A Dual-Loop Frequency-Feedforward PLL for Three-Phase VSC Transient Stability Improvement

A dual-loop frequency-feedforward phase-locked loop (DLFF-PLL) is proposed in this article. The proposed structure achieves angle and frequency tracking without integratorbased loop compensator commonly used in the conventional synchronous reference frame PLL (SRF-PLL). A dual-loop structure is proposed for tracking frequency and phase respectively, where a feedforward filter provides the identified frequency from the frequency-tracking loop to the phase-tracking loop to ensure synchronization. The proposed dual-loop structure enables a simple transient-stability-improving technique for grid-connected voltage-sourced converters (VSCs) by placing a saturation block in the frequency feedforward path. Under weak grid conditions, the PLL bandwidth is automatically reduced by saturating the frequency feedforward. As a result, DLFF-PLL with feedforward saturation effectively improves the transient stability of the VSC, e.g., when subjected to large phase and grid impedance jumps.