Yang Li

With the growing integration of renewable energy sources into power grids, the risks of oscillation caused by interactions between grid-tied inverters and the grids are becoming increasingly prominent. Although existing studies have made significant progress in inverter modeling and oscillatory stability analysis, most of them do not sufficiently consider complex mirror frequency coupling effects (MFCE) under unbalanced operating conditions, leading to unreliable models and erroneous stability analysis results. To address this inadequacy, this work develops a novel sequence impedance modeling scheme that can be widely applied to unbalanced operating conditions. In particular, taking a representative type of grid-forming inverter for instance, i.e., droop-controlled inverter (DCI), a single-input single-output sequence impedance modeling method based on harmonic linearization (HL) is proposed to comprehensively model both a given DCI and the connected grid. By accounting for multifrequency interactions within the DCI, this method captures MFCE and unbalanced factors, leading to a more accurate impedance model. Further, the dominant factors influencing system stability are identified with a combination of normalized sensitivity analysis and proportional weighting. Finally, the detailed impacts of these dominant factors on system stability margin under three typical unbalanced operating conditions are analyzed through the Bode criterion. The effectiveness and reliability of the whole scheme proposed in this work are validated on the constructed grid-connected droop-controlled experimental platform.

Due to extreme operational conditions, submarine DC power systems are prone to large-signal instability. Therefore, it’s essential to investigate the underlying physical mechanisms and develop effective stabilization strategies. This paper first establishes a large-signal model of the submarine DC power system. Incorporating the physical characteristics and control dynamics in the system, the large-signal stability boundary is defined by using the mixed potential function method, and key factors influencing the stability boundary are identified. Subsequently, the instability mechanisms, along with the effect mechanisms of these key factors’ variations on large-signal stability, are further analyzed based on the model of source-side and load-side power interactions. The analysis indicates that suppressing the transient reduction in the voltage conversion ratio of the submarine converter is critical to maintaining stability. Motivated by this insight, a switching frequency compensation strategy is proposed, and it demonstrates improved disturbance-rejection performance compared with conventional approaches. Finally, simulations and hardware-in-the-loop experiments are conducted to validate the stability analysis results and the effectiveness of the stability enhancement strategy under large disturbances.

The subsea direct current (dc) converter is prone to component failures, particularly in switching devices and input capacitors, primarily due to its installation within a tightly sealed, small-volume metal enclosure, which significantly impairs heat dissipation. To enhance the reliability of subsea input-series output-parallel (ISOP) dc converters, this article proposes a multilevel synergistic fault-tolerant strategy that takes into account the challenges of large input voltage fluctuations and limited electrical monitoring. Unlike conventional fixed-threshold methods, the proposed scheme leverages the dynamic voltage–current time-domain signatures of submodules during incipient faults for real-time monitoring and fault localization, thereby maintaining robustness against input voltage variations. Furthermore, a redundant bypass circuit based on current-limiting resistor multiplexing is incorporated to improve the traditional topology. When combined with a smooth redundancy switching mechanism, this approach effectively mitigates transient electrical stresses on healthy modules during fault transitions. These enhancements collectively ensure the operational safety of both the subsea converter and associated high-precision deep-sea equipment. Finally, a comprehensive fault-tolerant control scheme is developed to guarantee the reliable operation of the subsea dc power supply system following fault occurrences.

Due to the high incidence of faults in submarine cables, the rapid and accurate fault line detection is crucial for ensuring the safety and reliable operation of submarine MVDC power systems. In this paper, considering the fault response of shore station MMC, the transient and steady-state fault characteristics of meshed submarine MVDC power systems are analyzed. It reveals the differences in the direction and amplitude of high frequency superimposed currents (HFSCs) and low frequency superimposed currents (LFSCs) at both ends of the fault and non-fault lines. Then, by extracting transient energy from HFSC and monitoring changing trend of LFSC, signed binary state direction features of full frequency current are derived. These features are used to construct the protection criteria, which are robust against the change of fault scenario, fault starting speed and sampling frequency. Localized feature processing renders the protection accuracy immune to communication delay and synchronization error. Furthermore, the low bandwidth fault data exchange demand between line terminals is ensured by combining the fast submarine cabled communication and flexible space-ground-sea integrated network. Finally, simulation results on the modified North East Pacific Time-Integrated Undersea Networked Experiments (NEPTUNE) power system validate the effectiveness of the proposed scheme under various fault scenarios.

Due to the high fault occurrence probability of submarine cable caused by extreme working environment, accurate fault detection and location is very essential for submarine DC power systems. While electrical faults are usually accompanied by the failures of optical fiber communication, resulting in the poor reliability of the existing methods based on cabled communication. Therefore, we propose a communication-free fault location method using multiple incipient current features. First, the fault modeling and current loop decoupling of submarine DC power systems are carried out to obtain the time domain description of fault line current. According to the varying trends and rates of incipient fault current with fault distance and fault resistance, multiple time domain features are utilized to reflect the fault distance meanwhile weaken the adversity of transition resistance. Then, the segmented mapping model between selected features and fault distance are established by using least squares support vector machine regression (LSSVR) algorithm. Via the integrated fault features and segmented location models, the proposed method can cover the fault location of whole line, and offers the advantages of fast speed and high accuracy. Finally, PSCAD simulation and hardware-in-loop (HIL) experiment results validate the effectiveness of the proposed method under various fault conditions.

