TDLAS-Based Detection of Dissolved Methane in Power Transformer ...

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1

TDLAS-Based Detection of Dissolved Methane in Power Transformer Oil and Field Application Jun JIANG, Member, IEEE, Mingxin ZHAO, Guo-ming MA, Member, IEEE, Hong-Tu Song, Cheng-rong LI, Senior Member, IEEE, Xiao HAN, Chaohai ZHANG  Abstract— Methane is one of the indicative gases in power transformer oil, and the detection of methane dissolved in oil with high accuracy is of great importance for dissolved gases analysis (DGA) and fault diagnosis inside power transformers. Based on Beer-Lambert spectral absorption law, dissolved methane detection with tunable diode laser absorption spectrum (TDLAS) is proposed in this paper for the advantages of high sensitivity and resolution. On the basis of wavelength modulation spectroscopy (WMS), a specialized TDLAS system was established. To meet the actual needs of field testing, the anti-vibration design of integrated Herriott cell and gas pressure (P)/ temperature (T) setting are worked out. Photodetector (PD), collimator, and Herriott cell are integrated into one component to reduce to reduce the effects of vibration. It is investigated that the temperature has little effect on the second harmonic amplitude in the range of 30 ℃ ~ 50 ℃ , and the vacuum pressure is reasonably set at about 1 kPa. Experimental results showed that the resolution of sensitivity could be reached as 6.8 mV/(μL/L), the maximum deviation was less than ±4 μL/L, and the response time is less than 5 minutes. In the end, field application was also carried out, proving it is a prospective online sensing technique to serve oil-immersed power transformers better. Index Terms —Transformer oil; Oil-immersed transformers; Methane; Infrared absorption spectrum; Dissolved gases analysis

I. INTRODUCTION

O

il-immersed power transformers are the critical equipment to step up/down line voltage and transfer electrical energy in power grids. In-service failures of power transformers may occur due to electrical, mechanical or thermal faults, which are perilous to the power grids and social expenses, Manuscript received XXXX, 2017. This work was supported in part by This work was supported by Natural Science Foundation of Jiangsu Province (BK20170786), the Fundamental Research Funds for the Central Universities (NO. XCA17003-04, JB2015RCY02), National Natural Science Foundation of China (Grant No.51677070), Young Elite Scientists Sponsorship Program by CAST, initial funding of new faculty in Nanjing University of Aeronautics and Astronautics (Grant No. 90YAH16081) and the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources(Grant No. LAPS17012). Jun Jiang, Mingxin Zhao, Xiao Han and Chaohai Zhang are with the Center for more-electric-aircraft power system, Nanjing University of Aeronautics and Astronautics, Nanjing, P.R.China, 210016 (e-mail: [email protected], [email protected], [email protected], [email protected]); Guoming Ma and Chengrong Li are with are with the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, P.R.China,102206 and Beijing Key Laboratory of High Voltage and EMC, North China Electric Power University, Beijing, P.R.China, 102206 (e-mail: [email protected]; [email protected]); Hongtu Song is with the State grid Jinzhong power supply company, Jinzhong, P.R.China, 030600 (e-mail: [email protected]).

