System for detecting dissolved gas in transformer oil and calibration-free detection method

Abstract

The invention relates to a system for detecting dissolved gas in transformer oil and a calibration-free detection method, and the system mainly comprises a transformer oil degassing unit, a current controller, a temperature controller, a laser, a gas absorption cell, an optical etalon, a photoelectric detector and a microprocessor, the system outputs the concentration of the dissolved gas in the transformer oil through a calibration-free detection method. According to the calibration-free detection method, after logarithmic transformation processing is carried out on light intensity information obtained by the system, residual analysis is carried out on the light intensity information and the simulated spectrum absorptivity, and a detection result is output after an iterative convergence condition is met. The system for detecting the gas dissolved in the oil and the calibration-free detection technology can improve the detection limit of the gas and the dynamic detection range, and have the advantages of being free of maintenance for a long time, stable, reliable and the like.

Description

Technical Field

[0001] The present invention relates to the field of monitoring of oil-immersed transformer equipment in power systems, and particularly to a detection system and a detection method for fault gases of an oil-immersed transformer. Background Art

[0002] A transformer is an important hub device in a power system. The stable operation of the transformer is a prerequisite for improving power supply reliability and building a stable smart grid. Currently, the basic type of large transformers is oil-immersed transformers. Transformer oil is a mineral oil mainly composed of hydrocarbon compounds. When the transformer ages or fails, the transformer oil will crack to generate gases such as acetylene. Gas detection is of great significance for monitoring early transformer faults. Therefore, the technology for detecting dissolved gases in transformer oil plays an important role in monitoring the operation status of the transformer and preventing power outages in the power system caused by transformer failures.

[0003] Currently, the main methods for detecting dissolved gases in transformer oil include technologies such as gas chromatography and photoacoustic spectroscopy. Gas chromatography separates gases through a chromatographic column to obtain the concentration of gases in the oil. It has the advantages of mature technology and low cost, but has deficiencies such as poor detection repeatability, rapid attenuation of long-term detection performance, the need to regularly replace the chromatographic column, regularly replace the carrier gas, and the system needs to be calibrated. Photoacoustic spectroscopy detects the gas concentration by detecting the periodic fluctuations of the gas pressure through a microphone. It has the advantage of low cost, but is easily affected by external environmental factors such as the vibration of the transformer.

[0004] Laser absorption spectroscopy technology is based on Beer-Lambert's law: when a laser irradiates the gas to be measured, gas molecules will absorb the laser energy corresponding to the wavelength. By detecting the intensity of the outgoing light, the corresponding gas concentration information can be inverted. Since different gas molecules correspond to different absorption wavelengths, cross-interference between different gases in transformer oil can be avoided, and it has good detection accuracy. At the same time, this technology directly detects the outgoing light intensity information. Compared with the electro-optical-thermal-acoustic-electric multi-physical parameter conversion of photoacoustic spectroscopy technology, it reduces the intermediate conversion process and reduces the influence of environmental noise on the detection system, and has good system robustness.

[0005] Compared with gas chromatography technology, this technology eliminates the need to replace devices such as chromatographic columns, saving labor and time costs. However, this technology still needs to perform gas calibration to invert the gas concentration. However, since the components of the gas to be measured in the actual field environment are different from those of the standard gas, and the measurement environment may also change at any time, there may be certain deviations in the measurement data. Summary of the Invention

[0006] To solve the problems existing in the above laser absorption spectroscopy technology and make it have better detection stability in the monitoring of dissolved gases in transformer oil, the present invention provides a dissolved gas detection system in transformer oil based on a calibration-free detection technology.

[0007] To achieve the above object, the specific technical solutions provided by the present invention are as follows:

[0008] The first aspect of the present invention provides a dissolved gas detection system in transformer oil, including: a gas path part, an optical path part, and a circuit part;

[0009] The gas path part includes: a transformer oil degassing unit; the transformer oil degassing unit is used to separate the dissolved gases in the transformer oil and inject the gases into the optical path part for gas detection;

[0010] The optical path part includes: a laser, an optical fiber coupler, a gas absorption cell, an optical etalon, and a photodetector; the laser outputs tuned laser light, which is split by the optical fiber coupler, one path passes through the gas absorption cell, and the other path passes through the optical etalon; the photodetector receives the optical signals passing through the gas absorption cell and the optical etalon and performs photoelectric information conversion;

[0011] The circuit part includes: a current controller, a temperature controller, and a microprocessor; the current controller is used to output a current to tune the output frequency and light intensity of the laser; the temperature controller is used to control and monitor the temperature of the laser; the microprocessor is used to control the laser and analyze the concentration of dissolved gases in the transformer oil.

