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Finite control set model predictive control (FCS-MPC) chooses at each sampling period the input control vector state without using a modulator. Therefore, it belongs to the direct control family. As a consequence, the conventional implementation of FCS-MPC presents variable switching frequency. This makes difficult the design of the output filter. This article investigates the period control approach (PCA-MPC) to achieve a fixed switching pattern, similar to that of modulated strategies placed at a desired frequency. PCA-MPC cannot reach a reference switching frequency without a steady-state error. Nonetheless, it is more than capable of reaching a fixed switching pattern, with clean spectrum at a wide range of switching frequencies. This feature is used here to design an embedded control objective to reach the desired switching frequency. The implementation of the cost function is done through modification of PCA-MPC measurements. These modifications allow one to obtain fast and precise estimates of the switching frequency, which, in turn, permits a fast control. The performance of this control strategy is evaluated through simulation and corroborated by experimental validation on a two-level three-phase, power converter. [ABSTRACT FROM AUTHOR]

The nine papers in this special section focus on photovoltaic module and sub-module level power electronics. Grid connected photovoltaic energy systems have experienced an explosive growth over the last decade, with a cumulative installed capacity surpassing the 400 GW milestone as of 2017. Among PV system configurations, distributed module-level converter architectures can lead to a higher energy yield by mitigating partial shading, persistent shading (soiling, snow, bird droppings, and fallen leaves), mismatch, and aging, through a higher maximum power point tracking (MPPT) efficiency. Also, distributed electronics might be the key for implementing diagnostic and prognostic actions at a module level. Among these configurations, microinverters (also known as ac-module inverter), which connect a single PV module to the grid, and PV power optimizers, which are dc–dc converters performing the MPPT function at a module level, have attracted the academic and industrial interest in the last decade. So much so, that both microinverters and dc–dc power optimizers are commercialized by tens of companies around the world, with a great variety of circuit topologies, which comprise combinations of one or more power stages, interleaved converters, resonant converters, topologies with and without isolation, etc. [ABSTRACT FROM AUTHOR]

One of the main drawbacks with the use of finite control set model predictive control (FCS-MPC) in power converters is the variable switching frequency. When not taken into consideration, it can vary randomly depending mainly on the operating point of the system and the sampling time. This will produce a wide distributed voltage and current spectrum, causing audible noise, resonances, and poor steady-state behavior. To address this issue, a new algorithm, based on the control of the switching period within the cost function, is proposed in this paper. The algorithm shows to be very effective in achieving a regular commutation pattern, similar to pulse width modulation (PWM) techniques, which is evidenced even by similarities in the sideband harmonics generated in the voltage spectrum. The algorithm is easy to design and implement, demanding very low computation power, and enables a combination of the advantages of FCS-MPC with the benefits of a PWM-like power quality. Simulations and experimental results are presented to validate the proposed method, which are compared to previously reported FCS-MPC techniques dealing with the same challenge. [ABSTRACT FROM PUBLISHER]

This paper presents a new multilevel converter topology based on a variable or multilevel dc-link stage. This stage is shared by all inverter output phases, thus increasing the number of output voltage levels while reducing the overall number of active devices compared to traditional topologies; therefore, it is called a reduced multilevel converter (RMC). The converter is not capable of reducing the blocking voltage of the devices; hence, unlike other multilevel converters aimed at medium-voltage applications, this converter is interesting for low-voltage- and high-power-quality-demanding applications such as photovoltaic inverters, wind energy conversion systems, and uninterruptible power supplies. The main novelty behind the proposed concept is the basic dc cell used to generate the variable dc-link voltage, which includes a controlled path through the floating capacitors to provide the necessary degree of control to enable a shared multilevel dc link for all the output phases of the converter. The dc cells are connected in a multicell structure to increase the number of levels of the converter. A five-level version of the RMC topology is briefly compared to other five-level converters such as the five-level active neutral-point-clamped converter and the five-level flying capacitor converter. This paper presents the structure of the new topology with its operating principle, switching states, main characteristics, and experimental validation using finite-control-set model-predictive control. [ABSTRACT FROM PUBLISHER]

This paper explores a utility-scale photovoltaic (PV) plant configuration based on a dc–dc stage interfaced directly to a modular multilevel converter (MMC) used for an HVdc power station. Since PV systems are dc by nature, the proposed solution has several advantages, such as full dc operation and step-up transformers avoidance with modular and reliable configuration. The contributions of this paper are the experimental validation of the proposed utility-scale configuration, while the conventional modulation stage has been further enhanced by including an adaptive phase-shifted modulation allowing improved voltage and current waveforms injected to dc grid under unbalanced power string operation. The enhanced performance is obtained by reducing the carrier frequency component of the converter voltage, generating an improved voltage ripple. Finally, experimental validation during steady-state and dynamic operation are presented to illustrate the behavior of the dc–dc MMC converter managed by the proposed modulation and control scheme. [ABSTRACT FROM PUBLISHER]

