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    Basic knowledge of power transmission line

    Transmission line.png

    一、 Main equipment of power transmission line:

    The power transmission line is a power facility that uses insulators and corresponding hardware to suspend conductors and overhead ground wires on

    poles and towers, connect power plants and substations, and achieve the purpose of power transmission. It is mainly composed of conductor, overhead

    ground wire, insulator,hardware, tower, foundation, grounding device, etc.

    1. Conductor: its function is mainly to transmit electric energy. The line conductor shall have good conductivity, sufficient mechanical strength, vibration
    fatigue resistance and resistance to corrosion of chemical impurities in the air. It shall be a bundled conductor type composed of two or four conductors
    per phase.

    2. Overhead ground wire: mainly used for lightning protection. Due to the shielding of overhead ground wire to the conductor and the coupling between
    the conductor and overhead ground wire, the chance of lightning directly striking the conductor can be reduced. When lightning strikes the tower, part 

    of the lightning current can be diverted through the overhead ground wire, thus reducing the tower top potential and improving the lightning withstand 

    level. The overhead ground wire is usually galvanized steel strand. At present, good conductors such as steel cored aluminum strand and aluminum clad

    steel strand are often used to reduce power frequency overvoltage and secondary arc current in case of asymmetric short circuit. Optical cable composite

    overhead ground wire shall be used for those with communication function.

    3. Insulator: It refers to the object that fixes and suspends the conductor on the tower. Common insulators for power transmission lines include: disc 

    porcelain insulator,disc glass insulator and rod suspension composite insulator.

    (1) Disc porcelain insulator: domestic porcelain insulator has a high deterioration rate, which requires zero value detection and heavy maintenance. In

    the event of  lightning strike and pollution flashover, it is easy to cause string dropping accidents, which has been phased out.

    (2) Disc glass insulator: It has zero value self explosion, but the self explosion rate is very low (usually several ten thousandths). No inspection is required

     for maintenance. In case of self explosion of tempered glass, its residual mechanical strength still reaches more than 80% of the breaking force, and the safe

     operation of the line can still be ensured. In the event of lightning strike and pollution flashover, there will be no chain drop accident. It has been widely used

     in Grade I and II sewage areas.

    (3) Rod suspension composite insulator: it has the advantages of good anti pollution flashover performance, light weight, high mechanical strength, less 

    maintenance, etc., and has been widely used in Grade III and above pollution areas.

    4. Hardware

    Power transmission line fittings can be divided into: clamp type, connection fittings, connection fittings, protective fittings and pull wire fittings according 

    to their main performance and use.

    (1) Clamp type: suspension clamp: used to fix the conductor on the suspension insulator string of the tangent pole and tower, or hang the overhead ground

     wire on the overhead ground wire support of the tangent pole and tower.Strain clamp: it is used to fix the conductor or overhead ground wire on the strain 

    insulator string for anchoring. There are three categories of strain clamps, namely: bolt type strain clamps; Compression type strain clamp; Wedge clamp. 

    Bolt type strain clamp: it is used to fix the conductor by the friction effect generated by the vertical pressure of U-shaped screw and the wavy groove of the

    clamp. Compression type tension clamp: it is composed  of aluminum tube and steel anchor. The steel anchor is used to connect and anchor the steel core 

    of the steel cored aluminum strand, and then cover the aluminum tube  body to make the metal plastic deformation by pressure, so that the wire clamp and

    the conductor are combined as a whole. When hydraulic pressure is used, the steel mold with corresponding specifications shall be used for compression 

    with a hydraulic press. When explosive pressure is used, the wire clamp and conductor (overhead ground  wire) can be pressed into a whole by means of

    primary explosive pressure or secondary explosive pressure.Wedge clamp: used to install steel strand and fasten the stay wire of overhead ground wire 

    and stay tower. It uses the splitting force of wedge to lock the steel strand in the clamp.

    (2) Connecting hardware: connecting hardware is used to connect insulator string and tower, wire clamp and insulator string, overhead ground wire 

    clamp and tower. Commonly used connection hardware includes ball head hanging ring, bowl head hanging plate, U-shaped hanging ring,right angle 

    hanging plate, etc.

    (3) Connection fittings: used for the connection of conductors, overhead ground wires and jumpers of tension poles and towers. The finalized connection

     fittings include:clamp pressure connection fittings, hydraulic connection fittings, bolt connection fittings, explosive pressure connection fittings.

    (4) Protective hardware: shockproof hammer, armour rod and damping wire used to protect conductor and overhead ground wire from vibration;Spacer

     used to suppress subspan vibration; Shielding ring and grading ring used to protect insulator string from corona.

    (5) Hardware for stay wire: hardware for adjusting and stabilizing tower stay wire include: adjustable UT type clamp; Steel wire clamp, and double pulling

     wire connecting plate, etc.

    5. Tower:

    Towers are used to support overhead line conductors and overhead ground wires, and to ensure that there is sufficient safety distance between conductors

     and conductors, between conductors and overhead ground wires, between conductors and towers, and between conductors and the earth and crossing objects.

