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Modular multilevel converters (MMCs) represent the forefront of power converter technology, with wide-ranging applications across diverse industries. Over recent decades, substantial research and development efforts have been dedicated to enhancing MMCs' performance. A significant challenge in power conversion processes is the presence of total harmonic distortion (THD) in output waveforms, which can have adverse effects on electrical equipment. In response, extensive studies have been conducted to address THD-related challenges by refining the control and operation of MMCs. This study investigates the effect of the amplitude modulation index (Ma) on the total harmonic distortion (THD) in nine-level MMC output waves. For this, a standard three-phase and nine-level MMC model was built and simulated in MATLAB/ Simulink environment, and the Ma value was shifted between 0.1 and 1.5. The output current and voltage waves were analyzed, and the optimal limits for the Ma values yielding the lowest THD values were determined. The simulation outcomes reveal a crucial Ma range between 0.6 and 1.2, where THD is significantly minimized. Ma values below 0.6 introduce significant harmonic distortion in the voltage waves, while values surpassing 1.2 lead to appreciable harmonic distortion in the current wave. This study contributes valuable insights for engineers and researchers and aids in the refinement of MMC control strategies and the mitigation of THD-related challenges in power systems. [ABSTRACT FROM AUTHOR]

Summary: The intrinsic characteristics such as modular structure, filterless operation, and absence of DC‐link capacitor have made the modular multilevel converter (MMC) superior to the conventional multilevel converters. However, the submodule (SM) capacitor voltage balancing is crucial. This paper presents a voltage balance control method for MMC. The proposed method generates 2N + 1 levels (where N is the number of SMs per arm) which halves the step size of output voltage and improves the harmonic performance as compared to N + 1 level generation. It requires only one carrier per phase and no voltage and current measurements are needed for the implementation. Therefore, the proposed method makes the system cost‐effective and reduces the implementation issues related with carrier‐based methods substantially. It also alleviates the non‐uniform charge distribution problem with the phase disposition pulse width modulation (PDPWM) technique. Further, a detailed mathematical analysis is presented which enhances the understanding of balancing algorithm. In addition to this, the circulating current with 2N + 1 modulation is analyzed and a control technique is presented. The proposed voltage balancing method is simulated comprehensively in MATLAB/SIMULINK software and validated by the experimental results obtained from a laboratory prototype. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become popular for medium-voltage applications. During MMC operation, unavoidable circulating currents flow through its arms and cause extra losses in the converter system. To generate a reference for circulating current, different current injection methods to control the circulating current have been reported. The conventional current injection techniques used so far relies on a mathematical model that excludes the effect of arm inductor voltages in its design. For high-voltage applications with many submodules (SMs), the above exclusion does not make much difference. But for medium-voltage applications with few SMs, the applied arm inductor voltages may be significant for a given dc-link voltage. In this article, inductor voltages are included in the proposed circulating current model, and a new reference for circulating current is obtained resulting in reduced circulating current through MMC arms. The proposed method is analyzed in detail on both seven-level and nine-level three-phase MMC, where advantages of the 2N + 1 level phase disposition pulsewidth modulation (PD-PWM) scheme are retained. The simulation study is carried out in PLECS blockset to show the reduction in the rms magnitude of circulating current. The validation of the proposed technique in a lab-scale hardware prototype shows the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The active circulating current suppression may cause high-frequency current ripple in the dc link of modular multilevel converters (MMCs). In this article, a phase-shifted double-group multicarrier-based phase-disposition pulsewidth modulation strategy is proposed to eliminate the dc-link high-frequency current ripple of MMCs, where the fundamental frequency component and the second-order component of the arm reference are modulated, respectively, by two groups of carriers. By shifting the phase angles of double-group multicarrier in each carrier period, the high-frequency current ripple injected into the dc link of MMCs can be eliminated. The proposed strategy not only eliminates the dc-link current ripple, but also extends the active and reactive power region of MMCs in comparison with the existing method. Simulation and experimental studies are conducted, and their results verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

In this article, a new, simple, and generalized discontinuous pulse width modulation (PWM)-based submodule capacitor voltage balancing approach is proposed and generated within the carrier-based PWM for modular multilevel converters (MMCs). In this scheme, different variants of zero-sequence voltage (ZSV) component is injected into the pulse width modulator so that each arm of the MMC is virtually clamped to positive or negative dc bus during a specific interval to achieve discontinuous modulation. A simple approach based on the location of the voltage vector mapping to a two-level space-vector diagram is presented to determine various ZSV components. The effectiveness of the proposed scheme with a voltage balancing algorithm is demonstrated through PLECS simulations and experimentally verified using a hardware-in-the-loop-based system. The performance of MMC with different ZSV injections is holistically evaluated to determine the best possible sequence over a wide range of modulation index. [ABSTRACT FROM AUTHOR]

This article presents a simple pulse width modulator to achieve both $N$ + 1 and 2 $N$ + 1 modulation in a modular multilevel converter. The proposed implementation needs a single triangular waveform to control a modular multilevel converter with a large number of submodules. Hence, the proposed approach significantly reduces the required triangular carrier count and solves the practical implementation issues associated with the carrier-based pulse width modulation schemes in the digital control platforms. The performance of the proposed approach is validated with detailed experimental studies. The results show that the proposed strategy effectively generates a multilevel voltage waveform at the load terminals. Furthermore, the performance of the proposed approach is compared with the traditional phase-shifted carrier pulse width modulation schemes. In addition, it can be easily applied to the $n$ -level modular multilevel converter with any type of submodule configuration. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of its high modularity, availability, and high power quality. In this article, the dc-link current ripple of the three-phase MMC derived from the phase-disposition pulsewidth modulation and circulating current control is analyzed. A variable carrier phase-shifted angle control strategy under the two-group carriers-based modulation is proposed to suppress the dc-link current ripple of the three-phase MMC. Through the regulation of the carrier phase-shifted angles under the two-group carriers-based modulation in each carrier period, the high-frequency current injected into the dc link from the three phases of the MMC can be counteracted, which suppresses the dc-link current ripple of the MMC with simple algorithm, and therefore improves the dc-link power quality. Simulations and experimental studies of the MMC are conducted, and the results confirm the effectiveness of the proposed control strategy for the MMC. [ABSTRACT FROM AUTHOR]

