Modular multilevel resonant dc-dc converters (MMRDCs) are becoming promising candidates in medium-voltage dc (MVDC) power distribution systems, owing to their outstanding features, including high scalability and control flexibility. The fundamental-frequency quasi-square-wave (QSW) modulation is a widely used technique for MMRDC. However, in practical drive implementation, the dead time in submodule (SM) devices could introduce a delay in the arm output voltage relative to the ideal modulation signal, leading to a mismatch between the upper arm and lower arm output voltages. This mismatch can cause surge voltages and current distortion in the modular multilevel converter (MMC) arms, jeopardizing the safe and stable operation of the converter. This paper presents a detailed analysis of the influence of dead time on modulation under various operating conditions and proposes a dead-time compensation method to achieve ideal QSW arm voltages. The proposed modulation reduces undesired voltage and current stress and mitigates disturbances caused by high dv/dt. Finally, validation is provided through both simulation and experimental results from a 10 kV MMRDC prototype.

Modular multilevel converters (MMCs) have become an attractive solution for large-scale renewable energy integration. However, the complex internal dynamics of MMCs introduce challenges to system stability. To address the internal-dynamics driven (IDD) instability and enhance system stability, this article develops an internal-dynamics-guided (IDG) stabilization framework. Firstly, the origin of IDD instability is identified through internal-dynamics impedance characterization of MMCs, revealing that the capacitive behavior of internal dynamics may interact with inductive systems, leading to system instability. Building upon this mechanistic insight, a stabilization-design principle is established that compensates the destabilizing components of internal dynamics through targeted reshaping of the internal-dynamics impedance, thereby addressing the instability at its origin. Based on this principle, a systematic stabilization controller design methodology is developed, in which the control architecture and parameters are analytically derived from converter internal dynamics rather than relying on system impedance identification. As a result, the proposed IDG stabilization framework inherently avoids reliance on exact grid impedance and enhances stabilization robustness against system condition variations. Experimental results obtained under multiple operating conditions validate the effectiveness of the IDG stabilization framework in enhancing the stability margin and extending the stable operating region of MMC-based systems.

PPositively coupled inductors (PCIs) can be used to improve the filtering performance and power density of parallel inverters, but their saturation at high power may increase the harmonics and reduce the system stability. In this article, a design method for achieving the optimal coupling coefficient leveraging partial saturation is proposed, to maximize the filtering effect of PCIs while preserving system stability. Firstly, through analysis, inductor saturation means that the inductance decreases with higher current, which induces the incompatibility between control bandwidth and stability for different power. Secondly, the saturation characteristics of PCIs and uncoupled inductors (UCIs) are derived and compared, which reveals that PCIs are more prone to saturation than UCIs. In addition, once exceeding a threshold current, PCIs have even worse filtering effect than UCIs. The quantitative relationship between the threshold current and the coupling coefficient is then established. Based on the relationship, a design method of the coupling coefficient for PCIs to work in optimized partial saturation at high power is proposed to balance the system stability and filtering effect. Finally, simulations and experiments validate that the theoretical analysis and presented method achieve the optimal coupling coefficient giving consideration to both filtering performance and system stability.

With the rapid expansion of renewable energy systems in remote areas, high-voltage direct current (HVdc) systems are playing an increasingly vital role in integrating and transmitting renewable power over long distances. High-voltage, high-power dc/dc converters are essential for enabling interconnections within HVdc grids and facilitating efficient power transfer across multiple voltage levels. The T-type modular multilevel dc/dc converter (T-MMDC), which employs three arms of series-connected submodules (SMs), has emerged as a promising transformer-less solution for HVdc applications, enabling direct power conversion through SMs. This paper proposes a tri-arm coordinated quasi-square-wave (TA-QSW) modulation scheme for the T-MMDC. Building on the advantages of conventional QSW modulation in reducing power device count and improving power transfer capability, the proposed TA-QSW scheme actively coordinates the currents in all three arms during commutation. This eliminates the need for extra SMs in any single arm under conventional QSW when applied to T-MMDCs, and thereby resolves the coupled design trade-off among SM count, arm inductance, and current slew rate, ultimately enhancing the achievable power transfer capability under high-slew-rate conditions. The topology configuration, operating principles, parameter design, and control strategies of the TA-QSW modulation are comprehensively described. The feasibility and effectiveness of the proposed scheme are verified through detailed full-scale simulations and down-scaled experimental tests, confirming the validity of the theoretical analysis.

