The vertical distance between straight line joining points of consecutive supports and lowest point of conductor is called sag.
Sag is influenced by several factors, and it is crucial to consider these factors to ensure the safe and efficient operation of the transmission line.
Some of the key factors upon which sag in an overhead transmission line depends include:
- Tension in the Conductor: The tension in the conductor is a critical factor affecting sag. Higher tension generally results in less sag, while lower tension leads to increased sag.
- Temperature: Temperature affects the sag of the conductor by influencing its thermal expansion or contraction. High temperatures can cause the conductor to expand, leading to increased sag, while lower temperatures can result in contraction and reduced sag.
- Conductor Material: The type of material used for the conductor (aluminum, steel, or a combination) affects its thermal expansion characteristics, which in turn influences sag.
- Wind Speed and Direction: Wind can cause lateral forces on the conductor, affecting the sag. Higher wind speeds and changes in wind direction can lead to increased or decreased sag.
- Ice and Snow Load: Accumulation of ice or snow on the conductor adds weight and can lead to increased sag. This factor is particularly important in regions with cold climates.
- Span Length: The distance between two consecutive supporting structures (poles or towers) affects the sag. Longer spans generally result in increased sag.
- Conductor Diameter: The diameter of the conductor plays a role in determining its weight, affecting sag. Thicker conductors typically have more weight and, therefore, experience more sag.
- Load on the Conductor: The electrical load carried by the conductor influences the tension and, consequently, the sag. Higher electrical loads may result in higher tension and less sag.
- Terrain and Elevation Changes: The topography of the terrain and elevation changes along the transmission line route can impact sag. Steep slopes or changes in elevation may affect the tension in the conductor.
- Installation Conditions: Factors such as the method of installation, initial tensioning of the conductor, and the condition of the supporting structures can also influence sag.
Design engineers consider these factors collectively to determine the appropriate sag for a given transmission line, ensuring that it meets safety and performance standards under various operating conditions. Computer models and simulations are often used to optimize the design and account for these factors.