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Low-speed drive is one of the challenges for modular multilevel converters (MMCs) due to large capacitor voltage fluctuation. In this article, a cascaded MMC and cycloconverter (CCV) based machine drive system is proposed to ensure stable operation of medium-voltage and low-speed machine. The MMC provides medium-frequency ac voltage for the CCV, and the CCV converts the medium-frequency ac input to low-frequency ac output required by the machine. Detailed analysis about MMC's operation frequency, device current stress, and submodule (SM) capacitance are given in this article. Proposed drive system can operate at zero/low frequency under rated load torque, while the SM capacitance is much smaller than that in existing methods. Simulation and experimental studies are conducted, and the results verify the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

In this article, a low computational burden model predictive control (MPC) strategy without weighting factor is proposed for the single-phase cascaded H-bridge CHB converters. To reduce the switching state candidates, a hierarchy control algorithm is proposed. The grid current is controlled by selecting a subregion from the designed 2-D control plane, instead of the entire area. Two vectors are chosen in one sampling period for more accurate tracking. Then, the voltage balancing is achieved by selecting the optimal switching state from the subregion candidates to form the above two vectors. The cost function can be constructed of one variable: load voltage. Therefore, the weighting factor can be eliminated. No tuning or retuning processes are required in the proposed method. To reduce the computational time further, the principle of eliminating the switching state candidates operating the same voltage balancing performance is proposed. Conventional and proposed MPC methods are verified by experimental tests via a laboratory setup of a three-cell connected CHB converter. Steady-state and transient operations demonstrate that the proposed method guarantees less distortion grid current and shorter execution time (reduced from 15 to 3 $\mu s$). Fast response speed to variations in voltage reference and load resistance can be achieved t. [ABSTRACT FROM AUTHOR]

This article proposes a novel application method, inverse application of artificial intelligence (IAAI) for the control of power electronic converter systems. The proposed method can give the desired control coefficients/references in a simple way because, compared to conventional methods, IAAI only relies on a data-driven process with no need for an optimization process or substantial derivations. Noting that the IAAI approach uses artificial intelligence to provide feasible coefficients/references for the power converter control, rather than building a new controller. After illustrating the IAAI concept, a conventional application method of artificial neural network is discussed, an optimization-based design. Then, a two-source-converter microgrid case is studied to choose the best droop coefficients via the optimization-based approach. After that, the proposed IAAI method is employed for the same microgrid case to quickly find good droop coefficients. Furthermore, the IAAI method is applied to a modular multilevel converter (MMC) case, extending the MMC operation region under unbalanced grid faults. In the MMC case, both simulation and experimental online tests validate the operation, feasibility, and practicality of IAAI. [ABSTRACT FROM AUTHOR]

In this article a finite control set model predictive control, is used to track a current reference in a power converter connected to an $RL$ load. An artificial intelligence approach is presented for real-time determination of the weighting factor that regulates the average switching frequency, independently of the operating point. The article focuses on the design, training, and digital implementation of an artificial neural network (ANN) that can be developed in a low-cost control platform. It is presented a sampling and offline ANN training procedure, together with a low-cost hardware implementation based on integer quantization of the ANN. The abovementioned approach provides a standalone application, serving as a framework for the development of ANN applications for power-converters. The main advantage of the presented approach is that the ANN inference is executed in real time. In this way, the weighting factor is automatically updated in real-time, allowing the system to quickly adapt to any reference step changes, and consequently provide the desired behavior. Executing the setup in laboratory prototype confirmed the theoretical analysis and successful tracking of the reference frequency. [ABSTRACT FROM AUTHOR]

