Analysis of SF6 Solid Decomposition Products after Breaking Tests

Sulfur hexafluoride (SF6), a kind of gas with strong electronegativity, outstanding insulation property and arc quenching performance under a certain pressure, is widely used in medium and high voltage electrical equipment. Almost all circuit breakers of 126kV and above voltage level use SF6 gas as dielectric and arc quenching medium. SF6 gas will decompose to form sulfur fluorides and fluoride ions under the high temperature produced by arcing, and these intermediates will react with metallic elements from gas chamber or the materials of contacts to form solid state products adhering to gas chamber in the high temperature produced by arcing, which have effect on the insulation performance and breaking capability. Researches showed that the characteristics of SF6 gas decomposition products had relation to different type of latent failures, thus analyzing SF6 gas decomposition products is an effective method to status assessment and fault diagnosis of SF6 gas insulated equipment. However, most researches focused on the gaseous decomposition products but few on solid state decomposition products which result from the difficulty of detection and less information for discharging recognition. To research the component of the solid state SF6 decomposition products and to investigate the relationship between the form of solid state products and breaking tests, a 40.5kV SF6 circuit breaker was chosen as the test object, and a series of current breaking tests ranging from 6.5kA to 14.6kA were carried out in the high voltage synthesis circuit. After breaking tests, the SF6 circuit breaker was disassembled, solid state products was found in the arc quenching chamber and was analyzed by scanning electron microscope (SEM) and energy disperse spectroscopy (EDS). The result showed that the main metallic elements detected in the attachments and powders which were collected in different parts of the test object were Cu, Ag and W, and the non-metallic element detected is O and S; these elements existed in the form of metal-sulfide, oxide and fluoride, and the copper oxide accounted for 85% in mass fraction. In addition, trace amounts of Fe and Cr were found on the surface of filter washer. The mass fraction of $S$ element increased in the serious eroded area. From the experiment results, reaction mechanism of the formation process of solid products was analyzed according to the material consisting of gas chamber and the decomposition mechanism of SF6 gas. SF6 gas decomposed to sulfur fluorides of low valence and F atoms or ions, or further decomposed to $S$ atoms or ions and F atoms or ions under high temperature. The metal of Cu and Cr in the arc quenching chamber or contacts reacted with these SF6 decomposition intermediates to form metal sulfide and fluoride respectively. Further, trace amounts of water and oxygen which existed in the arc quenching chamber dissociated, O reacted with metal element in the arc quenching chamber to form the corresponding metal oxide. Through analyzing the components of solid state SF6 decomposition products, it can provide guidance for the maintenance, status assessment and fault diagnosis of SF6 circuit breaker in operation.