Dual mach-zehnder fiber sensing phase noise suppression system and method for GIS discharge defect detection

Abstract

This invention belongs to the field of power equipment operation and maintenance, specifically proposing a dual Mach-Zehnder fiber optic sensing phase noise suppression system and method for GIS discharge defect detection. The laser output light is fed into a first coupler and split into two paths: one path enters a second coupler and is split again into a sensing optical path and a reference optical path; the sensing fiber is split by a fifth coupler, and the reference fiber is split by a fourth coupler. The two split signals interfere at a sixth coupler to form a self-interference reference signal; the output light from the sensing and reference optical paths converges into a third coupler, generating clockwise interference; the other path split from the first coupler is guided in the opposite direction by a guide fiber into the sensing and reference optical paths, forming counterclockwise interference at the second coupler. Phase demodulation of the clockwise and counterclockwise interference signals is performed based on the self-interference reference signal. This invention achieves phase noise suppression based on a dual Mach-Zehnder fiber optic sensing architecture and is suitable for GIS discharge defect detection.

Description

Technical Field

[0001] This invention belongs to the field of power equipment operation and maintenance, and specifically relates to a dual Mach-Zehnder fiber optic sensing phase noise suppression system and method for detecting discharge defects in GIS. Background Technology

[0002] Gas-insulated circuit breakers (GIS) are core equipment in the power transmission network of hydropower stations, and their insulation condition directly affects the reliability of power supply. However, during the production, installation, and long-term operation of GIS, defects such as metal particles are inevitably generated or introduced. These defects can induce partial discharge under high electric fields, and continuous discharge will gradually degrade the insulation performance of the GIS, eventually leading to insulation breakdown accidents. Therefore, it is essential to detect partial discharge signals in GIS and identify defects as early as possible.

[0003] Among various partial discharge detection methods, acoustic emission is an important one. Because partial discharge diffracts acoustic signals, maintenance personnel often use piezoelectric ceramic (PZT) sensors, mounting them on the outer wall of the GIS cavity to capture these acoustic signals. However, PZT sensors can only perform single-point sensing and cannot form a distributed detection system through series multiplexing, resulting in low detection accuracy. In recent years, many research teams at home and abroad have begun to explore acoustic emission sensing technology based on fiber optic interferometry. One of the key research directions is to realize the multiplexing of fiber optic sensors to make up for the shortcomings of PZT.

[0004] Currently, fiber optic sensing multiplexing technologies for detecting acoustic emission signals from discharges are mainly divided into two categories: backscattering and forward interferometry. The former, typically represented by a phase-sensitive optical time-domain reflectometer (φ-OTDR), utilizes the interference between Rayleigh scattered light and intrinsic light in the optical fiber to achieve fully distributed sensing. This method is compact and has high spatial resolution. However, due to the weakness of Rayleigh scattered light, the φ-OTDR system has low sensitivity and can only detect large-amplitude acoustic signals during breakdown or severe discharge. In contrast, forward interferometry fiber optic sensing technology has the advantage of high sensitivity, making it more suitable for detecting weak acoustic emission signals from discharges. Based on differences in interference structure, they can be mainly divided into three types: dual Michelson, dual Mach-Zehnder, and unbalanced Mach-Sagnac. Among them, the dual Mach-Zehnder type has advantages such as fewer optical components and a simpler topology, and is currently the main fiber optic sensor multiplexing scheme, which has been implemented in perimeter security and large-structure health monitoring.

[0005] In the field of electrical equipment discharge defect detection, there are no research reports on acoustic emission detection using dual Mach-Zehnder fiber optic interferometers; all studies use single-channel Mach-Zehnder structures, which can only perform single-point detection of partial discharges. The main reason is that the optical path topology of dual-channel MZ fiber optic interferometers is more complex, resulting in higher noise levels. For example, the phase noise level of a typical single-channel MZ interferometer is about -100dB (0dB=1rad / √Hz), while the phase noise of a dual-channel MZ interferometer can reach -60dB. The acoustic emission signal intensity of partial discharge is far less than the disturbance intensity caused by intrusion events or structural damage, making it highly sensitive to the noise level of the sensing system. The noise level of existing dual-channel MZ interferometers masks the acoustic emission signal of the discharge, making it impossible to detect discharge defects or utilize the multiplexing advantage for multi-point synchronous detection. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a dual Mach-Zehnder fiber optic sensing phase noise suppression system and method for GIS discharge defect detection, which achieves noise suppression based on dual Mach-Zehnder fiber optic sensing to meet the needs of GIS discharge defect detection.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection, comprising a laser for generating narrow-linewidth continuous light to provide a light source for the system, the light entering a first coupler after passing through an isolator, the first coupler splitting the light into two optical signals, one of which enters a second coupler, the second coupler splitting the optical signal into two more paths, one entering a sensing optical path composed of sensing fibers, and the other entering a reference optical path consisting of a series acousto-optic modulator and a reference fiber, a fifth coupler connected in series on the sensing fiber, splitting the optical signal on the sensing fiber into two paths, one continuing to output along the sensing fiber, and the other converging into a sixth coupler, the reference fiber connected in series with a fifth coupler... The four-coupler splits the optical signal on the reference fiber into two paths. One path continues to be output along the reference fiber, while the other path merges into the sixth coupler. The two optical signals from the fifth and fourth couplers interfere with each other at the sixth coupler, generating a self-interference reference signal. This signal is then converted into an optical signal by the third photodetector. The light output from the sensing fiber and the reference fiber merges into the third coupler. After clockwise interference at the third coupler, the light is converted into an optical signal by the second photodetector. The other optical signal output from the first coupler enters the third coupler via the guide fiber. It then enters the sensing optical path and the reference optical path in reverse through the third coupler. After counterclockwise interference at the second coupler, the light is converted into an optical signal by the first photodetector. The signal processing unit is connected to the first photodetector, the second photodetector and the third photodetector respectively, and is used to perform phase demodulation on the clockwise interference signal and the counterclockwise interference signal according to the self-interference reference signal.

