Low Switching Frequency Research Articles

Design of Parallel Resonant Switched-Capacitor Equalizer for Series-Connected Battery Strings

The traditional pure switched-capacitor equalizer suffers from a large inrush current and low balance speed. An automatic parallel resonant switched-capacitor equalizer (PReSCE) for series-connected battery strings is proposed, which utilizes resonant switched-capacitor to eliminate the inrush current. The parallel ReSC converters not only minimizes output impedance at the low switching frequency, but delivers the excess energy to the low-voltage battery directly from the high-voltage battery in one cycle. Both of the two functions increase the balance speed. All of the switches are controlled by a pair of complementary pulsewidth modulation signals at a fixed operational frequency. Both simulation and experiment are used to verify the theoretical analysis and system feasibility of the proposed circuit. The results show that the PReSCE circuit eliminates the inrush current and increases the balance speed three times than the parallel pure switched-capacitor equalizer.

Analysis of a GaN based PWM AC-AC converter with an improved switch loss model

AC-AC converters have a wide range of use in industrial applications mostly with installed silicon switches. In last decade, GaN’s are served to market with their high efficiency and fast on-off times, hence implementation of GaN’s to any conventional power electronic circuit is a focus of interest. In this paper, effects of both GaN and MOS switches on efficiency of an AC-AC converter were analysed both numerically and practically. A new extended mathematical loss model proposed by the authors was used to calculate the switching-conduction losses. Laboratory tests were performed with same physical circuit for both switches having the same package. Calculated and measured efficiencies and loss results which are in great harmony proved that the proposed mathematical loss model is extraordinarily realistic. All-in-all, it was observed that GaN and MOSFET switches have both high efficiencies (greater than 96.5%) at low switching frequencies, even though they operate at low power levels. With increasing switching frequency, GaN’s efficiency is slightly reduced (less than 0.5%) while that of MOSFET decreased dramatically almost by 3%. Besides, it was seen that at low frequencies, reverse diode has a significant effect on efficiency of a fast switch, although switching-conduction losses are extremely low.

A Multi-Functional De-Icing Equipment Using Modular Parallel PWM Current Source Converters

For transmission line de-icing, a five level specific harmonic elimination (5l-she) modulation method for high-power parallel current source is proposed, which realizes the current balance of DC bridge arm and the suppression of common mode voltage, and ensures the power quality of grid connected current at low switching frequency. In addition the reactive power of the low voltage load can be compensated at the same time. Firstly, the switching states are classified according to the different mode lengths of PWM current, and the common mode voltage corresponding to each switching state is calculated. Secondly, the current sharing control strategy is established based on the analysis of the influence of redundant switching state on the current sharing of DC bridge arm. Then, the 5l-she waveform is constructed based on the switching state with lower common mode voltage, and the redundant switching states is optimized according to the current sharing strategy. Finally, the effectiveness of the proposed method is verified by simulation.

Optimized Harmonic Reduction PWM based Control Technique for Three-Phase quasi Z-Source Inverter

This paper proposes an optimized harmonic reduction pulse width modulation (HRPWM) control strategy for three-phase quasi Z-source inverter (qZSI). In traditional sinusoidal or space vector pulse width modulation techniques, the flexibility in adjustment of individual switching angles is not possible and thus, these techniques are not optimum choices for low switching frequency operations of high/medium power qZSI. In the proposed technique, adjustments of switching angles of HRPWM waveform are possible to achieve optimum performance. The optimum performance is targeted as maximization of boosting factor and simultaneous minimization of weighted total harmonic distortion (WTHD) at the output voltage of qZSI. The hybrid particle swarm optimization gravitational search algorithm (PSOGSA) is used for computation of optimum switching angles of suggested HRPWM waveform at various modulation indices. The obtained WTHDs up to 49th order harmonics and boosting factors of optimized HRPWM methodology are compared with that of the maximum boost control (MBC) technique for qZSI to justify superior performances of the suggested method in low switching frequency range. The proposed concept has been verified via simulation study. The experimentation (qZSI controlled by microcontroller) validates the working of optimized HRPWM based qZSI which agrees with software results.

