The project of the flexible direct transmission of renewable energy has become an inevitable development trend for the large-scale grid connection of renewable energy. Its two-terminal weakly fed AC system is often composed of 100% power electronic equipment, which leads to an essential transformation in fault characteristics and protection requirements. At present, in research, the traditional directional elements are limited by the negative-sequence control strategy, resulting in the decline of their sensitivity and reliability. Therefore, this paper proposes a model for identifying directional elements using composite electrical quantities that is not affected by the control strategy of the two-terminal weakly fed AC system and can reliably identify the fault direction. Firstly, the adaptability of traditional directional elements under the negative-sequence current suppression strategy on both sides of the system when faults occur in the AC line was analyzed. Secondly, based on the idea of model recognition, the model relationship of fault voltage and current in the case of ground faults and non-ground faults occurring at different locations was analyzed. Finally, a fitted voltage was constructed and the Kendall correlation coefficient was introduced to achieve fault direction discrimination. Simulation results demonstrate that the proposed pilot protection scheme can operate reliably under conditions of 300 Ω transition resistance and 25 dB noise interference.

The oscillation problem has emerged as one of the critical challenges confronting emerging power systems, particularly with the increasing penetration of grid-forming renewable energy sources. This trend can lead to the coexistence of multiple oscillation modes across a wide frequency range. To enhance the safety and stability of power systems, this paper proposes a wideband oscillation parameter identification method based on the multiple matching synchrosqueezing transform (MMSST), addressing the limitations of traditional time–frequency analysis techniques in accurately separating and extracting oscillation components during wideband parameter identification. The method first applies MMSST to decompose the measured oscillation signal into a set of intrinsic mode functions (IMFs). Subsequently, the Hilbert transform is applied to each IMF to extract the instantaneous frequency, amplitude, and initial phase, thereby achieving precise parameter identification of the oscillation signal. The validation study results demonstrate that the MMSST algorithm outperforms the empirical mode decomposition (EMD) and variational mode decomposition (VMD) algorithms in accurately extracting individual oscillation components and estimating their dynamic characteristics. Additionally, the proposed method achieves superior performance in terms of both accuracy and robustness when compared to the EMD and VMD algorithms.

The growing research interest in diode rectifier unit-based modular multilevel converter (DRU-MMC) configurations has made it imperative to analyze their potential impact on the short-circuit current characteristics of future AC grids. Most existing short-circuit current calculation approaches are oriented toward DC-side fault scenarios, while systematic research on AC-side fault analysis remains incomplete. To address this limitation, this work first illustrates the operating mechanism of the MMC. The short-circuit current at the fault point is decomposed into two independent components based on the superposition theorem. These components are: the current injected by the MMC and the current originating from the AC system. This work further explores the regulatory mechanism by which dq-axis limiting and fault-ride-through current limiting shape the MMC’s output current and derives an analytical equation for its amplitude. Furthermore, the phase correlation between the MMC-injected current and the AC system current under symmetrical fault conditions is clarified, and the computational formula for the aggregate short-circuit current is established. Then, considering the influence of transition resistance, the proposed method is verified to be applicable to both symmetrical metallic and non-metallic faults. Then, the symmetrical component method is used to analyze the sequence component of asymmetric fault short-circuit current, and a negative-sequence suppression (NSS) strategy is introduced. At the same time, considering the influence of transition resistance, the calculation formula of asymmetric metal and non-metal fault short-circuit current is derived. At the final stage of the study, a two-terminal simulation model is constructed in the PSCAD/EMTDC simulation environment. Comparative verification confirms that the results derived from theoretical calculation are in strong agreement with the simulation outcomes. The approach introduced here offers a favorable combination of simplicity and precision, rendering it highly suitable for practical engineering use. It reliably determines the short-circuit current under various fault conditions, thereby supporting fault-current analysis and the coordination of protective relays on the AC side of the receiving-end MMC in a DRU-MMC system.

Virtual Synchronous Generator (VSG)-based grid-forming inverters are widely employed to provide voltage and frequency support for enhancing power system stability in renewable-rich grids. However, their Point of Common Coupling (PCC) voltage quality deteriorates under harmonic perturbations caused by nonlinear loads, grid voltage distortions, and so on. In this paper, an adaptive repetitive voltage control strategy is proposed for VSG inverters to eliminate harmonic perturbations. The proposed method integrates a frequency-adaptive repetitive controller into the state-feedback voltage control loop to ensure accurate harmonic compensation in the presence of nonlinear loads, grid voltage distortions, and grid frequency variations. A plug-in design preserves the VSG’s inherent grid-forming capabilities while avoiding control loop redesign. Experimental results demonstrate that the proposed voltage control strategy can achieve fast, accurate, and robust PCC voltage regulation to offer voltage support and grid frequency support and provide accurate elimination of harmonic distortions under nonlinear rectifier loads, grid voltage distortions, and grid frequency variations.

The flexible DC transmission project of renewable energy has become an inevitable development trend for large-scale renewable energy grid connection. A two-terminal weak feed (TTWF) AC system is often composed of 100% power electronic equipment. The traditional fault control strategy adopted after a fault in the converter at both terminals of the line limits the fault current and controls the phase, resulting in a decrease in the time-domain distance protection performance. This paper first analyzes the adaptability challenges of time-domain distance protection in TTWF. Based on detailed fault characteristic studies, two improvement approaches are proposed: (1) accounting for phase control effects by equivalently modeling the fault impedance as a series combination of fault resistance and inductance; and (2) incorporating distributed capacitance effects through fault differential equation derivation based on π-type line equivalent models. A novel time-domain distance protection method is subsequently developed, comprehensively considering control strategy impacts and distributed capacitive currents. Simulation tests verify that the proposed method maintains reliable operation under severe conditions, including 300 Ω fault resistance and 30 dB white noise interference, demonstrating significantly improved resistance to fault impedance and noise compared to conventional solutions.