Dissolved gas detection device and detection method in transformer oil based on micro-nano optical fiber

Abstract

The present invention discloses a detection device and a detection method for dissolved gas in transformer oil based on micro-nano optical fiber. The detection device comprises a gas sensor based on micro-nano optical fiber, an interference phase demodulation unit and an inversion unit, a photothermal effect excitation unit for generating and amplifying excitation light, and an intensity modulation unit for applying sinusoidal modulation to the amplified excitation light; the sinusoidally modulated excitation light is coupled to the gas sensor to excite the fundamental mode and the first high-order mode, and an evanescent field is generated in the waist region to excite the gas in the oil to generate a photothermal effect to change the phase of the fundamental mode and the first high-order mode, thereby forming a mode phase difference; the detection light generated by the interference phase demodulation unit has a fundamental mode HE in the gas sensor; 11 With the first higher order mode HE 12 The interference is subjected to different degrees of phase modulation, and the modulated phase is converted into the peak-to-peak value of the interference signal; the inversion unit uses the fitting formula to invert the peak-to-peak value of the interference signal to obtain the concentration of the gas in the oil.

Description

Technical Field

[0001] The present invention relates to the technical field of detection of dissolved gas in transformer oil, and in particular to a photothermal spectrum detection device and method based on micro-nano optical fiber. Background Art

[0002] Insulating oil is an important insulating medium widely used in high-voltage equipment. It is often regarded as the "blood" of oil-filled electrical equipment, and mainly plays the role of insulation and cooling. However, when defects such as local overheating or discharge occur inside the electrical equipment, the insulating oil will decompose and produce small molecular gases, such as CH4, C2H2, C2H4, C2H6, etc. Therefore, conducting dissolved gas analysis (DGA) in oil and obtaining the dissolved gas components, concentrations and change trends is one of the most recognized and effective fault monitoring methods to evaluate the reliable operation of oil-filled equipment.

[0003] At present, the research on online monitoring technology of dissolved gas in oil for large oil-filled equipment (such as oil-immersed power transformers) has become relatively mature and has been widely promoted and applied, which has strongly supported the long-term operation and condition maintenance of power equipment. However, according to a large number of practical feedback, the existing online monitoring technology still has the following challenges: large detection deviation, serious false alarms and missed alarms; low-oil equipment cannot be detected online; and the detection accuracy still needs to be improved. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the photothermal spectrum detection device and method based on micro-nano optical fiber provided by the present invention solves the problem of large deviation in the existing online detection.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0006] In a first aspect, a device for detecting dissolved gas in transformer oil based on micro-nano optical fiber is provided, comprising:

[0007] A photothermal effect excitation unit is used to generate excitation light and amplify the excitation light for output;

[0008] An intensity modulation unit, used for applying sinusoidal modulation to the amplified excitation light;

[0009] The gas sensor based on micro-nano optical fiber is placed in an oil environment. It includes a transition zone symmetrical at both ends and a cylindrical waist zone located between the transition zones. The sinusoidally modulated excitation light is coupled into the gas sensor to excite the fundamental mode HE 11 With the first higher order mode HE 12 , and generate an evanescent field in the waist region, which excites the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The phase of , forming a mode phase difference;

[0010] Interferometric phase demodulation unit, used to generate detection light, the detection light exists in the fundamental mode HE in the gas sensor 11 With the first higher order mode HE 12 Interference, subjected to different degrees of phase modulation, and the modulation phase is converted into the peak-to-peak value of the interference signal;

[0011] The inversion unit is used to perform inversion according to the peak-to-peak value of the interference signal by using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration to obtain the concentration of the gas in the oil.

[0012] Furthermore, the waist diameter of the gas sensor is related to the fundamental mode HE 11 The relationship between them is:

[0013]

[0014] Waist diameter and first high-order mode HE 12 The relationship between them is:

[0015]

[0016] in, n eff1 is the waist diameter and the fundamental mode HE 11 The effective refractive index between n eff2 is the waist diameter and the first high-order mode HE 12 The effective refractive index between d is the waist diameter.

