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The modular multilevel matrix converter (M3C) is a potential frequency converter for low-frequency AC transmission. However, capacitor voltage control of high-voltage and large-capacity M3C is more difficult, especially for voltage balancing between branches. To solve this problem, this paper defines sequence circulating components and theoretically analyzes the influence mechanism of different sequence circulating components on branch capacitor voltage. A fully decoupled branch energy balancing control method based on four groups of sequence circulating components is proposed. This method can control capacitor voltages of nine branches in horizontal, vertical and diagonal directions. Considering influences of both circulating current and voltage, a cross decoupled control is designed to improve control precision. Simulation results are taken from a low-frequency transmission system based on PSCAD/EMTDC, and effectiveness and precision of the proposed branch energy balancing control method are verified in the case of nonuniform parameters and an unbalanced power system.

An Innovative Modular multilevel matrix converter (M3C) is proposed with reduced number of switching device owing to the improved efficiency, reduced cost and minimizes the size. Offshore Low-Frequency AC (LFAC) transmissions are economical with greater reliability for short and intermediate distance transmissions. Similar to HVDC, it increases the transmission capacity and also distance can be increased in LFAC.M3C is proposed as frequency converters for LFAC transmissions which link AC systems operating at 16.7 and 50 Hz. The double αβ0 transform control technique has been the most often used approach for decoupling control of input, output and circulating currents in such applications. The performances of this work's proposed modular multilevel matrix converters are analysed using simulation in MATLAB/SIMULINK software. [ABSTRACT FROM AUTHOR]

An Innovative Modular multilevel matrix converter (M3C) is proposed with reduced number of switching device owing to the improved efficiency, reduced cost and minimizes the size. Offshore Low-Frequency AC (LFAC) transmissions are economical with greater reliability for short and intermediate distance transmissions. Similar to HVDC, it increases the transmission capacity and also distance can be increased in LFAC.M3C is proposed as frequency converters for LFAC transmissions which link AC systems operating at 16.7 and 50 Hz. The double αβ0 transform control technique has been the most often used approach for decoupling control of input, output and circulating currents in such applications. The performances of this work’s proposed modular multilevel matrix converters are analysed using simulation in MATLAB/SIMULINK software.

Modular multilevel matrix converter (M3C) is a new topology of AC-AC power converter, which has very broad application in low frequency transmission system, high-power asynchronous motor speed regulation and low offshore wind power transmission. Due to the power coupling effect of the two frequencies, the capacitive voltage of M3C bridge arms is prone to instability when the grid voltage is asymmetric. Firstly, the arm power of M3C under unbalanced input conditions is calculated, and the power distribution law among the arms of two different power balance method is summarized. On this basis, the influence of low-frequency circulating currents on the arm power is analyzed. Under the premise of ensuring the balance of input and output power of the system, a capacitor voltage balance control strategy based on the construction of circulating currents is proposed, which avoids the introduction of negative sequence current on the grid side. Under unbalanced conditions, the capacitor voltage can be quickly balanced by the capacitor voltage closed-loop control and direct power compensation strategy. The constructed low-frequency circulating currents only flow inside the converter, and will not affect the decoupling operation of M3C input and output sides. The effectiveness of the proposed control strategy is verified by a 220 kV, 400 MW M3C system implemented in MATLAB.

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Modular multilevel matrix converters stand out for their performance in ac–ac high-power conversion. However, they require multiple control loops to govern the currents from both ac ports, the internal circulating currents, and the capacitor voltages. This article proposes a computationally efficient model predictive control (MPC) strategy based on a new converter modeling to exploit the phase-shifted pulsewidth modulation working principle fully, achieving four improvements: 1) control unification of both ac-ports currents, circulating currents, and capacitor voltages; 2) constrained optimization to safeguard the converter limits; 3) computational burden reduction and high scalability compared to standard MPC strategies; and 4) wide frequency operation with fast closed-loop transient responses, low harmonic distortion, and fixed switching frequency.

This article presents an analysis of selected characteristics of the modular multilevel matrix converter (M3C) operating a doubly fed induction generator (DFIG) in a flywheel energy storage system. A detailed analysis of the necessary electrical input and output quantities, as well as the identification of required internal currents and voltages, leads to a newly introduced iterative design process for the converter. This process ultimately provides information on the required number of submodules and demonstrates the overload capability of the converter in reactive power operation. Also, the short-circuit current contribution of the system for this unique configuration of M3C and DFIG is investigated for the first time. Taking into account the unique operating characteristics of the M3C, a loss analysis of the converter and the machine is performed, and a loss-optimized distribution of reactive power between the machine and the converter is proposed. The article is conducted analytically and is validated by experimental results on a small-scale demonstrator.

