In recent years, modular multilevel converters (MMCs) are very popular in medium/high-voltage motor drive systems and high-voltage direct-current (HVDC) applications. The conventional model predictive control (MPC) strategies for the MMC are not practical due to their substantial calculation requirements, especially under high number of voltage levels. To solve this problem, a fast MPC strategy combined with a submodule (SM)-voltage sorting balancing method is proposed based on the discrete mathematical model derived for the MMC in this paper. By optimizing the implementation of the control objectives and simplifying the rolling optimization, the proposed strategy is simpler to expand to a system with larger number of SMs, with the amount of calculation reduced and the advantages of the conventional MPC algorithm reserved. In addition, the proposed control can minimize the $dv/dt$ of the output voltages. Both steady-state and transient performances are evaluated by a down-scaled three-phase MMC prototype under various experimental conditions, which validates the proposed fast MPC strategy.

Design and Experimental Evaluation of Fast Model Predictive Control for Modular Multilevel Converters

In vehicle-to-grid applications, the battery charger of the electric vehicle (EV) needs to have a bidirectional power flow capability. Galvanic isolation is necessary for safety. An ac–dc bidirectional power converter with high-frequency isolation results in high power density, a key requirement for an on-board charger of an EV. Dual-active-bridge (DAB) converters are preferred in medium power and high voltage isolated dc–dc converters due to high power density and better efficiency. This paper presents a DAB-based three-phase ac–dc isolated converter with a novel modulation strategy that results in: 1) single-stage power conversion with no electrolytic capacitor, improving the reliability and power density; 2) open-loop power factor correction; 3) soft-switching of all semiconductor devices; and 4) a simple linear relationship between the control variable and the transferred active power. This paper presents a detailed analysis of the proposed operation, along with simulation results and experimental verification.

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A Bidirectional Soft-Switched DAB-Based Single-Stage Three-Phase AC–DC Converter for V2G Application

This paper presents a new topology for a fully bidirectional soft-switching solid-state transformer (S4T) The minimal topology, featuring 12 main devices and a high-frequency transformer, does not use an intermediate dc voltage link, and provides sinusoidal input and output voltages The S4T can be configured to interface with two- or multiterminal dc, single- or multiphase ac systems An auxiliary resonant circuit creates zero-voltage-switching conditions for main devices from no-load to full-load, and helps manage interactions with circuit parasitic elements The modularized structure allows series and/or parallel stacking of converter cells for high-voltage and high-power applications

Soft-Switching Solid-State Transformer (S4T)

This paper presents a new speed finite control set model predictive control algorithm, which has been applied to a permanent-magnet synchronous motor driven by a matrix converter (MC) This method replaces the classical cascaded control scheme with a single control law that controls the motor currents and speed Additionally, unlike classical MC modulation methods, the method allows direct control of the MC input currents The performance of the proposed work has been verified by simulation studies and experimental results

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Speed Finite Control Set Model Predictive Control of a PMSM Fed by Matrix Converter

Modular multilevel converter (MMC) is an emerging and highly attractive multilevel topology for medium- and high-voltage applications. This paper proposes an MMC-based power electronic transformer (PET) for dc distribution grid. The high-voltage side is an H-bridge MMC to generate a high-frequency adjustable-magnitude sinusoidal voltage, and the low-voltage side is paralleled multiwindings and rectifiers to allow high-load currents. As the number of MMC voltage levels increases, the complexity of the control system as well as the control algorithm increases largely. In order to simplify the control system and reduce the computation time, a hierarchical control system is designed for the MMC-based PET, and the relevant hierarchical control algorithm is also presented in this paper. The modulation method, capacitor voltage balancing method, and communication solution under the hierarchical control system are all discussed in detail. Simulation and experimental results are presented to demonstrate this system and control method.

