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Grid-following and Grid-forming Switching Control for Grid-connected Inverters Considering Small-signal Security Region

Qiping Lai, Yi Shen, Chen Shen

Abstract

In high-penetration renewable power systems with complex and highly variable operating scenarios, grid-connected inverters (GCIs) may transition between different control modes to adapt to diverse grid conditions. Among these, the switching between grid-following (GFL) and grid-forming (GFM) control modes is particularly critical. Nevertheless, safe and robust GFL-GFM switching control strategies for GCIs remain largely unexplored. To overcome this challenge, this paper establishes a full-order small-signal state-space model for the GFL-GFM switched system, precisely reflecting all internal circuit and control dynamics. Subsequently, the small-signal security region (SSSR) of the switched system is defined and characterized, followed by an in-depth investigation into the multi-parameter impacts on the SSSRs and internal stability margin distributions (ISMDs). Furthermore, a novel comprehensive stability index (CSI) is proposed by integrating the stability margin, parameter sensitivity, and boundary distance. Based on this CSI, a multi-objective adaptive GFL-GFM switching control strategy is designed to guarantee the dynamic security and robustness of the system. Finally, the proposed SSSR analysis method for the GFL-GFM switched system and the designed CSI-based switching control mechanism are validated through electromagnetic transient (EMT) simulations.

Grid-following and Grid-forming Switching Control for Grid-connected Inverters Considering Small-signal Security Region

Abstract

In high-penetration renewable power systems with complex and highly variable operating scenarios, grid-connected inverters (GCIs) may transition between different control modes to adapt to diverse grid conditions. Among these, the switching between grid-following (GFL) and grid-forming (GFM) control modes is particularly critical. Nevertheless, safe and robust GFL-GFM switching control strategies for GCIs remain largely unexplored. To overcome this challenge, this paper establishes a full-order small-signal state-space model for the GFL-GFM switched system, precisely reflecting all internal circuit and control dynamics. Subsequently, the small-signal security region (SSSR) of the switched system is defined and characterized, followed by an in-depth investigation into the multi-parameter impacts on the SSSRs and internal stability margin distributions (ISMDs). Furthermore, a novel comprehensive stability index (CSI) is proposed by integrating the stability margin, parameter sensitivity, and boundary distance. Based on this CSI, a multi-objective adaptive GFL-GFM switching control strategy is designed to guarantee the dynamic security and robustness of the system. Finally, the proposed SSSR analysis method for the GFL-GFM switched system and the designed CSI-based switching control mechanism are validated through electromagnetic transient (EMT) simulations.
Paper Structure (23 sections, 23 equations, 11 figures, 1 table)

This paper contains 23 sections, 23 equations, 11 figures, 1 table.

Table of Contents

  1. Introduction
  2. Switched System Model of the GCIs Under GFL and GFM Switching Control
  3. Mathematical Model of the Electrical Circuits
  4. Mathematical Model of the Control Modules
  5. Common Control Components
  6. GFL Control Loops
  7. GFM Control Loops
  8. Small-signal State-space Switched System Model
  9. SSSR Analysis for GFL-GFM Switched System
  10. Definition of the SSSR for Switched System
  11. Characterization of the SSSR for Switched System
  12. SSSRs and ISMDs of the GFL-GFM Switched System
  13. GFL Subsystem
  14. GFM Subsystem
  15. Switching Control Strategy Design for the GFL-GFM Switched System
  16. ...and 8 more sections

Figures (11)

  • Figure 1: The overall architecture diagram of GCIs under GFL and GFM switching control.
  • Figure 2: SSSRs and ISMDs for PI control parameters of the GFL subsystem under different SCRs. (a) and (b) OPL. (c) and (d) ICL. The black dots denote the boundary points. The colormap for $\mu$ indicates the system stability margins, while the solid lines inside SSSR represent stability margin contours.
  • Figure 3: SSSRs and ISMDs for operational parameters of the GFL subsystem under different control parameter settings.
  • Figure 4: SSSRs and ISMDs for PI control parameters of the GFM subsystem under different SCRs. (a) and (b) OVL. (c) and (d) ICL.
  • Figure 5: SSSRs and ISMDs for operational parameters of the GFM subsystem under different control parameter settings.
  • ...and 6 more figures