Electric reactor strain detection method based on Brillouin dynamic grating sensor

Abstract

The invention discloses a reactor strain detection method based on a Brillouin dynamic grating sensor. A transverse optical fiber sensor capable of monitoring pressure based on a Brillouin dynamic grating is designed and is wound on an electric reactor. The optical fiber sensor is surrounded by a large elliptical air hole and a small elliptical air hole, two layers of elliptical air holes are formed in each of the upper part and the lower part, and the two layers of circular air holes and the two layers of elliptical air holes are sequentially stacked along the y axis and are symmetric about the x axis. The pressure sensitivity of the sensor can reach 1.021 GHz / MPa within a large pressure range by applying the photonic crystal fiber disclosed by the invention. The Brillouin dynamic grating has the characteristics of separating pump light, probe light and reflected light, improving the measurement precision and spatial resolution and simultaneously realizing strain sensing, so that the Brillouin dynamic grating is applied to the reactor to monitor the strain on line; the strain tension borne in the encapsulation layer of the dry-type air-core reactor can be obtained; and whether cracks are generated at the encapsulation insulation part can be judged.

Description

Technical Field

[0001] The invention relates to the field of optical fiber technology, and in particular to a reactor strain detection method based on a Brillouin dynamic grating sensor. Background Art

[0002] Distributed fiber optic sensing technology based on Brillouin scattering is widely used in dams, bridges, roads, and medical fields due to its advantages in strain measurement, such as high precision, large measurement range, and high spatial resolution. However, traditional distributed Brillouin fiber optic sensing can only measure strain, with low measurement accuracy and problems such as cross-sensitivity. In response to these problems, researchers have proposed fiber optic sensing based on Brillouin dynamic grating (BDG), which effectively separates pump light, detection light, and reflected light, improves measurement accuracy and spatial resolution, and realizes simultaneous strain sensing. In general, two beams of pump light with the same polarization state are injected into the optical fiber, and the coherent acoustic field can be excited by the stimulated Brillouin scattering (SBS) effect, and then BDG is further formed. BDG technology makes up for the shortcomings of traditional distributed Brillouin fiber optic sensing technology. The generation and detection of BDG are two independent processes, which can shorten the detection pulse width to achieve the purpose of improving spatial resolution, and the obtained Brillouin gain spectrum will also be narrower.

[0003] As an inductive device for current limiting, voltage stabilization and reactive power compensation, dry-type air-core reactors have been widely used in power systems. In recent years, from the perspective of power grid operation and maintenance, reactor burnout accidents have often occurred, which has damaged the safety of the power system and caused great economic losses. There are many reasons for the burning of reactors, and encapsulation cracking is one of the main reasons. After the insulation cracks, the inter-turn insulation of the reactor is eroded by external conditions such as rain in the environment, and the inter-turn insulation is damaged. Once the inter-turn short circuit occurs, the loss increases sharply and the reactor will burn out quickly. The cracking is mainly caused by thermal stress. This force is caused by the uneven axial distribution of the reactor temperature. The generation of temperature gradient will cause different deformations of the material. When the reactor is subjected to this thermal stress for a long time, its encapsulation will crack.

[0004] Brillouin dynamic grating has the characteristics of separating pump light, detection light and reflected light, improving measurement accuracy and spatial resolution while realizing simultaneous strain sensing. Therefore, in order to solve the above problems, Brillouin dynamic grating is applied to the reactor to monitor strain online. The strain tensile force in the encapsulation layer of the dry-type air-core reactor can be obtained, which solves the problem of strain measurement inside the encapsulation layer of the dry-type reactor. Summary of the invention

[0005] The purpose of the present invention is to provide a reactor strain detection method based on a Brillouin dynamic grating sensor to solve the problems of low sensitivity and narrow measurement range in the prior art, and to apply it to the reactor to solve the problem of strain measurement inside the encapsulation layer of the dry reactor.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: Step 1: A reactor strain detection method based on Brillouin dynamic grating sensor is designed, wherein the cross section of the photonic crystal fiber amplifier includes a cladding and a core. The characteristic is that the cross section of the photonic crystal fiber amplifier is a multi-layered structure, the core area is surrounded by large and small elliptical air holes, and the upper and lower parts are two layers of elliptical air holes, two layers of circular air holes, and then two layers of elliptical air holes arranged in sequence along the y-axis. All air holes are stacked and arranged along the y-axis and are symmetrical about the x-axis.

[0007] Furthermore, the material of the optical fiber is SiO 2 .