especially for the raise of global energy internet (GEI) [1,2]. It is of great significance to monitor the health status through detection of dissolved gases in transformer oil due to decomposition of insulation oil under abnormal condition, known as dissolved gases analysis (DGA) [3,4]. Methane is one of the indicative gases in power transformer oil and associated with the presence of overheated oil or partial discharges. Generally speaking, at a hot spot with temperatures higher than 150 °C, methane is produced through complex chemical reactions and stay dissolved in the oil [5]. Thus, the detection of methane dissolved in oil with high accuracy is of great importance for dissolved gases analysis and incipient fault diagnosis inside power transformers [6-9]. Generally, methane is widely distributed in nature, and various of measurements have been researched and developed [10-13]. Different from the methane concentration detection at % level in most occasions, dissolved methane in oil is always at ppm (parts per million) level [14]. The high precision detection of methane dissolved in oil lies the foundation for analysis and judgment of transformer incipient fault. Usually, several measurement techniques have been employed in the field of dissolved gases analysis including methane detection, such as chromatography (GC), catalyst combustion, electrochemical, semiconductor, optical method, et al. [8, 15-18]. As the most common multi-component gas analysis for decades, GC is mainly suitable for offline and laboratory measurement. Catalyst combustion, electrochemical, semiconductor are techniques related to metal oxides, which have been widely used for the development of methane gas sensors. However, most of the metal oxides based sensors can only detect methane at high temperatures (higher than 300 °C) [5, 19]. Therefore, recent efforts have been focused on reducing the reacting temperature by doping different metals/alloys or making nanocomposites. Nevertheless, the drawbacks of cross sensitivity, complicated samples, poisoning risks and baseline drifts are hard to solve. Besides, chromatographic columns and gas sensors used in online monitoring devices of dissolved gases analysis in transformer oil easily suffer from the consumption of carrier gas and the failure of long stability. Due to the long oil-gas separation processing time in existed online techniques, every detection cycle takes hours or more. Optical sensing has the advantages of non-contact, simultaneous measurement, instant detection, natural safety, immunity to electromagnetic interference (EMI) [20-22], easily adapts to harsh environment like high temperature, heavy dusts and high humidity, and promotes the access to gases sensing. Typically, photoacoustic spectrometry (PAS)

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2 [23-25] is a sensitive gas analyzing method free of gas consumption and therefore overcome some disadvantages of the previous online monitoring methods. Unfortunately, this technique relies on the photoacoustic effect and is too sensitive to noise or vibration. Regarding online DGA monitoring, the detection would be affected because of the heavy noises and vibration interference in a substation environment seriously [26]. Accordingly, a novel detection technique is essential. Methane, as a typical flammable gas, is focused on the lower explosive limit (LEL) monitoring for gas-and-oil industries. The detection of methane based on non-dispersive infrared (NDIR) technology shows obvious advantages in terms of sensitivity, high zero stability, selectivity, high speed of response and so on [27-29], proving a promising reference in the field of dissolved methane measurement of power transformer oil. In view of the above consideration, methane detection based on tunable diode laser absorption spectrum (TDLAS) was proposed in reference. Different from PAS, TDLAS makes full use of pure spectrum absorption of individual gas, and the noises interference can be ignored. Since it has the advantages of high speed and high sensitivity, TDLAS technique has been applied in atmosphere detection previously [30, 31]. To investigate dissolved methane detection in power transformer oil, specialized laser with high precision and high reliability, Herriott cell with long optical path, and wavelength modulation spectrum are discussed in this manuscript. In the end, a methane detection system with a relatively high resolution for potential monitoring of dissolved methane in power transformers has been developed in the study. II. PRINCIPLE OF TUNABLE DIODE LASER ABSORPTION SPECTRUM The TDLAS line scan may either be a straightforward linear sweep of the wavelength (direct absorption) or include an additional modulation on top of the wavelength sweep (wavelength modulation spectroscopy, WMS) [13, 30]. The latter approach gives a better signal-to-noise ratio and clearer identification of the baseline beneath an absorption line, as shown in Fig.1.

value of injection current. To modulate the absorption spectrum, a cosine current is applied on the injection current

iic to drive LD.

iic  icen   iia cos mt

(1)

Where

i are central current value and amplitude of

icen and

 ia

cosine current.

m

is the modulated frequency.

