Low Switching Frequency Research Articles

An Averaged-Value Model of an Asymmetrical Hybrid Multi-Level Rectifier

The development and the validation of an averaged-value mathematical model of an asymmetrical hybrid multi-level rectifier is presented in this work. Such a rectifier is composed of a three-level T-type unidirectional rectifier and of a two-level inverter connected to an open-end winding electrical machine. The T-type rectifier, which supplies the load, operates at quite a low switching frequency in order to minimize inverter power losses. The two-level inverter is instead driven by a standard sinusoidal pulse width modulation (SPWM) technique to suitably shape the input current. The two-level inverter also plays a key role in actively balancing the voltage across the DC bus capacitors of the T-type rectifier, making unnecessary additional circuits. Such an asymmetrical structure achieves a higher efficiency compared to conventional PWM multilevel rectifiers, even considering extra power losses due to the auxiliary inverter. In spite of its advantageous features, the asymmetrical hybrid multi-level rectifier topology is a quite complex system, which requires suitable mathematical tools for control and optimization purposes. This paper intends to be a step in this direction by deriving an averaged-value mathematical model of the whole system, which is validated through comparison with other modeling approaches and experimental results. The paper is mainly focused on applications in the field of electrical power generation; however, the converter structure can be also exploited in a variety of grid-connected applications by replacing the generator with a transformer featuring an open-end secondary winding arrangement.

Grid Integration of Single-Stage SPV-STATCOM System Using Symmetric Cascaded Five-Level VSC

This work proposes a dual-function single-stage solar photovoltaic (SPV) grid-tied system with STATCOM (static compensator) capabilities using a three-phase five-level cascaded VSC (voltage source converter) operating at low switching frequency with low total harmonic distortion (THD) and low switching losses. The proposed SPV-STATCOM system works in three modes: i.e. Mode-1, in which only an active power is transferred to the grid, Mode-2, in which both active and reactive power are supplied to the grid and Mode-3, in which only reactive power is supplied to the grid, thereby utilizing the full capabilities of the SPV system. An incremental conductance algorithm is used to track power from SPV array which is fed to the grid. To synchronize the VSC to the grid, a decoupled current controller with feed-forward term and double decoupled synchronous reference frame phase-locked loop (DDSRF-PLL) is used. The use of a single-stage five-level cascaded VSC offers the advantages of low switching losses and the operation at high power and high voltage which results in an improvement in quality of power of the system. The simulated results demonstrate the system design and control scheme under varying conditions.

Performance Analysis of Multilevel Inverter with Reduced Number of Switches

Multilevel inverters are mainly used in high power and medium voltage applications to reduce the required voltage rating of the power semiconductor switching devices. Nowadays multilevel inverters are also preferred for various applications regardless of the power ratings because they can essentially realize lower harmonics with lower switching frequency and lower electromagnetic interference (EMI). However, it has some disadvantages such as increased number of components, complex Pulse Width Modulation control method, and voltage balancing problem. In this paper a new topology of cascaded multilevel inverter using reduced number of switches is introduced resulting in higher output voltage levels. There era five series connected H-bridges and the DC voltage is given in the ratio n0: n: n3:2n2:10n. The output voltage having 123 levels is obtained (61 positive voltage levels, 61 negative voltage levels and zero voltage levels). Reduced Total Harmonic Distortion (THD) makes them useful for electric vehicle, FACTS and has given option for various power applications. The proposed topology results in reduction of cost and has simplicity of control system. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels.

Energy Management for a Dual-motor Coupling Propulsion Electric Bus based on Model Predictive Control

Energy Management strategy is indispensable for the power allocation control and energy-saving performance of a dual-motor coupling propulsion electric bus (DMCEB). Considering the constraints and instantaneous optimization, a strategy based on model predictive control is proposed. Firstly, the stationary feature of the driving cycle trend is analyzed and a novel acceleration sign prediction process is explored based on auto-regressive moving average (ARMA) method. Secondly, two specific state transition probability matrices are established according to the acceleration sign and piecewise Markov chain model is utilized to predict velocity sequences in the 5-second horizon followed by prior process. The fluctuations are significantly moderated under the effect of acceleration sign prediction and the RMSE can be controlled within around 1.4119 km/h. At last, dynamic programming (DP) is adopted as the online rolling optimization part of the model predictive control (MPC) based strategy and DP-based results are used as the benchmark to evaluate the control effect. The simulation results show that, the energy economy based on the proposed strategy decreased by 21.4% comparing with the preliminary rule-based strategy and is only 6.8% worse than that based on DP. With the 85.75 kWh/100 km energy consumption performance and low mode switch frequency, the strategy is reasonable and suitable for electric bus.