[0008] In a preferred embodiment, the signal processing unit includes: a power divider, a first phase detection module, and a second phase detection module; The power divider is connected to the third photodetector and is used to divide the self-interference reference signal after photoelectric conversion into two paths to obtain the first reference signal and the second reference signal. The first phase detection module is connected to the first photodetector and the power divider respectively, and is used to receive the clockwise interference signal and the first reference signal, and output the phase difference signal between the clockwise interference signal and the first reference signal. The second phase detection module is connected to the second photodetector and the power divider respectively. It is used to receive the counterclockwise interference signal and the second reference signal, and output the phase difference signal between the counterclockwise interference signal and the second reference signal.

[0009] In a preferred embodiment, both the first phase detection module and the second phase detection module are AD8302 phase detection chips.

[0010] In a preferred embodiment, the self-interference reference signal carries the actual frequency shift information of the acousto-optic modulator.

[0011] In a preferred embodiment, the phase difference signal output by the first phase detection module is a phase change Δφ(t) characterizing the amount of phase change caused by the discharge acoustic emission signal in the first direction; the phase difference signal output by the second phase detection module is a phase change Δφ(t+τ) characterizing the amount of phase change caused by the discharge acoustic emission signal in the second direction; where τ is the time difference between the clockwise interference signal and the counterclockwise interference signal.

[0012] In the preferred embodiment, the time difference τ satisfies the following relationship: ; In the formula, L The total length of the sensing fiber or reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν The speed of light in an optical fiber; The spatial location of the acoustic emission signal's point of action is calculated based on the time difference τ. x The value.

[0013] In a preferred embodiment, the frequency of the self-interference reference signal output by the sixth coupler is consistent with the frequencies of the clockwise and counterclockwise interference signals, and can change synchronously with the actual frequency shift of the acousto-optic modulator, thereby achieving frequency synchronization.

[0014] In a preferred embodiment, the high-frequency carrier component introduced by the acousto-optic modulator is canceled out in the phase difference signal output by the first phase detection module and the second phase detection module, and only the low-frequency phase change information caused by the discharge acoustic emission signal is retained.

[0015] In a preferred embodiment, the frequency of the phase difference signal is on the order of tens of kHz, which is lower than the frequency of the original interference signal output by the first photodetector and the second photodetector.

[0016] In a preferred embodiment, the time difference τ is obtained by calculating the phase difference between the two signals using a cross-correlation algorithm. The spatial location of the acoustic signal's point of action is then obtained by substituting the time difference τ into the position calculation formula. x This enables spatial positioning of acoustic emission signals.

[0017] This invention discloses a dual Mach-Zehnder fiber optic sensor phase noise suppression method for GIS discharge defect detection, characterized by employing the system described in any one of claims 1 to 10 for noise suppression, comprising the following steps: S1. Light source output and splitting steps: The laser outputs a continuous optical signal, which is input into the first coupler after passing through the isolator. The first coupler splits the optical signal into two paths. S2. Dual Mach-Zehnder Interference Optical Path Transmission: One optical signal output from the first coupler is input to the second coupler, which then splits it into two paths. One path enters the sensing optical path with the sensing fiber, and the other path enters the reference optical path with the acousto-optic modulator and the reference fiber connected in series. The optical signals from the sensing optical path and the reference optical path interfere clockwise at the third coupler and are converted into clockwise interference electrical signals by the second photodetector. The other optical signal output from the first coupler is input to the third coupler via the guide fiber and enters the sensing optical path and the reference optical path in the reverse direction. Finally, it interferes counterclockwise at the second coupler and is converted into counterclockwise interference electrical signals by the first photodetector. S3. Self-interference reference signal generation: The fifth coupler draws out part of the optical signal from the sensing fiber, and the fourth coupler draws out part of the optical signal from the reference fiber. The two optical signals are input together into the sixth coupler to interfere and generate a self-interference reference optical signal, which is then converted into a self-interference reference electrical signal by the third photodetector. S4. Synchronous Phase Demodulation and Noise Suppression: Using the self-interference reference signal as the common demodulation standard, the clockwise and counterclockwise interference signals are demodulated to obtain two phase difference signals. The phase noise introduced by the frequency shift deviation of the acousto-optic modulator is suppressed by the frequency synchronization characteristics of the self-interference reference signal and the two interference signals.

