Search Results (316)

Modular multilevel converters (MMCs) are widely used in power-conversion applications, including distributed energy storage integration, because of their scalability, high efficiency, and reduced harmonic distortion. Integrating battery storage systems into MMC submodules using dual active bridge (DAB) converters provides electrical isolation and reduces voltage stress, harmonics, and common-mode issues. However, voltage fluctuations due to the battery state of charge can compromise the zero-voltage switching (ZVS) operation of a DAB and increase the reactive power circulation, leading to higher losses and reduced system performance. To address these challenges, this study investigated an active control strategy for submodule voltage regulation in an MMC with DAB-based battery integration. Assuming single-phase-shift modulation, two control strategies were evaluated. The first strategy regulated the DAB voltage on one side to match the battery voltage on the other, scaled by the high-frequency transformer turns ratio, which facilitated the ZVS operation and reduced the reactive power. The second strategy optimized this voltage to minimize the total power-conversion losses. The proposed control strategies improved the efficiency, particularly at low power levels, achieving several percentage points of improvement compared to maintaining a constant voltage. Full article

A quantitative analysis of the fault transient is critical for system resilience assessment and protection coordination. Focusing on hybrid modular multilevel converter (MMC)-based HVDC architecture with enhanced fault ride-through (FRT) capability, this study develops a mathematical calculation framework to quantify how controller configurations influence fault current profiles. Unlike conventional static topologies (e.g., RLC or fixed-voltage RL circuits), the proposed model integrates an RL network with a time-variant controlled voltage source, which can emulate closed-loop control response during the FRT transient. Then, the quantitative relationship is established to map the parameters of DC controllers to the fault current across diverse FRT strategies, including scenarios where control saturation dominates the transient response. Simulation studies conducted on a two-terminal MMC-HVDC architecture substantiate the efficacy and precision of the developed methodology. The proposed method enables the evaluation of DC fault behavior for hybrid MMCs, concurrently appraising FRT control strategies. Full article

Considering the lightweight requirement of modular multilevel converter (MMC), the implementation of arm multiplexing significantly improves submodule utilization and achieves remarkable lightweight performance. However, the challenges of overvoltage and energy imbalance during pole-to-ground fault still exist. To address these issues, this paper proposes a hybrid half-wave alternating MMC (HHA-MMC) and presents its fault ride-through strategy. First, a transient equivalent model based on topology and operation principles is established to analyze fault characteristics. Depending on the arm’s alternative multiplexing feature, the half-wave shift non-blocking fault ride-through strategy is proposed to eliminate system overvoltage and fault current. Furthermore, to eliminate energy imbalance caused by asymmetric operation during non-blocking transients, dual-modulation energy balancing control based on the third-harmonic current and the phase-shifted angle is introduced. This strategy ensures capacitor voltage balance while maintaining 50% rated power transmission during the fault period. Finally, simulations and experiments demonstrate that the lightweight HHA-MMC successfully accomplishes non-blocking pole-to-ground fault ride-through with balanced arm energy distribution, effectively enhancing power supply reliability. Full article

Circulating current control is a critical part of the Modular Multilevel Converter (MMC) control system. Existing control methods rely on arm current information obtained from complex current measurement devices. However, these devices are susceptible to failures, which can lead to distorted arm currents, increased peak arm current values, and higher losses. In extreme cases, this can result in system instability. This paper first analyzes four typical arm current measurement faults, i.e., constant gain faults, amplitude deviation faults, phase shift faults, and stuck faults. Then, a Kalman Filter (KF)-based fault detection method is proposed, which allows for the simultaneous monitoring status of all six arm current measurements. Moreover, to facilitate fault detection, the Moving Root Mean Square (MRMS) value of the observation residual is defined, which effectively detects faults while suppressing noise. The entire fault detection process takes less than 20 ms. Finally, the feasibility and effectiveness of the proposed method are validated through MATLAB/Simulink simulations and experimental results. Full article

