有机酸对油纸绝缘热老化及频域介电谱特性的影响

Influence of Organic Acids on Thermal Aging and Frequency Domain Dielectric Spectroscopy Characteristics of Oil-paper Insulation

为研究有机酸对油纸绝缘热老化及频域介电谱(FDS)的影响,在绝缘油中分别添加甲酸和硬脂酸至油中酸值均达到0.3×10-3(即中和1 g该绝缘油需0.3 mg KOH),再将初始水分质量分数σmc分别为0.2%和3.0%的绝缘纸置于不同的绝缘油试品中,测试不同温度下有机酸在油和纸之间的分布特性(采用酸的扩散特性和溶解特性来表征)。当有机酸达到稳态后,分别在20℃和70℃下对不同试品进行FDS测试,分析试品复介电常数实部ε'和虚部ε''的变化规律;再分别在70℃和100℃下对试品进行加速热老化试验,分析老化过程中的绝缘纸聚合度的变化趋势和老化机理。结果表明,甲酸易溶于绝缘纸,硬脂酸易溶于绝缘油。温度和σmc对酸在油纸绝缘中溶解特性的影响很小,但温度的升高会加速酸的扩散。甲酸对绝缘纸的老化具有显著影响,而硬脂酸基本无影响。低温时,甲酸对ε'和ε''均具有显著影响,而硬脂酸只对干燥绝缘纸的ε''具有显著影响;高温时,甲酸和硬脂酸分别对ε''和ε'的影响较大。因此利用FDS法评估油纸绝缘中的σmc时,需充分考虑有机酸的影响。

We investigated the influence of organic acids on thermal aging and frequency domain spectroscopy（FDS） of oil-paper insulation through experiments. Firstly, we added formic acid and stearic acid into two sets of insulation oil samples until their acidity both reached 0.3×10-3. Then, we placed insulating papers with initial mass fractions of water（σmc） of 0.2% and 0.3% into the insulation oil samples, respectively, and measured the distribution characteristics（featured by dissolution property and dissolution characteristic of acid） of organic acids between oil and paper at different temperatures. After the organic acids reached steady state, we made FDS tests of the samples at 20 ℃ and 70 ℃ and analyzed the variation of real part（ ε＇） and imaginary part（ ε＇＇） of the samples＇ complex permittivity. Then we put forward accelerated thermal aging test of the samples at 70 ℃ and 100 ℃, and analyzed the variation and aging mechanism of the degree of polymerization（DP） of the insulating papers during their aging processes. The results show that the formic acid and stearic acid dissolve easily in paper and oil, respectively. This dissolution in oil-paper insulation is hardly affected by temperature and σmc, except being accelerated by temperature rise. Formic acid significantly affects the aging of insulating paper, but stearic acid barely does. Moreover, formic acid has significant effects on ε＇ and ε＇＇ at low temperatures, while stearic acid only has visible influence on ε＇＇ of dry insulating paper. However, formic acid and stearic acid affects ε＇＇ and ε＇ greatly at high temperature, respectively. Therefore, when FDS is used to estimate σmc in oil-paper insulation, the influence of acid should be fully taken into consideration.