[0018] In a preferred embodiment, in step S3, a fifth coupler is connected in series on the sensing fiber, and a beam of optical signal is extracted from the sensing fiber through the fifth coupler; a fourth coupler is connected in series on the reference fiber, and a beam of optical signal is extracted from the reference fiber through the fourth coupler; the optical signal extracted from the fifth coupler and the optical signal extracted from the fourth coupler are input together into a sixth coupler to interfere and generate a self-interference reference optical signal.

[0019] In the preferred embodiment, step S5, signal acquisition and determination, is further included: acquiring two phase difference signals using a signal acquisition module, reflecting the acoustic emission signal generated by the GIS discharge defect through time-domain waveform; when the amplitude of the detected phase signal exceeds the lower limit of the system phase detection, it is determined to be a valid discharge acoustic emission signal.

[0020] In a preferred embodiment, the phase difference demodulation process in step S4 specifically includes: S41. Input the self-interference reference electrical signal output from the third photodetector into the power divider, and the power divider divides it into the first reference signal and the second reference signal. S42. Input the first reference signal and the counterclockwise interference electrical signal output by the first photodetector into the first phase detection module, demodulate the phase difference between the two signals, and obtain the first phase difference signal. The first phase difference signal characterizes the phase change Δφ(t) caused by the discharge acoustic emission signal acting on the sensing fiber. S43. Input the second reference signal and the clockwise interference electrical signal output by the second photodetector into the second phase detection module, demodulate the phase difference between the two signals, and obtain the second phase difference signal. The second phase difference signal characterizes the phase change Δφ(t+τ) caused by the discharge acoustic emission signal acting on the sensing fiber, where τ is the transmission time difference between the clockwise interference signal and the counterclockwise interference signal.

[0021] In the preferred embodiment, S44, spatial positioning of the acoustic emission signal, specifically involves: performing a cross-correlation algorithm on the first phase difference signal and the second phase difference signal to obtain the time difference τ between the two signals; substituting the time difference τ into the position calculation formula to solve for the spatial position of the acoustic signal's point of action. x The calculation formula is: ; In the formula, LThe total length of the sensing fiber or reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν is the speed of light in the optical fiber.

[0022] The present invention provides a dual Mach-Zehnder fiber optic sensing phase noise suppression system and method for detecting discharge defects in GIS, which has the following beneficial effects: 1. In a dual Mach-Zehnder fiber optic interferometric sensing system, partial optical signals are extracted from the sensing optical path and the reference optical path, respectively. These signals are then re-interfered via a coupler to form a self-interfering reference signal, which is then demodulated through photoelectric conversion. Unlike existing technologies that use function generators or digital signal sources to generate fixed-frequency reference signals, the reference signal of this invention originates directly from the original interferometric optical path, thus naturally carrying the actual frequency shift information of the acousto-optic modulator (AOM). It does not rely on an external frequency source, does not require pre-calibration of the actual frequency shift of the AOM, and the reference signal has completely consistent frequency characteristics with the sensing signal, eliminating phase noise introduced by the frequency shift deviation of the AOM.

[0023] 2. This invention utilizes a reference signal generated by a self-interference structure as a common reference signal. This reference signal is then input into a phase detection module along with interference signals generated by clockwise and counterclockwise transmission from a dual Mach-Zehnder system for phase difference demodulation, yielding two phase difference signals characterizing acoustic emission disturbances. Unlike traditional IQ demodulation, which requires simultaneous acquisition of multiple high-frequency interference signals and digital computation, this invention directly extracts the phase difference between the reference signal and the sensing signal using a phase detection chip. This automatically cancels out the high-frequency carrier introduced by the acousto-optic modulator during demodulation, retaining only the low-frequency phase change information caused by the acoustic emission signal.

