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The main insulation structure of the valve‐side winding of converter transformer bears the effect of AC and DC superimposed voltage during operation. The electric field intensity in the pressboard is much higher than that in the pressboard in the AC transformer. This condition increases the probability of failure of insulation structure. Therefore, the design and optimization of the main insulation structure of the valve‐side winding are keys to ensure the stable operation of converter transformer. This study establishes a model of the main insulation structure of the valve‐side winding of the ±1100 kV converter transformer. The influence of the insulating oil–paper configuration on the electric field distribution of the main insulation structure of the valve‐side winding is analysed, and then, an optimization idea is proposed. The oil–paper configuration includes two aspects: oil–paper insulation structural design and combination selection, that is, the influence of typical oil–paper structural design parameters and the aging state of different oil–paper combinations on the electric field distribution. Finally, the optimization idea of the oil–paper configuration for valve‐side winding of converter transformer is obtained, and the established model is optimized. [ABSTRACT FROM AUTHOR]

Compared to bushings with lower voltage levels, the ±1100 kV converter valve‐side bushing has thicker radial insulation and longer axial insulation resulting in a more uneven temperature distribution and more severe electric field distortion, especially under high‐ambient‐temperature and high‐current conditions. Improving the distribution of the electro‐thermal coupled field of the bushing is critical for the safe operation of converter transformers. This study discusses the distribution of electro‐thermal coupled field of a typical bushing and proposes a new method to evaluate the conductivity of multiple capacitor layers under inhomogeneous axial temperatures. The effects of four bushing designs are investigated as per the adjustment of the capacitor screen structure, the replacement of the current‐carrying structure, and the modification of resin‐impregnated paper material. The results show that merely adjusting the capacitor screens is not feasible due to size limitations. By decreasing heat accumulation, adopting a single‐tube structure and increasing the thermal conductivity of resin‐impregnated paper material can reduce electric field distortion to a certain extent. Unlike the other three methods, hierarchically controlling the material resistance effectively balances the internal and external electric fields. Simulation results demonstrate the feasibility of these methods to improve the distribution of the electro‐thermal coupled field in the bushing. [ABSTRACT FROM AUTHOR]

The frequency domain dielectric response method has been one of the most effective approaches for condition assessment of an electrical insulation system. The low‐frequency part is more effective for diagnosing the moisture content and aging status of oil‐paper insulation. To facilitate the study of the low‐frequency dielectric spectrum of oil‐paper insulation, a model applicable to the low‐frequency dielectric response of insulation oil is proposed in this study, which is based on the ionic motion of insulation oil. The creation of charge carriers under field test conditions mainly derives from the dissociation of ionic pairs. During testing, ionic motion of insulation oil is jointly affected by thermal diffusion and the electric field; moreover, dissociation and recombination of negative and positive ions also occur. Therefore, the above factors were all considered in the modelling process. The applicability of the model is verified against measured results. For further interpreting the proposed model, the characteristics of the ionic concentration distribution, the relation between ionic distribution and dielectric properties, and the application of the proposed model are analysed. [ABSTRACT FROM AUTHOR]

In a multilayer insulator made of two or more materials with different dielectric constants and conductivities, internal space change accumulates at their interfaces. If the local field is increased by the accumulated charge, the electric durability of the insulator should be decreased. In this paper, we describe the space charge accumulation measured when oil-impregnated PPLP (polypropylene laminated paper), which consists of three layers, kraft paper, polypropylene (PP), and kraft paper, was subjected to a strong dc electric field. The experimental results suggest that internal space charge accumulates at both surfaces of a PP layer and that the electric field in the kraft paper layers disappears. © 1998 Scripta Technica, Electr Eng Jpn, 124(1): 1–6, 1998 [ABSTRACT FROM AUTHOR]