Thin palladium film as a sensor of hydrogen gas dissolved in transformer oil

Thin palladium film as a sensor of hydrogen gas dissolved in transformer oil

Sensors and Actuators B 87 (2002) 82–87 Thin palladium film as a sensor of hydrogen gas dissolved in transformer oil Jerzy Bodzentaa,*, Bogusław Bura...

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Sensors and Actuators B 87 (2002) 82–87

Thin palladium film as a sensor of hydrogen gas dissolved in transformer oil Jerzy Bodzentaa,*, Bogusław Buraka, Zbigniew Gacekb, Wiesław P. Jakubika, Stanisław Kochowskia, Marian Urban´czyka a

b

Institute of Physics, Silesian University of Technology, Krzywoustego 2, 44-100 Gliwice, Poland Institute of Power Systems Engineering and Control, Silesian University of Technology, Krzywoustego 2, 44-100 Gliwice, Poland Received 4 February 2002; received in revised form 16 April 2002; accepted 16 May 2002

Abstract A sensor for the detection of hydrogen gas dissolved in the transformer oil is proposed. The absorption of hydrogen in thin palladium film causes changes in the electrical and optical properties of the film. The proposed structure can be simultaneously used as a resistance and optical sensor. The sensor has been tested for different hydrogen concentration and in different temperatures. The hydrogen concentration was varied from 200 to 1500 ppm (in the transformer oil) and the oil temperature was changed from 20 to 120 8C. The sensor exhibits good sensitivity for low hydrogen concentration and the long-term stability of parameters in the transformer oil up to 90 8C. The sensitivity and the reaction time of the sensor strongly depend on the operation temperature. It should be possible to use such a sensor structure for the continuous monitoring of electric power systems. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen sensors; Resistance sensors; Optical sensors; Hydrogen dissolved in oil

1. Introduction One accepted method for the early detection of incipient faults in power transformers is the analysis of gases dissolved in the transformer oil. The interpretation of the results of such an analysis is specified in IEC Publication [1]. Concentrations of following gases are measured: H2, CH4, C2H6, C2H4, C2H2, C3H8, C3H6, CO, and CO2. At present, the analysis is based on gas chromatography. A sample for analysis is taken from a transformer and is transported to a chromatographic laboratory, where the measurement is carried out. Using such a procedure, continuous monitoring of the transformer is practically impossible. Additionally, the period of time between the drawing of the sample and the measurement may lead to considerable measuring errors because of the drop of the hydrogen content in the oil during sample handling. The solution to these problems is the monitoring of dissolved gases directly in the transformer. For this purpose special sensors, which can work directly in transformer oil, are needed. The sensor for the detection of hydrogen gas dissolved in the oil is considered in this paper. The increase of hydrogen concentration * Corresponding author. E-mail address: [email protected] (J. Bodzenta).

testifies that partial discharges take place in the transformer. The acceptable content of hydrogen in transformers is from 260 to 500 ppm and depends on the transformer type. The majority of the hydrogen sensors described in the literature are based on changes of the physical properties of thin palladium films induced by hydrogen absorption [2]. The absorption of hydrogen by Pd alters the work function of this material. As a consequence of it, the hydrogen absorption modifies the optical and electrical properties of Pd. This effect is a basis for the construction of the majority of electrical and optical hydrogen sensors. A lot of examples of such sensors are described in the literature, but they are mostly oriented to either the detection of hydrogen gas or to the measurement of hydrogen concentration in gas mixtures (see e.g. [3–7]). Only a limited number of sensors are devoted to the detection of hydrogen gas dissolved in a liquid phase [8–10]. These sensors are mainly described in commercial sources. In this paper, a sensor for the determination of the hydrogen concentration directly in the transformer oil is described. A signal from the sensor can be measured either electrically through a registration of the resistance of thin Pd film or optically via a registration of the intensity of light reflected from the film.

0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 2 2 1 - 6

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Fig. 1. Geometry of the sensor.

2. Experimental

Fig. 3. The typical dependence of hydrogen in the hydrogen–nitrogen mixture on time.