[0012] Preferably, the transformer oil degassing unit adopts the dynamic headspace method to separate the dissolved gases in the transformer oil and inject the gases into the gas absorption cell through a gas pump for gas detection.

[0013] Preferably, the current controller is used to superimpose a scanning triangular wave and a modulating sine wave to output a current, thereby tuning the output frequency of the laser;

[0014] The output driving current is the superposition of a low-frequency scanning triangular wave and a high-frequency modulating sine wave. The frequency range of the scanning triangular wave is 1 - 100 Hz, and the frequency range of the modulating sine wave is 1 - 100 kHz.

[0015] Preferably, the temperature controller is used to maintain the consistency of the laser temperature; the output driving temperature is a constant temperature of 23 °C, and the fluctuation range is controlled within ±0.2 °C.

[0016] The second aspect of the present invention provides a calibration-free detection method for dissolved gases in transformer oil, based on the above-mentioned dissolved gas detection system in transformer oil, including the following steps:

[0017] Step 1: Set the modulation depth a of the laser. The laser outputs tuned laser light, which passes through a gas absorption cell without the gas to be measured. The first light intensity measurement value of the laser is obtained through a photodetector.

[0018] Step 2: When there is the gas to be measured in the gas absorption cell, the laser outputs tuned laser light. After being split by an optical fiber coupler, it passes through the gas absorption cell through the absorption of the gas to be measured and an optical etalon respectively, and the second light intensity measurement value and frequency of the laser are obtained through a photodetector.

[0019] Step 3: The microprocessor calculates the measured spectral absorptivity of the gas to be measured based on the first light intensity measurement value after passing through the gas absorption cell without the gas to be measured and the second light intensity measurement value after passing through the gas absorption cell with the gas to be measured; based on the pressure P of the gas absorption cell, the set concentration x of the gas to be measured, the length L of the gas absorption cell, the absorption line intensity S(T) of the gas to be measured at temperature T, and the laser frequency obtained in Step 2, the simulated spectral absorptivity of the gas to be measured is calculated.

[0020] Step 4: Extract the second harmonic information of the measured spectral absorptivity and the simulated spectral absorptivity of the gas to be measured obtained in Step 3, and obtain the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s ;

[0021] Step 5: Calculate the residual between the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s . If it is less than the set convergence condition ε, output the measured value x of the concentration of the gas to be measured l ; if it is not less than the set convergence condition ε, update the modulation depth a and the concentration x of the gas to be measured, return to Step 1, and repeat the above steps until the convergence condition is met.

[0022] Preferably, Step 1 includes:

[0023] Step 1.1: Purge the gas absorption cell with air through a transformer oil degassing unit to clean the miscellaneous gases in the gas absorption cell.

[0024] Step 1.2: After the cleaning of the miscellaneous gases in Step 1.1 is completed, wait for the gas in the gas absorption cell to stand for a set duration, and execute Step 1.3;

[0025] Step 1.3: The laser outputs tuned laser light. After passing through the gas absorption cell, it is detected by a photodetector that receives the optical signal, and the first light intensity measurement value I 0,m (t) is obtained.

[0026] Preferably, Step 2 includes:

[0027] Step 2.1: The transformer oil degassing unit evacuates the gas absorption cell, and then decomposes the gas in the transformer oil by dynamic headspace as the gas to be measured, which is introduced into the gas absorption cell and sealed.

[0028] Step 2.2: After the sealing in Step 2.1 is completed, when the set gas standing time of the gas absorption cell is reached, Step 2.3 is executed.

[0029] Step 2.3: The laser outputs the tuned laser, which passes through the gas absorption cell. After being absorbed by the gas to be measured, it is detected by the photodetector that receives the optical signal, and the second light intensity measurement value I t,m (t) is obtained; at the same time, the laser outputs the tuned laser, which passes through the other high-resolution optical etalon of the coupler and is detected by the photodetector that receives the optical signal, so as to obtain the discrete relationship between the laser frequency and time.

[0030] Step 2.4: The microprocessor obtains the modulation depth a and the discrete relationship between the laser frequency and time, and obtains the frequency v t,m (t) of the laser after passing through the optical etalon by fitting.

[0031] Preferably, in Step 3, the microprocessor takes the logarithm of the first light intensity measurement value I 0,m (t) after passing through the gas absorption cell without the gas to be measured and the second light intensity measurement value I t,m (t) after passing through the gas absorption cell with the gas to be measured to obtain the measured spectral absorptivity α t,m (υ(t)) of the gas to be measured, which is expressed by the following formula:

[0032] α t,m (υ(t)) = ln(I 0,m (t)) - ln(I t,m (t)) (8)

[0033] In the formula:

[0034] ln(·) represents the logarithmic function with the natural constant e as the base;

[0035] At the same time, the microprocessor calculates the simulated spectral absorptivity of the gas to be measured according to the pressure P of the gas absorption cell, the set concentration x of the gas to be measured, the length L of the gas absorption cell, the absorption line intensity S(T) of the gas to be measured at temperature T, and the laser frequency υ t,m (t) obtained in Step 2, which is expressed by the following formula:

[0036]

[0037] In the formula:

[0038] α t,s(υ(t)) represents the simulated spectral absorption rate function of the gas to be measured;

[0039] are the Fourier series expansion coefficients;

[0040] is the absorption line shape function of the absorption spectral line of the gas to be measured.

