Robust AC excitation converters for pumped storage with variable speed doubly fed induction machines

Variable speed pumped storage with doubly-fed induction machines (DFIM) can use a much smaller converter rating compared to solutions with a fully fed synchronous machine. The rated power of an AC excitation converter for DFIM can be up to 20% of the machine power. Nonetheless, converter operation with DFIM has challenges on the rotor-side inverter, such as low frequencies and high currents during grid-fault events. These operating conditions must be reliably covered by the AC excitation system. This work focuses on the excitation converter, the system design, and improvements for an industrial medium-voltage three-level converter with press-pack IGBTs. High-voltage power semiconductors with different packaging technologies are compared regarding current rating and surge-current capability. Thermal simulations of high-voltage diodes in the event of overcurrent during a grid fault indicate limitations of the system.