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This article presents detailed Markov models for the reliability assessment of multilevel neutral-point-clamped (NPC) converter leg topologies, incorporating their inherent fault-tolerance under open-circuit switch faults. The Markov models are generated and discussed in detail for the three-level and four-level active NPC (ANPC) cases, while the presented methodology can be applied to easily generate the models for a higher number of levels and other topology variants. In addition, this article also proposes an extremely fast calculation method to obtain the precise value of the system's mean time to failure from any given formulated system Markov model. This method is then applied to quantitatively compare the reliability of two-level, three-level, and four-level ANPC legs under switch open-circuit-guaranteed faults and varying degrees of device paralleling. The comparison reveals that multilevel ANPC leg topologies inherently present a potential for higher reliability than the conventional two-level leg, questioning the suitability of the traditional search for topologies with the minimum number of devices in order to improve reliability. Experimental results are presented to validate the fault-tolerance assumptions upon which the presented reliability models for the three-level and four-level ANPC legs are based. This article is accompanied by supplementary MATLAB scripts. [ABSTRACT FROM AUTHOR]

This paper studies a multilevel multiphase dc–ac conversion system configured by a neutral-point-clamped 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. [ABSTRACT FROM AUTHOR]

Several pulsewidth modulation (PWM) strategies have been proposed for three-phase multilevel converters. Typically, these PWM methods can be either space-vector or carrier-based implemented, where the carrier-based implementation usually presents a lower computation complexity. In this paper, a novel approach for the carrier-based implementation of different existing PWM methods is proposed for three- (3L) and four-level (4L) dc–ac converters, which can be easily extended to any number of levels. The main features of the proposed approach are its reduced computation complexity and computation time. On the other hand, the proposed approach suggests the derivation of novel PWM strategies which can potentially present improved performance features. As an example, two new discontinuous PWM (DPWM) strategies are proposed for the 4L converter. One of these DPWM strategies presents lower weighted total harmonic distortion than other previously proposed strategies. Simulation and experimental results verify the effectiveness of the proposed approach and novel DPWM strategies. [ABSTRACT FROM AUTHOR]

This paper studies the coupling between the capacitor voltage control loops of diode-clamped (or functionally equivalent) multilevel multileg (multiphase) dc–ac converters. From a complete model of the plant revealing the coupling, a simple approach consisting in multiplying the vector of control commands by a constant matrix is proposed to decouple the control problem and achieve a better controller performance. Simulation and experimental results are presented to prove the superior performance of the proposed decoupled control. [ABSTRACT FROM PUBLISHER]

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 is 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. [ABSTRACT FROM PUBLISHER]

This paper presents a capacitor voltage balancing control applicable to any multilevel dc–ac converter formed by a single set of series-connected capacitors implementing the dc link and semiconductor devices, such as the diode-clamped topology. The control is defined for any number of dc-link voltage levels and converter legs (for single-phase and multiphase systems), guaranteeing the capacitor voltage control for any modulation index value and load (from idle mode to full power). The associated control loop small-signal transfer function is presented, from which optimum compensator design guidelines are derived. The improvement in control performance is verified through simulation and experiments comparing with a previous balancing control strategy in a four-level three-phase dc–ac conversion system. The satisfactory control performance is also verified through simulation in a four-level five-phase dc–ac conversion system. [ABSTRACT FROM PUBLISHER]

Thanks to the inherent redundancy to generate the different output voltage levels, the multilevel active-clamped (MAC) topology presents an important fault-tolerance ability which makes it interesting for several applications. This paper presents an analysis of the fault-tolerance capacity of the MAC converter. Both open-circuit and short-circuit faults are considered, and the analysis is carried out under single-device and two simultaneous device faults. Switching strategies and different hardware modifications to overcome the limitations caused by faults are proposed. Experimental tests with a four-level MAC prototype are presented to validate the analysis. [ABSTRACT FROM PUBLISHER]

This paper presents a novel multilevel active-clamped converter topology, which is an extension to m levels of the three-level active neutral-point-clamped topology. The operating principle is established through the definition of a proper set of switching states and a transition strategy between adjacent switching states. The benefits of the proposed converter topology and control in comparison to alternative multilevel converter topologies are discussed. The simulation and experimental results of a simple four-level dc–dc converter configuration are presented to illustrate the converter performance features. The experimental results of a four-level three-phase dc–ac converter are also presented to further validate the proposed topology and operating principle. [ABSTRACT FROM AUTHOR]