[0041] Preferably, in step 4, the measured spectral absorption rate α t,m (υ(t)) of the gas to be measured in step 3 and the simulated spectral absorption rate function α t,s (υ(t)) of the gas to be measured are respectively sent to the digital lock-in amplifier module of the microprocessor to extract the second harmonic information of the signal, and the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s are obtained through a finite impulse response low-pass filter, and are expressed by the following formula:

[0042]

[0043]

[0044]

[0045] In the formula:

[0046] represents the outer product operation;

[0047] LPF represents the low-pass filter;

[0048] A L is the amplitude of the reference signal of the lock-in amplifier;

[0049] α t (t) represents the measured spectral absorption rate α t,m (v(t)) of the gas to be measured and the simulated spectral absorption rate function α t,s (v(t)) of the gas to be measured, which are respectively substituted during calculation;

[0050] is the phase of the reference signal of the lock-in amplifier, x 2f,t and y 2f,t are the cosine component and sine component output by the digital lock-in amplifier module;

[0051] S 2f,t represents the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s When the measured spectral absorption rate α t,m (υ(t)) of the gas to be measured is substituted into formula (10) and formula (11), formula (12) obtains the measured second harmonic information S2f,m ; When substituting Formulas (10) and (11) into the simulated spectral absorption rate function α t,s (υ(t)) of the gas to be measured, the simulated second harmonic information S is obtained from Formula (12). 2f,s ; Thus, the measured second harmonic information S can be obtained according to Formula (10). 2f,m and the simulated second harmonic information S 2f,s .

[0052] Preferably, in Step 5, updating the modulation depth a and the concentration x of the gas to be measured includes: first, fixing the modulation depth a and finding the optimal concentration x of the gas to be measured by the bisection method; then, fixing the optimal concentration x of the gas to be measured and finding the optimal modulation depth a by the bisection method.

[0053] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0054] The present invention uses laser absorption spectroscopy technology to detect the dissolved gases in transformer oil. The laser absorption spectroscopy technology eliminates the on-site application of consumables such as chromatographic columns and calibration gases, reducing the on-site maintenance cost; and this technology is not sensitive to external factors such as environmental vibration, improving the stability of the detection system. In addition, the calibration-free detection technology adopted by the present invention suppresses the second harmonic waveform distortion phenomenon caused by residual amplitude modulation, improving the detection limit and dynamic range of the dissolved gases in oil, and having advantages such as long-term maintenance-free, stable and reliable, etc. Description of the Drawings

[0055] Figure 1 is a schematic structural diagram of a dissolved gas detection system in transformer oil provided by an embodiment of the present invention;

[0056] Figure 1 In it: 108 is a current controller, 109 is a temperature controller, 101 is a laser, 102 is an optical fiber coupler, 103 is a gas absorption cell, 104 is an optical etalon, 105 is a transformer oil degassing unit, 106 is a photodetector, and 107 is a microprocessor.

[0057] Figure 2 is a logic flow chart of a gas calibration-free detection method provided by an embodiment of the present invention. Detailed Embodiments

[0058] In order to enable those skilled in the art to more clearly understand the solution and principle of the present invention, the following will be described in detail with reference to the drawings and specific embodiments. The described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. General substitutions well known to those skilled in the art are also covered by the protection scope of the present invention.

[0059] It should be noted that in the following, acetylene is used as the gas to be measured to introduce the implementation manners of the invention. However, the dissolved gas detection system for transformer oil provided by the present invention is applicable to the detection of any gas dissolved in transformer oil, such as but not limited to, methane, ethylene, ethane, carbon monoxide, carbon dioxide, etc. Detecting acetylene is only an example but not a restrictive implementation manner. Detecting acetylene or other gases dissolved in transformer oil using the dissolved gas detection system for transformer oil provided by the present invention falls within the scope of the present invention.

[0060] Embodiment 1 of the present application provides a dissolved gas detection system for transformer oil. Exemplarily, Figure 1 a dissolved acetylene detection system for transformer oil is shown. The system includes: a gas path part, an optical path part, and a circuit part.

