Grid-Following Factor-based Current Limiting Strategy for a Grid-Forming Inverter with Two Electromotive Forces

Abstract

The increasing integration of inverter-based resources (IBRs) poses significant challenges in maintaining frequency and voltage stability in modern power systems. Traditional grid-following (GFL) control lacks inertial support and offers limited adaptability under voltage disturbances. In response, grid-forming (GFM) inverters have been introduced to emulate the behavior of synchronous machines. However, conventional GFM approaches often suffer from inherent coupling among virtual inertia, damping, and droop parameters, and their performance can deteriorate due to the bandwidth limitations of cascaded control structures. To overcome these limitations, the current-referencing electrified synchronous machine (CURESYM) was proposed. This method employs two separate electromotive forces (EMFs) to independently manage synchronization and current reference tracking. As a result, it provides greater flexibility in tuning dynamic responses without being restricted by inertia and droop parameters. While CURESYM offers promising advantages, it still faces challenges such as overcurrent during faults and the emergence of low-frequency oscillations. To enhance the performance of CURESYM, this dissertation proposes a novel current-limiting strategy based on a grid-following factor. This factor dynamically modifies the synchronizing EMF during fault events in order to suppress initial current peaks while preserving the GFM characteristics as much as possible. Two methods are proposed for calculating this factor, using current and voltage vectors generated by CURESYM. Furthermore, a synchronization preserving mechanism is developed to ensure accurate frequency tracking under severe current-limiting conditions. This work also investigates the phenomenon of synchronous resonance (SR) in CURESYM. The source of SR is identified as the voltage difference between the synchronizing EMF and the point-of-connection (PoC) voltage. To mitigate this issue, a virtual resistance adjustment is applied to the admittance model within the EMF generator. This approach improves the damping characteristics of the CURESYM. The proposed methods are validated through hardware-in-the-loop simulation and laboratory-scale experiments using a 4.4 kVA inverter. The results demonstrate notable improvements in suppressing peak current, mitigating SR, and maintaining synchronization. Through these contributions, the dissertation provides a practical and efficient control framework that enhances the reliability and fault tolerance of GFM inverters in power systems with high IBR penetration.