With the acceleration of China's modernization process and the rapid development of industrial and agricultural production, land resources are becoming increasingly scarce, significantly restricting the selection of overhead transmission line corridors. Therefore, to adapt to the needs of power grid construction that is moderately ahead of China's industrial and agricultural development, increasing the transmission power per unit transmission corridor is urgently needed. Currently, adopting compact, large-section and expanded-diameter conductors, heat-resistant conductors, and new multi-circuit transmission technologies on the same tower are fundamental measures to improve the transmission power per unit transmission corridor of overhead transmission lines. 1. Compact Transmission Technology Compact transmission technology is a new type of transmission technology that uses methods such as reducing phase-to-phase spacing, optimizing conductor arrangement, and increasing the number of conductors in phase splitting sub-segments to change the line geometry, thereby compressing the land occupied by the line corridor and increasing its transmission capacity. 1.1 High Natural Power Compact Lines High natural power compact lines break through traditional design principles by increasing the spacing and number of splitting sub-segments, modifying the splitting structure, and optimizing conductor arrangement to increase conductor capacitance and reduce line surge impedance, thereby significantly improving transmission capacity. Typically, this can increase the transmission power of the line by 50% to 70%. The electric field strength and distribution on the surface of the sub-conductors in high natural power compact lines are much more uniform than those in conventional lines, and the utilization rate of the conductor surface is greatly improved. Radio interference and corona loss can be controlled at an acceptable level. However, because the sub-conductors in high natural power compact lines are arranged in elliptical or parabolic shapes, and the distances between adjacent sub-conductors vary, as do the distances between sub-conductors along the line, this requires the design and manufacture of various new special hardware structures. The conductor structure is often overly complex (especially asymmetrical arrangements). Furthermore, there is currently no operational experience regarding conductor galloping under uneven icing conditions, increasing the complexity of line construction, operation, maintenance, and repair. Therefore, the practical application of high natural power compact lines is not widespread domestically or internationally. 1.2 General Compact Lines The conductor structure and tower type of general compact lines do not undergo major changes. The only difference is the increase in the number of phase-split sub-conductors, the elimination of phase-to-phase columns on the towers, and the arrangement of all three phase conductors within the tower windows. Simultaneously, simple measures such as triangular arrangement of conductors and V-shaped insulator strings are used to limit the swaying amplitude and reduce the phase-to-phase distance. This typically increases the transmission power of the line by about 30%. The three-phase conductors of a compact transmission line generally use a triangular arrangement, which significantly reduces the line corridor and promotes uniform charge distribution. The phase conductor arrangement and split conductor structure are all symmetrically distributed, facilitating construction and maintenance, and there is long-term safe operation and maintenance experience both domestically and internationally. Currently, various conductive work can be performed on compact transmission lines, all while ensuring the safety of personnel. For shorter lines with lower overvoltage levels, conventional live-line work methods are sufficient; however, for longer lines with higher overvoltage levels, live-line work with protective gaps should be used to ensure the safety of personnel. my country has established standards for compact transmission lines, including the State Grid Corporation standard Q/GDW110-2003 "Technical Regulations for the Design of 500kV Compact Overhead Transmission Lines" and the power industry standard DL/T5217-2005 "Technical Regulations for the Design of 220～500kV Compact Overhead Transmission Lines." From the current technical perspective, the relevant provisions in these standards are correct. However, based on an analysis of domestic and international development, there is still room for further optimization of relevant parameters of compact transmission lines, such as split conductors, sub-conductor radii, and phase spacing. Considering the current trend of power development in my country, the construction of large-capacity power plants and long-distance transmission lines is progressing rapidly. Promoting and adopting compact transmission lines at all voltage levels is technically and economically appropriate, and is a feasible way to change and improve the transmission capacity of my country's power transmission system. Compact transmission technology increases line capacitance, reduces reactance, reduces surge impedance, and increases natural power. The three-phase charge balance is better (especially in a triangular arrangement), eliminating the need for transposition over hundreds of kilometers. The inrush current