| Converter transformer is one of the key equipments in the HVDC system. There is a high failure rate in the major insulation of valve side during predelivery test or on operation, for the stressed voltage in converter transformer is different from the one in the ordinary AC transformer, and there’s not enough operation technique data. Experts all of the word have made a lot of research on the insulation in the converter transformer, but few is about the influence of working condition on the insulation. Hence, it is necessary to lucubrate on the influence of working condition upon the major insulation of valve side in converter transformer.In order to study the influence of temperature and moisture on the electric field distribution of oil-paper insulation, the resistivity and relative dielectric constant was tested in oil and oil-impregnated pressboard separately, with temperatures and moistures. And the expressions of resistivity and relative dielectric constant were obtained by surface fitting. Combining with the derivation of circuit model, the equation set of electric field distribution was deduced, on the condition of different temperature and moisture. And the electric field distribution was analysed by simulation. The result shows that the bulk resistivity and relative dielectric constant both fall exponentially in oil and pressboard as temperature and moisture rise, but the variation range is very small in relative dielectric constant. The increase of temperatue results in the electric field strength rising in the oil but falling in the pressboard. The electric field strength varies intensively when moisture equilibrium is in the transient state than in the stable state, and the lower AC content is the more obvious the phenomenon is. When the moisture equilibrium is in the stable state, the electric strength rises up in the oil but falls down in the pressboard, as the moisture ascends, and the variation trend goes acceleratedly. It is found by summarizing test results that the variation rate of bulk resistivity is different in oil and pressboard as the temperature(or moisture) changed, which leads to a great influence of temperature(or moisture) on the electric-field distribution, in the oil-paper insulation under the low AC content compound voltage.In order to study on the influence of temperature and moisture on the BDV(breakdown voltage) of oil-paper insulation, the breakdown test was carried out with different temperatures, moistures and AC content AC+DC voltages separately, on the typical oil-paper insulation under plate electrodes, and the BDV characteristics were obtained. The breakdown mechanism was discussed in-depth under AC+DC voltage of typical oil-paper insulation, by comparing the breakdown curves under different conditions. The breakdown voltage estimation expression was concluded, and the possibility to optimize the oil-paper insulation structure was proposed under AC+DC electric field. The results indicate that, the BDV(breakdown voltage) rises up first and falls down later as the temperature increased, but the temperatue corresponding to peak voltage varies and the scope of BDV is different either. In all moistures, the BDV of oil-paper insulation rises up first and falls down later, as well, with the AC content increased. The BDV falls down with the moisture increased, and the falling range varies under different AC content voltages. It is found by analysing that the electric field strength in oil and pressboard is influenced by temperature(or moisture), meanwhile the dielectric strength is influenced by them too. Both of them above make the BDV changed.The electric field strength varies in the oil and pressboard as the AC content changed, and there is a difference between them in AC and DC field on the breakdown characteristics. Both of the two reasons above make the BDV vary with the AC content.To study on the influence of oil-flow electrification on the BDV, a test platform for the experiment of oil-flow electrification and breakdown characteristics was designed and established, and the oil-flow electrification and breakdown test was carried out with different velocities under complicated voltages, on the condition of oil flowing. The result shows that the BDV rises up first and falls down later as the AC content increased, at all the velocities, but the range of BDV is not the same. The BDV falls down with the velocity rising, but the descent range is not equal, and the descent rate is not the same at different velocities. The streaming current is increased as the velocity goes up at every AC+DC voltage, but the ascend range differs as the AC content changes. There is saturation in the streaming current as the velocity ascends, and the saturation velocity is increased as the AC content rises. After analyzing, it is deduced that the electrification layer is enlarged by the DC field, which leads to the oil-flow electrification going up with the applied voltage ascending. In the AC field, the half period time is much shorter than the time for electrification layer enlarged, which leads to there’s little influence on the degree of oil-flow electrification. The charge distributes nonlinearly in the electrification layer and the velocity distributes linearly in the laminar flow, both above lead to the charge were separated nonlinearly as the velocity increased linearly, which results in the saturation in the streaming current as velocity increased. There is some charge in the oil that couldn’t flow through the electrification area, because the electric field strength is too strong; which results in saturation velocity ascend with the DC content increased. The space charge on the interface of oil and paper is increased by electrification, which makes the electric strength increased both in the pressboard and oil. The electric field is distorted by the space charge and the breakdown voltage of oil varies with the velocity, both above lead to the BDV changing as the velocity rises, under different AC content voltages in the oil-paper combined insulation. |
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