Classification of power lines
There are multiple type of powerlines as their transmission voltage can range between 60 V for phone lines and more than 1000000 V for high power transmissions. Although different types of current are used.
Three-phase AC powerlines
Nearly all power lines in the world are three-phase AC. A table with the most common properties for identification of line types can be found here
|Designation||Voltage-Range||Type of pylons used||Ending points||Type of substations used||Wire count per system||Type of wires||Ground conductor use||Recommended classification|
|Low-voltage line||0 – 1000 V||Wooden, concrete, steel-tube, lattice poles, also poles on rooftops||Indoor substations, poles ( not surrounded by switchyard), walls of buildings||Indoor||4||single||no||minor_line|
|Medium-voltage line||1000 V – 50000 V||Wooden, concrete, steel-tube, lattice poles||Indoor substations, poles ( not surrounded by switchyard)||Indoor, pylon transformer||3||single||normally no, but exceptions exist||minor_line (up to about 45 kV only), line|
|High-voltage line||50000 V – 200000 V||lattice towers, sometimes steel-tube towers or wooden poles||Outdoor substations, rarely termination towers ( sometimes surrounded by switchyard) or indoor substations||Outdoor, indoor rarely||3||single ( double, triple or quadruple rarely)||normally yes||line|
|Extreme-high-voltage line||> 200000 V||lattice towers, sometimes steel-tube towers ( until 500 kV) or wooden poles ( until 345 kV)||Outdoor substations, rarely termination towers ( sometimes surrounded by switchyard) or indoor substations||Outdoor, indoor rarely||3||2 –8, single rarely > 250 kV||normally yes||line|
Power line towers (or in some places, poles) can carry multiple systems with different voltages. For classification as “line” or “minor_line”, the highest voltage is relevant. If a pylon designed for a high-voltage line carries just circuits of medium-voltage lines, it should be classified as “line” with the voltage value. If this value is not-known, choose “20 kV” or “15 kV”.
Sometimes the line towers of lines for more than 1 kV carry an insulated communication cable, which can be either a separate cable or be part of the ground conductor. On some lines built by EVS ( now EnBW) the communication cable hangs like a garland on the ground conductor or a separate rope.
Single-phase AC lines
In Germany, Switzerland, Austria, Norway, Sweden and the east coast of the United States, electric trains uses single-phase alternating current with a frequency other than those on the public power grid. These railways are either supplied from the public grid by converters in the substations of the railway or by a separate single-phase AC grid fed by their own power stations and converters from the public grid. These lines use 2 conductors per system and nearly always use ground conductors. Although used for railway purposes, such lines often do not run along railway lines. In some cases the lines are installed on the pylons/supports of the overhead wire of the railway, which is notably used in Amtrak's (ex-Pennsylvania Railroad) 138 kV single-phase transmission lines in the Northeast Corridor providing 12 kV 25 Hz traction power, where most run along the main line catenary supports.).
The following voltages and frequencies are in use:
- 110 kV, 16.7 Hz, in Germany and Austria
- 55 kV, 16.7 Hz, in the S-Bahn in Vienna (Austria), and Norway
- 66 kV, 16.7 Hz, in Switzerland
- 132 kV, 16.7 Hz, in Switzerland and Sweden
- 27.5 kV, 25 Hz, in Mariazeller Bahn, Austria
- 24 kV, 138 kV, 25 Hz, in the Northeast Corridor and part of the Philadelphia to Harrisburg Main Line (by Amtrak, then Pennsylvania Railroad) and suburban lines of SEPTA, in the United States)
Conductors are normally single, but lines with bundled conductors also exist, usually supplying power for high-speed railway lines (such as for the 15 kV 16.7 lines serving ICE trains in Germany)
Another type of single-phase lines called Single Wire Earth Return is often used in sparsely populated rural areas. Since it consists of just one wire and can have large distance between the power poles it allows for low cost electrification of areas with few consumers. The voltage is typically 12 kV or 19 kV.
For long-distance transmission and for sea-cable transmissions often DC is used. A good list of realized DC lines can be found on http://en.wikipedia.org/wiki/List_of_HVDC_projects DC lines usually have 2 conductors, however schemes with 1 or 3 conductors also exist. A special feature of DC lines is that at some schemes ground-return is used, wherefore the installation of a sophistically constructed special electrode is required, in order to avoid undesired electrochemical corrosion. These electrodes are connected with the line terminals by special lines, the such called electrode lines. These lines can but most not be installed on the towers of the main line, where they can serve as ground conductor, if they are mounted on insulators with lightning arrestor above the conductors on the main line. If electrode lines are installed on a separate line route, this line often looks like a medium voltage line, however with 1 or 2 conductors, but some electrode lines are installed on towers for higher voltages. While DC lines normally use as three-phase AC lines of comparable voltage ground-conductors and multiple wires, electrode lines use single wires and no ground conductors, although exceptions exist.
In some cases line towers carry power lines for different types of current. The following combinations are possible
- DC – single-phase AC
- DC – three-phase AC
- Single-phase AC – three-phase AC
- DC – single-phase AC – three phase AC
So far no lines for with DC and single-phase AC on the same towers are realized. Lines with DC and three-phase AC are extremely rare. Only the following examples were realized:
- Switchyard Volgograd-Hydroelectric Power Plant ( HVDC Volgograd-Donbass and 500 kV-lines): 48.8276097N 44.6677902 E - 48.8255471 N 44.6748617 E
- Electrode line of HVDC Square Butte on 230 kV line: 46.924703 N 92.69731 W - 46.775461 N 92.295005 W
- Electrode line of HVDC CU on 230 kV line: 47.3766768 N 101.164063 W - 47.3786166 N 101.1680863 W
- Electrode line of HVDC CU on medium-voltage line: 47.3744681 N 101.1632154 W - 47.375943 N 101.1632905 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.3123671 N 118.483134 W - 34.3144624 N 118.4909085 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.3090196 N 118.490344 W - 34.2813563 N 118.477071 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.2797533 N 118.4790281 W - 34.2301203 N 118.5432844 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.2799661 N 118.4790367 W - 34.2301417 N 118.543499 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.2285002 N 118.5437708 W - 34.1797571 N 118.5430871 W
- Electrode line of HVDC Pacific Intertie on 230 kV line: 34.1781036 N 118.5430229 W - 34.0693651 N 118.4889834 W
Lines with three-phase AC and single-phase AC circuits using the same towers are however common in Germany and Switzerland.
Phone lines can be implemented either as insulated cable carried by ( usually wooden) poles or as overhead line with bare wires on usually wooden poles. The latter is mostly in industrialized countries , usually only in use on lines built along non-electrified railways. The count of conductors installed on these poles is normally a multiple of 2, although single conductor lines also exist. In some cases 20 or more conductors exist.
Phone lines with insulated cables on wooden poles are common in rural areas for connections to farm yards and other remote customers.
Radio frequency lines
At some transmitters for long-, medium- and shortwave the power transmission from the transmitter building to the antenna is performed by a special coaxial overhead line. It consists of multiple wires, which are fixed on insulators inside a ring of multiple wires. As support structures for these line concrete or steel poles are used.
Some antennas like T-antennas or directional shortwave antennas look also like powerlines. Also antenna wires carried by rock anchors exist. Feeding lines of ground dipole antennas used for ELF-transmission like the Russian ZEVS look like ordinary powerlines for voltages between 10 kV and 100 kV, but may have only 1 or 2 conductors.