This paper develops a modulation based fault-tolerant (FT) strategy for restoring the operation of three-phase modular multilevel converters (MMCs) with faulty switches. This FT strategy is based on a proposed modified space vector modulation (SVM) technique that generates balanced line-to-line (line) voltages even in the case of a fault occurrence. In the postfault operation, the proposed strategy is able to restore the fundamental amplitude of the line to the neutral (load) voltages to that of the normal operation condition with a slightly increased voltage stress over the switches in the faulty phase. In this paper, first a brief background about MMCs and SVM technique is provided. Then, the proposed FT strategy and the modified SVM technique are presented. Finally, several simulation and experimental results are provided to validate operation of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents two capacitor voltage balancing (CVB) strategies for modular multilevel converter (MMC) applications. Both balancing schemes are based on model predictive control and are designed to efficiently solve a constrained optimal control problem, where the predicted capacitor voltage errors are included in the cost function with the demanded output voltage of a cluster being forced through an equality constraint. The first method proposed in this paper computes specific modulation indexes for each module using the explicit solution of a relaxed version of the original optimization problem. The second approach proposed in this paper reduces the complexity of the original problem by linearizing the objective function and using an optimal sorting network based on a greedy algorithm to solve this approximation. Considering the structures of both solution approaches, they are integrated into modulation schemes based on phase-shifted and level-shifted pulsewidth modulation algorithms, respectively. Experimental results obtained from a nine-cell single-phase MMC prototype demonstrate the good performance achieved with the proposed methodologies, as well as the implementation simplicity offered by the proposed CVB algorithms. [ABSTRACT FROM AUTHOR]

Standard pulsewidth modulation (PWM) schemes for modular multilevel converters (MMC) have neglected deviations of the module voltages from the mean module voltage of an MMC arm as well as the dynamic change in the module voltages since their last measurement and during the switching period. This induces an output voltage error. Large output voltage errors disturb the higher level closed-loop control of the MMC, such that control targets cannot be met anymore. The output voltage error can be reduced using advanced PWM methods that predict and include the dynamic change of the individual module voltages during a switching period in the calculations for the duty cycle. This paper shows measurement results obtained from an experimental medium-voltage prototype, proving that advanced methods lead to much lower output voltage errors than the standard methods, but cannot fulfill the expectations raised by the simulations documented in the literature. In order to overcome the limitations of the old method, a new method that includes the semiconductor voltage drop and parasitic resistances is introduced and evaluated with measurements in this paper. Furthermore, simulation results showing the influence of the considered modulation methods on the closed-loop current control performance are presented. [ABSTRACT FROM AUTHOR]

During the design phase of a modular multilevel converter (MMC), an accurate loss evaluation of the submodule (SM) plays an important role. In this paper, a method based on the analytical description of the MMC key waveforms that allows to directly obtain the average semiconductor and capacitor losses that each SM will experience is introduced, under different operating conditions or control schemes. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in steady state. The proposed method provides a great flexibility and a significant reduction of the simulation/computational time otherwise needed to evaluate SM losses under various operating conditions. [ABSTRACT FROM PUBLISHER]

Modular multilevel converter (MMC) is now the state-of-the-art converter topology for high-voltage dc transmission (HVDC) systems. Another potential application for MMC is medium-voltage (MV) high-power industrial ac drives. In high-power applications, thermal constraints of power semiconductor devices limit device switching frequency to a few hundred hertz. However, there exists a tradeoff between device switching frequency and harmonic distortion of converter output currents. Synchronous optimal pulsewidth (SOP) modulation is an emerging low-device switching-frequency-modulation technique for high-power converters, which maintains the quality of converter output currents. SOP technique has been successfully demonstrated for classical multilevel topologies and it has been proved that maximum-device switching frequency can be limited to rated fundamental frequency for seven or higher level inverters without compromising on the quality of output currents. However, implementation of SOP technique for MMC topology is still pending. One of the main challenges for control of MMC is to maintain floating-capacitor voltages around their nominal value. The goal of our study is to propose, analyze, and implement enhanced SOP technique for MMCs to achieve low-device switching-frequency operation, better quality of converter output currents, and maintain capacitor voltages around their nominal value. The proposed technique has been validated using low-power prototype of five-level MMC feeding an 1.5-kW induction motor. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is emerging to be a promising approach for medium- and high-voltage applications. This paper proposes a modified nearest level modulation (NLM) method for the MMC, with which the level number of ac output voltage increases to 2N + 1 (where N is the submodule numbers per arm) as great as that of the carrier-phase-shifted PWM and the improved submodule unified PWM methods. The level number is almost doubled and the height of the step in the step wave is halved. Therefore, the ac output voltage waveform quality using the proposed method is better than that using the conventional NLM method. Besides, the proposed method keeps the conventional NLM method's characteristics such as low switching frequency and simple implementation. The performance of the proposed method is verified by simulations and experiments using a laboratory-scale prototype. [ABSTRACT FROM AUTHOR]