For the voltage source inverters (VSIs) based on grid-forming control (GFM-VSIs) in the same renewable energy plant, it is prone to occur the interactive stability issues due to the stiff-connected conditions among GFM-VSIs. To address this issue, a stability improvement strategy for multiple GFM-VSIs is proposed to enable GFM-VSIs to be stably connected in parallel under near-area and stiff-connected conditions, which cannot be achieved by the traditional grid-forming control. By employing the 2-norm (H2), the output characteristics of the VSIs utilizing the hybrid controller to emulate GFM-VSIs are quantitatively described and compared. Thus, the necessity for installation of GFM-VSIs is further demonstrated. Furthermore, the interaction mechanism among GFM-VSIs is elucidated via the simplified stability criterion based on the common mode and differential mode. By applying the proposed strategy, the system stability can be maintained even GFM-VSIs are directly paralleled. Finally, the experimental tests with two parallel GFM-VSIs are conducted to validate the effectiveness of the proposed strategy.

The arm multiplexing alternate arm converter (AM-AAC) can reduce the number of submodule capacitors, but transient voltage spikes may appear at commutation instants when phase-shifted carrier pulse-width modulation (PSC-PWM) is used with a low submodule count. This paper analyzes the spike-generation mechanism from the discrete sampling behavior of symmetric regular sampling. The analysis shows that interleaved carrier phases produce nonuniform response delays among submodules, so the arm voltage follows a staircase transition rather than an ideal step change. After mirror folding by the commutation switches, this transition is coupled to the ac side as a voltage spike. Analytical expressions are derived for the spike duration and amplitude, revealing their dependence on the submodule number and phase-shift conduction angle. A suppression strategy is then proposed by shifting the carrier phase of the commutation switches so that commutation occurs near the midpoint of the spike duration. MATLAB R2024b simulations and RTDS hardware-in-the-loop experiments verify that the proposed method substantially reduces the spike amplitude without additional hardware, while PSC-PWM maintains lower output-current THD than nearest-level modulation in low-submodule AM-AAC applications.

Multiport hybrid dc circuit breakers (M-HCBs) are considered a suitable solution for protecting multiterminal direct current grids, as the expensive main breaker (MB) is shared among adjacent lines connected to a common bus. However, MBs in most M-HCBs still utilize expensive fully controlled semiconductor devices. In this article, a novel thyristor-based multiport HCB (T-MHCB) is proposed to solve this issue, and it has several other advantages. First, the fully controlled semiconductor devices in the MB are replaced with thyristors to inherit some advantages of thyristors. Second, an LC resonant circuit is employed in the proposed T-MHCB to turn off the thyristor in MB and avoid the overvoltage problem during the reverse recovery process. Third, through the LC resonant circuit, the capacitor can be restored to its initial state after the first current breaking process. As a result, the fast-reclosing protection function is provided with no need for additional power supply. Fourth, simple operation sequences reduce the control complexity and enhance its reliability. The topology introduction, operation principles, mathematical analysis, parameter design guidelines, simulation results, experimental results, and comparison study of the proposed T-MHCB are all presented.

Conventional modular multilevel converter (MMC) exhibits excellent output performance but suffers from numerous submodules (SMs) and substantial SM capacitance. This results in a bulky physical size, restricting its application in medium-voltage systems like flexible interconnection urban power grids. To alleviate the hassle, a flying-capacitor-type hybrid multilevel converter (FHC) is proposed in this paper. Each phase of the FHC consists of flying capacitors, selector switches, and full-bridge submodules (FBSMs). The flying capacitors and selector switches are applied to generate a four-level voltage at the arm midpoint, while the FBSMs are responsible for shaping the voltage. A phase shifted conduction strategy is introduced to maintain energy balance. Furthermore, a design methodology is presented to optimize the converter parameters. A closed-loop control system is also proposed to dynamically regulate energy fluctuations in the flying capacitors and FBSMs, thereby ensuring the arm total energy balance and flying capacitor energy balance. Compared to MMC, FHC is more compact by reducing the number and capacitance of SMs and the number of switches. Finally, both simulation and hardware experiment are conducted, which verify the correctness, feasibility, and effectiveness of the proposed converter topology, parameter design methodology, and control strategy.