Then, the instantaneous frequency output of LD can be expressed as

  cen   cos mt

(2)

Where

cen

and





are central frequency value and amplitude of

wavelength. The intensity of emitted laser is given in

I [ (t )]  I 0 [ (t )]  exp [ (t )]  c  L

(3)

In the application of tracing gas detection, it is can be seen that

[ (t )]  c  L 1

(4)

So the intensity of emitted laser is simplified as

I [ (t )]  I 0[ (t )]  1  [ (t )]  c  L

(5)

On ideal condition, suppose the output intensity of laser keep stable independent on the output wavelength. That is to say,

I 0 [ (t )]  I 0 (0 )  I 0

(6)

Set   mt , Eq.(6) can be written as 

I (cen , )=I 0  A n (cen )  cons(n )

(7)

n 0

Where An (cen ) is harmonic component;

n refers to the order of harmonic component. 2c  L



 (cen   cos  ) cons(n ) d  (8)  0 In which, the harmonic component can be obtained by lock-in amplifier. It is easy to find that there is certain relationship between gas concentration to be detected and harmonic component. A n (cen ) 

At the point of

cen , Taylor series of absorption coefficient is

calculated as

A n (cen ) 

Fig. 1. wave

Illustration of gas concentration measurement by second harmonic

In our case, the output of laser diode (LD) is controlled by the

21-n c  L n d n  n！ d n

 cen

(9)

The linear relationship of harmonic component and absorption coefficient is confirmed. Absorption coefficient of odd order harmonic component is zero at the central position, while even order is the maximum. Since the amplitude of harmonic signal decrease with increasing order, the 2nd harmonic component is selected as the detection signal in practice. Through the peak-peak value of 2nd harmonic signal from lock-in amplifier, the concentration information of to-be-detected gas can be inverted.

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3 Rather than the relative strength of the direct absorption spectroscopy, wavelength modulation spectroscopy (WMS) method is used for the measurement of harmonic signal, which is proportional to the gas concentration. Thus, it is independent on the background spectrum and improves noise rejection. III. EXPERIMENTAL SETUP OF DISSOLVED METHANE DETECTION A．Experimental setup The experimental system is made up of several main parts: oil container, Herriott cell, DFB (Distributed Feedback) laser, TDLAS controller, vacuum pump, collimator and photodetector (PD), as shown in Fig. 2. Among them, the tunable diode laser is customized at the central wavelength of 1653.72 nm (manufactured by Nanosystems and Technologies GmbH, Germany), which is mounted in a Butterfly with TEC and plugged by a FC/APC connector. The typical output power is 3 mW. Collimator of small spot size (less than 5 micrometers) and large area InGaAs photodiode (diameter of photosensitive area=1.5 mm) are used in the special sensing application. High spectral response in the region 800 nm to 1700 nm, and the spectral response can be seen in Fig. 3. With regard to the temperature stability of laser source, a WTC3243 thermoelectric (TEC) controller module is used. Vacuum pump is used to prepare vacuum environment of the system and exclude miscellaneous gas. Each time approximate 400 mL oil sample was prepared, of which 300 mL was used to carry out the oil gas separation and TDLAS detection.

Fig. 3.

In addition to a laser current driver and detector signal processing electronics for generating 2f spectra, the board also contains bias and conditioning circuitry for common temperature sensors (thermistor) and pressure sensors (amplified radiometric sensors). These environmental parameters enable the spectrum processing necessary to produce a gas concentration from the measured spectra. The methane absorbs near infrared spectrum so weakly that the strength order is close to 10-21 level. In addition, it is especially significant at near-infrared wavelengths, if absorption cross-sections are small. Multiple-pass (multi-pass) or long path absorption cells are usually used in spectroscopy absorption to measure low-concentration components or to observe weak spectra in gases. Different from other long-optical-path absorption cells like the White Cell, the Herriott Cell inherits the opto-mechanical stability qualities and is more stable to small perturbations. Moreover, Herriott Cell provides a relatively long absorption path in a compact design and it is much simpler than high-finesse optical cavities (which would typically require spatial mode matching, precise optical alignment, or resonant excitation), making it more suitable for engineering applications [32, 33]. With these considerations, we decide to utilize Herriott cell to methane sensing in our study.