[0061] The gas path part includes: a transformer oil degassing unit 105; the transformer oil degassing unit 105 separates the dissolved gas in the transformer oil by using the dynamic headspace method, and pumps the gas into the gas absorption cell 103 through a gas pump for gas detection. The transformer oil degassing unit 105 is also used to purge the residual gas in the gas absorption cell 103 and maintain the airtightness of the gas absorption cell 103.

[0062] The optical path part includes: a laser 101, an optical fiber coupler 102, a gas absorption cell 103, an optical etalon 104, and a photodetector 106 for receiving optical signals.

[0063] The laser 101 is a tunable semiconductor laser, and the working current range is 20 - 120 mA, and the working temperature range is 15 - 40 °C. The output frequency of the laser is driven by both current and temperature. The selection basis of the center frequency of the laser 101 is the absorption spectral line intensity and position of acetylene gas, and it is necessary to exclude the interference of air and other gases dissolved in the transformer oil. Therefore, a tunable laser 101 with a center frequency of 1532 nm is selected as the system light source.

[0064] It can be understood that if other gases are detected, similarly, under the condition of excluding the interference of air and other gases dissolved in the transformer oil, the center frequency of the tunable laser 101 is selected according to the absorption spectral line intensity and position of the gas to be measured.

[0065] Among them, the output frequency and output power of the laser 101 driven by a current controller are shown as follows:

[0066]

[0067]

[0068] In the formula:

[0069] v(t) represents the output frequency of the laser 101 at time t, which is the relationship between the output frequency of the tuned laser 101 and time;

[0070] represents the slow change in the laser frequency modulated by a triangular wave;

[0071] a is the modulation depth;

[0072] f m is the modulation frequency;

[0073] I0(t) is the optical intensity of the laser output from the tuned laser 101 at time t;

[0074] is the slow change in the optical intensity of the laser modulated by a triangular wave;

[0075] ΔI1 and ΔI2 are the linear and non - linear intensity modulations of the laser 101 respectively;

[0076] and are the phase differences between the frequency modulation and the linear and non - linear intensity modulations respectively.

[0077] The fiber coupler 102 splits light into the gas absorption cell channel and the optical etalon channel. Further, the splitting ratio of the fiber coupler 102 is 9:1. One path passes through the gas absorption cell 103 for gas concentration detection; one path passes through the etalon to obtain the discrete relationship between the laser frequency and time.

[0078] The gas absorption cell 103 uses a gas absorption cell with a long optical path of 300 cm and a small volume of 35 mL to detect acetylene gas.

[0079] The optical etalon 104 is used to obtain the discrete relationship between the laser frequency and time, and calibrates the measured laser frequency with a resolution of a free spectral range of 0.02 cm -1 .

[0080] The photodetector 106 is used to receive the optical signal and perform optoelectronic information conversion.

[0081] The circuit part includes: the current controller 108 of the laser, the temperature controller 109, and the microprocessor 107 for signal generation and demodulation.

[0082] The current controller 108 is used to superimpose a scanning triangular wave and a modulated sine wave to output a current, thereby tuning the output frequency of the laser; the output driving current is the superposition of a low-frequency scanning triangular wave and a high-frequency modulated sine wave. Preferably but not restrictively, the frequency range of the scanning triangular wave is 1 - 100 Hz. More preferably, in this embodiment, the frequency of the low-frequency scanning triangular wave is 10 Hz; the frequency range of the modulated sine wave is 1 - 100 kHz. More preferably, in this embodiment, the frequency of the high-frequency modulated sine wave is 10 kHz.

[0083] The temperature controller 109 is used to control and monitor the temperature of the laser, maintaining the consistency of the laser temperature; the output driving temperature is a constant temperature. Preferably but not restrictively, 23°C is selected in this embodiment, and the fluctuation range is controlled within ±0.2°C.

[0084] The microprocessor 107 is used to control the laser and analyze the concentration of dissolved gases in transformer oil, and demodulate the concentration of acetylene gas using a calibration-free gas detection method.

[0085] As Figure 2 shown, Embodiment 2 of the present invention provides a calibration-free detection method for dissolved gases in transformer oil. It should also be noted that in the following, acetylene is used as the gas to be measured to introduce the implementation manner of the invention. However, the calibration-free detection method provided by the present invention is applicable to the detection of any gas dissolved in any transformer oil. Detecting acetylene is only an example but not a restrictive implementation manner. Using the calibration-free detection method provided by the present invention to detect acetylene or other gases dissolved in transformer oil falls within the scope of the present invention.

[0086] The calibration-free detection method includes the following steps:

[0087] Step 1: When there is no acetylene gas in the gas absorption cell 103, set the modulation depth a of the laser 101. The laser 101 outputs the tuned laser, passes through the gas absorption cell 103 without acetylene gas, and obtains the first light intensity measurement value of the laser through the photodetector 106.

