Low Switching Frequency Research Articles

Robust nonlinear model predictive control for automatic train operation based on constraint tightening strategy

AbstractThis paper studies the problem of automatic train operation (ATO) robust nonlinear model predictive control under considering multiple objectives and constraints. After establishing a nonlinear multipoint model with uncertain bounded disturbance, a robust nonlinear model predictive control algorithm to meet the punctuality of train operation and energy consumption for ATO is proposed based on constraint tightening strategy. Moreover, theoretical analysis of the feasibility and stability for the speed loop system are presented. Then, with the objective of reference electromagnetic torque tracking and low switching frequency, a model predictive direct torque control algorithm with one‐step delay compensation is proposed. Feasibility of the proposed algorithm is ensured by using deadlock prediction method, and convergence analysis of the torque loop is given simultaneously. Lastly, the effectiveness of these two algorithms are verified by numerical simulation.

Extendable Switched-Capacitor Multilevel Inverter With Reduced Number of Components and Self-Balancing Capacitors

Multilevel inverters (MLIs) with self-balanced switched-capacitors (SC) have received wide recognition for increasing the reliability and efficiency of renewable energy and high-frequency power distribution systems. This article presents a new SC MLI structure using a reduced number of components. The proposed dual-source SC MLI can be extended in various ways to increase the voltage levels at the output. The SCs are self-balanced by using a suitable charging-discharging pattern, and thereby, voltage boosting is achieved. The operational analysis and features of the proposed 25-level MLI are delineated in detail. Comparative analysis with state-of-the-art MLIs in terms of the number of components, voltage stress, and cost factor demonstrate the merit of the proposed extendable SC MLIs. Extensive simulation of the proposed MLI structure is performed on the MATLAB/Simulink environment using both the low and high switching frequency control schemes. Furthermore, simulation results are validated experimentally by developing a prototype of the proposed MLI under load variations, frequency change condition, and variation in the modulation index.

Hybrid Modulation for Modular Voltage Source Inverters with Coupled Reactors

This paper proposes and discusses a concept of a hybrid modulation for the control of modular voltage source inverters with coupled reactors. The use of coupled reactors as the integrating elements leads to significant reduction in the size and weight of the circuit. The proposed modulation combines novel coarsely quantized pulse amplitude modulation (CQ-PAM) and innovative space-vector pulse width modulation (SVPWM). The former enjoys very low transistor switching frequency and low harmonic elimination, while the latter ensures high resolution of amplitude control. The SVPWM is based on the use of barycentric coordinates. The feasibility of the proposed solution is verified by simulations and laboratory tests of a 12-pulse modular voltage source inverters with two-level and three-level component inverters.

A Generic Scheduling Algorithm for Low-Frequency Switching in Modular Multilevel Converters With Parallel Functionality

Since the introduction of modular multilevel converters (MMC), various additional features have been introduced for MMCs, among them parallel inter-module connectivity. However, such a parallel mode requires appropriate modulation and switching strategies to exploit its full potential. Low-frequency switching modulation is widely used with half-bridge modules and a large body of research focusses on the optimum control and scheduling of these methods. However, so far, a suitable adaptation for MMCs with parallel mode is missing, and existing methods perform suboptimum. This article proposes a generic scheduling algorithm for simple and low-cost integration with low-frequency switching modulation. The proposed method reduces control complexity, minimizes power loss, and can be combined with most modulation techniques, including nearest-level modulation (NLM) and selective harmonic elimination. The text in this article provides a general algorithm for integrating this method with other control levels and analyzes its effect on MMCs with parallel functionality. Moreover, it examines the performance of the system with NLM combined with the proposed scheduler and compares it to other alternatives. Results show up to a 50% reduction in power loss with a similar modulation technique as well as significantly lower THD and power loss compared to optimized phase-shifted carrier modulation as another sensorless alternative method.

