Battery storage plant to improve energy saving and security of subway electric supply

One of the possible actions to improve the management of underground railway plants is the insertion of a battery storage in subway substations. This provides the following main benefits: increase in the recovered braking energy; increase in the system security, as the energy stored in the batteries allows the trains to reach the nearest subway station in case of failure of the main power supply; decrease in the power peak with consequent savings in running and in investment costs; and improvement of the voltage profile on the DC line which produces a decrease in losses. In this paper the operation of a conventional substation equipped with uncontrolled rectifiers is investigated when a device called a Static Watt Compensator (SWC), composed of a battery storage system with a fully controlled DC/DC converter, is connected to the DC bus-bar. A new Battery Supervision System (BSS), tested at the University of Pisa, is used for the on-line estimation of the battery state. For a case study related to the underground of Milan, line 3, a management strategy of the plant and a first economical feasibility are studied