Classification of power lines
Is based on a number of characteristics, the first of which is the type of current. There are: direct current lines (limited use, because direct current transmission is associated mainly with technical difficulties in creating effective, inexpensive devices to convert alternating current to direct current – at the beginning of the line, and direct current to alternating current – at the end of the line), three-phase alternating current (by the length of overhead lines are the most common in the world), multi-phase alternating current power lines (six- and twelve-phase) – have not been widely used.
One of the main characteristics of transmission lines is their carrying capacity, i.e. the maximum power that can be transferred through the line, taking into account the limiting factors. The power transmitted through a three-phase AC power line is related to its length, voltage and current load. Power transmission lines are subdivided into low-voltage (up to 1 kV) and high-voltage (over 1 kV) lines with a rated voltage of 3-35 kV, high (110-220 kV), ultra-high (330-750 kV) and ultra-high (over 1000 kV). The development of higher voltage levels is caused by the need to transmit increasing power flows to increasing distances and the desire to reduce losses from heating of overhead wires, which are proportional to the square of the current (e.g., the current will increase by 2 times, losses will increase by 4 times).
In terms of the number of parallel circuits laid along the common route, overhead lines can be single-circuit (AC overhead lines with one set, i.e. three phase conductors), double-circuit (overhead lines with two sets of phase conductors) and multi-circuit (overhead lines with more than two sets of phase conductors). In terms of topological characteristics, a distinction is made between radial (power comes from a single source), trunk lines (several branch lines), and branch lines (lines connected at one end to another power line at its intermediate point). In terms of their functional purpose, power transmission lines are distribution (lines of local power networks), feeder (lines of district networks that supply power to power supply centers of distribution networks), as well as system-forming and inter-system lines that directly connect different power systems and are intended for mutual exchange of power both in normal and emergency modes.