Typical Types of Overhead Transmission Lines
1. Heavy Ice-Covered Lines
Conductor icing occurs when raindrops freeze and form an ice layer on the conductor surface under specific climatic conditions. A transmission line is classified as "heavy ice-covered" when the ice thickness on conductors exceeds 20 mm. This condition increases the conductor’s sag and load, reduces its dynamic tension performance, and enlarges the conductor’s wind-facing area.
2. Large-Span Lines
During conductor erection, spans of exceptional length are unavoidable (e.g., crossing rivers, lakes, or straits). Lines with a crossing span over 1,000 meters or crossing tower height above 100 meters are generally defined as large-span lines. Due to their long spans, the horizontal wind load on conductors is much higher than that on conductors in ordinary spans. Additionally, the surface roughness of geographical environments like rivers and lakes is lower than that of ordinary ground, resulting in higher basic wind speeds and longer wind duration under the same conditions. Conductors used in such lines prioritize mechanical properties but are prone to aeolian vibration, which causes fatigue wear between clamps and conductors and increases the risk of conductor breakage.
3. Mountainous Lines with Large Elevation Differences
When overhead transmission lines pass through mountainous terrain—especially continuous high mountains and steep ridges—the elevation difference between front and rear spans of tangent towers becomes significant. Conventional suspension clamps cannot meet the required sag angle, easily causing local mechanical damage to conductors and leading to strand breakage.
Application Characteristics of PAR Fittings in Overhead Transmission Lines
1. Application in Ice-Covered Lines
For lines in heavy ice areas, icing significantly increases the vertical load and horizontal wind load on conductors. It also generates longitudinal tension when ice covers conductors unevenly, melts asynchronously, or even causes conductor galloping.
When designing overhead transmission lines in icy areas, suspension clamps must provide greater grip on conductors to resist the adverse effects of heavy icing. Preformed suspension clamps feature a high-performance rubber pad at the inner center (in direct contact with conductors) and long preformed rods wrapped around the rubber pad and conductor surface. This structure eliminates stress concentration points and ensures high grip. For example, test reports from the China Electric Power Research Institute (CEPRI, Report No.: K111S30663) show that the AGS5133 preformed suspension clamp achieves a grip of 65 kN, fully meeting the grip requirements for conductors in heavy ice areas and making it highly suitable for such lines.
In addition, preformed anti-galloping whips can be appropriately used on ice-covered lines. These whips consist of three parts: a clamping section, a support section, and an anti-galloping section. The anti-galloping section continuously changes the dynamic shape of the cable under wind action, disrupting the air flow around the conductor and thus preventing galloping. Furthermore, since the anti-galloping section is not tightly fixed to the conductor, it moves relative to (or even taps) the conductor when wind blows, achieving a certain de-icing effect during light icing.
2. Application in Large-Span Lines
Large-span transmission lines typically use conductors that are lightweight, small in cross-section, and high in mechanical strength (e.g., steel strands or aluminum alloy strands). A major drawback of these conductors is their poor resistance to aeolian vibration. Operational data and research indicate that aeolian vibration is the primary cause of conductor fatigue damage.
Traditional suspension clamps are bolt-type, which rely on bolt compression to maintain grip on conductors. These clamps are simple to design and produce, low in cost, and widely used. However, statistics from both domestic and international sources show that nearly all conductor fatigue damage occurs at the contact points between conductors and fittings. Traditional fittings easily create high dynamic and static compressive stress in the conductor contact area.
In contrast, preformed suspension clamps adopt an innovative structural design: while providing sufficient grip on conductors, they do not require bolts to act on the conductor. This increases the contact area between the clamp and conductor, eliminates stress concentration at the contact point, and effectively disperses the static pressure from the fitting over a large area. Additionally, wrapping preformed rods of a certain length around the conductor surface significantly enhances the bending stiffness of the conductor’s contact area and reduces dynamic stress.
