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Based on the distributed temperature sensing optical cable network of experimental study

In Electronic Infomation Category: B | on April 13,2011

Abstract: This paper proposes to increase a single fiber Bragg grating and ADS7822U datasheet and optical cables wound together, used for real-time temperature monitoring cable. Finite element analysis method, a temperature field model of optical cable. Using tunable laser as light source, a fiber over the same center wavelength lithography Bragg grating, which as a system-wide with the grating temperature sensors, optical cable lines in the temperature when an exception occurs, the reflected grating center wavelength shifted, by detecting the center wavelength of reflected light to determine the offset occurring temperature changes the size of the grating. Different positions of the grating the time required to return to the different optical signals, by detecting and ADS7822U price and calculating the return of light at different times, you can calculate the position of the grating temperature changes occur. The results show that the temperature sensitivity of grating can reach 11.4 pm / , grating temperature and ADS7822U suppliers and actual temperature measurement error of 3% range.

0 Introduction

Optical Cable (Optical Power Cable, OPC) is the same time, go the same way, the same transmission power and to the integration of optical information transmission medium, is the foundation for building intelligent grid. Placed in the optical cable perennial underground, its potential and shortcomings of aging is difficult to detect, with the increase in running time, it is possible because the cable overheating or short-circuit and cause a fire. Environment and in the presence of high voltage transmission voltage, high current, high magnetic field and other factors, which have a traditional electric type temperature sensors serious interference.

FBG (Fiber Bragg Grating, FBG) sensors in addition to a general fiber sensor temperature, corrosion resistance, etc., but also with wavelength encoding, anti-interference ability and other characteristics of the target can be achieved fast and accurate temperature measurements. The traditional method of distributed temperature fiber grating is the use of most broadband light source, through the grating center wavelength to detect changes in the return sensor information, so the number of gratings will be bandwidth limited broadband light source itself; and Sweden due to the power factors such as Lee scattering attenuation, signal to noise ratio is low, so the distance broadband light source will be limited.

This paper, a low-cost, practical solution, the system uses a tunable pulsed light source, it has power, energy concentration, etc., not only can greatly increase the transmission distance, but also broke through the broadband light source bandwidth limitations, to achieve a wide range of fiber grating sensor network. With fiber grating temperature measurement system other than the real-time monitoring of the system not only the location of the fiber grating temperature changes, but also accurate positioning of each optical fiber grating is located. Production and processing in the optical cable directly to the fiber grating, when added to the cable, you can easily run the cable status light to do real-time monitoring, fiber gratings and optical cable sync transfer program in the future development of optical network has broad prospects for development .

1 Temperature field analysis of optical cable

Light Ansys finite element software to analyze the temperature field of the cable. The basic idea of ??FEM is limited to a continuous structure into discrete units and each unit is set in a finite number of nodes, as a continuum nodes are connected only in the * body; same field function of the node selected value as the basic unknowns, and in each unit difference in the assumption of a similar function to that unit of field distribution function; and use some of the variational principle for solving the node to create the finite element equation of the unknown quantity, will infinite degrees of freedom of a continuous domain, discrete domain is transformed into degrees of freedom problem. Solutions can be used to set the node value and the interpolation function to determine the unit on the field as well as * function of the body, thereby complex regional and solving complex boundary problems cause great adaptability and flexibility, with higher accuracy. Therefore, using the finite element analysis of temperature distribution optical cable.

1.1 optical cable structure

Optical fiber communications cable is placed together with the high-voltage cables, while the integration of transport energy and information transmission medium. Optical cable model proposed by the Centre for a fiber Bragg grating, surrounded by three cables and a cable form. Three of cable cross-section of each cable core radius of 2 cm, central angle of 90 fan-shaped, fiber optic cable core cross-section of 2 cm diameter circular structure shown in Figure 1.

Figure 1 optical cable structure

1.2 Temperature field conduction differential equation

Cartesian three-dimensional temperature field in conduction differential equation to describe the general form:

Where: , c, and are the density of micro-element, heat capacity, thermal conductivity and per unit time per unit volume in the heat of formation heat source, t is time.

1.3 left, right and bottom boundary conditions

Infinite set of cables in the soil, using cylindrical coordinates to express on the field, then:

Where: T1, T2 are the temperature and soil temperature cable skin, r1, r2, respectively, for the cable diameter and soil diameter, is thermal conductivity, q the volume of heat.

1.4 on the boundary conditions

Surface soil and the air is natural convection heat transfer, heat transfer coefficient:

Where: d is the soil temperature, Nu = C (Gr Pr) n, Gr husband knows the number for the Glasgow, Pr is Prandtl number, look-up table may have parameters C and n values. Newtons formula based on heat transfer surface soil temperature gradient:

Where: T1, T2, respectively, for the soil surface and air temperature, the convective heat transfer coefficient, is the soil thermal conductivity. Calculated soil surface temperature gradient can be obtained after the soil surface temperature, because the cable section is symmetrical, so can be combined with heat conduction equation and boundary conditions on the cable cross-section of temperature field simulation.

Temperature distribution within the optical cable shown in Figure 2, Figure 2 shows the temperature field on the line y = x symmetry. Figure 3 shows the straight line y = x along the path of the temperature curve. The figure shows the location and cable grating internal temperature is very close, so the grating can be directly measured temperature reflect the temperature of the cable.

