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Long prediction horizon finite control set model predictive control (LPH-FCS-MPC) for power converters can be reformulated as a box-constrained integer-least squares (ILS) problem to find the optimal control action without requiring an exhaustive search. Instead, the solution can be found by means of a sphere decoding method that still presents several intricacies regarding its complexity and its variable computational cost. This paper provides a study of the computational behavior of this approach. Special emphasis is placed on how the generator matrix is calculated, either as a lower or an upper triangular structure. This choice decides whether the switching sequences are explored forward- or backward-in-time during the optimization process. In this work, it is explained how this selection holds a great impact on the computational burden of the algorithm. Similarly, it is also analyzed how the tuning of the FCS-MPC and system parameters also drastically impacts the computational cost.

Finite control set model predictive control (FCS-MPC) is a salient control method for power conversion systems that has recently enjoyed remarkable popularity. Several studies highlight the performance benefits that long prediction horizons achieve in terms of closed-loop stability, harmonic distortions, and switching losses. However, the practical implementation is not straightforward due to its inherently high computational burden. To overcome this obstacle, the control problem can be formulated as an integer least-squares optimization problem, which is equivalent to the closest point search or closest vector problem in lattices. Different techniques have been proposed in the literature to solve it, with the sphere decoding algorithm (SDA) standing out as the most popular choice to address the long prediction horizon FCS-MPC. However, the state of the art in this field offers solutions beyond the conventional SDA that will be described in this article alongside future trends and challenges in the topic.

The application of model predictive control in electrical drives has been studied extensively in the past decade. This article presents what the authors consider the most relevant contributions published in the last years, mainly focusing on three relevant issues: weighting factor calculation when multiple objectives are utilized in the cost function, current/torque harmonic distortion optimization when the power converter switching frequency is reduced, and robustness improvement under parameters uncertainties. Therefore, this article aims to enable readers to have a more precise overview while facilitating their future research work in this exciting area.

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.

This paper presents an experimental investigation of tip vortex formation noise produced by wall-mounted finite airfoils. Acoustic measurements were taken in the open jet anechoic wind tunnel at Brandenburg University of Technology with a planar 47-microphone array. A parameter space of eight airfoil models with variations in airfoil profile shape (thickness and camber) and tip geometry were tested at a range of angles of attack and Reynolds numbers. The acoustic survey shows that tip noise is a strong contributor to the overall airfoil noise signature at mid-to-high frequencies above 2-3 kHz. A critical frequency was defined to distinguish the effect of the thickness of the airfoil on the noise spectra, which increases with Reynolds number and appears constant with angle of attack. Above the critical frequency, a thicker airfoil is shown to produce a lower amplitude tip noise peak, especially at mid-to-high frequencies above 5 kHz. An increase in camber does not affect the frequency of the tip noise peak but increases the noise level, especially for f≥3 kHz and high angles of attack. Further, the presence of a rounded tip is found to be effective in decreasing the noise levels by up to 5.6 dB over the frequency range of 5-15 kHz. To explore the tip flowfield, velocity data were also obtained using planar particle image velocimetry in the anechoic wind tunnel at the University of New South Wales for a few targeted cases. The results show that the size of the vortex and its turbulence intensity are influenced by the tip geometry. Specifically, the vortex size for the cambered airfoils decreases along the streamwise direction, while for symmetric airfoils it increases from x/c=1.2 to 1.8. Collectively, these databases can be used to aid tip noise prediction models and validate computational fluid dynamic and computational aeroacoustic simulations.

Airfoil tip vortex formation noise is a significant noise source in many aerodynamic applications such as aircraft, fans, rotors and propellers. The data collection presented in this paper examines the effects of sinusoidal geometry and porosity on the tip vortex formation noise produced by finite length airfoils. The use of serrated and porous materials is inspired by silent owl-wings and is a promising approach to control flow-induced noise. Noise measurements have been taken using a 47-channel planar microphone array in the anechoic wind tunnel at the Brandenburg University of Technology. Over 2600 unique test cases with variations in sinusoidal tip geometry (amplitude and wavelength) and flat tip porosity were measured during the experimental campaign for a NACA0012 and NACA614 airfoil. The microphone data have been processed using acoustic beamforming software named Acoular to produce one-third-octave band tip noise spectra.

The vortex generated at the tip of an airfoil such as an aircraft wing, wind turbine blade, submarine fin or propeller blade can dominate its wake and be a significant source of unwanted noise. The data collection presented in this paper consists of measurements of tip vortex formation noise produced by finite length airfoils with flat and rounded tips. These data were obtained using the specialist aeroacoustic test facilities at the Brandenburg University of Technology (BTU) in Cottbus, Germany and a 47-channel planar microphone array. Over 1200 unique test cases with variations in airfoil profile shape, tip geometry, angle of attack and Reynolds number were measured during the experimental campaign. The dataset contains one-third-octave band tip noise spectra that have been processed using Acoular, a Python module for acoustic beamforming.