[0008] Step 2: Use simulation software to perform stress analysis on the photonic crystal fiber amplifier in step 1 when pressure is applied along the x-axis, along the y-axis, and in different directions, and obtain the change in birefringence frequency shift and pressure sensitivity under corresponding pressure.

[0009] Step 3: In the above optical fiber structure design scheme, the following beneficial effects are achieved:

[0010] When measuring the pressure range of 0-40MPa within 0-100℃, the pressure sensitivity in the x-axis direction is about -1.004GHz / MPa, and the pressure sensitivity in the y-axis direction is about 1.021GHz / MPa. Compared with the current best "sandwich" structure PCF sensor (sensitivity of 692MHz / MPa), the sensitivity is increased by about 328MHz / MPa.

[0011] Step 4: When the sensing fiber is subjected to pressure, it will produce elastic deformation, causing the elasto-optic effect, which will cause the birefringence coefficient of the fiber to change. The change in the birefringence coefficient will then cause the birefringence frequency shift to change. The change in the birefringence frequency shift caused by pressure can be expressed as:

[0012] Where: C P Bire is the pressure sensing coefficient, ΔP is the pressure change, C T Bireis the temperature sensing coefficient, and ΔT is the temperature change. Given the sensing coefficient, the birefringence frequency shift change and the temperature change can be measured, and the pressure change can be obtained based on the above data and formula (1), thereby realizing the perception of pressure.

[0013] The designed Brillouin dynamic grating-based transverse optical fiber sensor capable of monitoring pressure is arranged on the reactor, and stress analysis is performed on the overall encapsulation insulation of the hollow reactor. If the monitored vibration data does not show an obvious step rise, the overall encapsulation insulation of the hollow reactor will continue to be monitored in real time; if the monitored vibration data shows an obvious step rise, it proves that cracks have occurred in the encapsulation insulation and an alarm is issued, which effectively solves the problem of strain measurement inside the encapsulation layer of the dry-type reactor. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a system flow chart of the reactor strain detection method based on Brillouin dynamic grating sensor.

[0015] Figure 2 Schematic diagram of the cross section of a Brillouin dynamic grating sensor in a specific embodiment. In the figure: 1-substrate material, 2-outermost row of small elliptical air holes, 3-circular air holes, 4-innermost row of small elliptical air holes, 5-large elliptical air holes; a 1 - major axis of the large elliptical air hole, b 1 - the short axis of the large elliptical air hole, a- the long axis of the small elliptical air hole, b- the short axis of the small elliptical air hole, c 1 - Longitudinal hole spacing between small elliptical air holes, c 2 - Transverse hole spacing between small elliptical air holes, c 3 - The distance between the large elliptical air holes on the left and right sides of the fiber core, c 4 - Longitudinal hole spacing between small elliptical air holes, c 5 - Longitudinal hole spacing between circular air holes, c 6 - Transverse hole spacing between circular air holes, c 7 - The hole distance between the small elliptical air hole and the large elliptical air hole, c 8 - The hole spacing between the small elliptical air holes and the circular air holes, d 1 - diameter of the circular air hole, d- diameter of the photonic crystal fiber.

[0016] Figure 3 It is a block diagram of the experimental principle of the Brillouin dynamic grating sensor in a specific embodiment.

[0017] Figure 4a and Figure 4b They are respectively the changes in the cross section of the optical fiber when pressure is applied along the x-axis direction and the y-axis direction based on the Brillouin dynamic grating sensor in a specific embodiment.

[0018] Figure 5a and Figure 5b They are respectively diagrams showing the relationship between the birefringence coefficient and the pressure when the pressure is applied along the x-axis direction and the y-axis direction based on the Brillouin dynamic grating sensor in a specific embodiment.

[0019] Figure 6a and Figure 6b They are respectively the changes of the BDG reflection spectrum when pressure is applied along the x-axis direction and the y-axis direction based on the Brillouin dynamic grating sensor in a specific embodiment.

[0020] Figure 7 It is a graph showing the relationship between the optical fiber birefringence frequency shift and the pressure magnitude under different pressure directions based on the Brillouin dynamic grating sensor in a specific embodiment. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and by way of examples. The following examples are intended to explain the present invention but the present invention is not limited to the following examples.