[0017] The beneficial effects of the above technical solution are as follows: the existing technology focuses on macroscopic analysis of the performance of gas sensors, lacks in-depth research on its microscopic information such as parameters and modes, and this solution is based on the waist diameter of the gas sensor and the fundamental mode HE 11 With the first higher order mode HE 12 The quantitative optimization of the relationship, combined with the sensor design parameters and actual application data, can more flexibly improve the detection sensitivity and accuracy of the sensor under different environments by precisely controlling the waist diameter, thereby achieving significant optimization of the detection effect of dissolved gas in transformer oil.

[0018] Furthermore, when sinusoidal modulation is applied, the modulation frequency ,in f 0 is the frequency corresponding to the maximum interference phase amplitude, for f The difference between 0 and the frequency corresponding to the maximum interference phase amplitude × 90%.

[0019] The beneficial effects of the above technical solution are as follows: by introducing sinusoidal modulation and accurately setting the modulation frequency, the present solution can realize sensitive detection of interference signals, especially in f By finely controlling the frequency difference near the maximum interference phase amplitude corresponding to 0, the signal resolution and anti-interference ability of the system can be significantly improved, thereby ensuring the stability and accuracy of the gas sensor in complex environments.

[0020] Furthermore, the photothermal effect excitation unit includes a pump light source, an isolator, a filter and an optical fiber amplifier connected in sequence, and the intensity modulation unit includes a sinusoidal signal source, a radio frequency driver and an acousto-optic modulator connected in sequence, and the acousto-optic modulator applies sinusoidal modulation to the excitation light passing through the optical fiber amplifier.

[0021] The beneficial effects of the above technical solution are: by adopting a combination design of a pump light source, an isolator, a filter and a fiber amplifier in the photothermal effect excitation unit, the power and stability of the light source are effectively improved, ensuring the stable excitation of the photothermal effect. At the same time, the intensity modulation unit is composed of a sinusoidal signal source, a radio frequency driver and an acousto-optic modulator, which can perform precise sinusoidal modulation on the excitation light. This solution not only improves the modulation depth and modulation accuracy of the system, but also enhances the sensitivity and accuracy of the detection signal, thereby effectively improving the detection efficiency and overall performance, and is suitable for a variety of complex detection environments.

[0022] Furthermore, the interference phase demodulation unit is a ring-shaped Sagnac interference structure, which includes a detection light source, an isolator and a coupler connected in sequence. The coupler divides the detection light into three beams of light, one of which passes through the delay fiber, the 1550 end of the wavelength division multiplexer, the gas sensor based on micro-nano optical fiber and the polarization controller in sequence and returns to the coupler; one beam of light passes through the polarization controller, the wavelength division multiplexer and the delay fiber and returns to the coupler; another single-mode optical fiber input end of the coupler is connected to the filter and the photodetector in sequence; the acousto-optic modulator is connected to the wavelength division multiplexer to couple the sinusoidally modulated excitation light into the gas sensor.

[0023] The beneficial effects of the above technical solution are as follows: the annular Sagnac interference structure has the advantages of high sensitivity and high stability. Through the symmetry of the interference optical path, the optical signal propagates along two paths and performs phase interference, thereby improving the detection resolution of the system. At the same time, the annular Sagnac interference structure can be flexibly adapted to different types of optical sensors and has broad application prospects.

[0024] Furthermore, the preparation method of the gas sensor includes:

[0025] A1. Select a single-mode optical fiber, remove the coating layer of 5~7 cm on the single-mode optical fiber, wipe the remaining coating layer with alcohol, and then fix the single-mode optical fiber between the optical fiber clamps of the fusion splicer;

[0026] A2. Control the left and right motors of the fusion splicer to stretch the single-mode optical fiber until the stretched length is 100 μm.

[0027] A3. Setting the discharge intensity and discharge time of the welding machine electrode, and performing arc discharge on the central area where the coating layer is stripped, to prepare two straight cones with a taper;

[0028] A4. Place the straight cone in the grooves of the two optical fiber fixtures, align the thinnest part of the waist area with the center of the hydrogen flame, keep the straight cone in a stretched state, and fix the single-mode optical fibers at both ends of the straight cone with magnetic suction;

[0029] A5, set the hydrogen flow rate to 200 mL / min, the motor stretching speed to 0.25 mm / s, and the motor maximum stretching displacement to 1 ~ 3 mm;

[0030] A6. Use flame to heat the waist area. When the single-mode optical fiber becomes molten and softens, the left and right motors begin to move toward the two ends, and the straight cone gradually elongates and becomes thinner along the waist area to form a micro-nano optical fiber.

