From AC to DC: Going green with supergrids - environment - 11 March 2009 - New Scientist
The idea of using DC rather than AC for conventional power grids is being seriously considered for the first time since the Edison-Tesla feud. At that time, Edison favoured the mathematically and conceptually simpler DC despite the complexity of the equipment needed to drop voltages from transmission levels to those required for household use. This conversion problem limited the voltage that could be used in for transmission and, as a result, made the approach practical only for transmission over short distances.
Enter HVDC (High-Voltage Direct Current), currently being considered by both Europe and the US as a replacement for their current grid because, unlike the current AC-based grid highly variable energy sources like solar and wind would not affect the grid significantly. In AC systems, adding energy to the grid requires load-balancing to ensure that the power cycles are maintained. HVDC is currently used in situations where large amounts of power need to transmitted for unusually long distances, such as between New Zealand and the North and South islands. In these cases DC was favoured because DC, in fact, has much lower transmission loss over long distances, provided the voltages are sufficiently high.
All that being said, there are a number of technical challenges that need to be overcome before HVDC grids can be moved from specialty to general-purpose applications. Also, it is uncertain how much transforming our current system to HVDC would cost, despite optimistic studies that state the costs are not excessively high.