[0024] The above measures eliminate the need for synchronous high-speed acquisition of three high-frequency interference signals, reducing the number of data acquisition channels; they also reduce the signal sampling rate requirement, decrease the computational load of subsequent digital signal processing, and lower the system hardware cost. Attached Figure Description

[0025] The accompanying drawings, which are provided to further illustrate the invention and constitute a part of this invention, do not constitute an undue limitation thereof. In the drawings: Figure 1 This is the basic structure of a dual Mach-Zehnder fiber optic interferometric acoustic emission sensing system. Figure 2 This invention introduces a dual Mach-Zehnder fiber optic interferometric acoustic emission sensing system with a self-interference structure; Figure 3 This is a schematic diagram of the interference signal processing method; Figure 4 The diagram shows the phase noise time-domain representation of the DMZ system before and after noise suppression. In the figure, there is a laser 1, an isolator 2, a first coupler 3, a second coupler 4, a sensing fiber 5, an acousto-optic modulator 6, a reference fiber 7, a third coupler 8, a fourth coupler 9, a fifth coupler 10, a sixth coupler 11, a guide fiber 12, a first photodetector 13, a second photodetector 14, a third photodetector 15, a power divider 16, a first phase detection module 17, and a second phase detection module 18. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0027] Example 1: The basic structure of the dual Mach-Zehnder fiber interferometric acoustic emission sensing system is as follows: Figure 1 As shown. Its working principle is as follows: The light emitted from the narrow-linewidth laser is split into two beams at coupler 1# after passing through an isolator. One beam enters coupler 2# and is split again into two paths: one is the sensing optical path, and the other is the reference optical path connected in series with an acousto-optic modulator. These two beams interfere (clockwise) at coupler 3# and are then converted into an electro-optic signal by photodetector 2#, with the AC component of the output interference signal being U2. The other beam split from coupler 1# passes through a guiding fiber and enters the sensing and reference optical paths in the reverse direction via coupler 3#, ultimately interfering (clockwise) at coupler 1# again. This time, the photodetector 1# is again converted into an electro-optic signal, with the AC component of the output interference signal being U1. Figure 1 In this process, the acousto-optic modulator shifts the frequency of light passing through the sensing fiber, introducing a high-frequency carrier wave into the interference signal to eliminate interference fading. When the acoustic signal generated by the discharge acts on the sensing fiber, it causes a change in the fiber length, thereby altering the phase of the interference light. By demodulating the change in the phase of the interference light, the discharge acoustic emission signal can be obtained.

[0028] Assuming the interference light signal intensities are the same for clockwise and counterclockwise propagation, then: (1); (2); In the formula, The amplitude of the AC component of the interference optical signal. This is the frequency shift amount of the acousto-optic modulator. This represents the change in phase of the interference light caused by the external acoustic emission signal acting on the sensing fiber. and Let be the initial phases of the clockwise and counterclockwise interfering optical signals, respectively, and τ be the time difference between the two sets of interfering signals. Then: (3); In the formula, L Total length of sensing fiber / reference fiber, the sensing fiber and reference fiber are of equal length; To guide the length of the optical fiber; x ν is the distance from the point of action of the acoustic signal to coupler 2#; ν is the speed of light in the optical fiber.

[0029] The changes in phase of the clockwise and counterclockwise interfering light, Δφ(t) and Δφ(t+τ), can be obtained using the IQ demodulation algorithm, the specific process of which is as follows: Generate a set of orthogonal reference signals with the same frequency as the AC components U1 and U2 of the interference optical signal. and : (4); (5); In the formula, For the reference signal amplitude, we can take... R 0 = 1V.

[0030] The clockwise transmitted interference signal U1 is multiplied by R and R' respectively, and then passed through the cutoff frequency. f c f A low-pass filter with a value of 0 is used to filter out high-frequency components, resulting in two sets of baseband signals: (6); (7); After performing differentiation-cross multiplication on the two sets of baseband signals, calculate the difference and then divide by. , The sum of squares gives: (8); Assumption By keeping the equation constant and integrating equation (8), the phase change of the interference light can be obtained. : (9); Similarly, the phase of the interference light propagating in the opposite direction can be obtained. Δφ (t+τ).

[0031] Then, the time difference τ is obtained through the cross-correlation algorithm, and substituted into formula (3) to obtain the result. x This means that the spatial positioning of the point of action of the acoustic emission signal is achieved.

[0032] for Figure 1 The dual Mach-Zehnder interferometric fiber optic sensing system shown has an error factor in each component, introducing phase noise into the system. Considering the noise effect, the output optical signals of the two interferometers should be expressed as: (10); (11); Obviously, the amount of optical phase change caused by the discharge acoustic signal Submerged in phase noise During this period, the system is unable to detect and locate the discharge. This is mainly caused by the frequency shift deviation of the acousto-optic modulator (AOM). It is also affected by the jitter of the RF signal driving the AOM and the nonlinear effects of the acousto-optic crystal. Figure 1 The frequency shift of light in the sensing fiber in the AOM is not strictly equal to... f 0. Assume there is an offset in the AOM frequency shift. If an ideal quadrature reference signal is still used to multiply the interference signal in IQ demodulation, then phase noise will be introduced. It can be represented as: (12); because The frequency of the reference signal is constantly changing due to environmental influences, making it extremely difficult to manually correct the frequency during demodulation, and this will also affect the response speed of the sensing system.

