In this thesis, a fiber-optic sensor system is proposed for multi-stress condition monitoring of electric power systems. The proposed sensor system simultaneously measure strain, temperature, and vibration or ultrasonic waves through fiber Bragg grating (FBG) and fiber-optic interferometers by using a single light source.
Two main schemes are employed in the proposed sensor system, namely: (1) FBG sensors with fiber-optic Michelson interferometer to measure strain, temperature and vibration in the wind turbines; and (2) FBG sensors with fiber-optic Sagnac interferometer to measure strain, temperature and ultrasonic waves in the oil-filled power transformers.
The first scheme utilizes FBG and fiber-optic Michelson interferometer. In order to measure multiple stresses, the system uses a single broadband light source which addresses both types of sensors. It simplifies the optical setup and enhances the cost-effectiveness of condition monitoring system.
The setup for this scheme consists of broadband light source (BBS), a 1×2 optical coupler, a photodetector (PD) to compensate the output variation of BBS, FBGs (for measuring temperature and strain), an athermal-packaged FBG, a Michelson interferometer (for measuring vibration signal), another PD (to detect the output signal of interferometer), a spectrometer (for the reconstruction of FBGs), and data acquisition (DAQ) board (for signal analysis and processing).
The Michelson interferometer composed of an optical circulator, an athermal-packaged FBG, a 2×2 optical coupler, and two fiber-optic retroreflector. The interferometer arms are about 3 m long each while an athermal-packaged FBG with coherence length of about 3.28 mm is used to supply quasi-coherent light for Michelson interferometer demodulation. Seven FBGs are connected in series to one fiber line with six general FBGs for measuring strain distribution and a temperature FBG for temperature compensation and temperature measurement.
A wind turbine simulator was constructed for testing the proposed system for multi-stress condition monitoring. Experimental results demonstrated that the proposed sensor system was capable of measuring strain and temperature with measurement accuracy of 1 pm. 500 ∼ 2000 Hz vibration signals were also successfully analyzed by applying FFT signal processing to interference signal. Additionally, multi-point vibration measurement method utilizing arrayed waveguide grating (AWG) is suggested instead of using athermal-packaged FBG since the can acquire interference signals ten times stronger than the athermal-packaged FBG. The proposed method can diversify the interferometer sensors as much as each channel in the AWG.
The second scheme measures ultrasonic waves due to partial discharge in power transformers by utilizing fiber-optic Sagnac interferometer instead of fiber-optic Michelson interferometer. To detect ultrasonic signals, an optical fiber coil should be used for high measurement sensitivity. Since it is difficult to match the length of both arms of the Michelson interferometer within the coherence length, the Sagnac interferometer is used. The Sagnac interferometer can acquire interference signals from BBS with very low coherence length since the two lights are combined after passing through completely the same optical path length.
Since the proposed sensor system shares the light source with the FBGs, ultrasonic waves are detected using bend-insensitive fiber in order to ensure the output of the interferometer sensor. The Sagnac interferometer is composed of a 3×3 optical coupler, a fiber-optic sensing coil (17 m long), a delay coil (17 m long), two PDs for detecting the output signal of interferometer.
Experiments demonstrating the ultrasonic waves detection capability of the proposed scheme are performed in an oil tank test bed made up of acrylic plates with a cylindrical PZT acting as the ultrasonic sound source. It was observed that the proposed scheme is capable of detecting ultrasonic waves up to 200 kHz. The use of AWG for detecting multi-point ultrasound measurements is also suggested. The multi-point sensor systems can be used to estimate where partial discharge occurred since the sensors can be utilized for each of channel of the AWG. The applied ultrasonic waves that could be measured are up to 150 kHz.
The proposed sensor systems that simultaneously uses two types of optical sensors with a single light source simplifies the complexity of optical circuit while at the same time increasing the cost-efficiency of condition monitoring systems.