Figure 2 cross-section temperature field of optical cable node cloud

optical cable cross-section in Figure 3 the direction of the temperature curve y = x

2 demodulation principle and theoretical analysis

Experimental system is a new type of tunable lasers based on distributed optical fiber Bragg grating sensor system, the system diagram shown in Figure 4. The system has more than 1 550 nm central wavelength are the same grating, tested every 30 m area to place a package of grating. Because some of the most easy heat cable connector, so when using the cable connector in the optical cable with grating position at best, so real-time understanding of the operational status of some of the cable connector.

System selected is a tunable pulsed laser, the scanning period T = 0.25 s, as shown in Figure 5 (a) below. Scan range is 1 545 ~ 1 555 nm, laser pulse width is 0.18 nm, as shown in Figure 5 (b) below. Narrow-band pulse in the grating coupler sensor in the sent. Figure 5 (d) represents the temperature does not change if the grating, only 1 550 nm light will be reflected back. If the grating temperature changes, the grating center wavelength will also change accordingly, and the corresponding pulse of light will be reflected back. Figure 5 (c) and Figure 5 (e) correspond to the grating cooling and warming of the two different situations. Through the photodetector converts the light signal voltage waveform, high-speed acquisition card sampling rate to 500 Mps sampling of electrical signals. Data processing system to control the laser voltage signal sent, and compare these voltage signals and the collected signal. Use of industrial grade motherboard, the program calculates the offset of fiber grating, the offset of the linear change in temperature corresponding to the grating. Figure 5 (f) indicates the central wavelength corresponding to light signals of different signal map. In addition, the different positions of the optical signal to return the time difference is different from the adjacent grating interval is 200ns. And calculated by measuring the time interval to return light, temperature changes can be grating position and temperature variation.

Figure 4 Distributed Fiber Bragg Grating Sensor System Diagram

Figure 5, the wavelength tunable laser demodulation schematic

3 Experimental results and analysis

The light beam incident center engraved with the same wavelength grating of five, due to grating FBG1, FBG2, FBG3, FBG4 and FBG5 are all central wavelength 1 550 nm, so the reflected light are only FBG1, as shown in Figure 6 instructions. When the heat treatment of FBG2, FBG2 the center wavelength shifted to the right place, respectively, as shown in Figure 7 and Figure 8, this time FBG1 and FBG2 have received light and reflects back, the other point is not receiving light. When heated to a certain extent, FBG2 after the central wavelength will shift completely from the original center wavelength, shown in Figure 9. So, when the temperature can change the same center wavelength of the two completely separate grating, consistent with the above analysis.

Figure 6 FBGs at 25 C, the spectra

Figure 7 FBG2 in 35 C, the other at 25 C, the spectra

Figure 8 FBG2 in 45 C, the other at 25 C, the spectra

Figure 9 FBG2 at 55 C, the other at 25 C, the spectra

Photodetector will receive light signals into light current, and then zoom through a filter circuit into a voltage signal. Observed with an oscilloscope waveform in Figure 10, Figure 11 (a), Figure 11 (b), Figure 11 (c) below, the experiment shows that the system can accomplish the requirements of the wavelength demodulation. The center wavelength of FBG 1 550 nm compared to temperature experimental results show that the temperature sensitivity of grating can reach 11.4 pm / , grating temperature and actual temperature measurement error of 3% range.

Figure 10 FBGs is 25 C, the spectra

35 C

45 C

55 C

Figure 11 FBG2 in 35 C, 45 C, 55 C, the other at 25 C, the spectra

Prepared a 100 m long optical cable of 110 kV, with a number of its internal grating, to take one of the three points P1, P2, P3 as the key experiment, while at the location of each grating place a high accuracy of platinum resistance temperature sensor, fiber Bragg grating temperature as the control. Optical cable in power before the temperature is 25 , power immediately after the start time, the internal temperature stability in the optical cable every two minutes were read before the monitoring system and the platinum resistance temperature value measured. In the light after stable internal temperature of the cable, and then every ten minutes, respectively, measured by FBG read the data and the data measured by platinum resistance, and the data plotted curves are shown in Figure 12, Figure 13 and Figure 14 Fig. Figure 15 for the three measuring points of the fiber grating of the measured data and the platinum resistance deviation of the measured data curve.

Figure 12 P1 point temperature measurement curve

Figure 13 P2 point temperature measurement curve

Figure 14 P3 point temperature measurement curve

Figure 15 grating measured with platinum resistance temperature deviation of the measured temperature curve

Can be seen from the figure, the light cable in power after the temperature gradually rises, the time at about 49.5 remained stable, floating in a small area only. Analysis of temperature field by the cable shows, cable cable core temperature is about 49 , the data are close. Figure 15 shows the three measuring points of each raster data measured by the platinum resistance of the measured data is very close to the actual deviations are both within 0.6 . It can be seen in this experimental system of measurement accuracy is higher.

4 Conclusions

This temperature inside the optical characteristics of the cable study, the combination of heat conduction equation and boundary conditions, the use of Ansys temperature field inside the light cable to do a detailed analysis, and presents a tunable laser based on practical All the same grating distributed temperature monitoring system. The biggest advantages of the system is a breakthrough bandwidth limitations of the broadband light source, optical fiber by a continuous sculpted a large number of the same grating center wavelength, grating laser power only by the number of constraints, to achieve a number of different locations at the same time monitoring requirements. The simulation of a theoretical analysis of this view, the description of this scheme is feasible. After repeated experiments, the oscilloscope through the spectrometer and the spectra of the received voltage signal and verify the correctness of this method. The center wavelength of 1 550 nm fiber grating temperature comparing experimental results show that the grating temperature sensitivity of up to 11.4 pm / , grating temperature and actual temperature measurement error of 3% range, further evidence The system is suitable for distributed multi-point measurements.

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