Nine‐level high‐frequency inverter

In the high-frequency AC (HFAC) power distribution system, problems such as high switching frequency, a complicated circuit configuration and difficult parameter design still exist in the traditional inverters. A multi-level high-frequency inverter topology based on a forward converter is proposed in this study, which implements the electrical isolation of input and output. With the introduction of variable turns ratio technology, the nine-level output is achieved, which is conductive to the decrease of output harmonics. The selected harmonic elimination method is also adopted to further reduce the output harmonic distortion through the Fourier analysis of output waveforms. Besides, the HFAC output can be achieved at a lower switching frequency for the maximum switching frequency is only double the output frequency. The detailed analysis on operating principles and parameter design of the proposed inverter are provided in this study. Finally, the feasibility of the inverter is verified by the simulations and experiments.

Modified capacitor voltage balancing sorting algorithm for modular multilevel converter

The modular multilevel converter (MMC) with large-scale sub-modules has the advantage of simpler modulation, lower switching frequency, and lower harmonic component, thus would be very promising in voltage source converter (VSC)–high voltage direct current transmission systems. Conventional capacitor voltage balancing algorithm suffers from insufficient grouping and sorting techniques. Therefore, it might result in excessive computation and high switching frequency of power electronic devices. To address the problem, a modified capacitor voltage balancing sorting algorithm is proposed in this paper. The proposed algorithm could avoid sorting all the module capacitor voltages by selecting only a certain number of the largest or smallest module capacitor voltages, and thus reduces time complexity greatly without losing control precision. Furthermore, the proposed algorithm focuses on the sub-modules whose capacitor voltage exceeds the limits, while the switching states of the other sub-modules are maintained to some degrees by employing the maintaining factor. Therefore, the switching frequency of the power electronic devices is further reduced. The performance of the proposed algorithm is evaluated through a time-domain MMC–HVDC simulation in PSCAD/EMTDC. Results show that the proposed algorithm is able to balance the module voltage with lower computation and reduce the switching frequency of power devices significantly, without noticeably increasing the capacitor voltage ripples.

Low cost fast frequency switching driver for Acousto-Optic Modulators used in laser cooling

Abstract In many laser atom cooling experiments, Acousto-Optic Modulators (AOM) are used to modulate a laser beam frequency close to an electronic resonance of the atoms. Commercially available RF drivers for the AOMs tend to be expensive ($2k and above) and can also require supplementary equipment such as power supplies and amplifiers. This article presents a low cost, compact, re-configurable, and portable modulator driver system. This system can switch between two frequencies within a period of few hundreds of ns, while giving the user complete control over the frequency range and output power up to about 2 W. This fast switching allows to toggle a laser beam between two optical fibers which could be used for cooling, imaging, or optical pumping of the laser-cooled atoms.

Interleaved multi‐level power converter for a battery energy storage system

This study proposes an interleaved multi-level power converter (IMPC) for a battery energy storage system. The proposed IMPC is composed of two power-electronic legs. The first power-electronic leg is an interleaved multi-level leg that contains eight power-electronic switches. Four power-electronic switches constitute a two-phase interleaved switch set and these are switched in a high-frequency interleaved pulse-width modulation. The other four power-electronic switches constitute a level-selection switch set that determines the dc bus voltage for the two-phase interleaved switch set according to the magnitude of the utility voltage. The second power-electronic leg is the low-frequency switching leg, which is composed of two power-electronic switches. Since the proposed IMPC integrates the performances of the multi-level power converter and interleaved power converter, the effective switching frequency is twice the practical switching frequency and the output voltage has five voltage levels. The ripple of the output current is significantly attenuated and the power capacity of the output filter is reduced. The proposed IMPC can also be used to adjust the real and the reactive power flows, in order to control the charging/discharging of battery sets and to compensate for the reactive power. A hardware prototype is developed to verify the performance of the proposed IMPC.

