Impact and Mitigation of Current Saturation Algorithms in Grid-Forming Inverters on Power Swing Detection

TL;DR

This paper addresses how current-saturation algorithms in grid-forming IBRs distort power-swing impedance trajectories and threaten legacy PSB/OST protection. It provides a theoretical analysis of entering and exiting saturation for circular, d-axis priority, and q-axis priority CSAs, and characterizes the resulting full-cycle impedance trajectories. A constant, user-defined saturation angle β is proposed as an optimal CSA to maintain continuous trajectories during saturation transitions, with validation in MATLAB/Simulink. Overall, the work enhances the reliability of power-swing protection in grids with high penetration of GFM IBRs by mitigating CSA-induced malfunctions and exposing potential stability risks like SSEPs.

Abstract

Grid-forming (GFM) inverter-based resources (IBRs) are capable of emulating the external characteristics of synchronous generators (SGs) through the careful design of the control loops. However, the current limiter in the control loops of the GFM IBR poses challenges to the effectiveness of power swing detection functions designed for SG-based systems. Among various current limiting strategies, current saturation algorithms (CSAs) are widely employed for their strict current limiting capability, and are the focus of this paper. The paper presents a theoretical analysis of the conditions for entering and exiting the current saturation mode of the GFM IBR under three CSAs. The corresponding impedance trajectories observed by the relay on the GFM IBR side are investigated. The analysis results reveal that the unique impedance trajectories under these CSAs markedly differ from those associated with SGs. Moreover, it is demonstrated that the conventional power swing detection scheme may lose functionality due to the rapid movement of the trajectory. To mitigate this issue, an optimal current saturation strategy is proposed. Conclusions are validated through simulations in MATLAB/Simulink.

Figures (14)

• Figure 1: Grid-connected GFM IBR system. (a) System model and control structure. (b) Control block diagram.
• Figure 2: The new current references generated by the CSAs. (a) Circular. (b) D-Axis Priority. (c) Q-Axis Priority.
• Figure 3: The process of a change in the sign of $\bar{i}_{sq}$. (a) Before $v_{q}$ tracks $v_{q}^{\text{ref}}$. (b) After $v_{q}$ tracks $v_{q}^{\text{ref}}$, as indicated by the solid lines.
• Figure 4: Two scenarios for current exiting saturation under circular CSA control. The dashed lines represent the moment just before $v_{q}$ tracks $v_{q}^{\text{ref}}$, while the solid lines represent the moment just after $v_{q}$ tracks $v_{q}^{\text{ref}}$. (a) $\dot{V}_{g}$ rotates clockwise. (b) $\dot{V}_{g}$ rotates counterclockwise.
• Figure 5: A Single Machine GFM IBR Grid-Connected System.