In the following, we shall describe the sensor element based on thin Pd film, and the arrangement for sensor testing. 2.1. The sensor The sensor consists of a thin layer of palladium deposited on a glass plate. Firstly, copper electrodes for the electrical measurements are deposited. Then a Pd film is produced by vacuum evaporation. The Pd film is deposited either directly on a glass surface or on preliminary evaporated thin Cr film (5 nm). Use of the transient Cr film significantly improves adhesion of the Pd film. At the first stage of investigation sensors with different thickness of the Pd film were tested. The thickness varied from 5 to 100 nm. The best results were obtained for 12–15 nm thick films. Sensors with such films have sufficient sensitivity and acceptable response time, and are used in further investigations. Some sensors are additionally covered by temperature-resistant paint permeable to hydrogen. The geometry of the sensor is schematically shown in Fig. 1. 2.2. Experimental setup The sensor is tested in the experimental setup shown in Fig. 2. A special cell is constructed for investigation of the sensor in the oil. A mixture of nitrogen and hydrogen is

inserted into fresh transformer oil through a perforated tube, placed near the bottom of the cell. The composition of the mixture is controlled by Bronkhorst High-Tech mass-flow controllers. The total gas flow is 1.0 l/min, the hydrogen concentration in the mixture varies from 0.5 to 4.0%, which corresponds to the hydrogen concentration from 200 to 1500 ppm in the oil (measured by gas chromatography). Before the first application of the hydrogen, pure nitrogen is run through the oil for 1 h. Afterwards, mixtures with different concentrations of hydrogen are applied for 1 h each. The exposures for different mixtures are separated by a 1 h application of the nitrogen. The typical dependence of the hydrogen concentration in the mixture on time is shown in Fig. 3. The measuring cell is placed on an electric heater. The oil temperature may be changed from room temperature up to about 120 8C. The oil temperature is continuously monitored during measurement by a thermistor (Therm). The temperature stability is not worse than 1 8C. Measurements are carried out at room temperature, and at approximately 50 and 85 8C. Additionally, the aging effects of Pd films are examined at 120 8C. Investigations in higher temperatures are practically impossible because of fast degradation of the transformer oil. The sensor is pressed down to a window of the measuring cell with the Pd film exposed to the oil. A thin layer of the

Fig. 2. The schematic diagram of the experimental setup.

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Fig. 4. Signals from the resistance (solid line) and the optical (dashed line) sensors at 20 8C.

immersion oil is applied between the sensor and the window for better optical contact. The electric resistance of the Pd film is directly measured using prepared contacts. Changes to the optical properties of the film are examined through the intensity of light reflected at the glass–film interface. A laser-diode emitting at 670 mm is used as a light source for the optical measurements. The intensity of the light reflected from the Pd film is monitored by the photoresistor (PhR1). Light from the laser is preliminary polarized. The fact that light reflected from the glass is partially polarized is taking into account. A polarization plane is selected so as to minimize the intensity of light reflected from the cell window. It improves the signal to noise ratio. Additionally the light beam passes through a glass plate. The intensity of light reflected from this plate is measured using another photoresistor (PhR2). It makes it possible to take into account the influence of the instability of the light source during analysis of the reflection signal. The resistances of the Pd film, photoresistors and the thermistor are measured using data acquisition and a switch unit (Agilent 34970A) controlled by a PC with Agilent BenchLink Data Logger software. Data from all channels are collected every 10 s.

3. Results As mentioned above, during testing of the sensor, changes of the electrical resistance and the optical reflectance of the Pd film are measured simultaneously. Results obtained for a typical measuring cycle (Fig. 3) at 20 8C are shown in Fig. 4. Even for the lowest concentration of hydrogen, the appearance of this gas in the oil causes apparent changes in the resistance and the reflectance of the Pd film. The reaction time of the sensor at room temperature is long. After 1 h from the beginning of H2 application, the signal does not reach the steady value, and for concentrations higher than 1% the signals seem to be far from it. The reaction time rapidly decreases with temperature increase. In Fig. 5, the resistance signals measured for a 1 h application of 1% H2 at different temperatures are shown. At 50 8C, the reaction time is about 3 min, whereas at 85 8C the sensor reacts practically immediately (the reaction time is less than 1 min). In some experiments short spikes of hydrogen concentration caused by too wide an opening of the valve at the beginning of H2 application are registered. The second effect connected with the rise of the oil temperature is the drop of sensor sensitivity. As shown in Fig. 6, the sensitivity

Fig. 5. Signals from the resistance sensor for different temperatures of transformer oil. Changes in hydrogen concentration in the gas mixture are also shown.

