Four-level Converter Research Articles

Active Thermal Control in Neutral-Point-Clamped Multilevel Converters Based on Switching-Cell Arrays

Neutral-point-clamped multilevel converters are a suitable solution to the implementation of low–medium voltage and power applications at present, thanks to their intrinsic superior voltage and current quality. The conventional configurations of these converters present uneven power loss distribution, causing thermal stress in some power semiconductors, which weakens the power converter reliability. To overcome this, an implementation of the neutral-point-clamped multilevel converter based on a switching-cell array is introduced, adding redundant conduction paths on one side and more options to distribute the switching losses on the other side. An active thermal control is proposed to balance the temperature distribution in the converter. A four-level converter has been implemented to evaluate the proposed solution. The experimental results show that the proposed implementation and active thermal control presents an enhanced temperature distribution in the converter and, therefore, reduced thermal stress and better reliability.

Novel discontinuous PWM for dc-link voltage balancing of multilevel quasi-nested topologies

ABSTRACT The use of multilevel power converters for its application in high demanding industrial processes and grid connected renewable energy conversion systems has nowadays become the most suitable solution. The main advantages of these converter configurations are: reduction in the blocking voltage stress, lower harmonic ratings, smaller filters for output and input stages. The proposed quasi-nested topology merges the advantages of the classical multilevel topologies, such as the NPC and ANPC configurations, addressing less active switching devices, lower blocking voltage stress. A novel discontinuous modulation technique, with controlled discontinuous gain, is implemented in order to deal with the dc-link voltage unbalance. Simulation and experimental validation results in a scaled-down power 1,5 kW three-phase converter prototype under different operational conditions using different modulation index are presented. The proposed four-level converter topology allows generating an additional output voltage level with the same semiconductor count as conventional three-level inverters such as NPC and ANPC with superior waveform quality, with a THDv reduction of 34.54% in comparison to the clamped inverters. Experimental tests carried out in a 1.5 kW setup verify the feasibility of the proposed topology and the developed discontinuous modulation scheme, to achieve dc-link voltage compensation.

Corrections to “Dual T-Type Four-Level Converter” [Jun 20 5594-5600

Chen and Wang [1] proposed a dual T-Type four-level converter with a variable-carrier voltage balancing method. The capacitor voltage imbalance mechanism was revealed in [1, Sec. III], with a conclusion that the overdischarge of the middle capacitor is the root cause of the capacitor voltage imbalance issue. This has been experimentally validated by the articles [1], [2]. Although the conclusion is correct, the theoretical derivation of the average capacitor currents is incorrect as the input current and the duty cycle of every power switch were not considered in the previous version. As a correspondence, we would like to present the corrected equations of the average capacitor current calculation in Section III. In addition, there is one more correction to the capacitor voltage ripple description in Section V.

Pulse-Amplitude-Modulation Full-Bridge Diode-Clamped Multilevel LLC Resonant Converter Using Multi-Neighboring Reference Vector Discontinuous PWM

A full-bridge diode-clamped multilevel LLC resonant converter suitable for power conversion systems that use high input voltage, such as railway vehicles, is proposed in this paper. In order to eliminate the voltage deviations of the capacitors connected in series to the high voltage input DC link, a novel modulation strategy referred to as multi-neighboring reference vector discontinuous pulse-width modulation (MNRV DPWM) is proposed. Unlike the existing two-level resonant converter that varies the operating frequency to hold the output voltage constant, the proposed multilevel resonant converter modulates the amplitude of the fundamental wave input to a resonance tank while fixing the operating frequency at the resonance point. Therefore, the design of passive elements becomes easier, and stable operation is possible over a wide operating range with only one power conversion stage. In this paper, the control algorithm and operation characteristics of the newly proposed full-bridge diode-clamped four-level LLC resonant converter are analyzed in detail and design guidelines are presented. The feasibility of the proposed converter is verified through a simulation and an experiment with a prototype converter.

