Low Switching Frequency Research Articles

Control Scheme and Performance Analysis of Dual-Frequency Single-Phase Grid-Connected Inverter Interfaced With Weak and Distorted Grids

Grid impedance variation and voltage harmonics may cause instability of the grid-connected inverter system and reduce the grid current quality under weak grid. Switching losses and electromagnetic interference of inverter can be reduced by using lower switching frequency. However, it is a challenging task to achieve stable operation of LCL-type grid-connected inverter using the active damping method at the low switching frequency. In this paper, an improved control strategy is proposed for the dual-frequency inverter, which contains a power inverter and an auxiliary converter. For the proposed control strategy, the ability to suppress the influence of the grid voltage harmonics and the robustness against grid impedance variation are analyzed. In order to reduce the switching losses, the electrical energy is delivered to the power grid through the power inverter at a low switching frequency. By using the switching ripple compensation method and the grid voltage feedforward scheme, the switching current ripple caused by the power inverter and the current harmonics caused by the grid voltage harmonics can be suppressed. Since the filters used in the proposed inverter are all L type filters, the system stability is guaranteed under weak grid. The simulation and experimental results are provided to verify the effectiveness of the proposed grid-connected inverter system under weak grid.

Low-Switching-Loss Finite Control Set Model Predictive Current Control for IMs Considering Rotor-Related Inductance Mismatch

This paper develops a high-performance finite control set model predictive current control (FCS-MPCC) method for induction motors (IM) to ensure the system control performance over the low-switching-frequency (LSF) range, where the switching loss is low. The new controller is based on tripartite calculations in each control period and sliding mode (SM) rotor-related inductance observer. In detail, firstly, to solve the problem that the control performance of the traditional FCS-MPCC methods is low when they are used at low frequencies, tripartite calculations are employed in each control period to improve the prediction accuracy. By dividing one control period into three equal parts and executing the prediction algorithm in each subsection, the current estimation errors become lower compared to the conventional single-step Euler-discretization controller. Then, considering that the performance of an FCS-MPCC controller highly relies on the machine parameter values, but it is hard to directly obtain the accurate rotor-related inductances (mutual inductance and rotor inductance), this paper uses an online parameter observer based on the sliding mode (SM) principle to diagnose them in real time. It needs to be mentioned that when discussing the stability of the sigmoid-function-based SM observer, a new technique called estimation-error-limitation is initially adopted in this paper to simplify the analysis process. Finally, the proposed algorithms are verified by simulation, which is conducted on a three-phase IM at LSF situations.

ONLINE SHE TECHNIQUE FOR CASCADED MULTILEVEL INVERTERS WITH ADJUSTABLE DC SOURCES

This paper presents a selective harmonic elimination pulse-amplitude modulation (SHE-PAM) technique for medium- and high-power cascaded multilevel inverters, where the low switching frequency is of prime interest.

Acoustic Noise of Induction Motor With Low-Frequency Model Predictive Control

This paper studies the acoustic noise of induction motor with low switching frequency model predictive control and compares its noise performance with the traditional pulse width modulation with proportional-integral controllers. First, the relationship among the air-gap magnetic field, the electromagnetic force, and the acoustic noise is modeled to describe the influence of control algorithm on the noise characteristics of motors. As the counterpart with the traditional pulse width modulation control, a finite control set model predictive control strategy with low switching frequency is proposed. The steady-state performance and switching frequency constraint ability of the proposed model predictive control strategy are verified on the experimental platform. Second, the noise spectrums of these two control strategies are sampled and compared in different operating conditions, and the influence of the weighting factor on the noise spectrum is also discussed. It is proved by the experimental results that model predictive control has better noise performance than the traditional sinusoidal pulse width modulation under the same switching frequency and the total harmonic distortion of stator current.

