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Saturation-Informed Current-Limiting Control for Grid-Forming Converters

Maitraya Avadhut Desai, Xiuqiang He, Linbin Huang, Florian Dörfler

TL;DR

This paper addresses transient stability of grid-forming converters employing complex-droop control (dVOC) when current saturation occurs. It introduces a degree of saturation (DoS) based feedback and saturation-informed outer and inner-loop gains to sustain synchronization during faults, while maintaining a stable equivalent impedance. The authors derive existence conditions for current-saturated equilibria, establish Kron-reduced augmented-network relationships, and provide parametric stability criteria for single- and multi-converter grids as well as islanded microgrids. Validation via case studies demonstrates that the proposed approach preserves stability under faults and saturations, enabling reliable fault ride-through and facilitating stability analysis for networked GFM systems.

Abstract

In this paper, we investigate the transient stability of a state-of-the-art grid-forming complex-droop control (i.e., dispatchable virtual oscillator control, dVOC) under current saturation. We quantify the saturation level of a converter by introducing the concept of degree of saturation (DoS), and we propose a provably stable current-limiting control with saturation-informed feedback, which feeds the degree of saturation back to the inner voltage-control loop and the outer grid-forming loop. As a result, although the output current is saturated, the voltage phase angle can still be generated from an internal virtual voltage-source node that is governed by an equivalent complex-droop control. We prove that the proposed control achieves transient stability during current saturation under grid faults. We also provide parametric stability conditions for multi-converter systems under grid-connected and islanded scenarios. The stability performance of the current-limiting control is validated with various case studies.

Saturation-Informed Current-Limiting Control for Grid-Forming Converters

TL;DR

This paper addresses transient stability of grid-forming converters employing complex-droop control (dVOC) when current saturation occurs. It introduces a degree of saturation (DoS) based feedback and saturation-informed outer and inner-loop gains to sustain synchronization during faults, while maintaining a stable equivalent impedance. The authors derive existence conditions for current-saturated equilibria, establish Kron-reduced augmented-network relationships, and provide parametric stability criteria for single- and multi-converter grids as well as islanded microgrids. Validation via case studies demonstrates that the proposed approach preserves stability under faults and saturations, enabling reliable fault ride-through and facilitating stability analysis for networked GFM systems.

Abstract

In this paper, we investigate the transient stability of a state-of-the-art grid-forming complex-droop control (i.e., dispatchable virtual oscillator control, dVOC) under current saturation. We quantify the saturation level of a converter by introducing the concept of degree of saturation (DoS), and we propose a provably stable current-limiting control with saturation-informed feedback, which feeds the degree of saturation back to the inner voltage-control loop and the outer grid-forming loop. As a result, although the output current is saturated, the voltage phase angle can still be generated from an internal virtual voltage-source node that is governed by an equivalent complex-droop control. We prove that the proposed control achieves transient stability during current saturation under grid faults. We also provide parametric stability conditions for multi-converter systems under grid-connected and islanded scenarios. The stability performance of the current-limiting control is validated with various case studies.
Paper Structure (21 sections, 19 equations, 9 figures, 1 table)

This paper contains 21 sections, 19 equations, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. System Description and Equivalent Circuit
  3. Configuration of Complex-Droop Control
  4. Complex-Droop Control as GFM Reference Model
  5. Inner Controllers and Current Limiter
  6. Equivalent Circuit Under Current Saturation
  7. Saturation-Informed Complex-Droop Control
  8. Saturation-Informed Feedback Control Design
  9. Existence of Current-Saturated Steady-State Equilibrium
  10. Relationship Between Original and Augmented Networks
  11. Conditions for Transient Stability
  12. Single-Converter Grid-Connected Systems
  13. Multi-Converter Grid-Connected Systems
  14. Multi-Converter Islanded Microgrids
  15. Fault Ride-Through and Recovery
  16. ...and 6 more sections

Figures (9)

  • Figure 1: A grid-forming converter with complex-droop control (i.e., dVOC) and a circular current limiter.
  • Figure 2: (a) Current saturation leads to current-dependent virtual impedance in the equivalent circuit fan2022equivalent. (b) The proposed current-limiting control leads to a constant virtual impedance and a virtual voltage-source node with current-dependent adaptive magnitude under current saturation.
  • Figure 3: The proposed saturation-informed current-limiting control strategy scales up the current and voltage feedback signal by the DoS $\mu_{\rm f}$.
  • Figure 4: Kron reduction of the original network for (a) multi-converter grid-connected systems and (b) multi-converter islanded microgrids.
  • Figure 5: Comparison of the control strategies for a single-converter grid-connected system under a grid fault.
  • ...and 4 more figures

Theorems & Definitions (1)

  • Example 1