[0022] Figure 2 The cross-sectional view of the Brillouin dynamic grating sensor of the present invention is shown, and the substrate material 1 is SiO 2 The photonic crystal fiber amplifier is surrounded by two types of elliptical air holes, large and small. The upper and lower parts are composed of two layers of elliptical air holes, two layers of circular air holes and two layers of elliptical air holes arranged along the y axis. The major axis and minor axis of the large elliptical air hole are respectively a 1 、b 1 The major axis and minor axis of the small elliptical air hole are a and b respectively, and the longitudinal and transverse hole spacing of the small elliptical air hole are c respectively. 1 、c 2 , the distance between the large elliptical air holes on the left and right sides of the fiber core is c 3 , the hole spacing between the large elliptical air holes on the same side of the fiber core is c 4 , the longitudinal and transverse hole spacings between the circular air holes are c 5 、c 6 , the hole spacing between the small elliptical air hole and the large elliptical air hole is c 7 , the hole spacing between the small elliptical air hole and the circular air hole is c 8 , the diameter of the circular air hole is d 1 , the diameter of the photonic crystal fiber is d.

[0023] In this embodiment, based on the Brillouin dynamic grating sensor parameters: the major axis and minor axis a of the large elliptical air hole 1 、b 1 The lengths of the longitudinal and transverse holes of the small elliptical air hole are 2 μm and 0.6 μm respectively, the major and minor axes a and b of the small elliptical air hole are 0.8 μm and 0.4 μm respectively, and the lengths of the longitudinal and transverse holes of the small elliptical air hole are 0.1 、c 2 The spacing c between the large elliptical air holes on the left and right sides of the fiber core is 0.9 μm and 0.4 μm respectively. 3 Take 3μm, the hole spacing c between the large elliptical air holes on the same side of the fiber core 4 The longitudinal and transverse hole spacing c between circular air holes is 2.1 μm. 5 、c 6 The pore spacing c of the small elliptical air hole and the large elliptical air hole is 0.8 μm and 0.9 μm respectively. 7 Take 0.7 μm, the pore spacing c between the small elliptical air hole and the circular air hole 8 Take 0.6μm, the diameter of the circular air hole d 1 Take 0.8μm, and the diameter d of the photonic crystal fiber is 10μm.

[0024] The present invention can use the finite element method and combine the perfect matching layer boundary absorption condition to perform theoretical calculations to obtain the birefringence coefficient, BDG reflection spectrum and birefringence frequency shift of the present invention.

[0025] The experimental principle block diagram of the Brillouin dynamic grating sensor in this embodiment is as follows Figure 3 As shown:

[0026] In this Brillouin dynamic grating system, two pump lights are injected into the polarization-maintaining fiber from the x-axis direction under the action of the polarization controller to "write" the dynamic fiber grating. The narrow linewidth semiconductor laser is used to generate the probe light, which is first modulated into a pulse light by the electro-optic modulator 1, then amplified by the erbium-doped fiber amplifier and transmitted in the y-axis direction by the polarization controller, and then enters the polarization-maintaining fiber through the circulator and the polarization beam splitter to "read" the fiber grating spectrum. The pink thick line parts in the figure are all made of polarization-maintaining fiber, and the rest are made of single-mode fiber. The optical power meter 1 is used to detect the BOTDA signal. The optical power meter 2 is used to monitor the pump light 1 and the probe light power and the delay between the two. The optical power meter 3 is used to measure the reflection signal of the dynamic grating, that is, to read the birefringence spectrum.

[0027] In this embodiment, when the Brillouin dynamic grating sensor is subjected to force along the x-axis and y-axis at a pressure of 1 MPa, the change in the cross section of the optical fiber is as follows: Figure 4a and 4b As shown, observe Figure 4a and 4b We can get:

[0028] The Brillouin dynamic grating sensor of the present invention introduces an elliptical air hole, thereby destroying the original symmetrical structure of the PCF, making its structure in the y-axis and x-axis directions different, thereby generating a birefringence effect Δn=|n x eff -n y eff|, where n x eff is the effective refractive index for the x polarization state, n y eff is the effective refractive index for the y polarization state.

[0029] In this embodiment, when the Brillouin dynamic grating sensor applies pressure along the x-axis and the y-axis, the birefringence coefficient is as follows: Figure 5a and 5b As shown, observe Figure 5a and 5b We can get:

[0030] The Brillouin dynamic grating sensor according to the present invention deforms after being subjected to pressure. When the volume reference temperature T 0 =0℃, working temperature T 1 =0℃, no pressure applied, n x eff 、n y eff They are 1.294 891 935 063 30 and 1.276 288 805 901 13 respectively, and Δn is 0.018 603 129 162 170. When pressure is applied along the x-axis of the optical fiber, the birefringence index decreases linearly with the increase of pressure; when pressure is applied along the y-axis of the optical fiber, the birefringence index increases linearly with the increase of pressure. The reason is that the pressure changes the cross section of the optical fiber and thus changes the birefringence index.

