- Email: [email protected]
Detection of partial discharge in SF6 gas using a carbon nanotube-based gas sensor
Sensors and Actuators B 105 (2005) 164–169
Detection of partial discharge in SF6 gas using a carbon nanotube-based gas sensor Junya Suehiro∗ , Guangbin Zhou, Masanori Hara Department of Electrical and Electronic Systems Engineering, Graduate School of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 14 February 2004; received in revised form 7 May 2004; accepted 26 May 2004 Available online 2 July 2004
Abstract For electrical insulation diagnosis of gas-insulated switchgear (GIS), detection of partial discharge (PD) generated in sulfur hexafluoride (SF6 ) gas is important. This paper describes a new detection method of PD generated in SF6 gas using a gas sensor composed of multiwall carbon nanotubes (MWCNTs). The gas sensor was fabricated by electrokinetic manipulation of semiconducting MWCNTs using positive dielectrophoresis. The MWCNT gas sensor and a point-to-plane electrode system were placed in a steel tank filled with SF6 gas at atmospheric pressure. AC high voltage was applied to the point electrode in order to generate PD while the electrical impedance of the MWCNT sensor was monitored. When the PD was generated, the electrical conductance of the MWCNT sensor gradually increased. The sensor response was reversible and was influenced by the PD intensity as well as by the relative position of the sensor to the point electrode. It was suggested that the sensor conductance increase was caused by an electronic interaction between MWCNTs and non-identified oxidative decomposition products. The faint PD, which could not be sensed by gas detecting tubes, was successfully detected by the MWCNT sensor on a real time basis. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Gas sensor; Sulfur hexafluoride (SF6 ); Gas-insulated switchgear (GIS); Partial discharge (PD); Decomposition
1. Introduction Sulfur hexafluoride (SF6 ) gas is widely used as electrical insulator as well as arc-quenching medium in gas-insulated switchgear (GIS). SF6 is chemically inert and has high dielectric strength three times that of air at atmospheric pressure. If sharp edges or small contaminant particles accidentally exist in GIS, partial discharge (PD) may be generated around the points where the electric field is intensified. PD or corona discharge is defined as an electrical discharge phenomenon that is localized in high field region near electrodes or insulators. It is difficult to completely eliminate PD that may cause fatal damage to insulators and conductors. PD activity in GIS may eventually lead to a catastrophic breakdown or arc discharge. Unlike PD, the arc discharge completely bridges between the high voltage and grounded electrodes and carries much higher current. As a result, the arc discharge can give a serious damage to the GIS and the entire electric power distribution system. Early ∗ Corresponding author. Tel.: +81 92 642 3912; fax: +81 92 642 3964. E-mail address: [email protected] (J. Suehiro).
0925-4005/$ – see front matter. © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.05.050
detection of PD can provide a way to predict and avoid the fatal failure. So far, various PD detection methods have been proposed and investigated [1]. The most intensively studied methods are physical sensors, which can detect ultra-high frequency (UHF) electromagnetic wave, acoustic emission (AE) or optical emission generated by PD. The gas sampling method, which detects decomposition gaseous products, has been also regarded as effective technique. The gas sampling method has some advantages over the other types of the PD sensor. For example, it is less influenced by background electromagnetic or acoustic noises, and can be applicable to both of on-line and off-line PD monitoring. Since the decomposition products accumulate in a closed GIS tank depending on a total PD activity, the gas sampling method is suitable for long term trend-based monitoring. Up to now, decomposition products were successfully detected using a detection tube [2,3], a gas chromatography, a mass spectrometer [4] and an ion mobility spectrometer [5]. The detection tube is handy and can detect main decomposition products such as sulfur dioxide (SO2 ) or hydrogen fluoride (HF) at ppm-levels. However, the detection tube, which indicates the gas concentration by color changes, suffers from poor ac-
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
curacy and is not suitable for automated continuous remote PD monitoring. On the other hand, the gas chromatography and the spectrometers with high sensitivity are costly and suitable for laboratory-based analysis rather than continuous on-line monitoring. As a result, there is a strong demand for a new type of gas sensor [6–10], which is easy to use on-site and sensitive enough to detect PD generated in SF6 . In the development of a gas sensor for PD detection in GIS, there are two possible approaches to address the issue. The first approach is based on precise and quantitative analysis of decomposition components produced by PD. It is well known that SF4 , F2 , SOF2 , SO2 F2 , SOF4 , SO2 and HF are produced as a result of discharge-induced SF6 decomposition as well as by succeeding reactions with contaminants such as air or water vapor [2–13]. A sensing material, which responds to at least one of these components, can be employed as a transducer for PD detection. For example, ion-conductive solid electrolyte, which could cause an electrochemical reaction with HF to generate ionic current, has been successfully used as a SF6 decomposition gas sensor [8]. Since a target gas is explicitly determined, this method realizes precise sensor calibration for the target gas concentration. However, the sensor calibration for one gas component may not be equivalent to that for the entire PD activity because various decomposition products can be simultaneously generated in real GIS [2–13]. From such a point of view, the second method calibrates the sensor for the PD parameters such as charge, current or energy rather than for one peculiar decomposition gas concentration [6,7,10]. It is known that the amount of the decomposition products increases with discharge energy [3,11–13]. The PD energy calibration, in which the sensor is exposed to real decomposition products generated by PD in SF6 , may provide more practical data for evaluation of the sensor performance. In the present study, a new detection method of PD generated in SF6 gas was demonstrated using a gas sensor composed of carbon nanotubes (CNTs) [14]. Recently, interest in CNTs has been rapidly growing in various scientific and engineering fields. Especially, CNT-based gas sensors [15–20] have received considerable attention because of their outstanding properties such as faster response, higher sensitivity, lower operating temperature and a wider variety of detectable gas. The authors have proposed a new fabrication method of a gas sensor composed of multiwall carbon nanotubes (MWCNTs) using dielectrophoresis (DEP) [20]. The semiconducting MWCNTs dispersed in ethanol were trapped and enriched in an interdigitated microelectrode gap under action of positive DEP force that drove the MWCNTs to higher electric field region. Thus fabricated MWCNT sensor could detect ppm-levels of gases such as ammonia (NH3 ) or nitrogen dioxide (NO2 ) at room temperature. In this study, the MWCNT sensor was employed to detect PD generated in SF6 at atmospheric pressure. It was found that the MWCNT sensor enabled a sensitive and stable detection of PD in SF6 . The possible decompo-
165
sition products detected by the MWCNT sensor were also discussed.
