Power Systems are commonly planned and operated symmetrically or balanced. However, unsymmetrical situations may occurred. Symmetrical components theory is a commonly used method for power system analysis in such conditions. This method is a basic concept in many of power system problems such as fault analysis, protection, reactive power compensation, power system modeling and identification and etc. In this thesis, a new method for symmetrical component extraction is proposed based on reduced order generalized integrator (ROGI), which is called quadruple reduced order generalized integrator- frequency locked loop (QROGI-FLL). To achieve this aim, ROGI concept is developed and double- reduced order generalized integrator (DROGI) is presented. DROGI is applicable for single-phase power systems, therefore, the concept of QROGI is finally proposed, which is applied for the extraction of three-phase symmetrical components. Additionally, for the estimation of power system frequency, a frequency locked loop (FLL) is added to QROGI, so the final proposed algorithm is called QROGI-FLL. Symmetrical components are extracted and frequency is estimated accurately after an unsymmetrical situation. The proposed method has a low computational burden and its discretization is simple. Thus, it is an appropriate method for implementing with digital signal processing (DSP). Furthermore, as in symmetrical component extraction, the focus is on the positive component, the proposed method has the advantage of soft variation of positive component, during unsymmetrical situation. In fact, the shock-free operation of power system during a possible asymmetry will be achieved. The performance of the proposed method is evaluated by the MATLAB software simulations. The results show that, QROGI-FLL is able to extract symmetrical components efficiently, in all unsymmetrical situations such as amplitude and phase asymmetry and frequency variations either or not in presence of single-phase faults, harmonics and noise. Index Terms — Symmetrical Component Extraction, Frequency Estimation, unsymmetrical Power System, Quadruple Reduced Order Generalized Integrator- Frequency Locked Loop (QROGI-FLL)