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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]

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]

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]

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 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]

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]

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]

This paper proposes a novel control strategy for three-level neutral-point-clamped (NPC) power converter. The proposed control scheme consists of three control loops, i.e., instantaneous power tracking control loop, voltage regulation loop, and voltage balancing loop. First, in the power tracking control loop, a set of adaptive sliding mode controllers are established to drive the active and reactive power tracking to their desired values via radial basis function neural network technology. In the voltage regulation loop, an efficient but simple adaptive controller is designed to regulate dc-link output voltage where the load is considered as an external disturbance. Moreover, a composite controller is developed in the voltage balancing loop to ensure imbalance voltages between two dc-link capacitors close to zero, in which a reduced-order observer is used to estimate sinusoidal disturbance improving the converter performance. The effectiveness and superiority of the proposed control strategy for the NPC power converter are compared with other control schemes through experimental results. [ABSTRACT FROM AUTHOR]

A new approach to the control of three-phase two-level grid-connected power converters is proposed in this paper. The proposed control is an extended state observer (ESO)-based second order sliding mode (SOSM), which comprises two control loops: the outer loop is a voltage regulation loop, as well as inner loop is an instantaneous power tracking loop. The outer loop is accomplished by an ${H}_{\infty }$ controller plus an ESO, which is designed to regulate dc-link capacitor voltage of the converter and asymptotically reject external disturbances and parameter perturbations. The SOSM strategy is employed in the inner loop to drive the active and reactive power convergence to their desired values. Control objectives of nearly unity power factor and dc-link capacitor voltage regulation are simultaneously satisfied. Availability of the ESO-based SOSM is compared with the classic proportional-integral control in simulations, and the comparison implies that the proposed strategy not merely achieves an almost perfect tracking performance, but also provides a complete robustness against resistance load variation. [ABSTRACT FROM AUTHOR]

In the last decades, multilevel converters have been developed usually for medium-voltage high-power applications. They have become a mature solution for the increasing power demand of multiple applications such as renewable energy systems, power quality improvement, and motor drives. In this paper, the operation of multilevel converters is addressed focusing on control and modulation techniques for different well-known applications. The new developments are presented as an extension of conventional methods for two-level voltage-source converters which are still the mainstream solution for most cases. [ABSTRACT FROM PUBLISHER]

This paper proposes an extended state observer (ESO) based second-order sliding-mode (SOSM) control for three-phase two-level grid-connected power converters. The proposed control technique forces the input currents to track the desired values, which can indirectly regulate the output voltage while achieving a user-defined power factor. The presented approach has two control loops. A current control loop based on an SOSM and a dc-link voltage regulation loop which consists of an ESO plus SOSM. In this work, the load connected to the dc-link capacitor is considered as an external disturbance. An ESO is used to asymptotically reject this external disturbance. Therefore, its design is considered in the control law derivation to achieve a high performance. Theoretical analysis is given to show the closed-loop behavior of the proposed controller and experimental results are presented to validate the control algorithm under a real power converter prototype. [ABSTRACT FROM PUBLISHER]

In this work, a Model Based Adaptive Direct Power Control (MB-ADPC) with constant switching frequency for Three-Phase Three-Level Neutral Point Clamped (3L-NPC) converters is proposed. The rectifier and inverter operation mode are used to illustrate the flexibility of the proposed MB-ADPC controller. The control design process is based on the continuous averaged model of the system. Depending on the operation mode different control objectives have to be guaranteed. The proposed controller ensures the voltage regulation of the dc-link capacitors for the rectifier operation mode and to achieve voltage balance in the dc-link capacitors and the active and reactive power tracking for the rectifier and inverter operation modes. In addition, adaptive techniques are used to avoid system parameters uncertainties as smoothing inductors and grid frequency values. This work shows that the application of advanced control strategies based on the system model allows enhancing the performance of the overall system. The details of the controllers design process and the experimental results using a 50 kVA Three-Phase Three-Level NPC prototype are presented in this paper validating the proposed controllers. [ABSTRACT FROM PUBLISHER]

The papers in this special section focus on predictive control in power converters and electrical drives. With the fast microcontrollers available today, applications of predictive control in power converters and electrical drives are a very powerful and attractive alternative to classical controllers. The use of predictive control offers a number of advantages: very intuitive approach, no need for linear controllers and modulators, easy inclusion of nonlinearities and restrictions, etc. However, predictive control schemes in power converters and electrical drives have not yet been implemented in industrial applications. Nevertheless, after some further progress, it can be expected that the advantages of predictive algorithms will lead to an increased number of industrial applications in the future. [ABSTRACT FROM PUBLISHER]