Conical-spherical coupled cavity photoacoustic gas sensor for detecting acetylene dissolved in transformer oil

Abstract

The invention discloses a cone-ball coupled cavity photoacoustic gas sensor for detecting acetylene dissolved in transformer oil, relates to the field of testing or analyzing materials by using an optical means, and solves the problem that miniaturization and high sensitivity of an existing photoacoustic sensor are difficult to consider at the same time. An optical fiber collimator is arranged at one end of the shell, a sonic sensor is arranged at the other end of the shell, a buffer cavity, a first resonant cavity and a second resonant cavity are sequentially arranged in the shell between the optical fiber collimator and the sonic sensor, the end of the optical fiber collimator is located in the buffer cavity, and the center lines of the optical fiber collimator, the buffer cavity, the resonant cavities and the sonic sensor coincide. The buffer cavity is cylindrical, an air inlet and an air outlet are formed in the side wall of the buffer cavity, the first resonant cavity is in a circular truncated cone shape, and the second resonant cavity is in a segment shape. The buffer cavity, the first resonant cavity and the second resonant cavity are coupled to form a cylindrical-conical-spherical coupled cavity, so that limitation and focusing of acoustic energy are realized, and the amplitude of a photoacoustic signal is enhanced.

Description

Technical Field

[0001] This invention relates to the field of testing or analyzing materials by measuring their chemical or physical properties, and more particularly to the field of testing or analyzing materials using optical means, specifically a photoacoustic gas sensor for detecting multiple gases dissolved in transformer oil. Background Technology

[0002] During long-term operation, oil-immersed transformers are susceptible to localized overheating, short circuits, and discharge events due to thermal and electrical stresses. These events can cause degradation of insulating materials such as oil and paper, producing characteristic gases dissolved in the insulating oil. Acetylene is widely considered a key tracer for high-energy discharge events; therefore, continuous and accurate monitoring of acetylene is an indispensable part of transformer early warning and predictive maintenance.

[0003] Currently, various methods exist for analyzing dissolved gases in insulating oils. Traditional methods such as gas chromatography and electrochemical sensing are well-established, but they have limitations in online deployment. Column aging, gas cross-sensitivity, and baseline drift can compromise long-term stability and accuracy. Spectroscopic methods have emerged as a compelling alternative due to their non-contact nature, high selectivity, intrinsic safety, and immunity to electromagnetic interference. Tunable diode laser absorption spectroscopy offers quantitative precision, but it is susceptible to background fluctuations and optical path limitations, ultimately restricting its application.

[0004] In photoacoustic spectroscopy, the modulated light energy absorbed by gas molecules is converted into heat, causing their periodic thermoelastic expansion and contraction. This generates acoustic pressure waves within the photoacoustic cell, which are captured by an acoustic sensor, converting molecular absorption into an electrical signal. This technology is independent of optical path length, avoiding expensive photodetectors and facilitating the development of compact, low-cost, field-deployable gas analyzers for continuous, high-sensitivity monitoring. The sensitivity of photoacoustic spectroscopy depends on the synergistic performance of the two core components: the photoacoustic cell and the acoustic sensor. Resonant unit structures, especially compact T-shaped photoacoustic cells, reduce sample volume while enhancing acoustic amplification.

[0005] The development of acoustic sensors has progressed from traditional capacitive microphones to high-Q quartz tuning forks and fiber optic Fabry-Perot cantilever beam acoustic sensors, achieving high-sensitivity detection of trace gases. However, employing advanced optical microphones typically involves trade-offs in cost, demodulation complexity, and system stability. Therefore, realizing the detection of acetylene gas in oil using a photoacoustic cavity-enhanced photoacoustic acetylene gas sensor remains a key technology for power transformer diagnostics. Summary of the Invention

[0006] This invention provides a cone-sphere coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil, solving the problem of existing photoacoustic sensors struggling to balance miniaturization and high sensitivity.

[0007] The technical solution adopted in this invention is as follows: a cone-spherical coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil, comprising a housing, an optical fiber collimator at one end of the housing, and an acoustic wave sensor at the other end. Inside the housing between the optical fiber collimator and the acoustic wave sensor, a buffer cavity, a first resonant cavity, and a second resonant cavity are sequentially arranged, interconnected. The end of the optical fiber collimator is located within the buffer cavity, and the acoustic wave sensor is located at the end of the second resonant cavity opposite to the first resonant cavity. The centerlines of the optical fiber collimator, buffer cavity, resonant cavity, and acoustic wave sensor coincide. The buffer cavity is cylindrical, with an inlet and an outlet on its sidewall. The first resonant cavity is frustum-shaped, with the major circle radius of the frustum matching the radius of the buffer cavity. The second resonant cavity is spherical, with the height of the spherical cap greater than its radius, and the radius of the spherical cap greater than the minor circle radius of the frustum. The radius of the bottom surface of the spherical cap matches the minor circle radius of the frustum.

[0008] Specifically: the radius of the buffer cavity is 10mm and the height is 20mm.

