Energy Is On Board: Energy Storage and Other Alternatives in Modern Light Railways

The traditional concept of dc traction systems for light rail applications was based in a simple dc system that was fed by ac/dc noncontrolled diode rectifier substations connected to the ac distribution network. Low-energy efficiency and controllability were not a problem. However, with the massive implementation of regenerative braking technologies in light trains and trams, the development of an effective way to manage the recovered energy became an important issue. The regenerated power injected in the system by a train in braking mode could be used only in the case where another nearby train was in traction mode. Otherwise, the regenerated power was dissipated in the dc traction system or in the rheostatic braking equipment, decreasing the overall system energy efficiency. The need for more solutions that would allow more use of the regenerated energy and increased energy efficiency in the system was the driving force for new technological developments. First, proposals were for the use of reversible substations able to give back the dc energy surplus to the ac distribution system. Then, due to energy storage cost reductions, a combination of technologies such as insulated-gate bipolar transistor (IGBT)-based reversible substations with on-board and off-board accumulation systems evolved into a reality. The use of substation-level off-board accumulation systems can also avoid the installation of reversible infrastructure, providing a high degree of controllability over the dc traction system voltage. However, higher efficiency rates are achieved when the energy storage is installed on board. In such a case, the regenerated power is used in-situ, minimizing the transmission losses. One of the main benefits of the offboard accumulation approach is the fact that many trains equipped with regenerative braking systems can take advantage of a single device. On the other hand, the use of on-board accumulation leads us toward more flexible, efficient, and dependable systems. Therefore, some manufacturers are using these kinds of on-board accumulation systems to avoid the use of overhead line equipment and to reduce the visual impact, offering catenary-free solutions.