Furfural Detection System and Detection Method in Transformer Oil Based on Photothermal Lens Spectroscopy

Abstract

The present invention discloses a furfural detection system and method in transformer oil based on photothermal lens spectroscopy. A quantum cascade laser is used as the pump laser, and a helium-neon laser is used as the probe laser. A visible-infrared dichroic mirror, a chopper, a reflector, an attenuation adjuster, a plano-convex lens, a fixed optical path liquid cell, a variable aperture, a photodetector, a lock-in amplifier, and a host computer are provided, etc., realizing in-situ, rapid, accurate, and highly sensitive quantitative detection of trace furfural in transformer oil. Through the pump laser covering the mid-infrared characteristic absorption spectral line of furfural, after trace furfural molecules in transformer oil absorb the pump laser with an average power exceeding 100 mW, a strong photothermal lens effect is excited, significantly improving the detection performance. By using the photothermal lens effect generated by the absorption of mid-infrared light by furfural in transformer oil, in-situ and rapid detection of furfural in transformer oil is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of transformer oil detection, and particularly to a furfural detection system and method for transformer oil based on photothermal lens spectroscopy. Background Art

[0002] Power transformers are one of the most important power conversion equipment in the power system, and their operation reliability is of great significance for ensuring the safety and stability of the power grid. Under the action of electricity and heat, the oil-paper insulation inside the operating transformer gradually ages and generates substances such as CO, CO2, and furfural that reflect the degree of insulation deterioration. Among them, furfural molecules are only produced by the deterioration of cellulose-based paper insulation materials. Therefore, detecting the dissolved furfural content in transformer oil is one of the important technical means to evaluate the aging degree of the paper insulation inside the transformer. At present, the methods for determining the furfural content in transformer oil mainly include high-performance liquid chromatography, spectrophotometry, and colorimetry. These methods have high detection sensitivity, but they all require complex pretreatment such as extraction and distillation of transformer oil samples, and it is difficult to meet the requirements of live detection or in-situ detection.

[0003] Photothermal lens spectroscopy is mainly achieved by using the thermal lens effect of liquids. The thermal lens effect is a non-linear effect. When a laser with a specific wavelength irradiates the target liquid, the target molecules will absorb light energy, resulting in a local temperature increase. The change in temperature further changes the local refractive index, thus forming a liquid thermal lens. Currently, the liquid content analysis technology based on thermal lenses mainly adopts a single-beam configuration, and the light sources used mainly focus on the near-infrared band and the visible light band, which have limited applications in liquid content analysis and low detection sensitivity. Summary of the Invention

[0004] The object of the present invention is to address the problems existing in the prior art and provide a furfural detection system and method for transformer oil based on photothermal lens spectroscopy to achieve in-situ, rapid, accurate, and highly sensitive quantitative detection of trace furfural in transformer oil.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A furfural detection system for transformer oil based on photothermal lens spectroscopy, comprising:

[0007] A first laser, which is used to emit an infrared beam;

[0008] A second laser, which is used to emit a visible beam;

[0009] Visible-infrared dichroic mirror, a chopper, an infrared mirror, and an infrared plano-convex lens are sequentially arranged between the first laser and the visible-infrared dichroic mirror; an adjustable attenuator, a visible plano-convex lens, and a first visible mirror assembly are sequentially arranged between the second laser and the visible-infrared dichroic mirror;

[0010] A fixed optical path liquid cell is arranged in the output direction of the visible-infrared dichroic mirror, and the fixed optical path liquid cell is used to hold the transformer oil to be measured;

[0011] A second visible mirror assembly is arranged in the output direction of the fixed optical path liquid cell, a variable aperture is arranged in the output direction of the second visible mirror assembly, a photodetector is arranged in the output direction of the variable aperture, the photodetector is connected to a lock-in amplifier, and the lock-in amplifier is connected to a host computer.

[0012] This transformer oil furfural detection system is based on photothermal lens spectroscopy, using a quantum cascade laser as the pump laser and a helium-neon laser as the probe laser, realizing in-situ rapid, accurate, and highly sensitive quantitative detection of trace furfural in transformer oil; the first laser that covers the mid-infrared characteristic absorption line of furfural emits pump laser, and after trace furfural molecules in the transformer oil absorb the pump laser with an average power exceeding 100 mW, a strong photothermal lens effect is excited, significantly improving the detection performance; by using the photothermal lens effect generated by the absorption of mid-infrared light by furfural in the transformer oil, the in-situ and rapid detection of furfural in the transformer oil is realized; moreover, this detection system can detect transformer oils with different aging degrees, improving the applicability of its detection.

