Bidirectional AC-DC Modular Multilevel Converter with Electric Spring Functions for Stabilizing Renewable AC Power Grid at the Distribution Voltage Level

Bidirectional ac-dc power converters are essential in emerging smart grids with increasing renewable energy penetration. This paper presents a bidirectional ac-dc power converter system comprising modular multilevel converters (MMCs) and dual-active-bridge (DABs) with medium-frequency transformer isolation designed for linking an ac distribution voltage of 6.6 kV to a dc grid of 800 V for future electric vehicle (EV) charging infrastructure. The novel contributions include (1) a modular method to power EV charging infrastructures in multistorey carparks without mains-frequency transformers and (2) the incorporation of a front-end control with electric spring functions that enable the dc power grid with battery energy storage to interact dynamically with the ac power grid at the distribution voltage level to achieve instantaneous power balance and hence system stability. The long-term aim is to use large EV charging infrastructures to stabilize increasing intermittent renewable energy via the proposed ac-dc converter, consequently accelerating the adoption of large-scale renewable energy and EV as a complementary solution to combat climate change. This paper focuses on the bidirectional ac-dc converter of this overall idea based on the MMC and DAB technologies as an example. Results on the power converter operation level and ac microgrid level are included.