Frequency Scan Considerations for SSCI Analysis of Wind Power Plants

Frequency-dependent impedance scans, or simply frequency scans, are an integral part of the sub-synchronous control interaction (SSCI) study process for assessing stability of wind power plants near series compensated transmission lines. Frequency scans are often utilized as an important screening tool to evaluate small-signal stability and identify contingency cases requiring more detailed evaluation. Small-signal stability is the foundational element required to evaluate SSCI stability, as it encompasses the largest realm of potential events and likelihood. Although a small-signal approximation is typically valid, physical systems, and modern wind turbine generators in particular, are ultimately non-linear and the harmonic excitation signal used to conduct the scan itself must be carefully constructed to maintain validity of the small-signal approximation. Otherwise, the results obtained can be inaccurate and misleading. In this paper, two common frequency scanning methods are compared and validated using detailed time-domain models of wind turbines. Recommendations and guidelines are provided for conducting frequency scans and for interpreting results.