The modular multilevel converter (MMC) is currently one of the power converter topologies which has attracted more research and development worldwide. Its features, such as high quality of voltages and currents, high modularity and high voltage rating, have made the MMC a very good option for several applications including high-voltage dc (HVdc) transmission, static compensators (STATCOMs), and motor drives. However, its unique features such as the large number of submodules, floating capacitor voltages, and circulating currents require a dedicated control system able to manage the terminal variables, as well as the internal variables with high dynamical performance. In this paper, a review of the research and development achieved during the last years on MMCs is shown, focusing on the challenges and proposed solutions for this power converter still faces in terms of modeling, control, reliability, power topologies, and new applications.

Modular Multilevel Converters: Recent Achievements and Challenges

Due to the attractive advantages such as modularity, scalability and excellent power quality of modular multilevel converter (MMC), converters based on MMC could be a promising alternative solution for traction transformer. A novel MMC-based advanced co-phase traction power supply system is proposed in this paper to solve power quality issues and eliminate the neutral sections in the traditional traction power supply system. And in the proposed system, a DC power transmission system is designed, which provides convenient access for distributed energies benefiting the utilization of the natural resources such as solar energy and wind energy along railways. In order to ensure the normal operation of the proposed system, nearest-level modulation considering voltage balancing is designed for MMCs. The mathematic model three-phase MMC-based rectifier is derived in detail. Based on the mathematic model, dual current-loop control is designed for the rectifier. Besides, the parallel operating traction substations suffer circulating current issue. A droop control combining with double closed-loop control is designed to deal with the problem. The correctness and feasibility of the system and its modulation and control strategies is verified through simulation and a small-scale experiment.

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A Novel Advanced Traction Power Supply System Based on Modular Multilevel Converter

The utilization of silicon carbide (SiC) MOSFET instead of silicon (Si) IGBT can significantly improve the performance of many converters. However, a modular multilevel converter (MMC) completely based on the SiC MOSFET suffers from high cost and high conduction loss at the high power levels. To solve these issues, a SiC MOSFET and an Si IGBT hybrid MMC are proposed in this letter. In the new topology, only one full-bridge submodule (SM) of each arm uses SiC MOSFETs, while the other half-bridge SMs use Si IGBTs. Meanwhile, a specialized modulation scheme is proposed to move most of the switching actions from the Si SMs to the SiC SM. As a result, the advantages of SiC MOSFET and Si IGBT are both utilized, and the total loss and cost of the MMC are reduced. Finally, the experimental result proves the feasibility of the topology and modulation scheme, and the further loss analysis verifies the superiority of the hybrid MMC over others.

A SiC MOSFET and Si IGBT Hybrid Modular Multilevel Converter With Specialized Modulation Scheme

Multilevel inverters (MLIs) have proven superior performance in several applications, especially in photovoltaic (PV) applications. However, power switches in MLIs possess high failure rates due to unequal power losses distributions that shorten lifetime of the whole PV inverter. Therefore, a new pulsewidth modulation (PWM) methodology for loss balance in grid connected PV single-phase five-level inverters is presented in this paper. The proposed loss balance PWM (LBPWM) method is based on swapping the utilization of power switches through half or quarter of the fundamental line period based on the redundancies between the switching states in MLIs. In addition, the proposed LBPWM method achieves natural balance among voltages over the dc-link capacitors. The effectiveness and feasibility of the proposed LBPWM method are verified using the simulation environment and experimental prototype. Different operating conditions and points of PV inverters are investigated in addition to reliability evaluation of the PV inverter are provided. Moreover, the superior performance criterion of the proposed LBPWM methods are verified through comprehensive comparisons with the most prominent previously developed PWM methods in the literature. The generalization and implementation steps of the proposed LBPWM method are also provided.

Modulation Method for Improving Reliability of Multilevel T-Type Inverter in PV Systems

Using high voltage (HV) silicon carbide (SiC) power semiconductors in a modular multilevel converter (MMC) is promising because it results in fewer submodules and lower switching loss compared to conventional Si based solutions. The nearest level pulsewidth modulation (NL-PWM) is commonly used in the MMC for medium voltage applications. However, with the NL-PWM and existing voltage balancing control, there are many submodules that switch their modes in a control cycle, resulting in a high dv/dt during the deadtime of the power semiconductor, which could be multiple times of the dv/dt of the single device. This poses great challenges on the noise immunity and insulation design in the MMC using HV SiC devices, which have very fast switching speed. A novel voltage balancing control, which ensures only two submodules switch their modes in a control cycle, is proposed in this article, limiting the maximum dv/dt to the dv/dt of a single power semiconductor and also maintaining the voltage balance performance. The proposed voltage balancing control is experimentally validated in a 10-kV SiC mosfet based MMC with four submodules per arm.

