Average Switching Frequency Research Articles

Collaborative Mid-Point Voltage Regulation in Low-Switching-Frequency MPC for Three-Level NPC Inverters Fed Dual Three-Phase PMSM Drives

In this paper, the control strategy is studied for the neutral-point-clamped three-level (NPC-3L) dual three-phase permanent-magnet synchronous machine (PMSM) drives at low-switching-frequency operation. The finite-control-set (FCS) model predictive control (MPC) scheme is investigated as the basic MPC scheme, where the average switching frequency is established as the cost function. With the high sampling frequency operation, the discretization with both forward and backward Euler methods is utilized so that the two sets of three-phase windings are controlled separately. To further decrease the switching frequency while maintaining the current and voltage control performance, a collaborative mid-point voltage regulation is proposed. The collaborative regulation strategy fully employs the additional control degree of freedom in the dual three-phase drives by setting the two NPC-3L inverters to select small vectors with opposite polarities. Therefore, the situation is avoided where the upper or the lower capacitor in the DC link provides load currents for the two sets of windings simultaneously. Simulation results of a 2.45MW dual three-phase drive and experimental results on a laboratory prototype have been presented to verify the validity of the proposed collaborative mid-point voltage regulation strategy.

Virtual Subspace-Based DTC Strategy for Torque Ripple Minimization in Six-Phase Induction Motors

Classical switching-table-based direct torque control (ST-DTC) of six-phase induction machine (6PIM) drives is seriously penalized by current harmonics as well as torque ripples because of uncontrolled voltage vectors in the low-impedance non-energy subspace and hysteresis torque regulator, respectively. Hence, a virtual subspace-based DTC strategy is proposed to alleviate the impact of low-order current harmonics and torque ripples. The proposed virtual subspace includes 48 virtual voltage vectors (VVs) in the $\alpha -\beta $ subspace with average volt-seconds of zero in the $z_{1}-z_{2}$ subspace, which offers full extent of freedom degrees for 6PIM fed by a two-level voltage source inverter (VSI). The increased number of VVs allows the possibility to design up to nine-level hysteresis torque regulator to minimize the torque ripples. Nevertheless, in this paper, it is found that torque ripple, current harmonics, average switching frequency, and computational burden come to a compromise with a seven-level hysteresis torque regulator. The impact of these VVs on the performance of ST-DTC is experimentally studied and some details of hardware implementation are presented. The performance of the proposed scheme is verified by simulations as well as laboratory experiments, where a complete comparison with recent schemes is made to reveal the superiority of the proposed scheme.

Finite Control Set Model Predictive Direct Power Control of Single-Phase Three-Level PWM Rectifier Based on Satisfactory Optimization

In this paper, a finite control set model predictive direct power control (FCS-MPDPC) method based on satisfactory optimization, for single-phase three-level PWM rectifier, is proposed to achieve global optimization that takes into account all its objectives and constraints. Satisfaction optimization aims to obtain the satisfactory solution after coordination of multiple goals, instead of the optimal solution of a single goal. By replacing “optimal” with “satisfaction”, more control degrees of freedom are acquired, so that low-priority auxiliary control objectives can participate in the optimization process. Meanwhile, in order to enhance the description accuracy of FCS-MPDPC method for the future trend of PWM rectifier, the prediction time domain is extended from the traditional single-step to multi-step, and the average switching frequency model is established to realize the low switching frequency control. The satisfactory optimized FCS-MPDPC method proposed in this paper is compared, through simulation and experimental results, with the standard FCS-MPDPC method, which verifies the effectiveness and superiority of the proposed algorithm.

On Migratory Behavior in Video Consumption

Today’s video streaming market is crowded with various content providers (CPs). For individual CPs, understanding user behavior, in particular how users migrate among different CPs, is crucial for improving users’ on-site experience and the CP’s chance of success. In this article, we take a data-driven approach to analyze and model user migration behavior in video streaming, i.e., users switching content provider during active sessions. Based on a large ISP dataset over two months (6 major content providers, 3.8 million users, and 315 million video requests), we study common migration patterns and reasons of migration. We find that migratory behavior is prevalent: 66% of users switch CPs with an average switching frequency of 13%. In addition, migration behaviors are highly diverse: regardless large or small CPs, they all have dedicated groups of users who like to switch to them for certain types of videos. Regarding reasons of migration, we find CP service quality rarely causes migration, while a few popular videos play a bigger role. Nearly 60% of cross-site migrations are landed to 0.14% top videos. Finally, we validate our findings by building an accurate regression model to predict user migration frequency as well as user survey, and discuss the implications of our results to CPs.

