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DC lines are most useful when connecting large power systems across long distances. You would want the DC intertie to isolate both systems from frequency fluctuations in either system. DC lines themselves don't have many losses, but the converter stations on either end introduce more losses than a standard high voltage AC transformer. You only use DC if you have no other choice. DC lines also can be very expensive, due to all the equipment it takes for the conversion, and the operating costs are very high. Another problem with DC lines is that it is more difficult to interrupt fault currents. I am all for alternative energy, but you need to build the wind farms close to load centers. A really good solution would be to use wind in conjunction with pumped storage and nuclear(for base load). Again, all this would best be built near large load centers. Read about TVA's Raccoon mountain facility.