A Novel Voltage Balancing Control With dv/dt Reduction for 10-kV SiC MOSFET-Based Medium Voltage Modular Multilevel Converter

For modular multilevel converters (MMCs) applied to medium-voltage dc distribution grids, using the traditional nearest level modulation (NLM) as in HVdc systems can lead to severe current distortion due to significantly reduced module numbers. This paper proposes a hybrid modulation method combining NLM and pulse width modulation (PWM) where only one module per arm operates under PWM mode. The proposed nearest level PWM (NL-PWM) method not only significantly reduces the current distortion, but also avoids the complicated voltage balancing control in each module. The harmonic characteristics of NL-PWM are derived using double Fourier transform, which provides a theoretical basis for selecting the module number and switching frequency for medium-voltage applications in accordance with grid harmonic requirements. Finally, the harmonic characteristics and feasibility of the proposed modulation method are validated by simulation and experimental studies on an MMC with six modules per arm. The simulated and experimental results reveal that NL-PWM has better voltage and current harmonic characteristics over NLM and CPS-PWM, thereby suiting the application of MMC with few models.

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A Nearest Level PWM Method for the MMC in DC Distribution Grids

Memristor-based systems can exhibit the phenomenon of extreme multi-stability, which results in the coexistence of infinitely many attractors. However, most of the recently published literature focuses on the extreme multi-stability related to memristor initial conditions rather than non-memristor initial conditions. In this paper, we present a new five-dimensional (5-D) two-memristor-based jerk (TMJ) system and study complex dynamical effects induced by memristor and non-memristor initial conditions therein. Using multiple numerical methods, coupling-coefficient-reliant dynamical behaviors under different memristor initial conditions are disclosed, and the dynamical effects of the memristor initial conditions under different non-memristor initial conditions are revealed. The numerical results show that the dynamical behaviors of the 5-D TMJ system are not only dependent on the coupling coefficients, but also dependent on the memristor and non-memristor initial conditions. In addition, with the analog and digital implementations of the 5-D TMJ system, PSIM circuit simulations and microcontroller-based hardware experiments validate the numerical results.

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Initial-Condition Effects on a Two-Memristor-Based Jerk System

The five-dimensional (5-D) two-memristor-based jerk (TMJ) system can emerge the initial condition-dependent extreme multi-stability due to two zero roots caused by its plane equilibrium set. However, using conventional stability analysis methods, we cannot explain the kinetic mechanism of initial condition effects on the 5-D TMJ system. To this end, this paper establishes a three-dimensional (3-D) incremental integral reconstructed (IIR) model using an integral transformation method, and converts the internal initial conditions and plane equilibrium set of the original system into the explicit initial condition parameters (ICPs) and one or no equilibrium point of the reconstructed model respectively. With the explicit ICPs and determined equilibrium point of the reconstructed model, the memristor and non-memristor initial condition effects on the original system are thereby detected by the stability distributions and then confirmed by the kinetic distributions. The results show that the memristor and non-memristor initial conditions have powerful influence on the kinetic effects of the 5-D TMJ system, and its kinetic mechanism of initial condition effects can be explained effectively based on the 3-D IIR model. Besides, with analog circuit and digital circuit platform, the circuit simulations and hardware experiments verify the kinetic effects of the ICPs on the 3-D IIR model. 

Incremental integral reconstitution for detecting initial condition effects

To improve the security of mobile networks in the postquantum era, Dabra et al. recently proposed a lattice-based anonymous password-authenticated key exchange (LBA-PAKE) protocol for mobile devices. Especially, LBA-PAKE is claimed to support the key reuse. However, we find that LBA-PAKE is still vulnerable to the signal leakage attack when the master key is reused. We propose two strategies to reduce the needed number of queries in our attack. Compared to the method of Bindel et al., our method reduces the required queries by more than 75%. Our experiments show that breaking LBA-PAKE needs less than 2 min. Through analysis of why LBA-PAKE fails in their security proof, we further propose an improved protocol without incurring extra computation costs. The formal security analysis shows that our improved scheme supports all features of LBA-PAKE while thwarting the signal leakage attack. Moreover, the implementation of our improved protocol demonstrates its efficiency in mobile networks.

Further Analysis and Improvements of a Lattice-Based Anonymous PAKE Scheme

This work presents a two-dimensional (2-D) non-autonomous tabu learning single neuron (TLSN) model based on sinusoidal activation function (SAF), which can generate a class of multi-scroll chaotic attractors with parameters controlling the number of scrolls. The SAF-based TLSN (SAF-TLSN) model has a periodically alterable equilibrium point closely related to the model parameters. Its equilibrium trajectories with different stability types can be distributed in the phase plane, resulting in the generation of multi-scroll chaotic attractors. The multi-scroll chaotic attractors and their dynamical behaviors are investigated by phase plane portrait, maximal Lyapunov exponent, bifurcation diagram, and cross section. The numerical results demonstrate that each scroll of multi-scroll chaotic attractors is associated with the equilibrium trajectories composed of infinitely many unstable equilibrium points, and the scroll distributions of multi-scroll chaotic attractors are confined to the distribution ranges of equilibrium trajectories that are controlled by the model parameters. Besides, using a digital hardware platform, the SAF-TLSN model is implemented and the multi-scroll chaotic attractors with different scrolls are obtained experimentally to verify the numerical ones. 

Ruoyu Ding

Papers

A Nearest Level PWM Method for the MMC in DC Distribution Grids

Initial-Condition Effects on a Two-Memristor-Based Jerk System

Incremental integral reconstitution for detecting initial condition effects

Further Analysis and Improvements of a Lattice-Based Anonymous PAKE Scheme

Two-dimensional non-autonomous neuron model with parameter-controlled multi-scroll chaotic attractors