Marine energy conversion systems are highlighted among nonconventional renewable energy sources for their availability and energy density. Most marine energy conversion systems are based on multigenerator units or arrays, projected to inject power to the grid in the megawatt range. The current solution for regulating generators and managing power output is the use of two-level voltage-source converters in a back-to-back configuration for each generator. This work presents the model, control scheme, and experimental validation of a configuration, which is able to interface medium-voltage high-power multigenerator marine energy conversion systems to the grid at medium-voltage high-power levels. The configuration is based on a two-step rectifier/boost converter for each generator and a multilevel inverter that merges and manages the multiple power outputs from the multigenerator section of the system. The multilevel inverter is responsible for controlling power flow to the grid and provides regulated medium-voltage high-power injection. [ABSTRACT FROM AUTHOR]

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future. [ABSTRACT FROM PUBLISHER]

Multilevel cascaded H-bridge converters have found industrial application in the medium-voltage high-power range. In this paper, a generalized modulation technique for this type of converter based on a multidimensional control region is presented. Using the multidimensional control region, it is shown that all previous modulation techniques are particularized versions of the proposed method. Several possible solutions to develop a specific implementation of the modulation method are addressed in order to show the potential possibilities and the flexibility of the proposed technique. In addition, a feedforward version of this technique is also introduced to determine the switching sequence and the switching times, avoiding low harmonic distortion with unbalanced dc voltages. Experimental results are shown in order to validate the proposed concepts. [ABSTRACT FROM AUTHOR]

Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities. [ABSTRACT FROM AUTHOR]

This paper presents a detailed description of Finite Control Set Model Predictive Control (FCS-MPC) applied to power converters. Several key aspects related to this methodology are, in depth, presented and compared with traditional power converter control techniques, such as linear controllers with pulsewidth-modulation-based methods. The basic concepts, operating principles, control diagrams, and results are used to provide a comparison between the different control strategies. The analysis is performed on a traditional three-phase voltage source inverter, used as a simple and comprehensive reference frame. However, additional topologies and power systems are addressed to highlight differences, potentialities, and challenges of FCS-MPC. Among the conclusions are the feasibility and great potential of FCS-MPC due to present-day signal-processing capabilities, particularly for power systems with a reduced number of switching states and more complex operating principles, such as matrix converters. In addition, the possibility to address different or additional control objectives easily in a single cost function enables a simple, flexible, and improved performance controller for power-conversion systems. [ABSTRACT FROM AUTHOR]

Multilevel inverters have emerged as attractive high-power medium-voltage power-conversion systems. They are mainly controlled with high-frequency pulsewidth-modulation methods. This is not suitable for very high-power application due to significant switching losses. This paper presents an adaptive duty-cycle modulation algorithm that reduces the switching frequency to a minimum necessary to fulfill the dynamic requirements of the system. Switching losses are, therefore, strongly reduced. This is achieved by using the slope of the voltage reference to adapt the modulation period to ensure that only one-step change between two voltage levels. Simulation and experimental results are presented for a nine-level cascaded inverter. Voltage waveforms obtained for variable frequencies and amplitudes show similar switching patterns, with a reduced and near-constant number of commutations per cycle, regardless of the reference frequency and amplitude. [ABSTRACT FROM AUTHOR]

This paper proposes and validates model predictive control as an alternative control strategy for H-bridgeneutral-point-clamped (H-NPC) converters for single-phase grid-tied string photovoltaic (PV) systems. The presented control scheme achieves good quality current waveforms with unity power factor, dc-link voltage control, and neutral-point voltage minimization. Furthermore, the predictive controller has been further enhanced by including an average device switching frequency restriction and a $dv/dt$ mitigation. The main contribution of this paper is the avoidance of the potential leakage current due to parasitic capacitance of the PV modules by using a predictive model based control technique instead of modulated schemes and eliminating high-frequency common-mode voltage components. Experimental results during steady state and dynamic operation are presented to illustrate the behavior of the H-NPC converter commanded by the proposed control scheme. [ABSTRACT FROM AUTHOR]

This article presents a novel concept of a universal solar converter suitable for application in both in the dc or single-phase ac grids using the same terminals. The idea lies in the utilization of the same semiconductors in the dc–dc and in the dc–ac configuration, resulting in minimal redundancy. Possible semiconductor stages are considered. The particular attention is focused on the output filter design along with proper protection circuit selection for dc and ac grids. The design example and comparative analysis between dc–dc, dc–ac, and universal solutions are given. The experimental prototype of the universal solar converter that is rated for 3.6 kVA power in the ac mode and 5 kW in the dc mode is presented. The experimental results demonstrate the ability of operation in ac or dc grids with main correspondent modes. Possible fields of application along with main benefits are addressed in conclusions. [ABSTRACT FROM AUTHOR]