2. Experimental 2.1. Carbon nanotube-based gas sensor Details of fabrication method of the MWCNT gas sensor using positive DEP have been described elsewhere [20]. The semiconducting MWCNTs (purchased from Nilaco, Japan) had 20 nm average diameter, 5–20 m length and were suspended in ethanol. An interdigitated microelectrode of thin chrome film was patterned on a glass substrate (20 mm × 20 mm) by photolithography technique. The electrode finger had a castle-wall pattern and had 5 mm length and 5 m the minimum clearance. The 20 electrode fingers formed 19 castellated gaps. The DEP trapping of MWCNTs to the microelectrode was performed with ac voltage of 100 kHz frequency and 10 V amplitude (peak to peak value) while MWCNTs suspension continuously flowed over the microelectrode surface for 3 h. As shown in Fig. 1, MWCNTs were trapped by positive DEP in the electrode gap where the electric field strength became higher. After the DEP process, the ethanol was evaporated and the microelectrode retaining the MWCNTs was employed as a gas sensor. 2.2. Partial discharge detection A schematic of the PD detection experimental setup is depicted in Fig. 2. A point-to-plane electrode system (50 m point radius and 15 mm gap length, made of stainless steel) was vertically place in a model GIS steel tank (50 cm inner diameter, 90 cm long, 0.18 m3 inner volume). Before each test run, the tank was filled with new SF6 gas (99.99% purity, containing 10 ppm water vapor or less) at atmospheric pressure (0.1 MPa). The MWCNT gas sensor was placed
Fig. 1. SEM image of an MWCNT-based gas sensor. The MWCNTs were trapped onto a castellated microelectrode by positive dielectrophoresis.
166
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
Fig. 2. A schematic of experimental setup for PD detection in SF6 gas using a MWCNT gas sensor.
in the tank and electrically connected to the external measuring equipments via feedthrough connectors. The sensor was placed in the same plane as the plane electrode. The horizontal distance of the sensor from the electrode axis was varied from 5 to 40 cm along the horizontal axis of the cylindrical tank (position P1 , P2 , P3 in Fig. 2). The sensor impedance was continuously measured at room temperature and 100 kHz frequency using a lock-in amplifier (Model 7280, Perkin Elmer Instruments, USA) controlled by a PC. AC high voltage (60 Hz frequency) was applied to the point electrode to generate corona discharge (PD) at the point tip. PD signals were detected using a shunt resistance connected to the grounded plane electrode. The phase-resolved PD characteristics were obtained by a PD measuring system (MS/SPAC Model 310, Marubun Co., Japan) with the minimum detection level of 3 pC. Passive gas detection tubes (GASTEC Co., Japan) for SO2 (0.1 ppm sensitivity) and HF (0.5 ppm sensitivity) were also installed in the tank in order to monitor SO2 and HF that might be produced by PD [2–13]. The sample gas could disperse into the tube by diffusion without an aspirating pump. Color indication of these tubes, which was a measure of the gas concentration, could be observed through a glass window on the tank wall. This enabled continuous monitoring of the gas concentration during PD generation.
Fig. 3. The relative increase of the MWCNT gas Sensor Conductance Caused by PD in SF6 gas. The distance between the sensor and the point tip was 5 cm (position P1 in Fig. 2).
vated. When the voltage was switched off to stop the PD, the conductance stopped increasing and gradually decreased (Period B). When the voltage was raised back to the initial value, the conductance started to increase again at the same rate as before (Period C). It was also noted that the conductance increase was saturated at 52 S regardless the voltage level. This implies that the sensor saturates earlier with higher voltage. When the voltage V was lower than 30 kV, SO2 and HF were not detected by the detection tubes in 10 h. On the other hand, SO2 and HF were detected at ppm-levels by the detection tubes for V > 30 kV as shown in Fig. 4. The gas concentration increased almost linearly with elapsed time. The generation rate of SO2 and HF was about 0.2 and 0.4 ppm/h, respectively. When SF6 gas was evacuated and replaced with new gas after a series of experiments, the sensor conductance was returned to the initial value. Fig. 5 depicts results of similar measurements that were conducted changing the relative position of the sen-
3. Results Fig. 3 shows the conductance response of the MWCNT gas sensor measured at four different voltages higher than the PD inception voltage (about 8 kV, a root mean square value). The sensor did not respond to pure SF6 gas before PD was generated. Just after the PD onset, the sensor conductance gradually increased with elapsed time (Period A). The conductance increase rate became higher as the applied voltage became higher, namely, as the PD was more acti-
Fig. 4. Temporal change of SO2 and HF concentration detected by gas detection tubes. Applied voltage was 30 kV.
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
Fig. 5. Effects of the MWCNT gas sensor location on the sensor response. Applied voltage was 20 kV.
