Polarization Current Characteristics Of Multilayer Paperboard Oil-Paper Composite Insulation

The large-scale power transformer is the pivotal equipment of the electric system,the aging insulation is the main factor affecting its operating life.It is necessary to test the transformer insulation regularly.In recent years,the polarization-depolarization experiment method based on the intermediate point response has been widely used.Most scholars often use the equivalent model of single-layer oil-immersed insulating cardboard to study its polarization characteristics.The main insulation of the actual transformer is a multi-layer cardboard structure,so it is necessary to design a multi-layer cardboard composite insulation structure that is more suitable for the actual transformer.In addition,at present most laboratories often use several hundred to several thousand volts of applied voltage to test,applied on the multi-layer cardboard structure signal-to-noise ratio is not high,it is necessary to appropriately increase the level of voltage to explore polarization characteristics.Referring to the insulation structure of 110 k V power transformer,the paper designed the multilayer cardboard-oil-paper composite insulation model,prepared the new cardboard and aged cardboard samples and built an experiment platform.Depolarization experiments and repeated polarization experiments were conducted,and data were fitted using a three-branch extended Debye model,the influence of voltage level and aging state on the polarization characteristics of multilayer paperboard oil-paper composite insulation and the influence of voltage level,aging state and repolarization time on the repolarization characteristics of multilayer paperboard oil-paper composite insulation were investigated,the aging assessment are given and the mechanism is discussed.The results of polarization and depolarization tests show that the polarization current and depolarization current of the new paperboard sample are asymmetric under different voltage levels,and the asymmetry becomes more obvious with the increase of voltage levels,the polarization current and depolarization current of the aged sample are basically symmetrical.The results show that the maximum time constant of the aged paperboard sample with the same voltage is obviously smaller than that of the new paperboard sample,and the gap increases with the increase of voltage level.Using the change of the maximum time constant of unit voltage as the aging characteristic quantity can obviously distinguish paperboard aging.It is believed that the existence of hydroxyl adsorption charge is the fundamental reason for the asymmetry of polarization current and depolarization current.The results of repeated polarization test show that the second polarization current of new and aged paperboard samples in the first 1s is obviously higher than that of the first,and the difference increases with the increase of voltage level,however,the second polarization current in the middle and later segments of the curve is obviously smaller than the first polarization current.Under the same voltage level,the difference of two polarization current of aged paperboard is less than that of new paperboard.With the increase of the repeated polarization time,the difference of the two polarization currents of the new and aged paperboard samples decrease gradually.It is found that the difference of two polarization currents can be characterized by the difference of the current amplitude of the shortest polarization branch in the fitting parameters.It is considered that the difference between the two polarization currents is caused by the incomplete dissipation of the charge during depolarization.With the loss of hydroxyl,the charge adsorption capacity decreases and the difference decreases.As the depolarization process increases,the amount of residual charge decreases and the difference decreases.