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Fig. 6. The dependence of the normalized signal from the resistance sensor on the temperature. The hydrogen concentration in the mixture is 0.5%.

decreases by almost four times when the oil temperature rises from 25 to 110 8C. Similar effects were observed ZnO film as hydrogen sensor [11]. Explanation of these effects is a separate physical problem, and it is not the aim of this work. In our opinion, temperature should influence dynamical equilibrium between hydrogen absorbed in the Pd film and hydrogen concentration in the surrounding but this problem needs basic physical investigations. The aging of the sensor was also investigated over a period of a few months. It was found that investigated layers are stable in the transformer oil if the oil temperature does not exceed 90 8C. Fast aging of Pd films occurs at temperatures above 100 8C. The aging effects consist in growth of the resistance and the reflectance of investigated films. The aging rate may be reduced using a protective coating deposited on the Pd film. The coating layer must be stable in the transformer oil, non-conducting and, of course, transparent to the hydrogen. Satisfactory results are obtained from a temperature-resistant paint. Experimental results obtained for coated and non-coated Pd film used as a optical sensor are shown in Fig. 7. The deposition of the paint layer

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Fig. 8. The dependence of the signal from the resistance sensor on the concentration of hydrogen in the gas mixture.

results in a reduction of the sensitivity and in an increase in the reaction time, but the sensor shows practically no change in its characteristics during many experiments. Very important for practical applications is the dependence of the signal from the sensor in the steady state on the hydrogen concentration. As shown in Fig. 8, this dependence is not linear, but in the investigated range of concentrations the saturation effect does not occur.

4. Conclusions The problem of the measurement of hydrogen gas concentration in the transformer oil is significant for the detection and monitoring of incipient faults in power transformers. The functioning of the sensor proposed in this paper is based on the influence of hydrogen absorbed in thin Pd film on its electrical and optical properties. Both resistance and optical signals show clear dependence on the hydrogen concentration in the hydrogen–nitrogen mixture passing through the oil. The concentration of H2 dissolved in

Fig. 7. Signals from coated (solid line) and uncoated (dashed line) optical sensors at 50 8C. Changes in hydrogen concentration in the gas mixture are also shown.

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the oil has been measured independently by standard chromatographic method. The 2% content of H2 in the gas mixture corresponds to 600–650 ppm in the oil at room temperature. As it is mentioned in introduction, the acceptable content of hydrogen in transformers is from 260 to 500 ppm and depends on the transformer type. It follows that the investigated sensor has enough sensitivity for its considered application. A comparison of the resistance and optical signals demonstrates the higher sensitivity of the resistant sensor. Also, the signal to noise ratio for this sensor is better. But the main advantage of the optical sensor is its resistance to electromagnetic interferences. This feature is particularly important for applications in power transformers. The parameters of the optical sensor may be considerably improved if lockin signal detection is be used. The sensors show high sensitivity for changes in hydrogen concentration. Therefore, quantitative measurements should be possible. The problem which must be solved is the dependence of sensitivity on the temperature. Possible solutions are the stabilization of the sensor temperature or the use of correction curves. The sensor structure shows satisfactory long-term stability in the transformer oil for temperatures of up to about 90 8C. Above this temperature, relatively fast changes to the physical parameters of the Pd film have been observed. The stability of the sensor may be improved by covering the sensor with a protective layer. In our case, the application of a thin film of temperature-resistant paint gave good results. The protective layer also improves the mechanical stability of the sensor. The hydrogen sensor proposed in this paper has a very simple structure and demonstrates high sensitivity to hydrogen dissolved in transformer oil. It would seem possible to use such a sensor for the continuous monitoring of power transformers and other electrical power systems. This work aims at elaboration of the sensor for hydrogen dissolved in transformer oil and such sensor is realized. Investigation of physical effects connected with functioning of this device is the separate problem. Some authors of this paper are involved in investigations of phenomena proceeded in bilayer sensing structure described in [12]. Results of these investigations may also help to understand general processes occurring in Pd–H system.

Acknowledgements The authors acknowledge the Polish State Committee of Scientific Researches (KBN) for financial support under the Grant 8T10C03218. References [1] IEC Publication 599: Interpretation of the Analysis of Gases in Transformers and Other Oil-filled Electrical Equipment in Service.