Fig. 6. Voltage dependency of time-averaged PD power determined from phase-resolved PD analysis.
sor to the PD-emitting point electrode. As the sensor was placed farther from the PD source, the conductance increase became slower. However, the sensor could still detect PD even when it was located 40 cm from the point electrode. Based on the phase-resolved PD characteristics, timeaveraged PD power PA is given by [21]
167
not completely clear how the sensor responds to PD, the experimental results strongly suggest that decomposition products generated by PD are responsible for the sensor response due to the following reasons. It is obvious that the decomposition products are produced in the vicinity of the point tip where PD is generated. Then these products will diffuse into the surrounding space. This implies that concentration of the decomposition products is varied depending on the position and the elapsed time before it reaches an equilibrium state. Temporal and spatial variations of the sensor conductance shown in Figs. 3 and 5 are consistent with this explanation. When the decomposition products reach the sensor, they will interact with MWCNTs to modify the electrical properties. When the voltage is switched off, decomposition of SF6 stops and pre-produced decomposition will gradually diffuse away from the sensor as the concentration profile becomes uniform throughout the tank. It is known that the amount of the decomposition products increases with discharge energy [3,11–13]. The voltage dependency of PD power (Fig. 6) and the sensor response (Fig. 3) also suggest that the decomposition of SF6 gas is enhanced by increasing voltage. It is known that the conductance increase of a CNT gas sensor is almost proportional to the target gas concentration in a lower range and tends to saturate at higher concentration [16–20]. The conductance saturation, which was observed in a few hours with a higher voltage in Fig. 3, might be attributed to the sensor saturation rather than suppression of decomposition process. Fig. 7 depicts a relationship between the PD power PA and the initial increase rate of the sensor conductance, which can be determined from Fig. 3 at time t = 0. Since the concentration of decomposition products is low just after the PD inception, the conductance increase should be proportional to the decomposition concentration. That is, Fig. 7 implies that the decomposition rate is almost linearly increased with the PD power. This result is consistent with previously reported discharge energy dependency of decomposition products [3,11–13]. Fig. 7 may be useful
N
PA =
1 qi vi T
(1)
i=1
where vi is an absolute instantaneous value of applied ac voltage at the instant of apparent PD charge qi . N is the total number of PD pulses measured during a period T. As illustrated in Fig. 6, the more PD power was dissipated with higher applied voltage.
4. Discussion It has been experimentally proven that the MWCNT gas sensor could detect PD generated in SF6 gas. Although it is
Fig. 7. Initial increase rate of the MWCNT gas sensor conductance as a function of the PD power. The distance between the sensor and the point tip was 5 cm (position P1 in Fig. 2).
168
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
at 0.1 ppm-level. Although the responsible gas is not identified yet, the CNT-based gas sensor may be promising as a sensitive PD detection technique for SF6 gas insulated switchgears or transformers. Besides the high sensitivity, the MWCNT gas sensor has some other advantages. The position dependency of the sensor response (Fig. 5) may realize locating a PD source by installing several sensors in a GIS tank. By exchanging the partially decomposed SF6 gas with new one, the sensor can be initialized and repeatedly used. Since the sensor is small enough to be directly installed in the GIS tank, there is no need for the gas sampling for diagnosis.
5. Conclusions Fig. 8. Conductance response of the MWCNTs gas sensor to SO2 and HF. The sample gas was generated by a permeater employing argon as a carrier gas [20].
as a calibration curve for the PD power versus the sensor response, although it can be affected by the other condition such as sensor location, gas pressure and tank volume. Thus far, electrical discharge-induced decomposition process of SF6 gas has been intensively investigated [2–13]. The reaction process is considerably influenced by small amount of contamination such as water or air. It is widely accepted that SO2 and HF are major final products and useful as an indicator of PD activity in GIS. In fact, ppm-levels of SO2 and HF were detected also in this study (Fig. 4) when PD was activated by increasing voltage as high as 30 kV. The conductance change of CNT-based gas sensor has been attributed to their p-type semiconducting properties [15–18,20]. The adsorption of reducing gas molecules such as NH3 will inject electrons to the p-type MWCNTs and decrease the sensor conductance. In contrary to this, oxidative gas like NO2 will increase positive holes and the resultant sensor conductance. In order to check if the PD-induced conductance increase can be attributed to HF or SO2 , the sensor response to these gases was measured at ppm-level concentration. In the additional experiments, the same setup as the previous study [20], in which NH3 sensing was tested at various concentrations, was employed. As shown in Fig. 8, the MWCNT sensor was sensitive to HF and SO2 . However, both of them did not increase but decreased the sensor conductance as NH3 did [15,16,18,20]. This implies that HF and SO2 are reducing agents and that the PD-induced conductance increase is caused by adsorption of some other oxidative molecules. It seems that fluorine (F), which is very strong oxidant and can be produced by SF6 decomposition [3,9], may be one of responsible gases for the sensor response. At this point, however, it is unclear if highly reactive and attaching F molecules can reach the sensor before they are lost [9]. More detailed studies are required to clarify this point in future. As shown in Fig. 3, the MWCNT sensor could detect faint PD near the onset voltage, which could not be detected by detection tubes for HF and SO2 with sensitivity
Partial discharge detection in SF6 gas using a carbon nanotube-based gas sensor was demonstrated for the first time. The sensor was fabricated by positive dielectrophoresis of semiconducting MWCNTs on a microelectrode array. PD was detected as the sensor conductance increase depending on PD activity and the sensor location. The sensor could realize a real time detection of faint PD activities that detection tubes could not sense. Although the responsible product is not identified yet, it is suggested some oxidative decomposition products may cause the conductance increase of the MWCNT sensor. The sensor response was almost proportional to the PD power. In this work, the sensor design and selection of CNT (multiwall or singlewall, for example) have not been optimized yet. The authors are currently working on these subjects as well as identifying the decomposition products detected by the MWCNT sensor.