Modeling and Compensation for Dead-Time Effect in High Power IGBT/IGCT Converters with SHE-PWM Modulation

Research on applying selective harmonic elimination pulse width modulation (SHE-PWM) to high power converters has drawn tremendous interest, due to the advantages of low switching frequency and high output harmonic performance. In the fields of high power converters such as variable speed traction motor drives and static synchronous compensators (STATCOM), the adoption of high voltage but slow speed semiconductor devices, i.e., IGBT/IGCT, results in a longer dead time of several microseconds, which leads to a motor vibration in the former case or the distortion of grid current in the latter case. This paper analyzes in detail the mechanism of the dead-time effect on 3-level SHE-PWM with different operating conditions considered. For the first time, a general mathematical model describing the relationship between the dead time and harmonic distribution of SHE-PWM wave is established. Based on which an open-loop compensation method by inserting a margin time into the effective switching angles is proposed. Furthermore, a closed-loop controller that implements online adaptive adjustment of the margin time is designed in case of a variable frequency application. The effectiveness of the proposed method in different scenarios is verified through simulation results.

Active-Damping Disturbance-Rejection Control Strategy of LCL Grid-Connected Inverter Based on Inverter-Side-Current Feedback

In the medium- and high-power grid-connected photovoltaic systems, the controller bandwidth is constrained due to the low switching frequency. This issue limits the resonant frequency of the traditional proportional multiresonant (PMR) controller and then makes it difficult to suppress the main low-frequency harmonics in the grid-connected current. In addition, it is difficult for PMR to maintain good performance under severe operating conditions, such as the grid voltage containing the resonant-frequency harmonic and the grid voltage sagging. Based on the abovementioned practical problems and on-site requirements, a novel first-order active-disturbance-rejection active-damping control strategy based on the inverter-side-current feedback (ICF-ADRC-AD) for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> grid-connected inverter is proposed in this article. The standard form of the system block diagram is deduced, and the stability and immunity of the system are analyzed. Finally, the effectiveness and superiority of the proposed control strategy under different operating conditions are verified by simulation and experiment. The control strategy proposed in this article improves the suppression effect of low-frequency total harmonic and resonance and also improves the stability and immunity of the system.

Large virtual voltage vectors for direct controllers in six-phase electric drives

The appearance of virtual voltage vectors (VVs) has simultaneously provided two attractive features for multiphase drives: a simple and effective regulation of the x-y currents and a natural fault tolerance that avoids any control reconfiguration after an open-phase fault occurrence. VVs in six-phase drives were initially constructed using large and medium-large voltage vectors, and all subsequent works adopted this procedure without contemplating other alternatives. The use of medium-large vectors limits however the utilization of the dc-link voltage and brings undesired switching harmonics. This work goes back to the initial definition of the VVs and suggests using exclusively large (adjacent) voltage vectors to create the newly proposed large virtual voltage vectors (LVVs). Experimental results confirm that LVVs provide better current quality, lower switching frequency and better dc-link voltage utilization than standard VVs.

A Finite-Control-Set Linear Current Controller With Fast Transient Response and Low Switching Frequency for Grid-Tied Inverters

This article presents a linear current controller for a grid-tied inverter with an L or LCL filter that uses a finite-control-set (FCS) operation. Thanks to the FCS operation, the proposed controller provides a very fast transient response, similar to an FCS model-predictive controller (MPC), while needing a low switching frequency and a low computational load. Contrarily to MPC solutions, the proposal uses a linear control structure. Such a linear structure offers a straightforward implementation and stability assessment compared to an MPC, which usually requires complex optimization algorithms and Lyapunov functions that are a field of study in its own right. The proposal does not generate any switching or low-order harmonics. By design, the switching noise is evenly spread at all frequencies instead of it being concentrated at some harmonic frequencies, as in the case of pulsewidth-modulator-based controllers. This reduces the risk of exciting resonances in the utility grid and facilitates the operation of multiple inverters in parallel. In particular, the proposal is shown to achieve compliance with stringent grid harmonic codes using an L filter in a multimegawatt application, where previous proposals required an LCL filter with the same total inductance.