To deal with the interphase circulating-current problem of modular multilevel converters (MMCs) in multiphase wind power systems, a cooperative circulating-current suppression strategy based on a second-order generalized integrator (SOGI) and passivity-based control–integral sliding mode control (PBC-ISMC) is proposed in this paper. Firstly, a multiphase permanent magnet direct-drive wind power system topology without a step-up transformer is established. On this basis, SOGI is utilized to construct a circulating current extractor, which is utilized to accurately extract the double-frequency component in the circulating current, and, at the same time, effectively filter out the DC components and high-frequency noise. Secondly, passivity-based control (PBC), with its fast energy dissipation, and integral sliding mode control (ISMC), with its strong robustness, are combined to construct the PBC-ISMC circulating-current suppressor, which realizes the nonlinear decoupling and dynamic immunity of the circulating-current model. Finally, simulation results demonstrate that the proposed strategy significantly reduces the harmonic content of the circulating current, optimizes both the bridge-arm current and output current, and achieves superior suppression performance and dynamic response compared to traditional methods, thereby effectively enhancing system power quality and operational reliability. Full article

Power quality is a key issue in cold ironing (CI) systems, where a stable, clean power supply is essential to meet the needs of moored vessels. According to IEC/ISO/IEEE 80005-1, these systems must deliver high power at standardized voltages (6.6 kV or 11 kV) with minimal harmonic distortion in the presence of vessel load variability. This study proposes a model-free control strategy based on an intelligent proportional–integral (iPI) corrector with adaptive gain, applied to a three-phase modular multilevel converter (MMC) equipped with an LC filter. This architecture, adapted to distributed infrastructures, reduces the number of transformers required while guaranteeing high output voltages. The iPI strategy improves system robustness, dynamically compensates for disturbances, and ensures better power quality. A comparative analysis of three control strategies, proportional–integral (PI), intelligent proportional–integral (iPI), and intelligent proportional–integral adaptive (iPIa), performed in MATLAB/Simulink and complemented by experimental tests on the OPAL-RT platform, revealed a significant THD reduction of 1.18%, in accordance with the IEC/ISO/IEEE 80005-1 standard. These results confirm the effectiveness of the proposed method in meeting the requirements of CI systems. Full article

This paper proposes a multi-channel robust damping controller based on the static H∞ loop shaping method, specifically tailored for modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems with embedded energy storage. The controller is designed to suppress sub-synchronous oscillations, a critical issue in power systems. To optimize the controller’s performance, a genetic algorithm is employed to tune the weighting functions for robust control. Additionally, the TLS-ESPRIT (Total Least Squares–Estimation of Signal Parameters via Rotational Invariance Techniques) identification algorithm is utilized to clarify the system oscillation characteristics, thereby enhancing the controller’s effectiveness. Simulation results demonstrate that the sub-synchronous oscillation controller, designed based on the proposed robust control algorithm, achieves satisfactory oscillation suppression effects under various disturbances, underscoring its robustness. This study highlights the potential of MMC-HVDC systems with embedded energy storage in mitigating power grid oscillations, contributing to the advancement of power system stability and reliability. Full article

This paper addresses the challenge of poor dynamic performance in Modular Multilevel Converter-based High-Voltage Direct Current (MMC-HVDC) systems within weak power grids when conventional control strategies are applied. To enhance system performance, a novel grid-connected power control method integrating Virtual Synchronous Generators (VSGs) and Passivity-Based Control (PBC) is proposed. The passivity characteristics of the MMC and the roles of virtual inertia and damping in VSG control are thoroughly examined. Based on the passivity property of the MMC, PBC is implemented in the current inner loop, while VSG control, leveraging its unique working characteristics, is incorporated into the power outer loop. To further optimize performance, adaptive virtual inertia and damping compensation mechanisms, utilizing sigmoid functions, are introduced within the VSG framework. The synergistic operation of PBC and adaptive VSGs significantly improves the dynamic response and robustness of the MMC-HVDC system. The effectiveness and feasibility of the proposed method are validated through simulation experiments in MATLAB/Simulink, conducted under power variations, grid voltage variations, and load changes. Full article