The rapid evolution of power electronics toward high-voltage and high-power multilevel converters has created a significant gap between academic laboratory education and industrial practice. This paper presents a reconfigurable multi-topology inverter platform for power electronics education to bridge this gap. The proposed system features a topology-reuse hardware architecture that enables seamless switching among two-level (2L), neutral-point-clamped (NPC), and active neutral-point-clamped (ANPC) topologies. A per-unit normalization framework is introduced to establish a direct mapping between a 360 W laboratory platform and megawatt-scale engineering systems. Experimental results, including harmonic spectrum analysis under different modulation strategies and closed-loop dynamic response under grid disturbances, demonstrate strong agreement with theoretical predictions and large-scale simulations. An integrated theory– simulation–experiment–engineering closed-loop teaching paradigm is developed, significantly enhancing students’ system-level understanding and engineering intuition. The proposed platform provides an effective and scalable solution for modern power electronics education aligned with industrial requirements.

The modular multilevel resonant DC-DC converter (MMRDC) has become a promising solution for modular multilevel DC-DC converter (MMDC), with the advantage of reducing switching losses and improving conversion efficiency. In this paper, an arm voltage balancing control method for MMRDC is studied. First, the mechanism of arm voltage imbalance in MMRDC is investigated, focusing on the impact of asymmetrical arm impedance parameters. Then a novel arm voltage balancing control method is proposed, which actively adjusts the equivalent duty cycles of the upper and lower arm modulations based on the average capacitor voltages of the submodules. The effectiveness of the proposed method is validated through simulations on the Plexim/PLECS platform and experiments on an MMRDC prototype. Results demonstrate that the proposed control method effectively eliminates arm voltage imbalance, ensuring stable operation and preventing issues such as submodule overvoltage or undervoltage and resonant capacitor overvoltage, which can lead to component damage.

The modular multilevel converter (MMC) has emerged as a promising solution for large-scale renewable energy integration. However, its inherent internal dynamics pose challenges in ensuring and assessing system stability. This article focuses on addressing the internal-dynamics-driven (IDD) instability in MMC systems. Unlike existing studies that rely on the output impedance compressing the entire dynamics into an all-in-one transfer function with internal dynamics hidden, this article visualizes the MMC internal dynamics as a targeted equivalent impedance, which directly links internal control parameters to system stability and thus facilitates targeted control design. The analysis reveals that the capacitive characteristics shaped by internal dynamics cause instability when coupled with inductive grids. Moreover, circulating current control can broaden this capacitive frequency band and exacerbate the instability risk without targeted design. Based on this insight, an internal-dynamics-originated (IDO) stability region is derived as practical control design guidance. A targeted stabilization control design framework is then established, enabling system stability without precise prior knowledge of the system impedance. Finally, multicondition experiments validate the effectiveness of the framework, providing a generalizable guideline for stability enhancement in MMC-based renewable energy systems and other MMC-interfaced applications.

To address circulating current and output current harmonic challenges in parallel three-level T-type inverters, this study proposes a cost-effective solution utilizing positively coupled inductors (PCIs) with synchronized carriers. The common-mode equivalent inductor of PCIs enhances filtering of the output current harmonics with smaller inductor sizes, reducing cost compared to uncoupled inductors. The differential-mode equivalent inductor, in conjunction with synchronized carriers, effectively suppresses the high-frequency circulating current (HFCC) compared to interleaved carriers with negatively coupled inductors. The control strategies for output current regulation, low-frequency circulating current suppression, and neutral point voltage balancing are designed based on the decoupled mathematical models. Besides, the parameter design processes of PCIs and controllers are provided, and the impact of parameter inconsistency on HFCC and system stability is analyzed. Simulations and experiments validate that the proposed solution achieves superior performance in terms of circulating current suppression, low harmonic distortion, neutral point voltage balancing, and dynamic response.

The rising penetration of renewable energy sources promotes the development of the high-voltage direct current (HVdc) systems for large-scale power integration and transmission. To meet the future demands of meshed HVdc grids, high-voltage high-power dc-dc converters are required to serve as essential interfaces to bridge the HVdc links with different voltage levels. This article proposes a T-type modular multilevel converter (T-MMDC) composed of two energy buffering arms to transfer partial power and an active filtering arm for removing bulky passive filters. By utilizing the coordination of arms, the proposed T-MMDC not only enables flexible power regulation and bidirectional fault ride-through, but also has the advantages of cost-efficiency, small footprint, and high efficiency. The topology configuration, quantitative characterization of operating principle, analytical parameter design, and fault ride-through control scheme are presented sequentially, providing a straightforward and detailed guideline for engineers. Full-scale simulations from a 320 kV@0.5 GWcase and down-scale experiments from a 750 V@5 kW prototype confirm the effectiveness of the proposed operating principle and fault ride-through capability of T-MMDC and its good potential for HVdc grids.

