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Modular multilevel cascade converter (MMCC) is a family of the emerging multilevel converters that are configured with a cascaded connection of full-bridge submodules or half-bridge ones by distinct topological structures. So far, MMCC family can be classified by basic configurations and submodule types into single-star full-bridge, double-star half-bridge (modular multilevel converter (MMC)), double-star full-bridge, double-star half-bridge back-to-back (indirect MMC), triple-star full-bridge (modular multilevel matrix converter, M3C), single-delta full-bridge and double-delta full-bridge (Hexverter). This study introduces a generalised and standard analytical procedure for MMCC and deals with the double-star half-bridge case (MMC) as example. A particular defined circulating current with a clearer physical meaning is used to analyse the complicated branch current composition and branch energy fluctuation. A full mathematical model based on state-space equations is established for MMC and a corresponding energy-current control strategy is presented. The validity of the control design and effectiveness of MMC is confirmed by simulation and experiment.

There has been a lot of buzz about the recently designed modular multilevel converter (MMC), which is quickly becoming a viable technology for a wide range of high- and medium-power applications. The MMC provides emulous benefits such as excellent output quality, flexible modularity, simple scaling, and low rating demand for the power switches that make it stand out from the competition. MMC topologies, on the other hand, present a number of unique problems, such as reduced voltage stress, significantly lower semiconductor losses, and compactness. A reduced cost of operation for high-power systems such as high-voltage direct current transmission and distribution systems is attributable to these factors. The most significant contribution of this work is to examine MMC applications and various circuit configurations. There is a lot of work being done around the globe for its uses in photovoltaic devices, wind energy conversion, low-/medium-voltage drives, and so on. We will go deep into the topological derivatives, modeling, modulation methods, controlling provisions, fault-tolerant behavior, capacitor voltage balancing techniques of MMC, and a few other topics in this article in detail.

The Modular Multilevel Matrix Converter is a relatively new power converter topology appropriate for high-power Alternating Current (AC) to AC purposes. Several publications in the literature have highlighted the converter capabilities such as modularity, control flexibility, the possibility to include redundancy, and power quality. Nevertheless, the topology and control of this converter are relatively complex to design and implement, considering that the converter has a large number of cells and floating capacitors. Therefore multilayer nested control systems are required to maintain the capacitor voltage of each cell regulated within an acceptable range. There are no other review papers where the modelling, control systems and applications of the Modular Multilevel Matrix Converter are discussed. Hence, this paper aims to facilitate further research by presenting the technology related to the Modular Multilevel Matrix Converter, focusing on a comprehensive revision of the modelling and control strategies.

The modular multilevel matrix converter is a relatively new power converter topology suitable for high-power alternating current (AC)-to-AC applications. Several publications in the literature have highlighted the converter capabilities, such as full modularity, fault-redundancy, control flexibility and input/output power quality. However, the topology and control of this converter are relatively complex to realise, considering that the converter has a large number of power-cells and floating capacitors. To the best of the authors’ knowledge, there are no review papers where the applications of the modular multilevel matrix converter are discussed. Hence, this paper aims to provide a comprehensive review of the state-of-the-art of the modular multilevel matrix converter, focusing on implementation issues and applications. Guidelines to dimensioning the key components of this converter are described and compared to other modular multilevel topologies, highlighting the versatility and controllability of the converter in high-power applications. Additionally, the most popular applications for the modular multilevel matrix converter, such as wind turbines, grid connection and motor drives, are discussed based on analyses of simulation and experimental results. Finally, future trends and new opportunities for the use of the modular multilevel matrix converter in high-power AC-to-AC applications are identified.

In recent years, the small-signal stability of modular multilevel converter (MMC)-based high-voltage direct current (HVDC) systems has garnered significant attention. But little attention has been paid to the impact of extreme temperature weather, although it may change the parameters of outdoor devices and lead to oscillations in a weak system. To explore the impact of environmental temperature on the stability of the MMC-HVDC system, this paper firstly establishes a comprehensive harmonic state space (HSS) model, incorporating the effect of temperature on the impedance of AC and DC transmission lines based on the thermal balance equation. By comparing the theoretical and simulation results, the accuracy of the model is validated. Subsequently, the mechanisms through which extreme temperature conditions affect system stability were analyzed. The results indicate that under extreme high-temperature conditions, the impedance of the MMC is significantly affected, weakening system stability and potentially causing small-signal instability. In contrast, extreme low-temperature conditions show no noticeable impact on system stability.