    6. Foundation:

    The foundation is mainly used to stabilize the tower and can bear the uplift force, downforce and overturning moment generated by various loads of tower,

     conductor and overhead  ground wire.Prefabricated fabricated foundation shall be used for poles and stay wires. Cast in situ reinforced concrete foundation

     or concrete foundation should be used for iron  tower. If possible, the undisturbed foundation shall be preferred. Including: rock foundation, mechanically 

    expanded pile foundation,cut (half cut) foundation, explosive expanded pile foundation and bored pile foundation.

    7. Grounding device:

    It is mainly composed of the grounding downlead connecting the overhead ground wire and the grounding body (pole) buried in the tower ground.The 

    main function of the grounding  device is to rapidly diffuse and discharge the lightning current in the earth, so as to maintain a certain lightning withstand

    level of the line. The smaller the grounding resistance of  tower, the higher the lightning withstand level.

    二、 Terminology of power transmission lines

    1. Span: the horizontal straight distance between two adjacent towers, called span, is generally expressed in L.

    2. Sag: for horizontally erected lines, the vertical distance between the horizontal connecting line between two adjacent suspension points of the conductor

     and the lowest point of  the conductor is called sag or sag. Expressed by f.

    3. Distance limit: the minimum distance between the conductor and the ground or the crossed facilities. The minimum allowable distance from the lowest

     point of the general guidance line to the ground, usually expressed in h.

    4. Horizontal span: half of the sum of two adjacent spans is called horizontal span, which is commonly expressed as.

    5. Vertical span: the horizontal distance between the lowest points of the conductor between two adjacent spans, which is called vertical span and is

     usually expressed.

    6. Representative span: in a tension section, there are often multiple spans except for arc vertical spans. Due to different terrain and ground objects crossed

     by the conductor, the size of each span is not equal, the elevation of the suspension point of the conductor is also different, and the stress of the conductor 

    in each span is also different. However, the stress  and sag of the conductor are closely related to the span. When the span changes, the stress and sag of the

     conductor also change. If each span is calculated one by one, the mechanical calculation of the conductor will be difficult. However,the conductors of the 

    same phase in a tension section are tightened together during construction. Therefore, the horizontal tension of the conductor is equal in the whole tension

     section, that is, the conductor stress at the lowest point of the sag of each span is equal. We replace a multi span tension section with an equivalent imaginary

    span. This imaginary span that can express the whole mechanical law of tension is called representative span or regular span, and is represented by LO.

    7. Tower height: the vertical distance from the highest point of the tower to the ground, called tower height. It is indicated by H1.

    8. Tower nominal height: the vertical distance from the lowest cross arm of the tower to the ground is called tower nominal height, which is referred to as

     nominal height and is expressed in H2.

    9. Height of suspension point: the vertical distance from the suspension point of the conductor to the ground, which is called the height of the suspension

     point of the conductor and  is represented by H3.

    10. Line to line distance: the horizontal distance between two phases of conductors, called line to line distance, expressed in D.

    11. Root opening: the horizontal distance between the roots or tower feet of two electric poles, called root opening. It is represented by A.

    12. Protection angle of overhead ground wire: the included angle between the external connecting line of overhead ground wire and side conductor and

    the vertical line of overhead ground wire is called the protection angle of overhead ground wire. Expressed in.

    13. Burial depth of pole and tower: The depth of the electric pole (tower base) buried in the soil is called the buried depth of the pole and tower. It is expressed in h0.

    14. Jumper: the lead connecting the conductors on both sides of the load-bearing tower (tension, corner and terminal tower) is called jumper, also called 

    drain wire or bow wire.

    15. Initial elongation of conductor: permanent deformation (stretching along the axis of conductor) caused by the initial external tension of conductor is called

     initial elongation of conductor.

    16. Bundled conductor: one phase conductor is composed of multiple wires (2, 3, 4), which is called bundled conductor. It is equivalent to thickening the "

    equivalent diameter" of the  conductor, improving the electric field strength near the conductor, reducing corona loss, reducing radio interference,and improving

     the transmission capacity of the transmission line.

    17. Conductor transposition: the conductor arrangement of the power transmission line, except for the regular triangle arrangement, the distance between the

     three conductors is not equal. The reactance of the conductor depends on the distance between lines and the radius of the conductor.Therefore, if the conductor

     is not transposed, the three-phase impedance is  unbalanced. The longer the line is, the more serious the imbalance is.As a result, unbalanced voltage and current

     will be generated, which will adversely affect the operation of the generator and radio communication.The power transmission line design specification stipulates

     that "in the power network with the neutral point directly grounded, the power transmission line with a length of more than 100km shall be transposed".

     Conductor transposition is generally carried out in transposition tower.

    18. Conductor (ground) line vibration: in the line span, when overhead lines are subjected to the wind force perpendicular to the line direction,a stable vortex

     with a certain frequency  alternating up and down will be formed on the leeward side of overhead lines. Under the effect of the vortex lift component, the overhead

     lines will produce periodic oscillation in their vertical plane, which is called overhead line vibration.