| The insulation failure rate of the converter transformer as a key component of high-voltage direct current transmission has been high,which has caused serious damage to the safe operation of UHV direct current transmission engineering equipment that has been widely used at present.The main insulation of the converter transformer is an oil-pressboard composite insulation structure.Its insulation performance is determined by the transformer oil and the high-density laminated transformer pressboard.Oil and pressboard produced by different manufacturers have uneven insulation properties,which is one of the key factors affecting the safe and stable operation of converter transformers.Under the actual operating conditions of UHV engineering transformers,the internal oil and pressboard materials will gradually age due to the coupling effects of multiple physical fields such as large gradient temperature fields and electric fields.And affected by factors such as the interface charge of the oil-pressboard composite insulation structure,the DC electric field has a certain degree of uncertainty.Factors such as temperature and field strength have a significant impact on the insulation properties of oil and pressboard materials.Therefore,it is necessary to study the evolution of insulation properties,the distribution characteristics of space electric field and the accumulation interface charges of oil-pressboard under different aging times.In this paper.three kinds of transformer oils widely used at home and abroad are impregnated with the same pressboard.and subjected to high temperature heat aging at 90℃ it is taken out after 1 day,7 days,21 days,28 days,and 35 days respectively,Then,a variety of electrical performance parameters were measured at different temperatures,the electric field measurement platform based on the Kerr effect was used to measure the space electric field in the oil under different aging states,and the oil-pressboard interface charge accumulation was calculated according to the electric displacement vector formula of the medium interface condition.In addition,this paper also proposes a comprehensive evaluation method for the insulation performance of oil-pressboard materials based on the combination of the dispersion of samples and the expert scoring method based on the combination of subjective and objective weights.The evaluation conclusion is fair and convincing,which provides a basis for the material selection and optimization of insulation design.The research results obtained in this paper mainly include:(1)The electrical/dielectric performance evolution tests of the three oil-impregnated pressboards under different thermal aging times and different temperatures were carried out.and the performance evolution rules of the key insulation properties of the three oil-impregnated pressboards under long-term operating conditions were obtained.(2)Based on the Kerr effect electric field measurement platform,the distribution of electric field and charge in the oil of oil-pressboard composite insulation was carried out under different thermal aging times,and the space-time distribution characteristics of electric field and charge under different aging states of oil-pressboard insulation were obtained.(3)A comprehensive performance evaluation method based on oil-pressboard insulation materials is proposed,which combines the dielectric and electrical properties of pressboard with the electric field and charge characteristics of oil-pressboard composite insulation to form a total of 21 assessment indicators,covering the key performance of oil-pressboard insulation in converter transformers.and the advantages and disadvantages of the insulation performance between the three oil-impregnated pressboards were obtained in an all-round and objective way combined with the subjective experience of experts,and the evaluation results are relatively reliable. |
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