Power line is a metal conductor on which an electric current passes. Now worldwide alternating current (AC) is used. In most cases power supply is three-phase therefore the power line, as a rule, consists of three phases, each of which can include some wires. Designly power lines are deviede into overhead power lines and cable transmission lines.
The overhead power lines are placed over the earth surface at the safe height on special facilities, called supports. As a rule, the wire on overhead power lines has no surface insulation; insulation is at fixing the supports. On overhead power lines there are lighting protection systems.
- the relative low cost in comparison with the cable;
- repairability, is is not nesessary to carry out excavation for wire replacement, there is a direct view of the line condition.
High voltage line shortcomings:
- wide right-of-way: it is forbidden to put any constructions and to plant trees close to the high voltage line; trees across the width of the right-of-way are cut down when a line is laid through the wood;
- vulnerability to external influence: falling of trees and theft of wires. Because of vulnerability, one overhead power line is often equipped with two chains: the main and reserve;
- esthetic unattractiveness is one of the reasons of almost ubiquitous transition to a cable way of an electricity transmission in city line.
The Cable transmission lines are carried out underground. Electric cables have a various design, however it is possible to reveal common elements. Core of a cable are three electrically lead veins (on number of phases). As a rule, the lead cable core is protected by steel armor. From outer side the cable is covered with bitumen.
There are collector and brushless PM cable transmission lines.
In the first case the cable is laid in underground concrete channels — collectors. Through particular intervals line outputs are equipped on the surface as hatches — for ease of penetration of repair crews to the collector.
Brushless PM cable transmission lines are laid directly in the soil. Brushless Construction of brushless PM cable transmission line is significantly cheaper than one of the collector line, however its maintenance is more expensive because of unavailability of a cable.
- there is no right-of-way. Under condition of rather deep underlay, various constructions (including houses) can be built over the collector line;
- cable transmission lines do not spoil the look of a city landscape;
- reliable protection against external influence.
Shortcomings of cable transmission lines:
- high cost of construction and subsequent maintenance;
- cable transmission lines are less accessible to visual inspection of their condition, besides it's impossible to control brushless PM cable transmission lines at all.