With the increasing popularity of electric vehicles, there is an urgent demand to shorten the charging time, so the development of high-power charging stations with fast chargers is necessary to alleviate range anxiety for drivers. The charging station based on the neutral-point-clamped (NPC) converter can bring many merits, but it has unbalanced power problems in the bipolar dc bus. To solve this issue, comprehensive dc power balance management (PBM) in conjunction with high-power three-level dc-dc converter based fast charger is proposed in this paper. The active dc power balance management (APBM) is proposed to assist the central NPC converter in balancing power so that the additional balancing circuit is eliminated; while the passive dc power balance management (PPBM) is proposed to eliminate the fluctuating neutral-point currents and to ensure the balanced operation of fast chargers. The principles of APBM and PPBM are researched, the efficient integration between them is studied, and the overall control scheme for the fast charger is proposed. The power balance limits of APBM are explored, while the circulating currents of PPBM are analyzed. Simulation and experimental results are presented to verify the effectiveness of the proposed fast charger with PBM functions.

The development of high-power charging stations with fast chargers is a promising solution to shorten the charging time for electric vehicles (EVs). The neutral-point-clamped (NPC) converter-based bipolar-dc-bus-fed charging station brings many merits, but it has inherent voltage balance limits. To solve this issue, a voltage balance control (VBC) method based on a new modulation together with three-level (TL) dc-dc converter-based fast charger is proposed. Additionally, an effective VBC coordination between the TL dc-dc converter and the NPC converter is formulated. Through the proposed VBC coordination, the controllable balancing region is extended so that additional balancing circuits are eliminated. Meanwhile, the quality of the grid-side currents is improved as the NPC converter has more freedom to control currents. The low-frequency voltage fluctuations in dc buses are removed because the TL dc-dc converter performs most of the balancing tasks. Faster VBC perturbation performance is achieved due to higher available balancing current at TL dc-dc converter side. In addition, the voltage balance limits of both the TL dc-dc converter and the NPC converter are explored. The voltage balancing performances are compared when VBC is located at different sides. Simulation and experimental results are provided to verify the proposed VBC and the VBC coordination.

This paper proposes a bipolar-dc-bus Electric Vehicle (EV) charging station without a line-frequency transformer at the medium-voltage grid side. It is mainly composed of the Neutral Point Clamped (NPC) converter based central ac-dc station and the parallel isolated three-level DC-DC converter based fast chargers. Due to the elimination of the line-frequency transformer, the volume and cost of the system are reduced. Meanwhile, the isolation and step-down between high dc-bus voltages and low load voltages are performed through the high-frequency transformers, leading to increased voltage utilization of switching devices. In order to guarantee the bipolar dc buses balance and minimize the dc-bus voltage ripples, the 180°-interleaved operation for the two parallel isolated three-level DC-DC converters is proposed to make the total neutral-point current zero so that all fast chargers do not bring any unbalance problems. The proposed 180°-interleaved operating principle has been analyzed in detail, then simulation results are presented to verify its performance.

This paper proposes an effective voltage balance control (VBC) together with a new modulation technique for a high-power bidirectional three-level dc-dc converter based fast charger. The proposed VBC method can assist charging stations in balancing the dc-bus voltages so that additional balancing circuits are eliminated, leading to improved system efficiency. Meanwhile the proposed modulation method is not only intuitive to be implemented, but also have merits of reduced switching frequency, resulting in lower switching losses. The modulation principle is formulated, and its effective combination with the voltage balance control are analysed in detail. Based on which, the overall control scheme for the charging system is proposed. Simulation results are presented to validate the effectiveness of the proposed methods.