Experimental setup of TDLAS system

1653.72 nm

Responsivity (A/W)

Fig. 2.

Second harmonic (2f) spectrometer controller is integrated in a double-sided printed circuit board (PCB) for use with tunable diode lasers that operate central wavelength at drive currents below 200 mA. It is based on a field programmable gate array (FPGA) interfaced to an on-board flash for storage of nonvolatile parameters, as shown in Fig. 4. The board can also be used to control a laser for direct transmission operation by setting the 1f modulation amplitude to zero and monitoring only the DC channel output.

Fig. 4. Block diagram of TDLAS controller

Wavelength (nm) Spectral response of InGaAs photodiode

Reflection spot pattern of far-end and near-end mirrors is shown in Fig. 5, and the serial numbers represent the light reflected on the mirror sequence. Among them, 0/34 point is the location of the coupling window at the near-end side. The incident angle of laser should be appropriate, then the

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4 reflection light from point 33 to point 34 can emit from the coupling window, just opposite to the incident angle between X-axis. Emitted light travel out in different path and detected by a photodetector (PD). To reduce the sensitivity of vibration, we integrate the PD, collimator, and Herriott cell into one component. The multipass gas cell has been fabricated in the laboratory, its structure is shown in Fig. 6.

Fig. 5. Reflection spot pattern of far-end and near-end mirrors

Fig. 6.

damaged and there are more gases decomposed from the insulation material. That is to say, we can control the high voltage values and impulse times to prepare the oil samples with different concentrations of gases. As mentioned above, the oil samples were obtained through impulse high voltage generators by stressing various impulse times and voltage values. Every time, certain amount of pure standard transformer oil samples were prepared at different impulse fault severities and 40 mL of them were extracted for oil-gas separation according to conventional GC measurement, as shown in Fig. 7. In this study, nine types of oil samples (marked M0, M1, … , M8) were prepared. Among them, M0 is the fresh oil without any process for reference. It is obvious to see that the color of the oil samples from M1 to M8 becomes deep and black, which is to simulate the electric breakdown faults and aging degree in different levels. And the concentration of the oil samples also prove the varying degradation degrees.

Structure of integrated Herriott cell

The length of the gas cell is about 0.3 m, and the length of effective light path reaches 10.2 m approximately. Thus the fabricated cell has advantages of small size, long optical path, strong absorbance and high sensitivity. B. Procedures of experiment The subject power transformer oil is KARAMAY oil 25# (Type KI25X) in this study. According to related field routine, it was processed by vacuum degasification, moisture elimination, and purification to meet the quality standards of power transformers in site conditions. According to the field knowledge and experiences, the concentrations of dissolved gases represent the health status of transformer oil, regardless of fresh, stressed and aged oil. So it is an acceptable alternative solution to indicate the status of oil through controlling the concentration of the dissolved gases. Since the natural aging process of the oil lasts very long time at working conditions, as long as decades. Whereas, the insulation oil easily cracks into gases form and are dissolved in the oil especially in the case of thermal or electrical failures. And the rate of decomposition and the type of gases change during defective operation depends on the thermal overloading and/or electrical faults. According to the principle, an accelerated method is put forward in this study to prepare oil samples with the help of impulse high voltage generators. The higher impulse and more times of stressing impulse voltage applied on the insulation oil is, the more severe the oil gets

Fig. 7. Oil samples before oil-gas separation by conventional DGA (40 mL) with different impulse fault severities (heavy to light: from left to right)

The concentration of 7 typical gases were measured by conventional DGA method at first, and the equipment type was Agilent 7890B. And one of the detection was illustrated in Fig. 8.

Fig. 8.

