This factsheet describes the use of HVDC power converters for alternate current/direct current (AC/DC) conversion. HVDC converters are mainly divided between current source converters (CSC) and voltage source converters (VSC) technologies.
CSC have been commercially in use since the 50s, making it a well-established technology, based on the use of high power thyristors. The technology is widely used for back-to-back links between asynchronous grids. Due to its conversion technique, these converters absorb reactive power from the adjacent AC grid, generating harmonic currents having a negative impact on the quality of the electrical power. Therefore, reactive compensation and harmonic filters are required. As a result, the converter depends on the AC system’s voltage to ensure the correct operation. DC current flows unidirectionally, to reverse it in a CSC connection, the DC control system reverses the polarity of the DC voltage. DC cables are still limited to lower voltages and power levels . DC cables power capabilities are limited due to electrothermal heating and problems arising out of possible electrothermal instability of the insulation that could result in rapid failure under certain conditions of operation .
VSC technology is based on the use of Insulated Gate Bipolar Transistors (IGBT). VSC devices are self-commutating, making the operation of converter independent of the AC system’s voltage and a more controllable. An advantage over CSC is that it does not require reactive power compensation and less AC harmonic filtering, resulting in less ancillary equipment.
The VSC HVDC systems in service so far have been limited to lower voltages and power ratings than CSC systems due to its low overload capability, limited by the capability of the IGBT devices. Nevertheless, new projects such as NordLink and NorthSeaLink with a 1400MW rated power are under construction .