spie proceedings [spie spie photonics europe - brussels, belgium (monday 16 april 2012)] optical...

Demodulation of FBG sensors embedded in a fiber-optic Sagnac loop

Hyunjin Kim1, JuneHo Lee2, Jong-kil Lee3 and Minho Song1 1Division of Electronics Engineering, Chonbuk National University, Jeonju 561-756

TEL: +82-63-270-4285, FAX: +82-63-270-2394 [email protected]

2Department of Electrical Engineering, Hoseo University, Asan 336-795 3Department of Mechanical Engineering Education, Andong National University, Andong 760-749

ABSTRACT

For condition monitoring of large scale electrical power transformers, a fiber-optic multi-stress sensor system was constructed by combining fiber-optic acoustic sensors and fiber Bragg grating temperature sensors in a fiber-optic Sagnac interferometer. To separate the grating signals from the interferometer output, an attenuator was placed at an asymmetrical position in the Sagnac loop. By balancing the counter propagating light intensities with the attenuator, the background noises could be suppressed to obtain grating signals with enough signal-to-noise ratio. With the preliminary experiments, the temperature and the vibration information at multiple locations could be measured simultaneously with single optical circuit and signal processing unit.

Keywords: transformer, condition monitoring, FBG, Sagnac interferometer, temperature, vibration

1. INTRODUCTION

For decades, electric power consumption has increased consistently and the capacity of power facilities has grown in huge amount. Power suppliers and consumers require reliable power quality and stable operation of power facilities without any serious failures. For the purpose, the condition monitoring technology becomes more important in modern power industry. The condition monitoring is the process of monitoring parameters of condition in machinery. It enables to avoid possible power failures by alerting any significant changes which are indicative of developing failures detected by individual sensors and/or networks in the major power facilities, such as power transmission lines, transformers, GIS (gas insulated switchgear), distribution lines and so on. The transformer is one of the most common and expensive power facilities that need continuous condition monitoring. The major cause of transformer accidents is the damage of machine by natural degradation and vibration. There are several transformer condition monitoring techniques, such as dissolved gas analysis, degree of polymerization, furfural, recovery voltage measurement, tangent delta, insulation resistance test, partial discharge measurement, and so on [2-3]. Although these techniques have been widely used, most of them cannot be used while the facilities are in use due to the high potential and large current environment. Even the online sensors, for example piezoelectric transducer partial discharge sensors, are applied to the casings of the transformers to analyze the vibrations of the casings. However, in order to analyze the precise locations, it is desired to measure the vibrations or the temperature distribution from the inside locations of the transformers.

Considering all these things, fiber-optic sensors are ideally suited for these condition monitoring applications. The inherent dielectric nature of the optical fiber makes the sensors to be immune to electromagnetic wave noise, and insulation problems. Because they are small in dimension and chemically passive, it is easy to install the fiber-optic sensors at inside locations of power transformers, making it possible to measure different measurands while the facilities

Optical Sensing and Detection II, edited by Francis Berghmans, Anna Grazia Mignani, Piet De Moor, Proc. of SPIE Vol. 8439, 84392B 2012 SPIE CCC code: 0277-786X/12/$18 doi: 10.1117/12.923243

Proc. of SPIE Vol. 8439 84392B-1

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To show the temperature measuring capability, we placed FBG sensors in an oven and raised the temperature in 15 steps. Eight FBGs were placed in the Sagnac loop, and the temperature of 8 points were measured at the same time. Only two of them, sensors 5 and 8, are exposed to the temperature variations. Figure 7 shows measured temperature profiles of the 8 points with measurement error of 1.

(a) Sensor #1 (b) Sensor #2 (c) Sensor #3

(d) Sensor #4 (e) Sensor #5 (f) Sensor #6

(g) Sensor #7 (h) Sensor #8 Figure 7. Temperature measurements using 8 FBGs

In addition to the FBG sensor signal processing, the mandrel sensor signal was also analyzed by using a FFT analyzer. Figure 8 shows the frequency spectra of the signals measured with the proposed system. The acoustic signal was applied by driving cylindrical piezo-electric transducers. Figure 8(a) and (b) are the results when the transducers were driven by 35, 80 Hz and 35, 85 Hz of modulation frequencies. The other frequencies in the spectra are the harmonic components originating from the transformers internal structure.

4. CONCLUSION

We proposed a fiber-optic hybrid sensor system which can measure multi-stress of a power transformer. Due to the inherent advantages of the fiber-optic sensor, it is able to measure the temperature and the vibration signals from the

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several locations inside of a power transformer. FBG sensors and fiber-optic mandrel sensors were embedded in a fiber-optic Sagnac interferometer for temperature and vibration measurement, respectively. A fiber-optic wavelength-sweeping laser was constructed and used as the light source to address the sensor outputs simultaneously. An optical attenuator was used in the fiber laser cavity to separate the FBG sensor signals from that of the Sagnac sensor. With the experiments done with the prototype sensor system, we showed the feasibility of the suggested system in temperature and strain sensing applications. This hybrid-type multi-stress sensing system would be attractive especially for condition monitoring uses which require low-cost and small installation spaces.

(a) 35 Hz and 80 Hz (b) 35 Hz and 85 Hz

Figure 8. Acoustic signal analysis

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST)(No. 2011-0000896)

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