Highly Sensitive Detection of Methane Based on

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transformer oil have the problems of cross sensitivity and insecurity factor. ... methane, one of the most common methods is the thermal conductivity detector .... filter, and the 2nd harmonic wave signal is obtained by the sum of the square and ...
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Highly Sensitive Detection of Methane Based on Tunable Diode Laser Absorption Spectrum Jun Jiang, Guo-Ming Ma, Hong-Tu Song, C. R. Li

Ying-Ting Luo, Hong-Bin Wang

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing, P. R. China

Electric Power Research Institute of Guangdong Power Grid Co., Ltd Guangzhou, P. R. China

Abstract Traditional methane detection techniques in power transformer oil have the problems of cross sensitivity and insecurity factor. Based on Beer-Lambert spectral absorption law, low concentration methane detection with tunable diode laser absorption spectrum is proposed in this paper for the advantages of high sensitivity and resolution. Firstly, the central wavelength of 1653.72 nm in near infrared spectrum is selected. Then long optical length gas cell by means of refraction and reflection more than 20 times is built in the laboratory. At last, at different concentrations ranging from 50 1000 L/L, the harmonic signal waveforms are obtained. Experimental results show that the lower detection limit of this developed methane system can detect 1.28 L/L approximately and the deviation is less than 0.02 V, which proved to be a prospective sensing technique to be utilized in power transformers.

INTRODUCTION

Methane is widely distributed in nature, and methane is the most simple organic matter, which is the main component of natural gas, methane, gas, and so on. However, methane (CH4) dissolved in oil immersed power transformer oil is a kind of fault characteristic gases[1, 2]. The high precision detection dissolved in oil lay the premise and foundation for analysis and judgment of transformer incipient fault. Different from the methane concentration detection under mine at % level, dissolved methane in oil always at ppm (parts per million) level. So novel detection technique is essential.

(1) high selectivity, high resolution spectral technique, due to the "fingerprint" feature of the molecular spectrum, it is not affected by other gases. This feature has obvious advantages compared with other methods.

(3) it has the advantages of high speed and high sensitivity. The time resolution can be in millisecond level. this technology has been applied in the following fields: molecular spectroscopy, industrial process monitoring, diagnosis and analysis of combustion process, engine efficiency and vehicle emissions, explosion detection, trace gas monitoring, etc. It is taken into consideration that the near infrared laser is adopted for the mid-infrared laser is much expensive. To guarantee the high sensitivity, long optical length is used. At last, the key parameters including sensitivity and deviation is tested. The successful detection lay good foundation for methane detection in power transformer.

Traditionally, there are many methods for detecting methane, one of the most common methods is the thermal conductivity detector (TCD)[3-6]. The working principle of the thermal conductivity detector is based on different gases with different thermal conductivities. The process of detecting the concentration of gas is realized by the heat exchange and heat balance between the heat resistance and the gas being measured. Usually, the detection emits some heat energy dependent on the methane concentration in a dissolved gas

II.

The authors acknowledge the financial support of Fundamental Research Funds for the Central Universities, Research Fund for the Doctoral Program of Higher Education of China (RFDP), National Natural Science Foundation of China (Grant No. 51307052), Beijing Natural Science Foundation (Grant No. 3144035), 111 Project (B08013) of China and China Southern Power Grid.

978-1-4673-9220-4/16/$31.00 ©2016 IEEE

In this paper, methane detection based on tunable diode laser absorption spectrum is proposed for these advantages[710]:

(2) it is an effective universal technique for all active molecules that are active in the infrared, and the same instrument can be easily modified to measure the other components of the instrument, which only needs to change the laser and standard gas. Because of this characteristic, it is easy to change it into an instrument for measuring the number of components at the same time.

Keywords Power transformer; Oil-immersed; Methane; Modulation; Sensitivity

I.

analysis (DGA) equipment. Undoubtedly the test method is unsafe for the heat emission, and the cycle is relatively long. As well, semiconductor type sensors and catalytic combustion type sensors have some disadvantages, including easy poisoning and cross sensitivity, for the use in high voltage situation.

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PRINCIPLE OF OPTICAL DETECTION

A. Beer-Lambert Law When light passes through different components and different concentrations of gases, the light intensity can produce different degrees of attenuation. By measuring the intensity and attenuation of light, the change of gas concentration is reflected.

the natural frequency: 2+2 3=7571.1 cm-1 (1.33 m); 2 3=6037.8 cm-1 (1.65 m). Specifically, the absorption spectrum around 1.65 m is acquired from HITRAN 2012, as shown in Fig. 2.

