Recent Progress, Challenges and Outlooks of Insulation System in HVDC

In the modern sense, DC transmission involves converting three-phase AC into stable DC at a converter station, then transmitting it over DC transmission lines to another converter station, where it is inverted back into three-phase AC. The transmission distance for DC and AC at the same cost is about 800 km to 1000 km, but DC is more economical for longer distances. DC transmission has a large transmission capacity, reaching 3 GW to 12 GW, and a long transmission distance of up to 3000 km.

Today, more than 230 HVDC projects have been commissioned worldwide, including 60 back-to-back projects, 24 ultra-high-voltage (UHV) projects (±800 kV to ±1100 kV), and 45 extra-high-voltage (EHV) projects (±400 kV to ±660 kV). The total capacity of these HVDC projects exceeds 315 GW, of which 215 GW is in operation in China. In particular, the world’s only ±1100 kV HVDC transmission project with the highest voltage level, largest transmission capacity, and longest transmission distance was commissioned in September 2019 and is capable of delivering 12,000 MW of power over 3300 km.

As HVDC transmission projects play an increasingly critical role in modern power grids, the safety of these projects has become an important part of the associated research. The characteristics, aging, and structural design of DC insulation systems are different from those of AC systems. As the voltage level increases, the insulation system is exposed to a more complex operating environment, not only with regard to ensuring reliable operation under the influence of DC ionic and synthetic electric fields but also with regard to withstanding the effects of special climatic conditions. Fortunately, in recent years, researchers have paid close attention to HVDC insulation systems and have made great efforts in related research to ensure the prosperous development and safe and stable operation of HVDC projects.

The above introduction gave us the motivation and confidence to organize this Special Issue to present and disseminate the latest advances related to the phenomena, theory, design, modelling, application, and condition monitoring of all types of insulation systems, including materials, devices, and projects. This Special Issue, “Recent Progress, Challenges, and Outlooks Regarding HVDC Insulation Systems”, has received a good response. There are eight contributions in this Special Issue, including seven original research papers and one review paper.

In [1], the thermal aging characteristics of cross-linked polyethylene (XLPE) cables are investigated based on polarization and depolarization current measurements. Although XLPE cables are widely used in power transmission and distribution systems, their insulation properties are susceptible to degradation due to thermal aging. By using polarization and depolarization current measurements, the investigators show that when the XLPE cable is aged at 140 °C, the crystallinity of the insulation layer appears to increase and then decrease. At the same time, micron-sized voids appear on the surface of the XLPE as the aging time increases. In addition, the average DC conductivity increases from 2.26 × 10⁻¹⁶ S/m for new cables to 4.47 × 10⁻¹⁶ S/m after aging for 432 h, while the 0.1 Hz dielectric loss increases from 0.11% to 0.42%. According to the extended Debye model, thermal aging can damage the crystalline structure of XLPE; thus, the number of interfaces between the crystalline and amorphous regions of the material increases, resulting in structural damage and a decrease in the dielectric properties of the cable insulation.

The authors of [2] propose a sag calculation method that takes into account the temperature difference between the strands. Finite element analysis and experiments are used to analyze the temperature differences between each area and layer of the conductor as wind blows on the conductor, providing the basis for a correction calculation model for the conductor sag and a sag monitoring system design. Moreover, the method is applied to a ±400 kV transmission line in Qinghai Province, China, with good results. By considering the temperature difference in the conductor strands, the error of the sag calculation result is much smaller than that without considering the temperature difference in the conductor strands, and the maximum relative error is reduced from 7.86% to less than 2%.

Zhang et al. [3] investigate the delicate balance between the energy storage density of polypropylene (PP), a widely used dielectric, and dielectric loss. Such loss can lead to excessive heat generation, posing a threat to the operation of energy storage capacitors. In this study, PP grafted with glycidyl methacrylate (GMA) is used as a compatibilizer and incorporated into a PP/nano ZrO₂ blend to form a ternary system of PP/nano ZrO₂/PP-grafted GMA. The novel results of this work show that the ternary system not only ensures a high breakdown voltage (382.29 MV/m) but also has a high dielectric constant (2.67), thereby achieving an energy storage density of 1.7275 J/cm³ with low dielectric loss. This ternary system exhibits excellent energy storage performance, ease of fabrication, and stability.

In [4], accelerated aging experiments are performed on a specific model of an auxiliary switch under salt spray conditions using an existing test platform. The inrush voltage, trip voltage, inrush time, and trip time of the relay are analyzed as characteristic parameters affected by salt spray. The ingress of salt spray causes a decrease in the coil performance of the relay, which requires a higher voltage to provide the electromagnetic force required for operation, resulting in an increase in the operating voltage. A Genetic Algorithm–Back Propagation (GA-BP) Neural Network algorithm model is established to identify the relay status, and the detection accuracy reaches 91.8% after optimization. This work has important engineering application value for understanding the relay operation status of circuit breaker operating mechanisms in salt spray environments, predicting the lifespan of circuit breaker operating mechanism relays, and preventing circuit breaker operating mechanism failures.

The feature paper [5] of this Special Issue investigates the development characteristics, including the time-varying tendency of the discharge magnitude and repetition rate, of partial discharge in oil-pressboard insulation under constant positive and negative DC voltage. There are three stages in the development of partial discharge in a needle-plane oil-pressboard insulation mode: intensive discharge, silent burst, and breakdown. The partial discharge magnitude and repetition rate of all stages are determined via experiments. In addition, compared with positive charges, negative charges easily accumulate and are difficult to dissipate at the same voltage amplitude, leading to a weaker actual electric field at the needle tip and, thus, to partial discharges under negative DC voltage with a smaller magnitude and longer interval.

Zhu et al. [6] present a new method that combines the Volterra model of Variation Mode Decomposition (VMD) with singular value entropy to identify anchor damage faults in composite-fiber submarine cables. Compared with Empirical Mode Decomposition (EMD) and Ensemble Empirical Mode Decomposition (EEMD), the proposed method achieves a higher fault identification accuracy and effective identification.

A joint displacement control technique for inclined foundation piles and prestressed foundation tie beams is proposed in [7] to solve the problem of tower base displacement and durability degradation caused by environmental factors. In this study, a finite element model of an exposed pile transmission tower corresponding to the structural characteristics of an actual line tower is established based on the current situation of Tower 292 of the 800 kV Tianzhong line in Xinjiang, China, to analyze three different displacement control schemes under the combined effects of tower line load, ice cover load, and wind load, including changing the exposed pile height, changing the inclined pile tilt angle, and increasing the prestressed foundation tie beam. The results show that the combined displacement control technology can reduce the horizontal displacement of EHV tower foundations by more than 50%, which could greatly reduce the safety problems caused by tower displacement and effectively improve tower durability.

The only review paper [8] in this Special Issue presents the current status of research on the external insulation characteristics of post insulators, and it establishes and clarifies the concept of ‘pollution rain flashover’ of insulators. It also summarizes the research results on the pollution rain flashover of post insulators used in power stations in recent years, including the characteristics and mechanism of discharge, the parameters and factors influencing the flashover voltage, and their influence laws. Prevention methods are also discussed, and a brief outlook on future research is given.

Finally, I would like to thank the authors of all these papers for their contributions and cooperation. I would also like to thank the staff and reviewers for their efforts and contributions.