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
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.
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.
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.
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
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.