VSC-HVDC Transmission with Cascaded Two-Level Converters

Abstract

SUMMARY The two-level converter valve together with series-connected press-pack insulated-gate bipolar transistors (IGBTs) is a reliable and proven technology for transmission-scale converters. This has been put to use in a further development of high-voltage dc (HVDC) transmission employing voltage-source converters (VSCs). A cascaded two-level (CTL) converter consisting of several smaller two-level building blocks, also called cells, enables the creation of a nearly sinusoidal output voltage from the converter. The development of the CTL is the subject for this paper. Using technology modules developed and refined during the last 15 years, it has been possible to create a converter that addresses and solves many of the limitations of VSC-HVDC transmission while retaining all operational functionality. The technology is scalable up to the highest transmission voltages. Losses are reduced to roughly 1% per converter through a combination of methods. A method for removing the need for external protective equipment and circuitry in the valves is presented. The main control of the converter has been reused, although the control bandwidth, i.e., the speed, has been further improved.