is larger than that of conventional transmission lines. Compared with conventional circuits, compact transmission lines have the advantages of high natural transmission power and low imbalance, effectively addressing the contradiction between limited arable land and high transmission capacity demand. Compact transmission lines can improve natural transmission power and corridor utilization without significantly increasing line investment and electromagnetic field strength, and can especially effectively utilize valuable arable land resources and limited river crossings. Based on the characteristics and advantages of compact transmission lines, and to improve transmission capacity, alleviate corridor conflicts, reduce project costs, and protect the ecological environment, compact transmission lines will be comprehensively promoted and applied across all voltage levels of 110kV, 220kV, 330kV, and 500kV; especially in urban suburbs and densely populated areas where lines often require multiple detours. Given the current trend of power development in my country, the construction of large-capacity power plants and long-distance transmission lines is progressing rapidly. Promoting and adopting compact transmission lines in long-distance transmission is not only technically and economically suitable, but also a feasible way to improve and increase the transmission capacity of my country's power transmission system. In the future, compact transmission technology will exhibit a diversified development pattern. 2. Large-Cross-Section Conductor Transmission Technology. Large-section conductor transmission technology refers to a new type of transmission technology that uses conductors with larger cross-sections (such as 4×500mm², 4×630mm², and 4×800mm² for 500kV) instead of the conventional minimum cross-section controlled by economic current density (e.g., 300mm² for 220kV; 4×300mm² for 500kV) to significantly increase the transmission capacity of the line. Large-section conductors refer to conductors with cross-sections exceeding the conventional minimum cross-section controlled by economic current density. With increased conductor cross-section, the resistance per unit length of conductor decreases, and within the limits of thermal capacity, its allowable current carrying capacity increases, thereby increasing its transmission power. The use of large-section conductors can reduce the number of line corridors, saving land resources, which is a significant advantage given the increasing scarcity of arable land in my country today. As the conductor cross-section increases, the surface electric field strength of the transmission line decreases, and corona loss also decreases accordingly, while the ground electric field strength increases, but the increase is small and has little impact on the transmission line. Furthermore, wireless interference and noise pollution are also greatly reduced. Using large-section conductors in power transmission lines increases initial investment, but the design and construction of towers to withstand greater stress and loads, along with the production of hardware compatible with large-section conductors, are crucial for their widespread application and development. Currently, many wire and cable manufacturers in my country have the capacity to produce large-section conductors, and domestic construction equipment for large-section conductors has met engineering requirements. Significant progress has been made in the construction of large-section conductors, enabling independent erection of these conductors while fulfilling engineering requirements. While large-section conductors can increase transmission power, the increased conductor cross-section leads to increased loads on towers, greater difficulty in stringing, and higher investment costs. Therefore, when using large-section conductors, it is essential to consider the actual transmission capacity requirements of the line, allowing for a certain margin, and using appropriately sized conductors rather than blindly adopting excessively large cross-sections. Using large-section conductors not only significantly increases transmission power and reduces the number of line corridors, but also greatly reduces line losses due to decreased conductor resistance, lower surface electric field strength, and correspondingly reduced corona losses. Furthermore, for ultra-high voltage and extra-high voltage lines, it significantly reduces radio interference and noise pollution. Large-section conductor transmission lines have large transmission capacity and low power loss, but due to the difficulty in conductor production and construction, they consume a large amount of steel. Therefore, they are not suitable for widespread adoption at present. Based on the advantages and characteristics of large-section conductor transmission technology, it is best suited for short-distance transmission lines in areas with concentrated populations, high electricity demand, concentrated power flow, and high power density. It also shows good performance in some medium-to-short-distance transmission lines with large capacity output (such as at substations and power plant outlets). Furthermore, large-section conductor transmission technology is also suitable for ultra-high voltage direct current transmission. 