SUMMARY Since the Seventies line commutated converters based on thyristor technology have been the standard for all HVDC applications. This converter topology is, however, marked by its loworder harmonics and high reactive power consumption. Such measures as the use of filter circuits and switchable capacitor banks have been taken to reduce these effects. On the other hand, these converters are marked by low operational losses, robustness of the thyristors against surge currents as well as their overload capability. Therefore, line-commutated thyristor technology will remain best suited to bulk power transmission in future HVDC projects. Compared with the line-commutated technology, additional inherent advantages are obtained through the use of self-commutated converters. In this kind of converter topology, current commutation is independent of the line voltage. This is the case because commutation is purely based on the turn-off capability of the power semiconductors such as IGBTs (Insulated Gate Bipolar Transistor). These semiconductors have gained in importance over the years and, due to their reliability, they are most commonly used in such challenging applications as traction and industrial drives. The VSC (Voltage-Sourced Converter) is the most common type of self-commutated converters and has therefore become a standard in the applications mentioned above. The possibility to control active and reactive power independently within weak or even passive networks makes them attractive to power transmission and distribution applications. Most VSCs are based on two-level technology along with PWM (Pulse-Width Modulation) controls. This implies that the AC connection voltage can only be switched between two possible levels which are defined by the common DC link capacitor connected to the DC terminals. Through pulse-width modulation with high switching frequencies (up to kHz range), the desired RMS voltage can be controlled and even some discrete low order harmonics can be reduced or eliminated. The use in HVDC applications requires high voltage ratings of the converter which can be achieved by a series connection of power