[0088] In a preferred but not restrictive implementation manner of the present invention, Step 1 specifically includes:

[0089] Step 1.1: Blow air through the gas absorption cell 103 by the transformer oil degassing unit 105 to clean the residual gases in the gas absorption cell 103;

[0090] Step 1.2: After the cleaning of the residual gases in Step 1.1 is completed, wait for the gas in the gas absorption cell 103 to stand for a set time. Preferably but not limited to, after standing for 15 s, execute Step 1.3;

[0091] Step 1.3: The laser 101 outputs the tuned laser. After passing through the gas absorption cell 103, it is detected by the photodetector 106 that receives the optical signal, and a first light intensity measurement value is obtained, which is expressed by the following formula:

[0092] I 0,m (t) = K·I0(t) (3)

[0093] In the formula:

[0094] I0(t) represents the original light intensity value at time t, which is the light intensity of the laser output by the laser 101 after being tuned by the scanning triangular wave and the modulating sine wave;

[0095] I 0,m (t) represents the first light intensity measurement value at time t, which is the light intensity measurement value detected by the photodetector 106 after the laser output by the laser 101 passes through the gas absorption cell 103 without acetylene;

[0096] K is the gain of the measurement system, including: optical path loss, detector responsivity, and circuit amplification factor.

[0097] Among them, the output frequency and output power of the laser 101 driven by the current controller are expressed by the following formula:

[0098]

[0099]

[0100] In the formula:

[0101] v(t) represents the output frequency of the laser 101 at time t, which is the relationship between the output frequency of the tuned laser 101 and time;

[0102] represents the slow change of the laser frequency modulated by the triangular wave;

[0103] a represents the modulation depth, and the initial value of the modulation depth a is 0.15 cm -1 ;

[0104] f m represents the modulation frequency;

[0105] I0(t) represents the light intensity of the laser output by the laser 101 after being tuned at time t;

[0106] represents the slow change of the laser light intensity modulated by the triangular wave;

[0107] ΔI1 and ΔI2 respectively represent the linear and nonlinear intensity modulations of the laser 101;

[0108] and respectively represent the phase differences of frequency modulation and linear and nonlinear intensity modulation.

[0109] Step 2: When there is acetylene gas in the gas absorption cell 103, the laser 101 outputs the tuned laser, which is split by the fiber optic coupler and then passes through the gas absorption cell 103 through the acetylene gas absorption and the optical etalon 104, and then the second light intensity measurement value and frequency of the laser are obtained through the photodetector 106.

[0110] In a preferred but non-limiting embodiment of the present invention, Step 2 specifically includes:

[0111] Step 2.1: The transformer oil degassing unit 105 evacuates the gas absorption cell 103, and then decomposes the gas in the transformer oil into the gas absorption cell 103 through the dynamic headspace method and seals it;

[0112] Step 2.2: After the sealing in Step 2.1 is completed, after waiting for the set gas standing time of the gas absorption cell 103, preferably but not limited to, after standing for 15 s, Step 2.3 is executed;

[0113] Step 2.3: The laser 101 outputs the tuned laser, which passes through the gas absorption cell 103, and after being absorbed by the acetylene gas, is detected by the photodetector 106 that receives the optical signal, and the second light intensity measurement value is obtained, which is expressed by the following formula:

[0114] I t,m (t) = K·I0(t)·exp(-α(v(t))) (6)

[0115] In the formula:

[0116] I t,m (t) represents the second light intensity measurement value at time t, which is the light intensity measurement value detected by the photodetector 106 after the tuned laser output by the laser 101 passes through the gas absorption cell 103 and is absorbed by the acetylene gas;

[0117] I0(t) represents the original light intensity value at time t, which is the light intensity of the laser output after the laser 101 is tuned by the scanning triangular wave and the modulating sine wave;

[0118] exp(·) represents the exponential function of the natural constant e;

[0119] α(·) represents the spectral absorption rate function of acetylene gas in the laser scanning frequency region;

[0120] v(t) represents the output frequency of the laser 101 at time t, which is the variation relationship of the output frequency of the tuned laser 101 with time;

[0121] Meanwhile, the laser 101 outputs the tuned laser, which passes through another high-resolution etalon 104 of the coupler 102 and is detected by the photodetector 106 that receives the optical signal, for obtaining the discrete relationship between the laser frequency and time.

[0122] Step 2.4: The microprocessor 107 obtains the modulation depth a and the discrete relationship between the laser frequency and time, and obtains the frequency of the laser after passing through the etalon 104 by fitting, which is expressed by the following formula:

[0123]

[0124] In the formula:

[0125] υ t,m (t) represents the frequency of the laser after being absorbed by acetylene gas through the gas absorption cell 103;

[0126] represents the slow change amount of the laser frequency modulated by a triangular wave;

[0127] a represents the modulation depth;

[0128] f m represents the modulation frequency;

[0129] F(t) is the fitting function of the frequency scanning triangular wave. Preferably but not limited to, in this example, it is selected as a second-order polynomial.

