To address the inaccuracy of the conventional second-order model in describing the transient synchronization stability of doubly-fed induction generator (DFIG)-based wind turbines (WTs) connected to weak grids, this paper proposes a model of high accuracy, considering DC voltage dynamics. First, the limitations of the second-order model are revealed. Then, a novel model is established by integrating DC voltage dynamics and current dynamics. Further mechanism analysis shows that the slip ratio is the key parameter determining the impact of DC voltage dynamics: DC voltage dynamics enhance transient stability in super-synchronous mode but weaken it in sub-synchronous mode, in which ignoring such dynamics may lead to stability misjudgment. This study provides a reliable modeling tool for transient synchronization stability analysis of DFIG-based WTs.

The Dual-Active-Bridge (DAB) topology is widely used in solid-state transformers (SSTs) due to its electrical isolation, natural soft-switching intervals and modular symmetric structure. Traditional SSTs usually employ single phase-shifting (SPS) control, which may cause large peak currents and limit the transformer's overload capacity. To address this issue, this paper first establishes time - domain analysis equations for the DAB-based SST and investigates the feasibility of increasing output power while maintaining constant current stress by frequency adjustment. Then, a novel phase-frequency coordinated control strategy is proposed. This strategy can enhance the overload capacity to 1.3 p.u. without exceeding the safe current stress limit. A PLECS - based simulation model is built to verify the feasibility of the proposed control strategy. Finally, an experimental platform is constructed for validation. The experimental results demonstrate the strategy's overload capacity and show that it can reduce converter losses and significantly improve the transformer's operating efficiency.

An extended state observer (ESO) based active disturbance rejection control (ADRC) algorithm is presented for a single-phase power factor correction (PFC) converter in order to improve grid-side power quality under adverse grid conditions and enhance the dynamic performance of the DC output voltage. First, system overview including a LCL filtered PFC and its control strategy is carried out. On this basis, the model is augmented to design a dual-input single-output resonant extended state observer (RESO) whose resonant enhancement at specific frequencies facilitates the observation of periodic disturbances from the grid within the current-feedback loop. Meanwhile, well-known multi-proportion-resonant regulator (MPR) is also introduced as a fundamental part in parallel with RESO. For another aspect of voltage-feedback loop, a linear extended state observer (LESO) is developed to estimate the output power disturbances. It is able to improve disturbance rejection and transient performance under load variations through compensation on bases of disturbance estimation results. Finally, a simulation model is built up to verify the correctness and feasibility of the presented algorithm and the associated theoretical analysis.

In photovoltaic system, high voltage gain DC-DC converters are required. Hybrid active switched inductor (ASL) / passive switched capacitor (PSC) DC-DC converters are suitable for photovoltaic applications due to their high voltage gain characteristics. However, when the two inductors or the two parasitic capacitors of the switches in such ASL/PSC hybrid converters are mismatched, significant voltage oscillation occur across the switches, which increase the switch voltage stress. In addition, the ground potential difference between the input and output ports is not constant, which increases EMI issues. To address these issues, this article proposes a novel hybrid ASL/PSC converter. The proposed converter has no switch voltage oscillation and thus reduces the switch voltage stress. Moreover, the proposed converter has a constant ground potential difference. It also exhibits high voltage gain, lower voltage stress across the output diodes, and higher efficiency. To validate the theoretical analysis, a laboratory prototype was built and tested at an input voltage range of 20 V-60 V, an output voltage of 400 V, and a rated power of 200 W.

Unlike L-DAB converters, where startup safety primarily focuses on limiting peak inductor current, LC-DAB converters require additional attention due to the overvoltage issue of resonant capacitors, causing immediate dielectric breakdown and permanent damage. Therefore, limiting the peak resonant voltage should be prioritized as a critical condition for ensuring safe startup in LC-DAB converters, which has not yet been effectively solved. To address this issue, a closed-loop safe-fast startup strategy for LC-DAB converters is proposed in this article. By keeping the peak voltage of the resonant capacitor (v(rmax)) to its allowable maximum during the startup process, the proposed strategy ensures converter safety while maximizing output power, thereby accelerating the startup process. First, a time-domain model of the LC-DAB converter is established through state plane trajectory analysis. Based on this model, the optimal phase shift combination that limits v(rmax) while maximizing output power is derived. Furthermore, smooth variation of the phase-shift angles and online optimization over a wide voltage and load range can be realized based on real-time sampling of the input and output voltages for the proposed strategy, demonstrating strong engineering applicability. Experimental results show that, compared to conventional startup strategies for LC-DAB converters, the proposed method reduces startup time by up to 73.85% under safe resonant voltage limitation.