When a certain volume of uniform gas is irradiated by a laser beam with a specific wavelength , the laser spectrum and the absorption spectrum of the irradiated gas are overlapped, and the laser intensity is reduced.

Intensity (cm/mol, l10 )

1.5

Fig. 1. Schematic view of Beer-Lambert Law

0.6 0.3

1.6532

1.6534

1.6536

1.6538

Wavelength ( m)

1.6540

1.6542

According to the figure above, we can know that the highest absorption intensity of CH4 molecule at 1.65 m is in the order of 10-21. Moreover, the absorption spectrum at 1.65 m are less coincident with others. Based on the above discussion, the absorption spectrum of methane gas with 1653.72 nm (1.49 10-21 cm/mol) absorption line is selected.

(1)

I( ) and I0( ) are intensities of incident and emitted laser respectively, V. C is the concentration of gas to be detected, L/L.

C. Tunable Diode Laser Absorption Spectrum Tunable diode laser absorption spectrum technique is mainly used to measure the absorption line of a single or a few distance of the molecules, which can be used to change the narrow linewidth and wavelength of the tunable diode laser. In order to achieve the highest selectivity, analysis is generally carried out at low pressure, then the absorption lines will not be widened because of pressure. This method is proposed by Hinkley and Reid. It has developed into a very sensitive and common monitoring technique for trace gases in the atmosphere [11, 12].

L is the path length of the beam of light through the material sample, m. ( ) is the absorption coefficient per unit distance and per unit concentration of the gas, cm/mol. The Beer Lambert law in gas can also be written as

ln[I ( ) / I 0 ( )] ó ( )I L

1.49l10-21cm/mol

Fig. 2. Absorption spectrum of methane around 1.65 m

Where

C=

1653.72 nm

0.9

0.0 1.6530

The absorption of the gas in the light meets the BeerLambert law, and the formula can be expressed as

I ( )=I 0 ( ) I exp[ ó ( ) I C I L ]

1.2

(2)

Semiconductor laser is controlled by slow sawtooth laser current and sine wave signal, correspondingly, wavelength output and light intensity output is changed.

In Equ.(2), the concentration is easily obtained by the ratio between incident and emitted intensities.

The 2nd harmonic signal (2f signal) is extracted by locking phase. The phase locked loop can be used to eliminate the effect of the phase difference between the detected signal and reference signal. With two channels of sinusoidal and cosinoidal input, the frequency ratio of the reference signal is 2 times that of the modulation frequency. The two reference signals are respectively multiplied with the suction channel number, and the DC component is extracted by the low pass filter, and the 2nd harmonic wave signal is obtained by the sum of the square and root solving algorithm. The detection principle of 2nd harmonic signal is shown in Fig.3. The uncertainty and reliability of this instrument detection after a long time (~1 year) has been proved in reference[13].

B. Absorption Spectrum of Methane In the HITRAN (High Resolution Transmission Molecular Absorption Database) database, all of the absorption spectra of methane gas molecules can be queried, including natural frequency, combined frequency and frequency range. Among them, the natural frequency segment contains 4 wavelengths: -1 -1 1=3.43 m (2913.0 cm ), 2=6.53 m (1533.3 cm ), 3=3.31 -1 -1 m (3018.9 cm ), 4=7.66 m (1035.9 cm ). At the natural frequency of vibration, there are different gas absorption bands, which are in the mid infrared band. The relatively important frequency band absorption spectrum can be calculated according to the combination of

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Fig. 3. Illustration of methane concentration and 2nd harmonic wave signal. (a) output sawtooth signal of laser diode; (b) photodetector (PD) signal of background gas; (c) PD signal of methane gas; (d) the absorption signal; (e) harmonic wave output after lock-in amplifier. The horizontal axises of figure (a) ~ (d) are wavelengths (nm) and figure (e) is time (ms).

The variation of the modulation coefficient not only affects the height of the two harmonic, but also affects the width of the two harmonic. The width of the two harmonic signal is defined as the distance between the two valleys of the signal. Thus, within a whole detection cycle, the modulation coefficient should not be changed. III.

METHANE DETECTION AT LOW CONCENTRATION

A. Experimental Setup Tunable diode laser absorption spectrum method is a system mainly composed of optical part and electrical part. Especially, the optical path is very important according to Beer-Lambert Law. As shown in Fig.4, a collimator, two diaphragms and gas cell are assembled to guarantee the optical path of absorbing methane.

Fig. 5. Scanning signal and carrier signal detected by oscilloscope

The scanning frequency is set at 5 Hz and the carrier sinusoidal wave is about 30 kHz.