Fault diagnosis is one of the most important issues for a modular multilevel converter (MMC). However, conventional solutions are deficient in two aspects. Firstly, they lack the necessary feature information. Secondly, they are incapable of performing open-circuit fault diagnosis of the modular multilevel converter with the requisite degree of accuracy. To solve this problem, an intelligent diagnosis method is proposed to integrate the modal time–frequency diagram and FFT-CNN-BiGRU-Attention. By selecting the phase current and bridge arm voltage as the core fault parameters, the particle swarm algorithm is used to optimize the Variational Modal Decomposition parameters, and the fault signal is decomposed and reconstructed into sensitive feature components. The reconstructed signals are further transformed into modal time–frequency diagrams via continuous wavelet transform to fully retain the time–frequency domain features. In the model construction stage, the frequency–domain features are first extracted using the fast Fourier transform (FFT), and the local patterns are captured through a combination with a convolutional neural network; subsequently, the timing correlations are analyzed using bidirectional gated loop cells, and the Attention Mechanism is introduced to strengthen the key features. Simulations show that the proposed method achieves 98.63% accuracy in locating faulty insulated gate bipolar transistors (IGBTs) in the sub-module, with second-level real-time response capability. Compared with the recently published scheme, it maintains stable performance under complex working conditions such as noise interference and data imbalances, showing stronger robustness and practical value. This study provides a new idea for the intelligent operation and maintenance of power electronic devices, which can be extended to the fault diagnosis of other power equipment in the future. Full article

This paper presents a comprehensive review of High-Voltage Direct-Current (HVDC) systems, focusing on their technological evolution, fault characteristics, and advanced signal processing techniques for fault detection. The paper traces the development of HVDC links globally, highlighting the transition from mercury-arc valves to Insulated Gate Bipolar Transistor (IGBT)-based converters and showcasing operational projects in technologically advanced countries. A detailed comparison of converter technologies including line-commutated converters (LCCs), Voltage-Source Converters (VSCs), and Modular Multilevel Converters (MMCs) and pole configurations (monopolar, bipolar, homopolar, and MMC) is provided. The paper categorizes HVDC faults into AC, converter, and DC types, focusing on their primary locations and fault characteristics. Signal processing methods, including time-domain, frequency-domain, and time–frequency-domain approaches, are systematically compared, supported by relevant case studies. The review identifies critical research gaps in enhancing the reliability of fault detection, classification, and protection under diverse fault conditions, offering insights into future advancements in HVDC system resilience. Full article

Modular multilevel converter (MMC)-based power electronic transformers (PETs) present a promising solution for connecting AC/DC microgrids to facilitate renewable energy access. However, the capacitor ripple voltage in MMC-based PET submodules hinders volume optimization and power density enhancement, significantly limiting their application in distribution networks. To address this issue, this study introduces an optimized method for suppressing the submodule capacitor ripple voltage in MMC-based PET systems under normal and grid fault conditions. First, an MMC–PET topology featuring upper and lower arm coupling is proposed. Subsequently, a double-frequency circulating current injection strategy is incorporated on the MMC side to eliminate the double-frequency ripple voltage of the submodule capacitor. Furthermore, a phase-shifting control strategy is applied in the isolation stage of the dual-active bridge (DAB) to transfer the submodule capacitor selective ripple voltages to the isolation stage coupling link, effectively eliminating the fundamental frequency ripple voltage. The optimized approach successfully suppresses capacitor ripples without increasing current stress on the isolated-stage DAB switches, even under grid fault conditions, which are not addressed by existing ripple suppression methods, thereby reducing device size and cost while ensuring reliable operation. Specifically, the peak-to-peak submodule capacitor ripple voltage is reduced from 232 V to 10 V, and the peak current of the isolation-stage secondary-side switch is limited to ±90 A. The second harmonic ripple voltage on the LVDC bus can be decreased from ±5 V to ±1 V with the proposed method under the asymmetric grid voltage condition. Subsequently, a system simulation model is developed in MATLAB/Simulink. The simulation results validated the accuracy of the theoretical analysis and demonstrated the effectiveness of the proposed method. Full article

