The oil-pressboard insulation on the valve side of the converter transformer withstands pulsating square wave voltage. The voltage contains lots of DC components and high-order harmonics, which is likely to cause insulation failures. The partial discharge and space charge experiments of oil-impregnated pressboard under DC superimposed harmonic voltage are conducted to study the effects of harmonic frequency and DC components on the discharge and charge accumulation and to explore the correlation mechanism of charge dynamic behaviour and discharge. Firstly, it is found that high-order harmonic voltage promotes the partial discharge generation, but DC component suppresses the discharge. Moreover, the charge inside the pressboard is mainly injected in the same polarity and accumulates over time. The space charge density near the electrode increases with the increase of harmonic frequency, and the large amount of charge injection leads to electric field distortion. Meanwhile, the electric field direction changes faster, and the frequent charge injection and extraction behaviours make the charge recombination intensify, which promotes the discharge. Through the above analysis, the influence law of harmonic voltage on the failure of oil-pressboard insulation is obtained, providing a theoretical basis for the structural optimisation and condition assessment of the converter transformer insulation.

中文翻译：

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直流叠加谐波电压下油板绝缘局部放电及电荷积累特性研究

换流变压器阀侧的油压板绝缘可承受脉动方波电压。电压中含有大量直流分量和高次谐波，容易引起绝缘故障。开展直流叠加谐波电压下油浸纸板的局部放电和空间电荷实验，研究谐波频率和直流分量对放电和电荷积累的影响，探讨电荷动态行为与放电的相关机制。首先，发现高次谐波电压促进局部放电的产生，而直流分量则抑制放电。而且，纸板内部的电荷主要以相同极性注入并随着时间的推移而积累。电极附近的空间电荷密度随着谐波频率的增加而增加，大量的电荷注入导致电场畸变。同时，电场方向变化较快，频繁的电荷注入和引出行为使得电荷复合加剧，从而促进放电。通过上述分析，得出谐波电压对油压板绝缘失效的影响规律，为换流变压器绝缘结构优化和状态评估提供理论依据。