Due to the long delay of their control chains， voltage source converters （VSCs） in the high frequency band tend to show negative damping， which may give rise to high-frequency resonance when VSCs are connected to power systems. Flexible low-frequency alternating current transmission （LFAC） system adopts the modular multilevel matrix converter （M3C） as the core equipment and its high-frequency resonance risk needs to be evaluated. Based on practical engineering， this paper establishes the high-frequency impedance model of M3C. It evaluates the high-frequency resonance risk of the LFAC system under multiple operating modes through impedance analysis method. The operating conditions with high resonance risk are obtained， and the generation mechanism of the resonance is analyzed. Furthermore， a high-frequency resonance suppression strategy of installing an extra low-pass filter in the voltage feedforward control of M3C is proposed for practical engineering. Finally， the accuracy of the analysis results and the effectiveness of the proposed high-frequency resonance suppression strategy are verified by electromagnetic transient simulation.

Low-frequency transmission system (LFTS) based on modular multilevel matrix converter (M3C) has great application potential in offshore wind power transmission,urban power grid and long-distance power transmission. In order to study the startup process and the control strategies of LFTS and connect the converter station to the low-frequency transmission line without voltage or current impulse of M3C,a startup method of undisturbed grid connection is proposed. Firstly,the structure and operating principle of the LFTS are analyzed. Then,the selection principle of sub modules and the charging process of M3C are analyzed with two-terminal LFTS as the target. Taking the advantage that the blocked M3C does not exchange power with the low-frequency power grid when the capacitor is at rated voltage,the problems of over-voltage and over-current that tend to occur during LFTS startup can be solved by flexible control of M3C and reasonable design of startup strategy,without additional control devices and detecting the synchronization of the breaker. Finally,a real time digital simulation system (RTDS) is built to verify the proposed startup method. The simulation results show that converter stations connect to the low-frequency transmission line without voltage or current impulse. Also,the proposed strategy is correct and has a great prospect of the engineering application.

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage and high-power direct ac-to-ac power conversion applications. Fault tolerance ability is one of the advantages of the M3C. To further enhance the reliability of the M3C, this article proposes a novel branch current configuration method for branch fault conditions, which is available either one or two branches are failed. By deriving basic branch current configurations and analyzing branch dc power equations under branch fault conditions, feasible branch current configurations can be directly derived. In terms of minimizing the maximum peak branch current, the derived configuration is also the optimal one for the single branch fault condition. Compared with the existing method, the proposed method does not need to solve configuration coefficients of branch currents offline, which is automatically adaptive to different load conditions. An M3C prototype with three submodules each branch is built, and experimental results are presented to validate the proposed branch fault tolerance method. [ABSTRACT FROM AUTHOR]

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. Recent researches have proved its significant advantages for adjustable-speed motor drives compared with the back-to-back modular multilevel converter. However, the branch energy balancing in the M3C presents great challenge especially at critical-frequency points where the output frequency is close to zero or grid-side frequency. Generally, this balancing control depends on the appropriate injection of inner circulating currents and the common-mode voltage (CMV), whereas their values are hard to determine and optimize. In this paper, an optimization-based predictive control method is proposed to calculate the required circulating currents and the CMV. The proposed method features a broad frequency range balancing of capacitor voltages and no reactive power in the grid side. For operation at critical-frequency points, there is no increase on branch voltage stresses and limited increase on branch current stresses. A downscaled M3C system with 27 cells is designed and experiment results with the R–L load and induction motor load are presented to verify the proposed control method. [ABSTRACT FROM AUTHOR]

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. It features easy scalability, high-quality input and output waveforms, superior availability, etc. This letter aims to further increase the availability of the M3C when one or more of the nine branches are lost. A general nonlinear multivariable optimization model is set up to optimize the branch current configuration after the branch lost. The proposed method reduces system power capability loss after the branch failure. It achieves a smooth transition from a full M3C to a reduced topology without shutdown of the system. A M3C prototype with 27 cells is designed, and experiment results are presented to confirm the validity of this method. [ABSTRACT FROM AUTHOR]

The modular multilevel matrix converter (M3C) is a promising topology for medium-voltage, high-power applications. Due to the modular structure, it is scalable and capable to produce high quality output waveforms and can be fault tolerant. However, the M3C suffers from low frequency capacitor voltage fluctuation if the output frequency is close to the input voltage frequency, which limits its application in adjustable speed drive fields. This paper presents a theoretical analysis in the phasor domain to find the branch-energy equilibrium point of the M3C when operating with equal input and output frequency first. Then, a branch energy balancing control method based on branch current reallocation is proposed to equalize the energy stored in the nine converter branches. With the proposed method, the M3C can effectively suppress the capacitor voltage fluctuation without injecting common-mode voltage or applying reactive power to the input side. Experimental results are presented to validate the proposed method. [ABSTRACT FROM AUTHOR]