A High-Efficiency Single-Phase T-Type BCM Microinverter

This paper proposes a high-efficiency single-phase T-type boundary conduction mode (BCM) microinverter. The conventional full-bridge BCM microinverter has achieved zero voltage switching (ZVS) and thereby improved the efficiency, but it suffers from high switching losses under light load conditions. The proposed T-type BCM microinverter reserves ZVS and uses a multilevel technique to further decrease the switching losses. The BCM operation with multilevel technique will have too low switching frequency when the grid voltage approaches half of the dc link voltage. To solve this problem, this paper adopts a third operation mode for the T-type switching leg to maintain the switching frequency above a minimum value. The corresponding mode transitions are also detailed to ensure a smooth operation. Because of the turn- off delay of the freewheeling transistor, the actual lower current boundary deviates from the programmed one, which will distort output current. To address this issue, this paper also proposes a boundary compensation method. A prototype has been built for performance verification, which can test both full-bridge and T-type topology. Compared with the full-bridge BCM microinverter, the proposed T-type BCM microinverter has a higher efficiency over the whole load range.

Band-Based Multi-Step Predictive Torque Control Strategy for PMSM Drives

The multi-step model predictive control can effectively reduce the switching frequency of inverter and improve the efficiency of motor drive system. However, the conventional multi-step model predictive control algorithm is difficult to implement in a short sampling period because of high computational complexity. In this paper, a band-based multi-step predictive torque control (BM-PTC) strategy for permanent magnet synchronous motor (PMSM) drives is proposed. On the one hand, the simplified multi-step prediction model for tracking errors of torque and stator flux is derived, and the method of reducing candidate switching sequences by band constraints is proposed, thus the computational complexity of multi-step prediction can be significantly reduced. On the other hand, the steady-state control mode and transient-state control mode are designed respectively to ensure that the system has good steady-state and dynamic performance under low switching frequency. Based on a 6-kW PMSM, the feasibility and effectiveness of the proposed strategy are verified by experiments. The proposed strategy can reduce the switching frequency by more than 37% while ensuring high performance.

Control of a Type-IV Wind Turbine With the Capability of Robust Grid-Synchronization and Inertial Response for Weak Grid Stable Operation

With the increasing penetration of wind power, the effective inertia of the power system reduces. Besides, a series of interactive instability issues including sub-synchronous oscillations and harmonic oscillations were reported due to the weak grid effects. Broadly speaking, those issues are closely related to the PLL and its tuning. Therefore, recent literature make efforts to avoid using the PLL for converter control, e.g., the virtual synchronous generator (VSG) control. However, the VSG control usually employs multiple loops, where a fast switching frequency of the converter is required to decouple each loop's dynamic. This prerequisite is usually not met for wind power converters with high capacity and low switching frequency. To address these issues but also inherit the merits of a VSG on the grid-integration, this paper employs a new concept of PLL-less control and applies it to the Type-IV wind turbine, in which the grid-synchronization is realized by the dynamics of dc capacitor voltage. The virtual capacitor control is designed and added to the machine-side converter to eventually deliver adequate inertia to the grid. To justify the effectiveness of the proposed control, both the soft start-up validation and the thorough analysis of the overall small-signal stability are presented. Several concerns of vital importance regarding the virtual capacitor design and stabilization control are discussed, where the mechanism of stability is revealed through the complex-power coefficient-based analysis. On this basis, a stabilization control method is proposed, which can enlarge the stable range of virtual capacitor coefficient and enhance the inertial response effect. Finally, the performance of the proposed method on the inertial response and the weak grid operation is evaluated by time domain simulations in PSCAD/EMTDC, which is proven effective overall.

Analytical and Comparative Study of Different Types of Two-Leg Chopping Up Regulator

The main focus of this article is to analyze and simulate the two-leg parallel connection of a chopping up regulator with flattering inductive smoothers or with an interphasing centre-tap transformer supplied by a three-phase diode rectifier and a DC link in between. The article deals with the problem of reducing total harmonic distortion, minimizing THD and EMI with low switching frequency. The Simulated three phase a.c. load model is added at the end to investigate the current and voltage harmonics. The main objective of this paper is the investigation of the problem and active impact of replacing flattering inductive smoothers used to reduce voltcurrent fluctuating waveforms of the chopping up regulator by new topology known as interphasing centre-tap transformer with magnetic coupling. The comparison of these two variations of the study is then done based on their technical parameters and economical investment viewpoint. The considered technical parameters are to be current distribution into individual legs, amount of voltcurrent ripple and area of discontinuous currents. The investment costs governed by the material requirements are essential for transformer and smoother production design. The outcome of using the interphasing centre-tap transformer is successive cancelation of voltcurrent fluctuating waveforms generated at the output of the chopping up regulator. This is proved by an experiment with 35 in input and power chopping up400-V/90-A.Software simulations in Simplorer and Matlab/Simulink or software program and regimen prototypes have been arranged to confirm the results.

