Search Results (152)

To address the lack of inertia in full-power converter wind turbines and the inability of existing mechanical speed regulation technologies to achieve power smoothing without converters, this paper proposes a novel hybrid wind energy storage system integrating a Continuously Variable Transmission (CVT) and an electromechanical flywheel. This system establishes a cascaded topology featuring “CVT-based source-side speed regulation and electromechanical flywheel-based terminal power stabilization.” By utilizing the CVT for speed decoupling and introducing the flywheel via a planetary differential branch, the system retains physical inertia by eliminating large-capacity converters and overcomes the bottleneck of traditional mechanical transmissions, which struggle to balance constant frequency with stable power output. Simulation results demonstrate that the proposed system reduces the active power fluctuation range by 47.60% compared to the raw wind power capture. Moreover, the required capacity of the auxiliary motor is only about 15% of the rated power, reducing the reliance on power electronic converters by approximately 85% compared to full-power converter systems. Furthermore, during a grid voltage dip of 0.6 p.u., the system restricts rotor speed fluctuations to within 0.5%, significantly enhancing Low Voltage Ride-Through (LVRT) capability. Full article

In practical engineering, large-scale wind power integration typically requires long-distance transmission lines to deliver power to load centers. The resulting weak sending-end systems lack support from synchronous power sources. Under extreme weather conditions, the rapid increase in active power output caused by high wind power generation may lead to voltage instability. In existing projects, a phenomenon of repeated voltage fluctuations has been observed under fault-free system conditions. This phenomenon is induced by the coupling of the characteristics of weak sending-end systems and low-voltage ride-through (LVRT) discrimination mechanisms, posing a serious threat to the safe and stable operation of power grids. However, most existing studies focus on the analysis of voltage instability mechanisms and the optimization of control strategies for single devices, with insufficient consideration given to voltage fluctuation suppression methods under the coordinated operation of wind power and energy storage systems. Based on the actual scenario of energy storage configuration in wind farms, this paper improves the traditional LVRT discrimination mechanism and develops a coordinated voltage ride-through control strategy for permanent magnet synchronous generator (PMSG) wind turbines and energy storage batteries. It can effectively cope with unconventional operating conditions, such as repeated voltage ride-through and deep voltage ride-through that may occur under extreme meteorological conditions, and improve the safe and stable operation capability of wind farms. Using a hardware-in-the-loop (HIL) test platform, the coordinated voltage ride-through control strategy is verified. The test results indicate that it effectively enhances the wind–storage system’s voltage ride-through reliability and suppresses repeated voltage fluctuations. Full article

Recently, the installed generation capacity of wind energy has expanded significantly, and the doubly fed induction generator (DFIG) has gained a prominent position amongst wind generators owing to its superior performance. It is extremely vital to enhance the low-voltage ride-through (LVRT) capability for the wind turbine DFIG system because the DFIG is very sensitive to faults in the electrical grid. The major concept of LVRT is to keep the DFIG connected to the electrical grid in the case of an occurrence of grid voltage sags. The currents of rotor and DC-bus voltage rise during voltage dips, resulting in damage to the power electronic converters and the windings of the rotor. There are many protection approaches that deal with LVRT capability for the wind turbine DFIG system. A popular approach for DFIG protection is the crowbar technique. The resistance of the crowbar must be precisely chosen owing to its impact on both the currents of the rotor and DC-bus voltage, while also ensuring that the rotor speed does not exceed its maximum limit. Therefore, this paper aims to obtain the optimal values of crowbar resistance to minimize the crowbar energy losses and ensure stable DFIG operation during grid voltage dips. A recent optimization technique, the Starfish Optimization (SFO) algorithm, was used for cropping the optimal crowbar resistance for improving LVRT capability. To validate the accuracy of the results, the SFO results were compared to the well-known optimization algorithm, particle swarm optimizer (PSO). The performance of the wind turbine DFIG system was investigated by using Matlab/Simulink at a rated wind speed of 13 m/s. The results demonstrated that the increases in DC-link voltage and rotor speed were reduced by 42.5% and 45.8%, respectively. Full article