Typical gas chromatography detection of oil sample

Main procedures of TDLAS detection include preparation of oil samples, vacuum extraction, oil injection, oil gas separation, methane detection and data recording & processing. Practices proved that the developed system could complete the detection within 5 minutes each time, much more prompt than conventional methods. Therefore, it is quite attractive to online monitoring of insulation oil in power transformer. IV. EXPERIMENTAL RESULTS AND DISCUSSION A．Influences of Temperature and Pressure

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5

In the heating process, gas pressure in a slight increase, less impact on the test results can be ignored. As the temperature increases, the detected 2f signal amplitude gradually decreased, and amplitude attenuation reached about 45% at the temperature of higher than 50 ℃. High temperature gives more kinetic energy of the gas molecules, accordingly, excited photon energy consumes less and the absorption intensity decreases. In the temperature range of 30 ℃ ~ 50 ℃, the temperature has little effect on the second harmonic amplitude.

prolong the testing cycle. Therefore, the detection of pressure is reasonably set at about 1 kPa. 0.08

0.3 kPa 0.5 kPa 1.3 kPa 2.3 kPa 3.3 kPa 5.9 kPa

0.06 0.04

2f signal (V)

To investigate and predict the influences of temperature and pressure in this system, 500 μL/L methane detection based on TDLAS was carried out at different temperatures and pressures, as shown in Fig. 9 and Fig. 10. Wherein, 2f signal (A. U.) was normalized peak-peak value.

0.02 0.00 -0.02 -0.04 -0.06 -0.08

0

20

40

60

80

100

120

140

160

180

Time (ms) 1.0

0.8

30 ℃ 40 ℃ 50 ℃ 60 ℃ 70 ℃ 80 ℃

2f signal (V)

0.6 0.4

2f signal=0.011VP2-0.175VP+1.062 R2=0.997 (VP -Vacuum Pressure)

0.8 0.7 0.6 0.5 0.4 0.0

1.0

2.0

3.0

4.0

5.0

6.0

Vacuum Pressure (kPa)

0.2

Fig. 10. 500μL/L methane detection and fitting at different pressures 0.0 -0.2 -0.4

B. Dissolved methane detection in transformer oil 0

20

40

60

80

100

120

140

160

180

Time (ms) 1.0 0.9

2f signal (A.U.)

0.9

2f signal (A.U.)

The pressure of normal atmosphere is 101.3 kPa, and vacuum pressure refers to the real value in Herriott cell. As the pressure increases, the 2f signal showed a downward trend. The measured 2f signal at 1.3 kPa was about 18% higher than that of 5.9 kPa, and reduced by about 13% at the pressure of 0.3 kPa.

0.8 0.7

2f signal=-2.11E(-4)T2+1.39E(-2)T+0.769 R2=0.959

0.6 0.5 30

40

50

60

70

80

Temperature (℃) Fig. 9. 500μL/L methane detection and fitting at different temperatures

At low pressure, the magnitude of the detected second harmonic increased slightly. And low pressure also helps to improve the efficiency of oil and gas separation, so the low pressure is preferred. However, extremely low pressure was strict for tightness and high performance of vacuum pump system, moreover it would take more evacuation time and

Since the cross sensitivity of different gases have been investigated, the exposure to other gases makes little difference on the results of dissolved methane detection [29]. The following analysis and detection put emphasis on the 2 nd harmonic signal versus dissolved methane concentration. The linear relationship of harmonic component and absorption coefficient is calculated according to Equation (9). Then a linear fitting between peak-peak value of 2f signal and dissolved methane concentration is worked out, as shown in Fig.11. The R-square coefficient of this fitting is as high as 0.996. The slope value, 6.8 mV/(μL/L), can be seen as the resolution of the system sensitivity. With the help of the linear fitting and conventional GC measurement, the deviation values are easily to know. Subtracting the fitted values from the GC test values, the deviation errors are listed in Table. 1. The precision of the developed TDLAS methane system is proved that the maximum residual error was less than ±4 μL/L. In practice, through the peak-peak value of 2nd harmonic signal from lock-in amplifier, the concentration information of to-be-detected gas can be inverted. Actually, the sensitivity of the system can be adjusted through controlling the gain of lock-in amplifier. However,

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6 higher sensitivity with a large gain, the measuring span will be limited.

