DC-DC Converters for DC Transmission Grids

Abstract

This chapter describes DC-DC converters which are expected to become important components in large DC transmission grids. Similarly as with AC transformers in AC transmission systems, DC-DC converters enable interconnection between two DC systems with different voltage levels. However, some DC-DC converter topologies will provide a range of other functions including breaking DC circuits, power flow control and galvanic isolation. While the description of the DC-DC systems includes many design details, the application of DC-DC converters is still a concept, and the detailed design of these systems needs further in depth studies. The information provided in the chapter focuses on the functional requirements of the DC-DC systems, which are described in significant detail in this chapter. There are two families of DC-DC converters: Non-isolated DC-DC and Isolated DC-DC. Non-Isolated DC-DC is the lowest-cost topology, with the costs and losses expected to be 1.1–1.3 pu relative to a similarly rated single MMC VSC converter. The design, modeling and control of Non-Isolated DC-DC systems are decribed. The responses during DC faults are also analysed in some depth and DC fault simulation results are presented using a typical 600 MW DC-DC test system. The isolated DC-DC will have an internal AC transformer which provides galvanic isolation between two DC systems, while costs and losses are expected to be around 2 pu compared with a similarly rated MMC VSC converter. The design, modeling and control of this DC-DC topology is described based on the application of CIGRE’s DC grid test system. DC fault responses are also analysed and illustrated using simulation responses with a typical 600 MW test system. The DC-DC converter applications for interconnecting high voltage with medium voltage DC systems are also described. This application requires higher voltage step ratio and some specific DC-DC converter topologies are found to be more suitable than others. Typical applications would include interconnection to existing medium voltage DC systems like those with renewable energy plants.