wavelength. For this reason, wecan get the hydrogen concentration via wavelength shift.Pure Pd film is easily suffered from fatal fracture caused byits phase transition and hysteretic effect.20 Therefore,the novel sensor was fabricated by coating the FBG with aspecific composite Pd alloy membrane which would swellin the presence of hydrogen. Moreover, alloying Pd withother metals such as Ag,11,2123 structural stability of hydrogensensitive film can be improved. Ti and polyimide layers areacted as adhesive coatings to ensure connection between fiberand Pd alloy film.

    To avoid uneven coating, self-rotation of sampling trayat a speed of 30 rad/min and floating arrangement have beenassigned, as shown in Fig. 2. The fabrication procedures ofthe FBG-based hydrogen sensors include processing of FBG,polyimide coating and magnetron sputtering, shown as Fig. 3and Table I.

    FIG. 2. FBG arrangement on the sampling tray.

    FIG. 3. Fabrication process of FBG-based hydrogen sensor.

    Magnetron dual-sputtering was used during preparationfor Pd/Ag composite film. In the end, uniform films werecoated around the fiber and ratio of Pd/Ag was close to 3:1.Pure Pd film was sputtered at the outermost layer to stopoxidation of silver alloy and enhance sensitivity of hydrogen.In addition, to improve the sensitivity of trace hydrogendetection, a relatively thick coating was sputtered.

    III. TEMPERATURE CALIBRATION

    For the purpose of eliminating the temperature effect onthe hydrogen measurement, an unforced temperature compen-sation FBG sensor is necessary to be calibrated.

    The sensors were put in a temperature-controlled cabinet(GDW-100). The temperature was set from 10 C to 80 C ata step of 10 C and every temperature step was maintained 60min, the results of wavelength shifts were shown in Table II.

    Experimental results showed that the temperature sensi-tivity of naked FBG was 9.79 pm/C and that of Pd/Agsensor was 9.81 pm/C. The two values were so close that the

    TABLE I. Detailed parameters of magnetron-sputtering coating.

    Target material Pd Ag Ti

    Start power (W) 50 40 30Sputtering power (W) 60 20 30Start time (min) 10 10 5Sputtering time (min) 23 15 5Vacuum (Pa) 5104Working gas ArGas flow rate (sccm) 20Working pressure (Pa) 1.0Deposition rate (nm/min) 20 7 4Film thickness (nm) 560 20

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  • 045003-3 Ma et al. Rev. Sci. Instrum. 86, 045003 (2015)

    TABLE II. Wavelength shift of hydrogen sensor and the compensation FBGin different temperatures.

    Wavelength shift (pm)

    Temperature (C) Naked FBG Pd/Ag FBG

    10 0.00 0.0020 94.18 94.5330 188.09 186.2240 292.00 293.8750 393.06 395.0160 488.65 490.7470 587.04 589.6780 685.44 689.02

    temperature influence could be ignored by using temperaturecompensation sensor with high feasibility.

    IV. TEST IN TRANSFORMER OIL

    A. Experimental setup

    The fabricated FBG-based hydrogen sensor was testedon the platform of dissolved hydrogen in transformer oil. Asshown in Fig. 4, the MFC-2 (Mass Flow Controller) withrange of 0 2 slm (standard liter per minute) was chosen tocontrol nitrogen (99.999% purity), and the MFC-1 with rangeof 0 20 sccm (standard cubic centimeter per minute) wasselected to control hydrogen (99.999% purity). Both of themwere connected to the gas mixer. Finally, the sensors in thegas chamber were connected to the SM-130 demodulationequipment, manufactured by Micron Optics, to measure theFBG wavelength.

    MFCs were regulated to obtain a certain concentrationof nitrogen-hydrogen mixture and the gases were sent intothe transformer oil via perforated tube, so as to preventtransformer oil from flowing into the tube. Both bare andcoated FBGs were fixed under the perforated tube to eliminatethe stress influence caused by bubbles. Transformer oil in thecontainer was heated by conducting oil, while the temperaturewas controlled by ST-703 series intelligent PID (ProportionIntegration Differentiation) temperature controller.

