Since the beginning of the twentieth century, alternating current (ac) of frequency 50 or 60 Hz has been the dominant method for transmission and distribution of electric power. However, high-voltage direct-current (HVDC) transmission can provide economic solutions for very long distances and enables interconnections between unsynchronized ac networks to be realized. Worldwide, HVDC schemes totaling 59 GW with individual scheme ratings between 50 and 6300 MW have been installed, and many more applications for this technology are being considered.