Due to the high-performance requirements of power converters in industrial applications, sliding-mode control has drawn extensive research attention for its insensitivity against external disturbances. In order to ensure this advantage while suppressing the chattering problem caused by discontinuous switching functions, in this article, a fuzzy sliding-mode control (FSMC) strategy consisting of three control loops is proposed for three-phase three-level neutral-point-clamped active front-end rectifiers. In the power tracking loop, an improved super-twisting algorithm is proposed to enforce the tracking errors of active power and reactive power to bounded regions in finite time and guarantee the transient performance. Next, in the dc-link voltage regulation loop, the regulation error of dc-link voltage converges to a bounded region with a super-twisting extended state observer-based FSMC strategy. A simple but efficient proportional-integral controller is employed in dc-link voltage balancing loop to reduce the voltage difference between two dc-link capacitors. The proposed controller is characterized by attenuated chattering phenomenon and improved the dc-link voltage regulation performance, disturbance rejection ability, and power tracking performance. Experimental results are provided to verify the effectiveness and superiority of the proposed strategy. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS-MPC) strategies for power conversion devices benefit from extending the prediction horizon length. Solving this problem relies on the definition of the underlying integer least-squares problem. Sphere decoding algorithm (SDA) has been extensively used in previous works as an approach to solve this problem. In this article, a parallel and fully scalable K-best SDA hardware design is proposed as an alternative. The K-best SDA establishes a different breadth-first search strategy, which addresses some of the main drawbacks of the SDA. Through experimental tests based on an uninterruptible power supply, the K-best SDA performance for long prediction horizon FCS-MPC is assessed and verified. Results demonstrate key beneficial aspects through which the K-best SDA is capable of rendering an improved control performance when compared to the conventional SDA. [ABSTRACT FROM AUTHOR]

In this article, a nonlinear high-gain observer (NHGO)-based second-order sliding mode (SOSM) control strategy is proposed for the three-phase three-level neutral-point-clamped (NPC) converter. This controller applies the advanced SOSM algorithm both in the voltage regulation loop and in the power tracking loop, which provides a fast dynamic for the dc-link voltage, and also assures a good steady-state behavior for the NPC converter. Additionally, an NHGO technique is implemented in the voltage regulator combining with the SOSM algorithm. The conventional observer-based controllers suffer from the destructive effects of measurement noise, and it can only be addressed by diminishing the observer gain, which sacrifices the observer property. The NHGO technique adopts a time varying gain, that is, high gain in transient while low gain in steady state, which minimizes the adverse influence of measurement noise. The tuning method of the proposed NHGO-based SOSM controller is given to simplify the implementation process. Finally, the simulation and experimental results of the proposed control scheme for the NPC converter are given and compared with the conventional PI controller as well as the well-known linear extended state observer-based control method, which validates the feasibility and superiority of the proposed controller. [ABSTRACT FROM AUTHOR]

Modular converters based on the connection of multiple power cells have become a competitive solution for many applications, achieving high performance with reduced production and maintenance costs. However, their performance is degraded if the power modules present an unbalanced operation, which is a common case in some applications and/or when they implement active thermal control methods. Variable-angle carrier-based pulsewidth modulation (PWM) strategies face this challenge by minimizing the negative impact of unequal power sharing among the cells. They do this by improving the harmonic quality of the output waveforms, which has a direct impact on the lifetime of the output filters and/or components. [ABSTRACT FROM AUTHOR]

This article proposes a sliding-mode-observer (SMO)-based control strategy to regulate the dc-link voltage for a three-phase two-level active front end (AFE). The SMO is designed for the voltage control loop to estimate the external load which is abruptly connected to the AFE dc-link and consequently causes the dc-link voltage fluctuation. The estimated load value is used to compensate the voltage loop controller, therefore, the voltage loop gains more robustness against the load perturbation and its disturbing effect is greatly reduced. The effectiveness and advantage of the proposed control strategy has been verified through theoretical analysis, simulations, and the real-application experiments conducted on a 5 KVA laboratory AFE. The results show that the proposed control strategy provides obvious improvement of the dc-link voltage control performance comparing with the conventional PI controller and demonstrates stronger robustness against the operating point variations caused by the changes in external load and dc-link capacitance. [ABSTRACT FROM AUTHOR]

