<P> A high - voltage, direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems . For long - distance transmission, HVDC systems may be less expensive and suffer lower electrical losses . For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle . For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits of direct current links . HVDC uses voltages between 100 kV and 1,500 kV . </P> <P> HVDC allows power transmission between unsynchronized AC transmission systems . Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power . HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz . This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks . </P>

Difference between high voltage ac and high voltage dc
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