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Power Swing Trajectory Influenced by Virtual Impedance-Based Current-Limiting Strategy

Yanshu Niu, Zhe Yang, Bikash C. Pal

TL;DR

Power swing detection in SG-based systems relies on well-characterized impedance trajectories, but grid-forming IBRs with virtual-impedance current limiting produce fundamentally different trajectories. The authors develop a theoretical analysis of full-cycle impedance trajectories under variable VI and adaptive VI strategies, and validate them with MATLAB/Simulink simulations. They show that VI-induced trajectories depend on control parameters and can cause PSB and OST malfunctions, while also deteriorating transient stability. The work underscores the need to rethink protection settings and controller designs in high IBR-penetration grids to maintain reliable power-swing detection and system stability.

Abstract

Grid-forming (GFM) inverter-based resources (IBRs) can emulate the external characteristics of synchronous generators (SGs) through appropriate control loop design. However, in systems with GFM IBRs, the apparent impedance trajectory under current limitation differs significantly from that of SG-based systems due to the limited overcurrent capability of power electronic devices. This difference challenges the power swing detection functions of distance relays designed for SG-based systems. This paper presents a theoretical analysis of the apparent impedance trajectory over a full power swing cycle under two typical current-limiting strategies: variable virtual impedance (VI) and adaptive VI. The analysis reveals that the trajectory under VI current-limiting strategies differs significantly from that of a conventional SG. The results also indicate that the control parameters affect the characteristics of the trajectory. In addition, the new trajectories challenge conventional power swing detection functions, increasing the risk of malfunction. Furthermore, the implementation of VI leads to a deterioration in system stability. The theoretical analysis is further validated through simulations on the MATLAB/Simulink platform.

Power Swing Trajectory Influenced by Virtual Impedance-Based Current-Limiting Strategy

TL;DR

Power swing detection in SG-based systems relies on well-characterized impedance trajectories, but grid-forming IBRs with virtual-impedance current limiting produce fundamentally different trajectories. The authors develop a theoretical analysis of full-cycle impedance trajectories under variable VI and adaptive VI strategies, and validate them with MATLAB/Simulink simulations. They show that VI-induced trajectories depend on control parameters and can cause PSB and OST malfunctions, while also deteriorating transient stability. The work underscores the need to rethink protection settings and controller designs in high IBR-penetration grids to maintain reliable power-swing detection and system stability.

Abstract

Grid-forming (GFM) inverter-based resources (IBRs) can emulate the external characteristics of synchronous generators (SGs) through appropriate control loop design. However, in systems with GFM IBRs, the apparent impedance trajectory under current limitation differs significantly from that of SG-based systems due to the limited overcurrent capability of power electronic devices. This difference challenges the power swing detection functions of distance relays designed for SG-based systems. This paper presents a theoretical analysis of the apparent impedance trajectory over a full power swing cycle under two typical current-limiting strategies: variable virtual impedance (VI) and adaptive VI. The analysis reveals that the trajectory under VI current-limiting strategies differs significantly from that of a conventional SG. The results also indicate that the control parameters affect the characteristics of the trajectory. In addition, the new trajectories challenge conventional power swing detection functions, increasing the risk of malfunction. Furthermore, the implementation of VI leads to a deterioration in system stability. The theoretical analysis is further validated through simulations on the MATLAB/Simulink platform.

Paper Structure

This paper contains 23 sections, 32 equations, 14 figures, 1 table.

Table of Contents

  1. Introduction
  2. Current-Limiting Strategies for Grid-Forming Inverter Based on Virtual Impedance
  3. System Model
  4. Variable Virtual Impedance
  5. Adaptive Virtual Impedance
  6. Analysis of Power Swing Trajectories
  7. Trajectory in the Absence of Current Limitation
  8. Trajectory with the Variable Virtual Impedance Strategy
  9. Trajectory with the Adaptive Virtual Impedance Strategy
  10. Impact of Power Swing Trajectories on Power Swing Detection
  11. Overview of Power Swing Detection
  12. Power Swing Blocking Function
  13. Out-of-Step Tripping Function
  14. Rate of Trajectory Change
  15. Factors Affecting the Power Swing Trajectory
  16. ...and 8 more sections

Figures (14)

  • Figure 1: Grid-connected GFM IBR system. (a) System model and controller structure. (b) Cascaded control loops block diagram.
  • Figure 2: Control block diagram in adaptive VI strategy to obtain the voltage drop $\Delta V$ across the VI.
  • Figure 3: Single-machine grid-connected GFM IBR system.
  • Figure 4: Power swing trajectory in the absence of current limitation and for an SG.
  • Figure 5: Equivalent single-machine grid-connected GFM IBR system after implementing the VI current-limiting strategy.
  • ...and 9 more figures