The developed equipment was applied in field at a main power transformer (Rated power: 150/150/75 MVA; Rated voltage: 220±8×1.5%/121/11 kV) in China Southern Power Grid, as shown in Fig. 13. 220kV oil-immersed power transformer

Fig. 11. The linear fitting of 2nd harmonic signal and dissolved methane in power transformer oil TABLE 1 Deviation analysis of the TDLAS-based methane measurement

Oil samples M1

CH4 (μL/L) 8.7

2f value (V) 0.058

2f fitting value (V) 0.055

Deviation (µL/L) -3.5

M2

19.9

0.177

0.176

3.0

M3

42.1

0.282

0.282

-3.3

M4

46.2

0.342

0.341

1.2

M5

70.3

0.504

0.504

1.2

M6

79.6

0.586

0.586

3.9

M7

134.7

0.928

0.928

-0.4

M8

136.7

0.928

0.929

-2.1

Note: Deviation values are calculated with reference of conventional GC method.

C. Field application of the detection system Base on TDLAS principle, a new online DGA equipment was developed and installed to a power transformer in the field. There are several units in the new DGA equipment, including oil-gas extraction module, gas detection module, power module, et al., as shown in Fig. 12.

Fig. 12

Setup of developed equipment for practical engineering application

Existed detection equipment

Fig. 13

Developed detection equipment Field application in China Southern Power Grid

Compared with the traditional DGA equipment, the developed equipment has worked well since January in 2016 and the volume is just the 2/3 of the traditional one. It is clear that the developed dissolved methane technique successfully monitored the increasing concentration with DGA value, exhibiting it is an alternative for promising use in condition monitoring of power transformers. V. CONCLUSION Briefly, instead of conventional DGA method, a novel dissolved gas detection method based on tunable diode laser absorption spectrum (TDLAS) is presented in this paper. As one of the sensing techniques for quantitative measurements of gases, TDLAS works by scanning a single absorption line of the gas using a single narrow laser line, providing high sensitivity, resolution and fast measurement. Main procedures of TDLAS detection include preparation of oil samples, vacuum extraction, oil injection, oil gas separation, methane detection and data recording & processing. Environmental parameters and influences of temperature and vacuum pressure have been investigated in the laboratory. Transformer oil samples of different methane concentrations were prepared to be detected. Experimental results showed that the temperature has little effect on the second harmonic amplitude in the range of 30 ℃ ~ 50 ℃. The vacuum pressure is reasonably set at about 1 kPa. As well, it is very important to have a methane detector that can be applied straightly in the field and gives a quick measurement response within 5 minutes, while conventional methods need several hours or more. In addition, the resolution of sensitivity could be reached as 6.8 mV/(μL/L), the maximum residual error was less than 4 μL/L, satisfying the latest IEEE standard for dissolved gas detection of oil-immersed transformer.

