Polarization Characteristics Of Oil-pressboard Composite Insulation With Aging Pressboard Under DC And Polarity Reversal Voltage

Converter transformer is one of the most core equipment in the UHVDC transmission system,and its main insulation of the valve is subjected to DC and polarity reversal voltage during the insulation test and operation.The main insulation adopts oil-pressboard composite insulation structure,which forms composite insulation polarization and accumulates a large amount of space charge on the oil-pressboard interface under DC voltage.When the polarity of DC voltage is reversed,the accumulated charge in the early stage dissipates and the space charge of reverse polarity accumulates,resulting in high amplitude transient voltage.As the operation life of converter transformer increases,the pressboard of main insulation ages gradually,and the polarization characteristics of composite insulation will change.To improve the operation reliability of converter transformer,it is necessary to research the polarization characteristics of oil-pressboard composite insulation with different aging degree of pressboard under DC and polarity reversal voltage.In addition,it is also necessary to explore the aging assessment method of insulation pressboard to provide a basis for the converter transformer to quit operation.Three kinds of parallel-plane three-electrode test models are designed,which are the oil-impregnated pressboard covered high-voltage electrode model,covered low-voltage electrode model and separated oil gap model.The polarization currents of oil-pressboard composite insulation samples with different aging degree of pressboard are measured under different DC voltages and polarity reversal voltages.The polarization currents are analyzed by using the extended Debye model,the polarization mechanisms are discussed,and the characteristics of accumulated charge under DC voltage and electric field distribution under polarity reversal voltage are analyzed.Based on the polarization characteristics of oil-pressboard composite insulation,a test method of repeated application of DC voltage is proposed.The characteristics of repeated polarization current are studied experimentally and the aging condition of oil-pressboard composite insulation is assessed.Under DC voltage,the polarization absorption current value is significantly larger than the depolarization current value in the same time,and the polarization process and the depolarization process are asymmetric.The main reason is that the accumulated charges in the polarization process agglomerate to form larger charged colloidal particles,which makes the charges dissipate slowly in the depolarization process.The polarization current of oil-pressboard composite insulation is also related to the space position of insulation pressboard.The polarization current value of oil-impregnated pressboard covered high-voltage electrode model is the largest and the decay rate is the fastest,which has obvious polarity effect.As the aging time of pressboard increases,the polarity effect decreases.The adsorption of hydroxyl of insulation pressboard to negative charge in oil is the fundamental reason for the polarity effect of polarization current in oil-pressboard composite insulation.The steady-state charge changes linearly with the applied voltage,and the static capacitance of the oil-impregnated pressboard is a constant.The static capacitance of the oil-impregnated pressboard is larger than the displacement polarization capacitance,and the static capacitance when accumulating negative charges is larger than that when accumulating positive charges of the oil-impregnated pressboard.As the aging time of pressboard increases,the negative steady-state charge quantity decreases linearly,while the positive steady-state charge quantity remains basically the same,and the change law of static capacitance of oil-impregnated pressboard is consistent with that of steady-state charge quantity.Under the polarity reversal voltage,for the oil-impregnated pressboard covered high-voltage electrode model,the polarization current from negative to positive reversal is larger and the decay rate is faster than that from positive to negative reversal,and for the oil-impregnated pressboard covered low-voltage electrode model,the polarization current from negative to positive reversal is smaller and the decay rate is slower than that from positive to negative reversal.The polarization currents between two polarity reversals are asymmetric with the oil-impregnated pressboard covered electrode model.As the aging time of pressboard increases,the polarization currents tend to be symmetrical.For the oil-impregnated pressboard separated oil gap model,due to the symmetrical structure,the polarization current and decay rate of two polarity reversal are the same,which have symmetry.As the aging time of pressboard increases,the polarization current value becomes smaller.Under the polarity reversal voltage,the hydroxyl still has the adsorption characteristics to the negative charge in oil so that the negative charge accumulate quickly and dissipate slowly,which is the fundamental reason for the asymmetry of polarization currents between the two polarity reversals.The different between accumulation and dissipation rates of negative charges changes the insulation condition of transformer oil.The equivalent transient insulation resistance of the oil gap changes,which changes the voltage distribution relationship of the oil gap,resulting in different decay laws of the electric field in oil,and the electric field in oil at the moment of polarity reversal larger than twice in the initial DC electric field.Based on the asymmetry of polarization absorption current and depolarization current and the polarity effect of insulation pressboard,a repeated polarization current test method is proposed.There is obvious difference in the decay rate of the two polarization absorption currents,and the second polarization current decays significantly faster than the first polarization current.The difference between the decay rates of the two polarization absorption currents increases at first and then decreases with the repeated polarization time,and decreases with increasing aging degree of the pressboard.The fundamental reason for the difference in the decay rate of two polarization absorption currents is that the accumulated charge is not completely dissipated after the first polarization and depolarization,and the adsorption characteristics of hydroxyl on the negative charge in oil increase this difference.The ratio of the first normalized polarization charge to the second normalized polarization charge is proposed as a novel aging characteristic parameter of oil-pressboard composite insulation,and the aging condtion of oil-pressboard composite insulation is characterized by the maximum value of aging characteristic parameter under different repeated polarization time.