[0031] A7. Use a microscope to observe the uniformity of the waist diameter of the micro-nano optical fiber and measure the waist diameter of the optical fiber;

[0032] A8. When both the uniformity and the waist diameter meet the preset requirements, the preparation of the gas sensor is completed. When at least one of the uniformity and the waist diameter does not meet the preset requirements, return to step A6.

[0033] The beneficial effects of the above technical solution are as follows: by precisely controlling the stretching process and heating conditions of the optical fiber, this solution can produce a high-precision, uniform micro-nano optical fiber-based gas sensor; this process achieves strict control of the waist diameter and uniformity of the gas sensor, effectively improving the sensitivity and stability of the sensor. In addition, the method is simple in process and suitable for large-scale production of micro-nano optical fiber gas sensors, and has good application prospects.

[0034] Furthermore, the waist diameter of the gas sensor is 8-20 μm, the transition zone length is 350-5000 μm, and the operating wavelength is 1260nm~1675 nm.

[0035] The beneficial effects of the above technical solution are: the above setting of this solution can optimize the optical properties of the gas sensor and improve its sensitivity and response speed to different gases; this design enables the gas sensor to maintain efficient detection capabilities within a wide wavelength range, while ensuring that it has better stability and reliability under different environmental conditions.

[0036] Furthermore, the pump light source is a single-wavelength laser with a wavelength range of 1260nm~1675nm.

[0037] In a second aspect, a detection method of a dissolved gas detection device in transformer oil based on micro-nano optical fiber is provided, which comprises the steps of:

[0038] Generate a narrow linewidth laser with a wavelength of 1530.37 nm as the excitation light, amplify it through a fiber amplifier, and then apply sinusoidal modulation to the excitation light through an acousto-optic modulator;

[0039] The excitation light is coupled into the gas sensor based on micro-nano optical fiber through a wavelength division multiplexer to excite the fundamental mode HE 11 and the first higher order mode HE 12 , and produces an evanescent field in the waist region;

[0040] The evanescent field leaks into the oil, stimulating the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The phase of , forming a mode phase difference;

[0041] Amplified spontaneous emission broadband light source is used as the detection light. The detection light has the fundamental mode HE in the gas sensor. 11 and the first higher order mode HE 12 interference, subject to varying degrees of phase modulation;

[0042] A photodetector is used to convert phase modulation into peak-to-peak interference signal;

[0043] According to the peak-to-peak value of the interference signal, the concentration of the gas in the oil is obtained by inversion using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration.

[0044] Furthermore, the fitting formula is:

[0045] y =0.29 x- 0.51

[0046] in, y is the gas concentration; x is the peak-to-peak value of the interference signal.

[0047] The present invention has the following beneficial effects compared with the prior art:

[0048] 1. This scheme couples the sinusoidally modulated excitation light into the gas sensor to excite the fundamental mode HE 11 With the first higher order mode HE 12 , and generate an evanescent field in the waist region, which excites the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The gas information in the oil is encoded in the pattern phase difference here, and the gas concentration in the oil can be detected through phase demodulation and inversion.

[0049] 2. This scheme amplifies the excitation light and then performs sinusoidal modulation to reduce the loss of sensitivity caused by the strong attenuation of the excitation light by the oil, thereby making the gas sensor extremely sensitive and able to achieve highly sensitive concentration measurement in a liquid environment, which is especially suitable for low-concentration measurement. The gas sensor has a simple and stable structure and has broad application prospects.

[0050] 3. The evanescent field generated in the waist region of the gas sensor based on micro-nano optical fiber in this scheme has broadband characteristics, and has a strong evanescent field for light in the wavelength range of 1260nm~1675nm. Therefore, the sensor can effectively respond to light signals of various wavelengths, enhancing the system's detection capability and adaptability to various gases.