An interleaved LC resonant three-level Boost DC-DC converter is proposed in this article. Based on the traditional three-level boost (TLB) converter, this topology only adds a resonant capacitor. The resonant capacitor resonates with the inductor to achieve zero-current switching (ZCS) turn-on for two power switches. Meanwhile, the turn-off current and turn-off voltage of power switches are lower than those of other pulse frequency modulation (PFM) soft-switching TLB converters. Depending on whether the input voltage is greater than half of the output voltage, this converter can operate in Boost LC resonant mode and Buck LC resonant mode by PFM. Moreover, this converter features a small inductor and a narrow switching frequency range for a wide input voltage. The topological structure, operational principle, and design approach are introduced in this article. Finally, an experimental prototype was built, with 150–300 V input voltage, 400 V output voltage, and 40–70 kHz switching frequency. The experimental results validate the performance of the proposed converter.

Grid-forming (GFM) converters have emerged as essential technologies for ensuring the stability of power systems dominated by renewable energy sources. Nevertheless, their restricted overcurrent capacity may trigger protection during grid faults. While existing studies emphasize the design and optimization of overcurrent mitigation strategies, precise characterization of fault current and dynamic analysis of transient inrush current are still lacking, leading to occurrences of protection malfunction. To bridge this research gap, this article considers the impact of filter parameters and current loop parameters on the equivalent impedance of the converter, and on this basis, establishes a precise fault current characterization model for GFM converters. Based on the model, the key factors influencing inrush current can be analyzed, including voltage amplitude, impedance magnitude, and power angle. Finally, a precise virtual impedance-based inrush current suppression method is proposed, which effectively limits the inrush current to the desired level while ensuring system stability. Both theoretical analysis and experimental results are carried out to validate the robustness and adaptability of the proposed strategy under various grid strengths and grid voltage sags.

Grid-forming (GFM) converters are prone to significant overcurrent phenomena under grid fault conditions. The virtual impedance method can effectively limit overcurrent by enhancing the equivalent damping of the converter. However, the impact of the placement of virtual impedance on the transient stability of GFM converters is still absent in existing studies. To address this issue, this paper conducts a comparative study of two distinct virtual impedance integration strategies, based on the equivalent impedance and modulation voltage stiffness of the converter under grid voltage sags. The results demonstrate that the first strategy effectively increases the equivalent impedance, thereby achieving overcurrent suppression, whereas the second strategy exhibits limited effectiveness in overcurrent mitigation. Finally, experimental results validate the theoretical analysis, providing valuable insights for ensuring the secure operation of the GFM converter under fault conditions.

With the large-scale integration of renewable energy sources, grid-forming (GFM) converters with inherent voltage and frequency support capabilities have attracted significant attention. However, due to the limited overcurrent withstand capability of power electronic devices, the stable operation of GFM converters under grid faults such as grid voltage sags remains a critical challenge. To address this issue, this paper systematically investigates the mechanisms of power angle instability and overcurrent generation during grid faults by a unified equivalent impedance model. Based on this analysis, a comprehensive control strategy that simultaneously considers power angle stability and overcurrent suppression is proposed. By introducing an adaptive improved observer control (AIOC), the active power reference is adaptively adjusted to enhance the power angle stability of the system. Meanwhile, the voltage reference is dynamically regulated to effectively limit the fault current while enhancing the voltage support capability. Finally, comprehensive theoretical analysis and experimental validation are provided. The experimental results demonstrate that the proposed strategy is capable of ensuring power angle stability and limits the overcurrent to within 1.5 p.u. Meanwhile, the voltage magnitude is increased by approximately 6%. The results demonstrate the robustness and adaptability of the proposed method under various conditions.

With the large-scale integration of renewable energy sources and power electronic converters, grid-forming energy storage converter (GFM-ESC) with voltage and frequency support capabilities is emerging as an important component in power-electronic-dominated systems. Although GFM-ESCs are generally stable under weak grids, this article reveals that under ultra-weak grids, limitations in power transfer capability, specifically the static power transfer limit (SPTL) and dynamic power transfer limit (DPTL), can still cause stability issues and under-utilization of converter capacity. The SPTL is determined by grid impedance, while the DPTL further considers the control dynamics. It is first demonstrated that the SPTL exhibits asymmetry between discharging and charging modes, primarily attributed to grid resistance, which limits the power transfer in the charging mode. A third-order nonlinear dynamic model is then developed, with phase trajectory analysis used to evaluate the impact of various control parameter configurations on the DPTL. To address the instability induced by DPTL under ultra-weak grids, an adaptive voltage compensation strategy based on reactive power deviation is proposed, which improves both power transfer capability and capacity utilization. Finally, experimental results validate the theoretical analysis and the effectiveness of the proposed strategy.