Optimisation of speed control for switched reluctance motor using matrix converter

This paper proposes a new idea for converter-based speed control optimisation for switched reluctance motor. The main objective of the speed control technique is reduced torque ripples and improved control performance achieved using matrix converter-based PID controller. Speed control performance optimisation is attained using particle swarm optimisation algorithm. The potential benefits of matrix converter are flexible current profile and lowest switching frequency, hence, minimal loss. Simulation and analysis has been evaluated using MATLAB/Simulink with the reference speed 1,500 rpm within the time frame of 0.045 seconds.

An Improved Harmonics Mitigation Scheme for a Modular Multilevel Converter

Modular Multilevel Converters (MMC) are gaining importance because of their flexible structure, re-configurable property, and simplicity of operation. The operation of MMC at a low-switching frequency (LSF) helps in enhancing the performance of the converter. This paper proposes an improved harmonics mitigation scheme for a Multilevel DC-Link Inverter (MLDCLI), which is a variant of MMC. The proposed scheme is a modified version of the conventional Nearest Level Modulation (NLM) scheme, termed as modified Nearest Level Modulation (mNLM) Scheme. The proposed scheme is effective compared to NLM because of the choice of the switching angles obtained by the use of the algorithm proposed. The MLDCLI topology is operated for twelve different configurations, and mNLM is implemented on all the configurations. Using MATLAB software, the simulation results are validated, and the same is extended to a hardware prototype. The simulation and experimental results of NLM and mNLM schemes are compared. The effectiveness of the proposed scheme is evident by the reduced voltage THD, increased rms voltage, increased rms current, and increased output power.

Hybrid-Frequency Cascaded Full-Bridge Solid-State Transformer

Modular solid-state transformers (SSTs) are suitable for various voltage- and power-level applications. State-of-the-art modular-SST topologies contain a considerable quantity of high-frequency transformers (HFTs) and power switches, thereby increasing the costs, difficulty, and workloads of engineering. To overcome this challenge, a new SST topology family, namely, hybrid-frequency cascaded full-bridge SST (HCFB-SST), is established on the basis of the concept of hybrid-frequency modulation. In comparison with modular-multilevel converter SST (M 2 LC-SST) or CFB-SST, the HCFB-SST uses only 50%-66.7% and 1.11%-10% the device count of power switches and HFTs, respectively, in typical applications, thereby obtaining considerable cost savings. It also achieves a cost reduction of sub-module redundancy. In addition, HCFB-SSTs have simple engineering and compact structures. A hybrid-frequency modulation method is proposed for a low well-balanced average switching frequency and a high equivalent switching frequency without monitoring the bridge-arm current. Analysis and comparison of the semiconductor loss with existing topologies are presented. The simulation studies are conducted to validate the effective function of HCFB-SST and the proposed modulation method.

Dahlin-Based Fast and Robust Current Control of a PMSM in Case of Low Carrier Ratio

The low ratio of sampling frequency to electrical frequency (carrier ratio) is a challenging issue for applications, such as high-speed or high-power drives. In fact, high-speed drives have high fundamental frequency and high-power drives have low switching (sampling) frequency, leading to the same control challenges of having high dynamic and stability with a limited number of control actions in one period. Parameters mismatch occurs due to the converter non-linearities, deadtimes, temperature change, and saturation; hence, providing robust control is a key challenge. This paper starts reviewing the problems of controlling machines with a few switching instances per fundamental period and an analysis of the state-of-the-art current control strategies for permanent magnet synchronous motors (PMSMs). Three different control schemes are analyzed. The discrete-time PI current controller is providing good robustness, while the dynamic performance may not be sufficient for some applications. Second, the deadbeat control offers very fast dynamic performance but is weak under parameter mismatch. The novel application of the Dahlin controller offers a tradeoff between performance and stability. The control algorithms are evaluated based on the stability, robustness, and dynamic and steady-state performances. Their performance and sensitivity to parameter variation are analyzed through simulations, and the experimental measurements are proving the results.