[0051] 4. The gas sensor based on micro-nano optical fiber in the present invention has a high degree of adaptability and can be adapted to interference phase demodulation units such as annular Sagnac interference structure, linear Sagnac interference structure, Mach–Zehnder interference structure and Michelson interference structure. Therefore, the gas sensor can have good compatibility and application flexibility in different detection systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The principle block diagram of the photothermal spectrum detection device based on micro-nano optical fiber does not show the inversion unit.

[0053] Figure 2 Schematic diagram of an exemplary micro-nano optical fiber-based gas sensor of the present invention.

[0054] Figure 3 Graph 1 is the relationship between the peak-to-peak value of the interference signal and the modulation frequency of the exemplary micro-nano optical fiber-based gas sensor of the present invention.

[0055] Figure 4 The real-time response diagram of the intensity at different concentrations of the exemplary micro-nano optical fiber-based gas sensor of the present invention.

[0056] Figure 5 The linear fitting of the intensity of the exemplary micro-nano optical fiber-based gas sensor of the present invention at different concentrations. DETAILED DESCRIPTION

[0057] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0058] like Figure 1 As shown, the transformer oil dissolved gas detection device based on micro-nano optical fiber provided in the present scheme includes a gas sensor 41 based on micro-nano optical fiber, an interference phase demodulation unit 3 and an inversion unit, a photothermal effect excitation unit 1 for generating excitation light and amplifying the excitation light and outputting it; and an intensity modulation unit 2 for applying sinusoidal modulation to the amplified excitation light.

[0059] like Figure 2 As shown, a gas sensor 41 based on micro-nano optical fiber is placed in an oil environment 42. The oil environment 42 and the gas sensor 41 constitute a sensor unit 4. The gas sensor 41 includes a transition zone symmetrical at both ends and a cylindrical waist zone located between the transition zones. The excitation light after sinusoidal modulation is coupled into the gas sensor 41 to excite the fundamental mode HE 11 With the first higher order mode HE 12 , and generate an evanescent field in the waist region, which excites the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 phase, forming a mode phase difference.

[0060] The interference phase demodulation unit 3 is used to generate detection light, and the detection light has a fundamental mode HE in the gas sensor. 11 and the first higher order mode HE 12 The interference is subjected to different degrees of phase modulation, and the modulated phase is converted into a peak-to-peak value of the interference signal; the inversion unit is used to invert the peak-to-peak value of the interference signal using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration to obtain the concentration of the gas in the oil.

[0061] During implementation, the present scheme preferably comprises a photothermal effect excitation unit 1 including a pump light source 11, a first isolator 12, a first filter 13 and an optical fiber amplifier 14 connected in sequence, the pump light source 11 is a single-wavelength laser, preferably a tunable laser, and the excitation light wavelength range is 1260nm~1675nm.

[0062] The intensity modulation unit 2 includes a sinusoidal signal source 23, a radio frequency driver 22, and an acousto-optic modulator 21 connected in sequence, and the acousto-optic modulator 21 applies sinusoidal modulation to the excitation light passing through the optical fiber amplifier 14. In this solution, the modulation signal frequency of the acousto-optic modulator 21 is further preferably 20 Hz, and the tunable range of the tunable laser is 1480-1640 nm, and the operating wavelength of the tunable laser matches the operating wavelength of the gas sensor 41 based on micro-nano optical fiber.

[0063] In this scheme, when sinusoidal modulation is applied, the modulation frequency ,in f 0 is the frequency corresponding to the maximum interference phase amplitude, for f The difference between 0 and the frequency corresponding to the maximum interference phase amplitude × 90%.

[0064] like Figure 1 As shown, the interference phase demodulation unit is a ring-shaped Sagnac interference structure, which includes a detection light source 31, a second isolator 32 and a coupler 33 connected in sequence. The coupler 33 divides the detection light into three beams of light, one of which passes through the delay fiber 34, the 1550 end of the wavelength division multiplexer 35, the gas sensor 41 based on micro-nano optical fiber and the polarization controller 36 in sequence and returns to the coupler 33; one beam of light passes through the polarization controller 36, the wavelength division multiplexer 35 and the delay fiber 34 in sequence and returns to the coupler 33; another single-mode optical fiber input end of the coupler 33 is connected to the second filter 37 and the photodetector 38 in sequence; the acousto-optic modulator 21 is connected to the wavelength division multiplexer 35 to couple the sinusoidally modulated excitation light into the gas sensor 41.