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7 Other dissolved gases like acetylene, ethylene, et al. in the power transformer oil, are likely to be detected by the TDLAS-based techniques. The work gives an evidence to dissolved gases analysis by using TDLAS technique and more research of the dissolved gases should be done in the near future. ACKNOWLEDGEMENT The authors thank Prof. Zhenhui Du from School of Precision Instrument and Opto-electronics Engineering, Tianjin University for his advices on the TDLAS system. REFERENCES [1] P. Yi, T. Zhu, B. Jiang, R. Jin and B. Wang, "Deploying Energy Routers in an Energy Internet Based on Electric Vehicles," IEEE Transactions on Vehicular Technology, vol. 65, no. 6, pp. 4714-4725, 2016. [2] M. Singh, V. Khadkikar, A. Chandra and R. K. Varma, "Grid Interconnection of Renewable Energy Sources at the Distribution Level With Power-Quality Improvement Features," IEEE Transactions on Power Delivery, vol. 26, no. 1, pp. 307-315, Jan. 2011. [3] Ma, Guo-Ming, CR Li, RD Mu, J Jiang and YT Luo, "Fiber Bragg grating sensor for hydrogen detection in power transformers," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 21, no. 1, pp. 380-385, 2014 [4] Su Q., Mi C., Lai L. L., et al. A fuzzy dissolved gas analysis method for the diagnosis of multiple incipient faults in a transformer [J]. IEEE Transactions on Power Systems, vol. 15, no. 2, pp. 593-598, 2000 [5] Sun, Chenhu, Paul R. Ohodnicki, and Emma M. Stewart. "Chemical Sensing Strategies for Real-Time Monitoring of Transformer Oil: A Review," IEEE Sensors Journal, vol. 17, no. 18, pp. 5786-5806, 2017. [6] F. Jakob and J. J. Dukarm, "Thermodynamic Estimation of Transformer Fault Severity," IEEE Transactions on Power Delivery, vol. 30, no. 4, pp. 1941-1948, 2015. [7] S. A. Wani, M. U. Farooque, S. A. Khan, D. Gupta and M. A. Khan, "Fault severity determination in transformers using dissolved gas analysis(DGA)," 2015 Annual IEEE India Conference (INDICON), pp. 1-6, New Delhi, India, 2015 [8] M. Duval and J. Dukarm, "Improving the reliability of transformer gas-in-oil diagnosis," IEEE Electrical Insulation Magazine, vol. 21, no. 4, pp. 21-27, 2005. [9] R. Blue, D. G. Uttamchandani and O. Farish, "A novel optical sensor for the measurement of furfuraldehyde in transformer oil," IEEE Transactions on Instrumentation and Measurement, vol. 47, no. 4, pp. 964-966, 1998. [10] 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. [11] 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 Mars," Laser Applications to Chemical, Security and Environmental Analysis (LACSEA), paper no. LT6B, vol. 9, 2012. [12] C. V. Horii, "Tropospheric Reactive Nitrogen Speciation, Deposition, and Chemistry at Harvard Forest," PhD, Atmospheric Sciences Group, Harvard University, Boston, 2002. [13] 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. [14] "IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers," IEEE Std C57.104-2008 (Revision of IEEE Std C57.104-1991), pp. 1-36, 2009. [15] K. F. Thang, R. K. Aggarwal, A. J. McGrail and D. G. Esp, "Analysis of power transformer dissolved gas data using the self-organizing map," IEEE Transactions on Power Delivery, vol. 18, no. 4, pp. 1241-1248, 2003. [16] A. Somov, A. Baranov, D. Spirjakin, A. Spirjakin, V. Sleptsov, R. Passerone, “Deployment and evaluation of a wireless sensor network for methane leak detection,” Sensors and Actuators, A: Physics, vol. 202, pp. 217–225, 2013.