Analysis of Asymmetrical Multilevel Inverter Driving Induction Motor

In this paper thirteen level asymmetrical multilevel inverter (MLI) is presented. The topology produces thirteen level at the output using eight switches and three voltage sources. The main problem associated with power electronic converters are lower order harmonics. In this paper two control techniques are discussed, which are used to reduce the total harmonic distortion (THD) without using any filter circuit. Nearest level control (NLC) pulse-width modulation and selective harmonic elimination (SHE) technique has been used to generate gating pulse to get better output voltage. The generated pulse has low switching frequency which reduces the switching losses. A three phase MLI topology with reduced number of switches will improve the efficiency and reliability of the topology. Detailedanalysis of MLI topology has been carried out to show the special features of the topology. Comparison of different modulation technique has also been done and various simulation result have been included in this paper.

Compact Three Phase Multilevel Inverter for Low and Medium Power Photovoltaic Systems

A new three phase multilevel inverter with reduced number of components count is proposed in this paper. This inverter is designed using a single DC source per phase to generate multiple level output voltage which makes it suitable for low and medium voltage applications, including ac-coupled renewables or energy storages. A generalized circuit configuration is shown in this paper following which the number of output voltage level can be increased as per expectation. Although, each element endures the voltage stress equivalent to the input DC voltage, the value of total standing voltage (TSV) is reduced by the utilization of minimized number of components with respect to the number of series connected capacitors. Further, staircase modulation scheme is used to generate the switching signals. Hence, the proposed inverter can be operated at low switching frequency with optimal output current harmonic distortion which decreases switching losses and suppresses power factor falling. In order to validate the theoretical explanations and practical performances of the proposed inverter, the hypothesis is simulated for 9, 13 and 39 output voltage level inverters for three phase with a line voltage total harmonic distortion (THD) of 6.06%, 4.16% and 2.10% respectively in MATLAB/Simulink and a 5-level single phase laboratory prototype is implemented in the laboratory.

Sensorless Control of CSC-fed PMSM Drives with Low Switching frequency for Electrical Submersible Pump Application

In this article, current source converter (CSC)-fed permanent magnet synchronous motor (PMSM) drives are investigated for electrical submersible pump (ESP) applications. The CSC features motor-friendly waveform with low dv/dt , which is very suitable for long cable application. High-speed ESP by adopting a PMSM has higher efficiency and shorter length and diameter compared with the counterpart adopting induction motor. Sensorless control instead of mechanical sensor can further reduce the system size/cost and improve reliability due to the harsh downhole environment. However, low switching frequency and an inverter-side output filter cause great challenge in controller and observer design. These kinds of influence become more serious under a higher speed region due to the limited updates per fundamental cycle. To overcome these challenges, the dynamic capacitor voltage control is employed in a field-oriented control scheme to enhance the system dynamic response. Moreover, a discrete-time sliding mode observer based rotor position estimation method is proposed to reduce estimation error and improve parameter uncertainty robustness. The effectiveness of the proposed method is verified on a CSC-fed PMSM with simulation and experimental results.

Dual-Stage Control Structure for Multilevel Voltage Source Inverters

This work proposes an alternative for total harmonic distortion (THD) attenuation in power inverters by combining two different circuit stages. The Macro stage comprises of a Neutral-Point-Clamped (NPC) multilevel inverter operating in high voltage and low switching frequency. The Micro stage is a two-level three-phase inverter, which is faster and more accurate due to its higher frequency and limited voltage. In this dual-stage approach, the opposite characteristics of the converters lead the Micro stage to correct the distortions of the Macro. It is possible to work with small values of filtering components, lower than the ones needed in a one-stage approach. This way, the presented proposal aims to reduce the physical size of the converter and its costs. The NPC inverter works in open loop with its switching signals generated by optimized pulse patterns (OPP). For correct THD filtering, a resonant control strategy with a Notch filter is designed in the frequency domain, as well as a feedforward structure designed in time domain. Simulation results show that the proposed system keeps voltage THD in acceptable levels for IEEE Standards.