[0065] In this solution, the detection light source 31 is a broadband light source. The wavelength range of the detection light output by the detection light source 31 is within 1260nm~1675nm, and it can be a broadband light source in a certain section of the wavelength range. In this solution, the preferred broadband light source is an amplified spontaneous emission broadband light source with a range of 1525-1565nm. The operating wavelength of the amplified spontaneous emission broadband light source matches the operating wavelength of the gas sensor 41 based on micro-nano optical fiber. The operating wavelength range of the broadband light source is 1525-1565nm.

[0066] The gas sensor 41 based on micro-nano optical fiber is the core device of this solution, which is obtained by fusion taper drawing method. The drawing process includes the following steps:

[0067] A1. Select a single-mode optical fiber, remove the coating layer of 5~7 cm on the single-mode optical fiber, wipe the remaining coating layer with alcohol, and then fix the single-mode optical fiber between the optical fiber clamps of the fusion splicer;

[0068] A2. Control the left and right motors of the fusion splicer to stretch the single-mode optical fiber until the stretched length is 100 μm.

[0069] A3, setting the discharge intensity of the welding machine electrode to 20 and the discharge time to 300-900 ms, and performing arc discharge on the central area of ​​the stripped coating layer to prepare two straight cones with a taper;

[0070] A4. Place the straight cone in the grooves of the two optical fiber fixtures, align the thinnest part of the waist area with the center of the hydrogen flame, keep the straight cone in a stretched state, and fix the single-mode optical fibers at both ends of the straight cone with magnetic suction;

[0071] A5, set the hydrogen flow rate to 200 mL / min, the motor stretching speed to 0.25 mm / s, and the motor maximum stretching displacement to 1 ~ 3 mm;

[0072] A6. Use flame to heat the waist area. When the single-mode optical fiber becomes molten and softens, the left and right motors begin to move toward the two ends, and the straight cone gradually elongates and becomes thinner along the waist area to form a micro-nano optical fiber.

[0073] A7. Use a microscope to observe the uniformity of the waist diameter of the micro-nano optical fiber and measure the waist diameter of the optical fiber;

[0074] A8. When both the uniformity and the waist diameter meet the preset requirements, the preparation of the gas sensor 41 is completed. When at least one of the uniformity and the waist diameter does not meet the preset requirements, return to step A6.

[0075] The gas sensor 41 (41) based on micro-nano optical fiber finally prepared has a waist diameter of 8-20 μm, a transition zone length of 350-5000 μm, and an operating wavelength of 1260nm~1675 nm.

[0076] During implementation, the waist diameter of the gas sensor 41 prepared in this solution is preferably the same as the fundamental mode HE 11 The relationship between them is:

[0077]

[0078] Waist diameter and first high-order mode HE 12 The relationship between them is:

[0079]

[0080] in, n eff1 is the waist diameter and the fundamental mode HE 11 The effective refractive index between n eff2 is the waist diameter and the first high-order mode HE 12 The effective refractive index betweend is the waist diameter.

[0081] This solution also provides a method for detecting dissolved gas in transformer oil based on micro-nano optical fiber, which comprises the following steps:

[0082] Generate a narrow linewidth laser with a wavelength of 1530.37 nm as excitation light, and amplify it through a fiber amplifier 14, and then apply sinusoidal modulation to the excitation light through an acousto-optic modulator 21;

[0083] The excitation light is coupled into the gas sensor 41 based on the micro-nano optical fiber through the wavelength division multiplexer 35 to excite the fundamental mode HE 11 and the first higher order mode HE 12 , and produces an evanescent field in the waist region;

[0084] The evanescent field leaks into the oil, stimulating the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The phase of , forming a mode phase difference;

[0085] Amplified spontaneous emission broadband light source is used as the detection light. The detection light has the fundamental mode HE in the gas sensor. 11 and the first higher order mode HE 12 interference, subject to varying degrees of phase modulation;

[0086] The phase modulation is converted into a peak-to-peak value of the interference signal using a photodetector 38;

[0087] According to the peak-to-peak value of the interference signal, the concentration of the gas in the oil is obtained by inversion using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration.