[0033] This proposal suggests a dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection, such as... Figure 2 As shown, it includes: a laser for generating narrow linewidth continuous light to provide a light source for the system. In this embodiment, a narrow linewidth laser is used. After passing through an isolator, the light enters a first coupler. The first coupler splits the light into two optical signals. One optical signal enters a second coupler. The second coupler splits the optical signal into two paths again. One path enters a sensing optical path composed of sensing optical fibers, and the other path enters a reference optical path that connects an acousto-optic modulator and a reference optical fiber.

[0034] A fifth coupler is connected in series on the sensing fiber to split the optical signal on the sensing fiber into two paths. One path continues to be output along the sensing fiber, and the other path merges into the sixth coupler. A fourth coupler is connected in series on the reference fiber to split the optical signal on the reference fiber into two paths. One path continues to be output along the reference fiber, and the other path merges into the sixth coupler. The two optical signals from the fifth and fourth couplers interfere with each other at the sixth coupler to generate a self-interference reference signal. This self-interference reference signal is converted into an optical-electrical signal by the third photodetector. The self-interference reference signal carries the actual frequency shift information of the acousto-optic modulator.

[0035] The light output from the sensing fiber and the reference fiber converges into the third coupler. After clockwise interference occurs at the third coupler, the light is converted into an electrical signal by the second photodetector. The other optical signal output from the first coupler enters the third coupler through the guide fiber, and then enters the sensing optical path and the reference optical path in reverse through the third coupler. After counterclockwise interference occurs at the second coupler, the light is converted into an electrical signal by the first photodetector.

[0036] The frequency of the self-interference reference signal output by the sixth coupler is consistent with the frequencies of the clockwise and counterclockwise interference signals, and can change synchronously with the actual frequency shift of the acousto-optic modulator, thus achieving frequency synchronization.

[0037] The signal processing unit is connected to the first photodetector, the second photodetector and the third photodetector respectively, and is used to perform phase demodulation on the clockwise interference signal and the counterclockwise interference signal according to the self-interference reference signal.

[0038] The signal processing unit includes a power divider, a first phase detection module, and a second phase detection module, such as... Figure 3 As shown. To avoid the increased hardware cost and data processing difficulty caused by acquiring three signals, this solution uses two AD8302 phase detection chips to demodulate the phase difference between PD1# and PD3#, and PD2# and PD3# respectively, and output two signals.

[0039] The power divider is connected to the third photodetector and is used to split the photoelectrically converted self-interference reference signal into two paths, obtaining a first reference signal and a second reference signal. A first phase detection module is connected to both the first photodetector and the power divider, and is used to receive the clockwise interference signal and the first reference signal, and output the phase difference signal between the clockwise interference signal and the first reference signal. A second phase detection module is connected to both the second photodetector and the power divider, and is used to receive the counterclockwise interference signal and the second reference signal, and output the phase difference signal between the counterclockwise interference signal and the second reference signal. Both the first and second phase detection modules use AD8302 phase detection chips. The frequency of the phase difference signal is on the order of tens of kHz, lower than the frequency of the original interference signals output by the first and second photodetectors.

[0040] The phase difference signal output by the first phase detection module represents the phase change Δφ(t) caused by the discharge acoustic emission signal in the first direction; the phase difference signal output by the second phase detection module represents the phase change Δφ(t+τ) caused by the discharge acoustic emission signal in the second direction; where τ is the time difference between the clockwise and counterclockwise interference signals. In the phase difference signals output by the first and second phase detection modules, the high-frequency carrier component introduced by the acousto-optic modulator is canceled out, retaining only the low-frequency phase change information caused by the discharge acoustic emission signal.

[0041] The time difference τ satisfies the following relationship: ; In the formula, L Total length of sensing fiber / reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν is the speed of light in the optical fiber. The spatial location of the acoustic emission signal's point of action is calculated based on the time difference τ. x The value.

[0042] The time difference τ is obtained by calculating the phase difference between the two signals using a cross-correlation algorithm. Substituting the time difference τ into the position calculation formula, the spatial location of the acoustic signal's point of action is then obtained. x This enables spatial positioning of acoustic emission signals.

[0043] In practical application, a signal acquisition module is used to acquire two phase difference signals. An oscilloscope is selected for the signal acquisition module. The time-domain waveform reflects the acoustic emission signal generated by the GIS discharge defect. When the amplitude of the detected phase signal exceeds the lower limit of the system phase detection, it is determined to be a valid discharge acoustic emission signal. Then, the time difference τ is obtained through a cross-correlation algorithm. Substituting it into the above formula, x can be obtained, thus realizing the spatial positioning of the acoustic emission signal's point of action.

