Robust Adaptive Supplementary Control for Damping Weak-Grid SSOs Involving IBRs

Abstract: Subsynchronous oscillations (SSOs) involving grid-following converters (GFLCs) connected to weak grids are a relatively new phenomena observed in modern power systems. SSOs are further exacerbated when grids become weaker because lines are disconnected due to maintenance or following faults. Such undesirable oscillations have also led to curtailment of inverter-based resource (IBR) outputs. In contrast to most literature addressing the issue by retuning/redesigning of standard IBR controllers, we propose a robust adaptive supplementary control for damping of such SSOs while keeping standard controls unaltered. As a result, uncertainty in system conditions can be handled without negatively impacting the nominal IBR performance. To that end, the adaptive control law is derived for a GFLC connected to the grid, where the grid is modeled by the Thevenin's equivalent representation with uncertainty and disturbances. The theoretical result provides dissipativity certificate for the closed-loop error dynamics with sufficient conditions for stability. The effectiveness of the developed controller is validated with several case studies conducted on a single-GFLC-infinite-bus test system, the IEEE $2$-area test system, wherein some of the synchronous generators are replaced by GFLCs, and a modified IEEE $5$-area test system with two GFLCs. The findings demonstrate that under very weak grid conditions, the proposed robust adaptive control performs well in stabilizing SSO modes, which a classical state-feedback control method fails to address.

• The paper presents a robust adaptive control framework that stabilizes subsynchronous oscillations without altering standard GFLC structures.
• It provides analytical guarantees on performance through rigorous stability conditions that manage grid uncertainties and disturbances.
• Validated on both single and multi-machine systems, the approach significantly outperforms conventional state-feedback methods.

Analysis of Robust Adaptive Control for Damping Subsynchronous Oscillations in Weak Grids

The paper "Robust Adaptive Supplementary Control for Damping Weak-Grid SSOs Involving IBRs" by Ameli et al. addresses the growing challenge in modern power systems of subsynchronous oscillations (SSOs) associated with grid-following converters (GFLCs) linked to weak grid connections. This issue is becoming increasingly pertinent as the proliferation of inverter-based resources (IBRs) and grid fluctuations contribute to operational instability.

Motivation and Scope

The need for robust operational strategies in power grids is underscored by the rising integration of IBRs using GFLCs, which are pivotal for linking renewable energy sources to electrical networks. These converters, however, are prone to exacerbate SSOs when connected to weak grids, potentially leading to output curtailment and reduced grid reliability. Traditional strategies often involve retuning or redesigning standard IBR controllers to mitigate these oscillations, but such approaches might necessitate significant system changes and could impact nominal performance under typical conditions.

Approach and Contributions

This study proposes an innovative robust adaptive supplementary control that retains the standard control framework while addressing the uncertainties inherent in weak grids. The authors model the grid using Thevenin's equivalent representation, incorporating both parametric uncertainties and disturbances. This modeling approach allows the developed controller to stabilize SSO modes effectively by offering a dissipation certificate for the closed-loop dynamics.

Key Contributions

1. Robust Adaptive Control Framework: The paper provides a robust control solution capable of adapting to disturbances and uncertainties without modifying the existing GFLC structure.
2. Analytical Guarantees: Through rigorous theoretical development, the paper includes stability conditions, offering strong assurances of performance robustness against system uncertainties and perturbations.
3. Practical Implications: The proposed control system is validated through multiple test systems, including a single GFLC connected to an infinite bus and multi-machine systems incorporating GFLCs replacing synchronous generators. Results indicate significant improvements in controlling SSOs under weak-grid scenarios, which a classical state-feedback approach fails to achieve.

Validation and Results

The effectiveness of the proposed solution is rigorously tested through various scenarios:

• Single-GFLC-infinite bus systems demonstrate the adaptive control's ability to stabilize SSOs under changes in grid capacitance, exhibiting superior performance over conventional methods.
• Further validation on an IEEE 2-area test system confirms the robust adaptive control's capacity to maintain stability under increased power outputs and line maintenance conditions, where traditional methods sputter.

Implications and Future Directions

The research underscores its significance by offering a robust control approach that not only ensures stability in face of significant uncertainties but also integrates seamlessly with existing control architectures, eliminating the need for large-scale system redesigns—an attractive proposition for grid operators. The authors suggest extensions to decentralized control paradigms for multiple GFLCs, and future work could incorporate current limiting within internal control loops and disturbance estimation to optimize control efficacy further.

In conclusion, Ameli et al.'s work advances the discourse on managing SSOs in weak grids, presenting a promising pathway to securing grid stability amidst the increasing complexity of power networks dominated by renewable energy integrations.