[0009] Specifically: the large circle radius of the first resonant cavity is 10mm, the small circle radius is 0.6mm, and the height is 20mm.

[0010] Specifically: the radius of the second resonant cavity is 1.2 mm.

[0011] Specifically: the acoustic sensor is a capacitive microphone or an optical fiber-based acoustic sensor.

[0012] The beneficial effects of this invention are: the coupling of the first resonant cavity and the second resonant cavity to form a cone-sphere coupling cavity effectively realizes the limitation and focusing of acoustic energy, significantly enhances the amplitude of photoacoustic signal, thereby breaking the inherent trade-off between sensor miniaturization and performance.

[0013] The buffer cavity, the first resonant cavity, and the second resonant cavity are coupled to form a cylindrical-conical-spherical coupling cavity. This invention optimizes the geometry of the cylindrical-conical-spherical coupling cavity to generate four independent resonant frequencies: 5960 Hz, 9820 Hz, 13800 Hz, and 16800 Hz. These four resonant frequencies are respectively matched with the optimal detection frequencies for the four key characteristic gases dissolved in transformer oil: methane, acetylene, carbon monoxide, and carbon dioxide.

[0014] This invention employs wavelength-modulated photoacoustic spectroscopy, using DFB lasers with center wavelengths of 1653.7 nm (methane), 1532.8 nm (acetylene), 1568 nm (carbon monoxide), and 1580 nm (carbon dioxide) as pump sources. This enables highly selective and sensitive detection of various characteristic gases dissolved in transformer oil, providing a highly sensitive, field-deployable platform for dissolved gas analysis in transformer oil, thus possessing broad application prospects. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a structure according to an embodiment of the present invention.

[0016] Figure 2 This is the frequency response diagram of the present invention.

[0017] Reference numerals: housing 1, buffer cavity 1-1, first resonant cavity 1-2, second resonant cavity 1-3, air inlet 1-4, air outlet 1-5, fiber optic collimator 2, acoustic sensor 3. Detailed Implementation

[0018] The invention will now be further described with reference to the accompanying drawings.

[0019] This invention provides a photoacoustic gas sensor that is highly sensitive, compact, and stable over long periods. Specifically, this invention provides a cylindrical-cone-sphere coupled photoacoustic gas sensor for detecting various gases dissolved in transformer oil, rather than a conical-sphere coupled cavity photoacoustic gas sensor for detecting acetylene dissolved in transformer oil.

[0020] like Figure 1 As shown, the present invention includes a housing 1, with an optical fiber collimator 2 at one end and an acoustic wave sensor 3 at the other end. Figure 1 In the illustrated embodiment, a fiber optic collimator 2 is located at the left end of the housing 1, and an acoustic sensor 3 is located at the right end. A buffer cavity 1-1, a first resonant cavity 1-2, and a second resonant cavity 1-3 are sequentially arranged between the fiber optic collimator 2 and the acoustic sensor 3. The buffer cavity 1-1, the first resonant cavity 1-2, and the second resonant cavity 1-3 are all located inside the housing 1 and are interconnected. The first resonant cavity 1-2 and the second resonant cavity 1-3 are coupled to form a cone-sphere coupling cavity. The end of the fiber optic collimator 2 is located inside the buffer cavity 1-1. The fiber optic collimator 2 is used to collimate and couple the excitation laser beam from an external light source into the buffer cavity 1-1. The centerlines of the fiber optic collimator 2, the buffer cavity 1-1, the resonant cavity, and the acoustic sensor 3 coincide.

[0021] The sidewall of the buffer chamber 1-1 is provided with an air inlet 1-4 and an air outlet 1-5 for the introduction and discharge of the gas to be measured. For example, the air inlet 1-4 and the air outlet 1-5 are arranged opposite each other, that is, the air inlet 1-4 and the air outlet 1-5 are arranged along the same straight line, and this straight line is perpendicular to the center line of the buffer chamber 1-1. The air inlet 1-4 and the air outlet 1-5 are located on the sidewall of the buffer chamber 1-1 to ensure that they are located at the position of minimum sound pressure, thereby minimizing the damage to the integrity of the cavity's sound field and the quality factor (Q value).

[0022] The buffer cavity 1-1 also serves to ensure good collimation when the laser beam enters the first resonant cavity 1-2. The buffer cavity 1-1 is cylindrical, with a radius of 10 mm and a height of 20 mm.

[0023] The first resonant cavity 1-2 is used to smoothly transition acoustic wave energy from the buffer cavity 1-1 to the second resonant cavity 1-3 and to initially focus it. The first resonant cavity 1-2 is shaped like a frustum, with the radius of its major circle matching that of the buffer cavity 1-1. The major circle end of the first resonant cavity 1-2 is coupled to the buffer cavity 1-1. The two bases of the frustum are a major circle and a minor circle, and the distance between the major and minor circles is the height of the frustum. Specifically, the radius of the major circle of the first resonant cavity 1-2 is 10 mm, the radius of the minor circle is 0.6 mm, and the height is 20 mm.

