This paper presents detailed Markov models for the reliability assessment of multilevel neutral-pointclamped (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 higher number of levels and for other topology variants. In addition, this paper also proposes an extremely fast calculation method to obtain the precise value of the system 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 a 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 faulttolerance assumptions upon which the presented reliability models for the three-level and four-level ANPC legs are based. Index Terms-Neutral-point-clamped, Markov model, mean time to failure, multilevel, reliability. I. INTRODUCTIONELIABILITY of power electronics systems has become of primary importance to fully leverage the advantages that this technology offers [1]-[3]. In many applications, the power electronics subsystem is one of the weakest links from the reliability point of view and an unexpected sudden full system shutdown is not acceptable.Reliability research has traditionally focused on two main areas: modeling and methods to improve reliability.On the one hand, a significant effort has been devoted to the

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With the aim to increase standardization, reduce the cost, and obtain advanced performance features, the design of voltage-source power converter legs can be undertaken by combining several instances of a standard switching cell, properly connected in active neutral-point-clamped structures to reach the desired voltage and current ratings. These switching cells can be organized into switching-cell arrays. This design approach introduces several degrees of freedom into the design. Namely, the different options to interconnect the cells and the distribution of switching losses among these cells. This article aims to define an optimization problem to explore this design space. The design problem is formulated in different scenarios, involving different conversion configurations (dc-dc and dc-ac), different leg number of levels (two and three), and different types of available cells (standard and conduction-optimized in combination with switching-optimized). A weighted objective function is then defined in terms of leg simplicity, efficiency, and reliability. The value of the design variables that minimize the objective function with different sets of weighting factors are obtained under selected scenarios and operating conditions, to illustrate the flexibility of the converter design approach under study. The solution of the optimization problem is obtained using a surrogate optimization algorithm in MATLAB, well suited to quickly solve optimization problems involving a combination of integer design variables (the number of parallel switching cells in each converter leg position) and continuous design variables (the proportion of switching losses taken by each cell), together with linear and nonlinear constraints.

<p>Neutral-Point-Clamped multilevel converters are nowadays a suitable solution to implement low-medium voltage and power applications, thanks to their intrinsic superior voltage and current quality. The conventional configurations of these converters present uneven power loss distribution, causing thermal stress in some power semiconductors, which weakens the power converter reliability. For overcoming it, an implementation of the Neutral-Point-Clamped multilevel converter based on a Switching-Cell Array is introduced, adding redundant conduction paths on one side and more options to distribute the switching losses on the other side. An active thermal control is proposed here to balance the temperature distribution in the converter. A four-level converter has been implemented to evaluate the proposed solution. Experimental results show that the proposed implementation and active thermal control present enhanced temperature distribution in the converter and therefore reduced thermal stress and better reliability. </p>

<p>Neutral-Point-Clamped multilevel converters are nowadays a suitable solution to implement low-medium voltage and power applications, thanks to their intrinsic superior voltage and current quality. The conventional configurations of these converters present uneven power loss distribution, causing thermal stress in some power semiconductors, which weakens the power converter reliability. For overcoming it, an implementation of the Neutral-Point-Clamped multilevel converter based on a Switching-Cell Array is introduced, adding redundant conduction paths on one side and more options to distribute the switching losses on the other side. An active thermal control is proposed here to balance the temperature distribution in the converter. A four-level converter has been implemented to evaluate the proposed solution. Experimental results show that the proposed implementation and active thermal control present enhanced temperature distribution in the converter and therefore reduced thermal stress and better reliability. </p>