The construction of the future energy structure of China under the 2050 carbon-neutral vision requires compact direct current (DC) gas-insulation equipment as important nodes and solutions to support electric power transmission and distribution of long-distance and large-capacity. This paper reviews China's 10-year progress in DC gas-insulated equipment. Important progresses in basic research and industry perspective are presented, with related scientific issues and technical bottlenecks being discussed. The progress in DC gas-insulated equipment worldwide (Europe, Japan, America) is also reported briefly.

The valve winding in the converter transformer withstands several types of voltage, including the AC, DC, and the combined AC-DC voltage. It is important to consider the effects of voltage type to the electric field distribution. In this paper, a combined AC-DC source was constructed. The electric field distribution of oil paper insulation model was measured by Kerr electro-optic effect, under combined AC-DC voltage with different composite ratios of DC/AC. The results indicate that: 1) DC electric field distribution is asymmetric, which is caused by the difference of the charge accumulation status in different area. The electric field intensity in the left area accumulated negative charge is lower than that in the right area accumulated positive charge. 2) AC electric field distribution depends on the geometric structure and is symmetric. 3) The AC electric field intensity rises with the increase of AC component linearly. However, the relationship of the DC electric field and the DC component is complicated and non-linear in most area.

Abstract:Converter transformers are the key and the most important components in high voltage direct current (HVDC) power transmission systems. Statistics show that the failure rate of HVDC converter transformers is approximately twice of that of transformers in AC power systems. This paper presents an approach integrated with a two-dimensional cloud model and an entropy-based weight model to evaluate the condition of HVDC converter transformers. The integrated approach can describe the complexity of HVDC converter transformers and achieve an effective assessment of their condition. Data from electrical testing, DGA, oil testing, and visual inspection were chosen to form the double-level assessment index system. Analysis results show that the integrated approach is capable of providing a relevant and effective assessment which in turn, provides valuable information for the maintenance of HVDC converter transformers.

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