Grid-Forming Inverters: A Comparative Study of Different Control Strategies in Frequency and Time Domains
Authors: Nabil Mohammed; Harith Udawatte; Weihua Zhou; David J. Hill; Behrooz Bahrani
Abstract:
Grid-forming inverters (GFMIs) are recognized as critical enablers for the transition to power systems with high renewable energy penetration. Unlike grid-following inverters, which rely on phase-locked loops (PLLs) for synchronization and require a stable grid connection, GFMIs internally establish and regulate grid voltage and frequency. This capability allows them to operate stably in weak grid conditions and provide essential ancillary services, such as voltage and frequency support, inertia emulation, and power oscillation damping.
This article presents a comprehensive comparative study of four control strategies for GFMIs:
- Droop-Based GFMI: Mimics the droop characteristics of synchronous generators by adjusting frequency and voltage in response to active and reactive power imbalances. This approach ensures stable operation in both islanded and grid-connected modes, providing essential grid support functions such as frequency and voltage regulation. Its simplicity and reliability make it a widely adopted control strategy for grid-forming inverters.
- Virtual Synchronous Generator (VSG)-Based GFMI: Emulates the inertia and damping characteristics of synchronous machines, enhancing grid stability. By providing virtual inertia and damping, it improves frequency regulation and grid response to disturbances. It is particularly beneficial for weak grids and high-renewable penetration, offering stability in the absence of traditional synchronous generators.
- Compensated Generalized VSG (CGVSG)-Based GFMI: An advanced VSG variant that incorporates compensation techniques to enhance dynamic performance in both grid-connected and islanded modes. It ensures accurate power tracking in grid-connected mode with lower overshoots and shorter settling times compared to conventional VSG designs. In islanded mode, it provides enhanced virtual inertia to slow down the rate of change of frequency during disturbances.
- Adaptive VSG (AVSG)-Based GFMI: Introduces adaptive mechanisms based on online grid impedance estimation to optimize controller parameters in real-time. Operators can define specified dynamic performance metrics, such as settling time and damping ratio, ensuring consistent behavior under varying grid conditions. The AVSG enhances stability by eliminating oscillations, reducing overshoot, and achieving faster settling times compared to conventional fixed-parameter VSG designs, regardless of grid strength and grid impedance ratio, thereby improving system reliability.
The study evaluates these control strategies using both frequency-domain and time-domain analyses. In the frequency domain, impedance-based stability analysis is employed to assess small-signal stability and low-frequency oscillations across various grid strengths, impedance ratios, and operating points. The time-domain analysis, conducted using electromagnetic transient models, verifies the power tracking capabilities of each controller and their responses to changes in power references. Additionally, the robustness of the controllers is tested against external grid disturbances, such as frequency deviations, phase jumps, and voltage sags, under both weak and strong grid conditions.
The findings reveal the strengths and limitations of each control strategy, providing valuable insights for selecting the most suitable approach based on specific grid requirements and operational scenarios. This work contributes to the ongoing efforts to enhance the stability and reliability of power systems with high penetration of renewable energy sources.
