A change in direction – High voltage DC transmission and a continuation of Jack Harris’ discussion on AC versus DC supply
An American friend of mine read my September column and was upset to discover that her childhood hero Thomas Edison had been instrumental in persuading her country's adoption of the ghastly electric chair to execute prisoners. I had kept from her that an associate of Edison organised public demonstrations of the electrocution of dogs as part of the campaign to demonise AC current. I have recently learnt that Edison electrocuted an elephant and made a film of the event.
This month, as the final of my three-part coverage of AC versus DC, I would like, in a sense, to support Edison and draw attention to the virtues of DC supply, as well as to describe how high voltage DC transmission systems (HVDCs) may play a part in combating global warming.
As described last month, the ease with which AC can be raised to high voltages, transmitted over considerable distances and then stepped down to more practical voltage levels has resulted in its adoption for most national grid systems. However, the losses associated with this form of transmission, due to the inductance and capacitance of the circuit, can be quite large, particularly if the surrounding environment is conducting, such as water or metal pipes.
High voltage DC transmission systems
In the 1930s, the development, mainly in Sweden, of the high power mercury arc valve enabled high power currents to be rectified, transmitted over a high voltage DC line, and then inverted back to AC. A HVDC cable under the English Channel has joined the British and French electrical systems together since 1962. Today, rectifier-inverter systems are mostly thyristor based, and are becoming economic for transmission distances of over 700km by overhead cable, or 50km under the sea.
In his book Heat, George Monbiot points out how HVDC may find application for off-shore wind turbines because they could be 100km from the shore if connected by undersea HVDC cables. Not given to understatement, Monbiot concludes, ‘I am beginning to believe that HVDC cables could change the world'.
Dr Jürgen Schmid of the University of Kassel, Germany, is advocating a European-wide HVDC network to link-up the output of wind turbines. With such a system, where the wind is blowing would be irrelevant because it would always be blowing somewhere. If it was windy in Germany but not, for once, in Scotland, the current would flow towards Scotland, and, of course, vice-versa. Schmid has calculated that such a grid would meet at least 30% of Europe's power needs and could cover base-load requirements for the continent.
In Schmid's vision there is a special role for Norway, which has a superabundance of high level lakes which could act as pumped storage reservoirs (playing the same role as our Dinorwic scheme). When the wind is blowing all over Europe, the excess energy would pump water to the upper levels, to be released to drive hydro turbines as and when needed. Apparently, the capacity of Norway's high level reservoirs is so large that, should there be no wind in Europe, the hydro plants could meet Europe's energy requirements for up to a month. A group of Norwegian companies have started building a HVDC link between Scandinavia, The Netherlands and Germany - this could be the start of something exciting.