The leakage inductance of the transformer in a dual active bridge (DAB) dc–dc converter directly impacts the ac current waveforms and the power factor; thus, it can be considered a design requirement for the transformer. In the existing literature, a choice is made to either ensure soft switching in nominal power or to minimize the RMS current of the transformer. The inductance is typically obtained using optimization procedures. Implementing these optimizations is time-consuming, which can be avoided if a closed-form equation is derived for the optimum leakage inductance. In this paper, analytical formulas are derived to estimate the desired leakage inductance such that the highest RMS value of the current in the operation region of a DAB is kept to its minimum value. The accuracy and sensitivity of the analytical solutions are evaluated. It is shown that in a large design domain, the solution for the YY-connected MFT has a less than 3% error compared to the results obtained from an optimization engine. As an example of the importance of selecting the leakage inductance correctly, it is shown that for 11% deviations in the dc link voltages, a 10% deviation from the desired leakage inductance value can cause 2% higher RMS currents in the converter.

In this article, a three-phase modular multilevel converter (MMC) with three-level neutral point clamped converter (NPC) sub-modules (SMs) along with the placement of transformers in place of arm inductors is proposed for PV grid integration. Compared to the traditional MMCs, the proposed configuration reduces the voltage and power rating for the switches and the requirement of a high capacitor bank. In order to analyze the performance of the proposed converter arrangement, we have implemented four pulse width modulation schemes, such as Sine PWM with phase-level shifted carrier (SPWMLSC), Sine PWM with a phase-shifted carrier (SPWMPSC), Sine with the third harmonic injected level-shifted carrier (STHILSC), and Sine with the third harmonic injected phase-shifted carrier (STHIPSC). The proposed converter was simulated in the MATLAB/Simulink platform. Under normal and faulty operation, the results were presented with their performance indices of voltage and current harmonic distortion and sub-module capacitor voltage ripples at various modulation indices.

In this paper, the traveling wave protection issue of a hybrid high-voltage direct-current transmission line based on the line-commutated converter and modular multilevel converter is investigated. Generally, traveling wave protection based on voltage variation criterion, voltage variation rate criterion and current variation rate criterion is applied on hybrid high-voltage direct-current transmission lines as primary protection. There are two issues that should be addressed: (i) it has no fault direction identification capability which may cause wrong operation regarding external faults; and (ii) it does not consider the difference between line-commutated converter based rectifier station topology and modular multilevel converter based inverter station topology. Therefore, a novel traveling wave directional pilot protection principle for the hybrid high-voltage direct-current transmission line is proposed based on the S-transform. Firstly, the data processing capability of S-transform is described. Secondly, the typical traveling wave propagation process on a hybrid high-voltage direct-current transmission line is studied. Thirdly, a novel traveling wave fault direction identification principle is proposed. Eventually, based on PSCAD/EMTDC, a typical ±400 kV hybrid high-voltage direct-current transmission system is used for a case study to verify its robustness against fault location, fault resistance and fault type.

In MMC-HVDC (modular multilevel converter-based high-voltage direct current) applications, conventional control methods have defects such as complicated control and difficulty in controlling the internal energy of the converter. To ensure the safe and stable operation of an MMC-HVDC system, the problem of uneven internal energy distribution and increased fluctuations in the modular multilevel converter under asymmetrical network voltage conditions must be addressed. This paper has designed, a novel model predictive control (MPC) for MMC-HVDC applications. Through the proposed strategy, the switching states of all the MMC units can be optimized, which eliminates the circulating currents and achieves a voltage balance of the capacitor by redundant switching states. Moreover, an energy control circuit is established to adjust the DC bus power distribution in the MMC three-phase bridge arm. Thus, the symmetrical ac-side current can be realized, and the MMC internal energy imbalance caused by the transient process of the system can be avoided. Finally, the proposed novel predictive control strategy is tested via a case study. The obtained simulation and experimental results verify the effectiveness of the proposed control strategy.