Enabling Multistep Model Predictive Control for Transient Operation of Power Converters

Recently, an efficient multistep direct model predictive control (MPC) scheme for power converters has been proposed. It relies on the Sphere Decoding Algorithm (SDA) to solve the associated long-horizon optimal control problem. Since the SDA evaluates only a small number of candidate solutions to find the optimal one, a significant reduction in the average computational burden can be achieved compared to the basic exhaustive search approach. However, this is only true during steady-state operation. In fact, the SDA still requires a large execution time during transients. This paper shows that if not properly addressed, the dynamic performance of the system may be degraded, which clearly limits its practical application. To mitigate this issue, which particularly arises during transients, an efficient preconditioning approach for the SDA is proposed. This approach ensures that only a small number of candidate solutions are evaluated during both steady-state, and transients. This allows the multistep direct MPC to become a viable control alternative for power converters operating at low semiconductor switching frequencies, e.g., below 450 Hz. The proposal is validated using a grid-connected three-level converter as a case study. Both processor-in-the-loop simulations, and experimental results on a scaled-down 2.24 kVA laboratory setup are presented.

Predictive Direct Power Control of Doubly Fed Induction Generators to Reduce the Power Ripple During the Grid Synchronization

This paper presents a new method of predicting power based on predictive direct power control. This method reduces power ripple in a doubly fed induction generator, which makes it possible to have a smoother synchronization with the grid in low constant frequency switching. To achieve fast and smooth grid synchronization without any over current, the difference between the measure, frequency, and the phase of the stator voltage and the grid voltage should be minimized. This condition can be achieved by minimizing the error between active and reactive virtual power and their references. By predicting the virtual active and reactive power behavior and choosing different active voltage vectors and using each of them at different times among each period time, the best switching of the rotor side converter in doubly fed induction generator to have a smoother condition can be chosen. In this paper, a 15kw generator is simulated by the classic method which is direct power control, and this new method, predictive direct power control, and it will be shown how much the new method reduces the power ripple to have a smoother synchronization that doesn’t cause mechanical or electrical pressure for none of the grid and the doubly fed induction generator.

Optimum pulse-width modulation strategy for a symmetric cascaded multilevel inverter

This paper investigates for the most suitable PWM strategy for a symmetric cascaded multilevel inverter. The objective is to determine a PWM switching strategy which optimises inverter performance by ensuring higher DC link utilisation and minimum total harmonic distortion (THD) at low switching frequency to minimise switching power losses. Simulations carried out in MATLAB-Simulink environment has given an insight of inverter performance when carrier-based PWM techniques are applied. In this parametric evaluation, both odd and even multiples of 50 Hz modulating signal were applied for obtaining switching frequency for various multicarrier-based PWM strategies.

The Design of SVPWM Waveform Generator Based on CPLD_DSP

Space vector pulse width modulation (SVPWM) control technology could obtain quasi circular rotating magnetic field by switching the space voltage vector of the inverter. Under the condition of low switching frequency, the performance of AC motor was better than that of sine wave pulse width modulation. When this technology was applied to inverter control system, better output voltage waveform could be obtained than conventional six step waveform. And in order to further improve the control performance of the motor, it was very necessary to design SVPWM waveform generator. The design of SVPWM waveform generator based on Complex Programmable Logic Device and Digital Signal Processing (CPLD_DSP) was introduced in the article. The main idea of SVPWM was to synthesize reference voltage vector by switching space voltage vector of inverter. Time holding data and sector number could be calculated from coordinate switch of Voltage gathered from inverter by DSP. And the data was sent into CPLD. SVPWM waveform was generated by state machine programed in VHDL. Finally program was download in CPLD chip named EPM1270T144C5N. The transformation from time signal to SVPWM switch signal was finished successfully. The three phrase output signal had good coherence. Thus the controlling function of the motor control system was improved in a certain extent.

