HVDC enables secure and stable asynchronous interconnection of power networks that operate on different frequencies, or are otherwise incompatible. In addition, HVDC provides instant and precise control of the power flow.
Once installed, HVDC transmission systems become an integral part of the electrical power system, improving the overall stability, reliability and transmission capacity.
A number of HVDC links interconnect two AC systems that are not synchronous. When AC systems are to be connected, they must be synchronized. This means that they should operate at the same voltage and frequency, which can be difficult to achieve. Since HVDC is asynchronous it can adapt to any rated voltage and frequency it receives. Hence, HVDC is used to connect large AC systems in many parts of the world.
For example, the Nordel power system in Scandinavia is not synchronous with the UCTE grid in western continental Europe, even though the nominal frequencies are the same. And the power system of the eastern USA is not synchronous with that of western USA, Texas or Quebéc. There are also HVDC links between networks with different nominal frequencies (50 and 60 Hz) in for example Japan and South America.
There are no technical limits to the potential length of a HVDC cable. In a long AC cable transmission, the reactive power flow due to the large cable capacitance will limit the maximum possible transmission distance. With HVDC there is no such limitation; this is why, for very long cable links, HVDC is the only viable technical transmission alternative.
The 580-km long, ABB-built NorNed link is the world’s longest submarine high-voltage cable. It runs from southern Norway, crosses the North Sea and lands in The Netherlands. Read more about the NorNed link here.
Several HVDC links with very long submarine cables are being considered today, mainly in Europe. One example is Iceland - Europe.
A fundamental advantage of HVDC technology is the ease of controlling active power in the link.
In most HVDC links, the main control is based on constant power transfer. This property of HVDC has become more important in recent years, given the shrinking margins of power networks as electricity markets in many countries are deregulated.
In many cases, an HVDC link can also improve the performance of AC power systems by means of additional control facilities. Normally these controls are activated automatically as certain criteria are fulfilled. Automatic HVDC control functions include constant frequency control, redistribution of the power flow in the AC network, damping of power swings in the AC networks, etc. In many cases such additional control functions can make possible the safe increase of power transmission capability in AC transmission lines where stability is a limitation.
Today's advanced semiconductor technology, used in both power thyristors and microprocessors for control systems, has created almost unlimited control possibilities in HVDC transmission systems. Different software programs are used for different studies supporting these control options.
Normally a positive sequence program, for example PTI’s PSS/E program is used for load-flow and stability studies. For more detailed investigations of the performance of inner control loops of the converter and its interaction with a nearby network, a simulation is created in a full three-phase representation program such as PSCAD/EMTDC.
An HVDC transmission does not contribute to the short circuit current of the interconnected AC system.
When a high-power AC transmission is constructed from a power plant to a major load center, the short circuit current level will increase in the receiving system. High short circuit currents are becoming an increasingly difficult problem for many large cities, which may result in the need to replace existing circuit breakers and other equipment if the rating is too low.
But if new generating plants are connected to the load center using a DC link, the situation is quite different. The reason is an HVDC transmission does not contribute to the short circuit current of the interconnected AC system.