Aiming to improve the performance features of conventional two-level dual-active-bridge (DAB) converters, this paper presents a three-level neutral-point-clamped (NPC) DAB dc-dc converter. A general modulation pattern is initially defined, the dc-link capacitor voltage balancing is analyzed in detail, and a proper balancing control is designed. Then, a set of decoupled optimization problems are formulated as a function of the available modulation degrees of freedom to minimize the predominant converter losses. Finally, a simple and practical specific modulation strategy is provided resembling the optimum solutions. The good performance of the proposed three-level NPC DAB converter operated with the proposed modulation strategy and voltage balancing control is verified through simulation and experiments. The capacitor voltage balancing can be guaranteed for all operating conditions. In addition, it is concluded that the multilevel topology provides benefits compared with the conventional two-level DAB converter.Index Terms-Active neutral point clamped, bidirectional dcdc converter, capacitor voltage balancing, dual active bridge, efficiency optimization, multilevel converter.
NOMENCLATUREPhasor of the h-th harmonic of v z . i z z-side transformer current. I z,hPhasor of the h-th harmonic of i z . i p z,h Component of the h-th harmonic of i z in phase with the h-th harmonic of v z . i q z,h Component of the h-th harmonic of i z in quadrature with the h-th harmonic of v z . n Transformer turns ratio. d Primary-referred dc voltage gain. L Transformer leakage inductance. f s Switching frequency. t b Blanking time. α zi1 , α zi2z-side inner switching angles.
This paper studies a multilevel multiphase dcac conversion system configured by a neutral-pointclamped converter fed by multiple battery packs connected in series. A virtual-vector modulation is selected and a state-of-charge (SoC) balancing control is designed to be able to employ the full battery bank capacity, even under different battery initial SoC values or different battery nominal capacities. The SoC balancing among battery packs is accomplished through the multilevel converter operation in a lossless manner, by simply distributing the dc-to-ac power flow among the batteries according to their SoC. A simple average system model is also presented, which allows performing very fast system simulations over long periods of time and serves as a convenient tool to tune the compensator parameters. The satisfactory performance of the proposed system configuration and control, which can be applied with any number of levels and phases, has been verified through simulations and experiments in a four-level three-phase dc-ac converter fed by three Lithium-ion battery packs. The results prove the feasibility and advantages of the proposed system configuration, which can be used to implement conversion systems with different specifications combining several instances of a standard battery pack and a standard power semiconductor device.
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