[0013] Further, the first laser is a quantum cascade laser, and the quantum cascade laser includes a laser emission head and a quantum cascade laser driver connected by a cable. By adjusting the quantum cascade laser driver, the output infrared light wavelength of the quantum cascade laser is adjusted to realize the infrared light wavelength scanning function, and the emission spectrum range covers the mid-infrared absorption line of furfural; the second laser is a helium-neon laser.

[0014] Further, the chopper includes a chopper controller, a chopper base, and a chopping blade. The modulation frequency of the chopper is 20 Hz to 1 kHz. The external reference output terminal of the chopper controller is connected to the external reference input terminal of the lock-in amplifier to realize the synchronization of the demodulation frequency of the lock-in amplifier and the chopper frequency, reducing noise.

[0015] Further, the first visible mirror assembly includes at least two visible mirrors, and the visible light beam reaches the visible-infrared dichroic mirror after being reflected at least twice.

[0016] Further, the second visible light mirror assembly includes 4 to 8 visible light mirrors, and the visible light beam reaches the variable aperture after being reflected 3 to 7 times.

[0017] Further, the visible-infrared dichroic mirror is arranged obliquely. The visible light beam is incident on the visible-infrared dichroic mirror and reaches the fixed optical path liquid cell, and the infrared light beam is reflected behind the visible-infrared dichroic mirror and then reaches the fixed optical path liquid cell.

[0018] Further, the output port of the photodetector is connected to the input port of the lock-in amplifier through a BNC cable, and the output port of the lock-in amplifier is connected to the host computer through a USB cable.

[0019] Further, the fixed optical path liquid cell is clamped and fixed by two transparent window pieces, and then clamped and fixed by two hollow metal pieces through four metal bolts. There is a metal liquid injection port outside one of the hollow metal pieces.

[0020] Further, the transparent window pieces and the hollow metal pieces have concentric circular holes leading directly to the fixed optical path liquid cell, and the thickness of the fixed optical path liquid cell is 100 to 1000 μm.

[0021] A detection method for a furfural detection system in transformer oil based on photothermal lens spectroscopy, the detection method includes the following steps:

[0022] Pump the standard transformer oil into the fixed optical path liquid cell;

[0023] Turn on the first laser, the second laser, the lock-in amplifier, the photodetector, the chopper and the host computer;

[0024] Turn on the laser output knob of the second laser, adjust the pitching angles of the first visible light mirror assembly and the second visible light mirror assembly, and at the same time observe the time-domain signal in the oscilloscope window of the lock-in amplifier. Stop adjusting when the amplitude of the time-domain signal is the largest;

[0025] Adjust and set the modulation frequency of the chopper;

[0026] Turn on the laser output button of the first laser, and record the signal amplitude a1 demodulated by the lock-in amplifier;

[0027] Pump the furfural transformer oil containing a known trace furfural concentration into the fixed optical path liquid cell, stop pumping when the demodulation signal of the lock-in amplifier is stable, and at the same time record the signal amplitude a2 demodulated by the lock-in amplifier;

[0028] Perform linear fitting on the two sets of measured data a1, a2 and the corresponding furfural concentrations to obtain a standard curve;

[0029] Pump the transformer oil sample to be tested into the fixed optical path liquid cell, and stop pumping when the demodulation signal of the lock-in amplifier is stable. At the same time, record the signal amplitude a3 demodulated by the lock-in amplifier;