Ultra-high-voltage direct current (UHVDC) transmission serves as a vital method for long-distance transmission of renewable energy in China. Commutation failure represents a common fault type in UHVDC transmission systems, causing the sending-end bus voltage to exhibit a “low-to-high” characteristic. This phenomenon poses a high-voltage disconnection risk for renewable energy units at the sending end. The high-voltage ride-through criteria for renewable energy incorporate both time and voltage peak factors. However, existing research relies solely on the voltage peak metric to assess disconnection risks for renewable units, failing to determine the specific stability level of the voltage. Therefore, this paper considers the cumulative effect of voltage transients over time, constructing a mathematical model of transient voltage during the entire fault process of a UHVDC transmission system at the sending end under commutation failure. Subsequently, a transient voltage rise stability margin metric based on a multi-binary table is proposed to evaluate the system’s transient voltage rise stability margin from both time and voltage peak dimensions. Finally, the accuracy of the proposed mathematical model and evaluation metric is validated using the PSCAD/EMTDC simulation platform. Results indicate that following a commutation failure in a UHVDC system, under the scenario of overvoltage instability alone, a higher short-circuit ratio (SCR) correlates with a lower system rated voltage. This configuration enhances the voltage stability margin of the sending end grid, improves its transient voltage stability, and helps mitigate the risk of renewable energy units disconnecting from the sending end grid. Full article

Hybrid AC/DC microgrids (HMGs) are pivotal for integrating renewable resources, yet their stability and resilience are fundamentally constrained by the power electronic converters that interface them. This paper provides a critical review and synthesis of the co-dependent advancements in HMG converter topologies, control strategies, and fault management. Through a systematic analysis of the state of the art, this review examines the evolution from classical control to intelligent, software-defined converter functions. The analysis reveals a fundamental bifurcation in design philosophy between low-voltage (LV) and medium-voltage (MV) systems, driven by a trade-off between power density Gallium Nitride (GaN) and systemic reliability silicon carbide (SiC). Furthermore, it highlights the rise of virtualization, namely virtual Inertia control (VIC) and adaptive virtual impedance control (AVIDC), as a dominant paradigm to compensate for the physical limitations of low-inertia, resistive grids. Finally, this review identifies a critical, synergistic dependency in fault management, where ultra-fast solid-state circuit breakers (SSCBs) guarantee the survivability of vulnerable voltage source converters (VSCs), which in turn enables software-based resilience via fault ride-through (FRT). This synthesis concludes that the converter has become the intelligent nexus of the HMG and identifies the primary barriers to widespread adoption as the computational, economic, and standardization gaps in this new cyber–physical domain. Full article

With the increasing grid integration of wind turbines, mechanical reliability issues have become more critical. In particular, two-mass drivetrains undergo severe torsional vibrations due to abrupt torque fluctuations during low-voltage ride-through (LVRT) events. To address this, this paper proposes an adaptive damping coefficient determination method that differs from conventional pole-based approaches. The generator speed is decomposed into steady-state and transient components, and the maximum torsional angle is directly computed by integrating the transient component to derive the optimal damping coefficient. An adaptive algorithm adjusts this coefficient in real time according to operating conditions. The proposed approach is verified through PSCAD simulations of a 4.5 MW Type-4 permanent magnet synchronous generator (PMSG) wind turbine with a fully grid-decoupled back-to-back converter. Simulation cases combining active power levels (100% and 20%) with fault durations (20 ms and 400 ms) demonstrate that, compared with the conventional pole-based approach, applying the optimal damping coefficient reduces the maximum torsional angle by 30–37%, accelerates transient damping, and stabilizes speed and torque responses. The proposed method effectively mitigates drivetrain stress during LVRT, providing practical guidelines to enhance drivetrain reliability in Type-4 wind turbines. Full article

During system faults, power electronic converters in modern sustainable power systems activate low-voltage ride-through (LVRT) control strategies, which introduce second harmonic current into the power system. For transformer protection, the conventional inrush current identification method based on second harmonic current fails to adapt to the high harmonic conditions of electronic power-based sources in renewable energy systems. This paper proposes an identification scheme based on a modified MobileNetV4 (MNv4) architecture and multi-source electrical quantities. The experimental dataset is constructed through PSCAD simulation and engineering field data. The input feature combination including three-phase voltage, current and differential current is designed, which solves the defects of single feature in traditional methods. Experiments show that the MNv4 model delivers competitive performance in terms of accuracy and recall, while featuring a small number of parameters that make it suitable for resource-constrained embedded deployment. This research provides theoretical support and data paradigm for the engineering application of artificial intelligence in the field of relay protection. Full article