[0130] Step 3: The microprocessor 107 takes the logarithm of the first light intensity measurement value I 0,m (t) after passing through the acetylene-free gas absorption cell 103 and the second light intensity measurement value I t,m (t) after passing through the gas absorption cell 103 with acetylene gas to obtain the measured spectral absorptivity α t,m (υ(t)), which is expressed by the following formula:

[0131] α t,m (υ(t)) = ln(I 0,m (t)) - ln(I t,m (t)) (8)

[0132] In the formula:

[0133] ln(·) represents the logarithmic function with the natural constant e as the base;

[0134] Meanwhile, the microprocessor 107 calculates the simulated spectral absorptivity of acetylene gas according to the initial pressure P of the gas absorption cell 103, the set acetylene gas concentration x, the length L of the gas absorption cell, the absorption line intensity S(T) of acetylene gas at temperature T, and the laser frequency υ t,m (t) obtained in Step 2, which is expressed by the following formula:

[0135]

[0136] In the formula:

[0137] α t,s (υ(t)) represents the simulated spectral absorption rate function of acetylene gas;

[0138] are the Fourier series expansion coefficients;

[0139] is the absorption line shape function of the acetylene gas absorption line.

[0140] Preferably but not limitedly, in this example, the initial pressure P = 1 atm, the initial acetylene gas concentration x = 3 ppm, the gas absorption cell length L = 300 cm, the temperature T = 296 K, S(T) = 2.822×10 -3 cm -2 / atm, and the absorption line shape of the acetylene gas absorption line adopts the Voigt line shape function.

[0141] It should be noted that in step 3, the acetylene gas concentration x is set to iteratively update the acetylene gas concentration x by the calibration-free detection method in the present invention, and determine whether the convergence condition is satisfied in the subsequent steps. If the subsequent steps satisfy the convergence condition, then the acetylene gas concentration x used in the calculation in this iterative step 3, that is, the measured value x of the acetylene gas concentration to be measured l .

[0142] Step 4: Extract the second harmonic information of the measured spectral absorption rate of the acetylene gas and the simulated spectral absorption rate of the acetylene gas obtained in step 3, and obtain the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s .

[0143] In the preferred but non-limiting embodiment of the present invention, step 4 specifically includes:

[0144] Send the measured spectral absorption rate α t,m (υ(t)) of the acetylene gas in step 3 and the simulated spectral absorption rate function α t,s (υ(t)) of the acetylene gas into the digital lock-in amplifier module of the microprocessor 107 to extract the second harmonic information of the signal, and obtain the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s , which are expressed by the following formula:

[0145]

[0146]

[0147]

[0148] In the formula:

[0149] represents the outer product operation;

[0150] LPF represents a low-pass filter;

[0151] A L is the amplitude of the reference signal of the lock-in amplifier;

[0152] α t α(t) represents the measured spectral absorption rate α of acetylene gas t,m α(υ(t)) and the simulated spectral absorption rate function α of acetylene gas t,s α(v(t)), which are substituted respectively during calculation;

[0153] Υ is the phase of the reference signal of the lock-in amplifier, x 2f,t and y 2f,t are the cosine component and sine component output by the digital lock-in amplification module;

[0154] S 2f,t represents the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s , when substituting the measured spectral absorption rate α of acetylene gas into formula (10) and formula (11) t,m α(b(t)), formula (12) gives the measured second harmonic information S 2f,m ; when substituting the simulated spectral absorption rate function α of acetylene gas into formula (10) and formula (11) t,s α(υ(t)), formula (12) gives the simulated second harmonic information S 2f,s ; thus, according to formula (12), the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s can be obtained.

[0155] Step 5: Calculate the iteration. Calculate the residual between the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s which is expressed by the following formula:

[0156] ε t = |S 2f,m - S 2f,s | (13)

[0157] In the formula:

[0158] εt Represents the residual between the measured second harmonic information S 2f,m and the simulated second harmonic information S 2f,s ;

[0159] Set the convergence condition ε. When ε t ≥ε, the convergence condition is not satisfied. Update the modulation depth a and the initial acetylene gas concentration x, and return to step 1. Repeat the above steps until the convergence condition is satisfied, that is, output the measured value x of the acetylene gas concentration l .

[0160] In a preferred but non-limiting embodiment of the present invention, updating the modulation depth a and the initial acetylene gas concentration x includes: first, fixing the modulation depth a, and finding the optimal acetylene gas concentration x by the bisection method; further, the initial value of the modulation depth a is estimated according to the line type, and the adjustment step of the optimal acetylene gas concentration x is preferably but not limited to 0.1 ppm according to engineering requirements.