A comparative test was conducted on two sets of conductors—one equipped with traditional bolt-type suspension clamps and the other with preformed suspension clamps—under the same conditions. After tens of millions of vibration fatigue cycles, the former showed severe conductor strand breakage, while the latter had no conductor damage. This confirms that preformed suspension clamps have strong anti-fatigue performance and can effectively reduce fatigue loss in large-span transmission lines.
3. Application in Lines with Large Elevation Differences
Preformed suspension clamps are available in single-support and double-support types. Single-support clamps typically meet the requirement for conductor sag angles less than 30°, while double-support clamps can accommodate sag angles up to 60°. For transmission lines in mountainous areas, preformed suspension clamps fully satisfy the demand for large sag angles caused by elevation differences.
Moreover, lines with large elevation differences have large vertical spans and high vertical loads at suspension points. Compared with ordinary clamps, preformed suspension clamps disperse concentrated stress and provide greater grip on conductors, making them more advantageous for use in such lines.
Additionally, the large sag angles of these lines easily cause traditional vibration dampers to "slide" when conductors creep. Preformed vibration dampers, however, use preformed rods to fasten the damper to the conductor, effectively preventing sliding while maintaining vibration damping performance.
Additional Advantages of PAR Fittings
1. Easy Installation
Traditional fittings require specialized tools for installation, involving complex procedures and long installation times. Thanks to their unique structure, PAR fittings only require workers to wrap each preformed rod in the predetermined direction—no auxiliary tools are needed. This reduces installation difficulty and the load on workers when climbing towers, and allows for visual inspection of installation quality.
2. High Efficiency and Energy Savings
In China’s grid construction, ferrimagnetic power fittings (e.g., XGU-type suspension clamps) have been widely used due to their low material cost and easy processing. However, in-depth research has revealed that these fittings cause high energy consumption during line operation. To reduce fitting-related energy loss, energy-efficient aluminum alloy fittings are commonly used both domestically and internationally.
Most PAR fittings are made of aluminum alloy. Compared with ferrimagnetic fittings, aluminum alloy fittings not only meet the mechanical strength requirements of transmission lines but also offer significant energy-saving effects. According to a CEPRI energy consumption test report (Report No.: K111S30663) provided by PLP Beijing, the power loss of each traditional ferrimagnetic clamp is 58.8 W, while that of each anti-corona aluminum alloy clamp is -20.0 W, and each preformed AGS suspension clamp has a power loss of -27.1 W. These data indicate that aluminum alloy fittings have almost no power loss; instead, they increase the conductor’s conductive cross-section and reduce line transmission energy consumption. Among them, the preformed AGS suspension clamp achieves the lowest energy consumption—saving 146% more energy than each ferrimagnetic clamp and 35.5% more than each anti-corona aluminum alloy clamp.
3. Strong Corrosion Resistance
PAR fittings are usually made of the same material as the conductor surface. This effectively reduces electrochemical corrosion caused by different metals in electrical environments, extending the service life of power fittings and conductors in the humid, foggy, and acid-rain-prone natural environments of southern China.
4. Excellent Adaptability
PAR fittings are applicable not only to a series of commonly used conductors and ground wires (e.g., ACSR, aluminum-clad steel strands, aluminum alloy strands, ACSR aluminum alloy, and galvanized steel strands) but also to insulated conductors, high-temperature conductors, ADSS, and OPGW lines. Performance evaluations confirm that PAR fittings can be used in almost all overhead transmission lines.
5. Anti-Corona Performance
According to data provided by PLP Beijing, the ends of preformed rods for PAR fittings used in lines with voltage levels above 500 kV undergo special treatment, which effectively improves the fitting’s corona extinction voltage. For lines with voltage levels below 500 kV, the same special treatment can be applied upon request.
Conclusion
Power fittings are essential components of transmission lines, with wide application and large usage volumes. PAR fittings offer comprehensive performance advantages over traditional power fittings and are suitable for transmission lines under various working conditions. From the perspective of long-term line operation, the use of PAR fittings not only improves the safety and stability of transmission lines, reduces the adverse impact of fittings on conductors, but also lowers line operation energy consumption and extends service life.
Post time: 2025-01-31
