SUMMARY

VSC HVDC systems have a lot of interesting properties of which the most interesting is the possibility to make connections in or between networks by low weight extruded cables. These connections can be made at a cost that can even be comparable with overhead ac lines. In addition to full power flow control in both directions, the VSC HVDC systems can prevent fault propagation, increase low frequency stability, reduce network losses and increase voltage stability. These attractive features open interesting markets and applications where power shall be transported over long distances at large depths under demanding circumstances and where network stability, safety and reliability are of the highest priority. Today a maximum power handling of the system of at least 1000 MW at ±300 kV dc is attainable, thanks to synchronized development in four major technical areas: IGBT valves, DC cable system, main circuit of converter station and control system. This paper presents the development in those four areas. The VSC valves have increased current handling, lower losses, better cooling and more even voltage sharing so they may be built for drastically higher power rating than previously. A new larger presspack IGBT gives higher converter current handling. The cable system has been expanded in voltage and laying depth. Robust flexible and stiff joints have been developed for the polymeric extruded cables. Testing has proved 300 kVdc capability of the cable syste