Modular multilevel converters have become the prominent topology for medium- and high-voltage applications. The performance of these converters highly depends on the accuracy of the used modulation approach, for which the capacitor voltage of submodules (SM) are usually assumed to be equal. This article exhibits that ignoring the capacitor voltage differences among SMs adversely affects the system performance. This becomes more obvious the larger the capacitor voltage differences are. Hence, this article proposes a more accurate feedforward modulation approach that takes into account either the instantaneous capacitor voltage value and the real output voltage in the modulation stage. As a result, in applications where larger SM voltage differences are expected, the current distortion and control performance are improved. Particularly, switching–saving approaches benefit from this method as it enables their operation with reduced switching losses without the downsides of increased distortion due to capacitor voltage differences. The proposed approach is analyzed and compared with the nearest-level modulation and with the level-shift PWM. Simulations and experimental validation are presented to confirm the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

This article proposes a novel robust control strategy for three-phase power converters operated under unbalanced grid conditions. A consolidated control objective is obtained in the stationary $\alpha \beta$ frame, which can be flexibly adjusted according to the degree of oscillation in the active and reactive powers and the balance of the three-phase current. Based on the dynamics of the converter and control objective, a control scheme in a cascaded framework is presented, in which an adaptive observer is applied to estimate the positive and negative sequences of the grid voltage. In the current tracking loop, a super-twisting algorithm current controller coupled with a super-twisting differentiator is implemented to track the current references, featuring rapid dynamics, and improved robustness. Additionally, in the voltage regulation loop, an effective composite controller is developed to regulate the dc-link voltage, where a super-twisting observer is used to estimate the load disturbance, thereby improving the performance of the converter. The experimental results are provided to confirm the effectiveness and superiority of the proposed control strategy. [ABSTRACT FROM AUTHOR]

A finite control set model predictive control (FCS-MPC) strategy consists of a prediction model, a cost function and an optimization algorithm. Consequently, the performance of the FCS-MPC depends on the proper design of these three elements. This article assesses the influence of the prediction model of an uninterruptible power supply (UPS). Since the load connected to the voltage source inverter (VSI) affects the dynamic of the system state variables, the load dynamic should be included in the system model. This makes the design of the prediction model a challenge because the load connected to the VSI is generally unknown. To deal with this uncertainty, this work proposes an augmented prediction model based on a state observer that includes as many harmonic components as necessary to accurately represent the output current. The performance of the FCS-MPC for a UPS is evaluated in a laboratory prototype using the proposed and the conventional prediction models. Experimental results show that the proposed solution provides a more accurate representation of the output current, improving the system performance. [ABSTRACT FROM AUTHOR]

In this article, a two-stage control scheme consisting of an adaptive-gain second-order sliding mode (SOSM) controller and a switched high-gain observer (HGO) is proposed for the three-level neutral-point-clamped (NPC) converter. The adaptive-gain SOSM control method is applied both in the voltage regulation loop and instantaneous power tracking loop, thus, the boundary of the disturbance derivative does not need to be known a priori. Compared with the fixed-gain SOSM, it provides a faster dynamic and a better steady-state response for the NPC converter. On the other hand, the conventional disturbance compensation observer used in the power system suffers from the adverse effects of measurement noise, which limits the performance of the observer. A switched HGO is combined with the adaptive-gain SOSM controller in the voltage regulation loop to address this issue. By using a switched observer gain, the switched HGO greatly diminishes the performance degradation induced by the inevitable measurement noise. Finally, several experiments between the representative extended state observer-based SOSM control scheme and the proposed control method are carried out for a comparison. The results demonstrate the feasibility and effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

The cascaded H-bridge converter (CHB) is a very suitable solution with inherent modularity for many applications such as flexible ac transmission systems and motor drives. In this article, the discontinuous pulsewidth modulation (DPWM) by clamping some power cells is used in the CHB because it permits to reduce the switching losses in the power devices. However, this provokes a high harmonic distortion (basebands and sidebands harmonics) in the CHB output voltage compared with the quality obtained avoiding to clamp cells, just applying the conventional phase-shifted PWM (PS-PWM) technique. This has been studied for the three-cell CHB topology but the extension for an N-cell CHB with several clamped cells is a challenge that is addressed in this article. A generalized CHB harmonic voltage analysis applying the DPWM method is presented considering multiple clamped cells. The harmonic distortion mitigation target can be decoupled into two objectives. The proposed method cleverly groups the cells (one clamped + one or two nonclamped cells) to mitigate the basebands harmonic distortion. Simultaneously, a variable-angle PS-PWM technique is applied to the CHB in order to mitigate the sidebands harmonics content. Experimental results demonstrate the good performance of the proposed method. [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]