Model predictive torque control for permanent magnet synchronous motor based on dynamic finite-control-set using fuzzy control

Aiming at the problem of fixed candidate voltage vector (VV) in conventional finite-control-set (FCS) model predictive torque control (MPTC) of permanent magnet synchronous motor (PMSM), fuzzy control is use to determine dynamic candidate VVs based on effects of the value and angle of applying VV on stator flux and torque. The inputs of fuzzy controller are errors of stator flux and torque and outputs are 3 candidate VVs. Simulation results show compared with conventional FCS-MPTC, the proposed strategy can decrease stator flux and torque’s ripples and calculation burden of MPTC, but it will increase average switching frequency. In order to decrease average switching frequency, zero VV is also used as candidate VV, but it will decrease dynamic torque response. Thus the method that zero VV is used only at steady state is given. Simulation results show the control performance of the system remains basically unchanged and the average switching frequency is reduced.

Discrete Space Vector Modulation Based Model Predictive Flux Control With Reduced Switching Frequency for IM Drive

This article presents an optimal model predictive flux control (MPFC) for a two-level inverter fed induction motor. Integrating discrete SVM into FCS-MPFC enhances the performance of the IM drive. However, conventional DSVM-MPFC requires to enumerate and evaluate a higher number of virtual vectors in the prediction loop. In this paper, a high-efficient and low complexity voltage selection method is proposed to reduce the number of candidate voltage vectors from 38 to 15 without any suboptimality. Both steady-state and transient performances of the proposed method remain the same as the 38-vector based conventional DSVM-MPFC, producing the same cost-function values in all operating conditions. Furthermore, an online switching frequency reduction technique is proposed to achieve a minimum commutation per inverter vector change within each sampling cycle and between adjacent cycles. By appropriately arranging the sequence of real voltage vectors in each sampling cycle, a lower average switching frequency is achieved. The proposed switching frequency reduction method decreases the switching losses without compromising the performance of DSVM-MPFC as only the applied sequences of the real voltage vectors are optimized. Experimental studies are conducted to verify the effectiveness of the proposed algorithm.

Model Predictive Control of DC–DC SEPIC Converters With Autotuning Weighting Factor

In this article, a model predictive control (MPC) method for dc–dc single-ended primary-inductor converters with autotuning weighting factor capability is presented. The conventional MPC requires retuning of the weighting factor when the operating condition of the converter is changed. The effect of this change is observed as inability of the controller in maintaining the switching frequency constant. The weighting factor avoids excessive switching frequency in the dc–dc converters. Based on the relation between the inductor current ripple and switching frequency, an autotuning weighting factor based MPC is proposed. The effect of the weighting factor on the switching frequency is investigated. The proposed MPC eliminates the need for retuning the weighting factor when the operating point of the converter is changed. The proposed control strategy is verified experimentally under input voltage, load, and parameter variations. The results obtained from conventional and proposed MPC methods are compared. It is shown that the proposed MPC method controls the average switching frequency when the operating mode of the converter is changed. Furthermore, parameter mismatch results are presented for both conventional and proposed MPC methods.

A Comprehensive AC Current Ripple Analysis and Performance Enhancement via Discontinuous PWM in Three-Phase Four-Leg Grid-Connected Inverters

A complete analysis of the ac output current ripple in four-leg voltage source inverters considering multiple modulation schemes is provided. In detail, current ripple envelopes and peak-to-peak profiles have been determined in the whole fundamental period and a comprehensive method providing the current ripple rms has been achieved, all of them as a function of the modulation index. These characteristics have been determined for both phase and neutral currents, considering the most popular common-mode injection schemes. Particular attention has been paid to the performance of discontinuous pulse width modulation (DPWM) methods, including DPWMMAX and DPWMMIN, and their four most popular combinations DPWM0, DPWM1, DPWM2, and DPWM3. Furthermore, a comparison with a few continuous techniques (sinusoidal, centered pulse width modulations, and third harmonic injection) has been provided as well. Moreover, the average switching frequency and switching losses are analyzed, determining which PWM technique ensures minimum output current ripple within the linear modulation range at different assumptions. Numerical simulations and laboratory tests have been conducted to extensively verify all the analytical claims for all the considered PWM injections.