Circulating Current Suppression Strategy of Modular Multilevel Converter Based on Model Predictive Control

Modular Multilevel Converter (MMC) has the characteristics of high voltage level and low switching frequency. The traditional modular multilevel converter circulating current control strategy mostly adopts the PI control principle, and the parameter setting is complicated and the accuracy is not high, and the control process is more difficult. Model predictive control strategy is the optimal control method based on the model in the existing time domain. This paper proposes a Model Predictive Control (MPC) method based on carrier phase-shifted pulse width modulation (PSC-PWM) to suppress the circulating current, and output the optimal modulation wave through model prediction. Compared with the traditional control strategy, this strategy is simple to implement, does not require complex tuning calculations, and combines with the traditional capacitor voltage equalization strategy to obtain the output modulation wave. A 7-level MMC simulation control system is built in MATLAB / SIMLINK to verify the theory, comparing with existing control methods, it can be concluded that the proposed method has high calculation efficiency, good control accuracy and strong robustness.

Model Predictive Current Control of Multilevel Smart Load Based on MPUC7 Converter

Considering the poor power quality performance of a two-level capacitor-based electric spring (ES1), this article presents a high-power density multilevel ES1 configuration based on a seven-level modified packed U-cell converter (MPUC7-ES1). A novel control strategy based on finite control-set model predictive current control (FCS-MPCC) is also proposed for MPUC7-ES1 application. This algorithm is designed to predict the system behavior for all the conceivable switching vectors based on the discrete models of MPUC7-ES1 that is developed for the first time. To guarantee a desirable operation, the main control objectives are the MPUC7-ES1 current, auxiliary capacitors’ voltages, and switching frequency reduction. Compared with the conventional ES1 control methods, the proposed strategy has key merits including considering the dynamic models of ES1 converter, not requiring a modulator, and lower switching frequency. The operation and robustness of MPUC7-ES1 and the introduced nonlinear FCS-MPCC technique are also illustrated through extensive simulation and experimental results.

A Fast Convergent Homotopy Perturbation Method for Solving Selective Harmonics Elimination PWM Problem in Multi Level Inverter

Pulse width modulation (PWM) control for power converters have been vastly investigated and used in many industrial application. For medium voltage and high power applications, low switching frequency PWM techniques are preferred over high switching frequency-based PWM techniques. The preprogrammed low switching frequency-based PWM technique known as selective harmonics elimination (SHE) gives the better quality waveform at a lower switching frequency. The main difficulty in applying SHE PWM is in solving of non-linear transcendental equations for obtaining switching angles. Several methods have been proposed for computation of switching angles that include numerical techniques, optimization techniques, and algebraic techniques. However, the computation of switching angles is still a challenging task in the application of SHE PWM. In this paper, a novel fast convergent homotopy perturbation method (HPM) is proposed to compute switching angles for a multilevel inverter at a faster rate. The solutions obtained by the proposed technique is as accurate as obtained by the algebraic methods with no dependency on the initial guess. The proposed technique can compute the higher number of switching angles with multiple solutions in some modulation index range. A prototype is developed and the computed switching angles have been verified using field-programmable gate arrays (FPGA) controller to validate the results for practical applications.

High Quality NLM Method for Medium Voltage Hybrid Bridge MMC

Medium voltage hybrid bridge multilevel converter (MMC) usually has a lower number of links. Therefore, the traditional Nearest Level Modulation (NLM) method has the problems of low equivalent switching frequency and poor output quality. And the traditional Carrier Phase-shifted Pulse Width Modulation (CPS-PWM) modulation has problems such as difficulty in hybrid bridge control, difficulty in redundant configuration, and large amount of calculation. In response to the above problems, this paper proposes a new high-quality NLM method. Based on the traditional NLM modulation method, PWM modulation output and module voltage equalization are realized through the time difference between one module input and one module exit in each control cycle. It achieves the purpose of improving the equivalent switching frequency and output power quality, and at the same time has the advantages of the traditional NLM modulation method with small calculation amount, redundant configuration and hybrid bridge modulation. This article describes the three modulation methods and compares their effects. Finally, the theoretical analysis is verified by PSCAD simulation.