A Comprehensive Review of Capacitor Voltage Balancing Strategies for Multilevel Converters Under Selective Harmonic Elimination PWM

In recent decades, applications of selective harmonic elimination pulsewidth modulation (SHE-PWM) have been extended from two-level to multilevel converters (MLCs). For most MLC topologies, one of the main challenges of using SHE-PWM lies on capacitor voltage balancing, especially with very low switching frequency in high-power applications. Due to the broad variety of MLCs, it is of great difficulty to develop a generalized capacitor voltage balancing method under SHE-PWM that is suitable for all topologies. In order to further facilitate the industrial applications of SHE-PWM on MLCs, this article provides a comprehensive review of the state-of-the-art capacitor voltage balancing strategies for MLCs under SHE-PWM. The voltage balancing methods in this article include self-balancing control, charge amount regulation, zero-sequence harmonic adjustment, redundant switching angle sets adjustment, angle modification, selective harmonic elimination model predictive control, space voltage vectors adjustment, and redundant states adjustment. Detailed comparisons of the eight voltage balancing strategies under SHE-PWM are presented to facilitate the selections of the most suitable method for specific applications. Moreover, discussions on open questions and future trends of this topic are also presented, to motivate future research and explore new alternatives.

Modeling and Analysis of Inner Controls Effects on Damping and Synchronizing Torque Components in VSG-Controlled Converter

In virtual synchronous generator (VSG)-related discussions, the inner voltage and current controls are always omitted due to their fast dynamics. However, converters require a lower switching frequency as their power rating increases, which constrains the cascaded controls’ bandwidths. Hence, the interactions between the controls at different time-scales should be analyzed. Specifically, the aim is to reveal the effects of inner control loops on the VSG system from the torque perspective. First, this paper derives a linearized model of the internal voltage phase and amplitude, which accurately depicts the VSG system characteristics. Subsequently, based on this linearized model, we derive an equivalent single-input single-output motion-equation- based model. The proposed model can be used to investigate the change in damping and synchronizing torque components under low control bandwidths and weak grid conditions. Finally, simulation results are presented to validate the proposed modeling and effects analysis. Our model is general and can be used for any power ratings, and the study also provides a guide to designing control parameters for VSG systems.

State-of-the-art power electronics systems for solar-to-grid integration

Power generated by PV panels is highly vulnerable to uncertain weather conditions, and impedance connected to its terminals. Therefore, to maximize the energy productivity from panels by controlling output impedance, a power electronic converter capable of adopting maximum power point tracking (MPPT) technique is required. This paper includes a comprehensive review of basic and advanced MPPT techniques proposed to address the variations in temperature, irradiance, partial shading conditions.Power processing equipment such as dc/dc converters and inverters are mandatory in extracting power from PV panels and utilizing either for standalone systems or grid integration. Grid integration is a major focus where access to utility line ranging from domestic micro-inverters (<300 W) to solar generation (>MW). A centralized inverter topology interfaces a MW power rating PV farm consisting several parallel strings of series connected PV panels to the grid. This review article contributes on presenting an overview of the state-of-the-art power electronics systems for integration of PV panels to the grid. Various interfacing power electronic architectures covering micro-inverter, central inverter up to modular inverter approach operating at low switching frequency and high switching frequency with resonant and pulse-width modulated (PWM) soft-switching or hard-switching are reported. Various voltage-fed and current-fed multi-level inverters for such architectures are studied and discussed.

Modified Synchronized SVPWM Strategies to Reduce CMV for Three-Phase VSIs at Low Switching Frequency

The high common-mode voltage (CMV) generated by conventional synchronized space vector pulsewidth modulation (SVPWM) strategies may lead to bearing failure and electromagnetic interference problems. However, existing CMV reduction methods are based on asynchronized SVPWM, which will cause unacceptable harmonic distortion at low switching frequency. In this article, four modified synchronized SVPWM (MS-SVPWM) strategies are presented to reduce the CMV and improve the output performance of three-phase voltage source inverters (VSIs) at low switching frequency. In the proposed methods, only nonzero vectors are employed to reduce CMV, and the constraint conditions of preserving three-phase, half-wave, and quarter-wave symmetries of PWM waveforms in terms of active vectors are established. A precorrection scheme is proposed and applied to the presented MS-SVPWM strategies to achieve an accurate output voltage of inverters. Moreover, the linearity characteristics of the output voltage, the harmonic performance of line-line voltage and CMV, and the stator flux trajectories of the proposed strategies are investigated and compared. Experimental results for a 2.2-kW induction motor fed by a two-level VSI are presented to illustrate the effectiveness of the proposed MS-SVPWM strategies.