This paper proposes a novel architecture for plug-in electric vehicles (PEVs) dc charging station at the megawatt level, through the use of a grid-tied neutral point clamped (NPC) converter. The proposed bipolar dc structure reduces the step-down effort on the dc–dc fast chargers. In addition, this paper proposes a balancing mechanism that allows handling any difference on the dc loads while keeping the midpoint voltage accurately regulated. By formally defining the unbalance operation limit, the proposed control scheme is able to provide complementary balancing capabilities by the use of an additional NPC leg acting as a bidirectional dc–dc stage, simulating the minimal load condition and allowing the modulator to keep the control on the dc voltages under any load scenario. The proposed solution enables fast charging for PEVs concentrating several charging units into a central grid-tied converter. In this paper, simulation and experimental results are presented to validate the proposed charging station architecture. [ABSTRACT FROM PUBLISHER]

This paper presents a model predictive current control algorithm that is suitable for multilevel converters and its application to a three-phase cascaded H-bridge inverter. This control method uses a discrete-time model of the system to predict the future value of the current for all voltage vectors, and selects the vector which minimizes a cost function. Due to the large number of voltage vectors available in a multilevel inverter, a large number of calculations are needed, making difficult the implementation of this control in a standard control platform. A modified control strategy that considerably reduces the amount of calculations without affecting the system's performance is proposed. Experimental results for five- and nine-level inverters validate the proposed control algorithm. [ABSTRACT FROM AUTHOR]

This paper presents a simple and low-computational-cost modulation technique for multilevel cascaded H-bridge converters. The technique is based on geometrical considerations considering a unidimensional control region to determine the switching sequence and the corresponding switching times. In addition, a simple strategy to control the dc-voltage ratio between the H-bridges of the multilevel cascaded converter is presented. Examples for the two-cell topology are shown, but the proposed technique can be applied to develop modulation methods for a higher number of H-bridges. Experimental results are presented to validate the proposed technique. [ABSTRACT FROM AUTHOR]

This paper presents the application of direct torque control in an induction motor, using a multilevel cascaded inverter with separated dc sources. The control strategy operates with imposed switching frequency, improving torque behavior. The paper studies the theoretical concepts related to this method, like vector selection, state variables estimation, and commutation time calculation. In addition, this paper presents results for a three- and 11-level inverter-fed drive, from which it can be appreciated that the increase of levels of the load voltage reduces the torque ripple. [ABSTRACT FROM AUTHOR]

Modular converters such as the multilevel cascaded H-bridge (CHB) are an attractive option for multiple applications mainly because of inherent modularity and fault-tolerant operation. This article is focused on the CHB converter operating with unbalanced conditions (different dc voltages and/or modulation indexes). Under these circumstances, applying the conventional control and modulation strategies, the output voltage harmonic spectrum is degraded. In this article, a generalized variable-angle phase-shifted pulsewidth modulation (PS-PWM) technique for CHB converters with a large number of power modules (> 3) is presented. The method considers all possible cells’ combinations to form groups and assigns the role of each cell in the group. This cell role defines the identifier of the cell in the variable-angle PS-PWM technique. In the steady state, in each group of cells, the harmonic distortion of the CHB output voltage located at twice the carrier frequency ƒc is eliminated, while the distortion at 4ƒc is also diminished. Experimental results show how the proposed technique achieves superior harmonic performance without introducing any significant disadvantage. [ABSTRACT FROM AUTHOR]

Grid-connected photovoltaic (PV) systems can provide grid support features by implementing flexible power point tracking (FPPT) strategies. Conventionally, the FPPT algorithms are based on linear search algorithms, such as perturb and observe. These solutions offer good practical results but usually lead to oscillations around the steady-state operating point. The main contribution of this article is to propose a binary search based FPPT (BNS-FPPT) algorithm. BNS is a computer-science-based searching algorithm that performs a logarithmic search within a sorted data array until the target voltage value is found. Compared to sequential searching algorithms, the proposed BNS-FPPT algorithm conducts a nonlinear search and avoids waving around the final value, reducing the steady-state error. In this article, the basics of the BNS-FPPT are described in detail. In addition, experimental results are conducted on a laboratory setup to compare the BNS-FPPT performance against a state-of-the-art solution. The results show that compared to linear search-based FPPT algorithms, the proposed BNS-FPPT is able to provide the required grid support functionalities, and avoid waving around the final operation point. [ABSTRACT FROM AUTHOR]

Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method. [ABSTRACT FROM PUBLISHER]