[2] C. Christofides, A. Mandelis, Solid-state sensors for trace hydrogen gas detection, J. Appl. Phys. 68 (1990) R1–R30. [3] M.A. Butler, Micromirror optical-fiber hydrogen sensor, Sens. Actuators B 22 (1994) 155–163. [4] K. Kalli, A. Othonos, C. Christofides, A. Spetz, I. Lundstroem, Photomodulated thermoreflectance detection of hydrogen gas using optically thin palladium film on silicon oxide, Rev. Sci. Instrum. 69 (1998) 1505–1511. [5] M. Tabib-Azar, B. Sutapun, R. Petrick, A. Kazemi, Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interaction, Sens. Actuators B 56 (1999) 158–163. [6] A. Othonos, K. Kalli, D.P. Tsai, Optically thin palladium films on silicon-based substrates and nanostructure formation: effects of hydrogen, Appl. Surf. Sci. 161 (2000) 54–60. [7] A. Chtanov, M. Gal, Differential optical detection of hydrogen gas in the atmosphere, Sens. Actuators B 79 (2001) 196–199. [8] L.B. Kriksunov, D.D. MacDonald, Amperometric hydrogen sensor for high-temperature water, Sens. Actuators B 32 (1996) 57–60. [9] DCH Technology Inc., Sensors Used in Transformers, DCH Technology Press Release #6, 1998. [10] S.R. Lindgren, S.C. Pyke, M.A. Reynolds, On-line Transformer Load and Fault Analyzer for Determination of Reliable Overload Capacity, Micromonitors Inc., Scientific Papers, 1996. [11] G. Lu, N. Miura, N. Yamazoe, High temperature hydrogen sensor based on stabilized zirconia and a metal oxide electrode, Sens. Actuators B 35/36 (1996) 130–135. [12] W.P. Jakubik, M.W. Urban´ czyk, S. Kochowski, J. Bodzenta, Bilayer structure for hydrogen detection in a surface acoustic wave sensor system, Sens. Actuators B 82 (2002) 265–271.

Biographies Jerzy Bodzenta was born in 1961 in Ruda S´ la˛ ska (Poland). He graduated with an MSc in electronics from the Lvov Technical University (Ukraine) in 1985, and received his PhD and habilitation in electronics from the Institute of Fundamental Technological Researches of the Polish Academy of Sciences in Warsaw (Poland), in 1990 and 2001, respectively. He is on the staff of the Institute of Physics of the Technical University of Silesia. His research interests include the application of thermal waves in investigation of solids, and non-destructive testing and evaluation. Bogusław Burak was born in 1975 in Bolesławiec (Poland). He graduated with an MSc in applied physics in 1999 from the Silesian Technical University, Poland. At present, he finishes his PhD studies in electronics. His research interests include the application of thermal waves in investigation of solids. Zbigniew Gacek was born in 1942. He graduated with an MSc in electrical engineering in 1965, from the Silesian Technical University. In 1980, he received the PhD degree and in 1980, he habilitated in electrical engineering at the Silesian Technical University. In 1996, he was nominated to professor. He is a specialist in electric power systems. Wiesław P. Jakubik was born in 1964 in Cieszyn (Poland). He graduated with an MSc and a PhD degrees in applied physics (acoustoelectronics) in 1989 and 1998, respectively, from the Silesian Technical University, Poland. The subject matter of his doctor’s dissertation was surface acoustic wave propagation in selected phthalocyanine thin films. At present, he is involved in the SAW sensor technique and acousto-optic interaction in integrated technology. Stanisław Kochowski was born in 1946, in Stare Bielsko (Poland). He received his MSc and PhD degrees in physics from the University of Silesia (Katowice) in 1969 and 1976, respectively. Since 1969, he has worked at the Institute of Physics of the Silesian Technical University in

J. Bodzenta et al. / Sensors and Actuators B 87 (2002) 82–87 Gliwice. His interests include investigations into the electrical properties of semiconductor surfaces, as well as the technology of thin films of organic semiconductors and metals. Marian W. Urban´ czyk was born in 1948, in Katowice (Poland). He graduated with an MSc in electrical engineering in 1973, from the Silesian

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Technical University. In 1982, he received the PhD degree in applied physics from the Institute of Fundamental Technological Researches of the Polish Academy of Sciences in Warsaw. In 1999, he habilitated in microelectronics at the Technical University of Wrocław (Poland). At present, he is dealing with the SAW application in sensor systems and acousto-optic interaction in integrated optic devices.