Principle Of Dielectrometry Measurements Of Moisture Content In Pressboard And Research On The Measurement System

The task 《Principle of Dielectrometry Measurements of Moisture Content in Pressboard and Research on the Measurement System》 is a key problem to tackle in the electric power industry in China (S No.Y98010211) ,which is also a worldwide difficult problem. Since the presence of moisture in the transformer insulation can bring the transformer performance trouble by decreasing its insulate strength and shortening its service. Therefore, it is very important to monitor the moisture condition in pressboard during both manufacture and performance process of a transformer. The dielectrometry technique, which is not yet mature, can be used to detect moisture content in pressboard. In order to enrich this technique,from the mechanisms of dielectric polarization and loss, several aspects about dielectrometry technique to detect moisture content in pressboard are researched in this thesis, including the dielectrometry theories and development of the moisture one-line measurement system.The thesis presents the measurement principle and its disturbance factors, as well as the method to obtain the complex permittivity of pressboard with a lumped circuit model. If a capacitance system consists of the measured pressboard and a capacitive sensor,the outputs of the system are effected by dielectric characters of measured pressboard. As a function of moisture content, the complex permittivity are measured to inform the moisture content in pressboard.Aim at the relations of the complex permittivity of pressboard and moisture as well as temperature,according to the experiment data and master dielectric frequency spectra, new functional forms of moisture content for oil free pressboard versus complex permittivity, temperature and frequency are fitted, which are easier to use and more objective. With three models, the effective dielectric constant of pressboard can be calculated. The measurement errors of dielectric characters are analyzed completely .The method to measure moisture content of pressboard with single frequency is discussed, with corresponding models and fitting parameters, which provides a theory basis for the on-site measurement.The two key factors in development of the moisture one-line measurement system are capacitive sensors and interface cuicits.The figures of merit and design demands of the fringing field dielectrometry sensor are introduced. By considering the issues about dimensions of measured materials, conditions, electrodes, substrate, channel number, penetration depth, signal strength and installation, several kind of fringing field dielectrometry sensors are designed and worked out.and the design ideas of both float electrodes and stable montage force are also putted forward. The four developed moisture measurement sensors represent the developing tendencies with small size, large penetration depth, multi-wavelength and array pattern.The interface circuit performance effects the measurement results. Since the output signal from fringing field dielectrometry sensors are very low, the shield-drive, signal buffer and band-pass filter are adopted to improve SNR.With the signal gain and phase angle from a unique transform circuit, the complex permittivity of pressboard are obtained. The reliability and precision of the interface circuit can meet the needs of measurement of moisture content in pressboard.To obtain the moisture profile of pressboard,the multi-wavelength sensor and valid parameter estimation algorithms are necessary, the one-dimensional parameter estimation algorithms, including the equivalent node admittance algorithm,a new proposed by author, are presented. The moisture distribution measurement of pressboard in two dimensions is discussed as well.All research above can establish a stable foundation for the application of dielectrometry in moisture measurement.