Pressboard Water Line Monitoring System

The presence of moisture in the transformer insulation can bring the transformer performance trouble by decreasing its electric and mechanical strength. Therefore, it is very important to monitor the moisture condition in pressboard during both manufacture and performance process of a transformer. With the feasibility that the physical characters of material can be measured through dielectrometry technique, several aspects about dielectrometry technique to detect moisture content in pressboard are researched in this thesis, and an on-line measuring system of the trace moisture in transformer pressboard is developed.The fringing field dielectrometry sensor serves as a key factor in the moisture on-line measurement system. The figures of merit and design demands of the sensor are introduced. By designing the fringing field dielectrometry sensor, the issues about dimensions of measured materials, electrodes, substrate, channel number, penetration depth, signal strength and installation should be considered. The two developed moisture measurement sensors represent the developing tendencies with small size, large penetration depth.The interface circuit performance decides the measurement results. Since the output signal from fringing field dielectrometry sensors under a low frequency driving voltage 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 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.Single chip AT89S51 of 51 series is used as the core component in data-collecting system. With the help of software programmed in assemble language, the three voltage signals of amplitude, phase and temperature can be disposed and calculated, and the heating power supply of relay is controlled to start or stop in real time. Noting down amplitude voltage signals and phase voltage signals exported by interface circuit, with the weight value of pressboard the corresponding moisture value can be calculated, and a table reflecting the relation of voltage signals and moisture can be made. Encase the table in the ROM of single chip, moisture value can be get through checking the table and inserting values. And communications between single chip and PC are also discussed in this paper.The experiment results indicate that the whole design is feasible and reliable, and the presumed functions are conducted successfully. The trace moisture in pressboard can be measured actually by using the devised capacitive sensor and interface circuit. All research above can provide valuable help for measurement of trace moisture in transformer pressboard.