1. The source of corrosive sulfur in transformer insulating oil
The insulating oil used in the transformer power system is mainly refined from natural petroleum. The chemical composition of petroleum is relatively complex. It is neither composed of a single element nor a simple compound, but a mixture of multiple compounds composed of many elements.
Transformer oil manufacturers retain a small amount of sulfur in the oil, using its function as a natural oxidation inhibitor. Under normal circumstances, sulfur compounds can slow down the aging rate of the oil and prevent the formation of acidic substances and precipitates. The finished transformer oil The total sulfur content is generally below 0.5%. The sulfur and sulfide in the oil are divided into two categories: "active sulfur" and "inactive sulfur". Active sulfur mainly includes elemental sulfur, hydrogen sulfide, and mercaptan, which can directly react with metals. , has high corrosiveness, and the rest are called inactive sulfur, mainly including sulfide compounds, alkyl sulfides, cyclic sulfides and sulfonic acids. Higher levels of mercaptans are found in low-refined imported transformer oils of the type with no added antioxidants or with added trace amounts of antioxidants.
2. Harm of corrosive sulfur in transformer oil
1. The formation mechanism of cuprous sulfide in transformer oil The corrosion rate of mercaptan to copper increases with the increase of temperature. When the temperature increases from 60 °C to 120 °C, the corrosion rate of mercaptan to copper can increase about 16 times .
The formation of cuprous sulfide generally goes through three stages:
(1) The surface of the copper wire in the transformer is oxidized to form a cuprous oxide film.
(2) The cuprous oxide film reacts with thiols to generate copper thiolates that are soluble in oil.
(3) Copper mercaptide can be brought to various parts of the transformer by the transformer oil, but most of it is adsorbed and left on the insulating paper by the insulating paper. When the temperature is high, it will decompose to form cuprous sulfide Cu2S and oil-soluble organic residues . The reaction formula of the reaction process is as follows: 2Cu+O2→Cu2O Cu2O+2RSH→2CuSR+H2O 2CuSR→Cu2S+RSH where RSH represents a thiol, and R represents an alkyl group. At present, transformers above 220kv are all sealed with capsules or metal expanders, and have been vacuum degassed before being put into operation, but a small amount of oxygen will remain in the transformer oil. Standard and the dissolved ratio of oxygen and nitrogen in the transformer oil (about 3:7), the oxygen content in the oil is allowed to be 3000uL/L, so theoretically, the above reaction will occur in any oil-immersed transformer.
2. The influence of cuprous sulfide on the insulation performance of transformers
Cuprous sulfide is a micro-conductive substance, and its conductivity is much higher than that of insulating paper and transformer oil. The production of cuprous sulfide in the transformer is initially deposited on the surface of the copper conductor of the coil in the winding. At this time, the copper sulfide deposit is less harmful and is not enough to cause partial discharge and short-circuit accidents between turns and layers. However, when the transformer is running, with the flow of the transformer oil, the coil will be continuously scoured, and more cuprous sulfide deposits will be suspended in the transformer oil and adsorbed to the insulating cardboard. When the generated cuprous sulfide reaches At a certain amount, the cuprous sulfide adsorbed on the insulating cardboard will change the electric field distribution inside the transformer and reduce the voltage withstand strength of the coil in the winding. Under certain operating conditions, partial discharge occurs inside the winding, the insulating material is broken down, and Serious arc discharge, resulting in transformer burnout accident.