Four-Level Hybrid Neutral Point Clamped Converters

In terms of low-voltage (<; 1.5 kV) and high-power (>10 kW) industry applications, power converters without any flying capacitors are preferred to ensure high efficiency, high power density, and light weight. As for these requirements, this article proposes a four-level hybrid neutral point clamped (4L-HNPC) converter and a four-level hybrid active neutral point clamped (4L-HANPC) converter. Compared with the existing three-level and four-level converters, the proposed 4L-HNPC and 4L-HANPC converters show higher efficiency or more even loss distribution. Compared with five-level converters, the proposed converters show higher power density and lower weight due to the absence of flying capacitors. To solve the dc-link capacitor imbalance issue, this article proposes a variable-reference voltage-balancing control method that aims at controlling the middle capacitor voltage at 1/3 of the dc-link voltage via a proportional-integral (PI) regulator. Meanwhile, the other two capacitor voltages are automatically balanced. Compared with the existing voltage-balancing control methods, the proposed variable-reference voltage-balancing control method is more suitable for digital implementation as only one triangular carrier is used. Both simulation and experimental studies have been carried out to verify the proposed four-level converter topologies and variable-reference voltage-balancing control method.

Dual T-Type Four-Level Converter

This letter proposes a three-phase dual T-type four-level converter topology that consists of two T-type networks per phase leg. The proposed converter has only six power switches per phase leg without any clamping diodes or flying capacitors. Besides, it is a flexible topology as there are several bidirectional switch alternatives that can be used for the proposed converter. As for the common severe voltage imbalance issue of the proposed topology and other four-level converters with three series-connected capacitors, the voltage imbalance mechanism is first revealed, and then a variable-carrier based voltage-balance control method is proposed. It aims at controlling the middle capacitor voltage at 1/3 of the full dc-link voltage and the upper and lower capacitor voltages are automatically balanced. Experimental results have been carried out to verify the proposed topology and the proposed variable-carrier based voltage-balance control method.

A Fully Integrated Li-Ion-Compatible Hybrid Four-Level DC–DC Converter in 28-nm FDSOI

This paper presents a Li-ion-compatible fully integrated hybrid dc–dc converter implemented in 28-nm FDSOI. A modified four-level converter is proposed to achieve high efficiency while operating from the 2.8–4.2-V Li-ion battery range by using stacked 1.5-V transistors. In this paper, the proposed driver architecture exploits the internal nodes of the power stage as power/GND rails for drivers, eliminating the need for dedicated power rails, level shifters, or stacked drivers. The proposed converter operates in the discontinuous conduction mode (DCM) and regulates the output with constant ON-time pulse-frequency modulation (PFM), to achieve high efficiency across low load range suitable for wearable/Internet of Things (IoT) applications. The fabricated chip regulates a 0.6–1.2-V output from a 2.8–4.2-V input voltage over an output power range from $10~\mu \textrm {W}$ to 40 mW with 78% peak efficiency, and switching frequency up to 200 MHz. The converter occupies $1.5~\textrm {mm}^{2}$ of on-die area including a pair of 5-nF flying capacitors and a 3-nH inductor.

Intelligent SVM technique of a multi-level inverter for a DFIG-based wind turbine system

In spite of its several advantages, a traditional direct vector control (DVC) of doubly fed induction generators (DFIGs) driven by variable speed wind turbines has some drawbacks. In this work, a simple and fuzzy space vector modulation (4L-FSVM) is proposed to control the four-level converter in order to reduce total harmonic distortion (THD) of rotor current and powers ripples. The validity of the proposed control technique applied to the DFIG is verified by MATLAB/Simulink. The electromagnetic torque, stator reactive power, stator active power and rotor current are determined and compared with the traditional control scheme. Simulation results presented in this article show that the proposed control scheme reduces the THD value, electromagnetic torque ripples and powers ripples compared to conventional control under various operating conditions.

FPGA-Based Controller for a Permanent-Magnet Synchronous Motor Drive Based on a Four-Level Active-Clamped DC-AC Converter

This paper proposes a closed-loop control implementation fully-embedded into an FPGA for a permanent-magnet synchronous motor (PMSM) drive based on a four-level active-clamped converter. The proposed FPGA controller comprises a field-oriented control to drive the PMSM, a DC-link voltage balancing closed-loop control (VBC), and a virtual-vector-based modulator for a four-level active-clamped converter. The VBC and the modulator operate in consonance to preserve the DC-link capacitor voltages balanced. The FPGA design methodology is carefully described and the main aspects to achieve an optimal FPGA implementation using low resources are discussed. Experimental results under different operating conditions are presented to demonstrate the good performance and the feasibility of the proposed controller for motor-drive applications.