Wind energy has become a cornerstone of sustainable electricity generation, yet the reliable integration of wind energy conversion systems (WECSs) into modern grids remains challenged by dynamic variations in wind speed and stringent fault ride-through (FRT) requirements. Among the available technologies, the Doubly Fed Induction Generator (DFIG) and the Permanent Magnet Synchronous Generator (PMSG) dominate commercial applications; however, a comprehensive comparative assessment under diverse grid and fault scenarios is still limited. This study addresses this gap by systematically evaluating the performance of DFIG- and PMSG-based WECSs across three operating stages: (i) normal operation at constant speed, (ii) variable wind speed operation, and (iii) grid fault conditions including single-line-to-ground, line-to-line, and three-phase faults. To enhance fault resilience, a DC-link Braking Chopper is integrated into both systems, ensuring a fair evaluation of transient stability and compliance with low-voltage ride-through (LVRT) requirements. The analysis, performed using MATLAB/Simulink, focuses on active and reactive power, rotor speed, pitch angle, and DC-link voltage dynamics. The results reveal that PMSG exhibits smoother transient responses and lower overshoot compared to DFIG. Under fault conditions, the DC-link Braking Chopper effectively suppresses voltage spikes in both systems, with DFIG achieving faster reactive power recovery in line with grid code requirements, while PMSG ensures more stable rotor dynamics with lower oscillations. The findings highlight the complementary strengths of both technologies and provide useful insights for selecting appropriate WECS configurations to improve grid integration and fault ride-through capability. Full article

To address the critical issue of low reliability caused by fault impacts in large-scale wind farms transmitting power over long distances via flexible DC transmission systems, this study proposes a collaborative solution. First, a new protection scheme integrating variable quantity differential protection, steady-state quantity differential protection and zero-sequence differential protection is proposed. By establishing a refined model of a wind farm with a flexible DC system, the adaptability of the differential protection for the outgoing lines is checked. Simulation results show that the sensitivity of metallic faults within the protection zone is better than 3.0, and the protection reliably remains inactive for faults outside the protection zone. Second, an innovative fault ride-through strategy combining self-regulating resistor circuits with wind farm MPPT load reduction is proposed. During faults on the receiving grid, the DC voltage fluctuation is controlled within 1.05 p.u. through graded switching of resistor modules and dynamic power regulation. This solution offers both rapid response and smooth fault ride-through characteristics, significantly improving the feasibility and economic viability of wind farm integration via flexible DC transmission. Full article

The integration of large-scale distributed photovoltaic (PV) units into a microgrid poses critical challenges to transient stability. Developing an effective model of distributed PV generation systems is essential for stability analysis. However, detailed modeling of individual PV units leads to prohibitive computational costs. To address this issue, this paper proposes an equivalent model for distributed PV generation systems in a microgrid. By thoroughly analyzing the PV units’ responses during the low voltage ride-through (LVRT) process, the dominant active power responses are identified, and two segmentation thresholds for clustering are analytically derived. To improve engineering applicability of the proposed clustering method, one voltage-dip-dependent segmentation threshold is approximated. Moreover, for PV units exhibiting post-fault active power ramp recovery, an additional clustering based on average pre-fault steady-state active power is introduced to better represent the dynamic behaviors of actual distributed PV generation systems. On this basis, a four-machine equivalent model is proposed, which captures key dynamic characteristics while ensuring both computational efficiency and modeling accuracy. Extensive simulations under various operating conditions and fault scenarios verify the effectiveness of the proposed equivalent model in reproducing transient behavior of distributed PV generation systems in a microgrid. Full article

Grid-tied inverters must satisfy fault ride-through (FRT) requirements to ensure grid stability during voltage disturbances. However, most existing FRT-related studies have focused on reactive current injection or voltage support functions, with few addressing how the active power reference should be dynamically controlled during voltage dips. In addition, few systems enable bidirectional power transfer or provide comprehensive verification under deep voltage dips. To address this issue, this paper proposes an output power control method for FRT in a single-phase neutral-point-clamped (NPC) inverter-based PCS consisting of an input-parallel output-series (IPOS) dual-active-bridge (DAB) converter and a battery. The proposed PCS dynamically reduces the output power reference according to the retained voltage while maintaining the inverter current within the rated limit, thereby ensuring stable operation. Computer simulations were conducted using Altair PSIM to verify the effectiveness of the proposed method. The results confirmed that the PCS satisfied the FRT requirements for all post-fault voltage levels. The injected current returned to its pre-fault value within 20 ms and 90 ms for 20% and 0% voltage dips, respectively, complying with the required recovery times. The proposed control method enhances grid resilience and maintains power quality in single-phase low-voltage distribution systems. Full article

