Hybrid Threshold Virtual Impedance for Fault Current Limiting in Grid-Forming Converters

This paper presents a hybrid threshold virtual impedance method for grid-forming (GFM) converters to effectively limit current under balanced phase jumps and short-circuit faults. To this end, a virtual impedance method based on voltage information (VIv) is proposed to improve the current limiting capability of the standard threshold virtual impedance method for phase jumps (e.g., due to fault clearing or open circuit faults). To leverage the strengths of both threshold virtual impedance and VIv methods, an integrated hybrid threshold virtual impedance method is proposed, which enables reliable current limiting capabilities for both phase jumps and short-circuit faults. Electromagnetic transient (EMT) simulations demonstrate the improved fault ride-through (FRT) performance of the hybrid threshold virtual impedance method compared to threshold virtual impedance or VIv methods.

I. Introduction

As a result of sustainability concerns and increasing demand for electricity, massive integration of renewable energy into power systems has gained significant traction. Renewable energy sources are typically interfaced with the grid through power electronic converters, whose dynamic response and fault current characteristics significantly differ from synchronous machines. Due to their ability to impose a stable AC voltage waveform (e.g., frequency and magnitude) at their terminal and self-synchronize through the grid, grid-forming (GFM) converters are widely envisioned as the cornerstone of future grids with massive integration of inverter-based resources (IBRs) [1] –[3]. While GFM converters can reliably replicate ancillary services (e.g., frequency control) typically provided by synchronous machines, their limited overcurrent capability precludes replicating the fault response of synchronous machines. In addition, the fault characteristics of such converters are fully determined by their control. Therefore, an effective current limiting strategy is essential for GFM converters to ensure continuous support of the system and reliable power supply.