[17] X. Yu, R.-H. Lv, F. Song, C.-T. Zheng, Y.-D. Wang, “Pocket-sized non-dispersiveinfrared methane detection device using two-parameter temperature compensation,” Spectrosc. Lett., vol. 47, pp. 30-37, 2014. [18] M. Choudhary, V.N. Mishra, R. Dwivedi, “Pd-doped tin-oxide-based thick-film sensor array for detection of H2, CH4, and CO,” J. Electron. Mater. vol. 42, pp. 2793–2802, 2013. [19] N. M. Vuong et al., “Ni2O3-decorated SnO2 particulate films for methane gas sensors,” Sensors and Actuators B, Chemical, vol. 192, pp. 327–333, 2014. [20] Guo-ming Ma, Cheng-rong Li, Ying-ting Luo, Rui-duo Mu, Liang Wang, “High sensitive and reliable fiber Bragg grating hydrogen sensor for fault detection of power transformer,” Sensors and Actuators B: Chemical, vol. 169, pp. 195-198, 2012. [21] J. Jiang, H.-T. Song, G.-M. Ma, C. R. Li, Y.-T. Luo, and H.-B. Wang, " Highly Sensitive Detection of Methane Based on Tunable Diode Laser Absorption Spectrum," 2016 IEEE International Instrumentation and Measurement Technology Conference (I2MTC), pp. 104-108, Taibei, 2016 [22] I. Búa-Núñez, J. E. Posada-Román, J. Rubio-Serrano and J. A. Garcia-Souto, "Instrumentation System for Location of Partial Discharges Using Acoustic Detection With Piezoelectric Transducers and Optical Fiber Sensors," in IEEE Transactions on Instrumentation and Measurement, vol. 63, no. 5, pp. 1002-1013, 2014. [23] Mao, Zhixin, and Jingyu Wen. "Detection of dissolved gas in oil-insulated electrical apparatus by photoacoustic spectroscopy." Electrical Insulation Magazine, IEEE, vol. 31, pp. 7-14, 2015. [24] E. A. Mackenzie, J. Crossey, A. dePablo and W. Ferguson, "On-line monitoring and diagnostics for power transformers," Electrical Insulation (ISEI), Conference Record of the 2010 IEEE International Symposium on, pp. 1-5, San Diego, CA, 2010. [25] Zhiying Wu, Yinhai gong, Qingxu Yu, Photoacoustic spectroscopy detection and extraction of discharge feature gases in transformer oil based on 1.5 μ tunable fiber laser, Infrared Physics & Technology, vol. 58, no. 1, pp. 86-90, 2013. [26] Fan, Jingmin, Feng Wang, Qiuqin Sun, Feng Bin, Huisheng Ye, and Yuhan Liu. "An Online Monitoring System for Oil Immersed Power Transformer Based on SnO2 GC Detector with a New Quantification Approach." IEEE Sensors Journal, vol. 17, no. 20, pp. 6662-6671, 2017. [27] Fanchenko, Sergey, Alexander Baranov, Alexey Savkin, Andrey Petukhov, Karina Kalinina, Bizhigit Zhurtanov, and Mikhail Velikotny. "Non-dispersive LED-based methane open path detector capabilities." 2015 IEEE Workshop on Environmental, Energy and Structural Monitoring Systems (EESMS), pp. 146-151, 2015. [28] Makeenkov, Andrey, Igor Lapitskiy, Andrey Somov, and Alexander Baranov. "Flammable gases and vapors of flammable liquids: Monitoring with infrared sensor node." Sensors and Actuators B: Chemical, vol. 209, pp. 1102-1107, 2015. [29] Jiang, J., Ma, G.M., Song, H.T., Zhou, H.Y., Li, C.R., Wang, H.B., Luo, Y.T. and Wu, H. Tracing methane dissolved in transformer oil by tunable diode laser absorption spectrum. IEEE Transactions on Dielectrics and Electrical Insulation, vol. 23, no. 6, pp. 3435-3442, 2016. [30] Li C., Dong L., Zheng C. and Tittel, F. K., “Compact TDLAS based optical sensor for ppb-level ethane detection by use of a 3.34 μm room-temperature CW interband cascade laser,” Sensors and Actuators B: Chemical, vol. 232, pp. 188-194, 2016. [31] Xin F., Guo J., Sun J., Li J., Zhao C. and Liu, Z. “Research on atmospheric CO2 remote sensing with open-path tunable diode laser absorption spectroscopy and comparison methods,” Optical Engineering, vol. 56, no. 6, pp. 066113, 2017. [32]Herriott, D.R. and Schulte, H.J. Folded optical delay lines. Applied Optics, vol. 4, no. 8, pp.883-889, 1965. [33] Robert, C., 2007. Simple, stable, and compact multiple-reflection optical cell for very long optical paths. Applied Optics, vol. 46, no. 22, pp. 5408-5418, 2007.

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