Recent Progress, Challenges and Outlooks of Insulation System in HVDC: A Further Discussion

By 2025, China successfully commissioned more than 40 Ultra-High Voltage (UHV) power transmission and transformation projects, establishing the world’s largest and most technologically advanced UHV grid. The operational footprint of this network in terms of energy resource dispatching and allocation spans the entire nation. Driven by the mandate for socio-economic sustainable development, the proliferation of renewable energy integration has necessitated high-voltage infrastructure, which effectively mitigates the curtailment of intermittent power generation and resolves the geospatial mismatch between generation sources and load centers.

Especially in Southern China, numerous UHVDC (Ultra-High Voltage Direct Current) transmission projects serve as critical corridors delivering bulk clean hydropower to megacity load clusters. These large-capacity projects traverse distances exceeding 1000 km, operating in harsh geographical environments characterized by complex atmospheric conditions. Ensuring the secure and stable operation of these capital-intensive assets is inextricably linked to maintaining the integrity and dielectric performance of the insulation systems within an optimal state.

Having this in mind, we are pleased to launch the second edition of the Special Issue titled “Recent Progress, Challenges and Outlooks of Insulation System in HVDC.” This initiative is dedicated to disseminating state-of-the-art research findings regarding insulation coordination, failure mechanisms, and condition monitoring in HVDC transmission engineering, with the aim of fostering extensive and in-depth academic exchange among researchers.

The second edition of this Special Issue comprises a total of 10 contributions, including eight research articles and two review papers. The scope of these studies encompasses primary insulation equipment critical to HVDC engineering, such as circuit breakers, transmission line insulators, conductors, high-voltage bushings, power cables, and Gas Insulated Switchgear (GIS). The contributing authors present cutting-edge research findings addressing diverse insulation structures and characteristics, as well as the application of novel methodologies for the assessment and analysis of insulation performance.

Specifically, in Contribution 1, Hongping Shao et al. propose a novel detection technique for combined faults in circuit breaker operating mechanisms based on Deep Residual Networks (ResNet). Addressing the complex mechanical structure and potential failure sources of circuit breakers, this study innovatively incorporates spring-damping elements into the simulation analysis, thereby enabling the model to reflect real-world fault scenarios with higher fidelity. Furthermore, to tackle the challenge of small-sample data associated with mechanism faults, the research utilizes the ResNet50 architecture for spectrum data processing. By integrating the ReLU (Rectified Linear Unit) activation function, the proposed method achieves a fault identification accuracy exceeding 90%.

Moreover, the authors in paper Contribution 2 propose a methodology for diagnosing zero-value insulators on transmission lines. As zero-value insulators pose a long-term threat to the reliability of line insulation, this study leverages high-fidelity simulation results to develop a model based on Multi-Layer Perceptron (MLP) neural networks, capable of predicting the electric field intensity of defective zero-value insulators. Furthermore, a database characterizing the spatial electric field distribution of insulator strings containing zero-value defects was established, which has significant practical implications for the inspection and maintenance of external line insulation.

When it comes to the corona phenomenon on HVDC line conductors, Jules Simplice Djeumen et al. investigate the characteristics of space charge generated by DC corona through both experimental and simulation approaches Contribution 3. This study effectively integrates experimental data obtained from a corona cage with simulation results derived from Finite Element Method (FEM) models, enabling a comprehensive assessment of DC corona characteristics under varying ambient temperatures. Utilizing an indoor corona cage, the study tested different temperature gradients on standard models representing potential HVDC transmission lines in Southern Africa, acquiring critical data such as the Corona Inception Voltage (CIV).

Article Contribution 4 investigates the insulation reliability of epoxy resin-impregnated bushings for power transformers. Solid-insulation transformers have garnered significant attention in recent years. As critical insulation components within power transformers, epoxy resin-impregnated paper bushings are subjected to complex coupled multi-physical fields over the long term. Given the high manufacturing costs of these components, simulation analysis serves as an effective means to verify their insulation reliability. In this study, Daijun Liu et al. employ simulation to investigate the electric field distribution, thermal field distribution, and seismic performance verification of epoxy resin-impregnated paper bushings. In the study, the authors determine the maximum radial and axial electric field intensities for bushings of three different voltage levels, conduct thermal stability analysis under short-circuit conditions, and confirm that the bushing prototypes can effectively withstand seismic loads up to seismic intensity VIII, thereby satisfying product design requirements.

As the scale of the power grid expands, its structural complexity increases, and AC–DC hybrid grids inevitably face challenges regarding the interaction between HVDC and HVAC transmission lines. To address this, Jinyuan Xing et al. analyze the ground electric field of hybrid transmission lines Contribution 5. By adopting the preconditioned Krylov subspace method to solve discretized equations, the authors effectively calculate the spatial electric field when DC and AC components are coupled. This research provides valuable practical recommendations for determining critical design parameters—such as the optimal transmission tower configuration, minimum heights for towers and overhead conductors, and transmission corridor width—under various electromagnetic constraints.