[0031] In this embodiment, when the Brillouin dynamic grating sensor applies pressure along the x-axis and the y-axis, the BDG reflection spectrum is as follows: Figure 6a and 6b As shown, observe Figure 6a and 6b We can get:

[0032] When the temperature is fixed at 20°C and pressure is applied along the x-axis, the BDG reflection spectrum of the Brillouin dynamic grating sensor of the present invention gradually shifts to the left as the applied pressure increases, that is, the birefringence frequency shift gradually decreases; when pressure is applied along the y-axis, the BDG reflection spectrum gradually shifts to the right as the applied pressure increases, that is, the birefringence frequency shift gradually increases.

[0033] In this embodiment, the frequency shift of birefringence based on the Brillouin dynamic grating sensor changes with the applied force angle as shown in the figure. Figure 7 As shown, observe Figure 7 We can get:

[0034] The Brillouin dynamic grating sensor of the present invention has a negative pressure sensitivity when the force angle θ≤45° under different pressure directions; and a positive pressure sensitivity when the force angle θ>45°. It is more sensitive to the force angle and can maintain high sensitivity to pressure. If the pressure value is roughly known, the pressure sensitivity in the actual situation is combined with the relationship between the angle and the pressure sensitivity, and the pressure angle can be determined by interpolation.

[0035] The above-mentioned embodiment verifies the performance of the optical fiber sensor. The designed transverse optical fiber sensor based on Brillouin dynamic grating that can monitor pressure is arranged on the reactor, and stress analysis is performed on the overall encapsulation insulation of the hollow reactor. If the monitored vibration data does not show an obvious step rise, the overall encapsulation insulation of the hollow reactor continues to be monitored in real time; if the monitored vibration data shows an obvious step rise, it proves that cracks have occurred in the encapsulation insulation and an alarm is issued, which effectively solves the problem of strain measurement inside the encapsulation layer of the dry-type reactor.

[0036] The above is only one embodiment of the present invention, not all or the only embodiment. Any equivalent changes made to the technical solution of the present invention by ordinary technicians in this field after reading the specification of the present invention are covered by the claims of the present invention.

Claims

1. A method for detecting strain of a reactor based on a Brillouin dynamic grating sensor, characterized in that: The following steps are involved: Step 1: A reactor strain detection method based on Brillouin dynamic grating sensor is designed, which is characterized by: the cross-section of the photonic crystal fiber is a multi-layer structure, the core area is surrounded by large and small elliptical air holes, and the upper and lower parts are two layers of elliptical air holes, two layers of circular air holes and then two layers of elliptical air holes, which are arranged in sequence along the y-axis. Step 2: Use simulation software to perform stress analysis on the photonic crystal fiber amplifier in step 1 when pressure is applied along the x-axis, along the y-axis, and in different directions, and obtain the change in birefringence frequency shift and pressure sensitivity under corresponding pressure. Step 3: Calculate the strain using the following formula. Where: C P Bire is the pressure sensing coefficient, ΔP is the pressure change, C T Bire is the temperature sensing coefficient, ΔT is the temperature change; given the sensing coefficient, the birefringence frequency shift change and the temperature change can be measured, and the pressure change can be obtained based on the above data and formula (1), thereby realizing the perception of the reactor stress. Step 4: Arrange the designed Brillouin dynamic grating-based transverse fiber optic sensor that can monitor pressure on the reactor, and perform stress analysis on the overall encapsulation insulation of the hollow reactor. If the monitored vibration data does not show an obvious step rise, continue to monitor the overall encapsulation insulation of the hollow reactor in real time; if the monitored vibration data shows an obvious step rise, it proves that cracks have occurred in the encapsulation insulation and an alarm is issued.

2. The method for detecting strain of a reactor based on a Brillouin dynamic grating sensor according to claim 1, characterized in that: In the step 1, large and small elliptical air holes and circular air holes are alternately provided.

3. The method for detecting strain of a reactor based on a Brillouin dynamic grating sensor according to claim 1, characterized in that: The introduction of the elliptical air hole in step 1 destroys the original symmetrical structure of the photonic crystal fiber.

4. The method for detecting strain of a reactor based on a Brillouin dynamic grating sensor according to claim 1, characterized in that: In the step 1, the optical fiber material is pure SiO2.

5. The method for detecting strain of a reactor based on a Brillouin dynamic grating sensor according to claim 1, characterized in that: The external experimental equipment required in step 4 includes an ultra-narrow linewidth laser, a pulse signal generator, an optoelectronic modulator, a coupler, a circulator, an external optical fiber, a data acquisition card responsible for collecting return signals, a DC regulated power supply for the data acquisition card and each optoelectronic module, a remote client connected to the data acquisition card, and a data processing and recognition program.

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