Deadbeat Predictive Current Control for SPMSM at Low Switching Frequency With Moving Horizon Estimator

The deadbeat predictive current control (DPCC) of permanent-magnet synchronous motors (PMSMs) is well known for its simple structure and fast dynamic. However, it is sensitive to parameter mismatches and system delays, which becomes more serious at the low switching frequency. To handle such problems, this article proposes an accurate deadbeat control method based on an exact model of surface-mounted PMSM in a stationary two-phase frame with a moving horizon estimator (MHE). The motor model is obtained from differential equations, and transformations between stationary frame and rotating frame are reduced to eliminate the influence of the lag of electrical angle. MHE is utilized to estimate the back-electromotive force (EMF) and disturbance caused by parameter variations. The rotor flux is eliminated in the algorithm and the resistance and inductance mismatches are compensated by the estimator. Several simulations and experiments are performed in three methods-the conventional DPCC in rotating $dq $ -axis, DPCC in $\alpha \beta $ -axis with the accurate model, and the proposed method. These comparisons validate that the proposed method has good current tracking performance at low switching frequency and a strong robustness to parameter variations.

Direct AC voltage control for grid-forming inverters

Grid-forming inverters usually use inner cascaded controllers to regulate output AC voltage and converter output current. However, at the power transmission system level where the power inverter bandwidth is limited, i.e., low switching frequency, it is difficult to tune controller parameters to achieve the desired performances because of control loop interactions. In this paper, a direct AC voltage control-based state-feedback control is applied. Its control gains are tuned using a linear quadratic regulator. In addition, a sensitivity analysis is proposed to choose the right cost factors that allow the system to achieve the imposed specifications. Conventionally, a system based on direct AC voltage control has no restriction on the inverter current. Hence, in this paper, a threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent. The robustness of the proposed control is assessed for different short-circuit ratios using small-signal stability analysis. Then, it is checked in different grid topologies using time domain simulations. An experimental test bench is developed in order to validate the proposed control.

Performance Assessment of Modular Multilevel Half Bridge Inverter for High Power PV Application

This paper presents three-level Modular Multilevel Half Bridge Inverter to achieve lower total harmonic distortion and improved efficiency. Compared to conventional inverter, multilevel inverters provide output voltage of high quality and current waveform while using the low switching frequency. The use of multilevel inverters reduces the switching losses, size of semiconductor switches and the number of harmonic filters as compared to the traditional DC-AC inverters. This paper investigates different classification of multilevel inverters suitable for photovoltaic applications of high power based on efficiency and THD of the output line voltage and current waveform. Multilevel Inverter topologies are simulated as the same environment using MATLAB/Simulink.

Optimum pulse-width modulation strategy for a symmetric cascaded multilevel inverter

This paper investigates for the most suitable PWM strategy for a symmetric cascaded multilevel inverter. The objective is to determine a PWM switching strategy which optimises inverter performance by ensuring higher DC link utilisation and minimum total harmonic distortion (THD) at low switching frequency to minimise switching power losses. Simulations carried out in MATLAB-Simulink environment has given an insight of inverter performance when carrier-based PWM techniques are applied. In this parametric evaluation, both odd and even multiples of 50 Hz modulating signal were applied for obtaining switching frequency for various multicarrier-based PWM strategies.

Research on hybrid SHEPWM based on different switching patterns

<span>This paper presents a hybrid pulse width modulation (HPWM) strategy based on different switching patterns of selective harmonic elimination pulse width modulation (SHEPWM) for the three-level neutral point clamped (3L-NPC) converter. Specific low-order harmonics can be eliminated by SHEPWM at low switching frequency, while the remaining high-order harmonics can be selected to be simply filtered by additional hardware. Large oscillation waveform usually occurs in the transition instant between two diverse modulation situations, therefore switching between distinct switching patterns can be problematic if no effective means is taken, especially when the effect of smooth and fast transition at any time is considerable. A universal and valid control strategy, which maintains the high-quality output voltage and current, is proposed and implemented in this paper to address this issue. Simulation results obtained from MATLAB/SIMULINK are presented to analyze the performance and validate the feasibility and effectiveness of this control scheme.</span>