Hierarchical System Design and Control of an MMC-Based Power-Electronic Transformer

A solid-state transformer is a three-phase ac/ac converter with a high-frequency transformer. Due to advanced features like high power density, on demand var support and frequency regulation, solid-state transformer is an enabling technology for the modern power distribution system. It can also find application in high-power-density motor drives. The single-stage solid-state transformer considered in this paper is capable of bidirectional power flow and open loop power factor correction. This topology uses a minimum amount of copper and has relatively few semiconductor switches. One major problem in this converter is the commutation of leakage energy which results in power loss, reduction in switching frequency, loss of output voltage, and additional common-mode voltage switching. This paper presents a source-based commutation strategy along with a novel modulation technique resulting in elimination of additional snubber circuits, minimization of the frequency of leakage inductance commutation, recovery of the leakage energy, and soft switching of the output converter. The topology and its proposed control have been analyzed. Simulation and experimental results confirm the operation.

A Single-Stage Solid-State Transformer for PWM AC Drive With Source-Based Commutation of Leakage Energy

A filter is required to eliminate the high-frequency switching ripple present in the input current of a matrix converter (MC). Design of such a filter requires an estimation of the higher harmonic components present in the input current. This paper presents a simple closed-form analytical expression for the RMS input current ripple injected by the MC. The expression shows the variation with load power factor and is independent of the output frequency. This is used in a step-by-step procedure to design various input filter components from the specifications of allowable total harmonic distortion in the grid current and distortion in the input voltage. The MC is modeled for the grid frequency component in order to evaluate the design for input power factor and voltage drop across the filter. A damping resistance has been designed ensuring minimum ohmic loss. The analytical estimation of the ripple current and the proposed design procedure have been validated by simulations in MATLAB/Simulink and experiments on a laboratory prototype.

Systematic Input Filter Design of Matrix Converter by Analytical Estimation of RMS Current Ripple

One aspect of the disclosure includes a submodule topology for a modular multilevel converter. The submodule topology includes two electronic switches connected together with a first series connection terminal connecting the electronic switches in series, the series connected switches being connected in parallel with two capacitors connected together with a second series connection terminal connecting the capacitors in series. A bidirectional electronic switch connects the first series connection terminal with the second series connected terminal. An output voltage is obtained across the first series connected terminal and a common terminal formed by the parallel connection of the series connected switches with the series connected capacitors.

Modular converter with multilevel submodules

This paper targets the reduction of common-mode voltage in an indirect matrix converter (IMC) through an intelligent space-vector-based modulation technique. This mode of control results in lower d V /d t at the motor terminals, thereby, reducing voltage stress to windings; and reduced output voltage distortion resulting in lower machine losses. The conventional indirect space vector pulse-width modulation method of controlling matrix converters involves independent control of either the rectifying stage or the inverting stage of the converter. However, in this paper, by suitable selection of space vectors, the rectifying stage of the matrix converter generates different levels of virtual dc-link voltage. Therefore, the responsibility of formulating output voltages with a particular magnitude and frequency, which can be translated to different machine speeds, has been transferred solely to the rectifying stage of the IMC. Estimation of the degree of distortion in the three-phase output voltage is another facet discussed in this paper, which aids the sizing and designing of output passive filters. The analysis of output voltage distortion and the proposed modulation strategies have been substantiated by simulations in MATLAB/Simulink and experimentally verified on a scaled down laboratory prototype.

Modulation Techniques for Enhanced Reduction in Common-Mode Voltage and Output Voltage Distortion in Indirect Matrix Converters

LCL filter is becoming an attractive choice over conventional L filters for grid-connected voltage source inverters (VSI) due to smaller inductor size and better attenuation of the ripple components in the grid current. The modulation of the VSI generates switched voltages which results in distorted currents. In this paper, a simple closed form analytical expression is derived for the higher order switching components present in the inverter voltage. This is used in a systematic design procedure to design the LCL filter components for allowable grid current harmonics. A passive damping resistor is designed ensuring minimum power loss. The design is validated by simulations in MATLAB/Simulink and experiments on a laboratory prototype.

Ashish Kumar Sahoo

Papers

A Single-Stage Solid-State Transformer for PWM AC Drive With Source-Based Commutation of Leakage Energy

Systematic Input Filter Design of Matrix Converter by Analytical Estimation of RMS Current Ripple

Modular converter with multilevel submodules

Modulation Techniques for Enhanced Reduction in Common-Mode Voltage and Output Voltage Distortion in Indirect Matrix Converters

LCL filter design for grid-connected inverters by analytical estimation of PWM ripple voltage