Application Of SF₆ Decomposition Characteristics In Local Overheating Fault Diagnosis

With the continuous growth of national electricity consumption,in order to meet the needs of economic development,the scale of power grid construction continue to expand.SF₆ gas insulated equipment has been widely invested in the high-voltage and ultra-high-voltage fields due to its outstanding advantages such as small footprint,long service life,and high safety and reliability.However,the structure of electrical equipment is complicated,and there are latent defects in its interior,it will cause overheating failures under long-term operation.SF₆ will decompose to produce a variety of low-fluorine sulfides under the effect of failures,which can react with O and OH to form SO₂,SOF₂,SO₂F₂,H₂S and other gases,they will destroy the insulation performance of electrical equipment and shorten the service life of the equipment.In severe cases,it will cause sudden failures and threaten the operation safety of the power grid.The decomposition process of SF₆ is closely related to the development of faults,studying its decomposition characteristics can realize the prediction of the type and degree of development of gas-insulated equipment faults.This thesis firstly uses quantum chemistry theoretical calculation methods to study the decomposition process of SF₆:SO₂ is mainly produced by SOF₂ hydrolysis reaction,SO₂F decomposition reaction,SO₂F and SF₅ reaction,these reactions usually occur in the lower energy conditions;SOF₂ is obtained by the decomposition of SF₃OH and SOF₃;SO₂F₂ is mainly produced by the reaction of SO₂ and F₂,this path requires relatively high energy conditions,making the formation temperature of SO₂F₂ higher than other sulfide products;H₂S can be obtained by the reaction of S with 2 H atoms.When the insulating material is overheated,it provides sufficient H₂ and CO to form a strong reducing atmosphere,H₂ can react with SO₂ to form H₂S.This reaction requires high-temperature catalysis,making the temperature of H₂S generation very high.In addition,the formation path of typical carbon-containing products is analyzed:COS mainly comes from the reaction of CO with S or H₂S,and the reaction path related to the formation of COS needs to overcome a high potential barrier;CO and CO₂ come from the combination of C atoms and O ionized by H₂O or O₂ molecules,as well as the gradual conversion of hydrocarbon radical groups in the insulating material.Therefore,the content of CO and CO₂ under overheating of the insulating material is relatively high.CF₄ is mainly obtained by multi-step reaction of CH₃ and F.This thesis established a local overheating simulation experiment platform,carried out experiments on metallic overheating and insulating material overheating,and studied the influence of air pressure on the decomposition characteristics of SF₆under metallic overheating.The relevant conclusions obtained are as follows:in the temperature range of 200℃～360℃,the typical decomposition products produced under metallic overheating faults are SO₂,SOF₂,SO₂F₂,COS,H₂S,CO and CO₂.The temperature can accelerate the production of each product but the promotion effect is different.The air pressure can change the temperature field distribution in the gas chamber,and can increase the number of collisions of reaction activation molecules to affect the amount of product produced,but it will not change the types of SF₆decomposition products.The influence of air pressure needs to be considered when performing fault diagnosis.Two new products,CF₄ and H₂,are added when the insulating material is overheated,which can effectively distinguish the two types of faults.It is proposed to use the characteristic gas type method to preliminarily distinguish five typical faults,and construct the ratio C（SO₂+SOF₂）/C（CO+CO₂）range to further identify metallic overheating faults and insulating material overheating faults.A triangular diagnosis method based on the proportion of characteristic gas content is established,this method can predict the temperature level of metallic overheating failure and is less affected by air pressure.The mathematical relationship between the gas production rate and temperature of the 9 characteristic gases and the total amount of characteristic products under the overheating of the insulating material is given,and the function relationship between R-C（SO₂+SOF₂）/C（CO+CO₂）and temperature is obtained.These can accurately predict the hot spot temperature at the fault location.Based on this,the corresponding gas production rate attention value is set,and the gas production rate is used to observe the fault temperature and its development trend。...