    During the experimental process, pure nitrogen shouldbe imported to drive out air inside the platform for about half

    FIG. 4. Experimental setup of dissolved hydrogen detection in transformeroil.

    an hour at first. In addition, the flow rate of mixed gases hadbetter to be controlled under 1.5 slm to prevent FBG sensorsin the oil vibrating.

    B. Sensitivity test in low hydrogen concentrationat typical operation temperature

    A FBG hydrogen sensor with Pd/Ag composite film(Pd/Ag 400 nm, Pd 160 nm) and one with pure Pd film(560 nm) were tested simultaneously. To explore the sensitiveperformance, a comparative test in transformer oil was doneat the temperature of 60 C. Since 60 C was the transformerworking temperature, it was chosen to simulate the transformernormal operating condition.

    Then, various concentrations of hydrogen in nitrogenwere mainly controlled by MFCs and flowed into the trans-former oil to observe the different wavelength shifts. In theexperiments, continuous mixed gases with long enough timewere guaranteed to reach steady state. Oil samples were takenfor dissolved gas analysis (Agilent 7890B GC). The results ofDGA (dissolved gas analysis) and both Pd sensor and Pd/Aghydrogen sensor wavelength shifts were shown in Fig. 5.

    With the dissolved hydrogen in transformer oil increased,the wavelength of FBG shifted more, in accordance with thesensing principle. To get the sensitivity of hydrogen sensor,the data between wavelength shift and hydrogen value withconventional DGA method were fitted in Fig. 5. After calcula-tion, the result was in agreement with DGA and a relativelylinear relationship was fitted between wavelength shift anddissolved hydrogen concentration. The sensor sensitivity ofPd/Ag sensor was about 0.055 pm/(l/l) of dissolved hydrogenin power transformer oil at working temperature, and that ofpure Pd sensor was about 0.044 pm/(l/l).

    Compared to pure Pd film, Pd/Ag film was more sensitiveabout 25% for detecting dissolved hydrogen in oil under thesame condition.

    The field emission scanning electron microscope (FE-SEM) prepared by JSM-7001F was shown in Fig. 6. Protuber-ances and cracks could be observed after tens of absorptioncycles in hydrogen, in contrast, Pd/Ag had a better perfor-mance on inhibition of hydrogen embrittlement. Therefore,

    FIG. 5. Sensitivity test results at different hydrogen concentrations in oil (redcurve referred to Pd/Ag composite film sensor, blue curve referred to pure Pdfilm sensor).

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  • 045003-4 Ma et al. Rev. Sci. Instrum. 86, 045003 (2015)

    FIG. 6. Comparison of SEM morphology between (a) pure Pd film and (b)Pd/Ag film.

    the FBG hydrogen sensors reversibility and reliability can beimproved by using Pd/Ag and pure Pd composite film.

    Moreover, owing to no obvious cracks on the surface ofthe hydrogen sensitive film, the Pd/Ag composite film wasmore sensitive in sensing dissolved hydrogen. Besides, as anintermediate buffer layer, Pd/Ag film could avoid accumulateddefects that the monolayer Pd caused by volume expansion inthe presence of hydrogen. According to the latest IEEE Guidefor the Interpretation of Gases Generated in Oil-ImmersedTransformers14 and the permissible concentration of dissolvedhydrogen in the oil of a healthy transformer,24 limitation-of-detection below 150 l/l is acceptable and the sensitivityof FBG-based hydrogen sensor is good enough to meet theactual needs in the field. Approximate 18 l/l hydrogenconcentration could be detected by the developed FBG sensorat the typical operation temperature, it is acceptable.

    C. Correction curves at different temperature

    During the experiment, we found that the oil temperatureinfluenced the results. While the solubility of hydrogendissolved in the oil is independent on oil temperature. Inorder to provide a continuous monitoring of the health statusof the transformer, the sensor needs to be examined atdifferent temperature specifications. Thus , the oil temperaturewas set to range from 20 C to 80 C by adjusting thePID temperature controller, respectively, which simulated thetransformer working temperature and operational conditions.

