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 loss of small signal stability under parameter variation (e.g., fluctuation of loads and wind powers) can be ascribed to local bifurcations, i.e., saddle-node, Hopf, singularity-induced, and limit-induced bifurcations. Classic bifurcation calculation methods like the direct method and continuation method can only provide a bifurcation point or two-parameter bifurcation curve. Based on Galerkin method and the implicit function theorem, this paper proposes accurate polynomial approximations of bifurcation hypersurfaces in the multi-dimensional parameter space of interest. The proposed method can ensure high accuracy in this parameter space, and thus is preferable to the existing Taylor expansion-based local approximation method, given the intrinsic large-variation characteristic of wind powers. Acquired bifurcation hypersurfaces are immediately used to construct the small signal stability region of the continuous system, and then compute the stability margin for the operating point subjected to uncertainty of wind generation. Besides, the validity scope of the proposed method is analyzed, and the handling of limit-triggered equation switching as well as its computational difficulty is discussed. Computational results on the two-parameter 11-bus two-area and six-parameter IEEE 145-bus test systems validate the high accuracy and effectiveness of the proposed method.

As the photovoltaic (PV) penetration in the grid continues to rise, significant changes occur in the power source configuration of power systems, altering the short-term voltage stability characteristics of the grid. This paper theoretically analyzes the short-term voltage instability that occurs in power systems with PV penetration after large disturbances and proposes corresponding enhancement strategies. First, a reduced network is obtained by retaining voltage-sensitive nodes and simplifying other nodes. Then, based on the reduced network, a concise positive definite matrix is derived to equivalently investigate the singularity of the algebraic Jacobian. This positive definite matrix also allows for further decoupling of the influence of the power network and PV dynamics. Subsequently, the minimum eigenvalue of the positive definite matrix, defined as the Voltage Stability Index ( VSI ), is used to evaluate short-term voltage stability. When VSI equals 0, the algebraic Jacobian becomes singular, indicating the voltage collapse. Building upon this, voltage-vulnerable nodes are identified through the participation factors of VSI . Furthermore, it is proved that enhancing PV transmission lines can both improve short-term voltage stability. Finally, the effectiveness of the proposed index and method is validated using the 3-machine test system, 4-machine 2-area test system, and New England 39-bus test system.

A method based on the rational fractional representation is developed for parametric stability analysis of power systems, such as wind power integrated power systems. A fitted parametric state matrix with its parameters in rational fractions rather than traditional polynomials is shown to enjoy improved accuracy. The generalized linear fractional transformation (LFT) is used to develop the standard linear M(s)-Δ feedback system for robust stability analysis. The mapping theorem is then applied to compute the value set plots of the determinant of the return difference matrix, i.e., det(I - M(s)Δ) for the M(s)-Δ model. And the celebrated zero exclusion principle is directly extended to the value set plots of det(I - M(s)Δ), which reveals the robust stability of the power system with rational fractional uncertainty. The performance of the proposed method is tested on several systems with integrated wind powers including a classic 4-generator 11-bus test power system and a 547-machine 8647-bus model of the actual North China system.