3. Heat-resistant conductor transmission technology: Heat-resistant conductor transmission technology refers to a new type of transmission technology that uses heat-resistant conductors to increase the allowable temperature of the conductors, thereby increasing the current carried by the conductors and thus improving the transmission capacity of the line. Heat-resistant conductors have excellent electromechanical properties and maintain good mechanical characteristics even at high temperatures. Using heat-resistant conductors increases the allowable temperature of the conductors, thereby increasing the transmission capacity of the line. The use of heat-resistant conductors in overhead transmission lines is entirely feasible from both a technical analysis and practical engineering application perspective. Not only are there numerous wire and cable manufacturers offering products, but also matching hardware is available, meeting the current needs of my country's power development. Heat-resistant conductors are particularly suitable for high-current transmission in substations and power plants, as well as for capacity expansion and renovation of existing lines, saving on project investment. They are already being used in many projects in my country, operating well and achieving significant social and economic benefits. For long-distance power transmission, heat-resistant conductors must be comprehensively considered, especially regarding system stability, line losses, conductor prices, and the requirements for tower height due to significant variations in sag. my country already possesses the capability to research and produce general heat-resistant conductors and hardware, but further research is needed to develop even better heat-resistant conductors and reduce their price. With the continuous development of heat-resistant conductor technology, heat-resistant conductors with even higher allowable temperatures, better mechanical properties, and higher conductivity will emerge in the future. This will greatly promote the development of heat-resistant conductor power transmission technology. Heat-resistant conductor transmission technology is mature both theoretically and practically, and considering the future development of my country's power industry, it is bound to see significant growth. However, it should be noted that while heat-resistant conductors can increase transmission capacity, the increased conductor temperature leads to greater sag and line losses. Therefore, a careful balance must be struck when adopting this technology. The creep characteristics of conductors used in overhead transmission lines are a crucial indicator affecting the safe operation of the lines. Both exhibit similar creep characteristics at both room temperature and high temperatures. Regarding corrosion resistance, laboratory salt spray tests and outdoor atmospheric exposure tests have confirmed that there is no significant difference between heat-resistant aluminum alloy wire and ordinary hard aluminum wire. The biggest drawback of early-developed heat-resistant aluminum alloy wire was its higher resistivity (58% IACS) compared to ordinary hard aluminum wire. High-quality ordinary hard aluminum wire can achieve a conductivity of 61%. Replacing ordinary hard aluminum wire with heat-resistant aluminum alloy wire in traditional steel-cored aluminum stranded wire results in steel-cored heat-resistant aluminum alloy stranded wire (TACSR). While this type of conductor significantly improves heat resistance and current carrying capacity, its conductivity is lower than that of ordinary steel-cored aluminum stranded wire, and its resistance increases with operating temperature. This initially hindered its widespread adoption. Research has shown that titanium (Ti) and vanadium (V) can also improve the heat resistance of aluminum. Zirconium has the greatest impact on the conductivity of aluminum, followed by titanium and vanadium. The characteristics of aluminum alloy conductors (taking Al-Mg-Si heat-treated high-strength aluminum alloy conductors as an example), their application and development trends in advanced industrial countries worldwide, and their advantages over steel-cored aluminum stranded wires (such as high strength, light weight, and good conductivity (53% IACS)) are discussed. They were first used in overhead lines as early as 1921. With the improvement of power transmission and transformation technology and the development of transmission lines, aluminum alloy conductors have increasingly demonstrated their superiority. Internationally, they have been used since the 1950s, and are now widely adopted in Western Europe, Northern Europe, the United States, Canada, Japan, and other countries. In France, nearly 90% of transmission lines use aluminum alloy conductors, and in Japan, this figure has reached over 50%. The usage in Southeast Asian countries is increasing year by year (while in my country, the current usage is less than 1%). This is due to the unparalleled advantages of aluminum alloy conductors compared to steel-cored aluminum stranded wire. They hold a unique advantage in long-distance, large-span, and ultra-high-voltage transmission; in rural power grid renovation, they are an advanced and economical product, a replacement for ordinary steel-cored aluminum stranded wire, and also an energy-saving, material-saving, and land-saving product (saving transmission corridors). Currently, the problem of insufficient power transmission capacity in my country's power grid is very prominent. In