Quantifying Grid-Forming Behavior: Bridging Device-Level Dynamics and System-Level Strength

TL;DR

This paper introduces a unified framework with novel metrics to quantify and analyze grid-forming converter behavior and system strength, bridging device-level dynamics with system stability in power systems.

Contribution

It proposes the Forming Index for device-level GFM assessment and a new measure of system strength, linking GFM converter behavior to system stability.

Findings

The Forming Index effectively quantifies converter response to grid fluctuations.

A new measure captures multi-bus voltage stiffness and system strength.

GFM converters are formally proven to enhance overall system strength.

Abstract

Grid-forming (GFM) technology is widely regarded as a promising solution for future power systems dominated by power electronics. However, a precise method for quantifying GFM converter behavior and a universally accepted GFM definition remain elusive. Moreover, the impact of GFM on system stability is not precisely quantified, creating a significant disconnect between device and system levels. To address these gaps from a small-signal perspective, at the device level, we introduce a novel metric, the Forming Index (FI) to quantify a converter's response to grid voltage fluctuations. Rather than enumerating various control architectures, the FI provides a metric for the converter's GFM ability by quantifying its sensitivity to grid variations. At the system level, we propose a new quantitative measure of system strength that captures the multi-bus voltage stiffness, which quantifies the voltage and phase angle responses of multiple buses to current or power disturbances. We further extend and define this concept to grid strength and bus strength to identify weak areas within the system. Finally, we bridge the device and system levels by formally proving that GFM converters enhance system strength. Our proposed framework provides a unified benchmark for GFM converter design, optimal placement, and system stability assessment.