This paper presents a frequency support strategy for the diode rectifier unit (DRU)-high-voltage direct current (HVDC)-based offshore wind power integration system, which coordinates multiple power sources without communication to reduce receiving grid frequency fluctuations. First, based on the deduced DRU's frequency transfer characteristic, a fine-designed ripple carrying frequency information is superimposed on the HVDC link, transferring the onshore frequency to offshore wind turbines (WTs) via the DC ripple and coupled AC harmonic without communication. Second, multiple power sources are utilized for frequency support, including HVDC capacitance and grid-forming WTs combined with energy storage systems, and appropriate sources are activated in the order specified by the designed thresholds. Finally, the effectiveness of the proposed frequency support strategy is verified by simulations in PSCAD/EMTDC.

This paper analyzes the transient angle stability between synchronous generators (SGs) and grid-forming voltage source converters (GFM-VSCs), considering backward-swing dynamics. The impact mechanism of current saturation control (CSC) on the backward-swing and forward-swing stability is addressed. Based on the second-order rotor motion equation, the conventional energy-based method for transient stability evaluation becomes rough driven by the interactions between SG and GFM-VSC. To overcome the barrier, a forward-time integral (FTI) method is proposed for calculating the critical clearing time (CCT). We discover that the CSC can affect the stable and unstable equilibrium points of the post-fault system, leading to both the backward-swing and forward-swing synchronism instability. Based on the mechanism, an enhanced synchronization control (ESC) is proposed. It combines a modified frequency control with a current-limit strategy to avoid backward-swing instability while enhancing forward-swing stability. Time-domain simulation verifies the theoretical analysis as well as the effectiveness and robustness of the proposed ESC.