An Active Bypass Pulse Injection-Based Low Switching Frequency PWM Approach for Harmonic Compensation of Current-Source Converters

In this article, a bypass pulse injection-based pulsewidth modulation approach is proposed for mitigating line current harmonics of grid-tied current-source converters (CSCs) operating at a low switching frequency. First, the relationship between the CSC output harmonic current and the format of the bypass pulse is analyzed. Then, through online adjustment of both bypass pulse location and width, the seventh harmonic component of the output current is actively regulated to mitigate the corresponding seventh line harmonic current. Meanwhile, this action may be associated with additional amplification of the fifth output current. To deal with this issue, the unsymmetrical selective harmonic elimination (SHE) modulation with online adjustable switching angles is applied to further mitigate the fifth harmonic line current. It has been experimentally validated that with the application of both the bypass pulse injection and the unsymmetrical SHE-based modulation approaches, both the fifth and the seventh harmonic line currents can be simultaneously suppressed with ultralow switching frequency below 1 kHz.

Passivity-Based Model Predictive Control of Three-Level Inverter-Fed Induction Motor

Finite control set model predictive control (FCS-MPC) of three-level neutral point clamped inverter-fed induction motor has received wide concern recently, thanks to its low switching frequency and fast dynamics. However, the performance largely depends on the accurate measurement and parameters which determine the feedback and control loops, respectively. Besides, FCS-MPC demands a large number of resources to perform the exhaustive search, which indicates that the computation burden increases exponentially with the increasing switching states. Aiming at improving the robustness under the condition of unavoidable measuring noises and parameter variation as well as reducing the computational burden, a passivity-based model predictive control (PB-MPC) scheme is presented in this article. For an ideal system, the PB-MPC has a similar character as that of the FCS-MPC. In real applications, where noises and disturbances impact the system more or less, PB-MPC outperforms FCS-MPC due to the power shaping and damping injection inherited from passivity-based control which ensures the asymptotic stability. A Lyapunov function is designed to prove the stability of the proposed scheme. The stability and efficiency are verified by both simulation and experimental results.

Low-Frequency-Switching High-Frequency-Resonating Wireless Power Transfer

This article proposes and implements a multi-topological low-frequency-switching (LFS) high-frequency-resonating (HFR) wireless power transfer (WPT) system, which can significantly improve the system efficiency and fluctuations for specific HFR-WPT applications at equal power level. To adjust the HFR wireless power, high-frequency-switching phase-shift control (PSC) will increase the switching loss and operating temperature, thus inevitably degrading the system efficiency and stability. With the system optimization for suppressing current fluctuations, an LFS-PSC technology is deeply investigated to control wireless power while reducing the switching and conduction losses. Also, it can effectively mitigate the adverse impacts of dead zones on the power and efficiency losses. The system efficiencies using two exemplified WPT topologies can reach 89.53% and 91.62%, while they can be improved by 2.75% and 2.13% during power control. The theoretical analysis, simulation, and experimentation are given to verify the feasibility of the proposed systems using LFS-PSC.

Evolutionary algorithm based selective harmonic elimination for three-phase cascaded H-bridge multilevel inverters with optimized input sources

Multilevel inverters are finding wide application in electric drives, traction, flexible AC transmission systems (FACTS) and renewable energy systems. A cascaded H-bridge type multilevel inverter (CHBMLI) produces a near sinusoidal output voltage with lower switching stress and a higher conversion efficiency than the other types of MLIs. The Selective Harmonic Elimination (SHE) strategy is used to eliminate lower-order harmonic profiles and to regulate the fundamental component in the output voltage. SHE has the advantages of low switching frequency, low switching losses and low stress. In this paper, the modulation index and input voltage values are also considered as optimization variables along with the conventional switching angles to analyze the performance improvement in selective harmonic elimination. Heterogeneous Comprehensive Learning Particle Swarm Optimization (HCLPSO) and Gravitational Search Algorithm (GSA) algorithms are used to find the optimal switching angles, modulation index and input voltage source values for minimizing the lower-order harmonics present in the output voltage of seven-level and eleven-level CHBMLIs, while maintaining the fundamental component of the output voltage. The results obtained from MATLAB simulations and an experimental setup clearly indicate that the proposed HCLPSO-based multilevel inverter provides better performance when compared with GSA, firefly and Differential Search Algorithm (DSA)-based MLIs.