Modular multilevel converter with thyristor DC-link switch for full-torque variable-speed drives

The modular multilevel converter (MMC) is a promising topology for medium-voltage drive applications due to its high-quality output waveforms, low device switching frequency and voltage rating. However, the large cell capacitor voltage ripple is a severe challenge faced by MMC at low motor speeds. Recently, a hybrid MMC (HMMC) topology is proven to be a competitive solution because of its lower cell capacitor voltage ripple and no common-mode voltage (CMV) problem compared with other methods. However, the dc-link switch with IGBT limits HMMC to be applied for high-voltage applications. This paper uses the thyristor instead of IGBT as the dc-link switch. To ensure the thyristor can be softly turned on and safely turned off, a new control scheme is proposed. When using this proposed scheme, HMMC can also tolerate the failure of thyristor's turning-off without shutting down the system, improving the reliability effectively. The cell capacitor voltage ripple analysis is presented considering the effects of the thyristor switching process. In addition, a decoupled energy balancing control is utilized to suppress the fluctuation of dc current. Experimental results obtained from a 380 V/7.5 kW downscaled prototype validate the effectiveness of starting up a motor from the standby mode to rated speed with full-torque.

An Improved Low Switching Frequency Modulation Scheme for a Symmetric CCS-Multilevel Inverter

In this paper, a low switching frequency modulation scheme is proposed for the symmetrical configuration of cross-connected sources-based multilevel inverter (CCS-MLI). The proposed scheme, named modified nearest level modulation (mNLM), is an improvement over the conventional nearest level modulation (NLM) scheme for MLI. The implementation of mNLM, compared to the NLM, has shown a considerable reduction in total harmonic distortion (THD) at the ac output of MLI. The mNLM works through variations in the switching angles of the MLI. By suitably varying the switching angles, the THD content is brought to the minimum. An algorithm for the same is also proposed which enables the mNLM to be implemented on other MLI topologies. Simulation is carried out in MATLAB/Simulink environment and it is also extended to a hardware prototype. The effectiveness of the proposed modulation scheme is also seen in terms of improved inverter parameters such as rms voltage, rms current and generated power.

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.

An improved low switching frequency modulation scheme for a symmetric CCS-multilevel inverter

An improved low switching frequency modulation scheme for a symmetric CCS-multilevel inverter

Power quality enhancement of a hybrid energy source powered packed e-cell inverter using an intelligent optimization technique

The most installed Renewable Energy Sources (RES) in micro-grids (MG) are Photovoltaic (PV) power and wind power. Due to the intermittent behaviour of renewable sources, parallel operation of RES and battery storage known as hybrid system is important particularly in remote micro-grids to reduce the fuel consumption by diesel generators and continuity of supply to the load. In this paper, multilevel inverter called Packed E-Cell (PEC) is used for parallel operation of RES and battery storage optimally for micro-grid applications. The PEC requires less components compared to other Multi-level inverters (MLI) topology with relatively low total harmonic distortion (THD). Further, selective harmonic technique based on optimization principle is used to enhance the harmonic profile using low frequency switching technique. The 3rd and 5th harmonics are eliminated using Genetic Algorithm (GA) optimization technique. The simulation-based analysis is done using Simulink/MATLAB and the results obtained for THD in the output current and voltage are presented and discussed in the paper. A comparative analysis is also presented with high frequency modulation technique phase disposition pulse width modulation (PDPWM) technique. The experimental validation of the proposed scheme is done using Typhoon HIL (hardware in loop).

