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Synchronous condensers (SynCons) have been deployed in power grids penetrated by inverter-based resources (IBRs) worldwide to strengthen and stabilize the grids. This paper examines which machine parameters influence IBR weak grid stability and whether excitation systems also play a role. Four types of stability scenarios are examined, including transient stability, oscillations of a few Hz, oscillations near 9 Hz, and dynamic voltage stability. It is shown that the most influential machine parameter varies for the different types of stability issues. While minimization of field winding inductance (typically the major component of the machine transient reactance, X′d) can significantly improve transient stability, voltage stability, and low-frequency oscillatory stability, this parameter has no influence on relatively rapid oscillations. On the other hand, minimizing rotor damper winding inductance (typically the major component of the machine subtransient reactance, X′′d) improves the 9-Hz oscillation stability, but with insignificant influence on the other three types of stability. Furthermore, the excitation system characteristics show negligible influence for any of the scenarios. In addition to the simulation studies, we show how the operational reactances are associated with the machine's dq impedance viewed from the terminal bus and how a SynCon reduces the equivalent grid impedance, thereby improving weak grid stability. Finally, it is concluded that minimization of both transient and subtransient direct-axis reactances should help in a range of stability scenarios, while cautions should be taken when dealing with quadrature-axis transient reactances. 

While dq admittance models have shown to be very useful for stability analysis, extracting admittance models of inverter-based resources (IBRs) from the electromagnetic transient (EMT) simulation environment using frequency scans takes time. In this letter, a new perturbation method based on Gaussian pulses in combination with the system identification algorithms shows great promise for parametric dq admittance model extraction. We present the dq admittance model extracting method for a type-4 wind turbine. Challenges in implementing Gaussian pulse excitation are also pointed out. The extracted dq admittance model via the new method shows to have a high matching degree with the measurements obtained from frequency scans. 

Twenty-four centrifuge model tests have been conducted at nine different geotechnical centrifuge facilities around the world as part of the international LEAP effort (liquefaction experiments and analysis projects). All of the centrifuge models represent a 4 m deep 5 degree sloping submerged sand deposit. The mean effective PGA of the input motion for all of the experiments was approximately 0.15 g and the mean relative density was approximately 65%, but the effective PGA’s varied between about 0.07 g and 0.3 g, and the relative densities varied between about 40% and 75%. The test matrix was designed to enable experimental quantification of not only the median response but also the trend and sensitivity of the model response to density and shaking intensity. Quantification of the sensitivity of the response to initial conditions is a prerequisite for objective evaluation of the quality of the model test data. In other words, a discrepancy between two experiments should be evaluated after accounting for the uncertainty in the initial conditions and the sensitivity of the response to initial conditions. For the first time, a sufficient number of experiments has been performed on a similar problem to provide meaningful quantitative evaluation of the trend between PGA, density, and displacement. The sensitivity is quantified by the gradient of the trend and the uncertainty of the trend is quantified from the residuals between the fitting data and the trend.