[0044] Based on the original dual Mach-Zehnder interferometric fiber optic sensing system, light from the sensing fiber and the reference fiber is each drawn out via couplers C4 and C5, and interference occurs at coupler C6. This interference signal, after photoelectric conversion, serves as the reference signal in IQ demodulation. Since the reference signal originates from the original interferometric optical path, regardless of the AOM frequency shift, the frequency of the interference signal at coupler C6 will always remain consistent with the clockwise / counterclockwise interference signal (sensing signal), achieving synchronous frequency tracking.

[0045] Example 2: This embodiment provides a dual Mach-Zehnder fiber optic sensing phase noise suppression method for GIS discharge defect detection, including the following steps: S1. Light source output and splitting steps: The laser outputs a continuous optical signal, which is input to the first coupler after passing through the isolator. The first coupler splits the optical signal into two paths.

[0046] S2. Dual Mach-Zehnder Interference Optical Path Transmission: One optical signal output from the first coupler is input to the second coupler, which then splits it into two paths. One path enters the sensing optical path with the sensing fiber, and the other path enters the reference optical path with the acousto-optic modulator and the reference fiber connected in series. The optical signals from the sensing optical path and the reference optical path interfere clockwise at the third coupler and are converted into clockwise interference electrical signals by the second photodetector. The other optical signal output from the first coupler is input to the third coupler via the guide fiber and enters the sensing optical path and the reference optical path in the reverse direction. Finally, it interferes counterclockwise at the second coupler and is converted into counterclockwise interference electrical signals by the first photodetector.

[0047] S3. Self-interference reference signal generation: The fifth coupler draws part of the optical signal from the sensing fiber, and the fourth coupler draws part of the optical signal from the reference fiber. The two optical signals are input together into the sixth coupler to interfere and generate a self-interference reference optical signal, which is then converted into a self-interference reference electrical signal by the third photodetector.

[0048] A fifth coupler is connected in series on the sensing fiber, and a beam of optical signal is extracted from the sensing fiber through the fifth coupler; a fourth coupler is connected in series on the reference fiber, and a beam of optical signal is extracted from the reference fiber through the fourth coupler; the optical signal extracted from the fifth coupler and the optical signal extracted from the fourth coupler are input together into the sixth coupler to interfere and generate a self-interference reference optical signal.

[0049] S4. Synchronous Phase Demodulation and Noise Suppression: Using the self-interference reference signal as the common demodulation standard, the clockwise and counterclockwise interference signals are demodulated to obtain two phase difference signals. The phase noise introduced by the frequency shift deviation of the acousto-optic modulator is suppressed by the frequency synchronization characteristics of the self-interference reference signal and the two interference signals.

[0050] The phase difference demodulation process specifically includes: S41. Input the self-interference reference electrical signal output from the third photodetector into the power divider, and the power divider divides it into the first reference signal and the second reference signal. S42. Input the first reference signal and the counterclockwise interference electrical signal output by the first photodetector into the first phase detection module, demodulate the phase difference between the two signals, and obtain the first phase difference signal. The first phase difference signal characterizes the phase change Δφ(t) caused by the discharge acoustic emission signal acting on the sensing fiber. S43. Input the second reference signal and the clockwise interference electrical signal output by the second photodetector into the second phase detection module, demodulate the phase difference between the two signals, and obtain the second phase difference signal. The second phase difference signal characterizes the phase change Δφ(t+τ) caused by the discharge acoustic emission signal acting on the sensing fiber, where τ is the transmission time difference between the clockwise interference signal and the counterclockwise interference signal. S44. Spatial positioning of acoustic emission signals, specifically: A cross-correlation algorithm is used to calculate the time difference τ between the first and second phase difference signals. The time difference τ is then substituted into the position calculation formula to obtain the spatial location of the acoustic signal's point of action. x The calculation formula is: ; In the formula, L Total length of sensing fiber / reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν is the speed of light in the optical fiber.

[0051] S5. Signal Acquisition and Judgment: The signal acquisition module acquires two phase difference signals and reflects the acoustic emission signal generated by the GIS discharge defect through the time domain waveform; when the amplitude of the detected phase signal exceeds the lower limit of the system phase detection, it is determined to be a valid discharge acoustic emission signal.

[0052] Example 3: To verify the effect of reference signal frequency synchronization on suppressing phase noise in the sensing system, this study comparatively tested the phase noise of a dual Mach-Zehnder fiber interferometric acoustic emission sensing system with a self-interference structure. The results are as follows: Figure 4 As shown.

[0053] from Figure 4 It can be seen that after implementing reference signal frequency synchronization, the root mean square value of the system phase noise is 0.0036 rad, the mean noise signal is approximately 6.09 × 10⁻⁶ rad, and the standard deviation is 0.0035 rad. If the mean plus three times the standard deviation is taken as the lower limit of system detection, the lower limit of phase detection for the system using reference signal synchronization can be calculated to be approximately 0.0105 rad. In other words, when using the above system for partial discharge acoustic emission detection, a signal amplitude exceeding 0.0105 rad can be considered a valid signal. In contrast, the lower limit of phase detection for the system without noise suppression measures is approximately 0.6615 rad. In other words, through noise suppression, the lower limit of system detection is reduced by 98.4%.

