With the increasing demand for electricity, the transmission lines inevitably need to traverse steep mountainous terrains with significant elevation differences. Traditional high-low leg transmission towers are limited by the angle constraints of their main and diagonal elements. To overcome these limitations, new tower designs with transition sections have emerged in recent years, solving the problem of positioning the towers in terrains with large elevation differences. These new tower types have been applied to the transmission lines of 220 kV and 500 kV voltage levels. However, research on the ±800 kV ultra-high-voltage (UHV) transmission lines under heavy ice load remains a research gap. Southwestern China is rich in hydropower resources, but the natural conditions of the region, steep terrain, and severe icing, present significant challenges, with many areas classified as moderate to heavy ice regions. With the implementation of the west-to-east power transmission strategy of China, this new type of tower design with transition sections has promising application prospects for transmission corridors in this region. However, the failures of transmission lines caused by tower collapses and line breakages under ice load frequently occur in mountainous areas. Therefore, the structural and electrical safety of the new tower type, especially when applied to UHV lines under icing and ice-shedding conditions, urgently needs investigation.

A steep-span tension section of the ±800 kV UHV transmission line from Sichuan to Jiangsu was studied. The research employed a catenary model to simulate the shape of ground lines and conductor lines, and a finite element model of the transmission tower-line coupling system with transition sections was developed using ABAQUS software. The model accounts for line angles, as well as elevation differences; moreover, the structural dimensions and material property parameters were selected based on actual engineering data. The structural and electrical safety characteristics of the transmission tower-line coupling system under different icing and ice-shedding conditions were analyzed. Specifically, the stress distribution patterns of main and diagonal elements along the height of the tower during icing and ice-shedding were investigated, and the tension and safety factors of ground and conductor lines were evaluated; the displacement response and energy transfer mechanisms of ice-shedding were explored.

Results indicate that under the heavy icing condition of 30 mm, the stress ratios of the main and diagonal elements of the transmission tower are less than 0.4 and 0.3, respectively. The equivalent safety factors of the conductor and ground lines exceed 2.25, and the unbalanced tension complies with the code requirements, indicating a significant safety margin for the tower-line system. Compared to a uniform ice load condition, a non-uniform ice load condition significantly increases the stress on elements of the transmission tower, posing greater risks to the stability of the main and diagonal elements in transition section towers. However, the stresses remain within safe limits, and the main and diagonal elements remain stable. The weak positions of the transmission tower-line system with transition section include the main elements in the tower legs, the transition section and the cross-arms, and the diagonal elements of the ground line supports, which require special attention to the stability under heavy ice loads. The stress ratios along the height of the tower under non-uniform ice load conditions exhibit similar patterns to those under uniform ice load conditions, albeit with higher values. During ice-shedding under a 30 mm heavy ice load condition, the main and diagonal elements of the ±800 kV UHV transmission tower-line system with transition sections remain stable. The tensions of the ground and conductor lines all meet the minimum allowable safety factor, with only a few points slightly below the design safety factor value, maintaining adequate safety margins. The maximum vertical displacements of the ground and conductor lines of the ±800 kV UHV transmission tower-line system with transition sections during ice-shedding are 7.9 m and 5.2 m, respectively, which satisfy the requirements for air insulation clearances. However, for higher voltage levels or multi-phase transmission lines, electrical safety issues such as flashover induced by ice-shedding should be carefully considered. The ice-shedding process of ground and conductor lines exhibits typical damped cosine wave characteristics, with the maximum vertical displacement of a single line occurring in the first cycle, followed by gradual attenuation. Furthermore, when multiple lines unload ice simultaneously, the maximum jump height may not occur in the first cycle and may exceed the maximum displacement value of a single line, which requires significant attention. This indicates that energy transfer occurs between different lines during ice-shedding in the transmission tower-line coupling system. The jump height of a single line is influenced not only by its own response but also by energy transfer with other ground and conductor lines in the same span.

In conclusion, the coupling effects between transmission towers and transmission lines, as well as the complex dynamic response mechanisms induced by ice-shedding of multiple ground and conductor lines, should be fully considered in the analysis of transmission lines under heavy ice loads. The weak position of the transmission tower-line system with transition section is the main elements in the tower legs, transition section and cross-arms, and the diagonal elements of the ground line supports. Although the structure maintains adequate safety margins, it is essential to prevent flashover hazards and element instability to ensure the long-term reliability and safety of transmission lines in heavy ice regions.