A virtual resistance-based pre-synchronization control for grid-forming converters with seamless transfer capability

Abstract

Grid-forming converters (GFMCs) are expected to achieve the seamless transfer between islanded and grid-connected modes. However, the conventional virtual impedance-based pre-synchronization control (VI-PSC) will inevitably introduce a virtual impedance angle, leading to two issues: 1) This angle complicates virtual power estimations; 2) This angle may induce pre-synchronization failure during the transition from islanded mode to grid-connected mode. In this paper, a virtual resistance-based PSC (VR-PSC) is proposed to eliminate the adverse effects exerted by the virtual impedance angle for GFMCs, thereby enabling the reliable and seamless transfer. A comparison between VI-PSC and VR-PSC is conducted from two aspects of virtual power estimations and mapping relations between control targets and results, and then the pre-synchronization failure mechanism caused by virtual impedance angle is elucidated. Subsequently, a single-zero double-pole (SZDP) controller is developed, incorporating a phase margin constraint into its parameter design to avoid the low-pass filter (LPF)-related performance degradation. In this way, even if the LPF is ignored, the developed SZDP can still enhance the dynamic performance of pre-synchronization without compromising the stability of control loop. Finally, the proposed VR-PSC is applied to single and parallel GFMCs respectively, and the performance of VR-PSC is tested by using a hardware-in-the-loop experimental platform.