[0030] Substitute the signal amplitude a3 into the standard curve to obtain the furfural concentration value in the transformer oil sample to be tested.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The furfural detection system in this transformer oil is based on photothermal lens spectroscopy. Using a quantum cascade laser as the pump laser and a helium-neon laser as the probe laser, it realizes in-situ, rapid, accurate, and highly sensitive quantitative detection of trace furfural in transformer oil; 2. By emitting pump laser through the first laser covering the mid-infrared characteristic absorption spectral line of furfural, after trace furfural molecules in transformer oil absorb pump laser with an average power exceeding 100 mW, a strong photothermal lens effect is excited, significantly improving the detection performance; 3. Utilizing the photothermal lens effect generated by the absorption of mid-infrared light by furfural in transformer oil, it realizes in-situ and rapid detection of furfural in transformer oil; 4. This detection system can detect transformer oils with different aging degrees, improving the applicability of its detection; 5. Compared with high-performance liquid chromatography, Raman spectroscopy, etc., this detection method avoids complex pretreatment such as extraction and distillation of transformer oil samples; 6. By adjusting the optical path conditions, including the distance from the center of the fixed liquid cell to the silicon-biased photodetector, the distance from the helium-neon laser beam to the center of the fixed liquid cell, etc., the performance of the detection system can be significantly improved; 7. The absorption of infrared light by the transformer oil sample to be tested in the fixed optical path liquid cell will change the local temperature distribution of the transformer oil sample to be tested, and then change the local refractive index, thereby causing a divergence effect on the visible light passing through the transformer oil sample to be tested; Since the infrared light is modulated by the chopper to generate periodic infrared light pulses, the divergence effect on the visible light of the transformer oil sample to be tested will also change periodically, manifested as a periodic change in the intensity of the local visible light spot. The change in the visible light spot intensity is converted into a voltage signal by the photodetector, so that the upper computer can obtain the output signal of the detection system. Description of the Drawings

[0032] Figure 1 It is an overall layout schematic diagram of a furfural detection system in transformer oil based on photothermal lens spectroscopy according to the present invention;

[0033] Figure 2 It is a time-domain signal diagram when detecting furfural in transformer oil according to the present invention;

[0034] Figure 3Schematic diagram for obtaining the furfural concentration value in the transformer oil sample to be measured based on the standard curve of the present invention;

[0035] In the figure: 1. First laser; 2. Second laser; 3. Chopper; 4. First light-transmitting window; 5. Second light-transmitting window; 6. Fixed optical path liquid cell; 7. Visible-infrared dichroic mirror; 8-15. Visible light reflecting mirrors; 16. Infrared light reflecting mirror; 17. Adjustable attenuator; 18. Visible light plano-convex lens; 19. Infrared light plano-convex lens; 20. Variable aperture; 21. Photoelectric detector; 22. Lock-in amplifier; 23. Host computer; 24. BNC cable; 25. Visible light beam; 26. Infrared light beam. Specific embodiments

[0036] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0037] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "middle", "upper", "lower", "left", "right", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. Embodiment 1

[0038] As Figure 1 shown, a furfural detection system in transformer oil based on photothermal lens spectroscopy includes:

[0039] A first laser 1, and the first laser 1 is used to emit an infrared light beam;

[0040] A second laser 2, and the second laser 2 is used to emit visible light;

[0041] A visible-infrared dichroic mirror 7, a chopper 3, an infrared light reflecting mirror 16 and an infrared light plano-convex lens 19 are sequentially arranged between the first laser 1 and the visible-infrared dichroic mirror 7, and an adjustable attenuator 17, a visible light plano-convex lens 18 and a first visible light reflecting mirror assembly are sequentially arranged between the second laser 2 and the visible-infrared dichroic mirror 7;

[0042] A fixed optical path liquid cell 6 is disposed in the output direction of the visible-infrared dichroic mirror 7, and the fixed optical path liquid cell 6 is used to contain the transformer oil to be tested;

[0043] A second visible light reflecting mirror assembly is disposed in the output direction of the fixed optical path liquid cell 6. A variable aperture 20 is provided in the output direction of the second visible light reflecting mirror assembly. A photodetector 21 is provided in the output direction of the variable aperture 20. The photodetector 21 is connected to a lock-in amplifier 22, and the lock-in amplifier 22 is connected to a host computer 23.

[0044] This transformer oil furfural detection system is based on photothermal lens spectroscopy. Using a quantum cascade laser as the pump laser and a helium-neon laser as the probe laser, it realizes in-situ, rapid, accurate, and highly sensitive quantitative detection of trace furfural in transformer oil; by emitting pump laser through a first laser that covers the mid-infrared characteristic absorption spectrum line of furfural, after trace furfural molecules in the transformer oil absorb the pump laser with an average power exceeding 100 mW, a strong photothermal lens effect is excited, significantly improving the detection performance; using the photothermal lens effect generated by the absorption of mid-infrared light by furfural in the transformer oil, in-situ and rapid detection of furfural in the transformer oil is realized; moreover, this detection system can detect transformer oils with different aging degrees, improving the applicability of its detection.