In recent years, different advanced control methods have been successfully proposed as alternatives to conventional cascaded linear controllers for power converters in distributed generation systems and microgrids. The prime movers of this research are strong capabilities of advanced controllers to improve the dynamic performance and robustness of power electronic converters in these applications. This article first introduces the key roles and functionalities of voltage source converters in distributed generation systems and microgrids. Then, it describes how these functionalities are traditionally achieved by using linear controllers, and addresses their fundamental dynamic performance limitations. Afterward, the most prominent advanced control methods are overviewed. In this context, the implementation principles, advantages, and disadvantages of prominent model- and data-based advanced control methods are critically discussed and experimentally compared. This article ends with a discussion about promising research directions in the area of advanced control for power electronic converters. [ABSTRACT FROM AUTHOR]

Electrification has been a key component of technological progress and economic development since the industrial revolution. It has improved living conditions, spurred innovation, and increased efficiency across all sectors of our economy and all aspects of our lives. During the coming decades, electrification is expected to reach further and deeper into the transportation, building, and industry sectors, mainly motivated by the energy transition to a zero-carbonemission-based economy to mitigate climate change. [ABSTRACT FROM AUTHOR]

This article proposes a new control method for modular multilevel converter (MMC), which is based on a modification of the conventional nearest level control (NLC) scheme implemented by a model predictive control (MPC) technique. Applying the proposed method, the simplicity of the NLC is maintained while the current control ability in terms of both phase current and circulating current is enhanced by the MPC technique. A proper rounding function is used for each arm of the MMC instead of the conventional nearest integer function. The rounding function is determined by evaluating a cost function within the frame of MPC. To validate the proposed technique, a three-phase MMC prototype is employed. The conventional NLC-based control approach of MMC is also evaluated for comparative purposes. Results show that the proposed method exhibits excellent current-regulating ability. Compared with the conventional control scheme, the overall performance is improved without significantly increasing the computational burden or adding relevant complexity to the control system design. [ABSTRACT FROM AUTHOR]

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]

This paper presents voltage regulation and current tracking control strategies for three phase two-level grid-connected power converters. By using power-invariant Park’s transformation, an averaged mathematical model of power converters is obtained in dq synchronous reference frame. Then a novel control strategy using adaptive control and ${H}_{\infty }$ technique is proposed to regulate the dc-link output voltage as well as track a desired current reference for three-phase power rectifiers. More specifically, an efficient adaptive controller is established in the external loop for regulating dc-link output voltage in the presence of external disturbances. A set of ${H}_{\infty }$ controllers are designed in the internal loop to force the input currents track their desired values. Finally, simulation results obtained from the proposed control method are presented, analyzed, and compared with that of sliding mode control, and the superiority of the proposed control laws is verified. [ABSTRACT FROM AUTHOR]

Multilevel cascaded H-bridge (CHB) converters are an attractive solution for multiple applications like flexible ac transmission systems and solid-state smart transformers, among others. This article is focused on a CHB working with unbalanced conditions per cell (different cell dc voltages and/or cell voltage references). In this scenario, the phase-shifted pulsewidth modulation (PS-PWM) technique produces a harmonic distortion around twice the carrier frequency and its multiples in the CHB output voltage. In this article, a generalized variable-angle PS-PWM method is presented in order to mitigate this drawback improving the CHB output voltage harmonic spectrum. The proposed method is based on applying a mathematical search algorithm to determine the phase displacement angles to be applied to the PS-PWM technique. Experimental results show how this objective is achieved without introducing any disadvantage in the CHB operation. The method can be applied to CHB with any number of cells. [ABSTRACT FROM AUTHOR]

