As the global energy system accelerates its transition towards high penetration of renewable energy and high penetration of power electronic devices, regional power grids have undergone profound changes in their structural forms and component composition compared to traditional power grids. Conventional dynamic equivalencing methods struggle to balance modeling accuracy and computational efficiency simultaneously. To address this challenge, this paper focuses on the dynamic equivalencing of regional power grids and proposes a dynamic equivalencing scheme considering multiple feature constraints. First, based on the structural characteristics and the evolution of dynamic attributes of regional power grids, three key constraint conditions are identified: network topology, spatial characteristics of frequency response, and nodal residual voltage levels. Secondly, a comprehensive equivalencing scheme integrating multiple constraints is designed, which specifically includes delineating the retained region through multi-objective optimization, optimizing the internal system based on coherent aggregation and the current sinks reduction (CSR) method, and constructing a grey-box external equivalent model composed of synchronous generators and composite loads to accurately fit the electrical characteristics of the external power grid. Finally, the proposed methodology is validated on a Back-to-Back VSC-HVDC-connected regional power grid in Eastern Guangdong, China. Results demonstrate that the equivalent system reproduces the original power-flow profile and short-circuit capacity with negligible deviation, while its transient signatures under both AC and DC faults exhibit high consistency with those of the reference system.

High-Voltage Direct Current (HVDC) transmission systems have become a vital technology for modern electrical power networks, offering efficient, reliable, and long-distance energy transfer compared to conventional AC systems. This paper presents a comprehensive survey of HVDC systems, covering their evolution, working principles, converter technologies, and applications in today’s smart grids. It discusses major converter types—Line-Commutated Converters (LCC) and Voltage Source Converters (VSC)—and their roles in enabling bulk power transmission, renewable energy integration, and interconnection of asynchronous networks. The development of Multiterminal DC (MTDC) systems, including series, parallel, and ring configurations, is also explored for their enhanced controllability, scalability, and operational flexibility. Key advantages of HVDC systems such as reduced transmission losses, improved voltage stability, and lower environmental impact are analyzed, along with current challenges including DC fault management, converter losses, and control coordination. The survey highlights ongoing advancements in wide-bandgap semiconductor devices, intelligent control algorithms, and hybrid AC/DC grid architectures that are shaping the next generation of transmission systems. Future research directions focus on improving converter efficiency, protection schemes, and system interoperability to achieve flexible, resilient, and sustainable power transmission. Overall, HVDC technology stands as a cornerstone of modern smart grids, enabling efficient long-distance power transfer, renewable integration, and global energy connectivity.

Experimental investigation of the effects of current and voltage on the thermal characteristics of low-voltage DC fault arc

Hybrid cascaded multi-terminal HVDC systems represent a significant advancement in HVDC transmission technology. A notable real-world implementation of this concept is the bipolar hybrid cascaded multi-terminal high voltage direct current (MTDC) project in China, which successfully transmits hydropower from Baihetan to Jiangsu. This system combines MMCs for system support with LCCs for high-power transmission, offering both flexibility and efficiency in long-distance power delivery. This research explores the characteristics of main DC fault types in such systems, classifying faults based on sections and modes while analyzing their unique outcomes depending on DC fault locations. By focusing on the DC-side terminal behavior of the MMCs and LCCs, the main response processes to asymmetrical DC faults are investigated in detail. This study offers a detailed analysis of asymmetrical DC faults in bipolar HVDC systems, proposing a new classification based on fault characteristics such as current, voltage, active power, and reactive power. A supporting theoretical analysis is also presented. It identifies specific control demands needed for effective fault mitigation. PSCAD/EMTDC simulation results demonstrate that DC faults with similar characteristics can be consistently grouped into distinct categories by this new classification method. Each category is further linked to specific control demands, providing a strong basis for developing advanced protection strategies and practical solutions that enhance the stability and reliability of hybrid cascaded HVDC systems.