[0045] The first laser 1 and the second laser 2 can respectively emit different types of laser beams, which do not interfere with each other during use. The emitted beams respectively pass through corresponding processes and reach the visible-infrared dichroic mirror 7. The visible-infrared dichroic mirror 7 can not only separate and filter lights of different wavelengths, allowing lights of specific wavelengths to pass through, but also reflect the beams, making these two beams of light shoot towards the fixed optical path liquid cell together.

[0046] The setting of the chopper 3 can modulate the signal, suppress the noise signal, and improve the stability and reliability of the output infrared light. The visible light plano-convex lens 18 and the infrared light plano-convex lens 19 can converge Gaussian light or point light sources and convert them into Gaussian light. The infrared light reflecting mirror 16, the first visible light reflecting mirror assembly, and the second visible light reflecting mirror assembly can all change the propagation direction of the light beam and adjust the light beam direction. The setting of the visible light reflecting mirror assembly can also change the optical path.

[0047] The infrared light beam 26 emitted by the first laser 1 sequentially passes through the chopper 3, the infrared light mirror 16, and the infrared plano-convex lens 19 to reach the visible-infrared dichroic mirror 7, and after being reflected by the visible-infrared dichroic mirror 7, it reaches the fixed optical path liquid cell 6. Specifically, the infrared light with a specific frequency emitted by the first laser 1 is modulated by the chopper 3 to generate an infrared light pulse. The infrared light pulse changes its propagation direction after passing through the infrared light mirror 16 and passes through the infrared plano-convex lens 19. The focused infrared light pulse is reflected by the visible-infrared dichroic mirror 7 and shoots towards the transparent window and the fixed optical path liquid cell 6 to produce a thermal lens effect.

[0048] The visible light beam 25 emitted by the second laser 2 sequentially passes through the adjustable attenuator 17 (visible light adjustable attenuator), the visible light plano-convex lens 18, and the first visible light mirror assembly, and shoots towards the visible-infrared dichroic mirror 7. The visible-infrared dichroic mirror 7 will transmit the input visible light and reflect the input infrared light. Therefore, the visible light will sequentially pass through the visible-infrared dichroic mirror 7, the transparent window, and the fixed optical path liquid cell 6, then extend the optical path through the second visible light mirror assembly, and finally reach the photodetector 21 after passing through the variable aperture 20.

[0049] Specifically, the 632.8 nm visible light emitted by the second laser 2 is attenuated in light intensity by the adjustable attenuator 17, then focused by the visible light plano-convex lens 18, then sequentially passes through multiple visible light mirrors to enlarge the light spot, and then passes through the visible-infrared dichroic mirror and then through the transparent window and the fixed optical path liquid cell.

[0050] The infrared light will be absorbed by the furfural molecules in the transformer oil and excite a thermal lens effect, that is, a local negative lens is generated in the infrared light irradiation area. This negative lens will cause the visible light to converge and diverge. In order to amplify the change of the visible light, the visible light after passing through the fixed optical path liquid cell will be reflected by multiple visible light mirrors to extend the optical path. Subsequently, the visible light passes through the variable aperture to retain the area with the largest change in light intensity and passes through the aperture to shoot towards the photodetector.

[0051] The absorption of infrared light by the transformer oil sample to be measured in the fixed optical path liquid cell 6 will change the local temperature distribution of the transformer oil sample to be measured, and then change the local refractive index, thereby causing a divergence effect on the visible light passing through the transformer oil sample to be measured. Since the infrared light is modulated by the chopper 3 to generate periodic infrared light pulses, the divergence effect caused by the visible light of the transformer oil sample to be measured will also change periodically, manifested as a periodic change in the intensity of the local visible light spot. The change in the visible light spot intensity is converted into a voltage signal by the photodetector, and the time-domain waveform of the voltage signal can be presented in the form of a curve. The voltage signal is transmitted to the lock-in amplifier, and the lock-in amplifier can extract the photo-thermal lens spectral signal corresponding to the modulation frequency of the chopper, and finally transmitted to the host computer to obtain the output signal of the detection system.

