Condition monitoring of power transformers is of great importance for the timely detection of incipient faults to avoid potential malfunctioning. Transformer insulating oil contains about 70% of diagnostic information, and a dramatic rise in oil temperature may drastically reduce the lifetime of power transformers, and thus the temperature of the oil is considered the most crucial parameter that has to be monitored continuously in real-time. Compared to traditional temperature measurement methods used in transformer condition monitoring, distributed optical fiber sensors have inherent advantages of immunity to electromagnetic interference and insulation at high-voltage levels, and they offer spatially resolved temperature monitoring with high accuracy and sensitivity. In this study, optical fiber-based distributed temperature measurement of a fully energized 100 kVA distribution transformer is demonstrated by using two different techniques: Optical Frequency Domain Reflectometry (OFDR) and Fiber Bragg Grating (FBG) sensor array. The fiber sensors are robust for a safe long-term installation into oil-filled distribution transformers during manufacturing, and they can withstand heat runs, long-term hot oil immersion, and transformer vibration. The internal transformer temperature is monitored during standard thermal tests prior to installation on the distribution system. The test results show very good agreement between the standard thermocouple and proposed distributed fiber temperature sensors, providing transformer manufacturers with new insights into the distribution of temperatures internal to their commercial products.
KEYWORDS: Interferometers, Signal detection, Nonlinear optics, Optical fibers, Spatial resolution, Photodetectors, Temperature metrology, Signal processing
Swept Wavelength Interferometry (SWI) based on Tunable Laser Source (TLS) has various applications in optical measurement and imaging. Nonlinear tuning of the TLS is always a problem in SWI and require proper correction to enhance the spatial resolution and SNR of the signal. Typically, nonlinear tuning correction requires an extra (auxiliary) interferometer. A new type of Optical Frequency Domain Reflectometry (OFDR) arrangement was proposed in which auxiliary interferometer was integrated with the main interferometer and only a single detection channel was used instead of two. This new hardware design eliminated the need for an extra photodetector and an acquisition channel for the auxiliary interferometer. Single trace having beating signal from auxiliary and Rayleigh backscattering from the main interferometer was acquired in a single detection channel. Then the beating signal of the auxiliary interferometer was used to correct nonlinear tuning effects from Rayleigh backscattered signal of the main interferometer. The feasibility of new hardware design is demonstrated by correcting nonlinear tuning effects in a 50 meters long optical fiber and performing distributed strain and temperature sensing in the OFDR technique. Furthermore, an extension of the proposed new design is also described in this paper in which the auxiliary interferometer is replaced by a high reflection event inside the Fiber Under Test (FUT) created by femtosecond laser which makes the overall system design more compact and simpler.
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