[0088] The fitting formula is: y =0.29 x- 0.51, among which, y is the gas concentration; x is the peak-to-peak value of the interference signal.

[0089] In one example, the conditions for gas detection by the device are as follows: the excitation light intensity (the light intensity entering the wavelength division multiplexer) is maintained at 220 mW, and the light intensity is sinusoidally modulated in the range of 1 to 220 kHz, and a micro-nano optical fiber with a waist diameter of 13.1 μm is set to detect a 500 ppm acetylene oil sample, and the test conditions are all room temperature and atmospheric pressure.

[0090] In the above example, the relationship between the peak-to-peak value of the interference signal of the gas sensor 41 and the frequency is as follows: Figure 3 As shown, through Figure 3It can be seen that the peak-to-peak value of the interference signal reaches its maximum value at 15 Hz, and is not less than 90% of the maximum value in the frequency range of 5 to 50 Hz. This range is called the "high-efficiency zone", that is, the gas sensor can work with optimal performance at 5 to 50 Hz, which is the optimal range of the modulation frequency.

[0091] In another example, the diameter of the micro-nano fiber waist region is set to 13 μm, the length of the transition region is set to 1000 μm, the gas concentrations in the oil are 0.41 ppm, 0.99 ppm, 5.03 ppm and 10.42 ppm respectively, and the real-time monitoring results of dissolved gas in transformer oil based on photothermal spectroscopy of micro-nano fiber are carried out under the condition of modulation signal frequency of 20 Hz. The real-time response of the light intensity detected at different concentrations is shown in Figure 2. Figure 4 As shown, the linear fitting of the interference signal intensity detected by the gas sensor at different concentrations is as follows Figure 5 shown.

[0092] In summary, the used micro-nano optical fiber photothermal transformer oil gas sensor 41 has high sensitivity and can achieve highly sensitive refractive index measurement in a liquid environment. The gas sensor 41 has a simple and stable structure and has broad application prospects.

Claims

1. A device for detecting dissolved gas in transformer oil based on micro-nano optical fiber, characterized in that: include: A photothermal effect excitation unit is used to generate excitation light and amplify the excitation light for output; An intensity modulation unit, used for applying sinusoidal modulation to the amplified excitation light; The gas sensor based on micro-nano optical fiber is placed in an oil environment. It includes a transition zone symmetrical at both ends and a cylindrical waist zone located between the transition zones. The sinusoidally modulated excitation light is coupled into the gas sensor to excite the fundamental mode HE 11 With the first higher order mode HE 12 , and generate an evanescent field in the waist region, which excites the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The phase of , forming a mode phase difference; Interferometric phase demodulation unit, used to generate detection light, the detection light exists in the fundamental mode HE in the gas sensor 11 With the first higher order mode HE 12 Interference, subjected to different degrees of phase modulation, and the modulation phase is converted into the peak-to-peak value of the interference signal; An inversion unit is used to perform inversion according to the peak-to-peak value of the interference signal by using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration to obtain the concentration of the gas in the oil; The waist diameter of the gas sensor is related to the fundamental mode HE 11 The relationship between them is: Waist diameter and first high-order mode HE 12 The relationship between them is: in, n eff1 is the waist diameter and the fundamental mode HE 11 The effective refractive index between n eff2 is the waist diameter and the first high-order mode HE 12 The effective refractive index between d is the waist diameter; The photothermal effect excitation unit includes a pump light source, an isolator, a filter and an optical fiber amplifier connected in sequence, and the intensity modulation unit includes a sinusoidal signal source, a radio frequency driver and an acousto-optic modulator connected in sequence, and the acousto-optic modulator applies sinusoidal modulation to the excitation light passing through the optical fiber amplifier; The interference phase demodulation unit is a ring-shaped Sagnac interference structure, which includes a detection light source, an isolator and a coupler connected in sequence. The coupler divides the detection light into three beams of light, one of which is returned to the coupler through a delay optical fiber, a 1550 end of a wavelength division multiplexer, a gas sensor based on micro-nano optical fiber and a polarization controller in sequence; one beam of light is returned to the coupler through a polarization controller, a wavelength division multiplexer and a delay optical fiber in sequence; another single-mode optical fiber input end of the coupler is connected to a filter and a photodetector in sequence; the acousto-optic modulator is connected to the wavelength division multiplexer to couple the sinusoidally modulated excitation light into the gas sensor.

