The high penetration of converter-based renewables (CBRs) is challenging the stability and security of modern power systems, e.g., power balance, due to the fast fluctuation of renewables. Storage energies (SEs) are good choices for addressing the power balance issues, due to their flexibility of bi-directional power regulation. However, the bi-directional power regulation of SEs increases the complexity and difficulty for analyzing the CBRinduced small-signal stability issues in a multi-CBR system, which is one kind of key stability issues in modern power systems, especially in weak grids. It has not been theoretically revealed how the placement of SEs influences the CBR-induced small-signal stability issues, when the SEs absorb active power from grids. This paper aims to fill this gap. Based on modal decomposition theory, this paper explicitly demonstrates that the placement of SEs (when absorbing active power from grids) is equivalent to increasing the grid strength and thus enhancing the CBR-induced small-signal stability. Moreover, this paper investigates the optimal placement of SEs for effectively improving the small-signal stability by using a grid-strength-evaluation indicator, named as generalized shortcircuit ratio (gSCR). Finally, the analysis in this paper is validated based on a modified 39-bus test system.

This letter investigates a system with two identical grid-forming (GFM) converters to give a glimpse of the oscillatory stability characteristics in a power system dominated by multiple GFM converters. It is found that the control interaction of GFM converters can cause the oscillation issues under stiff grid conditions. Particularly, when the network topology is unsymmetrical, the oscillation stability is sensitive to all line impedances and external grid dynamics. When the network topology is symmetrical, the oscillation stability is sensitive to line impedances between converters and point of measurement, but is less sensitive to grid impedances and external grid dynamics.

    The increasing penetration of inverter-based resources (IBRs) in power systems has raised many concerns in terms of frequency stability. However, prior art analytical models for system frequency generally cannot precisely account for the impact of voltage dynamics, leading to potentially incorrect results. The main challenge lies in understanding system frequency in the case of non-constant bus voltages, and integrating the voltage dynamics of generation devices at different bus locations into the system frequency model. To address this issue, this article defines the voltage-influenced common-mode frequency (VCMF), serving as a system frequency analysis model considering voltage dynamics. The VCMF is derived through the decomposition of bus frequency responses, leveraging the connection between the consistent part of bus frequencies and the rotational invariance of power flow. The decomposition process introduces voltage dynamics into the system frequency response, represented as a global term that interconnects all devices through the power network. To address the complexity of this global term, an algebraic graph theory-based network partitioning method is introduced. This method effectively divides the globally coupled term into several locally coupled components, making the analysis of the VCMF more manageable. Finally, simulations are provided to validate the proposed methods.

The growing penetration of Inverter-Based Renewables (IBRs) that synchronize with the power grid through phase-lock loops (PLLs) has caused a set of small-signal stability issues. Commonly, these issues can be addressed by refining controllers’ design of IBRs, which however fails to be effective when power grids are operating under critical conditions. To this end, this paper presents a novel optimization model for coordinating active power outputs (i.e., operation adjustment) of IBRs while satisfying small-signal stability constraints (SSSCs). In particular, SSSCs are formulated based on a new metric that quantifies the small-signal stability from the viewpoint of grid strength, which is especially suitable for those “black-boxed” IBRs. To reduce the problem-solving complexity due to the inherent discontinuity and nonlinearity, a sequential solution approach is proposed to decompose the original optimization problem into a sequence of sub-optimization problems (SOPs). Also, a dynamical-region-adjustment method and a convex-relaxing method are integrated to ensure the existence of feasible solutions and further enhance the solution efficiency. Finally, the performance of the proposed method is verified based on a 11-IBR test power system.

The modern power gird is being transformed into a multiple converter system (MCS) driven by the increasing integration of converter-based resources (CBRs) such as wind and solar. In such an MCS, the dynamic interaction between converter controls and the power network may cause the small-signal stability issues. While static var generators (SVGs) are integrated into the MCS for providing voltage support, they may affect the small-signal stability. To understand the impact of SVGs, this paper investigates the small-signal stability of the MCS with SVGs. By constructing an equivalent model, the theoretical analysis finds two key factors affecting the small-signal stability when SVGs are connected to the MCS: 1) the topology and parameters of the power network interconnecting SVGs and CBRs; 2) the dynamic interaction between the SVGs and CBRs. Based on the results, a computationally efficient method is proposed to assess the small-signal stability of the MCS with SVGs. The analysis results and the proposed method are validated through eigenvalue analysis, and electromagnetic transient simulation.

The increasing penetration of renewable energy resources via power-electronic converters is turning the modern power grid into a multiple-converter system (MCS). In a MCS, the dynamics of converter-based resources (CBRs) are different from those of traditional synchronous machines, which poses great challenges to the grid planning and operation. One of the major challenges is the emerging small-signal stability problems resulting from the interaction between CBRs and the power network. These small-signal stability problems endanger the reliable and stable grid operation. To maintain grid stability, the capacity of CBR generation in the grid may be limited, which impedes the progress towards a sustainable future. To enhance the grid-accommodable capacity (GAC) of renewable generation while maintaining grid stability, this paper presents a semi-definite programming (SDP)-based method to assess GAC with small-signal stability constraints (GAC-SSSC) in a MCS. In the proposed method, the small-signal stability constraints are formulated by the smallest eigenvalue of a pertinent weighted Laplacian matrix of power network, so that the assessment complexity of GAC-SSSC is significantly reduced for a MCS with large-scale CBRs. It is theoretically proved that the derived SDP can find the optimal solution. The efficacy of the proposed method is demonstrated on a 39-bus test system.