June-Ho Lee

Accuracy improvement in peak positioning of spectrally distorted fiber Bragg grating sensors by Gaussian curve fitting

To improve measurement accuracy of spectrally distorted fiber Bragg grating temperature sensors, reflection profiles were curve fitted to Gaussian shapes, of which center positions were transformed into temperature information. By applying the Gaussian curve-fitting algorithm in a tunable bandpass filter demodulation scheme, approximately 0.3 degrees C temperature resolution was obtained with a severely distorted grating sensor, which was much better than that obtained using the highest peak search algorithm. A binary search was also used to retrieve the optimal fitting curves with the least amount of processing time.

Classification of PD patterns from multiple defects

In this work, two approaches were proposed for the recognition of partial discharge patterns. The first approach was neural network with back-propagation algorithm, and the second approach was angle calculation between two operator vectors. PD signals were detected using three electrode systems; IEC(b), needle-plane and CIGRE method II electrode system. Both of neural network and angle comparison method showed good recognition performance for the patterns similar to the trained patterns.

Investigation of Fiber Bragg Grating Temperature Sensor for Applications in Electric Power Systems

A reliable distributed temperature monitoring is very important for electric power systems because a power system failure will result in an enormous loss of life and property. Fiber Bragg grating (FBG) sensors, which have been studied intensively for last decade, can be very efficient tools for these applications because they are immune to EMI and can be highly multiplexed, which enables efficient quasi-distributed temperature sensing along tens of km range. We constructed a fiber-optic temperature sensor system with an array of 10 sensor gratings. The temperature-induced Bragg wavelength variations are accurately monitored by a scanned tunable wavelength filter. Differential measurement with temperature-stabilized reference gratings and a Gaussian curve-fitting algorithm has been used to enhance measurement accuracy, which obtained temperature resolution of ~0.6degC, and linearity error less than 0.4%

Distributed Fiber-Optic Temperature Sensor Network for Protection of Electric Power Systems

We developed a fiber-optic temperature sensor system, with 10 fiber Bragg gratings, for abnormal high-temperature monitoring in power systems. We used Gaussian line-fitting algorithm to compensate the spectrum distortion in the wavelength-scanned Farby-Perot filter demodulation scheme. Compared with highest-peak-detection method, the proposed algorithm substantially reduced measurement errors. The overall measurement error was less than 1[%] compared with the reference thermocouple and the linearity error was 0.37[%].

Electrical and mechanical properties of silicone rubber for high voltage insulation

This study was carried out to investigate the influences of low materials (the vinyl content of polydimethylsiloxane and reinforcing silica) on the electrical and mechanical properties of silicone rubber for high voltage insulation. When the content of vinyl group was increased, cross-linking density and hardness were increased and tensile strength, volume resistivity and tracking resistance were improved. The mechanical properties and electrical properties of silicone rubber reinforced with fumed silica were higher than these of silicone rubber reinforced with precipitated silica. It was found that electrical and mechanical properties of silicone rubber were influenced by the water contents of silica very much.

Analysis of partial discharge signals from multi-defect insulating systems

In practice, there may be various defects in an insulating system, so that the PD signals can be produces from these defect simultaneously. Regarding these situations, we have to discriminate the type of defect as well as to determine whether the PD occurs or not. In our research, some analysis results of the PD signals from multi-defects insulating system will be presented. We measure the PD signals using three kind of electrode system; IEC(b), Needle-Plane and mixed electrodes. To simulate multi-defect systems, we combine two electrode systems and apply test voltage simultaneously. Neural network, statistical analysis methods will be tested, and the possibilities and limitations of each method will be clarified.

Discrimination of phase-shifted partial discharge patterns by neural network using standardization method

In this work, a neural network algorithm using the standardization method was developed to discriminate phase-shifted partial discharge (PD) patterns. Previous efforts to recognize PD patterns have been performed by applying the neural network to the /spl phi/-q-n pattern, which is composed of phase angle /spl phi/, discharge magnitude q and repetition rate n of PD pulse. Considering PD measurement in the field, it is not so easy to acquire the information on absolute phase angles of PD pulses. As a consequence, one of the significant aspects in applying the neural network algorithm to practical systems, is to develop a method that can discriminate the phase-shifted /spl phi/-q-n patterns. Therefore, the authors have established a new method which can convert the phase-shifted /spl phi/-q-n patterns to a standardized /spl phi/-q-n pattern. This standardized /spl phi/-q-n pattern could be obtained by applying Fourier analysis and was not influenced by the phase-shift of PD patterns. As a result, the recognition performance of the neural network for the phase-shifted /spl phi/-q-n patterns was considerably improved by applying this new standardization method.<<ETX>>

Demodulation of FBG and Acoustic Sensors Embedded in a Fiber-Optic Sagnac Loop

When the fiber Bragg gratings are embedded in a fiber-optic Sagnac loop for measuring temperature or strain, it is difficult to separate the Bragg wavelengths. The transmitted light is mixed with the reflected Bragg wavelengths in the photo-detector, working as noises. To suppress the noises, we placed the FBG sensors and a fiber-optic attenuator at asymmetric positions in the loop. With the arrangement the reflected light became much bigger than the transmitted light, enabling the separation of the reflected Bragg wavelengths with almost the same signal-to-noise ratio of the FBG sensors outside the loop.

A fiber optic Sagnac multi-stress sensor system

Fiber Bragg grating (FBG) sensors are employed in a fiber-optic Sagnac interferometer sensor to measure multi-stress information of electric power systems. By using the hybrid sensor configuration, it was possible to measure the temperature and the vibration signal in an insulating transformer oil bath at the same time. A novel fiber-optic Sagnac interferometer design and a signal processing technique were used to separate the FBG sensor signals from the interference signal. The preliminary experimental results are presented to show the feasibility of the sensor system.

Current-induced Phase Demodulation Using a PWM Sampling for a Fiber-optic CT

In this work, we used PWM sampling for demodulation of a fiber-optic interferometric current transformer. The interference signal from a fiber-optic CT is sampled with PWM triggers that produce a 90-degree phase difference between two consecutively sampled signals. The current-induced phase is extracted by applying an arctangent demodulation and a phase unwrapping algorithm to the sampled signals. From experiments using the proposed demodulation, we obtained phase measurement accuracy and a linearity error, in AC current measurements, of ~2.35 mrad and 0.18%, respectively. The accuracy of the proposed method was compared with that of a lock-in amplifier demodulation, which showed only 0.36% difference. To compare the birefringence effects of different fiber-optic sensor coils, a flint glass fiber and a standard single-mode fiber were used under the same conditions. The flint glass fiber coil with a Faraday rotator mirror showed the best performance. Because of the simple hardware structure and signal processing, the proposed demodulation would be suitable for low-cost over-current monitoring in high voltage power systems.