2. The device for detecting dissolved gas in transformer oil based on micro-nano optical fiber according to claim 1, characterized in that: When sinusoidal modulation is applied, the modulation frequency ,in f 0 is the frequency corresponding to the maximum interference phase amplitude, for f The difference between 0 and the frequency corresponding to the maximum interference phase amplitude × 90%.

3. The device for detecting dissolved gas in transformer oil based on micro-nano optical fiber according to claim 1, characterized in that: The preparation method of the gas sensor comprises: A1. Select a single-mode optical fiber, remove the coating layer of 5~7 cm on the single-mode optical fiber, wipe the remaining coating layer with alcohol, and then fix the single-mode optical fiber between the optical fiber clamps of the fusion splicer; A2. Control the left and right motors of the fusion splicer to stretch the single-mode optical fiber until the stretched length is 100 μm. A3. Setting the discharge intensity and discharge time of the welding machine electrode, and performing arc discharge on the central area where the coating layer is stripped, to prepare two straight cones with a taper; A4. Place the straight cone in the grooves of the two optical fiber fixtures, align the thinnest part of the waist area with the center of the hydrogen flame, keep the straight cone in a stretched state, and fix the single-mode optical fibers at both ends of the straight cone with magnetic suction; A5, set the hydrogen flow rate to 200 mL / min, the motor stretching speed to 0.25 mm / s, and the motor maximum stretching displacement to 1 ~3 mm; A6. Use flame to heat the waist area. When the single-mode optical fiber becomes molten and softens, the left and right motors begin to move toward the two ends, and the straight cone gradually elongates and becomes thinner along the waist area to form a micro-nano optical fiber. A7. Use a microscope to observe the uniformity of the waist diameter of the micro-nano optical fiber and measure the waist diameter of the optical fiber; A8. When both the uniformity and the waist diameter meet the preset requirements, the preparation of the gas sensor is completed. When at least one of the uniformity and the waist diameter does not meet the preset requirements, return to step A6.

4. The device for detecting dissolved gas in transformer oil based on micro-nano optical fiber according to claim 1 or 3, characterized in that: The waist diameter of the gas sensor is 8-20 μm, the transition zone length is 350-5000 μm, and the operating wavelength is 1260nm~1675 nm.

5. The device for detecting dissolved gas in transformer oil based on micro-nano optical fiber according to claim 1, characterized in that: The pump light source is a single-wavelength laser with a wavelength range of 1260nm~1675nm.

6. A detection method for a detection device for detecting dissolved gas in transformer oil based on micro-nano optical fiber according to any one of claims 1 to 5, characterized in that: Includes steps: Generate a narrow linewidth laser with a wavelength of 1530.37 nm as the excitation light, amplify it through a fiber amplifier, and then apply sinusoidal modulation to the excitation light through an acousto-optic modulator; The excitation light is coupled into the gas sensor based on micro-nano optical fiber through a wavelength division multiplexer to excite the fundamental mode HE 11 and the first higher order mode HE 12 , and produces an evanescent field in the waist region; The evanescent field leaks into the oil, stimulating the gas in the oil to produce a photothermal effect to change the fundamental mode HE 11 With the first higher order mode HE 12 The phase of , forming a mode phase difference; Amplified spontaneous emission broadband light source is used as the detection light. The detection light has the fundamental mode HE in the gas sensor. 11 With the first higher order mode HE 12 interference, subject to varying degrees of phase modulation; A photodetector is used to convert phase modulation into peak-to-peak interference signal; According to the peak-to-peak value of the interference signal, the concentration of the gas in the oil is obtained by inversion using a fitting formula based on the peak-to-peak value of the interference signal and the gas concentration.

7. The detection method according to claim 6, characterized in that: The fitting formula is: y =0.29 x- 0.51 in, y is the gas concentration; x is the peak-to-peak value of the interference signal.

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