Identifying suitable DC system models for an independent design of VSC controllers in MTDC grids

In AC-multi-terminal DC (MTDC) systems, if converter controllers are designed individually, the interactive system modes stemming from the dynamic couplings among the converters and the DC system components may result in the instability of the interconnected system (INTSYS). This paper presents a comparative system-level study to determine the suitable models for an individual design of converter controllers to prevent INTSYS instability. The goal is to identify suitable individual system (INDSYS) models that do not contain the internal dynamics of adjacent converters and only include the coupling dynamics corresponding to the interactive system modes among the converters. The use of these models for control design will result in the dynamic response of the INTSYS being close to that of the INDSYS models. Furthermore, such models can ensure INTSYS stability when converter controllers are designed individually without requiring the complete model of the INTSYS or internal dynamics of the adjacent converters. Both the master–slave and the droop control modes of operation are analyzed, and specific INDSYS models among those available in the literature are recommended for each operation mode. Time-domain simulations in PSCAD validate the accuracy of the INDSYS models. Furthermore, eigenvalue analysis along with the participation factor and sensitivity analyses for a large set of control parameters are employed to evaluate the efficacy of the recommended models.