Electric Field In The Oil And Interface Charge Characteristics In Converter Transformer Oil-Paper Insulation System

Compared to AC power transmission, HVDC (High Voltage Direct Current) transmission has got rapid developments because of the unique advantages in the long-distance and high-capacity transmission, cross-sea power transmission and grid liaison. Converter transformer, as one of the important transmission equipment, is critical to safe and reliable operation of HVDC transmission system. The specificity of the converter transformer insulation design focused on two aspects:One is that the valve winding of converter transformer withstands the AC voltage, DC voltage and polarity reversal voltage, the electric field is more complex; the other is the uncertainty of the electric field distribution caused by the effects of the moisture in the oil and pressboard. For the above problems, this thesis studied the characteristics of the electric field and interface charge in the oil-paper insulation based on the electric field measurement.An on-line electric field measurement system was established based on the Kerr electric-optic effects. In order to improve the measuring accuracy of the DC electric field, the factors leading to the measurement errors were analyzed and discussed, the measures to reduce the errors were adopted. Sensitivity test of the measurement system was performed with the plate electrodes, the test indicated that the system is capable of measuring the DC field lower than20V/mm, the relative error is lower than5.4%。The measurement system can satisfy the measuring of DC, AC and polarity reversal electric field.According to the actual working condition of the converter transformer, the electric field in the oil was measured respectively under DC voltage, DC-AC hybrid voltage and polarity reversal voltage on the typical oil-paper insulation model.The polarity effect of the electric field transient process under DC voltage was found. The electric field decays faster under negative DC voltage than that under positive DC voltage. By the molecular structure analysis of the pressboard and the verification experiment, it can be proved that the pressboard has absorption to the negative charge in the oil, which directly results in the negative charge accumulates on the surface of the pressboard faster and decays slower than positive charge.The inconsistency of the electric field dynamic process under double polarity reversal voltage was found, the electric field analysis model was builded based on the interface charge. During the first polarity reversal, the electric field in the oil reduces to zero faster than the applied voltage and rises up to a magnitude1.46times as much as the initial electric field after polarity reversal; but for the second polarity reversal, the electric field drops to zero slower than the applied voltage and only increased to0.85of the initial electric field after the polarity reversal. Effects of the PR time on the electric field intensity after polarity reversal were studied for the first time. It can be found the the shorter of the PR time, the higher of the electric field in the oil after the polarity reversal, but the difference of the maximum electric field with different PR time is not obvious.Research also found the effects of the PR time on the electric field is also closed related to the oil resistivity.The asymmetric of the DC electric field distribution on the pressboards overlapping insulation model was found under AC-DC hybrid voltage. The DC electric field intensity of the oil in the right side is much stronger than that in the left side, but the AC electric field distribution is symmetric. Research also found the DC electric field intensity did’t rise up with the DC voltage linearly, compared with the linear dependence between the AC electric field and AC voltage. Through the verification test, it can be proved the difference of the interface charge accumulation in different position is the main reason for the DC electric field distribution.The effects of moisture on the electric field and interface charge were studied for the first time; the mechanism of moisture effects was analyzed. It was found that the increase of the moisture content in oil and pressboard leads to the decrease of the electric field in the oil and the interface charge density with time under both positive and negative DC voltage. By measurement of resistivity both of the oil and pressboard and analysis of the interface charge, it was proved that the increase of the moisture content leads to the decrease of resistivity ratio between the pressboard and the oil, which results in decrease of accumulated interface charge.The effects of the oil gap size on the electric field intensity under double oil gap insulation model were studied and the electric field analysis model based on the interface charge was proposed for the first time. The electric field is very depended on the positon and size of the oil gap. Under positive voltage, the electric field in the upper oil gap rises up with the time, the electric field in the lower oil gap decreases with time, the smaller of the upper oil gap, the stronger of the electric field in the upper oil gap. Under negative voltage, the status of electric field is opposite to that under positive voltage.