•Reliable PLL method to cope with grid unbalances.•Real time estimation of voltage symmetrical components and higher harmonics content.•Solution complexity can be tuned to the expected higher harmonics order. In this paper, a novel PLL technique based on Multi-Resonant disturbance Observer (MRO) is investigated. The proposed technique can achieve grid synchronization under distorted voltages and simultaneously quantify grid unbalances, including voltage sags and higher harmonics. Real-time quantification of grid condition is very important in estimating the overall power quality, which in turn has a drastic effect on power regulation. The proposed technique is easily applicable and expandable to any desired number of higher harmonics. In addition, we provide an in-depth analysis of the connection between original grid unbalances in the stationary reference frame and its image under dq transform. Finally, we propose a methodology capable of real-time estimation of positive and negative harmonic sequences based on the observed properties of fundamental and higher harmonics.