In this article, a model predictive control (MPC) strategy based on the optimal switching sequence (OSS) concept for a single-phase grid-connected H-bridge neutral-point-clamped (H-NPC) power converter is presented. The proposed OSS-MPC algorithm considers both the grid current tracking error and the dc-link capacitor voltage balance. Special emphasis is placed on the power converter control region in order to design suitable switching sequence candidates for this multiobjective control problem. Additionally, based on an analysis of the weighting factor effect over closed-loop performance, it is possible to demonstrate that this controller parameter is relatively easy to adjust. In fact, the weighting factor only affects the peak current during transients, with no effect over the steady-state performance. As a result, the proposed OSS-MPC provides a fast closed-loop dynamic to the H-NPC converter, which operates with a fixed switching frequency at all times. This predictive control strategy is experimentally validated in a 3.5-kVA laboratory setup. [ABSTRACT FROM AUTHOR]

This article proposes a closed-loop analytic filtering scheme for the low-frequency capacitor voltage ripple in cascaded H-bridge converters. Although this technique can be applied to a number of cascaded converter applications, this article focuses particularly on the low-capacitance cascaded H-bridge static compensator. Filtering of capacitor voltage signals is particularly important in this application, due to the presence of large low-frequency harmonic voltage ripple. The main novelty of the proposed algorithm in this article is the estimation of the capacitance value of each H-bridge. The closed-loop capacitance estimation method reduces steady-state error in estimated voltage ripple magnitude and phase, which is otherwise present in the available open-loop filters in the literature. Furthermore, fast dynamic response is achieved compared to filtering schemes based on low-pass and bandstop filters. Therefore, the proposed solution optimizes the tradeoff between filtering accuracy and transient response. It also mitigates parametric uncertainties, time delays, and harmonic contamination in the outer voltage control loop. Furthermore, the estimated capacitance is also useful to track the state of health of the dc-link capacitors. Experimental results on a seven-level 1-kVA cascaded H-bridge prototype are presented to demonstrate the comparative performance of the proposed filtering scheme and traditional approaches. [ABSTRACT FROM AUTHOR]

In this article, a linear extended state observer (LESO) based second-order sliding mode (SOSM) control strategy with the direct power control is proposed for a three-phase neutral-point-clamped (NPC) power converter connected to a dc microgrid. Comparing with the PI control method, the proposed approach implements the advanced SOSM controller into the voltage regulation loop and instantaneous power tracking loop to enhance the dynamic and steady state performance. Furthermore, saturation function is applied in the SOSM method to weaken the chattering phenomenon. On the other hand, since the dc load is regarded as an external disturbance, an efficient LESO is designed in the voltage regulation loop to reject this disturbance. The design process of the proposed control strategy is shown based on the continuous averaged model of the NPC converter. Finally, comparison experiments among PI, LESO-based PI, and proposed LESO-based SOSM control strategies are implemented, which validate the superiority of the proposed approach. [ABSTRACT FROM AUTHOR]

Cascaded H-bridge converter (CHB) is a multilevel topology that is a well-suited solution for multiple applications such as flexible ac transmission systems or motor drives. This paper is focused on a CHB where the cells present an aging mismatch. This can be caused by the maintenance operation which forces the replacement of some damaged cells of the converter with new or repaired ones. In this paper, a new improved approach of the active thermal control (ATC) of the CHB using discontinuous pulsewidth modulation (PWM) (D-PWM) is presented. The D-PWM technique is used to reduce the power losses of one cell reducing its average temperature in order to increase its remaining lifetime. However, the combination of D-PWM with traditional phase-shifted PWM (PS-PWM) introduces high harmonic distortion in the output voltage of the CHB converter at twice the carrier frequency. A detailed harmonic distortion analysis of the CHB output voltage when the D-PWM based ATC is active is presented. From this analysis, a modification of the traditional PS-PWM is derived to eliminate the harmonic distortion at twice the carrier frequency. Experimental results show how the ATC using D-PWM is achieved whereas the harmonic distortion around twice the carrier frequency is eliminated. [ABSTRACT FROM AUTHOR]