Gas Cell Collimator

B. Detection of Methane at Different Concentrations Two MFCs (Mass Flow Controller) are used to regulate gas flow of high-purity methane (0~50 sccm, standard-state cubic centimeter per minute) and nitrogen (0~2000 sccm). Fully mixed gases flow into gas cell in which the gas molecule absorbs the light at specific wavelength.

Diaphragm

1.0

N2 50 L/L 100 L/L 150 L/L 200 L/L 500 L/L 1000 L/L

2f signal voltage (V)

0.8 Fig. 4. Optical path of the methane detection

To make sure the high sensitivity of methane at low concentration, about 10 meter effective optical length is obtained by refraction and reflection more than 20 times. In addition, scanning signal and carrier sinusoidal wave is modulated together in the electrical control system. The waveform could be seen in Fig. 5.

0.6 0.4 0.2 0.0 -0.2 -0.4 400

800

1200

1600

2000

Index Fig. 6. 2f signal detection of methane at different concentration

Firstly, the background signal was detected by flowing pure nitrogen; then, different concentration of methane was

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flowed into the cell. The 2nd harmonic signal (the original signal subtract the background nitrogen signal) of methane gas from 50 1000 L/L is shown in Fig. 6

(3)

y ã 1.18 l10ó3 x +0.209

With the concentration increased, the measured peak-peak amplitude of signal got larger. The 2f signal successfully monitored the increase of the methane concentration.

Where, x refers to the methane concentration; y is the peak-peak voltage of 2f signal. The R-square coefficient of this fitting was 0.999.

C. Sensitivity Calculation To calculate the sensitivity the detection system, 2f signal peak-peak voltage is obtained from the detection waveform precisely. Here, the peak-peak voltage refers to the difference value of the maximum to the minimum points at certain concentration. The values are shown in Table. 1.

So we can get the fact that peak-peak voltage amplitude would increase 1.18 V with the concentration increasing every 0.1%. Fitted curve and fitted equation showed that the detection possessed good linearity respectively when concentration of hydrogen was at the low concentration rang.

TABLE I.

In addition, the noise amplitude of the detection was about 3 mV, then the SNR (signal to noise ratio) is

Peak-peak voltages at different concentration

Methane Concentration ( L/L) 50

2f signal peak-peak voltage (V) 0.2736

100

0.3083

150

0.3756

200

0.4465

300

0.5705

400

0.6902

500

0.8032

600

0.9267

700

1.0398

800

1.1705

900

1.2789

1000

1.3603

SNRdB ã 20lg(

Cmin ã

50 L / L ã 1.28 L / L 39.2

(5)

Thus, it is possible to detect the ppm-level concentration of the methane gas in the near infrared spectrum, which is proved to be relatively high limit of detection in comparison with other optical techniques[14-17]. According to the latest IEEE guide for the interpretation of gases generated in oil-immersed transformers [18], limitation-of-detection below 5 L/L is acceptable and the resolution of TDLAS-based methane system is good enough to meet the actual needs in the field. D. Deviation Deviation is the voltage difference between an observed 2f peak-peak voltage value and the calculated voltage value. Fig. 8 gives the deviation values at different methane concentration.

2f signal P-P voltage Fitting curve

1.2

(4)

Then the lower detection limit can be calculated as

To confirm the relationship between the methane concentration and the 2f signal peak-peak voltage, a linear curve is fitted in Fig. 7. 1.4

0.2736 ) ã 39.2dB 0.003

0.02

1.0 0.01

Deviation (V)

0.8 0.6 PPV=0.00118*Concentration+0.209 R2=0.999

0.4 0.2

0

200

400

600

Concentration ( L/L)

800

0.00

-0.01

1000 -0.02

Fig. 7. Fitting curve of 2f signal at different concentrations

0

200

400

600

Concentration ( L/L)