Acknowledgements The authors would like to thank Yasin Khan, Takashi Kurihara and Hiroshi Imakiire (Kyushu University) for their help in experimental works. Special thanks go to Professor Kiyoto Nishijima (Fukuoka University, Japan) for valuable discussion.
References [1] S.A. Boggs, F.Y. Chu, N. Fujimoto (Eds.), Gas Insulated Substations: Technology and Practice, Pergamon Press, New York, 1986, pp. 267–353. [2] S. Tominaga, H. Kuwahara, K. Hirooka, T. Yoshioka, SF6 gas-analysis technique and its application for evaluation of internal conditions in SF6 gas equipment, IEEE Trans. Power Appar. Syst. PAS-100 (1981) 4196–4206. [3] J.M. Braun, F.Y. Chu, Novel low-cost SF6 arcing byproduct detectors for field use in gas-insulated switchgear, IEEE Trans. Power Delivery 1 (1986) 81–86. [4] A. Derdouri, J. Casanovas, R. Hergli, R. Grob, J. Mathieu, Study of the decomposition of wet SF6 , subjected to 50-Hz ac corona discharges, J. Appl. Phys. 65 (1989) 1852–1857.
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169 [5] M. Wember, D. Peier, On-site diagnostic of the decomposition of sulfur hexafluoride due to partial discharges, in: Proceedings of the 13th International Symposium on High Voltage Engineering (13th ISH), Delft, The Netherlands, 25–29 August 2003, Paper No. O.29.01. [6] M. Kawamura, K. Ishii, S. Sato, Sensing of decomposition products in SF6 by use of a corner cube prism coated with a dye film as an optical sensor head, Jpn. J. Appl. Phys. 36 (1997) 4517–4520. [7] M. Kawamura, K. Ishii, S. Sato, Detection of arc- or corona-discharge in SF6 -insulated equipments by using an OTDR method and corner cube prisms coated with dye films, Sens. Actuat. B 50 (1998) 140– 144. [8] T. Minagawa, M. Kawada, S. Yamauchi, M. Kamei, C. Nishida, Development of SF6 decomposition gas sensor, Surf. Coat. Technol. 169/170 (2003) 643–645. [9] R. Irawan, G.B. Scelsi, G.A. Woolsey, Optical fiber sensing of SF6 degradation in high-voltage switchgear, J. Nonlin. Opt. Phys. 10 (2001) 181–195. [10] S. Kajita, S. Ushiroda, T. Yamada, Y. Kondo, K. Sato, H. Nakamura, Detection using tin evaporated thin film of the decomposition product gases created by a.c. corona discharge in SF6 gas, Trans. IEE Jpn. 117-B (1997) 139–140. [11] B. Belmadani, J. Casanovas, A.M. Casanovas, R. Grob, J. Mathieu, SF6 decomposition under power arcs—physical aspects, IEEE Trans. Elect. Insul. 26 (1991) 1163–1176. [12] B. Belmadani, J. Casanovas, A.M. Casanovas, SF6 decomposition under power arcs—chemical aspects, IEEE Trans. Elect. Insul. 26 (1991) 1177–1182. [13] F.Y. Chu, SF6 decomposition in gas-insulated equipment, IEEE Trans. Elect. Insul. EI-21 (1986) 693–725. [14] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [15] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [16] P. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. Dai, S. Peng, K.J. Cho, Toward large arrays of multiplex functionalized carbon nanotube sensors for highly sensitive and selective molecular detection, Nano Lett. 3 (2003) 347–351.
169
[17] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films, Appl. Phys. Lett. 82 (2003) 961–963. [18] K.G. Ong, K. Zeng, C.A. Grimes, A wireless, passive carbon nanotube-based gas sensor, IEEE Sensors J. 2 (2002) 82–88. [19] S. Chopra, A. Pham, J. Gaillard, A. Parker, M. Rao, Carbon nanotube-based resonant-circuit sensor for ammonia, Appl. Phys. Lett. 80 (2002) 4632–4634. [20] J. Suehiro, G. Zhou, M. Hara, Fabrication of a carbon nanotube-based gas sensor using dielectrophoresis and its application for ammonia detection by impedance spectroscopy, J. Phys. D: Appl. Phys. 36 (2003) L109–L114. [21] E. Kuffel, W.S. Zaengl, High-Voltage Engineering: Fundamentals, Pergamon Press, Oxford, 1984, pp. 433–462.
Biographies Junya Suehiro received his MS and Doctor degrees in electrical engineering from Kyushu University in 1985 and 1991, respectively. He was with the Nippon Steel Corporation from 1985 to 1988. Since 1988, he has been at Kyushu University and now an associate professor. His current research interests are applied electrostatics and high voltage engineering. Guangbin Zhou received a BS degrees in electrical engineering from Shenyang Institute of Technology, China in 1995. He was with the Shenyang Light Industries Research & Design Institute from 1995 to 1998. He received an MS degree in electrical engineering from the Kyushu University in 2002. Currently, he is a doctor course student in Kyushu University. His current research interests are dielectrophoretic manipulation of micro- and nanoscale-material including biological cells and carbon nanotubes. Masanori Hara received his MS and Doctor degrees in electrical engineering from Kyushu University in 1969 and 1972, respectively. Since 1986, he is a professor of Kyushu University. His current research interests are high voltage engineering, superconductivity engineering and pulsed power engineering.
