HVDC pumps-up the power

The demand for clean, renewable energy is changing the face of the power industry. For example, to get power from off-shore wind farms, rather than converting wind-generated electricity to high-voltage alternating current (HVAC), high-voltage direct current (HVDC) is a better fit. HVDC is the best option for underground and underwater transmission because the capacitance per unit of ac cable makes it impractical for lengths beyond about 100 km. In addition, direct current (dc) can transmit large amounts of power over long distances, with lower capital costs and lower losses than alternating current (ac). (In electric power transmission engineering, high voltage is usually considered any voltage over approximately 35 kV.)

A typical 600 MW HVDC installation.

HVAC transmission methods arose in the early 1900s because Thomas Edison's dc configuration worked well at low voltages but less well over long distances due to internal resistance (IR) loss. Another downside was that dc required the use of expensive voltage converters rather than simple transformers to get down to end-use voltages. In addition, associated technology, such as dc breakers for multi-terminal dc grids, had fallen behind other developments. (An HVDC grid is called “multi-terminal” when the dc system connects to more than two nodes on the ac network.) In fact, researchers are still trying to design a breaker able to force dc current to zero to avoid dangerous arcing during switching operations.

HVDC became more economical and therefore more attractive in the late 1960s with the development of new technologies which included high-power rectifiers such as mercury arc valves, and, since the mid-1970s, high-power thyristors.

Lack of technology, however, has not stopped Europe from using HVDC. In fact, one of the first commercial HVDC lines was a 98-km-long submarine cable with ground return between the island of Gotland and the Swedish mainland. It was built in 1954.

A screenshot of an example steady state and dynamic modeling of a VSC-based dc network.

Conventional HVDC configurations now fall under the categories of long-distance transmission via overhead lines >1,000 km long; submarine cable — e.g. undersea cable — > 100 km long; or what is called back-to-back transmission. A back-to-back dc facility connects two adjacent ac grids without a dc transmission line. The converters are in the same station. This configuration serves multiple purposes, such as trading power between asynchronous networks, stabilization of different layers of a grid, and control of power interchange.

Basic HVDC control functions

Although the power flow in a single dc link is relatively simple to control, it can be a conceptual challenge to coordinate a complex, multi-terminal dc grid.

Fortunately, PSS/E (power system simulation software for engineers) from Siemens Power Technologies International in the U.S. and in Germany lets users simulate both steady state and dynamic multi-terminal dc installations. The software uses the latest numerical algorithms to efficiently solve both large and small networks.

PSS/E models can simulate long-distance overhead dc line transmission and back-to-back facilities to analyze power flow control, improve system dynamic performance, and relieve congestion. With these models, planners can investigate the impact on steady state and dynamic performance of the systems when an HVDC link is added.