Low Complexity Model Predictive Flux Control for Three-Level Neutral-Point Clamped Inverter-Fed Induction Motor Drives Without Weighting Factor

Tuning of weighting factors is a very complex task in the implementation of conventional finite set predictive torque control (FS-PTC) for three-level neutral-point-clamped (3L-NPC) inverter fed IM drive. The choice of these weighting factors strongly influences the control objectives such as flux, torque, and neutral point voltage (NPV). Moreover, the computational burden is high as all 27 switching states are enumerated for the evaluation of a cost function involving multiple objectives. To reduce the complexity of the conventional FS-PTC, this article proposes a high-efficient single-vector-based predictive flux control for an IM drive fed by a 3L-NPC inverter. The proposed cascaded predictive flux and NPV control evaluates 13 vectors for outer predictive flux control and two switching states for the inner predictive NPV control. It significantly reduces the computational load as the number of enumerations is reduced by more than 50%. The nontrivial process of weighting factor calculation is no longer required. The complexity reduction and the elimination of weighting factors are achieved without sacrificing the transient and steady-state performances in terms of torque and stator flux responses, total harmonic distortion of stator currents, NPV variation, and average switching frequency, compared to the conventional method as verified by experimental results.

Unidirectional Asymmetric Hybrid Nine-Leg Rectifier With Floating H-Bridge Capacitors

This article proposes a unidirectional cascaded topology of a power rectifier. It is composed of a three-leg cell with half-controlled legs series connected with one H-bridge cell per phase. The dc links of the H-bridge cells operate with floating capacitors. The floating capacitor voltages regulation is solved by means of the common-mode voltage manipulation. Control and pulsewidth modulation strategies are presented to regulate the capacitor voltages and grid currents. The design of passive elements is also shown. The topology has its harmonic distortion content, semiconductor losses, and average switching frequency values compared to those of conventional solutions. Experimental results validate the proposed topology and strategy.

Predictive Direct Torque Control of Switched Reluctance Motor for Electric Vehicles Drives

The electrical drive systems utilized in Electric Vehicles (EVs) applications must be reliable and high performance. To providing these specifications, it is essential to design high-efficiency electric motors and develop high-performance controllers. This study introduces direct torque control of Switched Reluctance Motor (SRM) for electric vehicle applications using Model Predictive Control (MPC) technique. The direct torque control using MPC is proposed to maintain the motor torque and motor speed to tracking desired signals with a satisfactory response. In this study, the MPC algorithm was programmed in C- language, and the simulation tests were performed using a non-linear model of 6/4 - 60 kW SRM that is fed with the symmetrical converter. The proposed controller was tested under different load conditions to verify the robustness of the controller, as well as at variable speeds to investigate the tracking performance. Thanks to the proposed method, the SRM torque ripples, stator copper losses, and average switching frequency of the power converter can reduce effectively due to applying a cost function that combines multiple objectives. The obtained outcomes show the effectiveness of the suggested approach compared to conventional direct torque control techniques.

An optimal six vector switching pattern in matrix converters for reducing harmonics and switching loss

This paper analyses harmonic distortion caused by the narrow pulse problem in matrix converters, and presents an improved switching pattern that is based on the basic five vector switching pattern. With limited switching frequencies, the improved pattern solves the narrow pulse problem by applying an additional zero vector and adjusting the sequence of the four active vectors at the same time to minimize the switching times. Simulation results show that in the six vector switching pattern, a) the low-frequency harmonics caused by the narrow pulse problem is greatly reduced; b) the average switching frequency is reduced compared with the five vector pattern; c) the sampling frequency harmonics are apparently attenuated though a little 1/2 and 3/2 sampling frequency harmonics are produced; d) THD of output current is improved to the same level of the three zero vector switching pattern (TZVSP); e) The proposed switching pattern has benefit in both normal and unbalance input voltage conditions. Experimental results on a matrix converter platform verify the benefits of the six vector switching pattern. Faster dynamic, lower harmonics and lower loss can be achieved by using the proposed six vector switching pattern.