Under asymmetric low-voltage faults, the virtual synchronous generator (VSG) may experience negative-sequence current surges due to voltage imbalance. Such surges, combined with potential power angle instability, can lead to system transient collapse. To tackle these issues, this paper introduces a coordinated control strategy. It combines balanced current control with virtual positive-sequence power compensation (VPSPC). This approach suppresses negative-sequence current and enhances transient stability under asymmetric faults. The proposed strategy maintains a balanced three-phase output current during faults. It also enables fault ride-through without requiring extra fault detection, control mode switching, or adaptive power reference adjustments. A qualitative analysis explains how VPSPC works during fault ride-through. A quantitative evaluation examines how key parameters influence transient stability. Additionally, the strategy can inject reactive power under fault conditions and supports automatic system recovery after a fault. It is suitable for both symmetrical and asymmetrical low-voltage faults. Finally, hardware-in-the-loop tests and prototype experiments demonstrate the effectiveness of the proposed method.

The modular multilevel converter (MMC) is the mainstream high-voltage direct current transmission topology, but massive submodule (SM) capacitors and bulky arm inductors severely constrain its power density. To overcome these limitations, this letter proposes the modular alternate supporting converter (MASC). The three-phase legs of MASC alternately support the DC bus, while the remaining arms switch between conducting and blocking states. This unique operation naturally eradicates inter phase commutation, achieving an inductor-less architecture while retaining pure modularity. Concurrently, by inherently suppressing low-frequency power fluctuations, the proposed MASC reduces SM capacitance requirements by approximately 40% compared to conventional HB-MMCs, thereby yielding a 1.41-fold increase in overall power density. Experimental results validate the MASC topology for lightweight converter designs.

The cascaded submodule-based dc chopper (DCC) is a crucial fault ride-through device for remote offshore wind power systems utilizing voltage source converter-based high voltage direct current technology. In scenarios of significant surplus power, the traditional sinusoidal modulation strategy requires submodules (SMs) of DCC to provide a negative voltage to realize the SM capacitor charge-discharge balance. However, half-bridge SMs (HBSMs) cannot provide negative voltages, necessitating the inclusion of full-bridge SMs (FBSMs). A trapezoidal modulation strategy is proposed to dissipate the surplus power using exclusively half-bridge SMs. The proposed strategy consists of an amplitude-based trapezoidal modulation (ATM) mode and a frequency-based trapezoidal modulation (FTM) mode. The ATM mode prolongs the discharge time of the HBSM capacitor to compensate for the lack of FBSMs. The FTM adjusts the dissipated power by changing the trapezoidal wave frequency, working in tandem with ATM to handle the entire range of surplus power dissipation. The effectiveness of the proposed method is verified through simulation and downscaled hardware experiment results.

The dc chopper (DCC) plays a critical role in the fault ride-through (FRT) process of voltage source converter-based high voltage direct current (VSC-HVDC) systems. To reduce the cost of DCC, this paper proposes a device multiplexing DCC topology (DM DCC). Through device multiplexing, the proposed DM DCC achieves submodule (SM) activation/bypass and SM capacitor charging/discharging with only one insulated gate bipolar transistor (IGBT), thereby reducing the number of IGBTs by 50% compared to the traditional hybrid DCC. A control strategy is proposed for DM DCC to reduce the energy absorbed by the SM capacitor in one control period. Therefore, the capacity of the water-cooling system is reduced. Economic analysis indicates that the proposed scheme achieves at least 24.16% cost reduction compared to the traditional solutions. The effectiveness of the DM DCC and its control strategy is verified through simulation, hardware-in-the-loop (HIL), and downscaled hardware experiment results.

Parallel grid-following (GFL) converters exhibit poorer stability under weak grid conditions. When subjected to large disturbances, they tend to suffer from transient instability due to the weak or negative damping introduced by the synchronous reference frame phase-locked loop. This paper analyzes the transient instability mechanism of parallel GFL converters under weak grids: as disturbances intensify, the system's transient damping decreases to negative values. This problem is more prominent under weak grids, and there may even be no operating points satisfying static synchronization under extreme conditions. To address these, a virtual point of common coupling phase-locked loop (VPCC-PLL) is proposed. Its core lies in constant positive damping control and transient stability domain expansion, enabling stable operation when traditional PLLs fail to maintain static synchronization. Furthermore, VPCC-PLL requires no prior knowledge of line impedance parameters. The working principle, core characteristics, and parameter design of the VPCC-PLL are elaborated in detail. The proposed VPCC-PLL enhances transient stability while improving dynamic performance. Finally, simulations and experiments verify the effectiveness of the proposed VPCC-PLL.