ABSTRACTIn solidly grounded bipolar high‐voltage direct current (HVDC) grids, DC faults can cause rapid current surges due to inherent pole‐to‐pole coupling, posing significant risks to system stability. Traditional traveling‐wave protection methods, though unaffected by MMC control characteristics, face challenges in bipolar systems where coupling complicates fault identification and necessitates costly DC circuit breakers (DCCBs). To address these limitations, this paper proposes a hybrid modular multilevel converter (MMC) topology based on an improved dual half‐bridge submodule (IDHSM) with self‐clearing capability and a coordinated control‐protection (CCP) strategy. The proposed method enables dynamic adjustment of activated submodule ratios through an adaptive modulation coefficient, achieving a 14.93% reduction in DC voltage under a 300‐Ω fault resistance while maintaining arm current constraints. Compared with full‐bridge submodules (FBSM), this hybrid topology reduces IGBT counts per voltage level by 50%, lowering hardware costs. Simulation results on a four‐terminal HVDC grid demonstrate that the proposed strategy enables DC fault identification within 1 ms using modulus instantaneous average values, effectively addressing the sensitivity degradation caused by MMC‐based current limiting. Moreover, the DCCB breaking current is reduced by 30% (from 7.0 to 4.9 kA) through coordinated current limiting. The proposed strategy exhibits strong performance across a wide transition resistance range of 0.1–300 Ω. Moreover, a low‐voltage experimental platform is built to verify the effectiveness of the proposed protection strategy.

Abstract Modern power systems are evolving toward hybrid AC/DC distribution networks to enhance efficiency, improve renewable energy integration, and support bidirectional power flow. However, these systems present complex challenges for fault detection and localization due to the diversity of AC and DC fault characteristics and their intricate operational behavior. This paper proposes a novel federated learning (FL)-based fault analysis framework that enables privacy-preserving, decentralized training while handling bidirectional energy flow and inverter-based RES integration. The approach introduces a complete solution encompassing fault detection, classification and localization using feature-extracted voltage and current signals. Simulation results under various fault types, network topologies, and operational modes confirm the method’s robustness and real-time accuracy. The proposed framework contributes to intelligent, scalable, and secure fault management in hybrid AC/DC networks.

The large-scale integration of renewable energy and the high penetration of power electronic devices have led to a significant reduction in system inertia and short-circuit capacity. This is particularly manifested in the form of insufficient multiple renewable energy stations short-circuit ratio (MRSCR) and transient overvoltage issues following severe disturbances such as AC and DC faults, which greatly limit the power transfer capability of large renewable energy bases. To effectively mitigate these challenges, this paper proposes an optimal synchronous condenser deployment method tailored for large-scale renewable energy bases. The proposed mathematical model supports a hybrid centralized and distributed configuration of synchronous condensers with various capacities and manufacturers while considering practical engineering constraints such as short-circuit ratio, transient overvoltage, and available bays in renewable energy stations. A practical decomposition and iterative computation strategy is introduced to reduce the computational burden of transient stability simulations. Case studies based on a real-world system verify the effectiveness of the proposed method in determining the optimal configuration of synchronous condensers. The results demonstrate significant improvements in grid strength (MRSCR) and suppression of transient overvoltages, thereby enhancing the stability and transfer capability of renewable energy bases in weak-grid environments.

The development and utilization of large-scale offshore wind power (OWP) are critical measures for achieving global energy transition. To address the demands of future large-scale OWP centralized development and transmission, this study systematically investigates the influencing factors and construction principles for topology selection in offshore wind power high-voltage direct current (HVDC) transmission systems delivering power to load centers. First, under the context of expanding the offshore wind power transmission scale, the necessity of transmitting OWP via HVDC overhead lines directly to load centers after landing is theoretically discussed. Five key topological influencing factors are then analyzed: offshore wind power collection schemes, multi-terminal HVDC network configurations, DC fault isolation mechanisms, offshore converter station architectures, and voltage source converter HVDC (VSC-HVDC) receiving terminal landing modes. Corresponding topology construction principles for direct HVDC transmission to load centers are proposed to guide system design. Finally, the feasibility of the proposed principles is validated through a case study of a multi-terminal HVDC system integrated into an actual regional power grid, demonstrating practical applicability.

