Influence Of Corrosive Sulfur In The Oil On The Properties Of Oil-paper Insulation Under The Electro-thermal Impact

The safe and reliable power grid is depending on the normal operation of transformers. During the operation of the oil-immersed power transformers, the performance of the oil-paper insulation will deteriorate gradually under the influence of various factors. Besides the electric field, thermal oxidation and hydrolysis, the corrosive sulfur in the oil is also a factor results in the oil-paper insulation and equipment failure. The failures caused by the corrosive sulfur occur frequently, and it is a serious threat to reliability of transformer insulation. Domestic and foreign scholars have done a lot of research work on oil sulfur corrosion in oil, and have obtained certain achievements. But the influence of electric field on the sulfur corrosion process has not been reported, and influence of sulfur corrosion process on the properties of insulation oil and its mechanism has not been intensively studied.In this paper, the mineral oil with different corrosive sulfur content was prepared by adding dibenzyl disulfide(DBDS) into the oil. The paper copper windings were immersed in oil to carry out the sulfur corrosion experiments under the combined effects of electricity and heat. These experiments were focus on the influence of the AC and DC electric field on the extent of the sulfur corrosion in the oil, besides, the effect of sulfur corrosion on the properties of insulation oil was studied and the mechanism of the effect was explored. The main research achievements of this paper are as following:(1) No matter the AC or DC field could accelerate the sulfur corrosion in the oil. The copper sulfide deposition on the insulation paper and the dissolved copper in the oil were increased, meanwhile, the volume/surface resistivity of the winding insulation and the volume resistivity of the oil was reduced. When a stronger electric field was applied, there was a greater effect of electric field on the sulfur corrosion.(2) Compared with the AC electric filed, the influence of DC electric filed on the sulfur corrosion was more obvious. In addition, the effect of DC electric filed on the sulfur corrosion on two electrodes of Pig-tail winding was asymmetric, and the sulfur corrosion on the anode winding was more serious. There was more copper sulfide deposition on the anode winding, which leaded to the lower resistivity of the winding. On the contrary, there was slightly difference of the sulfur corrosion under AC electric field between two electrodes. The extent of the sulfur corrosion on the anode winding under DC electric filed was more serious than that under AC electric filed.(3) The sulfur corrosion accelerated the decline in the properties of insulation oil. Compared with the insulation oil which completed the experiments without DBDS, there was more dissolved copper content and oleic acid in the insulation oil experimented with adding DBDS. The oil after the sulfur corrosion experiment showed a higher dielectric loss and conductivity, the more DBDS added, the lower insulation properties it would be. Furthermore, the conductivity and ion concentration in the oil changed with the DBDS content, and the variations of these two parameters presented the same tendency. The increase of ion concentration in the oil was considered to be the main reason to the increase of the conductivity of insulation oil. Therefore, the sulfur corrosion affected the conductivity by increasing the ion concentration in the oil.