Oil–paper composite insulation (OPCI) is used as the main insulation material in the converter transformer. However, the dielectric properties of insulating oil and oil-impregnated paper are markedly mismatched under direct current (DC) voltage, resulting in an unbalanced distribution of voltage, which is considered to be a major cause of large insulation margin but frequent failures. Therefore, it is necessary to adjust the voltage distribution in OPCI to enhance its breakdown strength. In this work, surface-treated TiO₂ nanoparticles were employed to modify OPCI. The polarisation current tests revealed that by changing the additive amount of nanoparticles, the polarisation intensities of both oil and oil-impregnated paper could be adjusted. Based on this, a model taking relaxation polarisations into full consideration was established to analyse the voltage distribution in OPCI and predict the optimal concentration. The prepared OPCI with the concentration of 0.02 g/L has desired balanced voltage distribution due to the influence of nanoparticles on interface charge, and therefore high breakdown strengths under both DC voltage and polarity reversal voltage, which can be increased by 22.6% at most. This work puts forward a novel idea to improve insulation property of OPCI and provides a reference for broader application of nanomaterial in power equipment.

油纸复合绝缘（OPCI）被用作换流变压器的主要绝缘材料。但是，绝缘油和浸油纸的介电性能在直流（DC）电压下明显失配，导致电压分布不平衡，这被认为是绝缘裕度大但经常发生故障的主要原因。因此，有必要调整OPCI中的电压分布以提高其击穿强度。在这项工作中，表面处理的TiO ₂纳米颗粒用于修饰OPCI。极化电流测试表明，通过改变纳米颗粒的添加量，可以调节油和浸油纸的极化强度。在此基础上，建立了考虑弛豫极化的模型，以分析OPCI中的电压分布并预测最佳浓度。制备的OPCI浓度为0.02 由于纳米粒子对界面电荷的影响，g / L具有理想的平衡电压分布，因此在直流电压和极性反转电压下均具有较高的击穿强度，最多可提高22.6％。这项工作提出了提高OPCI绝缘性能的新思路，为纳米材料在电力设备中的广泛应用提供了参考。