While grid-forming (GFM) technology has gained increased attention for voltage-source converters (VSC) connected to low-inertial AC systems, there exists a gap in addressing their fault ride-through (FRT) capability under large grid disturbances. Specifically, the challenge lies in resolving the resynchronization issue of GFM-VSCs under current-limitation mode without relying on a phase-locked loop (PLL). To bridge this gap, this article proposes a novel approach called direct current-synchronization control (DCSC), which directly regulates the VSC current for synchronization. The validity of DCSC is substantiated by establishing equivalent relationships between current and power, as well as power and phase angle. Thus, achieving synchronization by controlling the VSC phase angle is made possible through the direct control of the VSC current. The stability boundary of DCSC is theoretically analyzed, concluding that DCSC has the same stability boundary as power synchronization control (PSC) with reactive power regulation in continued normal operations but a 90-degree stability boundary under large grid disturbances, irrespective of voltage magnitude. Additionally, boundary conditions for system stability assessment during large grid disturbances are established, and a control gain self-adaptability (CGSA) scheme is introduced to accelerate resynchronization after faults. Therefore, the DCSC scheme exhibits identical control dynamics to PSC with reactive power regulation in continued normal operations, while offering enhanced FRT performance under large grid disturbances. Experimental results validate the theoretical findings, affirming the effectiveness of the proposed control method.

The classic implementation of dispatchable virtual oscillator control (dVOC) in the stationary αβ frame is counterintuitive and complex for small-signal modeling of the controller. Additionally, it makes integrating advanced control loops into the dVOC controller challenging and cannot achieve zero reactive power tracking error. To address these issues, for the first time, a new implementation of the dVOC in the dq frame is proposed by transforming the dynamics of the instantaneous angular frequency and voltage magnitude of the dVOC-controlled voltage vector into the local synchronous rotating frame of the converter. This new implementation resolves the inconvenience associated with small-signal modeling, enhances the flexibility and compatibility of the controller, and eliminates reactive power tracking errors, without compromising the operational characteristics of the dVOC. These improvements are achieved through the control of DC variables and the direct regulation of the converter phase angle and voltage magnitude. The effectiveness of the new dVOC controller is validated through EMT simulations.