Fast Removal of Decaying Offset in Relaying Input Signals

In this paper, we propose a fast algorithm for removing of dc offset in current and voltage signals at the input of relaying devices. The method is based on phasor estimation in the input waves using a modified discrete Fourier transform (DFT). Thanks to proposed 3-state Least Error Squares (LES) algorithm the obtained outputs have specific symmetries which enables on-line control of suppressing the exponentially decaying dc components. The performance of the proposed algorithm is evaluated for faults on a 400 kV overhead transmission line. The Electromagnetic Transient Program (EMTP) is used to generate fault signals. The evaluation results indicate that adopting the proposed method in fault locators or relays can effectively suppress the adverse influence of exponentially decaying dc offsets reducing a transient delay to approximately a quarter a period.