Contents I. Introduction
When a fault occurs on power systems, not only are the fundamental components included in fault currents, but so are the decaying dc and harmonic components. But the protection system only uses the fundamental component for fault discrimination. Other components usually are obstacles in extracting the fundamental component from the current's waveform. In digital protection relaying, the discrete Fourier transform (DFT) is the most preferable method to extract the fundamental phasor quantities from waveforms [1]. DFT has immunity from harmonic components and has a relatively fast response time for the fundamental component calculation. However, the DFT is not immune from the dc component, and the decaying dc component in the fault current can cause undesirable oscillations in the DFT results [2], [3]. Since these undesirable oscillations can cause abnormal operation of the protection system, especially distance protection, practical digital relaying schemes require additional techniques to reduce the dc component effects in the DFT results.