Hybrid modular multilevel converters (HMMCs) have emerged as a promising solution for high- and medium-voltage applications. However, during AC-side startup, HMMCs face challenges of the inrush current and the capacitor voltage imbalance between full-bridge submodules (FBSMs) and half-bridge submodules due to their structural differences. This article proposes a closed-loop black start strategy to address these issues. First, the analysis of the internal control loop dynamics reveals that, when submodule capacitors are at low voltage levels, the inrush current and the charging control failure may occur due to limited power regulation capability. An enhanced three-stage startup method is proposed to fully control the charging current with utilizing the negative voltage of FBSMs. With the proposed strategy, the inrush current during the precharging period is suppressed. The strategy is built upon a conventional controller framework with minimal additional modifications, significantly simplifying the overall design and implementation. Furthermore, supported by periodic approximation and piecewise calculation, it is demonstrated that the capacitor voltage imbalance can be inherently eliminated during the whole startup process. Finally, simulation and experimental results validate the theoretical analysis and the effectiveness of the proposed strategy.

The modular multilevel DC-DC converter (MMDC) has seen increasing interest due to its potential in supporting renewable energy integration. Among the various configurations, the T-type transformerless MMDC offers advantages such as enhanced flexibility in power regulation and effective fault-blocking capabilities, making it a suitable solution for connecting DC systems with different voltage levels. In this work, an innovative modulation technique, namely the asymmetric quasi-square wave (AQSW) method, is introduced for the T-type transformerless MMDC. This approach reduces current stress compared to traditional modulation techniques, thus improving the overall system efficiency. The operating principles of the T-type transformerless MMDC are explained in detail, and the design parameters for the AQSW modulation method are outlined. Additionally, the current stress reduction achieved with the AQSW scheme is analyzed and compared with existing modulation techniques, demonstrating a notable improvement. The theoretical predictions are validated through 320kV/100kV@0.5GW simulations.

Modular multilevel-based DC-DC converters (MMDC) have gained increasing research interest for their advantages in renewable energy applications. Among the various configurations, the T-type MMDC offers advantages such as enhanced flexibility in power regulation and effective fault-blocking capabilities. Nonetheless, differing from conventional modular multilevel converters utilized in highvoltage direct current systems, due to the distinctive operating principles and modulation methods, the voltage and current stress distribution of MMDC is not clarified yet, potentially compromising the overall reliability and operational lifespan. Thus, it becomes crucial to deeply investigate the power losses distribution of semiconductor devices in MMDC to provide guidance on thermal design and active thermal control. In this paper, a comprehensive model is developed to accurately estimate the conduction and switching losses of submodules under quasi-square wave modulation. All theoretical analyses are validated through simulations.

Modular multilevel converter (MMC) is one of the most promising multilevel topologies in the high-voltage application, such as HVDC transmission system. However, the stability issues of MMC under grid-connected conditions have become increasingly prominent. A precise and concise equivalent impedance is crucial for stability analysis. In this paper, influence of internal dynamics on the external characteristics is analyzed and a novel control model considering internal dynamics and current control is proposed. Finally, the accuracy of the proposed control model is demonstrated.

This article proposes a magnetic-pole partitioned design concept for an axial flux permanent magnet (AFPM) motor. This concept newly serves as an effective bridge between the two key motor topology design elements of flux focusing and flux regulation. The partitioned magnetic poles play a couple of roles as flux producers and flux regulators, not only realizing high torque but also obtaining a widened speed regulation range with high efficiency. Then, a magnetic-pole partitioned dual stator AFPM (MPDS-AFPM) motor is presented. By considering the multioperating modes, the machine structure and flux regulation principle of the motor are discussed, where the motor can be efficiently designed. In addition, the sensitivity analysis and response surface (RS) evaluation are adopted to pick out the highly sensitive design parameters. Meanwhile, the multioperating mode optimization method with layered multimode weight (LMW) is proposed and conducted. Afterward, the performances of the motor before and after optimization are compared. Finally, a prototype is constructed and tested. Both the theoretical analysis and experimental results verify the effectiveness and reasonability of the proposed design method and the MPDS-AFPM motor.

The modular multilevel converter (MMC) has become a pivotal solution for renewable energy integration and HVdc transmission systems credited to its modular scalability, high efficiency, and low distortion. Nevertheless, the intricate internal dynamics, including circulating current and submodule capacitor voltage dynamics, have a significant impact on its control performance. To address this issue, this article proposes a novel equivalent impedance circuit model that incorporates coupled circulating current and capacitor dynamics to optimize ac voltage control for the MMC. Compared with the conventional models, this article: 1) considers the inherent circulating currents and capacitor voltage dynamics, as well as the close-loop voltage control and circulating current control; 2) derivation of a concise two-level converter circuit with series equivalent internal impedance to physically quantify the impact of internal dynamics on output behavior; 3) development of a model-based parameter optimization strategy for ac voltage control that incorporates internal dynamic interference. The proposed model enhances the accuracy of voltage control performance prediction compared to conventional models, enabling targeted controller tuning that improves voltage dynamic response and stability performance. Finally, detailed simulation and experiment results under various conditions verify the accuracy of the proposed model and the effectiveness of the control optimization strategy.

