Novel Method for Decaying DC Component Removal Based on Solving the Differential Equations

The fault current signal measured by digital relays contains the decaying direct-current (DDC) component and some higher-order harmonics in addition to the fundamental component. One important challenge of phasor estimation algorithms is the removal of the DDC component, as the presence of this component causes errors in the estimation of the fundamental component magnitude, which can result in malfunction of distance relays. To solve this problem, this paper proposes a new method based on solving the differential equations. Using this method, the fault current signal can be decomposed into minor components and its fundamental component can be properly extracted. After extracting the fundamental component, the discrete Fourier transform algorithm is used to estimate the current phasor. The proposed algorithm is firstly evaluated for different mathematical signals. Then, in the dynamic simulations performed using PSCAD/EMTDC software, the algorithm is employed as a phasor estimation function for a protection distance relay. Real fault data evaluations are also provided. In all cases, the algorithm is compared with some recent phasor estimation methods. The results of comparisons and evaluations confirm the speed and accuracy of the suggested algorithm in removing the DDC component and accurately estimating the fundamental component magnitude.