Of Transformer Insulation Aging And Life Assessment

Large numbers of power transformers around the world are approaching towards the end of their design life. They are very expensive to replace; however, some of these transformers are still in good condition and could be used for many more years. Clearly determining their condition would be of tremendous importance to the electricity industry.The aim of this work is to assess the quality of oil-paper insulation in transformers particularly the moisture content and ageing state of its solid component based on the measurement of polarization and depolarization current theory. The basic way to perform the measurement is to apply a DC voltage on the main insulation between the high and low voltage and the current is also recorded at the same time. A simple equivalent circuit is built to simulate the properties of the transformer's main insulation system by means of least square method which was proved to be an excellent tool to fulfill the calculation. The simple equivalent circuit determined from measured relaxation currents can be used to calculate all other dielectric responses such as the capacitance, polarization index, and dissipation factor in the time or frequency domain.The results show that some factors like charging voltage, charging time, geometry layout and the oil and pressboard condition may have some impact on the polarization and depolarization current. With time less than 100 second the oil properties will give a dominate affection on the current curve and at long times the amplitude of time dependence of these currents are influenced mainly by the response of pressboard. While the geometry layout will affect the middle part.It is also shown that for a homogeneous dielectric material it has a very linear property with increase of the applied voltage. More ageing information about the insulation could be got and the difference between the polarization and depolarization will be minimized with longer charging time. All of these above are demonstrated on the transformer samples.In this work we have measured and collect a lot of data about the conductivity and relative permittivity of some pressboard with different moisture content at different temperature. The oil and paper conductivity can be calculated by means of the measured polarization and depolarization so as to compare it with a database to get an overall evaluation of the aging stateof the transformer. A program debugged in VB is finished to perform the whole calculation process and other function such as the data's saving and input and the graph output.The PDC theory is not a new theory that has been known for a long time but put it on the diagnosis area is actually a breakthrough. There is still no a reliable and no-destructive diagnosis method to determine the remain life of an aged transformer. The PDC has a huge potential advantage and bright application future in our country.