Detection of partial discharge in SF6 gas using a carbon nanotube-based gas sensor Junya Suehiro∗ , Guangbin Zhou, Masanori Hara Department of Electrical and Electronic Systems Engineering, Graduate School of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 14 February 2004; received in revised form 7 May 2004; accepted 26 May 2004 Available online 2 July 2004
Abstract For electrical insulation diagnosis of gas-insulated switchgear (GIS), detection of partial discharge (PD) generated in sulfur hexafluoride (SF6 ) gas is important. This paper describes a new detection method of PD generated in SF6 gas using a gas sensor composed of multiwall carbon nanotubes (MWCNTs). The gas sensor was fabricated by electrokinetic manipulation of semiconducting MWCNTs using positive dielectrophoresis. The MWCNT gas sensor and a point-to-plane electrode system were placed in a steel tank filled with SF6 gas at atmospheric pressure. AC high voltage was applied to the point electrode in order to generate PD while the electrical impedance of the MWCNT sensor was monitored. When the PD was generated, the electrical conductance of the MWCNT sensor gradually increased. The sensor response was reversible and was influenced by the PD intensity as well as by the relative position of the sensor to the point electrode. It was suggested that the sensor conductance increase was caused by an electronic interaction between MWCNTs and non-identified oxidative decomposition products. The faint PD, which could not be sensed by gas detecting tubes, was successfully detected by the MWCNT sensor on a real time basis. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Gas sensor; Sulfur hexafluoride (SF6 ); Gas-insulated switchgear (GIS); Partial discharge (PD); Decomposition
1. Introduction Sulfur hexafluoride (SF6 ) gas is widely used as electrical insulator as well as arc-quenching medium in gas-insulated switchgear (GIS). SF6 is chemically inert and has high dielectric strength three times that of air at atmospheric pressure. If sharp edges or small contaminant particles accidentally exist in GIS, partial discharge (PD) may be generated around the points where the electric field is intensified. PD or corona discharge is defined as an electrical discharge phenomenon that is localized in high field region near electrodes or insulators. It is difficult to completely eliminate PD that may cause fatal damage to insulators and conductors. PD activity in GIS may eventually lead to a catastrophic breakdown or arc discharge. Unlike PD, the arc discharge completely bridges between the high voltage and grounded electrodes and carries much higher current. As a result, the arc discharge can give a serious damage to the GIS and the entire electric power distribution system. Early ∗ Corresponding author. Tel.: +81 92 642 3912; fax: +81 92 642 3964. E-mail address: [email protected] (J. Suehiro).
0925-4005/$ – see front matter. © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.05.050
detection of PD can provide a way to predict and avoid the fatal failure. So far, various PD detection methods have been proposed and investigated [1]. The most intensively studied methods are physical sensors, which can detect ultra-high frequency (UHF) electromagnetic wave, acoustic emission (AE) or optical emission generated by PD. The gas sampling method, which detects decomposition gaseous products, has been also regarded as effective technique. The gas sampling method has some advantages over the other types of the PD sensor. For example, it is less influenced by background electromagnetic or acoustic noises, and can be applicable to both of on-line and off-line PD monitoring. Since the decomposition products accumulate in a closed GIS tank depending on a total PD activity, the gas sampling method is suitable for long term trend-based monitoring. Up to now, decomposition products were successfully detected using a detection tube [2,3], a gas chromatography, a mass spectrometer [4] and an ion mobility spectrometer [5]. The detection tube is handy and can detect main decomposition products such as sulfur dioxide (SO2 ) or hydrogen fluoride (HF) at ppm-levels. However, the detection tube, which indicates the gas concentration by color changes, suffers from poor ac-
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
curacy and is not suitable for automated continuous remote PD monitoring. On the other hand, the gas chromatography and the spectrometers with high sensitivity are costly and suitable for laboratory-based analysis rather than continuous on-line monitoring. As a result, there is a strong demand for a new type of gas sensor [6–10], which is easy to use on-site and sensitive enough to detect PD generated in SF6 . In the development of a gas sensor for PD detection in GIS, there are two possible approaches to address the issue. The first approach is based on precise and quantitative analysis of decomposition components produced by PD. It is well known that SF4 , F2 , SOF2 , SO2 F2 , SOF4 , SO2 and HF are produced as a result of discharge-induced SF6 decomposition as well as by succeeding reactions with contaminants such as air or water vapor [2–13]. A sensing material, which responds to at least one of these components, can be employed as a transducer for PD detection. For example, ion-conductive solid electrolyte, which could cause an electrochemical reaction with HF to generate ionic current, has been successfully used as a SF6 decomposition gas sensor [8]. Since a target gas is explicitly determined, this method realizes precise sensor calibration for the target gas concentration. However, the sensor calibration for one gas component may not be equivalent to that for the entire PD activity because various decomposition products can be simultaneously generated in real GIS [2–13]. From such a point of view, the second method calibrates the sensor for the PD parameters such as charge, current or energy rather than for one peculiar decomposition gas concentration [6,7,10]. It is known that the amount of the decomposition products increases with discharge energy [3,11–13]. The PD energy calibration, in which the sensor is exposed to real decomposition products generated by PD in SF6 , may provide more practical data for evaluation of the sensor performance. In the present study, a new detection method of PD generated in SF6 gas was demonstrated using a gas sensor composed of carbon nanotubes (CNTs) [14]. Recently, interest in CNTs has been rapidly growing in various scientific and engineering fields. Especially, CNT-based gas sensors [15–20] have received considerable attention because of their outstanding properties such as faster response, higher sensitivity, lower operating temperature and a wider variety of detectable gas. The authors have proposed a new fabrication method of a gas sensor composed of multiwall carbon nanotubes (MWCNTs) using dielectrophoresis (DEP) [20]. The semiconducting MWCNTs dispersed in ethanol were trapped and enriched in an interdigitated microelectrode gap under action of positive DEP force that drove the MWCNTs to higher electric field region. Thus fabricated MWCNT sensor could detect ppm-levels of gases such as ammonia (NH3 ) or nitrogen dioxide (NO2 ) at room temperature. In this study, the MWCNT sensor was employed to detect PD generated in SF6 at atmospheric pressure. It was found that the MWCNT sensor enabled a sensitive and stable detection of PD in SF6 . The possible decompo-
165
sition products detected by the MWCNT sensor were also discussed.