This paper addresses the stability challenges of power-synchronization-based converters under strong grid conditions, with particular focus on single-phase converters deployed in localized, low-capacity applications where grid strength-induced instability is more prevalent. To overcome the limitations of conventional methods—such as synchronization instability due to PCC voltage stiffness in strong grids, and the delayed response or structural complexity caused by virtual impedance or mode switching schemes—a novel grid-forming (N-GFM) control strategy is proposed. Through systematic optimization of the control structure and parameters, the proposed method allows GFM converters to operate stably across a wide range of grid conditions, even when the short-circuit ratio (SCR) is infinite. Unlike conventional approaches relying on virtual impedance or control switching, the proposed strategy innovatively utilizes the inherent filter inductor to physically enhance the terminal inductive impedance, thereby effectively extending the system stability margin. By appropriately designing key parameters of the second-order band-pass filter and second-order general integrator, the implemented N-GFM controller demonstrates excellent dynamic and steady-state performance over wide SCR variations, achieving accurate power tracking, high-quality grid integration, and full-scenario operational adaptability. Simulation and experimental results validate the effectiveness of the proposed control strategy.

Isolated dc-dc converters have been widely studied and implemented in dc distribution grids and dc microgrids. However, conventional isolated dc-dc converter topologies demonstrate constrained fault tolerance capabilities during dc-side fault conditions. In this article, an open-winding transformer-based dc-dc converter topology, namely the open-winding quadruple active bridge (OW-QAB), is proposed, which offers reliable tolerance to dc side short-circuit faults, open-circuit faults, and open-circuit switching faults (OCSFs) without the need for additional dc circuit breakers. The operational principle of the OW-QAB topology is analyzed, and a control strategy based on the zero-point first-order linearized decoupling method is proposed to enable independent control of power at each port. Furthermore, an analysis of the fault tolerance mechanism of OW-QAB is conducted, and a corresponding fault tolerance strategy is proposed. The effectiveness of the proposed topology and strategy is validated through experimental results using a scaled-down prototype of OW-QAB.

This paper proposes a hierarchical control framework for MMC with integrated battery energy storage system (MMC-BESS), enabling decoupled three-port power regulation. Building on this capability, the system autonomously activates energy storage units to compensate for AC/DC power imbalance during DC bus faults, effectively maintaining grid-side power stability. Upon fault clearance, the system reverts to conventional MMC operation. Finally, the simulation results in Simulink verified the effectiveness of the control strategy.

The reliability of cascaded H-bridge converters utilizing centralized control methods is substantially limited by their inherent dependence on communication systems. This study proposes a decentralized control scheme for cascade H-bridge inverters, enabling independently power allocation and selfsynchronization for each H-bridge. The effectiveness is verified through simulation.

For grid-forming inverters, digital control is commonly used with a sampling process that converts the current and voltage signals into the discrete domain. Inverter admittance models in the continuous domain without considering the sampling process have reduced accuracy and may lead to false stability predictions. To address this issue, this paper proposes a hybrid admittance modeling approach considering the digital sampling process, which can improve the accuracy of the admittance model and thus is suitable for stability analysis for inverter-grid systems. The hybrid admittance model for grid-forming inverters is developed by considering the physical characteristics of sampling, the discrete nature of controllers, and the continuous characteristics of the inverter plant. The sampling of the transfer function with zero-order hold is equivalent to its step-invariant Z-transform, which can avoid the infinite sum due to sampling. Compared to state-of-the-art models, the proposed hybrid admittance model maintains high accuracy near and above the Nyquist frequency. Finally, the proposed method is validated through simulation and hardware-in-the-loop experiments.

The zero points of the determinant of the node admittance model (NAM) have been applied to address the stability issues in power systems with high-penetration converters. Yet, the NAM generally relies on the rotations, i.e., the dynamic phase of each converter, moreover, the frequency coupling effect is neglected via NAM. To fill this gap, this paper presents the concept of a unified alpha beta-frame complex-valued NAM, with which different converters as well as the passive components can be incorporated to form an s-domain model for a complex system. Additionally, the intrinsic relationship among the alpha beta-frame complex-valued NAM, the dq-frame NAM, and the sequence-domain NAM is revealed from a physical perspective. The proposed model merely relies on the initial phase of the voltage at each node and accurately reflects the frequency coupling effect. Furthermore, considering the frequency coupling, the consistency between the zero points of the determinant of alpha beta-frame complex-valued NAM and the eigenvalues derived from the state-space model is demonstrated. Finally, the effectiveness of the modeling approach and stability criterion is validated by applying them to the 3-machine system with different types of converters. Besides, the effect of the placement and capacity of converters based on grid-forming control on the stability performance in the multi-converter system is revealed.

