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OVERHEAD TRANSMISSION SYSTEM - Coggle Diagram
OVERHEAD TRANSMISSION SYSTEM
GENERAL
overhead transmission systems consist of transmission towers that support high voltage conductors of power lines.
CONDUCTORS
conductors (wires) comprise materials like copper and aluminum that readily conduct electric currents.
conductors used in transmission lines are usually aluminum placed over a steel core for reinforcement (ACSR-Aluminum core steel reinforced).
transmission line conductors are
not insulated
; insulation is provided by the air.
SHIELD CONDUCTORS/EARTH WIRE
earth wires or ground wires are bare conductors supported at the top of transmission towers.
the earth wire does not carry current and is solidly connected to earth at regular intervals.
the purpose of the earth wire is as a form of lightning protection, it intercepts direct lightning strikes which would otherwise strike the phase conductors below.
INSULATORS
conductors are connected to towers via insulators that support the conductors on the tower.
line insulators mechanically support line conductors while electrically isolating them from the support towers.
insulators must withstand normal operating voltage and surges due to switching and lightning.
insulators have commonly been comprised of porcelain or tough head glass, which need routine cleaning to eliminate dust build-up that can lead to insulator flashover and noise. newer insulators are composed of a polymer of silicon, which is lightweight and shattered resistant.
CIRCUITS AND BUNDLED CONDUCTORS
a circuit is made up of three phases (for alternating current).
towers can carry one or more circuits, depending on the design (e.g., single circuit vs double circuit towers).
for voltages up to 200kv, a phase is typically a single conductor (3 wires total per circuit).
for voltages over 200kv, bundled conductors are often used to increase the current-carrying capability of the line and reduce power loss. Bundled conductors consist of two or more conductor cables connected by non-conducting spacers.
COMPONENTS
Tower
: lattice steel tower, with concrete foundation, grounded.
Insulator
: V string holds two bundle conductors in each phase.
Conductor
: each conductor is stranded, steel re-enforced aluminum cable.
Foundation and Groundling
: steel-reinforced concrete foundation and groundling electrodes laced on the ground.
Shield conductors
: two grounded shield conductors protect against lightning.
GALLOPING TRANSMISSION LINES
since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety.
adverse weather conditions, such as high wind and low temperatures, can lead to power outrages.
wind speeds as low as 23 knots (43km/h) can permit conductors to encroach operating clearances, resulting in a flashover and loss of supply.
oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency and amplitude of oscillation.
TRANSMISSION TOWER
transmission towers
support high voltage conductors
of overhead power lines, from the generating station switchyard right up to the source substations and distribution substations located near populated areas.
transmission towers can come in many designs, the most common types are the
Lattice Steel Towers (STL)
and the
Tubular Steel Poles (TSP).
their shape, height and sturdiness (mechanical strength) depend on the stresses to which are exposed.
Towers do not transmit electricity themselves unless lightning strikes the ground wire strung along the top of the structure. This cable is designated to protect conductors by allowing lightning discharges to reach the ground through the tower.