Insulators

Protection and Insulation of Power Lines

Leakage pathIn dry air sparks can jump 1cm for every 10000VThus a spark can jump from a power line carrying 200 kV and its supporting structure if it is within a distance of 20 cmAn insulator is used to increase the distance between the power line and the supporting structure

Insulators

Insulators must support the conductors and withstand both the normal operating voltage and surges due to switching and lightning.

Insulators are broadly classified as either pin-type, which support the conductor above the structure, or suspension type, where the conductor hangs below the structure.

both types are commonly used for voltages up to about 33 kV. only suspension-type insulators are used at higher voltages

Pin type insulators

Pin type insulatorsIncreases the leakage pathThe greater the voltage the greater the number of discs usedUsually made of porcelain or toughened glass, with increasing use of glass-reinforced polymerThe shape reduces the likelihood of grime building up on the underside or getting wetGrime becomes a conductor when wetThe shape also increases the distance the current must travelThese both reduce the fire hazard

Suspension insulators

Suspension insulatorsmade of multiple units, with the number of unit insulator disks increasing at higher voltages. The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray. Longer insulators, with longer creepage distance for leakage current, are required in these cases. Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.

Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few mA) passes through the insulator. This warms the surface slightly and reduces the effect of fog and dirt accumulation. The semiconducting glaze also insures a more even distribution of voltage along the length of the chain of insulator units.

Lightning ProtectionLightning is a huge static electric spark created during a thunderstorm. Lightning strikes can be damaging to buildings and equipment, as well as dangerous to people.Earth line is a continuous cable which only carries a current if a fault develops

Static dischargers

Static dischargers

Earth or Ground WireThere are usually 2 wires at the top of high voltage transmission lines (or 1 on low voltage) which are connected to the earth Act as a lightning conductor to shield the current- carrying wires below, by intercepting the lightning strikeIf struck, the electricity will be conducted to the earth by the metal tower (or earth wire for wooden poles)Avoids current surges and damage to substations