A hybrid parallel-connected system with grid-following (GFL) and grid-forming (GFM) converters can be characterized as a high-order nonlinear dynamic system when performing transient synchronous stability analysis, which cannot be analyzed clearly by the existing method. The existing methods analyze the power angle for each converter individually, which fails to elucidate the transient coupling dynamic process of the power angles among different converters. To fill this gap, a 2-D phase portrait method is proposed by structure-preserving dimensionality reduction mapping, which can effectively reveal the interactive dynamics between GFM converter and GFL converter. The four types of factors, which impact the transient stabilization are elaborated by the proposed method. Furthermore, the influence of current limiting strategies on the transient stability is revealed by applying the 2-D phase portrait. Finally, experimental results are provided to verify the theoretical analysis.

Subsequent commutation failure (SCF) is an inherent issue for the line-commutated-converter based high voltage direct current (LCC-HVDC) in the receiving end. Nonlinear natures of LCC, such as the DC-AC conversion, commutation overlaps, relation between the transmitted power and grid voltage, etc. complicate the modeling and theoretical analysis for SCF under grid faults, which also poses challenges to the suppression of SCF. To address the above issues, the phase portraits analysis method is developed to depict the dynamics of SCF during grid faults. In the proposed analysis method, the large-signal model of the inverter is derived, hereafter, the dominant inducements of SCF, including control parameters, grid sag depth, and grid impedance are elaborated quantitatively. Moreover, an adaptive PI controller is designed for DC voltage control in both steady state and grid fault conditions, which can ensure the safe operation of the inverter under various degrees of grid fault. Then parametric boundaries for maintaining the highest DC voltage during transients are also obtained. Experimental results verify the effectiveness of the analysis and the proposed strategy.

Utility harmonic impedance estimation is critical for power quality assessment and improvement. Noninvasive methods without injecting harmonics are widely adopted to estimate utility harmonic impedance using natural load variations. However, background harmonic voltage fluctuations and abrupt harmonic impedance changes can lead to significant errors in utility harmonic impedance estimation. In this paper, a new noninvasive method is proposed to solve the above problem. To overcome the errors in harmonic impedance estimation caused by background harmonic voltage fluctuations, a time series clustering (TSC) method based on the cross-correlation principle is proposed to filter harmonic data. Moreover, an improved Pettitt method is proposed to identify the change points of harmonic impedance. Finally, the self-born weighted least squares (SBWLS) method is used to calculate harmonic impedance by iteratively weighting the anomalous data to weaken its influence, thereby improving the accuracy of the utility harmonic impedance. Simulation and field results validate the proposed method.

Thyristor-based solid-state circuit breakers (TCBs) have become a popular protection solution for DC microgrids, due to their low construction cost, high efficiency, and fast response time. Despite numerous TCBs being proposed, the majority of them still suffer from low controllability, low current interrupting reliability, slow fault isolation, poor reclosing protection, and high conduction loss. Inspired by these issues, a novel TCB with a three-winding coupled inductor is proposed in this paper. First, it could interrupt both operating and fault currents actively and reliably with its bidirectional symmetric construction, ensuring high controllability and reliability. Second, during the current interrupting process, the arrester immediately works after the proposed TCB acts, which makes the fault currents at both source and load sides drop, thus effectively suppressing the fault currents. Third, the capacitor and thyristors in the triggering circuits can automatically restore to their initial states after interrupting the current, thus ensuring a reclosing protection function with no extra operation sequences. Fourth, since the conducting branch only contains one set of parallel thyristors and an inductor coil, the conduction loss of the proposed TCB is low, thus ensuring effectiveness. The working principles, design guidelines, and experimental results are presented. The advantages of the proposed TCB are further demonstrated through comparison with some existing schemes.

Hybrid high voltage direct current transmission (HVDC) can avoid commutation failure of line commutated converter based HVDC (LCC-HVDC) and high costs from modular multilevel converter based HVDC (MMC-HVDC). The uncontrollability of reactive power from LCC results in the control performance of hybrid HVDC that is not comparable to MMC-HVDC. To address this shortcoming, a mathematical model with all AC and DC side characteristics of a three-terminal hybrid HVDC system is established. Then, the quantitative relationship between the reactive power, AC bus voltage, and control variables of three converter stations is revealed. Through this analysis, a coordinated control strategy that can achieve continuous and precise control of reactive power and AC bus voltage is proposed. This strategy is achieved by utilizing the self-regulating capabilities of the converter stations. Besides, the DC voltage is ensured to remain within a reasonable range through the coordinated operation of capacitor banks. Moreover, error feedback and gamma-kick are added to make the control objective more accurate. Finally, the simulation results indicate that the proposed reactive power coordination control strategy demonstrates ideal control performance