Coordinated fault ride-through strategy for DC faults of photovoltaic grid-connected MMC-HVDC systems

A Partial-Power Converter for Battery Energy Storage System With DC Fault Blocking Capability

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.

Abstract Power electronics switching devices played an important role in high-voltage DC circuit breaker development. Timely isolation of faulty portions of an HVDC transmission line from a healthy system is a basic requirement for a fault interruption. In this scenario, the integration of hybrid DC circuit breakers (HDCCBs) with wideband-gap semiconductor devices enables the effective management of high power, currents, and voltages. The SiC-MESFET and the GaN-HEMT are commonly used wideband-gap-based semiconductor devices. This paper introduces a fault interruption scheme for HVDC power systems, featuring the advancement of a hybrid DC circuit breaker. The proposed HDCCB design consists of two parts, one part is based on a VCB as a mechanical circuit breaker, and the second part involves electronic switches for fault interruption. The electronic switches are designed through the combination of GaN and HEMT to achieve fast switching to achieve rapid interruption of fault current. The system model is implemented through a Simulink model to perform a comparative analysis between the presented and existing protection topologies. Current commutation is achieved through the attainment of artificial zero current crossing to interrupt the DC fault. GaN-HEMT emerges as a more reliable and fast switching element compared to other electronic switches like Sic-MESFET as validated by the presented simulative results. The presented model shows better fault-clearing times of 2.2 ms and 2 ms for experimental parameters of (500 kV and 9kA) and (100 kV and 10kA), respectively. This fault-clearing time shows an improvement of 52.38% and 50% compared to the SiC-MESFET-based electronic switches used by the existing mechanisms. The outcomes of the proposed design are evaluated in terms of fault current, commutated current, and voltage across the commutated capacitor.

The increasing attention to medium-voltage direct current (MVDC) distribution networks is motivated by the need to efficiently connect renewable energy sources and DC loads. However, fast and reliable protection strategies remain a key challenge due to the rapid rise and high magnitude of DC fault currents. This paper proposes a protection strategy for MVDC distribution networks considering network reconfiguration. The strategy integrates a fault-detection scheme based on the product of the rate of change in current and voltage (ROCOC × ROCOV) and a fault-identification scheme based on the ratio of the magnitudes of the positive and negative pole voltages. In a radial topology, the sign of ROCOC × ROCOV provides selectivity between internal and external faults. In multi-terminal topologies under network reconfiguration, external faults can present characteristics similar to those of internal faults. To ensure selectivity, communication is introduced between protective relays that share the same protection zone. Thresholds were set without large-scale simulations. The protection strategy was implemented in PSCAD/EMTDC and evaluated in a 37.4 kV MVDC distribution network. The strategy was validated under various fault conditions in radial and multi-terminal MVDC distribution networks, demonstrating fast, sensitive, and selective performance. The proposed strategy can contribute to the stable operation of MVDC distribution networks.

Abstract Modular Multilevel Converters (MMC) are the key equipment in High Voltage Direct Current (HVDC) systems, but they still face challenges such as DC fault clearance and excessive computation for submodule capacitor voltage sequencing. This paper presents an improved full-bridge MMC (MF-MMC) with submodule capacitor voltage self-balancing and DC fault clearance capability. By using two reverse-series IGBTs, adjacent full-bridge submodules are reconfigured from series to parallel, establishing a parallel path between adjacent capacitors. The MF-MMC enables voltage self-balancing between capacitors of different submodules within the same arm without the need for monitoring the capacitor voltage, significantly reducing the complexity and computational load of the controller. At the same time, the parallel connection of capacitors reduces the output of individual capacitors, thereby decreasing voltage fluctuations. A dynamic distribution voltage balancing control strategy is proposed to optimize the switching of power devices, reducing overall switching losses. Finally, MATLAB experimental results validate the effectiveness of this topology in terms of submodule parallel capabilities and DC fault clearance.