2. Experimental 2.1. Carbon nanotube-based gas sensor Details of fabrication method of the MWCNT gas sensor using positive DEP have been described elsewhere [20]. The semiconducting MWCNTs (purchased from Nilaco, Japan) had 20 nm average diameter, 5–20 m length and were suspended in ethanol. An interdigitated microelectrode of thin chrome film was patterned on a glass substrate (20 mm × 20 mm) by photolithography technique. The electrode finger had a castle-wall pattern and had 5 mm length and 5 m the minimum clearance. The 20 electrode fingers formed 19 castellated gaps. The DEP trapping of MWCNTs to the microelectrode was performed with ac voltage of 100 kHz frequency and 10 V amplitude (peak to peak value) while MWCNTs suspension continuously flowed over the microelectrode surface for 3 h. As shown in Fig. 1, MWCNTs were trapped by positive DEP in the electrode gap where the electric field strength became higher. After the DEP process, the ethanol was evaporated and the microelectrode retaining the MWCNTs was employed as a gas sensor. 2.2. Partial discharge detection A schematic of the PD detection experimental setup is depicted in Fig. 2. A point-to-plane electrode system (50 m point radius and 15 mm gap length, made of stainless steel) was vertically place in a model GIS steel tank (50 cm inner diameter, 90 cm long, 0.18 m3 inner volume). Before each test run, the tank was filled with new SF6 gas (99.99% purity, containing 10 ppm water vapor or less) at atmospheric pressure (0.1 MPa). The MWCNT gas sensor was placed
Fig. 1. SEM image of an MWCNT-based gas sensor. The MWCNTs were trapped onto a castellated microelectrode by positive dielectrophoresis.
166
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
Fig. 2. A schematic of experimental setup for PD detection in SF6 gas using a MWCNT gas sensor.
in the tank and electrically connected to the external measuring equipments via feedthrough connectors. The sensor was placed in the same plane as the plane electrode. The horizontal distance of the sensor from the electrode axis was varied from 5 to 40 cm along the horizontal axis of the cylindrical tank (position P1 , P2 , P3 in Fig. 2). The sensor impedance was continuously measured at room temperature and 100 kHz frequency using a lock-in amplifier (Model 7280, Perkin Elmer Instruments, USA) controlled by a PC. AC high voltage (60 Hz frequency) was applied to the point electrode to generate corona discharge (PD) at the point tip. PD signals were detected using a shunt resistance connected to the grounded plane electrode. The phase-resolved PD characteristics were obtained by a PD measuring system (MS/SPAC Model 310, Marubun Co., Japan) with the minimum detection level of 3 pC. Passive gas detection tubes (GASTEC Co., Japan) for SO2 (0.1 ppm sensitivity) and HF (0.5 ppm sensitivity) were also installed in the tank in order to monitor SO2 and HF that might be produced by PD [2–13]. The sample gas could disperse into the tube by diffusion without an aspirating pump. Color indication of these tubes, which was a measure of the gas concentration, could be observed through a glass window on the tank wall. This enabled continuous monitoring of the gas concentration during PD generation.
Fig. 3. The relative increase of the MWCNT gas Sensor Conductance Caused by PD in SF6 gas. The distance between the sensor and the point tip was 5 cm (position P1 in Fig. 2).
vated. When the voltage was switched off to stop the PD, the conductance stopped increasing and gradually decreased (Period B). When the voltage was raised back to the initial value, the conductance started to increase again at the same rate as before (Period C). It was also noted that the conductance increase was saturated at 52 S regardless the voltage level. This implies that the sensor saturates earlier with higher voltage. When the voltage V was lower than 30 kV, SO2 and HF were not detected by the detection tubes in 10 h. On the other hand, SO2 and HF were detected at ppm-levels by the detection tubes for V > 30 kV as shown in Fig. 4. The gas concentration increased almost linearly with elapsed time. The generation rate of SO2 and HF was about 0.2 and 0.4 ppm/h, respectively. When SF6 gas was evacuated and replaced with new gas after a series of experiments, the sensor conductance was returned to the initial value. Fig. 5 depicts results of similar measurements that were conducted changing the relative position of the sen-
3. Results Fig. 3 shows the conductance response of the MWCNT gas sensor measured at four different voltages higher than the PD inception voltage (about 8 kV, a root mean square value). The sensor did not respond to pure SF6 gas before PD was generated. Just after the PD onset, the sensor conductance gradually increased with elapsed time (Period A). The conductance increase rate became higher as the applied voltage became higher, namely, as the PD was more acti-
Fig. 4. Temporal change of SO2 and HF concentration detected by gas detection tubes. Applied voltage was 30 kV.
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
Fig. 5. Effects of the MWCNT gas sensor location on the sensor response. Applied voltage was 20 kV.