the expansion and renovation of transmission lines, especially in economically developed and densely populated areas, various obstacles have been encountered due to the lack of corridor resources. Recently, the State Grid Corporation of China has clearly stated that provincial power departments must fully utilize existing power corridor resources, develop technologies to improve the temperature tolerance of transmission lines, adopt heat-resistant conductors, and enhance thermal stability to increase transmission capacity. Improving the operating temperature of transmission line conductors is considered a crucial means of power grid transformation. One important reason for the national decision to launch the ultra-high voltage (UHV) project is that one UHV corridor is equivalent to the transmission capacity of five extra-high voltage (EHV) corridors; on a single-circuit basis, the power transmission per unit corridor area can be increased by more than three times. In areas with narrow lines, simply replacing the conductors with heat-resistant aluminum alloy conductors of similar cross-section specifications can meet the increased transmission capacity requirements, essentially eliminating the need for land acquisition and tower replacement. This increases the transmission capacity of existing lines by 40% to 60%, saving significant engineering investment and valuable land resources required for tower erection, while also enabling rapid construction and generating substantial economic and social benefits. Experts point out that due to long-term insufficient investment in power grid construction, there is a significant backlog in power grid development. Bottlenecks are prevalent in inter-regional and inter-provincial interconnection lines, large power transmission lines, and load center input lines. Distribution network power supply capacity is also relatively insufficient, leading to problems of "inability to transmit, receive, and utilize power," significantly hindering the ability to meet the growing needs of people's production and daily life. Upgrading urban and rural power grids with 60% IACS heat-resistant aluminum alloy transmission conductors is economical, environmentally friendly, and increases transmission capacity. It is a demonstration project of "green manufacturing" with significant value for widespread application. Currently, the conductors used in my country's overhead transmission lines are still primarily traditional steel-cored aluminum stranded wire (LGJ). Due to its relatively poor heat resistance, its transmission capacity is limited, and transmission losses are significant. High-strength heat-resistant aluminum alloys, on the other hand, are aluminum alloy conductor materials with high tensile strength and excellent heat resistance. When made into conductors, they offer outstanding advantages such as high operating temperature, good heat resistance, large current carrying capacity, and high tensile strength. They are widely used abroad with good results. Ultra-high strength heat-resistant aluminum alloy conductors are used in the upgrading of old power lines, offering significant capacity expansion while saving substantial amounts of metal materials and infrastructure investment. They are particularly suitable for lines requiring high strength and large capacity, exhibiting unique advantages in long-distance, large-span, and ultra-high-voltage transmission. In power grid upgrading, they are an advanced and economical product, a replacement for ordinary steel-cored aluminum stranded wire, and also an energy-saving, material-saving, and land-saving product (saving transmission corridor space). For example, in large-span lines, they can significantly improve transmission capacity and line safety. The use of ultra-high strength heat-resistant aluminum alloy conductors will bring significant social and economic benefits. They will become a very important new type of overhead conductor in my country's large-span transmission lines. In my country's power line construction, especially in western regions, there are increasingly more areas with large spans or significant elevation differences, requiring conductors capable of transmitting large currents and withstanding high tension. Although the unit price of heat-resistant aluminum alloy conductors is 1.58 times that of ordinary conductors, the overall cost of lines using ordinary conductors is much lower. Because it can replace ordinary overhead conductors, saving land resources and simplifying the line structure by eliminating many components, it requires less investment on transmission lines with the same conductive cross-section. Especially in areas with narrow transmission corridors, only conductors with similar conductive cross-sections need to be replaced; tower replacement is generally unnecessary. This meets both strength and conductor-to-ground safety requirements, and allows for normal power transmission at temperatures up to 150℃ (compared to a maximum operating temperature of only 90℃ for ordinary conductors). It also increases transmission capacity by 40%–60%. The increased conductivity significantly reduces investment in line construction. Calculations show that a 1% difference in conductivity, under 220kV voltage conditions and economic current density transmission, translates to a reduction in power loss of approximately 200,000 yuan per kilometer based on current electricity prices.