Modular multilevel resonant dc-dc converter (MMRDCs) have garnered substantial research interest within the domain of medium-voltage dc to low-voltage dc applications. Nevertheless, compared with the traditional modular multilevel converter employed in high voltage dc transmission, due to the different operating principles and modulation techniques, the electrical and thermal stress distribution in MMRDC remains yet to be fully elucidated. In this article, the power losses of submodule (SM) devices in MMRDC are investigated comprehensively. A precise calculation model of the SM conduction and switching losses is established under different operation conditions. The calculations reveal the serious thermal imbalance features among the devices inside SM, which may curtail the device lifetime and thus threaten the converter reliability. Then, to alleviate the thermal imbalance, a modulation scheme by reassigning the current path inside the SM is proposed. Without extra hardware cost and influence on the output performance, the proposed modulation can mitigate the loss imbalance between the upper switch and lower switch significantly and reduce power losses of the most highly stressed device. Finally, the accuracy of the theoretical model and the efficacy of the thermal imbalance suppression modulation are corroborated by full-scaled simulation and an MV MMRDC laboratory prototype with 7-14 kV input and 300 V output.

This paper proposes a systematic design methodology for three-level neutral-point-clamped converters that emphasizes harmonic minimization and efficiency enhancement. The approach integrates hybrid topology optimization with advanced modulation strategies, enabling a balanced trade-off between waveform quality and switching losses. A comparative analysis of Sinusoidal Pulse Width Modulation and Space Vector Pulse Width Modulation demonstrates that Space Vector Pulse Width Modulation achieves superior harmonic suppression and higher DC-link voltage utilization. Furthermore, a scalable teaching and research platform is constructed using per-unit parameter normalization, which bridges laboratory small-scale laboratory experiments with industrial high-power benchmarks. This work addresses key limitations of existing power electronics training tools, offering a reliable and scalable solution suitable for both educational and industrial applications.

Modular multilevel DC-DC converter (MMDC) is the key energy conversion interface for the high-voltage dc grids. Among various configurations, the T-type MMDC is considered as a promising choice due to its compact structure and high transmission efficiency. The topology is based on modular multilevel architecture where the capacitor voltage balancing (CVB) strategy plays a vital role. This paper proposes a modified sorting-driving CVB strategy that uses the capacitor voltage unbalance as the control metric for T-type MMDC. The strategy effectively reduces the overall switching frequency of submodule arms while balancing switching activity among devices in the full-bridge arms. The topology and operating principle of the T-type MMDC are introduced, followed by the design of the proposed CVB strategy. The effectiveness and superiority of the proposed strategy in achieving low-frequency and well-balanced switching activity are validate through a high-power simulation model rated at 320 kV / 0.5 GW.

This paper presents a modular multilevel DC/DC converter which interconnects different DC voltage levels and enables the bidirectional flow of power. The DC/DC converter contains three staggered parallel subsystems, each of which consists of three arms and constitutes a T-type circuit, increasing the transmitted power by synthesizing the three-string AC component to make the external characteristic a direct flow. Based on injecting a common mode voltage across the arm, the required arm inductance is used to generate the basis for energy exchange. Meanwhile, its modular structure is ideally suited for medium and high voltage DC applications and allows for easy submodule(SM) replacement. Its central feature includes: 1) bidirectional flow of power and 2) internal energy complementation. A new degree of freedom is found to control the transmission of power. The operation of the MMDC is analyzed in the ideal case which means no internal energy loss is considered. Simulations performed in MATLAB/SIMULINK verified the validity of the MMDC operating principle and energy flow.

The modular multilevel resonant DC-DC converters (MMRDC) are becoming a promising solution for medium voltage DC-DC conversion, owing to high reliability, flexible adjustment and high conversion efficiency. In this paper, a flexible power control method of bidirectional modular multilevel resonant DC-DC converter (BMMRDC) is proposed for wide voltage range applications. First, the gain characteristics of the BMMRDC topology are analyzed, and the limitations of the switching frequency regulation control method in achieving a wide voltage range are studied. Then, a dual-loop power control method is proposed, which combines the regulation of the continuously inserted submodule number and the switching frequency. Switching frequency regulation enables continuously bidirectional power regulation within a small voltage range, while regulation of the continuously inserted submodule number allows operation within a wide voltage range. Furthermore, a typical design of the dual-loop control method is provided. The effectiveness of the proposed method is validated through simulations on the Plexim/PLECS platform and experiments on a BMMRDC prototype. The results demonstrate that the proposed control method achieves wide voltage range control on the basis of bidirectional power flow.