For large-scale photovoltaic (PV) systems, the multistring configuration is becoming more and more attractive compared with the classical central inverter, since it results in better energy yield by realizing distributed maximum power point tracking. Among the existing solutions, an attractive topology consists in a single dc bus bar collector cascaded H-bridge (CHB) inverter. Through the use of a single dc bus bar collector, the CHB inverter presents inherent balanced operation while the multistring PV-system is fully decoupled from the grid-tie inverter. This paper proposes the experimental validation of this structure on a reduced-size single-phase laboratory prototype. Results confirm the interest of the proposed PV multistring architecture. [ABSTRACT FROM AUTHOR]

In this paper, a predictive control scheme is proposed for the low-voltage ride-through (LVRT) enhancement of direct-driven permanent-magnet-synchronous-generator-based megawatt-level wind turbines. The proposed method uses the turbine–generator rotor inertia to store the surplus energy during the grid voltage dips. The power conversion system is realized using a three-phase diode-bridge rectifier, a three-level-boost converter, and a neutral-point-clamped (NPC) inverter. The wind turbine requirements, such as maximum power point tracking, net dc-bus voltage control, balancing of the dc capacitor voltages, and reactive power generation, are modeled as the reference control variables. The generator- and grid-side cost functions are defined to deal with these control objectives. During each sampling interval, the control goals are achieved based on the minimization of cost functions. The coordination of boost and NPC converters and the exchange of reference control variables during normal and LVRT operation are formulated such that the power converters operate in a safe mode while meeting the grid code requirements. Simulation and experimental results are presented to validate the proposed strategy. [ABSTRACT FROM PUBLISHER]

The increasing installed wind power capacity has caused wind power generation to become a significant percentage of the entire electric power generation. As a consequence, the power system operators have included wind power plants regulation to improve the control of the overall power system, both in steady-state and transient operation. Therefore, wind power systems are required to verify the grid connection requirements stated by the power system operators. In presence of grid voltage dips, the low voltage ride-through (LVRT) requirement compliance produces a mismatch between the generated active power and the active power delivered to the grid. The conventional solution assumes that the active power surplus is dissipated in a dc-link resistor. In this paper, a control scheme for the back-to-back neutral-point-clamped converter is proposed. Under grid voltage dip, the controllers for generator-side and grid-side converters work concurrently to meet the LVRT requirement by storing the active power surplus in the turbine-generator mechanical system inertia while keeping constant the dc-link voltage. Simulation and experimental results verify the proposed control scheme. [ABSTRACT FROM PUBLISHER]

This paper, along with an earlier published paper as Part 1, provides a comprehensive review of the state of the art of high-power converters (above 1 MW) for adjustable-speed ac drives. In this highly active area, different converter topologies have been developed for various drive applications in the industry. Due to its extensive coverage, the subject is divided into two parts: multilevel voltage source and current source converter topologies. This paper is focused on the second part and covers the current source converter technologies, including pulsewidth-modulated current-source inverters (CSIs) and load-commutated inverters. In addition, this paper also addresses the present status of the direct converter, which is also known as cycloconverter (CCV). This paper focuses on the latest CSI and CCV technologies and an overview of the commonly used modulation schemes. It also provides the latest technological advances and future trends in CSI- and CCV-fed large drives. This paper serves as a useful reference for academic researchers and practicing engineers in the field of power converters and adjustable-speed drives. [ABSTRACT FROM AUTHOR]

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point- clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications. [ABSTRACT FROM AUTHOR]

This paper presents the different technologies used in the generation of large controlled currents, in the kiloamperes range. After a brief review of processes requiring large currents, the paper discusses the working principles of thyristor phase-controlled rectifiers commonly used in these applications. Chopper-rectifiers using high-current insulated gate bipolar transistors are introduced as an alternative being considered in re. cent projects. The pulsewidth-modulated current-source rectifier, currently used in medium-voltage motor drives, is also analyzed as a future alternative for rectification in industrial processes. In addition, this paper presents the most important requirements and specifications to be considered in the applications of these high-power units. A system comparison is developed between thyristor and chopper-rectifiers in terms of quality of control, harmonics, power factor, losses, and efficiency. [ABSTRACT FROM AUTHOR]

This paper presents the application of high-power three-level active-front-end rectifiers to regenerate energy in a downhill conveyor system. The selective harmonic elimination method is used to eliminate harmonics 11 and 13, working with very low switching frequency, where six-pulse harmonic orders 6κ ± 1 are eliminated by the delta-wye connection of the transformer. In this way, the input current at the mains is highly sinusoidal with small harmonics starting at frequencies of order 23 and 25. Resonances have been detected, originated mainly by the capacitances of feeding cables and noneliminated harmonics, which produce high-voltage distortion. Theoretical and field measurements present the problem and the solution by using a specially designed high-pass power filter. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editors discuss various reports within the issue on topics including a diode-clamped converter, modulation methods for hybrid multilevel converters, and paralleled multi-level inverters and a space-vector-based hysteresis current control.