Modulation strategies of three-level neutral-point-clamped (3L-NPC) converters to suppress switching losses and common-mode (CM) voltage have gained widespread attention. However, existing methods primarily focus on unit power factor (PF) conditions, leading to optimization failure over a wide PF operating range. To address this limitation, this article proposes a novel discontinuous pulsewidth modulation (DPWM) strategy that effectively minimizes both switching losses and CM voltage over the full range of leading and lagging PF angles. For switching loss reduction, a new region division method is introduced based on current magnitude boundaries and the modulation index. For peak-to-peak CM-voltage reduction, an optimal combination of clamping phases and voltage levels is analytically derived and systematically analyzed, restricting the peak-to-peak CM voltage to U-DC/3 for all PF conditions. Furthermore, a simplified zero-sequence injection flowchart is developed to facilitate the rapid real-time implementation of the algorithm. Comprehensive performance evaluations, including switching losses, CM voltage characteristics, and neutral point NP voltage self-balancing capability, demonstrate the superiority of the proposed method over traditional PWM methods. Finally, simulation and experimental results validate the accuracy and effectiveness of the proposed method.

The CLLC converter is an attractive option for battery charging applications due to their bidirectional power flow and soft-switching characteristics. However, the voltage gain curve of the CLLC converter is flat in the buck region, and existing variable-frequency or phase-shift variable-frequency modulation still require a wide switching frequency range, which increases the switching and core losses. To address this issue, a variable frequency - phase shift - duty cycle (VF-PSD) modulation is proposed in this article. The proposed modulation features narrow frequency range, full ZVS range, and lower RMS current by introducing asymmetric duty cycle control. Notably, when voltage gain M <= 0.5, the system automatically switches to half-bridge mode, further decreasing switching loss and conduction loss. A multiharmonic model is employed to indirectly analyze the time-domain characteristics of the CLLC converter under VF-PSD. Then, the optimal combinations of modulation variables are obtained through an offline particle swarm optimization algorithm. Finally, the advantages of VF-PSD modulation are verified through a 1-kW CLLC converter experimental prototype.

As the penetration of photovoltaic (PV) sources in the power system continues to rise, it is important for PV converters to operate in grid-forming (GFM) mode. Since the dc voltage is not controlled by the PV side, GFM PVs should have capabilities for dc voltage regulation and ac side support. However, most research on GFM PVs has concentrated on the stability related to dc-link voltage synchronization control (DVSC), neglecting the impact of ac voltage amplitude control (AVC) on the stability of GFM PVs. In this article, a new low-frequency oscillation issue in GFM PVs is first identified. To investigate the destabilization mechanism of this phenomenon, a small-signal model is derived, revealing that the low-pass filter of the reactive power introduced in the AVC loop and the AVC loop coupled into the DVSC loop are two key factors leading to instability. This article proposes an effective approach to suppress the low-frequency oscillations, which feeds forward reactive power dynamics into the DVSC loop through a proportional gain. This strategy not only addresses low-frequency oscillations but also improves the transient performance of the GFM PVs. Moreover, this strategy enables the GFM converter to operate stably across a wide range of short-circuit ratios. Finally, simulation and experimental results validate the accuracy of the theoretical analysis and the effectiveness of the proposed method.

Grid-forming energy storage converters (GFM-ESC) are essential for stabilizing power electronic-based grids owing to their voltage and frequency support capabilities. However, current research primarily focuses on the discharging mode of GFM-ESC, while grid-supporting characteristics in the charging mode remain underexplored. This article first identifies a phenomenon that the GFM-ESC is stable in the discharging mode but exhibits low-frequency resonances in the charging mode. To investigate the instability mechanism of this oscillation, a reduced-order small-signal impedance model for GFM-ESC is developed, revealing that the current matrix, resulting from the dynamics of the reactive power loop, is the primary factor causing instability. Therefore, this article proposes an asymmetric matrix control strategy based on q-axis voltage feedforward, which optimizes the reactive power loop to reshape the current matrix, thereby counteracting its negative impact on stability and ensuring stable operation across different modes. Finally, simulation and experimental results validate the correctness and effectiveness of the proposed control strategy.

The three-level neutral-point-clamped (3L-NPC) converter is a key building block in solid-state transformers (SST) due to its capability of handling high voltage levels with reduced device stress and low harmonic distortion. However, existing pulse width modulation (PWM) methods face a tradeoff between switching losses and output current ripple. To overcome this limitation, this paper proposes a novel hybrid modulation strategy that dynamically combines space vector PWM (SVPWM) and discontinuous PWM (DPWM) according to the current ripple magnitude in different regions. A unified analytical expression of the zero-sequence injected voltage is derived to simplify implementation. Finally, simulations based on PLECS-platform are conducted to validate the accuracy and effectiveness of the proposed method in achieving global optimization of switching losses and current ripple performance.