    To observe the different temperature influence on theperformance of Pd/Ag sensor, 7 oil samples about 380 l/ldissolved hydrogen were prepared and detected. With the timegoing on, the wavelength shift was shown in Fig. 7 in order.And the data of response time and balanced wavelength shiftswere concluded in Table III.

    In this article, response time is calculated from thehydrogen flowing into oil to FBG reaching 90% maximumwavelength shift.

    FIG. 7. FBG hydrogen sensor with Pd/Ag composite film at 20 C 80 C(dotted line showed the steady state in different situations).

    Obviously, with temperature increasing, the wavelengthshift decreased under the same concentration, and so did thesensitivity of the sensor. Also, the response of the sensorbecame instant, which actually was effective to preventdefaults from worsening in overheat transformer oil. Thephenomenon can be explained as follows: it is a chemistryprocess that the Pd contact with hydrogen and generatehydride. The hydride decomposing to Pd and H is anendothermic process, thus the surrounding temperature risewill promote the decomposition process and decrease thehydrogen absorbing ability of Pd.

    Then, the response time of Pd/Ag coating sensor wasabout 6 h at room temperature, 0.4 h at 60 C, and 0.3 h at80 C. Compared with the conventional equilibrium time formembrane technology, the response of proposed sensor wasmuch more simple, timely and accurate. On the other hand,although the sensitivity decreased in the condition of 80 C, itis still acceptable.

    We can see that both the response time and sensitivitydecreased with the temperature and increased with Pd/Agsensing head. To analyze the dissolved hydrogen concen-tration and estimate response time of the sensor at differentcondition, correction curves was fitted as shown in Fig. 8.

    As shown in Fig. 8, wavelength shift and response timeunder increasing oil temperature slightly decreased and werebest described by an exponential function of the relationship.The response time, under the same temperature range, seemsto be more vulnerable to temperature change than sensitivity.Although both sensitivity and response time are susceptible totemperature, the problem can be solved by stabilization of the

    TABLE III. Results of DGA and FBG in transformer oil at differenttemperatures.

    Temperature (C) Response time (min) Wavelength shift (pm)

    20 388 67.230 191 59.640 110 49.450 45 37.960 24 29.370 20 27.280 20 23.5

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  • 045003-5 Ma et al. Rev. Sci. Instrum. 86, 045003 (2015)

    FIG. 8. The fitting curves of oil temperature (T) vs. response time (tr) andwavelength shift () (correction curves).

    sensor temperature and utilization of the fitted curves. Thus,the proposed hydrogen sensor can be expanded to monitorhealth status under on-site situation.

    V. CONCLUSIONS

    A novel FBG-based dissolved hydrogen sensor coatedPd/Ag composite film is fabricated and investigated in thisarticle, aiming to monitor health status of power transformeroil. A relatively high sensitivity of 0.050 pm/(l/l) andinstant response time about 0.4 h at operating temperatureare achieved. Moreover, the composite sensing film showssatisfactory mechanical performance. In addition, the factorof temperature is taken into full consideration, and then thecorrection curves of response time and sensitivity are obtainedby experiments for the first time, which proves a potentialutilization in power transformers at different temperaturecondition.

    With distinguished advantages compared with conven-tional DGA technique, the novel FBG-based sensor can detectthe dissolved hydrogen directly without separating hydrogenfrom oil and even locate the defect or partial dischargetimely and easily. Thus, the proposed sensor provides an idealsolution for tracing and monitoring dissolved hydrogen inpower transformers for the field testing in the near future.

    ACKNOWLEDGMENTS

    The authors thank Li-hong Cui from Center for Testing& Analyzing of Materials (School of Materials Science and

    Engineering, Tsinghua University) for her advices on theSEM analysis of films. The authors acknowledge the financialsupport of Fundamental Research Funds for the CentralUniversities, Research Fund for the Doctoral Program ofHigher Education of China (RFDP), National Natural ScienceFoundation of China (Grant No. 51307052), Beijing NaturalScience Foundation (Grant No. 3144035) and China SouthernPower Grid.

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