Lessons Learned

HVdc Transmission and Integration into an AC Grid

High voltage, direct current (HVdc) transmission has been in commercial operation since the 1950's. The vast majority of HVdc systems have two terminals, one acts as a rectifier and transfers power from an AC system to the DC system and the other acts as an inverter and transfers power from the dc system to ac system. Some HVdc system always transfer power the same direction from a remote power source to a load center. Others can transfer power in either direction at different times of the year with the converters exchanging rectification and inversion roles. The dominant technology used for HVdc has been line current commutated (LCC) systems utilizing thyristor-based technology. LCC systems require significant reactive support from the ac system at the rectifier and inverter terminals as well as current harmonic filtering. LCC HVdc systems are capable of high capacity in terms of voltage levels and power flows. There are LCC systems in operation today at voltage levels of ±800 kV (one conductor at +800 kV and the other at -800 kV) and 8000 MW. In addition, there are plans for future HVdc systems operating at ±1100 kV and 10,000 MW. More

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