Owing to their inherent ability to provide voltage and frequency regulation, grid-forming (GFM) converters have become a focal point in modern power systems. Nevertheless, maintaining transient stability under severe grid disturbances remains a critical challenge. To suppress fault-induced overcurrent, this paper adopts an adaptive virtual impedance-based current limiting strategy. Building upon this, a novel transient synchronization enhancement approach is proposed, which dynamically adjusts the active power reference in response to fault conditions. Additionally, to reinforce the voltage supporting performance, an advanced reactive power control framework incorporating output voltage feedback compensation is developed. Comprehensive experimental validations confirm that the presented methodology substantially elevates both the synchronization stability margin and the output voltage magnitude, thereby validating the correctness and effectiveness of the proposed strategy.

Single-stage AC-DC dual active bridge (SSDAB) converters in energy storage microgrids often face overload conditions due to AC load fluctuations, leading to excessive inductor current stress and potential device damage. Existing phase-shift controls optimize current stress but fail to prevent overloads exceeding safe current limits. To resolve this limitation, this paper first investigates the feasibility of increasing output power while maintaining constant current stress via frequency regulation. A novel phase–frequency coordinated control strategy is subsequently proposed, which integrates frequency modulation into both TPS and EPS controls under light- and heavy-load conditions, respectively. This approach extends the overload power range to 3.0 per unit (p.u.) without surpassing safe current stress limits. Such a scheme also features online closed-loop control, smaller RMS current and capacitor ripple current that lowers overload hardware costs. Experimental validation, based on a SSDAB platform with monolithic bidirectional GaN on the AC side, demonstrates that the proposed control extends the achievable overload power capability from 673 W (1.0 p.u) to 2020 W (3.0 p.u.) under safe current limits, while also reducing the maximum temperature of the magnetic components from 53°C to 47°C, outperforming conventional phase-shift controls.

Most studies on grid-forming (GFM) converters have focused on two-level topologies, whereas three-level converters are more prevalent in industrial applications. Compared to two-level structures, three-level converters require additional neutral-point (NP) voltage balancing control. The strong coupling between the nonlinear dynamics of the NP voltage and the GFM control loops presents a critical challenge in the study of three-level GFM converters. Specifically: 1) the inherent NP voltage imbalance in three-level topologies leads to output voltage distortion, degrading the grid-support capability of GFM converters; and 2) the closed-loop voltage regulation in GFM control further exacerbates the voltage distortion caused by NP imbalance, creating an unstable feedback loop. To address this challenge, this article develops a linearized model of a three-level GFM converter that comprehensively integrates the GFM control loops and NP voltage dynamics. Based on this model, theoretical analysis reveals that the zero-sequence control in the GFM control loop is the decisive factor influencing NP voltage balance. Building on this insight, a novel decoupling control strategy is proposed to effectively eliminate the coupling between zero-sequence control and NP voltage dynamics. The proposed method demonstrates robust NP voltage balancing performance under various conditions. Finally, experimental results validate the accuracy of the theoretical model and the effectiveness of the proposed control scheme.

In photovoltaic (PV) systems, microinverters play a pivotal role by converting DC power from solar panels into AC power for the grid. Compared to conventional two-stage topologies, single-stage microinverters features higher power density and superior efficiency. Consequently, the matrix-based dual-active-bridge (DAB) microinverter utilizing bidirectional-GaN devices has emerged as a promising solution. To further optimize the efficiency of the matrix DAB, this paper proposes a triple-phase-shift (TPS) control strategy, which guarantees at least zero-current switching (ZCS) for all switches. Furthermore, an inductor network comprising dual inductors and magnetizing inductance is designed to compensate for the current and widen the zero-voltage switching (ZVS) range. Finally, simulation results validate the effectiveness of the proposed control strategy and magnetizing inductance design, demonstrating the enhancement of ZVS range.

Balancing current stress minimization across the full load range against control complexity remains a challenge for the steady-state operation of dual active bridge (DAB) DC-DC converters. To address above issues, this paper employs the differential extremum method to conduct a detailed analysis of the current stress expression under DPS (VF-DPS) control, and a variable-frequency plus dual phase-shift control is proposed in this paper, by which the control complexity can be simplified and the current-stress under heavy load can be further optimized with frequency regulation. An experimental platform was constructed to validate these findings. The results demonstrate that the proposed VF-DPS control could effectively reduce the heavy-load’s current stress with simplified computation effort, therefore minimizing switching turn-off losses and significantly enhancing the operational efficiency during heavy-load operation.