[0054] On the other hand, this scheme uses two AD8302 phase detection chips to demodulate the phase difference between PD1# and PD3#, and PD2# and PD3#, respectively. The AD8302 outputs the phase difference between the two input signals, which can subtract the high-frequency carriers in the interference signals to obtain the low-frequency phase difference signal. Specifically, assuming the signal of PD1# is represented as... ; The signal of PD3# is represented as follows: : The signal output by AD8302 can be expressed as: : because This is caused by the emission signal of the discharge sound, with a frequency of approximately tens of kHz, which is much lower than the signal frequency of the original PD1 and PD3 output signals (because the AOM shift frequency is usually in the range of several MHz to tens of MHz, i.e., f0 is in the range of several MHz to tens of MHz). Therefore, without increasing the number of signal acquisition channels, the signal sampling rate can be further reduced, the difficulty of data acquisition and processing can be reduced, and it is more conducive to the engineering application of the sensing system.

[0055] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection, characterized in that, include: A laser is used to generate narrow-linewidth continuous light to provide a light source for the system. After passing through an isolator, the light enters the first coupler, which splits the light into two optical signals. One signal enters the second coupler, which further splits the signal into two paths: one enters the sensing optical path composed of sensing fibers, and the other enters the reference optical path connected in series with an acousto-optic modulator and a reference fiber. A fifth coupler is connected in series on the sensing fiber, splitting the optical signal therein into two paths: one continues along the sensing fiber output, and the other merges into a sixth coupler. A fourth coupler is connected in series on the reference fiber, splitting the optical signal therein into two paths: one continues along the reference fiber output. Another path merges into the sixth coupler. The two optical signals from the fifth and fourth couplers interfere with each other at the sixth coupler, generating a self-interference reference signal. This signal is then converted into an optical-electric signal by the third photodetector. The light output from the sensing fiber and the reference fiber merges into the third coupler. After clockwise interference at the third coupler, the light is converted into an optical-electric signal by the second photodetector. The other optical signal output from the first coupler enters the third coupler via the guide fiber. It then enters the sensing optical path and the reference optical path in reverse through the third coupler. After counterclockwise interference at the second coupler, the light is converted into an optical-electric signal by the first photodetector. The signal processing unit is connected to the first photodetector, the second photodetector and the third photodetector respectively, and is used to perform phase demodulation on the clockwise interference signal and the counterclockwise interference signal according to the self-interference reference signal.

2. The dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 1, characterized in that, The signal processing unit includes: a power divider, a first phase detection module, and a second phase detection module; The power divider is connected to the third photodetector and is used to divide the self-interference reference signal after photoelectric conversion into two paths to obtain the first reference signal and the second reference signal. The first phase detection module is connected to the first photodetector and the power divider respectively, and is used to receive the clockwise interference signal and the first reference signal, and output the phase difference signal between the clockwise interference signal and the first reference signal. The second phase detection module is connected to the second photodetector and the power divider respectively. It is used to receive the counterclockwise interference signal and the second reference signal, and output the phase difference signal between the counterclockwise interference signal and the second reference signal.

3. The dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 2, characterized in that, Both the first phase detection module and the second phase detection module are AD8302 phase detection chips.

4. The dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 1, characterized in that, The self-interference reference signal carries the actual frequency shift information of the acousto-optic modulator.

5. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 2, characterized in that, The phase difference signal output by the first phase detection module is the phase change Δφ(t) caused by the discharge acoustic emission signal in the first direction; the phase difference signal output by the second phase detection module is the phase change Δφ(t+τ) caused by the discharge acoustic emission signal in the second direction; where τ is the time difference between the clockwise interference signal and the counterclockwise interference signal.

6. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 5, characterized in that, The time difference τ satisfies the following relationship: ； In the formula, L The total length of the sensing fiber or reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν The speed of light in an optical fiber; The spatial location of the acoustic emission signal's point of action is calculated based on the time difference τ. x The value.

7. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 1, characterized in that, The frequency of the self-interference reference signal output by the sixth coupler is consistent with the frequencies of the clockwise and counterclockwise interference signals, and can change synchronously with the actual frequency shift of the acousto-optic modulator, thus achieving frequency synchronization.

8. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 2, characterized in that, In the phase difference signal output by the first phase detection module and the second phase detection module, the high-frequency carrier component introduced by the acousto-optic modulator is canceled out, and only the low-frequency phase change information caused by the discharge acoustic emission signal is retained.

9. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 8, characterized in that, The frequency of the phase difference signal is on the order of tens of kHz, which is lower than the frequency of the original interference signal output by the first photodetector and the second photodetector.

