This thesis presents a synchronization algorithm aimed to provide an estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of an unbalanced and distorted three-phase reference signal. It does not require transformation of variables into the synchronous frame coordinates as other conventionalphase-locked loop (PLL) schemes. Thus it is not based on the phase angle detection. Instead the angular frequency is detected and used for synchronization purposes. The design of UH-PLL, is based on a complete description of a three-phase signal which involves both positive and negative sequences in stationary coordinates of the fundamental and harmonic components. Also it includes an explicit harmonic compensation mechanism to alleviate the effect of harmonic distortion and unbalances in the supply. Therefore UH-PLL is intended to perform properly under severe unbalanced conditions, subject to distortion caused by low harmonics, and is insensitive against angular frequency variations, and sags and swells in the three-phase reference signal.