[0052] Further, the first laser 1 is a quantum cascade laser. The quantum cascade laser includes a laser emitting head and a quantum cascade laser driver connected by a cable. By adjusting the quantum cascade laser driver, the output infrared light wavelength of the quantum cascade laser is adjusted to achieve the infrared light wavelength scanning function, and its central wavelength is 1703.3 cm -1 , and the emission spectrum range covers the mid-infrared absorption spectral line of furfural; the second laser is a helium-neon laser, which can emit visible light.

[0053] Further, the chopper 3 includes a chopper controller, a chopper base and a chopping blade. The modulation frequency of the chopper 3 is 20 Hz to 1 kHz. The external reference output terminal of the chopper controller is connected to the external reference input terminal of the lock-in amplifier 22 to synchronize the demodulation frequency of the lock-in amplifier with the chopper frequency and reduce noise.

[0054] In this embodiment, the first visible light mirror assembly includes two visible light mirrors (8 and 9). The visible light beam reaches the visible-infrared dichroic mirror after one reflection, and the visible light mirror at this place can enlarge the light spot.

[0055] The second visible light mirror assembly includes 6 visible light mirrors (10 to 15). The visible light beam reaches the variable aperture after 5 reflections. The multiple visible light mirrors at this place can not only enlarge the visible light, but also extend the optical path, and can adjust the path and direction of the visible light by adjusting the pitching angle of the visible light mirror. The visible light mirror is usually arranged on an adjustable optical bracket.

[0056] Further, the visible-infrared dichroic mirror 7 is arranged obliquely. The visible light beam passes through the visible-infrared dichroic mirror 7 and reaches the fixed optical path liquid cell 6, and the infrared light beam is reflected behind the visible-infrared dichroic mirror 7 and then reaches the fixed optical path liquid cell.

[0057] Further, the output port of the photodetector 21 is connected to the input port of the lock-in amplifier 22 through a BNC cable, and the output port of the lock-in amplifier 22 is connected to the host computer 23 through a USB cable. The voltage signal is transmitted to the lock-in amplifier through the BNC cable 24. The lock-in amplifier 22 can extract the photo-thermal lens spectral signal corresponding to the modulation frequency of the chopper 3, and finally transmit it to the host computer through the USB cable to obtain the output signal of the detection system.

[0058] Further, the fixed optical path liquid cell 6 is clamped and fixed by two transparent window plates (transparent window plate one 4 and transparent window plate two 5), and then clamped and fixed by two hollow metal sheets through four metal bolts. Two fixed metal liquid injection ports with an inner diameter of 4 mm are arranged on the outside of one of the hollow metal sheets.

[0059] Further, there are two concentric circular holes in the transparent window plate and the hollow metal sheet leading directly to the fixed optical path liquid cell for introducing the transformer oil to be measured. The transformer oil can be evenly pumped into the fixed optical path liquid cell through a peristaltic pump and a fluororubber peristaltic pump tube with an outer diameter of 4 mm.

[0060] The thickness of the fixed optical path liquid cell 6 is 100 - 1000 μm, and the thickness of the fixed optical path liquid cell can be 100 μm, 200 μm, 500 μm, 1000 μm and other lengths according to the detection performance requirements.

[0061] Further, the material of the transparent window plate can be selected from zinc selenide, calcium fluoride, potassium bromide, etc. Embodiment 2

[0062] A detection method for detecting furfural in transformer oil based on photo-thermal lens spectroscopy, combined with Figures 1-3 As shown, the detection method includes the following steps:

[0063] S1: Pump the standard transformer oil into the fixed optical path liquid cell 6; specifically, pour the standard transformer oil into a 50 ml beaker, put the fluororubber peristaltic pump tube into the beaker, turn on the peristaltic pump and adjust it to an appropriate speed, and slowly pump the standard transformer oil into the fixed optical path liquid cell 6. After the standard transformer oil flows out from the other side of the fluororubber peristaltic pump tube, turn off the peristaltic pump.

[0064] S2: Turn on the power supplies of devices such as the first laser 1, the second laser 2, the lock-in amplifier 22, the photodetector 21, the chopper 3 and the host computer 23.