Multichannel spectrum access based on reinforcement learning in cognitive internet of things

With the development of Internet of Things(IoT), the demands for communication spectrum have increased rapidly, resulting in the shortage of limited spectrum resources. Cognitive IoT (CIoT) based on cognitive radio (CR) can improve the spectrum utilization by accessing the idle spectrum licensed to a primary user (PU). In this paper, a multichannel spectrum access scheme based on reinforcement learning (RL) is proposed to improve the spectrum access of CIoT, wherein the CIoT can use multiple channels for transmissions to reduce the communication interruptions. The channels are ranked in the decreasing order of their predicted idle probabilities, which can make the CIoT find enough idle channels quickly via decreasing the number of sensing operations and spectrum handoffs. The simulation results show that our proposed scheme is superior to the single-channel spectrum access scheme in terms of throughput, communication interruption, average collision probability and average spectrum switching frequency.

Switching frequency reduction technique for DSVM‐based predictive torque control of induction motor

This study proposes a new strategy to decrease the switching frequency of the predictive torque control–discrete space vector modulation (PTC–DSVM) method, which is a combination of the PTC scheme and the DSVM. The principle of the PTC–DSVM method is based on selecting a sequence of voltage vectors, called virtual voltage vectors, instead of a single voltage vector and applying it to the power converter in prefixed time intervals. The proposed switching frequency reduction method operates based on appropriately arranging the voltage vectors forming the virtual voltage vector so that, on average, a maximum of one commutation per inverter switching state change is achieved. As a result of the suggested method, the average switching frequency is reduced in all operating conditions, while the performance of the proposed method remains as satisfactory as the conventional approach. The reduction of switching frequency also results in a decrease in the switching losses. The proposed method is an offline approach and selects the appropriate application order for the real voltage vectors based on the described rules, which do not impose any further computational load to the control algorithm. In order to validate the proposed method performance, the control algorithm has been implemented on a laboratory prototype.

Model Predictive Current Control of Dual Inverter Fed Open-End Winding PMSM Based on Fast Vector Selection

Aiming at the problem of model predictive current for the dual two-level inverter needs to optimize the objective function of all 19 voltage vectors, which makes the calculation of optimization too large for practical application. A MPCC scheme for open-winding pmsm based on voltage vector preselection was proposed. First, the pre-selection of the voltage vector is performed by calculating the reference voltage vector using the deadbeat solution. And establish a voltage vector based objective function to achieve predictive current control and simplify the control algorithm. Next, the switching frequency constraint is introduced into the objective function to select the optimal switching state and reduce the average switching frequency. Finally, the simulation is verified and the effectiveness of the method is proved.

Direct Torque Control for Induction Motors Based on Minimum Voltage Vector Error

The traditional direct toque control (TDTC) for induction motors (IM) is afflicted by large torque ripple, high current total harmonic distortion (THD), and variable switching frequency. Recently, some improved direct torque control (DTC) methods have been proposed to address the above problems, but they often suffer from obscure concept, intensive computation, and poor robustness. To further improve the performance, this article proposes a DTC method based on minimum voltage vector (VV) error. The proposed strategy effectively optimizes the duty ratio of fundamental VV to minimize the error between reference VV and final VV imposed on motor terminals. The optimization algorithm dose not increase the system's complexity much and can be intuitively understood by a graphical interpretation. Experiment comparisons between the proposed strategy and some existing DTC methods are conducted on a 0.55-kW IM platform. The proposed DTC method has proven to achieve better steady-state performance, while retaining fast dynamic response of TDTC. Furthermore, the proposed method can obtain almost constant average switching frequency in the entire speed range, especially with rated load, due to the utilization of proportional-integral type torque regulator.