An Integrated Switched Reluctance Motor Drive Topology With Voltage-Boosting and On-Board Charging Capabilities for Plug-In Hybrid Electric Vehicles (PHEVs)

This paper presents a new topology of switched reluctance motor (SRM) drives for plug-in hybrid electric vehicles. Six operating modes can be achieved as a result. Three of those modes are applied for driving and the others for charging. During the driving mode, the topology can be converted to a four-level one. During the charging mode, two battery packs are charged in parallel with the boost circuit by the external ac source or generators. The main contributions of the proposed topology are as follows. First, a four-level converter is formed by adopting different ways of connection (series or parallel) of the two battery packs (or the two capacitors), which can accelerate the excitation and demagnetization procedures of SRMs. Moreover, the topology contributes to decreasing the switching losses and extending the constant torque region to improve the drive performance as well. Second, the state of charge of the two battery packs (or the voltages of two capacitors) can keep balance on their own by parallel operation of the two battery packs. Furthermore, a detailed comparison between two drive modes of the proposed topology and asymmetric half bridge inverters is undertaken with simulation and experimental studies, the results of which demonstrate the validity of the proposed topology.

A Space Virtual-Vector Modulation With Voltage Balance Control for Nested Neutral-Point Clamped Converter Under Low Output Frequency Conditions

The recently-developed nested neutral-point clamped (NNPC) converter is a four-level converter for medium-voltage high-power applications. It has some merits such as operating over wide voltage ranges (2.4-7.2 kV) without switching devices in series and less diodes or capacitors when compared with classical four-level topologies. But it has severe voltage balancing problems under low output frequency conditions when conventional space vector modulation (SVM) or sinusoidal pulse width modulation (SPWM) is applied, which limits its wide applications. In order to solve this issue, a space virtual-vector modulation (SVVM) along with a voltage balance control (VBC) algorithm is proposed in this paper. It helps reduce the voltage ripples greatly and maintain them within an acceptable region under low output frequency conditions. Therefore, it guarantees even voltage stresses among switching devices and extends the NNPC converter's applications. Principles of the proposed SVVM and VBC algorithms are elaborated in detail, and the voltage balancing performance is investigated and compared with the cases where conventional SVM and SPWM with the same VBC are applied. The relationship curves between voltage ripples and flying capacitance, output power, and output frequency have been explored and compared for both conventional SVM and proposed SVVM. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

High-Power Multilevel Step-Up DC/DC Converter for Offshore Wind Energy Systems

This paper presents a multilevel step-up dc/dc converter for dc-grid-based offshore wind energy conversion applications. The main features of the proposed multilevel converter are as follows: 1) the voltage stress of the power devices can be reduced, making it suitable for high-voltage applications; 2) the boost inductor can be reduced significantly, leading to the outstanding dynamic performance; and 3) high-voltage gain can be obtained and thus is well-suited for high-gain applications. Theoretical analysis was carried out for a four-level step-up converter, based on which, the converter can be extended to any arbitrary number of levels. The proposed four-level converter was verified by a simulation and evaluated by comparing with a conventional multilevel dc/dc converter in terms of the component stress and power device count. The developed topology was implemented on a 2-kW prototype converter to evaluate its feasibility.

New Integrated Multilevel Converter for Switched Reluctance Motor Drives in Plug-in Hybrid Electric Vehicles With Flexible Energy Conversion

This paper presents an integrated multilevel converter of switched reluctance motors (SRMs) fed by a modular front-end circuit for plug-in hybrid electric vehicle (PHEV) applications. Several operating modes can be achieved by changing the on-off states of the switches in the front-end circuit. In generator driving mode, the battery bank is employed to elevate the phase voltage for fast excitation and demagnetization. In battery driving mode, the converter is reconfigured as a four-level converter, and the capacitor is used as an additional charge capacitor to produce multilevel voltage outputs, which enhances the torque capability. The operating modes of the proposed drive are explained and the phase current and voltage are analyzed in details. The battery charging is naturally achieved by the demagnetization current in motoring mode and by the regenerative current in braking mode. Moreover, the battery can be charged by the external AC source or generator through the proposed converter when the vehicle is in standstill condition. The SRM-based PHEV can operate at different speeds by coordinating the power flow between the generator and battery. Simulation in MATLAB/Simulink and experiments on a three-phase 12/8 SRM confirm the effectiveness of the proposed converter topology.

