| Transformers are the key equipment in a power grid,and the safety of which plays a key role in guarantying the reliability of power supply.Assessing the aging degree of the transformer insulation system accurately and duly can provide reliable basis for the operation,maintaining,overhauling and replacing of power transformers.Furfural content in oil is a currently widely used aging indicator of transformer insulation paper.However,the existing aging estimation method based on furfural content in oil has apparent defects.The threshold value of furfural content given by the existing method is too high,and the aging assessment result is not in accordance with the actual transformer aging condition.In order to correct the defects of the existing furfural-based aging assessment method,the causes of defects were firstly analyzed in this paper.Then pertinent studies were conducted combing the theoretical derivation and experimental analysis,including: furfural partitioning ratio in oil-paper system with different temperatures,moisture contents and aging degrees,furfural dynamic diffusion process in oil-paper system and diffusion balance time,furfural correction method after oil replacement,and quantitative relation between cellulose chain scission and furfural generation.Based on the above four research results,a new aging assessment method of transformer winding hotspot insulation based on furfural analysis was proposed,which corrected the defects of the existing method.The main innovative research results achieved in this paper include the following aspects.(1)Furfural partitioning model between oil and paper with different temperatures,moisture contents and aging degrees was constructed.The effects of temperature,moisture and aging on furfural partitioning in oil-paper system were studied based on furfural equilibrium experiment.Results showed that the rising of temperature and moisture,as well as the deepening of aging would cause furfural migrating from paper to oil.Furfural distribution ratios between oil and paper under different conditions were calculated.Then a furfural partitioning model was constructed with multiple linear regression method.With this model,the effect of furfural partitioning on furfural content in oil and insulation paper aging assessment was analyzed.(2)Furfural diffusion dynamic model with different temperatures and aging degrees was constructed.Based on the Fick's second law and furfural diffusion experiment,furfural dynamic diffusion process in oil-paper system was analyzed.The effects of temperature and aging on furfural diffusion were investigated.Results showed that furfural diffused faster at higher temperatures and deeper aging degrees,thus the diffusion balance time was shorter.Combining the temperature and aging effects,a furfural dynamic diffusion model was constructed,and furfural diffusion balance time under different conditions was given.(3)Furfural correction method after oil replacement was constructed.The effect of oil replacement on furfural content in oil and insulation aging assessment was investigated.Results showed that oil replacement would cause remarkable furfural loss and severe deviation of aging assessment,and the deviation would be accumulated when several times of oil replacement was conducted.Furfural contents in oil before and after oil replacement were compared,based on which a furfural correction method after oil replacement was conducted and verified.The furfural correction method could be used to correct the furfural loss caused by oil replacement.(4)Aging assessment method of transformer winding hotspot insulation based on furfural analysis was constructed.The mechanisms of cellulose chain scission and furfural generation of insulation paper were investigated.Quantitative relation between furfural production and the degree of polymerization of insulation paper was obtained.On this basis,combining with furfural partitioning model,furfural dynamic diffusion model,and furfural correction method after oil replacement,a new aging assessment method of transformer winding hotspot insulation based on furfural analysis was constructed and verified.This method could directly assess the aging condition of transformer winding hotspot insulation with furfural analysis,and improve the accuracy of transformer aging assessment. |
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