Electric Vehicle Charging Station with an Energy Storage Stage for Bipolar DC Bus

This paper proposes a balancing approach for an electric vehicle bipolar dc charging station at the megawatt level, enabled by a grid-tied neutral point clamped converter. The work uses the presence of an energy storage stage with access to both of the dc buses to perform the complementary balance. This is aiming to reduce the hardware requirements of the system and maximize the usage of the ESS, whose main function is to perform the energy management related tasks. To meet this purpose, a three-level dc-dc interface is employed, allowing tocompensate the dc currents with a single ESS. In order to prevent the appearance of even-order harmonics in the input current during asymmetrical operation, an alternative switching sequence for the central converter is proposed. Results indicate that, without altering dramatically the charging process of the ESS, it is possible to cover the whole load scenario without the need of a balancing circuit. This allows the use of off-the-shelf products both for the rectifier and the fast chargers. In this paper, simulation and experimental results are presented to validate the proposed balancing strategy.