THIẾT KẾ BỘ THU PHÁT BĂNG X CÓ KHẢ NĂNG CHUYỂN TẦN NHANH TỪ XUNG TỚI XUNG CHO RA ĐA XUNG TƯƠNG CAN

The paper offers several technical solutions to design X-band transceiver with improvements to overcome the limitations of the conventional transceiver which is low frequency switching rate. In this solution, we suggest using DDS (Direct Digital Synthesizer) to perform two functions simultaneously: modulating and changing the frequency of the probe signal. This design has been implemented on hardware devices and integrated in a fully functional radar system. Experimental results approved that the new radar transceiver has met the requirements of fast frequency switching from pulse to pulse, ensuring the noise factorwas not exceeding 5dB and sensitivity of -100dBm. In field deployment, the radar system was capable of detecting sea targets at a distance up to35 nautical miles.

Tuning Method of Resonant Current Controller With DC Elimination for PWM Rectifiers in Electric Multiple Units

Current control in electric multiple unit (EMU) pulsewidth-modulated (PWM) rectifiers is crucial for the system's stable operation and fast transient response. It should not only regulate the sinusoidal current under very low switching frequency but also eliminate dc current avoiding transformer core saturation. Therefore, proportional integral (PI) plus vector proportional integral with phase compensation (VPIc) controller is chosen for the current control in such single-phase rectifiers. However, the gains of PI and VPIc are coupled and determine the system performance together, invalidating the traditional gain tuning method. Therefore, a gain tuning method for PI + VPIc based on Nichols chart is proposed in this article, which decouples the two gains and simplifies the gain tuning into the tuning of gain margin and phase crossover frequency, while settling time and overshoot are considered for the optimal gain tuning. The designed system has good transient responses, stability, and robustness. The experimental results validate the theoretical analysis of the tuning method on a laboratory prototype and the effectiveness of the current control on the EMU PWM rectifiers.

Analysis and design of L + LCL‐filtered dual‐frequency single‐phase grid‐connected inverter

To increase the efficiency of the grid-connected inverter, this study proposes an L + LCL-filtered dual-frequency single-phase grid-connected inverter. The proposed inverter consists of the low-frequency unit and high-frequency unit. The low-frequency unit transmits power to the grid at the low switching frequency. The high-frequency unit uses the feed-forward method to eliminate grid switching current ripples without current harmonics detection. The passive damping LCL filter is used to attenuate high-frequency unit switching harmonics. The operation principle of the proposed inverter is analysed, and the parameter design method of the inverter is introduced. An experimental prototype is built to test the performances of the inverter. Simulation and experimental results show that the grid current harmonics are effectively suppressed, and the efficiency of the proposed inverter can be higher than that of the L-type inverter and LCL-type inverter.

Model and control of the DC–DC modular multilevel converter with DC fault tolerance

ABSTRACT This paper presents the modelling of the DC–DC multilevel modular converter (DC–DC MMC) with half-bridge sub-modules and the control based on the inversion of its model. The DC–DC MMC presents many advantages such as its modularity, the absence of capacitors on the DC-bus voltage and a low switching frequency. This topology also preserves the intrinsic disadvantages of the MMC as the complexity controlling due to the large number of semiconductors and state variables to control. The control of this converter cannot be symmetrical due to the interconnection of the two parts by an internal AC grid. The control strategy of one part of the DC–DC MMC uses the conventional control scheme with current controls and stored energy control. The second one uses the energy control and produces the waveform of the three-phase internal AC bus voltage linking the two parts of the converter. The explicit control for the generation of internal AC voltages guarantees the correct operation of the converter even in a critical DC-voltage dip on DC buses. Thus, it avoids the need of a DC circuit breaker or the use of full-bridge MMC sub-modules. The validity of the proposed control is verified by simulation.