Three-Phase Step-Up Multilevel Inverter With Self-Balanced Switched-Capacitor

This article presents a new three-phase multilevel inverter with boosting capability for low-voltage applications like electric vehicles and renewable energy sources. This inverter is fed by a single dc voltage source and each phase involves two low-voltage transistors, two high-voltage transistors, two diodes, and two capacitors. Except that the two high-voltage transistors withstand twice the dc input voltage, all other components are rated to the dc input voltage. With phase-disposition pulsewidth modulation, the two types of high- and low-voltage transistors operate in low and high switching frequencies, respectively. This is very beneficial for reducing switching losses and selecting semiconductor switches. The two capacitors are connected in parallel and series alternately with the dc source resulting in high ac output voltage with multiple levels, self-balanced capacitor voltages as well as low voltage ripples. The topology, operation principle, capacitors' voltage ripples and power loss are analyzed in detail. Both simulation and experimental results are provided to demonstrate the feasibility of the proposed inverter.

Problem Formulations, Solving Strategies, Implementation Methods & Applications of Selective Harmonic Elimination for Multilevel Converters

Multilevel converters are commonly recommended for converting DC power to AC as per load requirements owing to several advantages such as common mode voltage, less distorted input current, low switching frequency and reduced THD. However, the reliability, performance and efficiency of MLIs depend in particular on the switching technique used. The Selective Harmonic Elimination PWM approach is more efficient among existing modulation techniques to remove undesired harmonics from the output of MLI, since it operates at low switching frequency, particularly at fundamental frequency. The fundamental switching frequency operation of SHEPWM reduce the switching losses and improves inverter efficiency. The principal problem, however, is the empirical approach to non-linear equations. Numerous algorithms have been developed and implemented over the last decades including computational approaches, analytical methods, algebraic methods and optimization algorithms to solve the SHE equations and to eliminate the unintended harmonics. This article presented various aspects of SHE problem formulations, the comprehensive philosophy of operation of various SHE problem-solving approaches and the application of multilevel inverters. PWM waveforms, with single and multiple transitions, are examined and shown at each voltage level for solution frameworks that suit multiple solutions for pulse width modulation to allow researchers gain a deeper understanding and solution of SHE problems.

Model Predictive Control With Reduced Common-Mode Current for Transformerless Current-Source PMSM Drives

The integrated dc-link choke can replace the bulky transformer in current-source converter (CSC)-fed motor drive systems to bear the common-mode voltage (CMV). However, the common-mode (CM) resonance will be excited by the specific harmonics in the CMV, which causes excessive CM current in the loop. Moreover, due to the high-order filter, the $LC$ / $CL$ resonance is prone to be induced at the rectifier and inverter sides, respectively. In this article, the model predictive control (MPC) scheme is proposed to tackle the CM resonance and filter resonance simultaneously. In the low-speed region, besides restraining the peak-to-peak (PTP) magnitude of the CMV at the rectifier side, the third-order harmonic of the CMV generated by the inverter is extracted and then penalized in the cost function to further suppress the CM current. With the increase of the motor speed, the control objective of the inverter side controller switches to the PTP magnitude of CMV suppression because of the alleviation of the CM resonance. The capacitor voltage of both rectifier and inverter sides are regulated via the cost function as well, which can mitigate the current harmonic distortion and improve system stability. The simulation with 1MVA rated power and the scaled-down experiment shows that the proposed scheme can suppress both the CM resonance and the $LC$ / $CL$ resonance effectively with a low switching frequency.

Transformerless Stacked Active Bridge Converters: Analysis, Properties, and Synthesis