10. A dual Mach-Zehnder fiber optic sensing phase noise suppression system for GIS discharge defect detection according to claim 6, characterized in that, The time difference τ is obtained by calculating the phase difference between the two signals using a cross-correlation algorithm. The spatial location of the sound signal's point of action is then obtained by substituting the time difference τ into the position calculation formula. x This enables spatial positioning of acoustic emission signals.

11. A method for suppressing phase noise using dual Mach-Zehnder fiber optic sensing for detecting discharge defects in GIS, characterized in that, Noise suppression using the system according to any one of claims 1 to 10 includes the following steps: S1. Light source output and splitting steps: The laser outputs a continuous optical signal, which is input into the first coupler after passing through the isolator. The first coupler splits the optical signal into two paths. S2. Dual Mach-Zehnder Interference Optical Path Transmission: One optical signal output from the first coupler is input to the second coupler, which then splits it into two paths. One path enters the sensing optical path with the sensing fiber, and the other path enters the reference optical path with the acousto-optic modulator and the reference fiber connected in series. The optical signals from the sensing optical path and the reference optical path interfere clockwise at the third coupler and are converted into clockwise interference electrical signals by the second photodetector. The other optical signal output from the first coupler is input to the third coupler via the guide fiber and enters the sensing optical path and the reference optical path in the reverse direction. Finally, it interferes counterclockwise at the second coupler and is converted into counterclockwise interference electrical signals by the first photodetector. S3. Self-interference reference signal generation: The fifth coupler draws out part of the optical signal from the sensing fiber, and the fourth coupler draws out part of the optical signal from the reference fiber. The two optical signals are input together into the sixth coupler to interfere and generate a self-interference reference optical signal, which is then converted into a self-interference reference electrical signal by the third photodetector. S4. Synchronous Phase Demodulation and Noise Suppression: Using the self-interference reference signal as the common demodulation standard, the clockwise and counterclockwise interference signals are demodulated to obtain two phase difference signals. The phase noise introduced by the frequency shift deviation of the acousto-optic modulator is suppressed by the frequency synchronization characteristics of the self-interference reference signal and the two interference signals.

12. The method for suppressing phase noise using dual Mach-Zehnder fiber optic sensing for GIS discharge defect detection according to claim 11, characterized in that, In step S3, a fifth coupler is connected in series on the sensing fiber, and a beam of optical signal is extracted from the sensing fiber through the fifth coupler; a fourth coupler is connected in series on the reference fiber, and a beam of optical signal is extracted from the reference fiber through the fourth coupler; the optical signal extracted from the fifth coupler and the optical signal extracted from the fourth coupler are input together into the sixth coupler to interfere and generate a self-interference reference optical signal.

13. A method for suppressing phase noise using dual Mach-Zehnder fiber optic sensing for GIS discharge defect detection, as described in claim 11, is characterized in that... It also includes step S5, signal acquisition and determination, which is as follows: the signal acquisition module acquires two phase difference signals, and the acoustic emission signal generated by the GIS discharge defect is reflected by the time domain waveform; when the amplitude of the detected phase signal exceeds the lower limit of the system phase detection, it is determined to be a valid discharge acoustic emission signal.

14. A method for suppressing phase noise using dual Mach-Zehnder fiber optic sensing for GIS discharge defect detection, as described in claim 11, is characterized in that... The phase difference demodulation process in step S4 specifically includes: S41. Input the self-interference reference electrical signal output from the third photodetector into the power divider, and the power divider divides it into the first reference signal and the second reference signal. S42. Input the first reference signal and the counterclockwise interference electrical signal output by the first photodetector into the first phase detection module, demodulate the phase difference between the two signals, and obtain the first phase difference signal. The first phase difference signal characterizes the phase change Δφ(t) caused by the discharge acoustic emission signal acting on the sensing fiber. S43. Input the second reference signal and the clockwise interference electrical signal output by the second photodetector into the second phase detection module, demodulate the phase difference between the two signals, and obtain the second phase difference signal. The second phase difference signal characterizes the phase change Δφ(t+τ) caused by the discharge acoustic emission signal acting on the sensing fiber, where τ is the transmission time difference between the clockwise interference signal and the counterclockwise interference signal.

15. A method for suppressing phase noise using dual Mach-Zehnder fiber optic sensing for GIS discharge defect detection, as described in claim 14, is characterized in that... It also includes S44, spatial positioning of acoustic emission signals, which specifically involves: using a cross-correlation algorithm to calculate the time difference τ between the first and second phase difference signals, and then substituting the time difference τ into the position calculation formula to obtain the spatial position of the acoustic signal's point of action. x The calculation formula is: ； In the formula, L The total length of the sensing fiber or reference fiber. L 0 represents the length of the guiding fiber. x The distance from the point of application of the acoustic signal to the second coupler. ν is the speed of light in the optical fiber.

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