Fig. 6. Voltage dependency of time-averaged PD power determined from phase-resolved PD analysis.
sor to the PD-emitting point electrode. As the sensor was placed farther from the PD source, the conductance increase became slower. However, the sensor could still detect PD even when it was located 40 cm from the point electrode. Based on the phase-resolved PD characteristics, timeaveraged PD power PA is given by [21]
167
not completely clear how the sensor responds to PD, the experimental results strongly suggest that decomposition products generated by PD are responsible for the sensor response due to the following reasons. It is obvious that the decomposition products are produced in the vicinity of the point tip where PD is generated. Then these products will diffuse into the surrounding space. This implies that concentration of the decomposition products is varied depending on the position and the elapsed time before it reaches an equilibrium state. Temporal and spatial variations of the sensor conductance shown in Figs. 3 and 5 are consistent with this explanation. When the decomposition products reach the sensor, they will interact with MWCNTs to modify the electrical properties. When the voltage is switched off, decomposition of SF6 stops and pre-produced decomposition will gradually diffuse away from the sensor as the concentration profile becomes uniform throughout the tank. It is known that the amount of the decomposition products increases with discharge energy [3,11–13]. The voltage dependency of PD power (Fig. 6) and the sensor response (Fig. 3) also suggest that the decomposition of SF6 gas is enhanced by increasing voltage. It is known that the conductance increase of a CNT gas sensor is almost proportional to the target gas concentration in a lower range and tends to saturate at higher concentration [16–20]. The conductance saturation, which was observed in a few hours with a higher voltage in Fig. 3, might be attributed to the sensor saturation rather than suppression of decomposition process. Fig. 7 depicts a relationship between the PD power PA and the initial increase rate of the sensor conductance, which can be determined from Fig. 3 at time t = 0. Since the concentration of decomposition products is low just after the PD inception, the conductance increase should be proportional to the decomposition concentration. That is, Fig. 7 implies that the decomposition rate is almost linearly increased with the PD power. This result is consistent with previously reported discharge energy dependency of decomposition products [3,11–13]. Fig. 7 may be useful
N
PA =
1 qi vi T
(1)
i=1
where vi is an absolute instantaneous value of applied ac voltage at the instant of apparent PD charge qi . N is the total number of PD pulses measured during a period T. As illustrated in Fig. 6, the more PD power was dissipated with higher applied voltage.
4. Discussion It has been experimentally proven that the MWCNT gas sensor could detect PD generated in SF6 gas. Although it is
Fig. 7. Initial increase rate of the MWCNT gas sensor conductance as a function of the PD power. The distance between the sensor and the point tip was 5 cm (position P1 in Fig. 2).
168
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169
at 0.1 ppm-level. Although the responsible gas is not identified yet, the CNT-based gas sensor may be promising as a sensitive PD detection technique for SF6 gas insulated switchgears or transformers. Besides the high sensitivity, the MWCNT gas sensor has some other advantages. The position dependency of the sensor response (Fig. 5) may realize locating a PD source by installing several sensors in a GIS tank. By exchanging the partially decomposed SF6 gas with new one, the sensor can be initialized and repeatedly used. Since the sensor is small enough to be directly installed in the GIS tank, there is no need for the gas sampling for diagnosis.
5. Conclusions Fig. 8. Conductance response of the MWCNTs gas sensor to SO2 and HF. The sample gas was generated by a permeater employing argon as a carrier gas [20].
as a calibration curve for the PD power versus the sensor response, although it can be affected by the other condition such as sensor location, gas pressure and tank volume. Thus far, electrical discharge-induced decomposition process of SF6 gas has been intensively investigated [2–13]. The reaction process is considerably influenced by small amount of contamination such as water or air. It is widely accepted that SO2 and HF are major final products and useful as an indicator of PD activity in GIS. In fact, ppm-levels of SO2 and HF were detected also in this study (Fig. 4) when PD was activated by increasing voltage as high as 30 kV. The conductance change of CNT-based gas sensor has been attributed to their p-type semiconducting properties [15–18,20]. The adsorption of reducing gas molecules such as NH3 will inject electrons to the p-type MWCNTs and decrease the sensor conductance. In contrary to this, oxidative gas like NO2 will increase positive holes and the resultant sensor conductance. In order to check if the PD-induced conductance increase can be attributed to HF or SO2 , the sensor response to these gases was measured at ppm-level concentration. In the additional experiments, the same setup as the previous study [20], in which NH3 sensing was tested at various concentrations, was employed. As shown in Fig. 8, the MWCNT sensor was sensitive to HF and SO2 . However, both of them did not increase but decreased the sensor conductance as NH3 did [15,16,18,20]. This implies that HF and SO2 are reducing agents and that the PD-induced conductance increase is caused by adsorption of some other oxidative molecules. It seems that fluorine (F), which is very strong oxidant and can be produced by SF6 decomposition [3,9], may be one of responsible gases for the sensor response. At this point, however, it is unclear if highly reactive and attaching F molecules can reach the sensor before they are lost [9]. More detailed studies are required to clarify this point in future. As shown in Fig. 3, the MWCNT sensor could detect faint PD near the onset voltage, which could not be detected by detection tubes for HF and SO2 with sensitivity
Partial discharge detection in SF6 gas using a carbon nanotube-based gas sensor was demonstrated for the first time. The sensor was fabricated by positive dielectrophoresis of semiconducting MWCNTs on a microelectrode array. PD was detected as the sensor conductance increase depending on PD activity and the sensor location. The sensor could realize a real time detection of faint PD activities that detection tubes could not sense. Although the responsible product is not identified yet, it is suggested some oxidative decomposition products may cause the conductance increase of the MWCNT sensor. The sensor response was almost proportional to the PD power. In this work, the sensor design and selection of CNT (multiwall or singlewall, for example) have not been optimized yet. The authors are currently working on these subjects as well as identifying the decomposition products detected by the MWCNT sensor.