[0024] The second resonant cavity 1-3 is coupled to the small circular end of the first resonant cavity 1-2. The second resonant cavity 1-3 is shaped like a spherical cap, with its height greater than its radius, and its radius greater than the radius of the small circle of the corresponding frustum. The radius of the bottom surface of the spherical cap is the same as the radius of the small circle of the frustum. The preferred radius of the second resonant cavity 1-3 is 1.2 mm. As a key part of the acoustic resonant cavity, the second resonant cavity 1-3 is used to achieve high-level confinement and focusing of sound wave energy, ensuring that the sound wave reaches maximum sound pressure at the sound wave sensor 3.

[0025] The aforementioned buffer cavity 1-1, first resonant cavity 1-2, and second resonant cavity 1-3 are coupled to form a cylindrical-conical-spherical coupling cavity. Their dimensions are all optimized optimal design parameters. Under these conditions, the cavity can maintain extremely high photoacoustic signal gain at the resonant frequency.

[0026] The acoustic wave sensor 3 is located at the end of the second resonant cavity 1-3, that is, at the end of the second resonant cavity 1-3 opposite to the first resonant cavity 1-2. This position is the optimized region of maximum sound pressure, used to collect the photoacoustic signal amplified by the cone-sphere coupling cavity resonance. The acoustic wave sensor 3 can be a high-performance condenser microphone or an optical fiber-based acoustic wave sensor, and its function is to convert periodic sound pressure waves into electrical signals.

[0027] The excitation laser enters the inner cavity of the housing 1 through the fiber collimator 2. The gas molecules being measured absorb and modulate the light energy, generating an acoustic signal. This invention optimizes the dimensions of the cylindrical-conical-spherical coupling cavity, significantly amplifying the photoacoustic signal at a specific resonant frequency. The acoustic sensor 3 acquires the signal at the position of maximum sound pressure, and through subsequent wavelength modulation spectral demodulation technology, highly sensitive quantitative analysis of the target gas concentration can be achieved.

[0028] Figure 2 This is the frequency response diagram of the present invention. The structure and dimensions of its photoacoustic cell have been optimized to generate four main resonant frequencies: 5960 Hz, 9820 Hz, 13800 Hz, and 16800 Hz, corresponding to the optimal operating frequencies for detecting methane, acetylene, carbon monoxide, and carbon dioxide, respectively. Combined with wavelength modulation spectral demodulation technology, this invention achieves extremely low detection limits for trace multi-gases while maintaining a compact size, providing a feasible monitoring solution for online, high-precision, and high-sensitivity detection of dissolved gases in transformer oil in power systems.

Claims

1. A cone-spherical coupled cavity photoacoustic gas sensor for detecting acetylene dissolved in transformer oil, comprising a housing (1), one end of which is provided with an optical fiber collimator (2), and the other end with an acoustic wave sensor (3). Inside the housing (1) between the optical fiber collimator (2) and the acoustic wave sensor (3), there are sequentially arranged a buffer cavity (1-1), a first resonant cavity (1-2), and a second resonant cavity (1-3). The buffer cavity (1-1), the first resonant cavity (1-2), and the second resonant cavity (1-3) are interconnected. The end of the optical fiber collimator (2) is located inside the buffer cavity (1-1), and the acoustic wave sensor (3) is located at the end of the second resonant cavity (1-3) opposite to the first resonant cavity (1-2). The center lines of the optical fiber collimator (2), the buffer cavity (1-1), the resonant cavity, and the acoustic wave sensor (3) coincide. The buffer cavity (1-1) is cylindrical, and the side wall of the buffer cavity (1-1) is provided with an inlet (1-4) and an outlet (1-5). The sensor is characterized by: The first resonant cavity (1-2) is shaped like a frustum, and the radius of the major circle of the frustum is the same as the radius of the buffer cavity (1-1). The second resonant cavity (1-3) is shaped like a spherical cap, and the height of the spherical cap is greater than the radius of the spherical cap. The radius of the spherical cap is greater than the radius of the minor circle of the frustum, and the radius of the bottom surface of the spherical cap is the same as the radius of the minor circle of the frustum.

2. The cone-spherical coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil as described in claim 1, characterized in that: The buffer cavity (1-1) has a radius of 10 mm and a height of 20 mm.

3. The cone-sphere coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil as described in claim 1, characterized in that: The first resonant cavity (1-2) has a large circle radius of 10 mm, a small circle radius of 0.6 mm, and a height of 20 mm.

4. The cone-spherical coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil as described in claim 1, characterized in that: The radius of the second resonant cavity (1-3) is 1.2 mm.

5. The cone-sphere coupled cavity photoacoustic gas sensor for detecting dissolved acetylene in transformer oil as described in any one of claims 1 to 4, characterized in that: The acoustic wave sensor (3) is a capacitive microphone or an optical fiber-based acoustic wave sensor.

Images