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Introduction

1.1 Background

Power cables are key equipment for achieving high‐capacity power transmission. The earliest cables can be traced back to the 19th century. In its development history of over 150 years, the voltage level and transmission capacity of cables have continuously increased, making important contributions to the development of society and economy. Especially after entering the 21st century, with the advancement of internationalization and the development of new energy sources such as offshore wind power, submarine cables are considered the most important means of transmission technology. Figure 1.1 is the schematic diagram of the grid connection and transmission of large‐scale offshore wind power based on the high‐voltage and large‐capacity power cable systems.

Power cable insulation is a protective medium that isolates high potential and is crucial for ensuring the safe and reliable operation of cable transmission systems. In other words, cable insulation needs to have high breakdown field strength, low dielectric loss, resistance to electrical tree discharge, and long service life, as well as good heat resistance, flexibility, and mechanical strength. Cable insulation materials are constantly advancing and developing in pursuit of better performance. The initial cable insulation used oil‐filled (OF)‐insulated cables and later developed into mass‐impregnated (MI) traditional or polypropylene‐laminated paper (PPLP)‐insulated cables. OF‐insulated cables have the advantages of safety, stability, reliability, and long service life. However, the length of OF‐insulated cables is greatly limited due to the need for oil supply equipment, and there are also environmental pollution hazards. Compared with OF‐insulated cables, MI‐insulated cables do not require oil supply equipment, break the length limit of OF‐insulated cables, and have no environmental hazards. They have higher insulation performance and heat resistance and are recognized as the most reliable type of high‐voltage cable insulation at that time. Until the 1950s, with the emergence of plastic extruded cable insulation represented by cross‐linked polyethylene (XLPE), cable insulation had a new development direction. Compared to the first two, extruded insulation has advantages such as high‐temperature resistance, high transmission power density, high strength, lightweight, environmental protection, and easy installation. Therefore, it has been widely applied and developed after its release. In 1957, General Electric (GE) Company produced the earliest XLPE AC cable by using the advanced production processes in chemistry. In 1999, the world's first commercialized XLPE‐extruded DC cable was applied in the Gotland Island II Project in Sweden. The voltage level of this DC cable was ± 80 kV, the transmission power was 50 MW, and the cable length was 140 km. The advantages and disadvantages of the three types of cable insulation are compared in Table 1.1, and Figure 1.2 indicates the cable typologies that can be conveniently used in relation to the length, power, and voltage of the link and the type of transmission system.

Figure 1.1 Schematic diagram of the grid connection and transmission of large‐scale offshore wind power based on the high‐voltage and large‐capacity power cable systems.

At present, the most commonly used extruded insulation material for power cables is cross‐linked polyethylene insulation. Its main producers include Dow Corporation in the United States and Nordic Chemical Company, which basically occupy the world market for DC cable insulation materials. The “superclean” XLPE insulation material launched by Nordic Chemical can meet the insulation material performance requirements of 500 kV high‐voltage AC and ±640 kV high‐voltage DC cables [1].

Table 1.1 Advantages and disadvantages of OF‐, MI‐, and XLPE‐insulated cables.

Figure 1.2 Cable typologies that can be conveniently used in relation to the length, power, and voltage of the link and the type of transmission system.

However, with the widespread application of XLPE, its shortcomings are constantly exposed. (i) The production process of XLPE high‐voltage DC cables requires controlling the environment and temperature of the cross‐linking process, resulting in complex production processes, high‐energy consumption, and low‐production efficiency. (ii) After the XLPE cable is extruded, a long‐term degassing treatment is required to reduce the impact of crosslinking by‐products on DC conductivity and space charge accumulation. (iii) The cross‐linking by‐products, mainly including acetophenone, cumyl alcohol, and α‐methylstyrene, accelerates the aging of cable insulation, which makes it difficult to achieve a lifetime of 40 years [2]. (iv) XLPE is a thermosetting polymer, and after cable retirement, the insulation cannot be melted again for processing and utilization [3], making it difficult to recover and treat, which has a significant adverse impact on the environment. Given the many issues mentioned above, the application of XLPE insulation material in high‐voltage DC cables has significant limitations, and the development of thermoplastic insulation materials is a new research direction.

Thermoplastic‐insulated cables have many advantages, such as no cross‐linking process is required in the production process, simple process, and low‐energy consumption, no need for degassing, shortened production cycle, no cross‐linking by‐products (which does not affect the electrical performance of insulation), and recyclable by melting. Since the 1970s, thermoplastic insulation materials such as low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE) have been used in high‐voltage cable insulation, but their heat resistance level or mechanical properties do not fully meet the requirements of high‐voltage DC cables. Polypropylene (PP) material has excellent insulation, heat resistance, mechanical properties, and recyclability, making it suitable for the manufacturing of high‐voltage DC cables. The melting point of PP is about 40–50% higher than that of polyethylene, and the long‐term working temperature is higher than that of XLPE [4]. Good heat resistance is of great significance for improving the working temperature and voltage of cables. PP is a nonpolar material with high breakdown field strength (≥100 kV/mm under AC voltage and ≥300 kV/mm under DC voltage) and high‐volume resistivity (~1016 Ω m), which does not change significantly with temperature. Under the same insulation thickness, the PP‐insulated cable system has higher operating voltage, higher long‐term operating temperature, and larger transmission capacity of the line, compared with that with XLPE insulation. In another word, PP cable insulation can reduce the thickness of the insulation layer and increase the current carrying capacity of the cable system. PP has a high charge injection threshold and less space charge accumulation [5]. PP‐insulated cables can reduce the energy consumption of the cable cross‐linking process by 1000 kWh/km and reduce carbon emissions by 80% throughout their entire life cycle, according to data from Prysmian. In summary, studying PP as an insulation material for high‐voltage DC cables has broad application prospects.

As early as the 1990s, scholars began to study the electrical properties of PP as an insulation material for high‐voltage DC cables. Scholars from Osaka University collaborated with Mitsubishi Cable to prepare 600 V and 22 kV cables using PP as insulation. The study found that the AC breakdown field strength and dielectric loss of PP cables at different temperatures met the requirements of the cables. Further research on 22 kV cables found that the lifespan of cables with PP as the main insulation is higher than that of XLPE, and the suppression effect on water trees is better than that of XLPE [6]. In 2010, Prysmian released a high‐performance thermoplastic elastomer (HPTE) insulation material developed based on PP materials. P‐Laser cables...