Mechanisms And Testing Of Moisture Content Measurement Of Transformer Pressboard

The project < Mechanisms and Testing of Moisture Content Measurement of Transformer Pressboard > is one of the key problems to tackle in the electric power industry in China (No.98010211), which is also one of the worldwide problems. The presence of moisture in transformer pressboard has a detrimental effect on insulation life by lowering electrical breakdown strength and thermal endurance. It is fundamental important to have the capability of accurately measuring and monitoring the moisture of pressboard to keep the moisture content in a suitable range for the manufacture and operating of power transformer industry.The dielectrometry technique is good at detecting moisture content in pressboard, but it is not mature yet According the mechanisms of dielectric polarization and loss, several aspects on dielectrometry are studied in this dissertation, including the dielectrometry theories and disturbance factors.The dissertation presents the principle of dielectrometry and its disturbance factors, and the methods to obtain the complex permittivity with a lumped circuit model. The moisture affects the dielectric properties of pressboard. Accurately monitoring and controlling the moisture of pressboard for the manufacture of power transformer industry can be obtained by the measurement of the complex permittivity of pressboard.The three key factors in the development of the on-line moisture measurement system are capacitor sensors, interface cicuit and software. The design demands and technique requirements of fringing field capacitor and the circuit are introduced. In order to obtain the permittivity and moisture content, the software is designed to calculate the gain and phase of signal.The method to measure moisture content of pressboard with single frequency is studied. The effects of frequency and temperature to the signal of capacitive sensor are analyzed According to the relations between the polarization and loss of pressboard, a model of effects of frequency and temperature to signal is developed. A calibrated expression relating permittivity and moisture content of oil-free pressboard is presented by using experiment data and dielectric frequency spectra.A modeling study of the drying kinetics is presented, which provides a theoretic basis for the on-line moisture measurement The models developed for mass transfer inside the pressboard is validated for a one-dimensional case and a two-dimensional case. The diffusion process is studied by the data obtained by sample weighing. The moisture concentration curves acquired in classical mass loss drying experiments show that the law of diffusion process is an exponential decay expression, and drying rate is a process of wavy decay.