| Insulation fault is the main reason of accidents occurred in transformer which is the core of energy conversion and transmission in power grid.The insulation of oil-immersed transformer is mainly composed of mineral oil and insulation paper or insulation cardboard at present.In the long-term running,oil-paper suffers from thermal and electrical stresses,which deteriorate the insulating performance.A test platform is designed for the oil-paper insulation aging under thermal and electrical stresses.Mineral oil and ordinary insulation paper are chosen to be the test samples and placed in the oil tank for the accelerated aging experiment under 140℃.According to related standards,some characteristic parameters,such as the degree of polymerization,breakdown voltage of paper,acid value and furfural content in oil are measured.There are three groups of aging tests carried out under different stresses.One is accelerated thermal aging test,the second is combined thermo-electrical aging test with uniform electric field to explore the aging characteristics of oil-paper insulation under thermal and electrical stresses,the third is accelerated aging test with discontinuous corona discharge in oil.The experiment results show that oil aging is promoted by the electrical field.The increase rate of acid value in oil under combined thermo-electrical stress is much faster than that under single thermal stress.In addition,electric field accelerates the aging of insulation paper,leading to the rapid increase of furfural content in oil and the rapid decrease of DP.But discontinuous corona discharge has lower influence on it.To thermal aging and combined thermo-electrical aging under uniform electric field,the breakdown voltage of paper has no obvious relationship with its aging condition.However,corona discharge has a significant impact on the electric strength of the insulation paper which is broke down more easily with the accumulation of corona discharge in oil. |
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