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Power Margin Ratio -- A Large-Signal System Strength Metric for Inverter-Based Resources-Dominated Power Systems

Zitian Qiu, Yunjie Gu

TL;DR

The paper introduces Power Margin Ratio (PMR) as a large-signal grid-strength metric for inverter-based resources-dominated power systems, addressing SCR limitations in multi-infeed scenarios. PMR is defined as $PMR_k = \frac{P_{max,k}}{P_{IBR}}$ and is computed via a modified power-flow approach that accounts for IBR control modes (GFL, grid-supporting, and GFM); it reduces to SCR in single-plant-infinite-bus systems but captures inter-IBR interactions in more complex networks. The theoretical foundation links PMR to dynamical-system concepts, using center-manifold reduction and fold bifurcation to relate PMR to the size of the region of attraction (ROA) and the distance between SEP and UEP. Case studies (SPIB and two-IBR systems) demonstrate that PMR can reveal grid strength and control-mode benefits that SCR may miss, while also highlighting that PMR is not a strict transient stability certificate. Overall, PMR provides a practical, static, yet information-rich metric for assessing large-signal grid strength across multi-infeed, IBR-dominated systems and can inform planning and control decisions.

Abstract

As the growing penetration of inverter-based resources (IBRs) in modern power systems, the system strength is decreasing. Due to the inherent difference in short-circuit capacity contributions of synchronous generators and inverters, the short-circuit ratio is not a one-size-fit-all metric to assess the system strength. Following the distinct dynamic behavior of the IBR in small- and large-signal disturbance, the system strength is separated accordingly. To address the large-signal system strength assessment, a control type-dependent metric, Power Margin Ratio (PMR), is proposed in this paper. PMR is defined as the ratio between the maximum power that can be injected to the system without causing any instability and the nominal power of the IBR. It can be obtained via power flow calculation with a modified algorithm. The theoretical foundation of PMR is established from the viewpoint of dynamical systems. PMR is identical to SCR for the single-plant-infinite-bus system, while presents advancement for multi-infeed power systems. Comprehensive case studies and discussions have validated that PMR reveals the large-signal system strength from a static perspective.

Power Margin Ratio -- A Large-Signal System Strength Metric for Inverter-Based Resources-Dominated Power Systems

TL;DR

The paper introduces Power Margin Ratio (PMR) as a large-signal grid-strength metric for inverter-based resources-dominated power systems, addressing SCR limitations in multi-infeed scenarios. PMR is defined as and is computed via a modified power-flow approach that accounts for IBR control modes (GFL, grid-supporting, and GFM); it reduces to SCR in single-plant-infinite-bus systems but captures inter-IBR interactions in more complex networks. The theoretical foundation links PMR to dynamical-system concepts, using center-manifold reduction and fold bifurcation to relate PMR to the size of the region of attraction (ROA) and the distance between SEP and UEP. Case studies (SPIB and two-IBR systems) demonstrate that PMR can reveal grid strength and control-mode benefits that SCR may miss, while also highlighting that PMR is not a strict transient stability certificate. Overall, PMR provides a practical, static, yet information-rich metric for assessing large-signal grid strength across multi-infeed, IBR-dominated systems and can inform planning and control decisions.

Abstract

As the growing penetration of inverter-based resources (IBRs) in modern power systems, the system strength is decreasing. Due to the inherent difference in short-circuit capacity contributions of synchronous generators and inverters, the short-circuit ratio is not a one-size-fit-all metric to assess the system strength. Following the distinct dynamic behavior of the IBR in small- and large-signal disturbance, the system strength is separated accordingly. To address the large-signal system strength assessment, a control type-dependent metric, Power Margin Ratio (PMR), is proposed in this paper. PMR is defined as the ratio between the maximum power that can be injected to the system without causing any instability and the nominal power of the IBR. It can be obtained via power flow calculation with a modified algorithm. The theoretical foundation of PMR is established from the viewpoint of dynamical systems. PMR is identical to SCR for the single-plant-infinite-bus system, while presents advancement for multi-infeed power systems. Comprehensive case studies and discussions have validated that PMR reveals the large-signal system strength from a static perspective.
Paper Structure (19 sections, 35 equations, 13 figures, 5 tables)

This paper contains 19 sections, 35 equations, 13 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Distinction between small- and large-signal problems
  3. Power margin ratio
  4. Short-circuit ratio
  5. Power margin ratio
  6. Relationship between PMR and SCR
  7. Discussion on PMR
  8. Theoretical background of PMR
  9. A graphical interpretation
  10. The relationship between PMR and stability region
  11. Case studies
  12. Single-plant-infinite-bus system
  13. Two-IBR-infinite-bus system
  14. High SCR while the system is prone to be unstable
  15. Low SCR while the system is strong
  16. ...and 4 more sections

Figures (13)

  • Figure 1: Single line diagram of the single-plant-infinite-bus system
  • Figure 2: Voltage at Bus $\#3$ based on EMT simulations of the SPIB system under (a) small disturbance and (b) large disturbance.
  • Figure 3: A typical structure of an IBR-dominated power system.
  • Figure 4: Circuit representation of a plant connected to an infinite bus.
  • Figure 5: Illustration of $P-\delta$ curve of the single-plant-infinite-bus system.
  • ...and 8 more figures