A Virtual Admittance-Based Fault Current Limiting Method for Grid-Forming Inverters

TL;DR

This work tackles fault current limiting for grid-forming inverters in high-renewable grids, where large fault currents and phase jumps challenge conventional control. It introduces a hybrid threshold virtual admittance (HTVA) method that adapts ideas from TVI and VIv into a virtual admittance framework, avoiding current differentiation and exploiting phase information to bound currents in a single-loop GFM structure. The method employs dynamic resistance via variable transient virtual resistance and a high-pass filter to balance transient damping with steady-state performance, with nominal values set for $R_{HTVA}$ and $L_{HTVA}$ in steady state. EMT simulations in MATLAB/Simulink demonstrate that HTVA can effectively limit currents under both three-phase faults and phase-jump disturbances, offering a robust alternative to existing TVA and VAv approaches for single-loop GFM inverters and enabling safer operation in modern grids.

Abstract

Inverter-based resources (IBRs) are a key component in the ongoing modernization of power systems, with grid-forming (GFM) inverters playing a central role. Effective fault current limiting is a major challenge to modernizing power systems through increased penetration of GFM inverters. Due to their voltage-source nature, GFM inverters offer no direct control over the output current and, therefore, are susceptible to high fault currents. This vulnerability is especially pronounced during large phase jumps, a condition overlooked by most fault current limiting methods. This paper proposes a hybrid fault current limiting method implemented through a virtual admittance by leveraging the advantages of two virtual impedance (VI)-based methods tailored for three-phase faults and phase jump disturbances. Electromagnetic transient simulations conducted in MATLAB-Simulink demonstrate the method's effectiveness across various disturbances, validating its potential in single-loop GFM structures.

Figures (6)

• Figure 1: Control diagram of a GFM inverter with current limiting capabilities connected to the AC Grid.
• Figure 2: Simplified quasi-steady-state model of a GFM converter with virtual impedance-based current limiting.
• Figure 3: Control diagram of the proposed HTVA-based current limiting.
• Figure 4: Test system with the GFM inverter connected to an infinite bus through step-up transformers and transmission lines.
• Figure 5: Comparison of transient responses for the VIv, TVI, and proposed HTVI methods, showing $\left\lVert i_c \right\rVert$, $\left\lVert v_o \right\rVert$, $\left\lVert v_{gfm} - v_{o} \right\rVert$, and $X_{vi}$ during the three-phase fault event.