FIBER OPTIC SENSOR SYSTEM FOR STRESS MONITORING IN POWER CABLES

Contents
What is OFS
Basics of Optical fiber
Raman Backscattering
Fiber Bragg grating
Stress monitoring by OFS
Advantages
Conclusion

[attachment=7378]

FIBER OPTIC SENSORS
Optical fiber sensor: A sensor that measures a physical quantity based on its modulation on the intensity, spectrum, phase, or polarization of light traveling through an optical fiber.

To measure physical parameters such as strain, temperature, pressure, velocity, and acceleration

Optical fibers: strands of glass that transmit light over long distances (wire in electrical systems)

Light: transmitted by continuous internal reflections in optical fibers (electron in electrical systems)

OPTICAL FIBRE
Total Internal Reflection
Singlemode and multimode optical fibers
PRINCIPLE
TYPE OF FIBER OPTIC SENSORS
Extrinsic or Hybrid Fiber Optic Sensors
Intrinsic or All-Fiber Optic Sensors
Raman Backscattering
Inelastic scattering (whereas Rayleigh scattering is elastic and is predominant)
Occur with a change in vibrational, rotational or electronic energy of a molecule
Stokes scattering :the atom or molecule absorbs energy; the emitted photon has less energy than the absorbed photon .
Anti-Stokes scattering :the atom or molecule loses energy; the emitted photon has more energy than the absorbed photon.



Comparison of Rayleigh, Stokes and anti-Stokes scattering
Fiber Bragg grating
wavelength-specific reflector
constructed in a short segment of optical fiber
reflects particular wavelengths of light and transmits all others
periodic variation
to the refractive
index of the fiber
core
Schematic of the fiber stress sensor system
Working
Supercontinuum(range:400-1600nm) obtained by pumping a PCF by a nanosecond microchip laser(λ=1064nm,t=1ns, Epulse=10 µ J).
Supercontinuum illuminates FBG.
Part of laser radiation(1064 or 532nm) is coupled to standard single mode fiber for measuring time resolved raman backscattering signals.
Temperature measurement
Time resolved Raman backscatttering used.
anti-Stoke photons temp dependent(unlike Stoke photons)
Ratio of anti-Stoke and Stoke intensity gives temp
Both detected by 2 photomultiplier tubes combined with a bandpass filter.
Time resolved single photon counting.
Spatial resolution:5cm

Squeezing measurement
Increased pressure on power cable(therefore also on the fiber) results in loss of Backscattered Raman light.
By measuring time resolved Raman intensities gives spatially resolved information on squeezing.
Spatial resolution is 5cm.

Backscattered stokes light of an 18m long fiber squeezed at position 16m.



Bending and Torsion(by FBG)
Supercontinuum provides the broadband source.
FBG provided by point to point femtosecond laser inscription.
The FBG reflects a defined wavelength which is given by the grating period Δ & the effective index neff of refraction: λbragg=2.neff. Δ
In this case, a FBG for the wavelength = 1550.8nm was written by the femtosecond laser(λ=800nm, f=1kHz,t=130fs, Epulse=1µJ)
Bending and Torsion(by FBG) cont.
Bending the fiber which is integrated into the power cable, results in a change of the grating period and thus different wavelengths from the broadband source will be reflected and can easily monitored.
Bragg wavelength is given as:
λbragg=2.neff . Δ
neff-effective index of refraction
Δ- grating period
A FBG sensor was glued on the jacket of a power cable and the reflected light was measured for different cablepositions
What Does F.O.S. Look Like?
Strain Gage



Embeddable Strain Gage






Pressure Transducer
Displacement Transducer








Temperature Transducer
ADVANTAGES
GENERAL USES
Measurement of physical properties such as strain, displacement, temperature, pressure, velocity, and acceleration in structures of any shape or size

Monitoring the physical health of structures in real time

Damage detection

Used in multifunctional structures, in which a combination of smart materials, actuators and sensors work together to produce specific action
Conclusion
The sensor system described in this paper will be installed into a 6 MW wind power plant for monitoring stress in power cables.
Optical fiber sensors are superior to electric sensors in terms of sensitivity, immunity to EM radiation..
They can work at very high temperatures.
Constantly evolving technology.
It has got numerous applications.


References
Fiber Optic Sensor System for Stress Monitoring in Power Cables by J. Burgmeier, W. Schippers, and W. Schade Clausthal University of Technology, LaserAnwendungsCentrum, Energy Campus, Am Stollen 19, 38640 Goslar, Germany
Optical Fiber Communications : John M.Senior
bluerrimages/Overview_of_FOS2.pdf