HVDC - High Voltage Direct Current
A high-voltage, direct current (HVDC) electric power transmission system (also called a power super highway or an electrical super highway) uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems.[5] 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 charge and discharge the cable to 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 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 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.
Power Transmission was initially carried out in the early 1880s using Direct Current (DC). With the availability of transformers (for stepping up the voltage for transmission over long distances and for stepping down the voltage for safe use), the development of robust induction motor (to serve the users of rotary power), the availability of the superior synchronous generator, and the facilities of converting AC to DC when required, AC gradually replaced DC. However in 1928, arising out of the introduction of grid control to the mercury vapour rectifier around 1903, electronic devices began to show real prospects for high voltage direct current (HVDC) transmission, because of the ability of these devices for rectification and inversion. The most significant contribution to HVDC came when the Gotland Scheme in Sweden was commissioned in 1954 to he the World's first commercial HVDC transmission system. This was capable of transmitting 20 MW of power at a voltage of -100 kV and consisted of a single 96 km cable with sea return.
1) Technical Merits of HVDC:
The advantages of a DC link over an AC link are:
⚫A DC link allows power transmission between AC networks with different frequencies or networks. which cannot be synchronized, for other reasons.
⚫ Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of a DC overhead line or cable. The conductor cross section is fully utilized because there is no skin effect.
For a long cable connection, eg. beyond 40 km, HVDC will in most cases offer the only technical solution because of the high charging current of an AC cable. This is of particular interest for transmission across open sea or into large cities where a DC cable may provide the only possible solution.
1 A digital control system provides accurate and fast control of the active power flow. 2 Fast modulation of DC transmission power can be used to damp power oscillations in an AC grid and thus improve the system stability. 2) Economic considerations:
For a given transmission task, feasibility studies are carried out before the final decision on implementation of an HVAC or HVDC system can be taken. Fig. 1 shows a typical cost comparis
curve between AC and DC transmission considering
⚫AC vs. DC station terminal costs
AC vs. DC line costs
AC vs. DC capitalized value of losses
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