The high voltage DC (HVDC) transmission is big power long distance DC transmission. The technology is used to transmit the electric power via overhead lines and submarine cables. Meanwhile, it can also connect independent electrical systems if traditional AC connection is not workable.
II. HVDC Transmission vs AC Transmission
Characteristic of power transmission: In order to meet the requirements of stability, measures such as series compensation and static compensation are often taken, and even transmission power voltage has to be raised. The DC transmission has no phase and power angle, so it is not necessary to consider the issue of stability. That is an important characteristic of DC transmission, but also an advantage.
Self-protection capability in case of line breakdown: If AC line single phase is grounded, it takes 0.4-0.8sec to eliminate the breakdown. Coupled with switch-on time, the time totals 0.6-1sec. If DC line single polarity is grounded, thyristor valves shut at once and the voltage reduces to zero, which forces the DC current to zero. Hence, it takes 0.2-0.35sec to recover from DC line single polarity breakdown.
Overload capability: The AC transmission line has strong capability to run without pause and its maximum capacity tends to be determined by stability limit. The DC line also has a certain overload capability and it is converter station that will be restricted. Generally speaking, it can be divided into three categories: 2h overload capability, 10sec overload capability and fixed overload capability. As for the overload, AC is more flexible. If the greater overload capability is needed for DC, consideration should be given in advance during the selection of model.
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