[0065] S3: Turn on the laser output knob of the second laser 2 (helium-neon laser), adjust the pitch angles of the first visible light mirror assembly and the second visible light mirror assembly, and simultaneously observe the time-domain signal in the oscilloscope window of the lock-in amplifier 22. Stop adjusting when the amplitude of the time-domain signal is the largest.

[0066] S4: Adjust and set the modulation frequency of the chopper 3; set an appropriate chopper modulation frequency through the chopper controller, generally 30 Hz.

[0067] S5: Turn on the laser output button of the first laser 1 (quantum cascade laser), record the signal amplitude a1 demodulated by the lock-in amplifier 22, and pump out the standard transformer oil in the fixed optical path liquid cell 6.

[0068] S6: Pump the furfural transformer oil containing a known trace furfural concentration into the fixed optical path liquid cell 6. Stop pumping when the demodulation signal of the lock-in amplifier 22 is stable, and simultaneously record the signal amplitude a2 demodulated by the lock-in amplifier 22;

[0069] Specifically, pour the furfural / transformer oil containing a known trace furfural concentration into another 50 ml beaker, place the fluororubber peristaltic pump tube into the beaker, turn on the peristaltic pump and adjust it to an appropriate rotation speed, and slowly pump the furfural / transformer oil containing a known furfural concentration into the fixed optical path liquid cell 6. When the demodulation signal of the lock-in amplifier 22 is stable, turn off the peristaltic pump and simultaneously record the demodulation signal amplitude a2 of the lock-in amplifier 22, and pump out the furfural transformer oil containing a known trace furfural concentration in the fixed optical path liquid cell 6.

[0070] S7: Perform linear fitting on the two groups of measured data a1, a2 and the corresponding furfural concentrations to obtain a standard curve.

[0071] S8: Pump the transformer oil sample to be measured into the fixed optical path liquid cell 6. Specifically, pour the transformer oil sample to be measured into another 50 ml beaker, place the fluororubber peristaltic pump tube into the beaker, turn on the peristaltic pump and adjust it to an appropriate rotation speed, and slowly pump the transformer oil sample to be measured into the fixed optical path liquid cell 6. Stop pumping when the demodulation signal of the lock-in amplifier 22 is stable, and simultaneously record the signal amplitude a3 demodulated by the lock-in amplifier 22.

[0072] S9: Substitute a3 into the standard curve obtained in S7 to obtain the furfural concentration value in the transformer oil sample to be measured.

[0073] This detection method emits pump laser through a quantum cascade laser that covers the mid-infrared characteristic absorption spectral line of furfural. After trace furfural molecules in transformer oil absorb the pump laser with an average power exceeding 100 mW, a strong thermal lens effect is excited, significantly improving the detection performance. By utilizing the thermal lens effect generated by the absorption of mid-infrared light by furfural in transformer oil, in-situ and rapid detection of furfural in transformer oil is achieved. Compared with high-performance liquid chromatography, Raman spectroscopy, etc., this detection method avoids complex pretreatment such as extraction and distillation of transformer oil samples. By adjusting the optical path conditions, including the distance from the center of the fixed liquid cell to the silicon-biased photodetector, the distance of the helium-neon laser beam to the center of the fixed liquid cell, etc., the performance of the detection system can be significantly improved. To reduce the differential influence of surrounding environmental factors (including temperature, mechanical vibration, etc.) on the furfural detection by the detection system at each on-site measurement, a standard curve is calibrated with a known oil sample before each measurement, so as to avoid directly using the known standard curve obtained from experiments and ensure the consistency of the measurement results of trace furfural concentration.

