Braking energy recovery by Modular Multilevel Converters in MVDC Railway Electrification Systems

DC railway electrification systems traditionally rely on transformer-rectifier groups to inject power into the catenary. This technology is well established and can be up-scaled for the upcoming medium voltage DC lines (MVDC) that are proposed to operate at 9 kVdc. However, the operation of MVDC railway lines could be reinforced by means of four quadrant substations. As such, Modular Multilevel Converters (MMCs) are envisioned as a serious alternative. They are expected to provide an outstanding flexibility in terms of load flow control, owing to their voltage regulation capability and their bi-directional nature. Additionally, they would improve the interconnection with AC utility networks, by providing robust isolation of perturbations and independent control of the active and reactive power. Due to their four quadrant operation, MMCs are also expected to be extremely beneficial during faults (e.g. short-circuits) and significantly increase the resilience of the system. This paper illustrates to what extent MMCs could be used to recover the energy regenerated by locomotives while braking. To this end, the work was carried on a Real-Time Hardware-In-the-Loop (RT-HIL) replica of a section of a railway line.