Overcurrent Limitation and Enhanced Fault Current Utilization by Grid-Forming Inverter Using Hybrid Methods During Short Circuit Faults

The next-generation grid-forming (GFM) inverters face significant over-current and over-voltage challenges during grid faults due to their inherent voltage source behaviour behind impedance. These challenges are mitigated by simultaneously reconstructing power, voltage, or current references fed to different control layers in a coordinated way, known as the hybrid method. However, existing hybrid methods fail to 1) ensure maximum fault current utilization and 2) provide improved post-fault recovery. This paper proposes two new hybrid methods to address these limitations. The first method employs a novel non-linear transient virtual resistance (NLTVR) combined with reconstructed power references for symmetrical faults. An extension of NLTVR, referred to as unified NLTVR (UNLTVR), is introduced to handle both symmetrical and asymmetrical faults. By integrating NLTVR or UNLTVR with reconstructed power references based on grid fault severity, the inverter operates near its maximum current limit during faults while preserving GFM functionality, ensuring smooth post-fault power recovery without exceeding reference values. The second method combines the sinusoidal current reference limiter (SCRL) with the proposed UNLTVR. The proposed schemes are implemented in a droop-controlled GFM inverter modelled in the stationary reference frame. The effectiveness of the proposed methods is demonstrated through MATLAB/Simulink simulations of symmetrical and asymmetrical faults, supported by a Controller Hardware-in-the-Loop (CHIL) experiment. The proposed methods are also compared with conventional current-limiting methods. It is revealed that the SCRL achieves maximum fault current capability faster, while the combined efforts of the UNLTVR help the GFM inverter to experience a seamless post-fault recovery.