To address the security and stability requirements of renewable energy clusters under extreme weather conditions, this study investigates the coordinated voltage support mechanisms between grid-following and grid-forming converters. This paper proposes an emergency control strategy suitable for such scenarios. First, a reactive power-voltage control architecture for new energy units is constructed to clarify the information interaction process. A mode-based coordinated strategy is designed: during steady-state voltage support, grid-following units adopt reactive power-voltage droop control for voltage regulation, while grid-forming units achieve autonomous support based on the virtual synchronous generator algorithm. During low-voltage ride-through, both types of units are controlled to output corresponding reactive power according to the depth of voltage drop until the voltage is restored. Hardware-in-the-loop simulation verification shows that under steady-state conditions, the strategy meets the voltage control accuracy requirements, and partial grid-forming transformation can reduce voltage overshoot and accelerate stabilization. During low-voltage ride-through, grid-forming transformation can reduce voltage fluctuations, shorten adjustment time, and mitigate reactive inrush current, effectively enhancing the voltage support capability of renewable energy plants. Full article

The growing penetration of renewable energy has reduced system inertia and damping, threatening grid stability. This paper proposes a novel control strategy that seamlessly integrates virtual synchronous generator (VSG) emulation with low-voltage ride-through (LVRT) capability for direct-drive permanent magnet synchronous generators (PMSGs). The unified control framework enables simultaneous inertia support during frequency disturbances and compliant reactive current injection during voltage sags—eliminating mode switching. Furthermore, the proposed strategy has been validated through both a single-machine model and actual wind farm topology. Results demonstrate that the strategy successfully achieves VSG control functionality while simultaneously meeting LVRT requirements. Full article

In this paper, a nonlinear power transfer matrix model is presented for power control of Permanent Magnet Synchronous Generator (PMSG) wind turbines, incorporating the DC link dynamics to account for transient stability, thereby clarifying the technical aspect and purpose. The rising penetration of wind turbines (WTs) into the power grid necessitates that they remain connected during and after faults to ensure system reliability. During voltage dips, the stator and grid-side converter (GSC) of a permanent magnet synchronous generator (PMSG) system are directly impacted by the sudden voltage changes. These disturbances can induce large transient voltages and currents in the stator, which in turn may lead to uncontrolled current flow in the rotor circuit and stress the converter components. Moreover, Low Voltage Ride-Through (LVRT) is a critical requirement for grid connection to Wind Energy Conversion Systems (WECS). It ensures that WTs remain connected and operational during short periods of grid voltage dips (faults), instead of disconnecting immediately. This capability is essential for maintaining grid stability. However, in this paper, the authors propose an LVRT scheme for a grid-connected PMSG-based WECS. A sequence of attempts was performed to validate the effectiveness of the proposed control scheme under fault conditions and to improve its overall performance. Full article

As offshore wind farms continue to scale up in both distance and capacity, the design of transmission systems has become a critical factor in the effective development and utilization of offshore wind energy. In response to the growing trend of larger wind turbine volume and the densification of offshore platforms, this paper presents a design methodology for compact transmission system tailored to large-scale offshore wind farms, with a focus on the collection system and reactive power control. Firstly, the feasibility of 66 kV single-stage collection system and a unified reactive power compensation scheme using wind turbines and Modular Multilevel Converter (MMC) is analyzed. On this basis, a compact transmission scheme based on MMC-HVDC is proposed for large-scale wind farms. Secondly, a cooperative reactive power control strategy is introduced, leveraging the reactive power regulation capabilities of both wind turbines and MMC. This approach enhances the system’s reactive power and voltage regulation capabilities, as well as its low-voltage ride-through (LVRT) performance. Finally, the effectiveness of the proposed transmission scheme and reactive power control strategy is validated through simulations, and a techno-economic comparison is made with conventional transmission systems. Full article