Transmission line towers are tall structures used for the transmission of electrical power. Their structural characteristics are primarily based on various types of spatial truss structures. The members of these towers are mainly composed of single equilateral angle steel or combined angle steel. The materials typically used are Q235 (A3F) and Q345 (16Mn).
The connections between the members are made using coarse bolts, which connect the components through shear forces. The entire tower is constructed from angle steel, connecting steel plates, and bolts. Some individual components, such as the tower base, are welded together from several steel plates to form a composite unit. This design allows for hot-dip galvanization for corrosion protection, making transportation and construction assembly very convenient.
Transmission line towers can be classified based on their shape and purpose. Generally, they are divided into five shapes: cup-shaped, cat-head shaped, upright-shaped, cantilever-shaped, and barrel-shaped. Based on their function, they can be categorized into tension towers, straight-line towers, angle towers, phase-changing towers (for changing the position of conductors), terminal towers, and crossing towers.
Straight-Line Towers: These are used in the straight sections of transmission lines.
Tension Towers: These are installed to handle the tension in the conductors.
Angle Towers: These are placed at the points where the transmission line changes direction.
Crossing Towers: Higher towers are set up on both sides of any crossing object to ensure clearance.
Phase-Changing Towers: These are installed at regular intervals to balance the impedance of the three conductors.
Terminal Towers: These are located at the connection points between transmission lines and substations.
Types Based on Structural Materials
Transmission line towers are primarily made from reinforced concrete poles and steel towers. They can also be classified into self-supporting towers and guyed towers based on their structural stability.
From the existing transmission lines in China, it is common to use steel towers for voltage levels above 110kV, while reinforced concrete poles are typically used for voltage levels below 66kV. Guy wires are employed to balance the lateral loads and tension in the conductors, reducing the bending moment at the base of the tower. This use of guy wires can also decrease material consumption and lower the overall cost of the transmission line. Guyed towers are particularly common in flat terrain.
The selection of tower type and shape should be based on calculations that meet electrical requirements while considering the voltage level, number of circuits, terrain, and geological conditions. It is essential to choose a tower form that is suitable for the specific project, ultimately selecting a design that is both technically advanced and economically reasonable through a comparative analysis.
Transmission lines can be classified based on their installation methods into overhead transmission lines, power cable transmission lines, and gas-insulated metal-enclosed transmission lines.
Overhead Transmission Lines: These typically use uninsulated bare conductors, supported by towers on the ground, with the conductors suspended from the towers using insulators.
Power Cable Transmission Lines: These are generally buried underground or laid in cable trenches or tunnels, consisting of cables along with accessories, auxiliary equipment, and facilities installed on the cables.
Gas-Insulated Metal-Enclosed Transmission Lines (GIL): This method uses metal conductive rods for transmission, completely enclosed within a grounded metal shell. It employs pressurized gas (usually SF6 gas) for insulation, ensuring stability and safety during current transmission.
Due to the high costs of cables and GIL, most transmission lines currently use overhead lines.
Transmission lines can also be classified by voltage levels into high voltage, extra high voltage, and ultra-high voltage lines. In China, the voltage levels for transmission lines include: 35kV, 66kV, 110kV, 220kV, 330kV, 500kV, 750kV, 1000kV, ±500kV, ±660kV, ±800kV, and ±1100kV.
Based on the type of current transmitted, lines can be categorized into AC and DC lines:
AC Lines:
High Voltage (HV) Lines: 35~220kV
Extra High Voltage (EHV) Lines: 330~750kV
Ultra High Voltage (UHV) Lines: Above 750kV
DC Lines:
High Voltage (HV) Lines: ±400kV, ±500kV
Ultra High Voltage (UHV) Lines: ±800kV and above
Generally, the greater the capacity for transmitting electrical energy, the higher the voltage level of the line used. Utilizing ultra-high voltage transmission can effectively reduce line losses, lower the cost per unit of transmission capacity, minimize land occupation, and promote environmental sustainability, thereby making full use of transmission corridors and providing significant economic and social benefits.
Based on the number of circuits, lines can be classified as single-circuit, double-circuit, or multi-circuit lines.
Based on the distance between phase conductors, lines can be categorized as conventional lines or compact lines.
Post time: Oct-31-2024