Acknowledgements The authors would like to thank Yasin Khan, Takashi Kurihara and Hiroshi Imakiire (Kyushu University) for their help in experimental works. Special thanks go to Professor Kiyoto Nishijima (Fukuoka University, Japan) for valuable discussion.
References [1] S.A. Boggs, F.Y. Chu, N. Fujimoto (Eds.), Gas Insulated Substations: Technology and Practice, Pergamon Press, New York, 1986, pp. 267–353. [2] S. Tominaga, H. Kuwahara, K. Hirooka, T. Yoshioka, SF6 gas-analysis technique and its application for evaluation of internal conditions in SF6 gas equipment, IEEE Trans. Power Appar. Syst. PAS-100 (1981) 4196–4206. [3] J.M. Braun, F.Y. Chu, Novel low-cost SF6 arcing byproduct detectors for field use in gas-insulated switchgear, IEEE Trans. Power Delivery 1 (1986) 81–86. [4] A. Derdouri, J. Casanovas, R. Hergli, R. Grob, J. Mathieu, Study of the decomposition of wet SF6 , subjected to 50-Hz ac corona discharges, J. Appl. Phys. 65 (1989) 1852–1857.
J. Suehiro et al. / Sensors and Actuators B 105 (2005) 164–169 [5] M. Wember, D. Peier, On-site diagnostic of the decomposition of sulfur hexafluoride due to partial discharges, in: Proceedings of the 13th International Symposium on High Voltage Engineering (13th ISH), Delft, The Netherlands, 25–29 August 2003, Paper No. O.29.01. [6] M. Kawamura, K. Ishii, S. Sato, Sensing of decomposition products in SF6 by use of a corner cube prism coated with a dye film as an optical sensor head, Jpn. J. Appl. Phys. 36 (1997) 4517–4520. [7] M. Kawamura, K. Ishii, S. Sato, Detection of arc- or corona-discharge in SF6 -insulated equipments by using an OTDR method and corner cube prisms coated with dye films, Sens. Actuat. B 50 (1998) 140– 144. [8] T. Minagawa, M. Kawada, S. Yamauchi, M. Kamei, C. Nishida, Development of SF6 decomposition gas sensor, Surf. Coat. Technol. 169/170 (2003) 643–645. [9] R. Irawan, G.B. Scelsi, G.A. Woolsey, Optical fiber sensing of SF6 degradation in high-voltage switchgear, J. Nonlin. Opt. Phys. 10 (2001) 181–195. [10] S. Kajita, S. Ushiroda, T. Yamada, Y. Kondo, K. Sato, H. Nakamura, Detection using tin evaporated thin film of the decomposition product gases created by a.c. corona discharge in SF6 gas, Trans. IEE Jpn. 117-B (1997) 139–140. [11] B. Belmadani, J. Casanovas, A.M. Casanovas, R. Grob, J. Mathieu, SF6 decomposition under power arcs—physical aspects, IEEE Trans. Elect. Insul. 26 (1991) 1163–1176. [12] B. Belmadani, J. Casanovas, A.M. Casanovas, SF6 decomposition under power arcs—chemical aspects, IEEE Trans. Elect. Insul. 26 (1991) 1177–1182. [13] F.Y. Chu, SF6 decomposition in gas-insulated equipment, IEEE Trans. Elect. Insul. EI-21 (1986) 693–725. [14] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [15] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [16] P. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. Dai, S. Peng, K.J. Cho, Toward large arrays of multiplex functionalized carbon nanotube sensors for highly sensitive and selective molecular detection, Nano Lett. 3 (2003) 347–351.
169
[17] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films, Appl. Phys. Lett. 82 (2003) 961–963. [18] K.G. Ong, K. Zeng, C.A. Grimes, A wireless, passive carbon nanotube-based gas sensor, IEEE Sensors J. 2 (2002) 82–88. [19] S. Chopra, A. Pham, J. Gaillard, A. Parker, M. Rao, Carbon nanotube-based resonant-circuit sensor for ammonia, Appl. Phys. Lett. 80 (2002) 4632–4634. [20] J. Suehiro, G. Zhou, M. Hara, Fabrication of a carbon nanotube-based gas sensor using dielectrophoresis and its application for ammonia detection by impedance spectroscopy, J. Phys. D: Appl. Phys. 36 (2003) L109–L114. [21] E. Kuffel, W.S. Zaengl, High-Voltage Engineering: Fundamentals, Pergamon Press, Oxford, 1984, pp. 433–462.
Biographies Junya Suehiro received his MS and Doctor degrees in electrical engineering from Kyushu University in 1985 and 1991, respectively. He was with the Nippon Steel Corporation from 1985 to 1988. Since 1988, he has been at Kyushu University and now an associate professor. His current research interests are applied electrostatics and high voltage engineering. Guangbin Zhou received a BS degrees in electrical engineering from Shenyang Institute of Technology, China in 1995. He was with the Shenyang Light Industries Research & Design Institute from 1995 to 1998. He received an MS degree in electrical engineering from the Kyushu University in 2002. Currently, he is a doctor course student in Kyushu University. His current research interests are dielectrophoretic manipulation of micro- and nanoscale-material including biological cells and carbon nanotubes. Masanori Hara received his MS and Doctor degrees in electrical engineering from Kyushu University in 1969 and 1972, respectively. Since 1986, he is a professor of Kyushu University. His current research interests are high voltage engineering, superconductivity engineering and pulsed power engineering.