Fitted equation when concentration of methane at the range of 50 1000 L/L is

Fig. 8. Deviation values at different concentration

107

800

1000

Mars," Laser Applications to Chemical, Security and Environmental Analysis (LACSEA), paper no. LT6B, vol. 9, 2012. [13] C. V. Horii, "Tropospheric Reactive Nitrogen Speciation, Deposition, and Chemistry at Harvard Forest," PhD, Atmospheric Sciences Group, Harvard University, Boston, 2002. [14] J. Kamieniak, E. P. Randviir, and C. E. Banks, "The latest developments in the analytical sensing of methane," TrAC Trends in Analytical Chemistry, vol. 73, pp. 146-157, 2015. [15] W.-C. Lai, S. Chakravarty, X. Wang, C. Lin, and R. T. Chen, "On-chip methane sensing by near-IR absorption signatures in a photonic crystal slot waveguide," Optics Letters, vol. 36, pp. 984-986, 2011. [16] R. Kan, W. Liu, Y. Zhang, J. Liu, M. Wang, D. Chen, et al., "Large scale gas leakage monitoring with tunable diode laser absorption spectroscopy," Chinese Optics Letters, vol. 4, pp. 116-118, 2006. [17]M.-C. Caumon, P. Robert, E. Laverret, A. Tarantola, A. Randi, J. Pironon, et al., "Determination of methane content in NaCl H2O fluid inclusions by Raman spectroscopy. Calibration and application to the external part of the Central Alps (Switzerland)," Chemical Geology, vol. 378 379, pp. 52-61, 2014. [18] "IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers," IEEE Std C57.104-2008 (Revision of IEEE Std C57.1041991), pp. 1-36, 2009.

The distribution range the deviation limits to 0.02 V. Generally speaking, small deviation and low measurement error is guaranteed. IV.

CONCLUSION

(1) Due to the mid-infrared laser is such expensive, the central wavelength of 1653.72 nm in near infrared spectrum is selected. Then long optical length gas cell by means of refraction and reflection more than 20 times is built in the laboratory. At last, at different concentrations ranging from 50 1000 L/L, the harmonic signal waveforms are obtained. The lower detection limit of this developed methane system is estimated as 1.28 L/L and the deviation is less than 0.02 V. High sensitivity of ppm-level concentration in near infrared is achieved. (2) Based on based on Beer-Lambert spectral absorption law, low concentration methane detection with tunable diode laser absorption spectrum is proposed in this paper for the advantages of high sensitivity and resolution. The successful detection lay good foundation for methane detection in power transformer. REFERENCES [1] A. Emsley and G. Stevens, "Review of chemical indicators of degradation of cellulosic electrical paper insulation in oil-filled transformers," IEE Proceedings-Science, Measurement and Technology, vol. 141, pp. 324334, 1994. [2] S.-w. Fei, M.-J. Wang, Y.-b. Miao, J. Tu, and C.-l. Liu, "Particle swarm optimization-based support vector machine for forecasting dissolved gases content in power transformer oil," Energy Conversion and Management, vol. 50, pp. 1604-1609, 2009. [3] Y. Inoue, K. Suganuma, M. Kamba, and M. Kikkawa, "Development of oil-dissolved hydrogen gas detector for diagnosis of transformers," Power Delivery, IEEE Transactions on, vol. 5, pp. 226-232, 1990. [4] M. Duval, "Dissolved gas analysis: It can save your transformer," Electrical Insulation Magazine, IEEE, vol. 5, pp. 22-27, 1989. [5] L. W. Pierce, "An investigation of the thermal performance of an oil filled transformer winding," Power Delivery, IEEE Transactions on, vol. 7, pp. 1347-1358, 1992. [6] J. Ding, X. Li, J. Cao, L. Sheng, L. Yin, and X. Xu, "New sensor for gases dissolved in transformer oil based on solid oxide fuel cell," Sensors and Actuators B: Chemical, vol. 202, pp. 232-239, 2014. [7] A. Sane, A. Satija, R. P. Lucht, and J. P. Gore, "Simultaneous CO concentration and temperature measurements using tunable diode laser absorption spectroscopy near 2.3 m," Applied Physics B, vol. 117, pp. 718, 2014. [8] H. R. Melroy, E. M. Adkins, M. J. Pause, and J. H. Miller, "Species measurements in a nitrogen-diluted, ethylene air diffusion flame using direct sampling mass spectrometry and tunable diode laser absorption spectroscopy," Proceedings of the Combustion Institute, vol. 35, pp. 3749-3755, 2015. [9] D. R. Bowling, S. D. Sargent, B. D. Tanner, and J. R. Ehleringer, "Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem atmosphere CO 2 exchange," Agricultural and Forest Meteorology, vol. 118, pp. 1-19, 2003. [10] T. Iseki, H. Tai, and K. Kimura, "A portable remote methane sensor using a tunable diode laser," Measurement Science and Technology, vol. 11, p. 594, 2000. [11] Q. Gao, Y. Zhang, J. Yu, S. Wu, Z. Zhang, F. Zheng, et al., "Tunable multi-mode diode laser absorption spectroscopy for methane detection," Sensors and Actuators A: Physical, vol. 199, pp. 106-110, 2013. [12] Y. Chen, S. Mahdi, and G. Anderson, "A Double FFT Procedure to Improve the Sensitivity of a Tunable Diode Laser Open Path Spectrometer for Sensing Local Atmospheric Gas Concentrations on

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