Design optimization of switching-cell-array-based power converters

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.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.Postprint (author's final draft

Electric vehicle powertrains with modular battery banks tied to multilevel NPC inverters

Nowadays, the internal combustion engine in vehicles is being replaced by electric motors, giving way to the electric vehicle, which results in reduced environmental impact, higher efficiency and lower emission of greenhouse gases. The powertrain of an electric vehicle is its most prominent subsystem, with the batteries and traction inverter being key components. Thus, due to their relevance, advances in the design of both components are of paramount importance. In this paper, the potential benefits achieved through a powertrain design approach based on combining a modular battery bank with multilevel NPC traction inverter topologies were analyzed, in comparison to a conventional two-level powertrain design. Several aspects were analyzed: modularity, complexity, battery-pack state-of-charge balancing, inverter loss, motor ac voltage harmonic distortion, motor common-mode voltage and reliability. Particularly, from the comparison study developed under the selected design scenario, the proposed design approach, based on modular battery packs and multilevel technology, shows a potential reduction of up to 55% in inverter losses, up to 65% in motor ac-voltage total harmonic distortion, and up to 75% in rms common-mode voltage.Peer ReviewedPostprint (published version

Pulsewidth modulations for the comprehensive capacitor voltage balance of n-level three-leg diode-clamped converters

In the previous literature, the introduction of the virtual-space-vector (VV) concept for the three-level, three-leg neutral-point-clamped converter has led to the definition of pulsewidth modulation (PWM) strategies, guaranteeing a dc-link capacitor voltage balance in every switching cycle under any type of load, with the only requirement being that the addition of the three phase currents equals zero. This paper presents the definition of the VVs for the general case of an n-level converter, suggests guidelines for designing VV PWM strategies, and provides the expressions of the leg duty-ratio waveforms corresponding to this family of PWMs for an easy implementation.Modulations defined upon these vectors enable the use of diode-clamped topologies with passive front-ends. The performance of these converters operated with the proposed PWMs is compared to the performance of alternative designs through analysis, simulation, and experiments.Postprint (published version

Control methods for low voltage ride-through compliance in grid-connected NPC converter based wind power systems using predictive control

This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.In presence of grid voltage dips, Low Voltage Ride-Through (LVRT) requirements demand the wind power plant to remain connected to the grid, helping the network to keep voltage and frequency stable. Neutral-Point-Clamped (NPC) converters are appropriate for wind power systems, because the current trend of increasing voltage levels. Predictive current control presents as fast dynamic response and accurate reference tracking as other well established control methods, while providing more flexibility. In this work, three different control strategies are applied to the grid-side NPC converter, in order to fulfil LVRT requirements, which are implemented with the predictive current control technique. Dc-link neutral point voltage is kept balanced by the predictive control algorithm, using the redundant switching states of the NPC converter. Simulation results confirm the validity of the proposed control approach.Peer ReviewedPostprint (published version

Comparison of modulations and dc-link balance control strategies for a multibattery charger system based on a three-level dual-active-bridge power converter,

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper focuses on the study of a charger for two batteries connected in series. From the three-phase grid, the batteries are charged through a three-level neutral-point-clamped (NPC) ac-dc converter in cascade with a three-level NPC dualactive-bridge converter. The system provides galvanic isolation and allows bidirectional power flow. A simple control strategy to charge the batteries is considered, based on the regulation of the common- and differential-mode components of the batteries charging currents. In addition, the proposed control system regulates the total dc-link voltage and the dc-link voltage balance in the two systems dc-links. This work is particularly focused on the comparison of the charger performance under two competitive ac-dc converter modulations, in terms of the ac-side voltage harmonic content, the number of switching transitions, the dc-link voltage balance, and the charging current control capacity. Simulation results with the performance comparison are provided and the merits and demerits of each option are highlighted.This publication is part of Grant DPI2017-89153-P, funded by MCIN/AEI/10.13039/501100011033 and by ERDF A way of making Europe.Peer ReviewedPostprint (author's final draft

