Frequency Stability of Synchronous Machines and Grid-Forming Power Converters

TL;DR

This paper investigates how grid-forming converters can enhance frequency stability in low-inertia power systems with high renewable penetration, analyzing their interactions with synchronous machines and control limitations.

Contribution

It provides a comprehensive case study on the impact of grid-forming converters, including qualitative analysis and insights into control limitations and system stability.

Findings

Grid-forming converters positively impact frequency stability.

Control limitations significantly influence converter and system behavior.

Interactions between converters and synchronous machines can lead to instability.

Abstract

An inevitable consequence of the global power system transition towards nearly 100% renewable-based generation is the loss of conventional bulk generation by synchronous machines, their inertia, and accompanying frequency and voltage control mechanisms. This gradual transformation of the power system to a low-inertia system leads to critical challenges in maintaining system stability. Novel control techniques for converters, so-called grid-forming strategies, are expected to address these challenges and replicate functionalities that so far have been provided by synchronous machines. This article presents a low-inertia case study that includes synchronous machines and converters controlled under various grid-forming techniques. In this work 1) the positive impact of the grid-forming converters on the frequency stability of synchronous machines is highlighted, 2) a qualitative analysis which provides insights into the frequency stability of the system is presented, 3) we explore the behavior of the grid-forming controls when imposing the converter dc and ac current limitations, 4) the importance of the dc dynamics in grid-forming control design as well as the critical need for an effective ac current limitation scheme are reported, and lastly 5) we analyze how and when the interaction between the fast grid-forming converter and the slow synchronous machine dynamics can contribute to the system instability