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HVDC (High Voltage Direct Current)

A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems.

The HVDC technology can be basically represented by the combination of a DC (Direct Current) circuit with two power electronics converters, each one at a link terminal, for AC/DC and DC/AC conversion. The DC circuit can consist of a cable, a line or simply a capacitor. The first HVDC installations date back to 50's; nowadays, HVDC technologies are worldwide widespread and used counting on a long operational experience.

In fact, this technology exhibits characteristics that have already made it widely attractive over High Voltage Alternating Current (HVAC) transmission for specific applications, such as long distance power transmission, long submarine cable links and interconnection of asynchronous systems. Thanks to its speed and flexibility, the HVDC technology is able to provide the transmission system with several advantages such as: transfer capacity enhancement, power flow control, transient stability improvement, power oscillation damping, voltage stability and control, rejection of cascading disturbances, absence of reactive power.

Recent advances in power electronics, coupled with HVDC traditional features, may lead to a further deployment of this technology to improve system operation and support the development of onshore and, possibly, offshore European transmission grids.

The main requirement in a power transmission system is the precise control of active and reactive power flow to maintain the system voltage stability. This is achieved through an electronic converter and its ability of converting electrical energy from AC to DC or vice versa. There are basically two configuration types of three-phase converters possible for this conversion process, Current Source Converters (CSC) and Voltage Source Converters (VSC). Modern HVDC transmission systems can utilize either traditional CSC or VSC as the basic conversion workhorse.


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