Identifying and Assessing Small-Signal Stability of Grid-Tied VSCs Considering Multiple Modes

As grid interfaces of renewable resources in weak grids, voltage source converters (VSCs) commonly cause low-frequency and sub-synchronous frequency oscillation issues that are mainly related to dc-link voltage control (DVC) and phase-locked loop (PLL). To investigate these oscillation issues, many scholars have proposed frequency-domain methods with different input-output variables. However, selecting improper input-output variables may lead to incorrect analysis results, which has rarely been discussed. To fill this gap, we propose a method for identifying VSC-induced oscillation modes and choosing the proper input-output variables for stability margin assessment. First, this study proposes a three-terminal admittance model suitable for analyzing both DVC- and PLL-dominated modes. This proposed model can also be evaluated for "black-boxed" VSCs. On this basis, we reveal that different admittance channels reflect different input-output characteristics, and two admittance-based stability criteria are proposed for DVC- and PLL-dominated modes. Moreover, a sensitivity indicator-based method and a corresponding approachable small-signal stability assessment procedure are proposed to identify the proper input-output variables for these two oscillation modes. The validity of the analysis results is verified through simulations in MATLAB/Simulink.