[0074] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. Detection method of a furfural detection system in transformer oil based on photothermal lens spectroscopy, characterized in that, The furfural detection system includes: A first laser, which is used to emit an infrared beam; A second laser, which is used to emit a visible beam; A visible-infrared dichroic mirror. Between the first laser and the visible-infrared dichroic mirror, a chopper, an infrared mirror, and an infrared plano-convex lens are sequentially arranged. Between the second laser and the visible-infrared dichroic mirror, an adjustable attenuator, a visible plano-convex lens, and a first visible mirror assembly are sequentially arranged; A fixed optical path liquid cell arranged in the output direction of the visible-infrared dichroic mirror, and the fixed optical path liquid cell is used to hold the transformer oil to be tested; A second visible mirror assembly arranged in the output direction of the fixed optical path liquid cell. A variable aperture is arranged in the output direction of the second visible mirror assembly, and a photodetector is arranged in the output direction of the variable aperture. The photodetector is connected to a lock-in amplifier, and the lock-in amplifier is connected to a host computer; The detection method includes the following steps: Pump the standard transformer oil into the fixed optical path liquid cell; Turn on the first laser, the second laser, the lock-in amplifier, the photodetector, the chopper, and the host computer; Turn on the laser output knob of the second laser, adjust the pitching angles of the first visible mirror assembly and the second visible mirror assembly, and at the same time observe the time-domain signal in the oscilloscope window of the lock-in amplifier. Stop adjusting when the amplitude of the time-domain signal is the largest; Adjust and set the modulation frequency of the chopper; Turn on the laser output button of the first laser, and record the signal amplitude a1 demodulated by the lock-in amplifier; Pump the furfural transformer oil containing a known trace furfural concentration into the fixed optical path liquid cell. Stop pumping when the demodulation signal of the lock-in amplifier is stable, and at the same time record the signal amplitude a2 demodulated by the lock-in amplifier; Perform linear fitting on the two groups of measured data a1, a2 and the corresponding furfural concentrations to obtain a standard curve; Pump the transformer oil sample to be tested into the fixed optical path liquid cell. Stop pumping when the demodulation signal of the lock-in amplifier is stable, and at the same time record the signal amplitude a3 demodulated by the lock-in amplifier; Substitute the signal amplitude a3 into the standard curve to obtain the furfural concentration value in the transformer oil sample to be tested.

2. The detection method of the furfural detection system in transformer oil based on the photothermal lens spectrum according to claim 1, wherein, The first laser is a quantum cascade laser. The quantum cascade laser includes a laser emission head and a quantum cascade laser driver connected by a cable. The output infrared light wavelength of the quantum cascade laser is adjusted by adjusting the quantum cascade laser driver; the second laser is a helium-neon laser.

3. The detection method of the furfural detection system in transformer oil based on photothermal lens spectroscopy according to claim 1, characterized in that The chopper includes a chopper controller, a chopper base, and a chopping blade. The modulation frequency of the chopper is 20 Hz to 1 kHz, and the external reference output terminal of the chopper controller is connected to the external reference input terminal of the lock-in amplifier.

4. The detection method of the furfural detection system in transformer oil based on photothermal lens spectroscopy according to claim 1, wherein, The first visible mirror assembly includes at least two visible mirrors, and the visible beam reaches the visible-infrared dichroic mirror after at least two reflections.

5. The detection method of the furfural detection system in transformer oil based on photothermal lens spectroscopy according to claim 1, characterized in that, The second visible light mirror assembly includes 4 to 8 visible light mirrors, and the visible light beam reaches the variable aperture after 4 to 8 reflections.

6. The detection method of the furfural detection system in transformer oil based on the photo-thermal lens spectrum according to claim 1, characterized in that, The visible-infrared dichroic mirror is arranged obliquely. The visible light beam is incident on the visible-infrared dichroic mirror and reaches the fixed optical path liquid cell, and the infrared light beam is reflected behind the visible-infrared dichroic mirror and then reaches the fixed optical path liquid cell.

7. The detection method of the furfural detection system in transformer oil based on the photo-thermal lens spectrum according to claim 1, characterized in that, The output port of the photodetector is connected to the input port of the lock-in amplifier through a BNC cable, and the output port of the lock-in amplifier is connected to the host computer through a USB cable.

8. The detection method of the furfural detection system in transformer oil based on the photo-thermal lens spectrum according to claim 1, characterized in that, The fixed optical path liquid cell is clamped and fixed by two transparent window plates, and then clamped and fixed by two hollow metal sheets through four metal bolts. There is a metal liquid injection port on the outside of one of the hollow metal sheets.

9. The detection method of the furfural detection system in transformer oil based on photothermal lens spectroscopy according to claim 8, characterized in that, The transparent window plate and the hollow metal sheet have concentric circular holes leading to the fixed optical path liquid cell, and the thickness of the fixed optical path liquid cell is 100 to 1000 μm.

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