Breakdown Characteristics of Oil-paper Interface in Converter Transformer

Abstract

The converter transformer plays a very important role in the HDVC transmission system. Because the impact of ac, dc, ac and dc, impulse voltage and polar reverse voltage during operation, the main insulation on the valve side has a high fault ratio, which results in major life and property safety accident. In the insulation faults of the converter transformer, the partial discharge (oil breakdown) along oil-paper interface is one of the key reasons for those faults. At present, many scholars have done research on related topics, but most structures of their experimental equipments are deficient, and the experimental data needs to be improved. Therefore, it is particularly necessary to study deeply the character of the surface breakdown on oilpaper insulation inside the transformer. The main research in this paper is as follows: (1) On the basis of other studies, this research had made some appropriate improvements on the structure of experimental equipment. In order to generate a uniform dc electric field, a new experimental device was designed. In this design, plane-plane electrodes were adopted to ensure the sample is only applied to the tangential electric field, which avoids the influence of the normal electric field. (2) In order to study the influence of oil-paper interface on the oil-gap breakdown voltage when the partial discharge occur, the breakdown voltage of oil-gap of pure oil and the oil placed new cardboard were respectively tested. And the data of different thickness in two cases were measured. The result showed that the new cardboard had little effect on the breakdown voltage of oil gap at the oil-paper interface. (3) Considering that the breakdown voltage at the oil-paper interface will be affected by the aging degree of the cardboard, since the breakdown voltage of the oil gap after placing the cardboard with different aging degrees was tested respectively and compared with the results of pure oil. The experimental result showed that aging degree had little effect on the breakdown voltage. (4) The relationship between oil temperature and breakdown voltage was also investigated. (5) A method of increasing DC voltage at a constant speed was adopted in this experiment. This paper also discussed the influence of different rates of voltage rise on the experimental result. (6) Finally, the test results showed that the structure of the electrode should is perfected.