High Voltage direct current (HVDC) technology
An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current.
"Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice"
Thyristors are controllable semiconductors that can carry very high currents and can block very high voltages. They are connected is series to form a thyristor valve, which allows electricity to flow during the positive half of the alternating current voltage cycle but not during the negative half.
"Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation"
🔺 HVDC transmission lines can either be single pole or bipolar, although most are bipolar, that is, they use two conductors operating at different polarities such as +/-500 kV.
HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation, cross-linked polyethylene. Although synchronous HVAC transmission is normally preferred because of its flexibility, historically there have been a number of applications where HVDC technology has advantages:
1 The need to transmit large amounts of power (>500 mW) over very long distances ( >500 km), where the large electrical angle across long HVAC transmission lines (due to their impedances) would result in an unstable system.
🔺 Examples of this application are the 1,800 mW ABC Project, where the transmission delivers the power to approximately 930 km away; the 3,000 mW system from the Three Gorges project to Shanghai in China, approximately 1,000 km distant; and the 1,456 km long, 1,920 mW line from the Cabora Bassa project in Mozambique to Apollo, in South Africa. In the United States the 3,100 mW Pacific HVDC Intertie (PDCI) connects the Pacific Northwest (Celilo Converter Station) with the Los Angeles area (Sylmar Converter Station) by a 1,361 km line.
2. The need to transmit power across long distances of water, where there is no method of providing the intermediate voltage compensation that HVAC requires.
3. When HVAC interties would not have enough capacity to withstand the electrical swings that would occur between two systems.
4. The need to connect two existing systems in an asynchronous manner to prevent losses of a block of generation in one system from causing transmission overloads in the other system if connected with HVAC.
5. Connection of electrical systems that operate at different frequencies. These applications are referred to as back-to-back ties. An example is HVDC ties between England and France.
6 Provision of isolation from short-circuit contributors from adjacent systems since dc does not transmit short-circuit currents from one system to another.
There is increasing interest in the use of HVDC technology to facilitate the new markets.
HVDC provides direct control of the power flow and is there-fore a better way for providing contractual transmission services. Some have suggested that dividing the large synchronous areas in the United States into smaller areas interconnected by HVDC will eliminate coordination problems between regions, will provide better local control, and will reduce short-circuit duties, significantly reducing costs.