Polarization And Depolarization Characteristics Of Oil-paper Composite Insulation Under Dc And Reverse Polarity Voltage

Oil-immersed power transformers and converter transformers play important roles in UHV long-distance transmission and DC transmission systems,and their stable operation is critical to the safety of the entire power system.Most of main insulation of the oil-immersed transformers is oil-paper composite insulation structure.In order to explore the polarization and depolarization characteristics of oil-paper composite insulation under different applied voltages and structures,a large gap oil-paper composite insulation structure model was designed and a high-voltage test platform was built.Under 50 k V,60 k V,70 k V,80 k V,90 k V applied voltage,three different oil-paper composite insulation structures with cardboard placed in the middle of the two electrodes,the positive electrode side and the negative electrode side were tested by multi-factors.Through these tests,it is found that under each of the above conditions of applied voltage and insulation structure,the relaxation time of the polarization and depolarization current of the oil-paper composite insulation increases gradually.In this paper,a new polarization and depolarization current attenuation function is proposed and used to fit the current data.And the change rule of characteristic parameters is found: The initial value of polarization and depolarization current has a positive exponential growth characteristic with the applied voltage and a negative exponential growth characteristic with the electrode gap.The current attenuation function is not affected by the applied voltage level,but it is related to the insulation structure closely.Moreover,for different insulation structures,the initial value and the variation rate of the relaxation time change obviously,but their change rules are different.The power index can be regarded as a constant under each of the applied voltage levels and insulation structures mentioned above.Under the same electrode gap and applied voltage level,when the cardboard is placed at different positions,the attenuation laws of polarization current and depolarization current both change,and the difference between the two attenuation laws also changes.But polarization current value is always larger than the depolarization current value.It is obtained that the polarization process of oil-paper composite insulation and the depolarization process are asymmetric,and there are obvious polar characteristics affected by the characteristics of the insulation structure.The repeated polarization test further confirms that the depolarization of the oilpaper composite insulation is a slow process.The longer the interval between repeated polarizations is,the slower the current decays during polarization.However,the interval has no effect on the initial value of the polarization current and the depolarization process.It can be found by comparing the two polarization currents in a polarity reversal test that,when the cardboard is placed in the center,the absolute values of the polarization currents under negative-positive reverse voltage and positive-negative reverse voltage are similar,and the two polarization current curves have a symmetric relationship about the coordinate axis.And with the cardboard placed on the negative electrode side,the absolute value of the polarization current under negative-positive reverse voltage is smaller than that under positive-negative reverse voltage.When the cardboard is placed on the positive electrode side,the conclusion is opposite.And under last two insulation structures mentioned above,there is no symmetric relationship between the two current curves.Therefore,the polarity characteristic also exists under the polarity inversion voltage.