Joint frequency and phasor estimation in unbalanced three-phase power systems

The problem of joint off-nominal frequency and phasor estimation in an unbalanced three-phase power system using a phasor measurement unit (PMU) is considered. Voltage unbalance arises when magnitudes are different from each other or when phases are unequally spaced. A general model for the zero, positive, and negative sequences from a PMU measurement at off-nominal frequencies is presented. Then, the maximum likelihood (ML) estimator is developed under an unbalanced condition for joint symmetrical components phasors and frequency estimation. Finally, a low complexity technique is developed for the estimation of frequency deviation based a new autoregressive (AR) representation of the innovation process for the symmetrical component.