The driver design for N2O gas detection system based on tunable interband cascade laser

In this paper, the driver circuit for N2O gas detection system based on tunable interband cascade laser (ICL) is developed. Considering the influence of power supply stability on the digital-analog hybrid drive circuit of tunable diode laser absorption spectroscopy (TDALS), the high-efficiency TPS5430 is used to design the positive and negative power supply circuit. The large electrolytic capacitor + post-stage LC filter combination filter is used to effectively filter out high and low frequency ripple and switching noise. The use of thick high current trace + via + multilayer printed circuit board (PCB) design makes the line temperature rise smaller, more stable and durable, and uses high frequency shielding inductance to effectively reduce radiation interference to ensure the stability of the drive. The STM32F407, a highperformance microcontroller based on the ARM Cortex-M4 core, is used as the master control chip and generates a sawtooth scanning signal. The direct digital synthesizer (DDS) chip ICL8038 is used to generate a sinusoidal modulated signal of a specific frequency. The two signals are superimposed by a reverse addition circuit, and the laser drive signal is generated by a developed positive feedback balanced voltagecurrent conversion circuit. Experimental results show that the driver circuit can well meet the drive development requirements of N2O gas detection systems based on tunable interband cascade laser.

Low Switching Frequency Based Asymmetrical Multilevel Inverter Topology With Reduced Switch Count

The inceptions of multilevel inverters (MLI) have caught the attention of researchers for medium and high power applications. However, there has always been a need for a topology with a lower number of device count for higher efficiency and reliability. A new single-phase MLI topology has been proposed in this paper to reduce the number of switches in the circuit and obtain higher voltage level at the output. The basic unit of the proposed topology produces 13 levels at the output with three dc voltage sources and eight switches. Three extentions of the basic unit have been proposed in this paper. A detailed analysis of the proposed topology has been carried out to show the superiority of the proposed converter with respect to the other existing MLI topologies. Power loss analysis has been done using PLECS software, resulting in a maximum efficiency of 98.5%. Nearest level control (NLC) pulse-width modulation technique has been used to produce gate pulses for the switches to achieve better output voltage waveform. The various simulation results have been performed in the PLECS software and a laboratory setup has been used to show the feasibility of the proposed MLI topology.

Low-Frequency Harmonic Elimination Technique in Cascaded H-Bridges Multilevel Inverters for Renewable Energy Applications

This paper proposes a fundamental frequency selective harmonic elimination (SHE) technique for medium and high power three phase cascaded H-bridges (CHB) l -level inverters, where low switching frequency assumes an important role. Respect to classical SHE methods, it is capable to delete more harmonics from the output voltage waveform and to provide lower total harmonic distortion (THD) values. A mathematical proof is given to demostrate that the undeleted harmonics are only those of order n = 2 k ( l + r ) ± 1 , k = 1 , 2 , . . . where r is an integer parameter that assumes the values − 2 , − 1 , 0 defining three different sets of switching angles, that are provided in the whole range of modulation index. The method assumes different dc voltage sources and pulses patterns, defined for the three sets of switching angles. Simulated results are shown to confirm the validity of the proposed procedure. For l = 5 and l = 7 , the results obtained are compared with those carried out by applying the classical SHE technique at the same conditions. Very good performances, in terms of number of deleted harmonics and THD, are observed when r = 0 . In comparison to classical SHE, the proposed method reduces computational complexity because it must not solve non linear equations.

Selective Harmonic Mitigation Based Self-Elimination of Triplen Harmonics for Single-Phase Five-Level Inverters

In this paper, a modified selective harmonic mitigation pulse amplitude modulation (SHM-PAM) is presented to be capable of canceling all triplen harmonic orders and suitable for single-phase application of five-level type of voltage source inverters. To this end, a new constraint is established for the two switching angles ( α 1, α2 ) to derive the new formula for the harmonics’ amplitude, which results in self-elimination of all triplen harmonics (e.g., 3rd, 9th, 15th, …). The fifth and seventh harmonic orders are mitigated through normal operation of the proposed SHM-PAM technique. It is also shown that the proposed technique is extendable to other multilevel voltage waveforms and a flowchart of self-elimination of all triplen harmonics has been presented. Mathematical analysis supported by experimental investigations show the desired performance of the proposed SHM-PAM algorithm on a two-cell single-phase cascaded H-bridge inverter as a typical five-level configuration in dealing with linear and nonlinear loads. Then, it is demonstrated that the maximum number of harmonic orders would be controlled with the minimum number of available angles in a low switching frequency voltage waveform.