The increasing penetration of renewable energy resources is changing AC grid dynamics. Potential dynamic stability issues such as small signal instability may occur when these resources are connected to a grid with low strength, i.e., a weak grid. In this paper, the relationships between grid strength measured with short circuit ratio (SCR) and small signal stability and between critical SCR (CSCR) and stability boundary are analytically studied in a single-infeed power electronic system (SIPES). Based on this study, the relationships are extended to a MIPES by eigenvalue decomposition techniques for defining the so-called generalized short circuit ratio (gSCR) and critical gSCR (CgSCR). The defined gSCR and CgSCR are the generalized representation of SCR and CSCR for SIPESs in MIPESs. The gSCR can assess the grid strength of a MIPES in terms of its small signal stability while the CgSCR can characterize the stability boundary. In addition, the condition of CgSCR=CSCR is satisfied for homogeneous power electronic devices (PEDs) in a MIPES. This enables the proposed method based on the gSCR to simplify the stability analysis of MIPESs into the stability study of SIPESs. The efficacy of the proposed gSCR and CgSCR is validated with eigenvalue analysis and time-domain simulation.

The increasing penetration of renewable energy sources (RESs) can challenge both power system planners and operators to maintain system reliability. Potential power system stability issues may arise when a large amount of RESs are connected to a weak power system. The short circuit ratio (SCR) with some modifications has been used to analyze power system strength. However, the existing SCR-based methods for system strength assessment neglect real electrical network connections among multiple RESs, which may not reflect the effect of the interactions among multiple RESs at different sites on system strength. To account for the interactions among electrically interconnected RESs, the so-called site-dependent SCR (SDSCR) is proposed in this paper by analyzing the relationship between system strength and voltage stability. The efficacy of the proposed SDSCR is demonstrated through numerical simulation case studies on the IEEE 39-bus system.

In this paper, a generalized short circuit ratio (gSCR) is pro-posed for assessing the strength of multi-infeed LCC-HVDC (MIDC) systems by the theoretical analysis of the relationship between static voltage stability and grid strength. The gSCR can resolve the ambiguity when applying the concept of short circuit ratio (SCR) for single-infeed LCC-HVDC (SIDC) systems to MIDC systems. Moreover, the gSCR is the generalized presen-tation of SCR in MIDC systems. Based on the gSCR, our defined critical gSCR (CgSCR) and boundary gSCR (BgSCR) for MIDC systems have the same functions as the critical SCR (CSCR) and boundary SCR (BSCR) used in grid strength analysis for SIDC systems. Also, the CgSCR and BgSCR have the same values as those of CSCR and BSCR (i.e., $CgSCR=CSCR\approx2$ and $BgSCR=BSCR\approx3$ ) to characterize the stability boundaries of MIDC systems. This allows the analysis experience of using the SCR, CSCR, and BSCR for SIDC systems can be easily transplanted to the application of the gSCR, CgSCR and BgSCR for MIDC systems. The efficacy of the proposed gSCR along with CgSCR and BgSCR is demonstrated by comparisons them with other indices of grid strength assessment for MIDC systems.

The increasing penetration of renewable resources into the power network through grid-following converters has increased the risks of small-signal instability resulting from the interaction between the converters and the power network. It is challenging to assess the small-signal stability in a power system with high penetration of renewable resources due to the complex interaction between a large number of grid-following converters and 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 such challenges, this paper proposes a method for assessing the small-signal stability in a heterogeneous multi-grid-following-converter system based on grid strength analysis. It is first theoretically proved that the small-signal stability of a heterogeneous multi-grid-following-converter system can be characterized by an equivalent homogeneous one, where all interconnected grid-following converters have the same control configurations and parameters. The equivalent homogeneous system can be decoupled into a set of subsystems for reducing the complexity of the small-signal stability assessment of the original heterogeneous system. On this basis, it is derived that the small-signal stability and stability margin of the heterogeneous system can be estimated based on the generalized short-circuit ratio (gSCR), and the critical gSCR (CgSCR). As a result, a gSCR-based method is proposed for assessing the small-signal stability of heterogeneous multi-converter systems. The efficacy of the proposed method is validated by both modal analysis and electromagnetic transient simulations on two heterogeneous multiple-converter systems with different network topologies and a large-scale practical power system.

The growing integration of converter-interfaced generators (CIGs) has caused small-signal stability issues driven by the converter control interaction of CIGs, especially in weak grids. Grid strength assessment is an important tool for fast identifying the stability issues. However, the existing techniques are ineffective in a multi-CIG system under actual operating conditions (MCIGS-AO). It is challenging for the small-signal stability analysis in a MCIGS-AO while considering different terminal voltages and power outputs for CIGs coupled with their different control parameters and configurations. The black-box converter modeling further increases the analysis complexity. This paper proposes a method for fast identifying the small-signal stability issues in a MCIGS-AO via grid strength assessment. First, we leverage the multiple-time scale analysis technique to study converter control dynamics with a focus on the stability issues mainly caused by converter phase-lock loops. Then, we transform a MCIGS-AO into a set of simple subsystems for the small-signal stability analysis. According to the analysis results, we propose the generalized operational short-circuit ratio (gOSCR) and critical gOSCR (i.e., CgOSCR) to assess grid strength in terms of the small-signal stability. With the gOSCR and CgOSCR, our method is proposed, and it is applicable even when converter details are unknown. The proposed method is validated on a modified IEEE 39-bus system and a practical power system with high wind generation.