The addition of an output LC filter to a voltage source inverter (VSI) allows the generation of high-quality output voltages with reduced harmonic distortion content. For this reason, this topology is widely used for uninterruptible power supply (UPS) applications. To control this system, finite control set model predictive control (FCS-MPC) has been assessed in previous research with promising results. In this paper, the dq-synchronous reference frame (dq-SRF) formulation of FCS-MPC and a state observer for an UPS system are presented and experimentally validated. The advantage of this proposal lies in the simplification of the observer design by considering a deadbeat observer along with the addition of a low-pass filter. In this way, the observer bandwidth is simply determined by the cutoff frequency of the low-pass filter. Simulation and experimental results are compared to assess the performance of the proposed solution.

Power electronics converters use switching elements to manipulate voltage and current waveforms. This enables the interconnection of components having different requirements, e.g., when incorporating renewable energy sources into the grid. The use of switching elements may lead to high energy efficiency. However, switching dynamical systems are difficult to analyse and design. In this chapter, we outline how model predictive control concepts can be used in power electronics and electrical drives. Special emphasis is given on the finite-set nature of manipulated variables and associated stability and optimization issues. For particular classes of system models, we discuss practical algorithms, which make long-horizon predictive control suitable for power electronics applicationsFrom a control systems perspective, power electronic systems give rise to in- trinsically challenging design problems. Specifically, three major challenges can be identified

© 2019 IEEE. Recently, a computational issue of sphere decoding algorithm (SDA) during transient operation of multistep model predictive control has been addressed in [1] and achieved its real-time implementation in [2] for a medium-voltage electrical drive system. This is achieved by projecting the unconstrained solution onto the convex-hull of the finite control set during transient operation. Therefore, a new initial sphere that guarantees feasibility and includes a significant smaller number of candidate solutions is obtained. This reduces the computation time required to solve the optimization problem. However, the reduction of the computational burden comes at the expense of (mild) suboptimal results [3]. This paper analyses the possibility to obtain a suboptimal solution by the SDA based optimization during transient operation. To deal with this suboptimality issue, this work explores the possibility to enlarge the convex-hull, whose size is by definition tied to the original finite control set. Therefore, in this work, the convex-hull is treated as a SDA initialization parameter during transient operation. As will be demonstrated, enlarging the convex-hull size reduces the possibility to obtain a suboptimal solution during the transient operation retaining, thus, the optimality during the whole converter operation.

In this paper, a new method is presented to correct for the distortions in beamforming maps caused by wind-tunnel shear layers during aeroacoustic testing. The shear layer correction method can be used for any convex shear layer shape including circles, ellipses, triangles, rectangles, octagons and squares. After deriving the methodology and proving its equivalence to one-dimensional methods, the shear layer correction method is used to correct the beamforming maps obtained experimentally from an airfoil placed in the potential core of a circular jet. The results show that the method can successfully remove the distorting effects of the shear layer on the airfoil trailing edge source distribution. Thus a new method is available to researchers to correct for the effects of shear layers in three-dimensions, which may also be extended to three-dimensional beamforming.

© 1986-2012 IEEE. This paper analyzes the limitations of a reduced-order model for modular multilevel converters (MMCs) by elucidating the relation between its accuracy, operating frequency, and converter parameters. A reduced-order simplifies the analysis of the MMC and thereby may provide additional information about the converter behavior. However, the accuracy of such model depends on several factors. In this paper, the effect of approximating the converter as a continuous system by neglecting quantization issues associated with having a finite number of modules is studied in detail. The analysis is done based on Fourier-series approximations with which it is possible to elucidate the relationship between the resonant frequencies of the MMC and the error of the reduced-order model. With the Fourier approximation, it is also possible to characterize resonant frequencies of the converter, both numerically and analytically, in terms of the converter parameters. The results can serve as a tool to identify situations when the reduced-order model produces good and also less accurate approximations especially when a low number of modules is available.

© 1972-2012 IEEE. In this paper, we present modifications to the sphere decoder initially introduced in the work of Geyer and Quevedo and modified in the work of Karamanakos et al. that significantly reduce the computation times during transients. The relative position of the unconstrained solution of the integer quadratic program underlying model predictive control (MPC) with respect to the convex hull of the (truncated) lattice of integer points is examined. If it is found that the unconstrained solution does not lie within the convex hull - a phenomenon that is observed mostly during transients - then a projection is performed onto the convex hull. By doing so, a new sphere that guarantees feasibility and includes a significant smaller number of candidate solutions is computed. This reduces the computation time by up to three orders of magnitude when solving the optimization problem at hand. Nonetheless, the reduction of the computational burden comes at a cost of (mild) suboptimal results. The effectiveness of the proposed algorithm is tested with a variable speed drive system consisting of a three-level neutral point clamped voltage source inverter and a medium-voltage induction machine. Based on the presented results, the sphere decoding algorithm with the proposed refinements maintains the very fast transient responses inherent to direct MPC. Moreover, it is observed that the occasional implementation of suboptimal solutions does not lead to a deterioration of the system performance.