Power cables constitute a vital mode of power transmission; the advancement of HVDC cables is constrained by the properties of insulation materials, with dielectric interfaces often representing the weakest link in the insulation system. Accordingly, Article Contribution 6 focuses on the performance of insulation interfaces in HVDC cable accessories. The authors propose a novel interface coating agent based on Polycyclic Aromatic Hydrocarbon (PAH)-modified silicone oil to enhance the insulation performance of HVDC cable accessories. This study investigates the DC breakdown characteristics of the insulation dielectric interface under conditions of corona and thermal cycling aging. By integrating results from breakdown tests, infrared spectroscopy, and microstructural analysis, the insulation aging mechanisms are elucidated. The research reveals that silicone oil modified with PAHs can significantly increase the breakdown voltage. Furthermore, Quantum Chemical Calculations (QCCs) identified 2,4-dihydroxybenzophenone (C₁₃H₁₀O₃) as the optimal additive for the interface coating.

Furthermore, Xiaolai Li et al. investigated the impact of a unique form of insulator surface contamination—“green algae”—on the surface characteristics of porcelain insulators Contribution 7. In the warm and humid mountainous regions of Southwest China, algae adherence to DC insulators significantly affects surface wettability, thereby increasing the risk of flashover under extreme weather conditions. This study proposes a method to measure the water absorption of the contamination layer based on surface conductivity, which enables the quantitative evaluation of the algae’s influence on surface wettability characteristics. The results indicate that naturally deposited pollution has a negligible effect on the saturated water absorption of the contamination layer, whereas the presence of algae significantly alters the wetting behavior, leading to an increase in saturated water absorption. This finding provides profound insights into the role of biological contamination in the pollution flashover process of insulators in high-humidity regions.

Paper Contribution 8 investigates the mechanisms of electric field enhancement and interface synergistic effects in the insulation failure of SF₆/epoxy resin insulation systems. Through experiments conducted in SF₆ gas gaps with a length of 36 mm and pressures ranging from 0.1 to 0.4 MPa, the study measured the AC breakdown voltages of different post insulator samples under Dielectric-Initiated Breakdown (DIBD) and Electrode-Initiated Breakdown (EIBD), thereby analyzing the critical role of the solid dielectric surface during the initial stages of gas–solid interface discharge. Experimental results indicate that, under comparable maximum electric field (E_max) conditions, the AC breakdown voltage required for DIBD is significantly lower than that for EIBD. This phenomenon can be attributed to three primary factors: the modulation of electric field distribution by the dielectric material and shielding electrodes; the fact that microscopic irregularities on the dielectric surface induce stronger local electric field enhancement compared to similar features on metal electrodes; and the desorption process near high-field regions adjacent to the dielectric surface, which significantly enhances electron multiplication during the gas discharge process. This study enriches the understanding of interface insulation performance in gas-insulated systems and provides valuable references for engineering insulation design.

The final papers, Contributions 9 and 10, are review papers. In paper Contribution 9, Haohua Hu et al. systematically review and summarize the breakdown characteristics and discharge mechanisms of overhead transmission line gaps under vegetation fire conditions. HVDC transmission lines traversing forest areas are commonplace, and wildfires pose a significant threat to the operational stability of these lines. This study reviews extensive research on gap breakdown tests and discharge mechanisms in simulated wildfire conditions, while also analyzing and summarizing physical parameter measurement methods commonly used in current experiments. Furthermore, the paper clarifies the breakdown characteristics and discharge mechanisms derived from existing experiments and numerical simulations under various influencing factors, highlighting the applicability and limitations of these findings. In the end, the authors provide an outlook on future work in this research direction.

As for paper Contribution 10, the authors review the application of digital twin technology in the full lifecycle management of high-voltage bushings. The study defines a five-dimensional digital twin framework encompassing the entire lifecycle of high-voltage power equipment (design, manufacturing, operation and maintenance, and decommissioning). It deeply explores the application paradigms of digital twins in typical scenarios and categorizes core enabling technologies, including multi-physics coupled modeling, multi-source heterogeneous data fusion, and data-driven model updating and condition assessment. In the last part, the paper identifies the current challenges facing digital twins for high-voltage power equipment in terms of data, models, standards, and costs, and provides an outlook on future research directions.

Finally, I would like to express my sincere gratitude to all the authors for their contributions and cooperation, as well as extend my special appreciation to the editorial staff and reviewers for their dedicated efforts. Insulation performance is a critical attribute of high-voltage power equipment. Currently, with the progressive integration of technologies such as big data, artificial intelligence (AI), the Internet of Things (IoT), and advanced sensing into high-voltage power equipment, the design, installation, operation, maintenance, and management of power infrastructure are continuously evolving towards digitalization and intelligence. Consequently, we will place greater emphasis on research achievements in this field in the future, aiming to provide enhanced opportunities and platforms to foster more extensive and effective academic exchange.