Comparative Analysis of DITC Based Switched Reluctance Motor Using Asymmetric Converter and Four-Level Converter

High torque ripple is the main draw of Switched Reluctance Motor (SRM) compared to other conventional motor. Torque ripples can be minimized by Direct Instantaneous Torque Control (DITC) technique. DITC responds against the torque error instantaneously by using hysteresis torque controller and regulate the torque output of the motor within hysteresis band.This paper compares the performance of Switched Reluctance Motor in terms of torque ripples with DITC strategy for both Asymmetric converter and Four-level converter. Asymmetric converter has three states namely magnetization, freewheeling and demagnetiztion states. With four-level converter, fast magnetization and fast demagnetization are also possible. Thus, the current build up and decay time is reduced which improves the dynamic performance. SRM is simulated using DITC scheme with Asymmetric converter and Four-level converter in MATLAB/SIMULINK to analyze torque ripples and it is observed that DITC with four level converter is better than that with Assymetrical converter.

Comparative Analysis of DITC Based Switched Reluctance Motor Using Asymmetric Converter and Four-Level Converter

Comparative Analysis of DITC Based Switched Reluctance Motor Using Asymmetric Converter and Four-Level Converter

Comparative Analysis of DITC Based Switched Reluctance Motor Using Asymmetric Converter and Four-Level Converter

Comparative Analysis of DITC Based Switched Reluctance Motor Using Asymmetric Converter and Four-Level Converter

Development of the C-dump topology-based four-level converter for switched reluctance motor drives

Demand for brushless and commutator-less motor drives is increasing day by day for industrial applications. So improvement in switched reluctance motor (SRM) drives remains an important research area due to cheap and maintenance free motor construction. In literature, various converter topologies and their control strategies are described, however the C-dump-based converter topologies have gained interest owing to its ability to retrieve the stored magnetic energy. This can be further utilised to achieve desirable performance characteristics. The paper addresses the development of a drive system for a four phase, 8/6 switched reluctance motor. The drive system consists of a power circuit and its control scheme. The power components are designed for a 4-phase, 3 hp, 8/6 SRM which is used to obtain the experimental results. A simple PI-based speed control scheme is implemented which is suitable for low power applications such as cooling fan, blower, etc. A C-dump-based converter is proposed which allows reduction of switching losses, especially in the low speed range and it eliminates energy intake from the source during energy discharge process.

Finite Control-Set Model Predictive Control (FCS-MPC) of Nested Neutral Point-Clamped (NNPC) Converter

This paper proposes a model predictive control (MPC) strategy for a nested neutral point-clamped (NNPC) converter to control output currents and voltages of flying capacitors. The NNPC converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4–7.2 KV) without the need for connecting power semiconductor in series, high quality output voltage, less number of components compared to other classical four-level topologies. A discrete-time model of the converter is presented and all the control objectives are formulated in terms of the switching states. During each sampling interval, the predicted variables are assessed by the cost function and the best switching state which gives minimum value for the cost function is selected and applied to the converter gating terminals. The performances of the NNPC converter and predictive control scheme are verified through MATLAB/Simulink simulations and their feasibility is evaluated experimentally.

A Novel and Simple Single-Phase Modulator for the Nested Neutral-Point Clamped (NNPC) Converter

The nested neutral-point clamped (NNPC) converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4–7.2 KV) without the need for connecting the power semiconductor in series, high quality output voltage, and less number of components in compare to other classical four-level topologies. The control and balance of the flying capacitors (FCs) of the NNPC converter can be done by different control techniques taking advantage of the large number of redundant switching states. This paper presents a simple single-phase modulator for the NNPC converter, which can be applied to each phase of a three-phase NNPC converter. The proposed simple technique can control and balance the FCs to their desired values. Performance of the proposed technique under different operating conditions is investigated in the MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally.

Low-Volume PFC Rectifier Based on Nonsymmetric Multilevel Boost Converter

This paper introduces a digitally controlled single-phase rectifier with power factor correction (PFC) based on a modified three-level boost converter topology. In comparison with the conventional boost-based systems, the new PFC rectifier has about three times smaller inductor and significantly lower switching losses. The improvements are achieved by replacing the output capacitor of the boost converter with a nonsymmetric active capacitive divider, with a 3:1 division ratio, and by utilizing the downstream converter stage for the capacitive divider's center-tap voltage regulation. The nonsymmetric voltage divider and applied switching sequence effectively provide four-level converter behavior using the same number of components as three-level converters. As a result, a 66% reduction of the inductor compared to the conventional boost-based PFC and a 33% compared to the standard three-level solutions operating at the same effective switching frequencies are also achieved. Experimental results obtained with a 400 W, 200 kHz, universal input voltage (85 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> -265 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> ) PFC prototype demonstrate three times smaller inductor current ripple than that of the conventional boost converter allowing for the same inductor reduction. Efficiency improvements of up to a 6% are also demonstrated.