Abstract
A converter transformer is important primary equipment in a DC transmission project. The voltage on the valve side winding is complex when the equipment is running, including DC, AC, and AC–DC combined voltage. The insulation structure of the valve side winding of a converter transformer is an oil-paper insulation structure, which may have a variety of defects in the manufacturing stage and daily use, resulting in partial discharge. Therefore, it is the key to studying the partial discharge characteristics and mechanism of oil-paper insulation under AC–DC combined voltage. In this paper, we build a two-dimensional air gap model of oil-paper-insulated pressboard considering the actual particles and actual reaction based on the fluid model. The characteristics and evolution mechanism of partial discharge (PD) in pressboard under different AC/DC combined voltages are studied by numerical simulation. The results show that when the DC component increases, the polarity effect of partial discharge is more obvious, while the potential and discharge intensity in the air gap decrease. Further analysis revealed that the DC component in the combined voltage accumulated a large number of surface charges on the surface of the air gap, and the space charge distribution was more uniform and dispersed, which generated an electric field with opposite polarity to the DC component in the air gap and, then, inhibited the development of local discharge in the paperboard. The results of the simulation are consistent with the previous experimental phenomena, and the mechanism analysis of the simulation results also verifies the previous analysis on the mechanism of experimental phenomena. This will lay a theoretical foundation for the further study of partial discharge phenomenon of oil-paper insulation structures in practical operation in the future.
Highlights
In recent years, with the development of DC transmission projects, the voltage level of UHV DC in China has increased year by year, which has played an important role in solving the problems of large-capacity and long-distance transmission in China
It is of great practical significance to study the characteristics and evolution mechanism of partial discharge under an AC/DC composite voltage to maintain the safe operation of the HVDC system [4,5,6,7,8,9,10,11,12]
Based on the experimental phenomena, scholars have analyzed the physical mechanism of partial discharge under combined voltage, but the analysis is still at the phenomenon level and cannot describe the microphysical process of partial discharge in the air gap
Summary
With the development of DC transmission projects, the voltage level of UHV DC in China has increased year by year, which has played an important role in solving the problems of large-capacity and long-distance transmission in China. It is of great practical significance to study the characteristics and evolution mechanism of partial discharge under an AC/DC composite voltage to maintain the safe operation of the HVDC system [4,5,6,7,8,9,10,11,12]. Based on the experimental phenomena, scholars have analyzed the physical mechanism of partial discharge under combined voltage, but the analysis is still at the phenomenon level and cannot describe the microphysical process of partial discharge in the air gap. Sha simulated the oil-paper insulation structure, but the overall model is relatively simple and failed to reveal the microphysical evolution process of partial discharge in the air gap [13,14,15,16,17,18,19,20]. The total amplitude of the applied AC–DC combined voltage of the model has always been 1.49 kV, but the AC–DC component proportions of the composite voltage are 1:1, 1:3, 1:5, and 1:7, respectively
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