Research On Measuring Concentration Of Methane Dissolved In Transformer Oil Based On Spectrum Analysis Method

Techniques for on-line monitoring of oil-immersed power transformer play an important role in keeping the reliability and stability of electricity supply. Dissolved gas analysis in oil-immersed power transformer is an effective way for monitoring and diagnosing the early malfunction and preventing catastrophic accident. It has formed national criterion and is widely applied for practical production.In this paper a new technique is adopted-spectrum analysis method, which is completely different from traditional chromatogram analysis method. When oil-immersed power transformer gets in trouble, some gas will be produced from the oil. Spectrum analysis can monitor the gas on line with high sensitivity. Optical fiber gas sensing technology is a developing and novel technique, which has a wide applicable prospect in industrial production, environment monitoring and medical science. In recent years, with the development of optical fiber sensing technology, the research of optical fiber gas sensor has been seriously regarded all over the world. The signal detected by optical fiber gas sensors is carried by light wave, which has no influence on the measured object. The sensor that has good independence performance itself can be adapted to all kinds of surroundings. The optical fiber sensing system with the sensors, which can be connected with computers conveniently, can realize many functions request and intellectualized need. They can match with remote measure technology to measure and control in long distance.Methane is a main kind of gas produced in the oil of oil-immersed power transformer when the transformer gets breaks down, this paper focus on a method to monitor it totally using fiber sensors. Based on absorption of lightby the vibrational rotational energy levels of methane molecules at near-IR, the concentration of methane can be obtained by modulating wavelength. This method can be applied to monitor other gases, too.The existing methods of monitoring dissolved gases in oil-immersed power transformer are presented in the beginning of this paper, then their strongpoints,weakpoints and their principles are discussed. Compared with others, spectrum analysis method's advantages are revealed. The principles of spectrum analysis method and chromatogram analysis method are compared in detail, and then the theory of gas molecule absorption spectrum,the gas molecule's absorption line shape and broadening of the spectrum are introduced. In succession, methane's absorption spectrum near 1.6μm is dissertated, including absorption intensity,half bandwidth of wavelength and position of spectrum.In the system discussed in this paper 1.65μm distributed feedback semiconductor laser diode (DFB LD) is adopted. Signal gained is stable due to DFB LD and modulated wavelength technique, and precision has been enhanced. Bragg grating and piezoelectric ceramic is the key part for wavelength modulating, their characteristics are described in this paper , such as influence voltage brings to piezoelectric ceramic and influence stress brings to Bragg wavelength .Because photo detector turns photo signals to electric signals, it is a indispensable component in fiber communication. Considered quantum effect,sensitiveness,response of spectrum and frequency, etc, photodiode is chosen as optoelectronic detector.For the signals are very weak, the lock-in amplifier is employed to extractthe signals in the circuit design. It can inhabit the noise at most and maximize the ratio of signal-to-noise.The last part is about experiment. First the spectrum of distributed feedback semiconductor laser diode has been tested for many times, according to the data obtained an optimum condition is chosen for DFB LD. Referred to the deterioration test of fiber circuit, possibility of long distance transfer using fiber is discussed. Second harmonics is adopted to detect the signals, the advantage is discussed in detail when second harmonics is used in full wavelength modulated detecting system.Using wavelet noise reduced technique to the system, the spectrum of some interfering gas is filtered and smoothed. The noise is dealt with. The situation, height and area of peak of the spectrum are almost not changed. It means the experiment data is unanimous with theory, and fully proves that the designed system is feasible.