WikiProject Power networks/HVDC
An HVDC is a powerline 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 https://en.wikipedia.org/wiki/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.