The LLC resonant converter offers several advantages, including compact size, low losses, and high efficiency. These attributes make it an ideal choice for use in seafloor observation networks where space is limited and efficient heat dissipation is crucial. However, the reliability of the LLC converter is compromised due to the susceptibility of the primary switch to failure. In response to this issue, this article proposes a precise open-circuit fault diagnosis (OCFD) method for phase-shifted LLC resonant converters using the integral of the bridge arm midpoint voltage, aiming to identify the fault location and facilitate maintenance. First, the characteristics of the primary-side switch fault in the LLC resonant converter are analyzed in detail, and the characteristic quantity required for fault diagnosis is selected. Subsequently, the OCFD method is introduced, which exclusively depends on the integral value of the voltage at the midpoint of the primary-side switching bridge arm. Following this, a diagnostic circuit is designed to process the bridge-arm midpoint voltage and precisely pinpoint the faulty switch. Finally, the experimental verification is performed on the LLC resonant converter prototype. The results demonstrate that the proposed method can swiftly detect and locate faults within two switching cycles when the LLC converter operates at 100 kHz.

Grid-connected converter should transmit power from distributed generation to the grid as much as possible, even under unbalanced conditions. However, under the necessary constraints of the point of common coupling (PCC) voltage support and current limitation, it is difficult for the converter to transmit more power to the grid. To fill this gap, this article provides a mechanism analysis of the power transfer considering the zero-sequence voltage influence. Moreover, a new control strategy is proposed to improve the power transfer capability under unbalanced conditions. It is found that the phase difference between the PCC voltage and the grid voltage in the positive- and negative-sequence network is a key factor affecting power transfer capability, while playing an important role in the accurate calculation of the PCC voltage amplitude. Based on the key factor, we propose a new control strategy to improve the power transfer capability of the grid-connected converter. The proposed strategy can flexibly control the phase difference and transfer more power to the grid under the premise of current limiting and voltage support. Finally, the proposed voltage support strategy is validated with an experimental study.

The fault ride through (FRT) control of inverter-based distributed generators (IBDGs) is quickly activated after a fault to limit the output current within 2 per unit. There is a phase jump phenomenon at the commencement of FRT control. However, the low fault current level and time-varying phase could make it difficult to guarantee the rapidity and the selectivity of islanded microgrid protection. In this paper, the transient characteristics of islanded microgrids are analyzed considering the influence of IBDG fault response and flexible network topology. The solution reveals the phase jump direction and magnitude difference between internal and external fault currents. Then, the energy value, the skewness value of directional morphological gradient, and peak polarity of incipient fault current are applied to formulate the protection criteria. The proposed protection scheme offers the following features: faulted phase selection capability, insensitivity to fault inception angle, adaptation to changes in the network topology, noise immunity with signal to noise ratio (SNR) down to 1dB, and low communication bandwidth requirements. The simulation results verify the effectiveness of proposed scheme for various fault conditions.

Grid-forming converters not only provide voltage and frequency support for remote islanded microgrids but also induce increasing instability risks during disturbances caused by typhoons, rainstorms, etc. With the assistance of LEO satellite internet, a data-driven predictive control (DPC) strategy is proposed to enhance the stability and resilience of remote islanded microgrids against widespread and frequent disturbances. Owing to the high-speed and wide-bandwidth communication services provided by the LEO satellite internet, the proposed DPC can establish timely and reliable data interaction among participating converters with acceptable latency, and then a two-layer gate recurrent unit (GRU) neural network is applied to predict the converters' dynamics precisely despite the parameter mismatch drawbacks of mathematic models. Furthermore, the proposed DPC achieves the derived consensus dynamically among converters such that the coherence and consistency are guaranteed effectively. Extensive case studies on practical remote islanded microgrids have been conducted to validate the effectiveness of the proposed DPC in coping with various types of disturbances and oscillations, effectively ensuring the system stability with considering the delays induced by the LEO satellite internet

With massive electronic inverters, microgrids are threatened by the instability problems caused by the impedance interactions among inverters and the network. For the microgrids with black-box inverters (whose parameters are unknown due to industry secrets), it is hard to assess, much less enhance, the stability of such systems. This article proposes a systematic impedance-based stability assessment and enhancement method for the microgrids with black-box inverters. First, the return-ratio matrix G _net of the system with both current-controlled and voltage-controlled inverters is formulated based on the nodal admittance matrix. And then, the sensitivities of the critical eigenvalues of G _net are calculated with respect to individual admittances/impedances of inverters, which can identify the “trouble maker(s).” Moreover, the low voltage active damper (LVAD) is proposed for the stability enhancement of the system. An eigenvalue perturbation sensitivity analysis method is presented to calculate the sensitivities of the critical eigenvalues with respect to nodal parallel admittances, which identifies the optimal installation position for LVAD, and accordingly provides the guidance for the design of LVAD. The effectiveness of the proposed method is verified using a modified IEEE 6-bus system in PSACD/EMTDC and RT-Lab platforms.