Predictive control of a back-to-back NPC converter-based wind power system

© 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksAs wind power technology points to increase power ratings, the implementation based on a permanent-magnet synchronous generator (PMSG) with a full-power converter is expanding its market share. Multilevel converters, as for example, neutral-point clamped (NPC) converters, are therefore well suited for this application. Predictive current control presents similar dynamic response and reference tracking than other well-established control methods, but working at lower switching frequencies, and providing extensive flexibility to apply either online or offline different control laws to the same plant. In this work, the predictive current control is applied to both sides of the back-to-back NPC converter connecting a permanent-magnet synchronous wind power generator to the grid. DC-link neutral-point balance is achieved by means of the predictive control algorithm, which considers the redundant switching states of the back-to-back NPC converter. Reduced number of commutations, current spectrum control, and compliance with the low-voltage ride-through (LVRT) requirement are carried out with the predictive control. The obtained experimental results confirm the suitability of the proposed control approach.Peer ReviewedPostprint (author's final draft

Multibattery charger system based on a three-level dual-active-bridge power converter

© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.A charger for two batteries connected in series is presented in this work. From the three-phase grid, the batteries are charged through a three-level neutral-point-clamped ac-dc converter in cascade with a three-level dual active bridge converter. The system provides galvanic isolation and allows bidirectional power flow. A simple control strategy to charge the batteries is presented, based on the regulation of the commonand differential-mode components of the batteries’ charging currents. With this control approach, each battery bank can be charged independently, allowing it to reach full battery bank capacity, even under different battery initial state-of-charge values or different battery nominal capacities. Moreover, the proposed control system also regulates the total dc-link voltage and the dc-link voltage balance in both dc-links of the system. The simulation results verify the feasibility of the proposed implementation and control system approach.This work was supported by the Ministerio de Economía, Competitividad, Spain, under Grant DPI2017-89153-P (AEI/FEDER, UE).Peer ReviewedPostprint (published version

Fast Reliability Assessment of Neutral-Point-Clamped Topologies through Markov Models

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.Peer Reviewed© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other work

Control scheme for low voltage ride-through compliance in back-to-back NPC converter based wind power systems

The increasing influence of wind power in the power system operation has caused the power system operators to include wind power plants in the control of the overall power system, both in steady-state and transient operation. Therefore, the grid connection requirements of the power system operators involve wind power systems. Low voltage ride-through requirement is probably the most demanding grid connection requirement to meet, at least from the point of view of the wind energy conversion system. In presence of grid voltage dips, the low voltage ride-through compliance produces a mismatch between the generated active power and the active power delivered to the grid. The management of this mismatch supposes a challenge for the wind energy conversion system. In this work, a control scheme for the back-to-back neutral-pointclamped converter is proposed. Under grid voltage dip, the controllers for generator-side and grid-side converters work concurrently to meet the low voltage ride-through requirement by storing the active power surplus in the inertia of the generator and keeping constant the dc-link voltage. Simulation results verify the proposed control scheme.Peer ReviewedPostprint (published version

Model predictive current control of grid-connected neutral-point-clamped converters to meet low-voltage ride-through requirements

The low-voltage ride through (LVRT) requirement demands the wind power plants to remain connected to the grid in the presence of grid voltage dips, actively helping the network overall control to keep network voltage and frequency stable. Wind power technology points to increase power ratings. Hence, multilevel converters, as for example, neutral-point-clamped (NPC) converters, are well suited for this application. Predictive current control presents similar dynamic response and reference tracking than other well-established control methods, but working at lower switching frequencies. In this paper, the predictive current control is applied to the grid-side NPC converter as part of a wind energy conversion system, in order to fulfill the LVRT requirements. DC-link neutral-point balance is also achieved by means of the predictive control algorithm, which considers the redundant switching states of the NPC converter. Simulation and experimental results confirm the validity of the proposed control approach.Postprint (author’s final draft