Modern HVDC: state of the art and development trends

After the first commercial HVDC transmission between Gotland and the mainland of Sweden (1954), the whole HVDC technology has experienced a dramatic development. Thyristor valves and IGBT valves have replaced mercury arc valves, air insulated reactors have replaced oil immersed ones. Traditional commutation is being helped by series capacitors, or altogether replaced by PWM technology. Valves are moving out of conventional valve halls to become self-standing, outdoor units. Traditional filters have become double tuned ones and then, for more stringent requirements, on the DC side, active, while on the AC side the move has been towards a high Q factor, electronically tuned filters. Vacuum tubes have replaced relays, to be in turn replaced by transistors, which then progressed to integrated circuits. Functions have been replaced by digital control. The interface for the operators has gone from mimic panels to work stations, greatly improving the flexibility and available information. Fibre optics are replacing more and more of the hard-wired communication in the stations