[0161] Then, find the modulation depth a in the same way, set reasonable upper and lower limits according to the actual situation, and the step size is generally 0.01 cm -1 .

[0162] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can make many variations without departing from the spirit and scope of the present invention as defined by the claims, and these all belong to the protection scope of the present invention.

Claims

1. A dissolved gas detection system in transformer oil, comprising: An air path part, an optical path part and an electric circuit part; characterized in that: The air path part includes: a transformer oil degassing unit (105); the transformer oil degassing unit (105) is used to separate the dissolved gas in the transformer oil and inject the gas into the optical path part for gas detection; The optical path part includes: a laser (101), an optical fiber coupler (102), a gas absorption cell (103), an optical etalon (104) and a photodetector (106); the laser (101) outputs tuned laser light, which is split by the optical fiber coupler (102), one path passes through the gas absorption cell (103), and the other path passes through the optical etalon (104); the photodetector (106) receives the optical signals passing through the gas absorption cell (103) and the optical etalon (104) and performs optoelectronic information conversion; The electric circuit part includes: a current controller (108), a temperature controller (109) and a microprocessor (107); the current controller (108) is used to output a current to tune the output frequency and light intensity of the laser (101); the temperature controller (109) is used to control and monitor the temperature of the laser; the microprocessor (107) is used to control the laser and analyze the concentration of dissolved gas in the transformer oil.

2. A dissolved gas detection system in transformer oil according to claim 1, characterized in that: The transformer oil degassing unit (105) adopts the dynamic headspace method to separate the dissolved gas in the transformer oil and inject the gas into the gas absorption cell (103) through a gas pump for gas detection.

3. A dissolved gas detection system in transformer oil according to claim 1, characterized in that: The current controller (108) is used to superimpose a scanning triangular wave and a modulating sine wave to output a current, thereby tuning the output frequency of the laser; The output drive current is the superposition of a low-frequency scanning triangular wave and a high-frequency modulating sine wave, the frequency range of the scanning triangular wave is 1 - 100 Hz, and the frequency range of the modulating sine wave is 1 - 100 kHz.

4. A dissolved gas detection system in transformer oil according to any one of claims 1 to 3, characterized in that: The temperature controller (109) is used to maintain the consistency of the laser temperature; the output drive temperature is a constant temperature of 23 °C, and the fluctuation range is controlled within ±0.2 °C.

5. A method for detecting dissolved gases in transformer oil without calibration, based on the dissolved gas detection system for transformer oil according to any one of claims 1 to 4, characterized in that, It includes the following steps: Step 1: Set the modulation depth a of the laser (101), the laser (101) outputs tuned laser light, passes through the gas absorption cell (103) without the gas to be measured, and obtains the first light intensity measurement value of the laser through the photodetector (106); Step 2: When there is a gas to be measured in the gas absorption cell (103), the laser (101) outputs tuned laser light, which is split by the optical fiber coupler (102), and respectively passes through the gas absorption cell (103) after being absorbed by the gas to be measured and the optical etalon (104), and obtains the second light intensity measurement value and frequency of the laser through the photodetector (106); Step 3: The microprocessor (107) calculates the measured spectral absorptivity of the gas to be measured based on the first light intensity measurement value after passing through the non-gas absorption cell to be measured (103) and the second light intensity measurement value after passing through the gas absorption cell to be measured (103); based on the pressure P of the gas absorption cell (103), the set concentration x of the gas to be measured, the length L of the gas absorption cell, the line intensity S(T) of the absorption spectrum of the gas to be measured at temperature T, and the laser frequency obtained in Step 2, it calculates the simulated spectral absorptivity of the gas to be measured; Step 4: Extract the second harmonic information of the measured spectral absorptivity of the gas to be measured obtained in Step 3 and twice the simulated spectral absorptivity of the gas to be measured, and obtain the measured second harmonic information S through a finite impulse response low-pass filter 2f，m and the simulated second harmonic information S2 f，s ; Step 5: Calculate the residual between the measured second harmonic information S2 f，m and the simulated second harmonic information S 2f，s If the residual is less than the set convergence condition ε, output the measured value x of the concentration of the gas to be measured l ; if it is not less than the set convergence condition ε, update the modulation depth a and the concentration x of the gas to be measured, return to Step 1, and repeat the above steps until the convergence condition is met.

