The smart grid, also known as the "energy internet," is renowned for its robust structure, widespread interconnectivity, high intelligence, and open interaction. It is considered a fundamental platform for the Third Industrial Revolution and plays a crucial role in its global development. A robust and optimized power communication system is essential for the realization of a smart grid. To meet the specific communication needs of power systems, special power optical cables have been developed.
Specialized power fiber optic cables are fiber optic composite cables that are installed on power lines and suspended from power poles and towers, utilizing the unique resources of the power system. They fully leverage the overhead power line corridors. These cables are constructed by tightly integrating with the power grid structure, making them economical, reliable, fast, and safe. They are less susceptible to external damage and have high reliability. Although their initial cost is relatively high, their construction cost is lower. Installed on various power poles and towers of different voltage levels, these cables have special requirements for their electrical, mechanical, and fiber characteristics compared to ordinary optical cables.
Special optical cables rely on the power system's own line resources, avoiding conflicts and entanglements with the outside world in terms of frequency resources, routing coordination, and electromagnetic compatibility, thus having great initiative and flexibility.
According to the laying and deployment method, special power optical cables are divided into several types, such as optical fiber composite overhead ground wire OPGW, optical fiber composite overhead phase wire OPPC, all-metal self-supporting optical cable MASS, all-dielectric self-supporting optical cable ADSS, ground wire bundled optical cable ADL, and ground wire wrapped optical cable GWWOP.
OPGW optical cables are mainly used on 500kV, 220kV, and 110kV voltage level lines. Due to factors such as power outages and safety concerns, they are mostly used on newly constructed lines. The outstanding feature of OPGW (Overhead Ground Wire Composite Optical Cable) is that it combines communication optical cables and overhead ground wires on high-voltage transmission lines into a single unit. It integrates optical cable technology and transmission line technology, becoming a multi-functional overhead ground wire that serves as a lightning protection wire, an overhead optical cable, and a shielding wire. It simultaneously completes the construction of communication lines while high-voltage transmission lines are being built, making it highly suitable for newly constructed transmission lines. Common OPGW structures mainly fall into three categories: aluminum tube type, aluminum skeleton type, and (stainless) steel tube type.
The applicable characteristics of OPGW are: (1) High voltage lines exceeding 110kV with large spans (generally above 250M); (2) Easy to maintain, easy to solve line crossing problems, and its mechanical characteristics can meet the requirements of large line crossings; (3) The outer layer of OPGW is metal armor, which has no effect on high voltage erosion and degradation; (4) OPGW must be shut down during construction, and the power outage loss is large, so OPGW should be used in newly built high voltage lines above 110kV; (5) Among the performance indicators of OPGW, the larger the short circuit current, the more it is necessary to use good conductors for armor, which reduces the tensile strength accordingly. Under the condition of a certain tensile strength, to increase the short circuit current capacity, the metal cross-sectional area must be increased, which leads to an increase in cable diameter and cable weight, thus raising safety issues for the strength of line towers.
OPGW is susceptible to short-circuit faults. When a short-circuit current from a line fault impacts the OPGW optical cable, the stainless steel units experience instantaneous high temperatures. Therefore, it is necessary to increase the short-circuit current capacity of the optical cable to reduce the impact of short-circuit faults. According to Q=I²t, the heat resistance of OPGW can also be improved by limiting the magnitude and duration of the actual short-circuit current.
Besides short-circuit faults, lightning strikes are another factor causing instantaneous high temperatures in OPGW optical cables. Compared to short-circuit faults, lightning strikes have a much higher instantaneous current intensity but a shorter duration. Therefore, the heat capacity caused by a lightning strike is less than that of a short circuit. However, while the short-circuit current acts on the entire metal cross-section of the OPGW optical cable, the lightning strike current is confined to a small segment of one or more metal filaments. This concentrated energy results in a high temperature on that small segment of filament sufficient to partially or completely melt it. This is the main reason why lightning strikes cause strand breakage in OPGW optical cables.
The main fault in OPGW optical cable applications is strand breakage caused by lightning strikes. Current solutions include: 1) Developing lightning-resistant outer strand materials. Brazil developed a high-lightning-resistant OPGW in 2000, using a high-grade galvanized steel wire for the outer strands and an aluminum tube protecting the optical fiber. High-grade galvanized steel requires significant energy to melt under lightning strikes. 2) Using aluminum-clad steel wire for the outer strands whenever possible, and increasing the thickness of the aluminum cladding. 3) Maximizing the design air gap between the outer and inner strands to prevent heat transfer. 4) Using a larger outer strand diameter with the same materials. Once the materials and structure of the OPGW optical cable are determined, its lightning resistance characteristics are also determined.
The all-dielectric self-supporting optical cable (ADSS) utilizes high-strength aramid yarn with high elastic modulus as the tensile element during manufacturing. Simultaneously, its small geometric dimensions and weight (only one-third that of ordinary optical cables) allow for direct mounting on appropriate locations on power poles with minimal additional load. The maximum span can reach 1500m. Its outer sheath undergoes neutral ionization impregnation treatment, giving the cable strong resistance to electro-corrosion and ensuring its lifespan in strong electric fields. Made of non-metallic materials, the cable boasts excellent insulation, preventing lightning strikes and ensuring uninterrupted operation even during power line failures. Utilizing existing power poles, construction can proceed without power interruption, and co-location with power lines reduces project costs. The various ADSS optical cable structures can be broadly categorized into two main types: the central tube type and the stranded type.
ADSS optical cables are widely used on 220kV, 110kV, and 35kV power transmission lines, especially on existing lines. They can meet the requirements of large spans and large sags of power transmission lines. The standard ADSS design can have up to 144 cores. Its features are: (1) The theoretical value of fiber tension in ADSS is zero; (2) ADSS optical cables are fully insulated, and can be used for live work during installation and line maintenance, which can greatly reduce power outage losses; (3) The expansion rate of ADSS can remain unchanged over a wide range of temperature differences, and it has stable optical properties under extreme temperatures; (4) The erosion-resistant ADSS optical cable can reduce the electro-corrosion of the optical cable by the high voltage induced electric field; (5) ADSS optical cables have a small diameter and light weight, which can reduce the impact of ice and wind on the optical cable, and its impact on the strength of the tower is also small; (6) ADSS adopts new materials and a smooth shape design, which gives it superior aerodynamic characteristics.
Because ADSS optical cables operate near high-voltage conductors, where strong electric fields exist, electrolytic corrosion is highly likely to cause cable damage. Current solutions include: 1. Prioritizing tracking-resistant jackets for lines of different voltage levels; for lines of 110kV and above, tracking-resistant materials must be used. 2. For 110kV and above lines, anti-corrosion coils can be considered to effectively reduce the electric field between the fittings and the cable surface, reducing leakage current. 3. For 220kV and above lines and towers with strong suspension field, vibration dampers are much safer than vibration whips for preventing creepage corrosion of the cable sheath. 4. Controlling the spatial potential at the cable suspension point: not exceeding 15kV for 110kV and not exceeding 20kV for 220kV.
