The necessary transition from fossil energy sources to clean electrical energy requires the introduction of electric systems with efficient controllability and seamless power conversion. For the electricity grid, this means controllable reactive power, controllable real power (battery storage) and large, controllable direct-current (DC) networks. Rainer Marquardt dedicated his career to solving this challenge, developing technology that has since become a global standard used in electricity grids, offshore wind integration and long-distance power transmission.