Frequency Stability of Synchronous Machines and Grid-Forming Power Converters (2003.04715v1)

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

• The paper demonstrates that grid-forming converters improve key frequency stability metrics, including frequency nadir and RoCoF, in low-inertia power systems.
• The paper uses qualitative case studies to analyze the dynamic interactions between synchronous machines and grid-forming controls, revealing both stabilizing and destabilizing effects.
• The paper emphasizes the need for integrating robust ac current limitations with effective dc dynamic control to ensure reliable operation in renewable-dominated grids.

Frequency Stability of Synchronous Machines and Grid-Forming Power Converters

This paper presents an in-depth analysis of the frequency stability challenges of future power systems transitioning towards 100% renewable energy generation. As synchronous machines (SMs) are gradually replaced, the network's inertia decreases, raising significant concerns over system stability. The authors propose that grid-forming power converters present a viable solution to these emerging stability issues. The paper demonstrates the role of various grid-forming control strategies in ensuring system stability in low-inertia systems.

The paper is structured around a case paper on a system with synchronous machines and grid-forming converters, investigating the effectiveness of various grid-forming controls. The specific contributions include the following key points:

1. Contribution of Grid-Forming Converters: The paper highlights the positive impact of grid-forming converters (GFCs) on the frequency stability of synchronous machines. The converters improve frequency metrics such as frequency nadir and the rate of change of frequency (RoCoF).
2. Qualitative Analysis: The paper provides a qualitative analysis of frequency stability, drawing insights from case studies. It explores how different grid-forming controls can interact with SM dynamics, including potential destabilizing interactions due to differing response times.
3. Dynamic Characteristics and Control Limitations: By exploring the behavior of grid-forming controls under various limitations, the paper emphasizes the importance of dc dynamics and the necessity for effective ac current limitation schemes to prevent system instability.
4. Stability Metrics: The research employs well-established metrics, such as frequency nadir and RoCoF, to assess the performance of different control strategies. These metrics illuminate the potential for GFCs to provide enhancements to system stability over traditional SM systems.
5. Future Grid Configurations: The analysis is extended to scenarios where the system comprises solely of GFCs and investigates the resilience and robustness of these no-inertia systems against load disturbances compared to mixed SM-GFC systems.

The research suggests that although SMs have been integral to traditional power systems, the swift responses enabled by GFCs are pivotal for future low-inertia systems. However, the results also highlight the potential adverse interactions between GFCs and SMs owing to the discrepancies between their dynamic responses. The paper calls for careful consideration of the synchronization and control strategies for GFCs to mitigate these interactions effectively.

The implications of this paper are significant for both the theoretical understanding and practical deployment of inverter-based resources in power systems. From an operational perspective, the research supports the design and implementation of control strategies that leverage the fast dynamics of GFCs for primary frequency control. Theoretically, the paper’s insights into frequency stability can guide future research towards developing more robust integration strategies for high-penetration converter-dominated grids.

In summary, as the transition towards renewable energy grows, grid-forming converters emerge as crucial components for maintaining frequency stability in a depleting inertia environment. This paper underscores the need for continued exploration of grid-forming control strategies, particularly in light of their interactions with slower synchronous machine dynamics, to ensure seamless operation and stability in future power grids. Continued research in this field will likely focus on exploring comprehensive current limitation strategies, seamless grid integration, and hybrid control systems combining different aspects of GFC and SM strategies to harness their collective advantages.