In this thesis, a fiber-optic laser sensor system is proposed for possible safety and condition monitoring of large scale electrical power systems, such as high voltage transformers, power cables, and wind turbine generators. Two main schemes have been devised and employed in the fiber-optic sensor system; a wavelength-sweeping fiber laser combined with a spectrometer demodulator and a polarization scrambling on to the fiber laser sensor cavity.
The usage of a spectrometer which consists of a diffraction grating and a 512-pixel photo-diode array for demodulation of fiber Bragg grating sensors has solved the nonlinearity problem of conventional wavelength-sweeping laser sensor system. The wavelength-tuning filter in the laser source shows nonlinear action against the driving signal, which may cause substantial measurement errors when the sensor outputs are processed with tunable bandpass filter demodulation algorithm based on wavelength-to-time domain transformation. By employing a spectrometer, the wavelength-encoded sensor signals are transformed to pixel positions, that is wavelength-to-spatial domain transformation, and the peak locations linearly correspond to the Bragg wavelengths, regardless of the nonlinearities in the wavelength tuning mechanism of the fiber laser. A linear and more sensitive measurement is possible in the measurement of temperature and/or strain distribution along the wide range power systems.
In order to improve the signal-to-noise ratio of the spectrometer output, a fiber laser sensor system was also investigated and constructed. It uses the fiber Bragg gratings as wavelength-determining filters as well as sensors in the multi-wavelength fiber laser configuration. It does not use any scanning filters and only the Bragg wavelengths are amplified and detected, generating no back scattering noises which happens in the wavelength-sweeping laser sensor system. In this configuration, multiple wavelengths are generated and demodulated into measurand information without quenching the photo-diode array, improving the signal-to-noise ratio. However, the instability in the multi-wavelength lasing environment leads to measurement data missing for positions which correspond to unstable sensors. It is difficult to sustain robust lasing conditions in this vulnerable cavity configuration. A polarization scrambling, therefore, was employed in the laser cavity in order to periodically perturb its lasing condition rather than trying to stabilize it. Combined with a spectrometer demodulation, the polarization scrambling proved to be exhibit much better data acquisition capability, because the scrambling assured at least one lasing event in each scrambling cycle.
The feasibility of the proposed fiber-optic sensor schemes has been intensively investigated and fully verified in the experiments conducted with downscaled power system simulators.