In examples, a computing system is configured to detect rotor imbalance at wind turbines by (1) obtaining sets of historical-vibration data for turbines, each set comprising vibration data captured by a given turbine's multi-dimensional sensor, (2) deriving a rotor-imbalance-detection model by: (a) for each turbine, (i) transforming time segments of the turbine's historical-vibration dataset into a frequency-domain representation, and (ii) for each time segment, using the frequency-domain representation for the time segment to derive a set of harmonic-mode values for at least one frequency-range of interest, thereby deriving a time-series set of harmonic-mode values for the turbine, and (b) performing an evaluation of the time-series sets for the turbines, thereby deriving the rotor-imbalance-detection model, (3) based on received vibration data for a given turbine from a reference time, executing the derived model, thereby detecting a rotor imbalance at the given turbine, and (4) transmitting a notification of the rotor imbalance.