Predictive Duty Cycle Control for Four-Leg Inverters With ${LC}$ Output Filter

Three-phase four-leg inverter is a well-known solution to handle unbalanced and nonlinear loading conditions in three-phase transformerless uninterruptible power supply applications. For such power converters, finite control set model predictive control (FCS-MPC) is considered as a promising control alternative due to its simplicity, fast dynamic response, and capability to include nonlinearities and constraints. However, the absence of modulator in FCS-MPC implementation leads to several shortcomings, such as higher load voltage total harmonic distortion (THD), large tracking error of control objectives, and variable switching frequency. Furthermore, the fourth leg operates with a higher average switching frequency than the other three main legs, which complicates the $LC$ filter design. To overcome the aforementioned shortcomings, a new predictive duty cycle control is proposed in this article. Extensive experimental results are shown to verify that the proposed method can achieve enhanced steady-state performance with significant reduced load voltage THD and tracking error under different operating conditions, such as nonlinear single-/three-phase unbalanced load and single-phase short-circuit fault, while preserving the fast dynamic performance of FCS-MPC. Steady-state and dynamic performances of the proposed method are compared to FCS-MPC and double-vector MPC approaches to validate the superiority of the proposed scheme.

Optimized Switching Finite Control Set Model Predictive Control of NPC Single-Phase Three-Level Rectifiers

A new model predictive control scheme, optimized switching finite control set model predictive control, is studied in this article for neutral-point-clamped (NPC) single-phase three-level pulsewidth modulation (PWM) rectifiers. To ensure the actual stability of the power converter, the ac-side current error of single-phase PWM rectifier and the neutral point voltage of the converter are both converged to the bounded invariant set, and the adjustment of system performance is thus realized by setting the bounded invariant set. Based on the NPC single-phase three-level PWM rectifier model, the ac-side current, the neutral point voltage, and the current error in the future time can be predicted in advance. The above objectives are achieved by optimizing the selection of switch combinations. The advantage of the proposed method is that it can reduce the average switching frequency of the converter when the ac-side current error and the neutral point voltage of the converter are controlled within a certain range, which makes it suitable for the application of high-power single-phase PWM rectifier.

Modified Single-Edge SVPWM Technique to Reduce the Switching Losses and Increase PWM Harmonics Frequency for Three-Phase VSIs

This article proposed a modified single-edge space vector pulsewidth modulation (MS-SVPWM) technique by adjusting the zero vectors of saw-tooth carrier SVPWM technique for three-phase voltage source inverters to reduce the switching losses and increase the PWM noises frequency. Mathematical analysis and implementation of the proposed MS-SVPWM are demonstrated in detail. The differences in PWM noises reduction between MS-SVPWM and modified SVPWM technique are compared by time-varying PWM noises analysis, computer simulation, and experimental results. With the same average switching frequency of conventional SVPWM, MS-SVPWM could increase the PWM harmonics frequency by 50%. Employing the same carrier frequency, MS-SVPWM could reduce average switching frequency by 33% compared with conventional SVPWM technique, and the switching losses could be reduced by more than 25% when the phase angle is less than 60°. Reduction of switching losses could ease the cooling requirements and result in greater compactness of the system, especially vital concern in high-power applications and limited-space environments. The detailed experimental results confirm the effectiveness of the proposed MS-SVPWM at last.

Critical Switching in Globally Attractive Chimeras

We report on a new type of chimera state that attracts almost all initial conditions and exhibits power-law switching behavior in networks of coupled oscillators. Such switching chimeras consist of two symmetric configurations, which we refer to as subchimeras, in which one cluster is synchronized and the other is incoherent. Despite each subchimera being linearly stable, switching chimeras are extremely sensitive to noise: arbitrarily small noise triggers and sustains persistent switching between the two symmetric subchimeras. The average switching frequency scales as a power law with the noise intensity, which is in contrast with the exponential scaling observed in typical stochastic transitions. Rigorous numerical analysis reveals that the power-law switching behavior originates from intermingled basins of attraction associated with the two subchimeras, which in turn are induced by chaos and symmetry in the system. The theoretical results are supported by experiments on coupled optoelectronic oscillators, which demonstrate the generality and robustness of switching chimeras.