An HVDC System Based on OWT-DMMC With DC Fault Ride-Through Capability

Accurate and reliable DC fault location methods can significantly enhance the resilience and reliability of offshore MMC-based multi-terminal direct current (MTDC) grids. This research proposes a novel double-ended DC fault location method using the GWO-VMD and the Hilbert transform. First, the propagation of traveling waves (TWs) in the faulty line-mode network is described, leading to the theoretical expressions for the Backward Line-mode Voltage TWs (BLVTWs). Next, the Grey Wolf Optimizer-Variational Mode Decomposition (GWO-VMD) algorithm is employed to extract the high-frequency components contained in the measured BLVTWs, and the Hilbert transform is used to obtain the Instantaneous Energy Spectrum (IES) of the specific IMFs, allowing for the determination of the arrival times of the TWs. Numerous simulations in the PSCAD/EMTDC and RTDS environments confirm that the fault location method is accurate under various sampling frequencies and for close-in faults. Comparison studies also demonstrate that the proposed method is more accurate than existing classical TW-based methods. Additionally, the RTDS tests show that the method can withstand noise disturbances of at least 30 dB and has the potential to be applied in real projects.

Abstract Given the significant quantity of submodules (SMs) employed in modular multilevel converters (MMCs), a lightweight MMC named CM-MMC (Capacitor multiplexing MMC) is proposed in this paper that can cut down SM count by 25% by arm reuse, significantly reducing system costs while ensuring efficient operation and DC fault current interruption capability. However, its increased bridge arm number may lead to increased redundancy cost since enough redundant SMs must be equipped for each bridge arm. Enhancing the CM-MMC’s reliability while reducing the redundancy cost, an innovative redundancy sharing scheme is proposed, enabling the upper, middle, and lower arms to utilize shared redundant capacitors. Additionally, a simulation model in Matlab/Simulink is created, and the proposed topology’s feasibility is validated through simulation results.

ABSTRACT In this article, the Modular Multilevel Converter (MMC) having a modified mixed-cell Submodule (SM) with improved DC-side fault current blocking capability for the High Voltage Direct Current (HVDC) system is presented. This improved design reduces the number of MMC SMs by one-third times compared to the traditional MMC, yet maintains a high level of output power quality. A mixed cell constitutes a series connected Half-Bridge (HB) and modified Full-Bridge (FB) in an SM. It comprises a power diode, two asymmetrically charged isolated capacitors, and five IGBTs with antiparallel diodes. Isolated capacitors are charged according to the binary Geometry Propagation (GP) ratio in an SM so that it can generate a maximum four-level output voltage. The proposed SM features a reduced device count, lesser converter level faults and losses, higher efficiency, low voltage and current harmonic distortion, DC fault blocking capability, etc. The dimensioning of the proposed MMC converter is discussed in detail. Also, a comparison of the proposed modified mixed-cell SM with other SM topologies in terms of power losses, component count, output voltage level, and DC fault-blocking capability is discussed. Finally, the feasibility of the proposed topology is validated by both simulation and experiments.

This work presents the design and analysis of an optimized Proportional-Integral-Derivative (PID) controller for photovoltaic (PV)-based microgrids integrated into power systems. Conventional PI controllers often suffer from issues such as prolonged oscillation time, high amplitude responses, excessive overshoot, and persistent steady-state errors—particularly during fault conditions in PV microgrids. To address these limitations, this study aims to introduce and evaluate an optimized PID controller that enhances system responsiveness and improves stability under both DC and AC fault conditions. The proposed controller is designed to maintain current regulation stability during outages caused by unsymmetrical faults, considering scenarios with varying load demands and transmission line lengths. A PI controller is implemented within the current regulation loop, and the gains of the DC/DC boost converter are tuned using a trial-and-error approach to ensure stable current flow during faults. Comprehensive stability and performance evaluations are conducted using Bode plots and pole-zero mapping techniques in MATLAB/Simulink to validate the effectiveness of the control strategy. The performance of the optimized PID controller is compared against a conventional PID controller under multiple scenarios. The results demonstrate improved dynamic response, reliability, and system robustness. Overall, the proposed control design, tuning methodology, and analytical validation under unsymmetrical fault conditions confirm its suitability for enhancing PV-based microgrid operations.

Research on overcurrent mechanism and suppression strategy for MMC-HVDC DC fault recovery