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 | Type of pylons | Ending points | Type of substations | Conductors per system | Type of conductors | Ground conductor | 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 rarely, quadruple) | 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 Hz 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.
DC lines
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.
Hybrid lines
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.
Additional note on HVDC lines
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An HVDC is a power line where for transmission DC instead of AC is used. As DC cannot be transformed like AC, AC has to be transformed to the desired voltage level, rectified and at the end of the line converted into AC again. As this requires expensive equipment the use of DC for power transmission is normally done only in the following case:
- interconnecting power grids of different frequencies or phase counts or which are not synchronized
- very long ( > 500 km) overhead lines
- very long ( > 50 km) cables
For the first case also plants are used, in which the conversion process into DC and back again into AC is done in the same plant. One calls these plants "back-to-back stations".
HVDC transmissions are either monopolar or bipolar. At a monopolar transmission one pole is grounded and sometimes even the ground is used as second conductor while the other has a high potential against ground. At bipolar lines one pole has positive potential to ground and the other negative. The ground or a conductor grounded at a single point, which is called metallic return, is used as common return of both poles.
The location of grounding points, which can be situated on land or in the sea, must be chosen very carefully, in order to prevent electrochemical corrosion. Therefore lines, the such called electrode lines, between the grounding point and the converter stations are required. The voltage level of these lines is not constant. It is the product of line current and ground resistance plus resistance of electrode line. In ideal systems it would be o volts, but in practice it may got values until 2 kV.
Electrode lines can be realized as overhead line or as underground cable. They can be installed on separate towers or use the towers carrying the high-voltage pole(s) of the HVDC transmission where they can, as they run to ground, also serve as ground conductor, whereby they have to be fixed on insulators equipped with lightning arrestors. In some cases (HVDC Pacific-Intertie, HVDC CU, HVDC Square Butte and HVDC Vancouver Island), the electrode line is installed on towers of an AC line. The installation of conductors of the high voltage poles of DC lines on AC towers was so far only realized in the switchyard of HVDC Volgograd-Donbass at Volgograd and at HVDC Kontiskan at Stenkullen, Sweden, whereby latter towers do not exist any more. However in China HVDC Hubei - Shanghai and HVDC Gezhouba - Shanghai, use in most part of its length the same towers.
As switches for HVDC are poorly available, so far HVDC is used nearly only for point-to-point transmissions and most realized systems consist of just 2 stations interconnected by the line. There are also a few multiterminal transmissions, but at no realized scheme more than 2 HVDC lines depart.
A good list of HVDC schemes can be found on list of HVDC projects. Most of these schemes are mapped at least in overhead sections already on OpenStreetMap. However no map of all existing HVDC schemes exist in the internet as there are no detailed maps of all schemes or groups of schemes.
Phone lines
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.