To address the stability problem related to grid-connected modular multilevel converter high voltage direct current (MMC HVDC) connected to weak alternative current (AC) systems, the short-circuit ratio (SCR) that affects the stability of the system was analyzed first. Short-circuit ratios with SCR values greater than 1.3 were obtained, and the system could still operate stably. By applying the theoretical equations of classical circuits, it has been theoretically proven that for the constant active power and constant AC voltage control modes on the weak system side, after the flexible direct current enters the weak system mode, the power must be reduced to ensure the stable operation of the system. Combined with the actual situation of the north channel of the Chongqing–Hubei back-to-back MMC HVDC project, which is connected to the weak system mode, measures such as the optimization of the control mode and the improvement of control functions in the weak system mode were proposed, and simulation calculations and real time digital simulator (RTDS) simulation verifications were carried out. These control strategies have been applied to the Chongqing–Hubei MMC HVDC project, and on-site verification tests have been conducted to ensure stable operation in the weak system mode. Full article

As renewable energy sources are integrated into power grids on a large scale, modular multilevel converter-high voltage direct current (MMC-HVDC) systems face two significant challenges: traditional PI (proportional integral) controllers have limited dynamic regulation capabilities due to their fixed parameters, while improved PI controllers encounter implementation difficulties stemming from the complexity of their control strategies. This article proposes a dual-agent adaptive control framework based on the twin delayed deep deterministic policy gradient (TD3) algorithm. This framework facilitates the dynamic adjustment of PI parameters for both voltage and current dual-loop control and capacitor voltage balancing, utilizing a collaboratively optimized agent architecture without reliance on complex control logic or precise mathematical models. Simulation results demonstrate that, compared with fixed-parameter PI controllers, the proposed method significantly reduces DC voltage regulation time while achieving precise dynamic balance control of capacitor voltage and effective suppression of circulating current, thereby notably enhancing system stability and dynamic response characteristics. This approach offers new solutions for dynamic optimization control in MMC-HVDC systems. Full article

A diode-based rectifier (DR) is an attractive transmission technology for offshore wind farms, which reduces the volume of large bulk platforms. A novel parallel–series DC wind farm based on a distributed DR is proposed, which meets the requirements of high voltage and high power with an isolation capability from other units. The coupling mechanism between a modular multilevel converter (MMC) and a DR has been built, and the coordinate control strategy for the whole system has been proposed based on the MMC triple control targets with intermediate variables. Under the proposed control strategy, the system automatically operates at maximum power point tracking (MPPT). The feasibility of topology and the effectiveness of the control strategy are verified under start-up, power fluctuation, onshore alternating current (AC) fault, and direct current (DC) fault based on the power systems computer-aided design (PSCAD)/electromagnetic transients including direct current (EMTDC) simulation. Full article

This paper investigates the optimal sliding mode control (SMC) of modular multilevel converters (MMCs) by considering control input constraints. Conventional SMC methods for MMCs typically overlook the system’s constraints. To address this, an optimal SMC approach that incorporates control input constraints through quadratic programming (QP) is proposed. The control design problem is formulated in a constrained optimization framework and solved using the infeasible active-set (IAS) method to efficiently achieve the optimal solution. By applying optimal SMC, this work contributes to the advancement of SMC strategies for MMCs by addressing both constraints and performance optimization in a systematic way. This is particularly relevant for real-world applications, where controllers may temporarily exceed their limits before enforcing constraints. To validate the proposed approach, a comparative analysis with conventional SMC methods is performed, and simulation results confirm that the proposed approach provides improved performance. Full article