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A robust technique for online estimation of the fundamental frequency of both balanced and unbalanced three-phase power systems is proposed. This is achieved by extending the recently introduced iterative frequency estimation method based on minimum variance distortionless response (MVDR) spectrum , in order to enhance its robustness in unbalanced system conditions. The approach is made optimal for the second-order noncircular nature of the unbalanced complex-valued system voltage, by combining the iterative MVDR (I-MVDR) frequency estimation and the complete available (augmented) second-order statistics. Such an approach makes it possible to eliminate the otherwise unavoidable estimation bias in unbalanced system conditions. It is also shown that the proposed method approaches the theoretical Cramer-Rao lower bound (CRLB), which we rigorously derive for the vector parameter in power systems. Simulations over a range of unbalanced conditions, including voltage sags, the presence of higher-order harmonics, and for real-world unbalanced power systems, support the analysis.