Conversion course

While European governments have been putting significant funding into the construction of offshore wind farms, they will only be a viable power source for consumers if transmission systems are efficient.

Current offshore wind farms are constructed relatively close to the coast, but larger farms planned for the future will need to be built further out to sea — meaning they will need a better method to transfer power.

That's why German engineering company

has invested millions in better power transmission systems for this market, paying particular attention to high-voltage direct current (HVDC) transmission technology, which is an alternative to the currently used alternating-current (AC) systems as a means for the bulk transmission of power.

The huge advantage of HVDC is the ability to transmit large amounts of power over very long distances at much lower capital costs and with greatly reduced power losses than AC.

Recently the company launched HVDC Plus, a system based on a new generation of power converters using voltage-sourced converter (VSC) technology. According to Siemens the system is able to install DC links up to the gigawatt range where conventional, line-commutated converters are still used exclusively today.

This system differs from line-commutated converter technology because it operates with power semi- conductors that can be disconnected. As a result, the commutation processes in the converter run independently of the system voltage.

HVDC technology has undergone constant development since its first commercial applications in the 1950s. The systems are always needed when the conventional method of transporting power in alternating current form comes up against its technical and economic limits.

It is a good choice for offshore farms because of its suitability for under- water cable links. Distances for AC transmission by submarine cables is restricted to 37-50 miles (60-80km) because of the charging and discharging of the cable capacitances and the ensuing energy losses. Current HVDC systems are possible with submarine cable links longer than 400 miles and a power rating of up to a gigawatt. But Siemens says cable lengths of up to 810 miles are already being planned.

HVDC Plus owes much of its improved capabilities to its converter topology, which was created by Modular Multilevel Converter (MMC). The MMC topology used in this system differs in design from current VSC topologies. The individual converter legs of an HVDC Plus system consist of submodules connected in series, each containing a semiconductor switching element connected to a DC storage capacitor. 'Semiconductor elements at each capacitor make it possible to switch the voltage of a given capacitor in a series connection on and off individually,' said Joerg Dorn, head of development for HVDC Plus. 'This enables the converter to generate the AC voltages in small increments.'

He explained that in normal operation, no more than one level per converter leg switches at any given time. As a result, the AC voltages can be adjusted, achieving a DC voltage with very little ripple. This minimises the number of generated harmonics, and in most cases completely eliminates the need for AC filters.

'What's more,' said Dorn, 'the small, relatively shallow voltage jumps that do occur cause very little radiant or conducted high-frequency interference.' He added that the low switching frequency of the individual semi- conductors results in very small switching losses and better efficiency.

The system is also well prepared to handle internal and external faults. Problems such as a short circuit between the two DC poles of the transmission line are reliably managed due to the very small current rise rates.

One of the greatest achievements with HVDC, said Dorn, is its scalability. 'The overall design can be engineered very flexibly,' he said. 'The converter station can be adapted to the local needs, and depending on these, the design can favour a more vertical or more horizontal construction.'