Abstract-The paper addresses the impact that time varying angular frequencies observed in electrical signals can have on the calculation and separation of components from the instantaneous electric power signal. Instantaneous power theories provide various methods for calculating the instantaneous power components in an electrical network. These methods are based on the basic assumption of constant fundamental frequency and harmonics that are multiple of the fundamental frequency. Recent field measurements in isolated electrical systems have however reported the existence of time varying angular frequencies or instantaneous frequencies. This new observation will affect the very foundation of the established methods for instantaneous power calculation and components separation. This paper analyses the separation of instantaneous average and oscillatory components of powers by using linear and nonlinear filtering approaches in systems that exhibit time varying angular frequencies. The results of this comparison reveals the limitations of the assumption of fundamental and harmonic frequency when using linear filtering techniques in the presence of time varying angular frequencies. Non-linear filtering may offer a more robust and accurate estimation of the instantaneous values of powers and a power quality assessment that better reflects the actual system conditions.