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

TL;DR

This paper introduces the Power Margin Ratio (PMR), a new large-signal system strength metric for inverter-based resources in power systems, addressing limitations of traditional measures like SCR.

Contribution

The paper proposes PMR as a control type-dependent metric for large-signal stability assessment, extending system strength evaluation to multi-infeed power systems.

Findings

PMR equals SCR in single-plant-infinite-bus systems

PMR provides a static perspective on large-signal system strength

Case studies validate PMR's effectiveness in multi-infeed systems

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.