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A model based approach to optimize controls of a large LCI VSD for minimal grid-side sub-synchronous torsional interaction

Local power generation units in an industrial grid have a certain risk for interacting with power electronics loads (drives, VAR compensators, grid interties), especially if these are of comparable power. Interaction is most prone to happen at the lowest torsional natural frequencies of the power generation equipment. The paper reports, how the level of such sub-synchronous torsional interaction (SSTI) is affected by the dynamics of the controls of a large VSD. Unlike other factors, namely the mechanical properties of the generator and the grid impedances, the damping effect of the VSD's control system can be adjusted to some extend via parameterization. A linear model based approach is reported and demonstrated with a 20MW drive installation, which allows optimizing sub-synchronous torsional damping while maintaining the necessary dynamic response to load- and reference-changes in the VSD system. In the proposed approach, only the critical frequencies of the generator and the driven equipment have to be known, while grid data and influence of other loads needs is segregated from the optimization.