Current Control Strategies for Parallel-Form Switch-Linear Hybrid Envelope Tracking Power Supply With Two Switched-Mode Converters to Achieve Optimal Power Allocation

Switch-linear hybrid (SLH) envelope tracking (ET) power supply is one of the mainstream structures for achieving both high efficiency and high bandwidth. When tracking a high bandwidth envelope signal, the overall efficiency of SLH ET power supply with the single switched-mode converter is limited. This paper adopts two switched-mode converters with different tracking bandwidths instead of the single switched-mode converter. The switched-mode converter with low tracking bandwidth is called slow switched-mode converter (SSMC), which aims for providing the low-frequency load current components with low slew rate; while the switched-mode converter with high tracking bandwidth is called fast switched-mode converter (FSMC), which copes with the high-frequency load current components with high slew rate. The rest load current components are processed by the linear amplifier. The class-A linear amplifier is employed for reducing the complexity of the circuits, which works as a voltage regulator that does not sink the load current. The SSMC provides the majority of the load current with low switching frequency. Thus, the switching loss of the SSMC is not high. The FSMC is operated at high switching frequency but provides the minority of the load current, and thus the switching loss of the FSMC is not high. The valley-point current control and peak current control with critical continuous current mode are proposed for the SSMC and FSMC, respectively, achieving the desired power allocation among the SSMC, FSMC, and the linear amplifier. Finally, a prototype is fabricated in the lab aiming for tracking a 10-MHz envelope signal and 20-W average output power. Experimental results are provided to verify the effectiveness of the proposed configuration and its current control strategies. The efficiency of the proposed SLH power supply is 77.0%, a 4.5% efficiency improvement compared with the SLH ET power supply with the single switched-mode converter.

Comparison Performance of Nine Level Multilevel Inverter Using Bipolar and Unipolar Switching Schemes

Multilevel inverter structures have the advantages due to the ablility to operate at high dc-bus voltage. It is able to operate at higher power applications by using series connection of switching devices. In addition, multilevel inverter can generate output voltage with very low harmonic distortion and synthesis a staircase voltage waveform by having multiple voltage levels. Various multilevel inverter topologies are introduced; however, cascaded multilevel inverter is commonly used because it is easy to be implemented. In addition, multilevel inverter topologies are able to operate at lower switching frequency and it produces higher efficiency by producing lower switching dissipation. In this paper, MATLAB/Simulink is used to model the seven levels cascaded multilevel PWM inverter by using bipolar and unipolar switching techniques. The analysis of the cascaded multilevel inverter will be done on different types of non-linear loads such as R-load, RL-load and RC-load The THD of voltages and currents on each type of loads that are done and finally the results are compared.

Improved Full-Order Adaptive Observer for Sensorless Induction Motor Control in Railway Traction Systems Under Low-Switching Frequency

In the applications of medium- to high-power railway traction systems, the switching frequency and controller sampling rate are relatively low due to the requirements for low losses, high reliability, and limited processor power. These limitations greatly hinder the sensorless control of induction motors, especially in the high-speed operation range. In this paper, a full-order adaptive observer based on an improved discrete model is proposed, which significantly improves the system stability and control performance over the entire speed range. The feedback gain matrix of the presented control scheme is developed in a discrete time domain, ensuring that all the poles of the closed-loop observer are within the stable region. Moreover, a speed estimation mechanism based on Lyapunov’s approach is designed in the synchronous rotating reference frame to further improve the precision of speed observation. Rigorous simulation studies and experimental tests of a benchmark induction motor in railway traction system demonstrate that the improved adaptive observer achieves superior sensorless control performance over the conventional adaptive observer.