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.

The ever-increasing adoption of power electronic converters for renewable integration and energy-saving applications has weaken grid and caused new types of small-signal stability issues, such as sub-/super-synchronous oscillations, resulting from the interaction between the fast-acting converter control and the power network. It is challenging to analyze the small-signal stability issues by grid strength assessment due to the complex interaction between many converter-interfaced generators (CIGs) and the power network in a multi-CIG system (MCIGS). This complexity is further increased when considering different non-rated operating conditions of each CIG, such as their actual power injections and terminal voltages, which manifest the heterogeneity in the MCIGS. Moreover, in such a heterogeneous MCIGS, it is difficult to characterize the small-signal stability boundary under non-rated operating conditions. To address the challenges, this paper derives generalized operational short-circuit ratio (gOSCR) and critical gOSCR (i.e., CgOSCR) by the small-signal stability analysis in such a heterogeneous MCIGS. Based on gOSCR and CgOSCR, a method is proposed for grid strength assessment to identify the small-signal stability issues in such a heterogeneous MCIGS. This proposed method can be used when CIGs are modeled by either black-box or white-box modeling in a MCIGS under non-rated operating conditions. More importantly, the proposed method can also be used to assess grid strength in terms of the static voltage stability in a MCIGS under non-rated operating conditions, since the static voltage stability issue is a special case of the small-signal stability issue with focus on zero frequency band for converter controls. The efficacy of the proposed method is demonstrated in the IEEE 39-bus system and a practical power system with a large-scale renewable integration.

Temporary overvoltage (TOV) is becoming one challenge for the integration of significant power electronics resources into a power system. To assess the risk of TOV in a Multi-Infeed Power Electronic System (MIPES), a worst-case estimation approach is proposed to assess TOV risk. The paper starts with analyzing the mechanism of TOV, which reveals that TOV is dominated by excessive reactive power resulting from control lagging of converters. Then, the principle of approximating the TOV for a Voltage Source Converter (VSC) connecting to an infinite bus is introduced within the context of the RMS model. After that, a methodology based on the concept of generalized short-circuit ratio for overvoltage (gSCR-TOV) is introduced to quantify the risk of TOV in a multi-bus system. The intent of this concept is to permit screening to identify those nodes at greatest need for further EMT assessment. To localize these nodes, a concept of weighted sensitivity matrix is further proposed to pinpoint the bus subject to highest TOV in a MIPES. Finally, case studies of EMT time-domain simulations are carried out and verifies the effectiveness of the proposed methodologies.

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.

While renewable resources are increasingly integrated into the electric power grid, the small-signal instability risk may be induced by grid-following converters using phase-locked loops (PLLs) for grid synchronization, especially under weak grid conditions. The analysis of the instability mechanism is complex in a multi-converter system due to the dynamic interaction between PLL-based converters and the power network. The analysis complexity is further increased in a heterogeneous multi-converter system (HMCS), where all converters have different control configurations and parameters from different manufacturers. To understand how the different PLL dynamics collectively affect the stability of the HMCS, this paper analytically derives the small-signal stability boundary condition of the HMCS dominated by the PLL dynamics (HMCS-DPLL). The derived stability boundary allows us to obtain analytical results about how the stability of the HMCS-DPLL is affected by grid strength, converter operating conditions, different PLL control parameters and the interaction among different PLLs. Based on the stability boundary condition, a computationally efficient method is also proposed to identify the design rationality of PLL control parameters as well as the small-signal stability and stability margin of the HMCS-DPLL. The analytical results and proposed method are validated by modal analysis and electromagnetic transient simulation with detailed models on a 9-converter heterogeneous system.

The increasing penetration of renewable resources into the power network through grid-following converters has increased the risks of oscillation instability. In such a power system, it is challenging for assessing the small-signal stability due to the complex interaction among converters interconnected through the power network. Moreover, the assessment complexity is further increased in a heterogeneous multi-converter system, where the interconnected converters have different control configurations or parameters from different manufacturers. To tackle the challenges, this paper proposes a method for the small-signal stability analysis of a heterogeneous multi-converter system. To this end, it is first theoretically proved that the small-signal stability of a heterogeneous system can be characterized by an equivalent homogeneous one, where all interconnected converters have the same control configurations and parameters. This can reduce the complexity of the small-signal stability assessment of the original heterogeneous system by decoupling the equivalent homogeneous system into a set of subsystems. To further reduce the assessment complexity, it is derived that the small-signal stability and stability margin of the heterogeneous system can be estimated based on the smallest eigenvalue of a weighted Laplacian matrix of the power network. Based on the analysis results, a scalable method is developed for assessing the small-signal stability of heterogeneous multi-converter systems. The efficacy of the proposed method is validated by performing both modal analysis and time-domain simulations on two heterogeneous multiple-converter systems with different network sizes and converter numbers.

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.