In this paper, the PD properties of an ester oil/PB and a mineral oil/PB system undergoing a moisture transient are investigated using a needle-plane electrode under natural cooling. The temperature, moisture concentration and water activity of the oil are continuously monitored. Moreover, an improved moisture diffusion model is developed by using COMSOL Multiphysics to verify the experimental results. The measurement results show that there is a sharp reduction of partial discharge inception voltage (PDIV) or a sudden increase of PD intensity in the process of natural cooling for both ester oil and mineral oil system. However, the PDs extinguish gradually after the occurrence of the minimum of PDIV. For the cooling mode selected in this study, the minimum of PDIV for the ester oil system appears in the third hour after cooling begins, while it is about in the second hour for the mineral oil system. In addition, the simulation results are consistent with the experimental data.

中文翻译：

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水分迁移对冷却状态下电力变压器油纸板表面放电的影响

在本文中，使用针平面电极在自然冷却下研究了酯油/PB 和矿物油/PB 系统在水分瞬变过程中的局部放电特性。持续监测油的温度、水分浓度和水分活度。此外，还使用 ​​COMSOL Multiphysics 开发了改进的水分扩散模型来验证实验结果。测量结果表明，酯油和矿物油系统在自然冷却过程中局部放电起始电压（PDIV）急剧下降或局部放电强度突然增加。然而，PDIV 的最小值出现后，PDs 会逐渐熄灭。对于本研究中选择的冷却模式，酯油系统的 PDIV 最小值出现在冷却开始后的第三小时，而矿物油系统大约在第二个小时。此外，模拟结果与实验数据一致。