This article presents analysis, properties, and systematic synthesis of a new class of hybrid dc–dc converters named transformerless stacked active bridge (TSAB) converters. The TSAB converters, which are obtained from switched-capacitor (SC) converters by insertion of small ac inductors, inherit advantages of parent SC converters together with conversion and control characteristics similar to transformer-isolated dual-active-bridge (DAB) dc–dc converters. Features of TSAB converters include soft charging and discharging of capacitors, zero voltage switching, low peak and rms current stresses, low energy storage requirement of magnetic components, and regulation capabilities using simple phase-shift control. Based on a network-theoretic approach, an algebraic representation of TSAB converters yields general results for dc characteristics and component stresses. Furthermore, a systematic synthesis approach is formulated allowing construction of TSAB converter topologies starting from various two-phase SC converters. Synthesis results are presented for TSAB converters with two inductors obtained from well-known SC parent topologies, including Dickson, ladder, stacked-ladder, doubler, and Fibonacci. Experimental results are summarized for 48-to-12 V doubler, 48-to-12 V Dickson, and 36-to-12 V ladder TSAB prototypes. It is shown how the TSAB prototypes use small ac inductors (tens to hundreds of nanohenries) while operating at relatively low switching frequencies (150–200 kHz) and have measured efficiency above 98% over wide load range.

The Loss Analysis and Efficiency Optimization of Power Inverter Based on SiC mosfets Under the High-Switching Frequency

Due to its low loss and high switching frequency, the silicon carbide metal oxide field effect transistors (SiC mosfet s) are more suitable as switching devices in power inverter for electric vehicles. However, the power losses of power inverter would be dramatically increased with the rise of switching frequency, which would result in the limitation of using the high switching frequency performance of SiC mosfet . In view of this case, this article would systematically analyze the power losses distribution of power inverter and further optimize its efficiency under the high switching frequency. It is demonstrated that the power losses of power inverter are linearly increased with the rise of switching frequency, which is mainly caused by the switching losses of mosfet chips increment. Based on this situation, an adaptive discontinuous pulsewidth modulation (ADPWM) is proposed to optimize the power inverter efficiency under the high switching frequency, which can effectively reduce the total power losses of power inverter and improve its efficiency compared with that of the conventional PWM strategies. Finally, the power losses and efficiency of power inverter with the proposed ADPWM strategy is verified by the experimental results.

LCL resonant current depression control strategy against pulse width modulated harmonic voltage under low switching frequency

The LCL filter is widely used in grid connected converters for good harmonic mitigation performance. With pulse width modulation strategy, converter terminal voltage contains various sideband harmonic components together with expected components corresponding to the modulation wave, especially under a low switching frequency. Considerable resonant current can be generated if the undesired sideband harmonic voltage is located around the LCL resonant frequency. With the background of a medium voltage three-level wind power converter, where the switching frequency is as low as 1.5 kHz, a resonant current depression strategy is proposed against undesired sideband harmonic voltage under low switching frequency. Additional compensators are adopted in the converter current loop. Depending on the system mathematical model, two solutions, a Butterworth low pass filter and a notch filter, based on phase compensation and amplitude compensation, are proposed in this paper. It is shown that the Butterworth low pass filter exhibits better resonant current depression performance, while the notch filter performance is better in terms of system stability. Simulation and experimental results validated the proposed control method.

A Direct Carried-Based PWM Scheme With Reduced Switching Harmonics and Common-Mode Voltage for Current Source Converter

The performance of a current source converter (CSC) is greatly affected by its adopted modulation schemes. Among various modulation schemes, the carrier-based pulsewidth modulation (CB-PWM) has gained increasing attention owing to its inherent autosequencing process. However, existing CB-PWM schemes suffer from either high switching harmonics in ac current or high low-order common-mode voltage (CMV), and sometimes, both, which restrains the application of this promising scheme. To address this issue, this article explores the modulation mechanism of CB-PWM for a CSC. Based on that, this article proposes a direct CB-PWM scheme. Its implementation, mechanism, and characteristics have been developed and presented in detail. It is shown that the proposed scheme enjoys inherent low switching frequency, easy implementation, as well as high dc current utilization. More importantly, compared with existing CB-PWMs, the proposed scheme can simultaneously reduce the switching harmonics of ac current and the low-order CMV for a CSC. These superior performances can bring potential benefits to scale down the ac filter and common-mode choke for practical CSC applications. Effectiveness of the proposed scheme is verified experimentally with a CSC operating as an inverter.