Phase locked loop (PLL) is one of the signal processing tools that has applications in a variety of fields, such as communication systems, control of power systems, power electronic for the purposes such as synchronization, phase detection and estimation of fundamental component of the input sinusoidal signal. The conventional structure of the PLL consists of three blocks, phase detector, loop filter and voltage controlled oscillator. Voltage controlled oscillator generates a sinusoidal signal whose phase angle is synchronous with the phase of input signal, therefore PLL can estimate phase and frequency of input sinusoidal signal. In this regard, in recent years, enhanced phase locked loop was introduced that has better performance compared with PLL. This would ascertain through adding a loop to the conventional PLL that provides the estimation of input signal amplitude. One phase synchronous refrence frame PLL is a simple structure of phase locked loop that is used for estimating the frequency and amplitude of a sinusoidal signal, too. The second order generalized integrator (SOGI) is also a structure for estimating amplitude, phase and the frequency of the sinusoidal signal. Quadrature phase locked loop is another structure for estimating the frequency on the basis of PLL and it is capable of providing the estimation of fundamental component. In this research, it is shown that the one phase PLL structures are mathematically equivalent. In the domain of three phase, three phase PLLs are used for estimating the fundamental component of the input signal. The most common algorithm in the domain of threephase is the three phase PLL or three phase synchronous refrence frame PLL which is widely used for estimating the components of symmetrical component in three phase power systems. In recent years, new PLL structures were suggested based on the generalization of one phase algorithms. A collection of these structures, is generalization of one phase enhanced PLL to the algorithm for estimating the three phase signal parameters that leads to the introducing the three phase magnitude-phase locked loop (MPLL). Another generalization of this structure is led to introducing the threephase PLL with seven order equations that named three phase enhanced PLL. In three phase domain, also a structure, namely threephase SOGI with frequency locked loop (SOGI-FLL), with generalization of second order generalized integrator was introduced. This structure composed of three SOGI sub filter and a first order differential equation that estimates frequency of three phase sinusoidal signals. In this thesis, three phase algorithms and their equivalency are studied. This study shows that threephaseMPLL with three phase synchronous refrence frame PLL and three phase pseudo linear enhanced PLL with three phase SOGI-FLL are equivalent. The performance of one phase PLL, three Phase PLL and their equivalents were examined by computer simulations. Key words: enhanced phase locked loop, second order generalized integrator, three-phase phase locked loop