6. A method for calibrating-free detection of dissolved gases in transformer oil according to claim 5, characterized in that: Step 1 includes: Step 1.1: The gas absorption cell (103) is purged with air by the transformer oil degassing unit (105) to clean the residual gas in the gas absorption cell (103); Step 1.2: After the cleaning of the residual gas in Step 1.1 is completed, when the gas in the gas absorption cell (103) has been static for a set time, Step 1.3 is executed; Step 1.3: The laser (101) outputs the tuned laser. After passing through the gas absorption cell (103), it is detected by the photodetector (106) that receives the optical signal, and the first light intensity measurement value I 0，m (t) is obtained.

7. A method for calibrating-free detection of dissolved gases in transformer oil according to claim 5, characterized in that: Step 2 includes: Step 2.1: The transformer oil degassing unit (105) evacuates the gas absorption cell (103), and then decomposes the gas in the transformer oil by the dynamic headspace method as the gas to be measured, which is introduced into the gas absorption cell (103) and sealed; Step 2.2: After the sealing in Step 2.1 is completed, when the set gas static time of the gas absorption cell (103) is reached, Step 2.3 is executed; Step 2.3: The laser (101) outputs the tuned laser, which passes through the gas absorption cell (103). After being absorbed by the gas to be measured, it is detected by the photodetector (106) that receives the optical signal, and the second light intensity measurement value I t，m (t) is obtained; meanwhile, the laser (101) outputs the tuned laser, which passes through the other high-resolution etalons (104) of the coupler (102) and is detected by the photodetector (106) that receives the optical signal, so as to obtain the discrete relationship between the laser frequency and time; Step 2.4: The microprocessor (107) obtains the modulation depth a and the discrete relationship between the laser frequency and time, and obtains the frequency υ t，m (t) after the laser passes through the etalon (104) by fitting.

8. A method for calibrating-free detection of dissolved gases in transformer oil according to claim 7, characterized in that: In step 3, the microprocessor (107) takes the logarithm of the first light intensity measurement value I 0，m (t) after passing through the absorption cell (103) without the gas to be measured and the second light intensity measurement value I t，m (t) after passing through the absorption cell (103) with the gas to be measured, and obtains the measured spectral absorptivity α t，m (υ(t)), which is expressed by the following formula: α t，m (v(t)) = ln(I 0，m (t)) - ln(I t，m (t)) (8) In the formula: ln(·) represents the logarithmic function with the natural constant e as the base; Meanwhile, the microprocessor (107) calculates the simulated spectral absorptivity of the gas to be measured based on the pressure P of the gas absorption cell (103), the set concentration x of the gas to be measured, the length L of the gas absorption cell, the line intensity S(T) of the absorption spectrum of the gas to be measured at the temperature T, and the laser frequency υ t，m (t) obtained in step 2, which is expressed by the following formula: In the formula: α t，s (υ(t)) represents the simulated spectral absorption rate function of the gas to be measured; are the Fourier series expansion coefficients; is the absorption line shape function of the absorption spectral line of the gas to be measured.

9. A method for calibrating-free detection of dissolved gases in transformer oil according to claim 8, characterized in that: In step 4, the measured spectral absorption rate α t，m (v(t)) of the gas to be measured in step 3 and the simulated spectral absorption rate function α t，s (υ(t)) are respectively sent to the digital lock-in amplifier module of the microprocessor (107) to extract the second harmonic information of the signal, and the measured second harmonic information S 2f，m and the simulated second harmonic information S 2f，s are obtained, which are expressed by the following formula: In the formula: represents the outer product operation; LPF represents a low-pass filter; A L is the amplitude of the reference signal of the lock-in amplifier; α t (t) represents the measured spectral absorption rate α of the gas to be measured t，m (υ(t)) and the simulated spectral absorption rate function α of the gas to be measured t，s (υ(t)), which are substituted respectively during calculation; γ is the phase of the reference signal of the lock-in amplifier, and x 2f，t and y 2f，t are the cosine component and the sine component output by the digital lock-in amplification module; S 2f，t Represents the measured second harmonic information S 2f，m and the simulated second harmonic information S 2f，s , substituting formula (10) and formula (11) into the measured spectral absorption rate α t，m (υ(t)) of the gas to be measured, formula (12) obtains the measured second harmonic information S 2f，m ; substituting formula (10) and formula (11) into the simulated spectral absorption rate function α t，s (υ(t)) of the gas to be measured, formula (12) obtains the simulated second harmonic information S 2f，s ; thus, according to formula (10), the measured second harmonic information S 2f，m and the simulated second harmonic information S 2f，s can be obtained.

10. A method for calibrating-free detection of dissolved gases in transformer oil according to any one of claims 5 to 9, characterized in that: Updating the modulation depth a and the concentration x of the gas to be measured in Step 5 includes: first fixing the modulation depth a, and finding the optimal concentration x of the gas to be measured by the bisection method; then fixing the optimal concentration x of the gas to be measured, and finding the optimal modulation depth a by the bisection method.

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