T&D World Magazine

New Voltage-Sourced HVDC Converter Technology Introduced

Siemens Power Transmission and Distribution (PTD) launched a high-voltage direct current transmission (HVDC) system on the market, based on a new generation of converters using voltage-sourced converter (VSC) technology. The HVDC Plus system is suitable for direct current links up to the 1,000 MW power range where line-commutated converters are still used exclusively today. In contrast to line-commutated converter technology, the HVDC-Plus system operates with power semiconductors which have both turn-on and turn-off capability. As a result, the commutation processes in the converter are independent from the AC system voltage. Next to other applications the transmission system allows the low-loss transport of electrical energy from offshore wind farms to the coast and the economical and environmentally-friendly supply of power to oil platforms from the AC system on the mainland.

The high-voltage direct-current transmission system HVDC Plus makes use of all the advantages offered by self-commutated voltage-sourced converter technology. This includes grid access to very weak AC systems as well as supplying passive networks. Active and reactive power can be controlled independently. The capability of very rapid control and protection actions of the converter makes the system highly dynamic, which is necessary especially for AC faults and system disturbances. Last but not least, the black-start capability function enables the HVDC system to restart a collapsed network.

HVDC Plus operates with an innovative multilevel converter concept, which offers in comparison to existing VSC solutions additional significant benefits. Amongst others these are low losses due to low switching frequencies, full modular design and therewith a straightforward scalability. In addition to the operation as back-to-back link and as cable transmission, HVDC Plus can also be used in combination with overhead lines.
Offshore wind farms in the power range of a few hundred megawatts usually demand for particularly high requirements of power transmission. Many wind farms are located offshore over a hundred kilometers from the AC system on the coast. This generally exceeds the economical and technical limits of AC-based cable transmission systems and calls for new DC transmission concepts, for example based on the HVDC Plus system.
Oil platforms, which have a high power demand, also require a high level of power quality for the transmission if they are to be supplied from the mainland and not locally as in the past. Power delivery from the mainland not only increases the availability of the electric supply on the drilling rigs but also renders the maintenance and servicing work unnecessary for the small power plants currently used on the platforms. This also eliminates environmentally harmful CO2 and NOX emissions from the small power plants usually used at sea.

Submarine cables are used exclusively for power transmission across the sea. However, the transport of power in the form of alternating current via cable is limited to a length of about 80 to 120 kilometers for technical and economical reasons, depending on the power to be transmitted. For this reason, direct-current transmission is the preferred solution.

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